DOKK Library

Linux Administration I - System and Users

Authors tuxcademy

License CC-BY-SA-4.0

                                                                    Version 4.0

Linux Administration I
           System and Users

                    $ echo tux
                    $ ls
                    $ /bin/su -

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      Linux Administration I        System and Users
      Revision: adm1:067839db3bb6bd7f:2015-08-08
                adm1:33e55eeadba676a3:2015-08-08 1–13, B

      © 2015 Linup Front GmbH           Darmstadt, Germany
      © 2015 tuxcademy (Anselm Lingnau)             Darmstadt, Germany ⋅
      Linux penguin “Tux” © Larry Ewing (CC-BY licence)

      All representations and information contained in this document have been com-
      piled to the best of our knowledge and carefully tested. However, mistakes cannot
      be ruled out completely. To the extent of applicable law, the authors and the tux-
      cademy project assume no responsibility or liability resulting in any way from the
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      Authors: Thomas Erker, Anselm Lingnau
      Technical Editor: Anselm Lingnau ⟨ ⟩
      English Translation: Anselm Lingnau
      Typeset in Palatino, Optima and DejaVu Sans Mono
                                                                                                  $ echo tux
                                                                                                  $ ls
                                                                                                  $ /bin/su -


1 System Administration                                                                      13
1.1 Introductory Remarks . . . . . . .           .   .   .   .   .   .   .   .   .   .   .   14
1.2 The Privileged root Account . . . . .        .   .   .   .   .   .   .   .   .   .   .   14
1.3 Obtaining Administrator Privileges . .       .   .   .   .   .   .   .   .   .   .   .   16
1.4 Distribution-specific Administrative Tools    .   .   .   .   .   .   .   .   .   .   .   18

2 User Administration                                                                        21
2.1 Basics . . . . . . . . . . . . . . . . . .                       .   .   .   .   .   .   22
   2.1.1 Why Users? . . . . . . . . . . . . . .                      .   .   .   .   .   .   22
   2.1.2 Users and Groups . . . . . . . . . . .                      .   .   .   .   .   .   23
   2.1.3 People and Pseudo-Users . . . . . . . . .                   .   .   .   .   .   .   25
2.2 User and Group Information . . . . . . . . . .                   .   .   .   .   .   .   25
   2.2.1 The /etc/passwd File . . . . . . . . . . .                  .   .   .   .   .   .   25
   2.2.2 The /etc/shadow File . . . . . . . . . . .                  .   .   .   .   .   .   28
   2.2.3 The /etc/group File . . . . . . . . . . .                   .   .   .   .   .   .   30
   2.2.4 The /etc/gshadow File . . . . . . . . . . .                 .   .   .   .   .   .   31
   2.2.5 The getent Command . . . . . . . . . .                      .   .   .   .   .   .   32
2.3 Managing User Accounts and Group Information . .                 .   .   .   .   .   .   32
   2.3.1 Creating User Accounts . . . . . . . . .                    .   .   .   .   .   .   33
   2.3.2 The passwd Command . . . . . . . . . .                      .   .   .   .   .   .   34
   2.3.3 Deleting User Accounts . . . . . . . . .                    .   .   .   .   .   .   36
   2.3.4 Changing User Accounts and Group Assignment                 .   .   .   .   .   .   36
   2.3.5 Changing User Information Directly—vipw . . .               .   .   .   .   .   .   37
   2.3.6 Creating, Changing and Deleting Groups . . .                .   .   .   .   .   .   37

3 Access Control                                                                             41
3.1 The Linux Access Control System . . . . . . . . .                    .   .   .   .   .   42
3.2 Access Control For Files And Directories . . . . . .                 .   .   .   .   .   42
   3.2.1 The Basics . . . . . . . . . . . . . . .                        .   .   .   .   .   42
   3.2.2 Inspecting and Changing Access Permissions. . .                 .   .   .   .   .   43
   3.2.3 Specifying File Owners and Groups—chown and chgrp               .   .   .   .   .   44
   3.2.4 The umask . . . . . . . . . . . . . . .                         .   .   .   .   .   45
3.3 Access Control Lists (ACLs) . . . . . . . . . . .                    .   .   .   .   .   47
3.4 Process Ownership . . . . . . . . . . . . . .                        .   .   .   .   .   47
3.5 Special Permissions for Executable Files . . . . . . .               .   .   .   .   .   47
3.6 Special Permissions for Directories . . . . . . . .                  .   .   .   .   .   48
3.7 File Attributes . . . . . . . . . . . . . . . .                      .   .   .   .   .   50

4 Process Management                                                                         53
4.1 What Is A Process? . . . . . . .         .   .   .   .   .   .   .   .   .   .   .   .   54
4.2 Process States . . . . . . . . .         .   .   .   .   .   .   .   .   .   .   .   .   55
4.3 Process Information—ps . . . . .         .   .   .   .   .   .   .   .   .   .   .   .   56
4.4 Processes in a Tree—pstree . . . .       .   .   .   .   .   .   .   .   .   .   .   .   57
4.5 Controlling Processes—kill and killall   .   .   .   .   .   .   .   .   .   .   .   .   58
4.6 pgrep and pkill . . . . . . . . .        .   .   .   .   .   .   .   .   .   .   .   .   59
4.7 Process Priorities—nice and renice . .   .   .   .   .   .   .   .   .   .   .   .   .   61
4                                                                                          Contents

    4.8   Further Process Management Commands—nohup and top                    . . . . . 61

    5 Hardware                                                                                     63
    5.1 Fundamentals . . . . . . . .           .   .   .   .   .   .   .   .   .   .   .   .   .   64
    5.2 Linux and PCI (Express) . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   65
       5.2.1 USB. . . . . . . . . .            .   .   .   .   .   .   .   .   .   .   .   .   .   67
    5.3 Peripherals . . . . . . . . .          .   .   .   .   .   .   .   .   .   .   .   .   .   69
       5.3.1 Overview . . . . . . .            .   .   .   .   .   .   .   .   .   .   .   .   .   69
       5.3.2 Devices and Drivers . . . .       .   .   .   .   .   .   .   .   .   .   .   .   .   70
       5.3.3 The /sys Directory . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   72
       5.3.4 udev . . . . . . . . .            .   .   .   .   .   .   .   .   .   .   .   .   .   73
       5.3.5 Device Integration and D-Bus      .   .   .   .   .   .   .   .   .   .   .   .   .   74

    6 Hard Disks (and Other Secondary Storage)                                                     77
    6.1 Fundamentals . . . . . . . . . . .                 .   .   .   .   .   .   .   .   .   .   78
    6.2 Bus Systems for Mass Storage . . . . .             .   .   .   .   .   .   .   .   .   .   78
    6.3 Partitioning . . . . . . . . . . . .               .   .   .   .   .   .   .   .   .   .   81
       6.3.1 Fundamentals . . . . . . . . .                .   .   .   .   .   .   .   .   .   .   81
       6.3.2 The Traditional Method (MBR) . . .            .   .   .   .   .   .   .   .   .   .   82
       6.3.3 The Modern Method (GPT) . . . .               .   .   .   .   .   .   .   .   .   .   83
    6.4 Linux and Mass Storage . . . . . . .               .   .   .   .   .   .   .   .   .   .   84
    6.5 Partitioning Disks. . . . . . . . . .              .   .   .   .   .   .   .   .   .   .   86
       6.5.1 Fundamentals . . . . . . . . .                .   .   .   .   .   .   .   .   .   .   86
       6.5.2 Partitioning Disks Using fdisk . . .          .   .   .   .   .   .   .   .   .   .   88
       6.5.3 Formatting Disks using GNU parted .           .   .   .   .   .   .   .   .   .   .   91
       6.5.4 gdisk . . . . . . . . . . . .                 .   .   .   .   .   .   .   .   .   .   92
       6.5.5 More Partitioning Tools . . . . .             .   .   .   .   .   .   .   .   .   .   93
    6.6 Loop Devices and kpartx . . . . . . .              .   .   .   .   .   .   .   .   .   .   93
    6.7 The Logical Volume Manager (LVM) . . .             .   .   .   .   .   .   .   .   .   .   95

    7 File Systems: Care and Feeding                                                                99
    7.1 Creating a Linux File System . .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   100
       7.1.1 Overview . . . . . .          .   .   .   .   .   .   .   .   .   .   .   .   .   .   100
       7.1.2 The ext File Systems . . .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   102
       7.1.3 ReiserFS . . . . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   .   110
       7.1.4 XFS . . . . . . . . .         .   .   .   .   .   .   .   .   .   .   .   .   .   .   111
       7.1.5 Btrfs . . . . . . . .         .   .   .   .   .   .   .   .   .   .   .   .   .   .   113
       7.1.6 Even More File Systems .      .   .   .   .   .   .   .   .   .   .   .   .   .   .   114
       7.1.7 Swap space . . . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   .   115
    7.2 Mounting File Systems . . . .      .   .   .   .   .   .   .   .   .   .   .   .   .   .   116
       7.2.1 Basics . . . . . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   .   116
       7.2.2 The mount Command . . .       .   .   .   .   .   .   .   .   .   .   .   .   .   .   116
       7.2.3 Labels and UUIDs . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   .   118
    7.3 The dd Command . . . . . .         .   .   .   .   .   .   .   .   .   .   .   .   .   .   120
    7.4 Disk Quotas . . . . . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   .   121
       7.4.1 Basics . . . . . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   .   121
       7.4.2 User Quotas (ext and XFS)     .   .   .   .   .   .   .   .   .   .   .   .   .   .   121
       7.4.3 Group Quotas (ext and XFS)    .   .   .   .   .   .   .   .   .   .   .   .   .   .   123

    8 Booting Linux                                                                                127
    8.1 Fundamentals . . . . . . . .           .   .   .   .   .   .   .   .   .   .   .   .   .   128
    8.2 GRUB Legacy . . . . . . . .            .   .   .   .   .   .   .   .   .   .   .   .   .   131
       8.2.1 GRUB Basics . . . . . .           .   .   .   .   .   .   .   .   .   .   .   .   .   131
       8.2.2 GRUB Legacy Configuration .        .   .   .   .   .   .   .   .   .   .   .   .   .   132
       8.2.3 GRUB Legacy Installation . .      .   .   .   .   .   .   .   .   .   .   .   .   .   133
       8.2.4 GRUB 2 . . . . . . . .            .   .   .   .   .   .   .   .   .   .   .   .   .   134
       8.2.5 Security Advice . . . . .         .   .   .   .   .   .   .   .   .   .   .   .   .   135

8.3     Kernel Parameters . . . . . . . . .                 .   .   .   .   .   .   .   .   .   .   135
8.4     System Startup Problems . . . . . . .               .   .   .   .   .   .   .   .   .   .   137
      8.4.1 Troubleshooting . . . . . . . .                 .   .   .   .   .   .   .   .   .   .   137
      8.4.2 Typical Problems . . . . . . . .                .   .   .   .   .   .   .   .   .   .   137
      8.4.3 Rescue systems and Live Distributions           .   .   .   .   .   .   .   .   .   .   139

9 System-V Init and the Init Process                                                                141
9.1 The Init Process . . . . . .            .   .   .   .   .   .   .   .   .   .   .   .   .   .   142
9.2 System-V Init . . . . . . .             .   .   .   .   .   .   .   .   .   .   .   .   .   .   142
9.3 Upstart . . . . . . . . .               .   .   .   .   .   .   .   .   .   .   .   .   .   .   148
9.4 Shutting Down the System . .            .   .   .   .   .   .   .   .   .   .   .   .   .   .   150

10 Systemd                                                                                          155
10.1 Overview. . . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   156
10.2 Unit Files . . . . . .     .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   157
10.3 Unit Types . . . . .       .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   161
10.4 Dependencies . . . .       .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   162
10.5 Targets. . . . . . .       .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   164
10.6 The systemctl Command      .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   166
10.7 Installing Units. . . .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   169

11 Dynamic (AKA Shared) Libraries                                                                   171
11.1 Compiling and Installing Software . . .                .   .   .   .   .   .   .   .   .   .   172
11.2 Dynamic Libraries In Practice . . . . .                .   .   .   .   .   .   .   .   .   .   174
11.3 Installing and Locating Dynamic Libraries .            .   .   .   .   .   .   .   .   .   .   176
11.4 Dynamic Library Versioning . . . . . .                 .   .   .   .   .   .   .   .   .   .   177

12 Software Package Management Using Debian Tools                                                   181
12.1 Overview. . . . . . . . . . . . . . . .                            .   .   .   .   .   .   .   182
12.2 The Basis: dpkg . . . . . . . . . . . . . .                        .   .   .   .   .   .   .   182
    12.2.1 Debian Packages . . . . . . . . . . .                        .   .   .   .   .   .   .   182
    12.2.2 Package Installation . . . . . . . . . .                     .   .   .   .   .   .   .   183
    12.2.3 Deleting Packages . . . . . . . . . .                        .   .   .   .   .   .   .   184
    12.2.4 Debian Packages and Source Code . . . .                      .   .   .   .   .   .   .   185
    12.2.5 Package Information. . . . . . . . . .                       .   .   .   .   .   .   .   185
    12.2.6 Package Verification . . . . . . . . . .                      .   .   .   .   .   .   .   188
12.3 Debian Package Management: The Next Generation                     .   .   .   .   .   .   .   189
    12.3.1 APT . . . . . . . . . . . . . . .                            .   .   .   .   .   .   .   189
    12.3.2 Package Installation Using apt-get . . . . .                 .   .   .   .   .   .   .   189
    12.3.3 Information About Packages . . . . . . .                     .   .   .   .   .   .   .   191
    12.3.4 aptitude . . . . . . . . . . . . . .                         .   .   .   .   .   .   .   192
12.4 Debian Package Integrity . . . . . . . . . .                       .   .   .   .   .   .   .   194
12.5 The debconf Infrastructure . . . . . . . . .                       .   .   .   .   .   .   .   195
12.6 alien : Software From Different Worlds . . . . .                    .   .   .   .   .   .   .   196

13 Package Management with RPM and YUM                                                              199
13.1 Introduction. . . . . . . . . . . . . . .                          .   .   .   .   .   .   .   200
13.2 Package Management Using rpm . . . . . . . .                       .   .   .   .   .   .   .   201
    13.2.1 Installation and Update . . . . . . . .                      .   .   .   .   .   .   .   201
    13.2.2 Deinstalling Packages . . . . . . . . .                      .   .   .   .   .   .   .   201
    13.2.3 Database and Package Queries . . . . . .                     .   .   .   .   .   .   .   202
    13.2.4 Package Verification . . . . . . . . . .                      .   .   .   .   .   .   .   204
    13.2.5 The rpm2cpio Program . . . . . . . . .                       .   .   .   .   .   .   .   204
13.3 YUM . . . . . . . . . . . . . . . . .                              .   .   .   .   .   .   .   205
    13.3.1 Overview . . . . . . . . . . . . .                           .   .   .   .   .   .   .   205
    13.3.2 Package Repositories . . . . . . . . .                       .   .   .   .   .   .   .   205
    13.3.3 Installing and Removing Packages Using YUM                   .   .   .   .   .   .   .   206
    13.3.4 Information About Packages . . . . . . .                     .   .   .   .   .   .   .   208
    13.3.5 Downloading Packages. . . . . . . . .                        .   .   .   .   .   .   .   210
6                                                                                           Contents

    A Sample Solutions                                                                              211

    B LPIC-1 Certification                                                                           219
    B.1 Overview. . . . . . . .         .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   219
    B.2 Exam LPI-101 . . . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   219
    B.3 Exam LPI-102 . . . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   220
    B.4 LPI Objectives In This Manual   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   221

    C Command Index                                                                                 229

    Index                                                                                           233
                                                                                          $ echo tux
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List of Tables

 3.1   The most important file attributes . . . . . . . . . . . . . . . . . . . .     50

 5.1   USB standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   68

 6.1   Different SCSI variants . . . . . . . . . . . . . . . . . . . . . . . . . .    80
 6.2   Partition types for Linux (hexadecimal) . . . . . . . . . . . . . . . .       82
 6.3   Partition type GUIDs for GPT (excerpt) . . . . . . . . . . . . . . . .        84

 10.1 Common targets for systemd (selection) . . . . . . . . . . . . . . . . 164
 10.2 Compatibility targets for System-V init . . . . . . . . . . . . . . . . . 165
                                                                                        $ echo tux
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List of Figures

 4.1   The relationship between various process states . . . . . . . . . . .       55

 5.1   Output of lspci on a typical x86-based PC . . . . . . . . . . . . . . .     66
 5.2   The usbview program . . . . . . . . . . . . . . . . . . . . . . . . . . .   70

 7.1   The /etc/fstab file (example) . . . . . . . . . . . . . . . . . . . . . . . 117

 9.1   A typical /etc/inittab file (excerpt) . . . . . . . . . . . . . . . . . . . 143
 9.2   Upstart configuration file for job rsyslog . . . . . . . . . . . . . . . . 149

 10.1 A systemd unit file: console- getty.service . . . . . . . . . . . . . . . . 159

 12.1 The aptitude program . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
                                                                                            $ echo tux
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This manual is an introduction to Linux system administration. Based on knowl-
edge about using Linux, it conveys the most important theoretical and practical
knowledge for the setup and operation of a standalone Linux-based computer.
    The manual is geared towards users who have knowledge and experience using
Linux or Unix systems at a level comparable to the tuxcademy manual Introduction
to Linux for Users and Administrators and are looking for a compact but extensive in-
troduction to system administration. Prerequisites are confidence using the shell
and a text editor as well as experience with the common command line tools of a
Linux system.
    This manual covers, after an introduction to the significance and problems
of system administration, the basics of process, user account, and access control
management, the management of disk partitions, file systems, and quotas, com-
mon boot loaders, the system start and shutdown process, PC hardware, and li-
brary and package management.
    The successful completion of this course or comparable knowledge are neces-
sary in order to take part in further Linux study and to obtain Linux Professional
Institute certification.
    This courseware package is designed to support the training course as effi-
ciently as possible, by presenting the material in a dense, extensive format for
reading along, revision or preparation. The material is divided in self-contained
chapters detailing a part of the curriculum; a chapter’s goals and prerequisites chapters
are summarized clearly at its beginning, while at the end there is a summary and goals
(where appropriate) pointers to additional literature or web pages with further prerequisites

B Additional material or background information is marked by the “light-
  bulb” icon at the beginning of a paragraph. Occasionally these paragraphs
  make use of concepts that are really explained only later in the courseware,
  in order to establish a broader context of the material just introduced; these
  “lightbulb” paragraphs may be fully understandable only when the course-
  ware package is perused for a second time after the actual course.

A Paragraphs with the “caution sign” direct your attention to possible prob-
  lems or issues requiring particular care. Watch out for the dangerous bends!

C Most chapters also contain exercises, which are marked with a “pencil” icon exercises
  at the beginning of each paragraph. The exercises are numbered, and sam-
  ple solutions for the most important ones are given at the end of the course-
  ware package. Each exercise features a level of difficulty in brackets. Exer-
  cises marked with an exclamation point (“!”) are especially recommended.

   Excerpts from configuration files, command examples and examples of com-
puter output appear in typewriter type . In multiline dialogs between the user and
the computer, user input is given in bold typewriter type in order to avoid misun-
derstandings. The “” symbol appears where part of a command’s output
had to be omitted. Occasionally, additional line breaks had to be added to make
things fit; these appear as “
12                                                                                                   Preface

                           ”. When command syntax is discussed, words enclosed in angle brack-
                         ets (“⟨Word⟩”) denote “variables” that can assume different values; material in
                         brackets (“[-f ⟨file⟩]”) is optional. Alternatives are separated using a vertical bar
                         (“-a |-b ”).
     Important concepts      Important concepts are emphasized using “marginal notes” so they can be eas-
             definitions ily located; definitions of important terms appear in bold type in the text as well
                         as in the margin.
                             References to the literature and to interesting web pages appear as “[GPL91]”
                         in the text and are cross-referenced in detail at the end of each chapter.
                             We endeavour to provide courseware that is as up-to-date, complete and error-
                         free as possible. In spite of this, problems or inaccuracies may creep in. If you
                         notice something that you think could be improved, please do let us know, e.g.,
                         by sending e-mail to

                        (For simplicity, please quote the title of the courseware package, the revision ID
                        on the back of the title page and the page number(s) in question.) Thank you very

                        LPIC-1 Certification
                        These training materials are part of a recommended curriculum for LPIC-1 prepa-
                        ration. Refer to Appendix B for further information.
                                                                                                     $ echo tux
                                                                                                     $ ls
                                                                                                     $ /bin/su -

System Administration

1.1     Introductory Remarks . . . . . . .          .   .   .   .   .   .   .   .   .   .   .   14
1.2     The Privileged root Account . . . . .       .   .   .   .   .   .   .   .   .   .   .   14
1.3     Obtaining Administrator Privileges . .      .   .   .   .   .   .   .   .   .   .   .   16
1.4     Distribution-specific Administrative Tools   .   .   .   .   .   .   .   .   .   .   .   18

      • Reviewing a system administrator’s tasks
      • Being able to log on as the system administrator
      • Being able to assess the advantages and disadvantage of (graphical) admin-
        istration tools

      • Basic Linux skills
      • Administration skills for other operating systems are helpful

adm1-grundlagen.tex   (33e55eeadba676a3 )
14                                                                                       1 System Administration

                          1.1    Introductory Remarks
                       As a mere user of a Linux system, you are well off: You sit down in front of your
                       computer, everything is configured correctly, all the hardware is supported and
                       works. You have no care in the world since you can call upon a system adminis-
                       trator who will handle all administrative tasks for you promptly and thoroughly
                       (that’s what we wish your environment is like, anyway).
                          Should you be (or strive to be) the system administrator yourself—within your
                       company or the privacy of your home—then you have your work cut out for you:
                       You must install and configure the system and connect any peripherals. Having
                       done that, you need to keep the system running, for example by checking the sys-
                       tem logs for unusual events, regularly getting rid of old log files, making backup
                       copies, installing new software and updating existing programs, and so on.
                          Today, in the age of Linux distributions with luxurious installation tools, sys-
                       tem installation is no longer rocket science. However, an ambitious administrator
                       can spend lots of time mobilising every last resource on their system. In general,
               changes system administration mostly takes place when a noticeable change occurs, for
                       example when new hardware or software is to be integrated, new users arrive or
                       existing ones disappear, or hardware problems arise.

                  Tools   B Many Linux distributions these days contain specialised tools to facilitate
                            system administration. These tools perform different tasks ranging from
                            user management and creating file systems to complete system updates.
                            Utilities like these can make these tasks a lot easier but sometimes a lot more
                            difficult. Standard procedures are simplified but for specialised settings you
                            should know the exact relationships between system components. Further-
                            more, most of these tools are only available for certain distributions.

                            The administration of a Linux system, as of any other computer system, re-
           responsibility quires a considerable amount of responsibility and care. You should not see your-
                      self as a demigod (at least) but as a service provider. No matter whether you are
                      the only system administrator—say, on your own computer—or working in a team
        communication of colleagues to support a company network: communication is paramount. You
                      should get used to documenting configuration changes and other administrative
                      decisions in order to be able to retrace them later. The Linux way of directly edit-
                      ing text files makes this convenient, since you can comment configuration settings
                      right where they are made (a luxury not usually enjoyed with graphical adminis-
                      tration tools). Do so.

                          1.2    The Privileged root Account
                          For many tasks, the system administrator needs special privileges. Accordingly,
                          he can make use of a special user account called root . As root , a user is the so-called
              super user super user. In brief: He may do anything.
                             The normal file permissions and security precautions do not apply to root . He
     unlimited privileges has allowing him nearly unbounded access to all data, devices and system compo-
                          nents. He can institute system changes that all other users are prohibited from by
                          the Linux kernel’s security mechanisms. This means that, as root , you can change
                          every file on the system no matter who it belongs to. While normal users cannot
                          wreak damage (e. g., by destroying file systems or manipulating other users’ files),
                          root is not thus constrained.

                          B In many cases, these extensive system administrator privileges are really
                            a liability. For example, when making backup copies it is necessary to be
                            able to read all files on the system. However, this by no means implies that
                            the person making the backup (possibly an intern) should be empowered to
                            open all files on the system with a text editor, to read them or change them—
                            or start a network service which might be accessible from anywhere in the
1.2 The Privileged root Account                                                                                       15

      world. There are various ways of giving out administrator privileges only in
      controlled circumstances (such as sudo , a system which lets normal users ex- sudo
      ecute certain commands using administrator privileges), of selectively giv-
      ing particular privileges to individual process rather than operating on an
      “all or nothing” principle (cue POSIX capabilities), or of doing away with POSIX capabilities
      the idea of an “omnipotent” system administrator completely (for instance,
      SELinux—“security-enhanced Linux”—a freely available software package SELinux
      by the American intelligence agency, NSA, contains a “role-based” access
      control system that can get by without an omnipotent system administra-

    Why does Linux contain security precautions in the first place? The most im-          Why Security?
portant reason is for users to be able to determine the access privileges that apply
to their own files. By setting permission bits (using the chmod command), users
can ascertain that certain files may be read, written to or executed by certain oth-
ers (or no) users. This helps safeguard the privacy and integrity of their data. You
would certainly not approve of other users being able to read your private e-mail
or change the source code of an important program behind your back.
    The security mechanisms are also supposed to keep users from damaging the
system. Access to many of the device files in /dev corresponding to hardware com-         Access control for devices
ponents such as hard disks is constrained by the system. If normal users could ac-
cess disk storage directly, all sorts of mayhem might occur (a user might overwrite
the complete content of a disk or, having obtained information about the layout
of the filesystem on the disk, access files that are none of his business). Instead,
the system forces normal users to access the disks via the file system and protects
their data in that way.
    It is important to stress that damage is seldom caused on purpose. The system’s
security mechanisms serve mostly to save users from unintentional mistakes and
misunderstandings; only in the second instance are they meant to protect the pri-
vacy of users and data.
    On the system, users can be pooled into groups to which you may assign their         groups
own access privileges. For example, a team of software developers could have
read and write permission to a number of files, while other users are not allowed
to change these files. Every user can determine for their own files how permissive
or restrictive access to them should be.
    The security mechanisms also prevent normal users from performing certain
actions such as the invocation of specific system calls from a program. For exam-         Privileged system calls
ple, there is a system call that will halt the system, which is executed by programs
such as shutdown when the system is to be powered down or rebooted. If normal
users were allowed to invoke this routine from their own programs, they could
inadvertently (or intentionally) stop the system at any time.
    The administrator frequently needs to circumvent these security mechanisms
in order to maintain the system or install updated software versions. The root
account is meant to allow exactly this. A good administrator can do his work
without regard for the usual access permissions and other constraints, since these
do not apply to root . The root account is not better than a normal user account
because it has more privileges; the restriction of these privileges to root is a secu-
rity measure. Since the operating system’s reasonable and helpful protection and
security mechanisms do not apply to the system administrator, working as root
is very risky. You should therefore use root to execute only those commands that
really require the privileges.

B Many of the security problems of other popular operating systems can be
  traced back to the fact that normal users generally enjoy administrator priv-
  ileges. Thus, programs such as “worms” or “Trojan horses”, which users
  often execute by accident, find it easy to establish themselves on the sys-
  tem. With a Linux system that is correctly installed and operated, this is
  hardly possible since users read their e-mail without administrator privi-
16                                                                                           1 System Administration

                                       leges, but administrator privileges are required for all system-wide config-
                                       uration changes.

                                 B Of course, Linux is not magically immune against malicious pests like
                                   “mail worms”; somebody could write and make popular a mail program
                                   that would execute “active content” such as scripts or binary programs
                                   within messages like some such programs do on other operating systems.
                                   On Linux, such a “malicious” program from elsewhere could remove all
                                   the caller’s files or try to introduce “Trojan” code to his environment, but
                                   it could not harm other users nor the system itself—unless it exploited a
                                   security vulnerability in Linux that would let a local user gain administrator
                                   privileges “through the back door” (such vulnerabilities are detected now
                                   and again, and patches are promptly published which you should install in
                                   a timely manner).

                                 C 1.1 [2] What is the difference between a user and an administrator? Name
                                   examples for tasks and actions (and suitable commands) that are typically
                                   performed from a user account and the root account, respectively.

                                 C 1.2 [!1] Why should you, as a normal user, not use the root account for your
                                   daily work?

                                 C 1.3 [W]hat about access control on your computer at home? Do you work
                                   from an administrator account?

                                 1.3     Obtaining Administrator Privileges
                                 There are two ways of obtaining administrator privileges:
                                    1. You can log in as user root directly. After entering the correct root password
                                       you will obtain a shell with administrator privileges. However, you should
                                       avoid logging in to the GUI as root , since then all graphical applications in-
                                       cluding the X server would run with root privileges, which is not necessary
                                       and can lead to security problems. Nor should direct root logins be allowed
                                       across the network.

                                       B You can determine which terminals are eligible for direct root login
                                         by listing them in the /etc/securetty file. The default setting is usually
                                         “all virtual consoles and /dev/ttyS0 ” (the latter for users of the “serial

                                    2. You can, from a normal shell, use the su command to obtain a new shell with
                                       administrator privileges. su , like login , asks for a password and opens the
                                       root shell only after the correct root password has been input. In GUIs like
                                       KDE there are similar methods.
                                 (See also Introduction to Linux for Users and Administrators.)
     Single-user systems, too!       Even if a Linux system is used by a single person only, it makes sense to create
                                 a normal account for this user. During everyday work on the system as root , most
                                 of the kernel’s normal security precautions are circumvented. That way errors can
                                 occur that impact on the whole system. You can avoid this danger by logging into
                                 your normal account and starting a root shell via “/bin/su - ” if and when required.

                                 B Using su , you can also assume the identity of arbitrary other users (here hugo )
                                   by invoking it like
1.3 Obtaining Administrator Privileges                                                                                17

       $ /bin/su - hugo

      You need to know the target user’s password unless you are calling su as
      user root .
   The second method is preferable to the first for another reason, too: If you use
the su command to become root after logging in to your own account, su creates a
message like
Apr   1 08:18:21 HOST su: (to root) user1 on /dev/tty2

in the system log (such as /var/log/messages ). This entry means that user user1 suc- system log
cessfully executed su to become root on terminal 2. If you log in as root directly,
no such message is logged; there is no way of figuring out which user has fooled
around with the root account. On a system with several administrators it is often
important to retrace who entered the su command when.
      Ubuntu is one of the “newfangled” distributions that deprecate–and, in the
      default setup, even disable—logging in as root . Instead, particular users
      may use the sudo mechanism to execute individual commands with admin-
      istrator privileges. Upon installation, you are asked to create a “normal”
      user account, and that user account is automatically endowed with “indi-
      rect” administrator privileges.
      When installing Debian GNU/Linux, you can choose between assigning a
      password to the root account and thereby enabling direct administrator lo-
      gins, and declining this and, as on Ubuntu, giving sudo -based administrator
      privileges to the first unprivileged user account created as part of the instal-
      lation process.
   On many systems, the shell prompt differs between root and the other users. shell prompt
The classic root prompt contains a hash mark (# ), while other users see a prompt
containing a dollar sign ($ ) or greater-than sign (> ). The # prompt is supposed
to remind you that you are root with all ensuing privileges. However, the shell
prompt is easily changed, and it is your call whether to follow this convention or

B Of course, if you are using sudo , you never get to see a prompt for root .
    Like all powerful tools, the root account can be abused. Therefore it is impor-     Misuse of root
tant for you as the system administrator too keep the root password secret. It
should only be passed on to users who are trusted both professionally and per-
sonally (or who can be held responsible for their actions). If you are the sole user
of the system this problem does not apply to you.
    Too many cooks spoil the broth! This principle also applies to system admin-        Administration: alone or by
istration. The main benefit of “private” use of the root account is not that the         many
possibility of misuse is minimised (even though this is surely a consequence).
More importantly, root as the sole user of the root account knows the complete
system configuration. If somebody besides the administrator can, for example,
change important system files, then the system configuration could be changed
without the administrator’s knowledge. In a commercial environment, it is nec-
essary to have several suitably privileged employees for various reasons—for ex-
ample, safeguarding system operation during holidays or sudden severe illness
of the administrator—; this requires close cooperation and communication.
    If there is only one system administrator who is responsible for system con-
figuration, you can be sure that one person really knows what is going on on the
system (at least in theory), and the question of accountability also has an obvi-       accountability
ous asnwer. The more users have access to root , the greater is the probability that
somebody will commit an error as root at some stage. Even if all users with root
access possess suitable administration skills, mistakes can happen to anybody.
Prudence and thorough training are the only precautions against accidents.
18                                                               1 System Administration

           There are a few other useful tools for team-based system administration.
           For example, Debian GNU/Linux and Ubuntu support a package called
           etckeeper , which allows storing the complete content of the /etc directory in
           a revision control system such as Git or Mercurial. Revision control systems
           (which we cannot cover in detail here) make it possible to track changes to
           files in a directory hierarchy in a very detailed manner, to comment them
           and, if necessary, to undo them. With Git or Mercurial it is even possible to
           store a copy of the /etc directory on a completely different computer and to
           keep it in sync automatically—great protection from accidents.

     C 1.4 [2] What methods exist to obtain administrator rights? Which method
       is better? Why?

     C 1.5 [!2] On a conventionally configured system, how can you recognise
       whether you are working as root ?

     C 1.6 [2] Log in as a normal user (e. g., test ). Change over to root and back to
       test . How do you work best if you frequently need to change between both
       these accounts (for example, to check on the results of a new configuration)?

     C 1.7 [!2] Log in as a normal user and change to root using su . Where do you
       find a log entry documenting this change? Look at that message.

     1.4    Distribution-specific Administrative Tools
     Many Linux distributions try to stand out in the crowd by providing more or less
     ingenious tools that are supposed to simplify system administration. These tools
     are usually tailored to the distributions in question. Here are a few comments
     about typical specimens:

           A familiar sight to SUSE administrators is “YaST”, the graphical adminis-
           tration interface of the SUSE distributions (it also runs on a text screen). It
           allows the extensive configuration of many aspects of the system either by
           directly changing the configuration files concerned or by manipulating ab-
           stract configuration files below /etc/sysconfig which are then used to adapt
           the real configuration files by means of the SuSEconfig tool. For some tasks
           such as network configuration, the files below /etc/sysconfig are the actual
           configuration files.

           Unfortunately, YaST is not a silver bullet for all problems of system admin-
           istration. Even though many aspects of the system are amenable to YaST-
           based administration, important settings may not be accessible via YaST, or
           the YaST modules in question simply do not work correctly. The danger
           zone starts where you try to administer the computer partly through YaST
           and partly through changing configuration files manually: Yast does exer-
           cise some care not to overwrite your changes (which wasn’t the case in the
           past—up till SuSe 6 or so, YaST and SuSEconfig used to be quite reckless),
           but will then not perform its own changes such that they really take effect in
           the system. In other places, manual changes to the configuration files will
           actually show up in YaST. Hence you have to have some “insider knowl-
           edge” and experience in order to assess which configuration files you may
           change directly and which your grubby fingers had better not touch.

           Some time ago, Novell released the YaST source code under the GPL (in
           SUSE’s time it used to be available but not under a “free” licence). However,
           so far no other distribution of consequence has adapted YaST to its purposes,
           let alone made it a standard tool (SUSE fashion).
1.4 Distribution-specific Administrative Tools                                         19

B The Webmin package by Jamie Cameron ( ) allows the
  convenient administration of various Linux distributions (or Unix versions)
  via a web-based interface. Webmin is very extensive and offers special fa-
  cilities for administering “virtual” servers (for web hosters and their cus-
  tomers). However you may have to install it yourself, since most distribu-
  tions do not provide it. Webmin manages its own users, which means that
  you can extend administrator privileges to users who do not have interac-
  tive system access. (Whether that is a smart idea is a completely different

   Most administration tools like YaST and Webmin share the same disadvan-
   • They are not extensive enough to take over all aspects of system administra-
     tions, and as an administrator you have to have detailed knowledge of their
     limits in order to be able to decide where to intervene manually.
   • They make system administration possible for people whose expertise is
     not adequate to assess the possible consequences of their actions or to find
     and correct mistakes. Creating a user account using an administration tool
     is certainly not a critical job and surely more convenient than editing four
     different system files using vi , but other tasks such as configuring a fire-
     wall or mail server are not suitable for laypeople even using a convenient
     administration tool. The danger is that inexperienced administrators will
     use an administration tool to attempt tasks which do not look more com-
     plicated than others but which, without adequate background knowledge,
     may endanger the safety and/or reliability of the system.
   • They usually do not offer a facility to version control or document any
     changes made, and thus complicate teamwork and auditing by requiring
     logs to be kept externally.
   • They are often intransparent, i. e., they do not provide documentation about
     the actual steps they take on the system to perform administrative tasks.
     This keeps the knowledge about the necessary procedures buried in the pro-
     grams; as the administrator you have no direct way of “learning” from the
     programs like you could by observing an experienced administrator. Thus
     the adminstration tools keep you artificially stupid.
   • As an extension of the previous point: If you need to administer several
     computers, common administration tools force you to execute the same
     steps repeatedly on every single machine. Many times it would be more
     convenient to write a shell script automating the required procedure, and to
     execute it automatically on every computer using, e. g., the “secure shell”,
     but the administration tool does not tell you what to put into this shell
     script. Therefore, viewed in a larger context, their use is inefficient.
    From various practical considerations like these we would like to recommend
against relying too much on the “convenient” administration tools provided by
the distributions. They are very much like training wheels on a bicycle: They
work effectively against falling over too early and provide a very large sense of
achievement very quickly, but the longer the little ones zoom about with them, the
more difficult it becomes to get them used to “proper” bike-riding (here: doing
administration in the actual configuration files, including all advantages such as
documentation, transparency, auditing, team capability, transportability, …).
    Excessive dependence on an administration tool also leads to excessive depen-
dence on the distribution featuring that tool. This may not seem like a real liabil-
ity, but on the other hand one of the more important advantages of Linux is the fact
that there are multiple independent vendors. So, if one day you should be fed up
with the SUSE distributions (for whatever reason) and want to move over to Red
Hat or Debian GNU/Linux, it would be very inconvenient if your administrators
20                                                             1 System Administration

     knew only YaST and had to relearn Linux administration from scratch. (Third-
     party administration tools like Webmin do not exhibit this problem to the same

     C 1.8 [!2] Does your distribution provide an administration tool (such as
       YaST)? What can you do with it?

     C 1.9 [3] (Continuation of the previous exercise—when working through the
       manual for the second time.) Find out how your administration tool works.
       Can you change the system configuration manually so the administration
       tool will notice your changes? Only under some circumstances?

     C 1.10 [!1] Administration tools like Webmin are potentially accessible to ev-
       erybody with a browser. Which advantages and disadvantages result from

        • Every computer installation needs a certain amount of system administra-
          tion. In big companies, universities and similar institutions these services
          are provided by (teams of) full-time administrators; in smaller companies
          or private households, (some) users usually serve as administrators.
        • Linux systems are, on the whole, straightforward to administer. Work arises
          mostly during the initial installation and, during normal operation, when
          the configuration changes noticeably.
        • On Linux systems, there usually is a privileged user account called root , to
          which the normal security mechanisms do not apply.
        • As an administrator, one should not work as root exclusively, but use a nor-
          mal user account and assume root privileges only if necessary.
        • Administration tools such as YaST or Webmin can help perform some ad-
          ministrative duties, but are no substitute for administrator expertise and
          may have other disadvantages as well.
                                                                                         $ echo tux
                                                                                         $ ls
                                                                                         $ /bin/su -

User Administration

2.1  Basics . . . . . . . . . . . . . . . . . .             .   .   .   .   .   .   22
   2.1.1 Why Users? . . . . . . . . . . . . . .             .   .   .   .   .   .   22
   2.1.2 Users and Groups . . . . . . . . . . .             .   .   .   .   .   .   23
   2.1.3 People and Pseudo-Users . . . . . . . . .          .   .   .   .   .   .   25
2.2 User and Group Information . . . . . . . . . .          .   .   .   .   .   .   25
   2.2.1 The /etc/passwd File . . . . . . . . . . .         .   .   .   .   .   .   25
   2.2.2 The /etc/shadow File . . . . . . . . . . .         .   .   .   .   .   .   28
   2.2.3 The /etc/group File . . . . . . . . . . .          .   .   .   .   .   .   30
   2.2.4 The /etc/gshadow File . . . . . . . . . . .        .   .   .   .   .   .   31
   2.2.5 The getent Command . . . . . . . . . .             .   .   .   .   .   .   32
2.3 Managing User Accounts and Group Information . .        .   .   .   .   .   .   32
   2.3.1 Creating User Accounts . . . . . . . . .           .   .   .   .   .   .   33
   2.3.2 The passwd Command . . . . . . . . . .             .   .   .   .   .   .   34
   2.3.3 Deleting User Accounts . . . . . . . . .           .   .   .   .   .   .   36
   2.3.4 Changing User Accounts and Group Assignment        .   .   .   .   .   .   36
   2.3.5 Changing User Information Directly—vipw . . .      .   .   .   .   .   .   37
   2.3.6 Creating, Changing and Deleting Groups . . .       .   .   .   .   .   .   37

      • Understanding the user and group concepts of Linux
      • Knowing how user and group information is stored on Linux
      • Being able to use the user and group administration commands

      • Knowledge about handling configuration files

adm1-benutzer.tex   (33e55eeadba676a3 )
22                                                                                2 User Administration

                     2.1     Basics
                     2.1.1   Why Users?
                     Computers used to be large and expensive, but today an office workplace without
                     its own PC (“personal computer”) is nearly inconceivable, and a computer is likely
                     to be encountered in most domestic “dens” as well. And while it may be sufficient
                     for a family to agree that Dad, Mom and the kids will put their files into different
                     directories, this will no longer do in companies or universities—once shared disk
                     space or other facilities are provided by central servers accessible to many users,
                     the computer system must be able to distinguish between different users and to
                     assign different access rights to them. After all, Ms Jones from the Development
                     Division has as little business looking at the company’s payroll data as Mr Smith
                     from Human Resources has accessing the detailed plans for next year’s products.
                     And a measure of privacy may be desired even at home—the Christmas present
                     list or teenage daughter’s diary (erstwhile fitted with a lock) should not be open
                     to prying eyes as a matter of course.

                     B We shall be discounting the fact that teenage daughter’s diary may be visible
                       to the entire world on Facebook (or some such); and even if that is the case,
                       the entire world should surely not be allowed to write to teenage daughter’s
                       dairy. (Which is why even Facebook supports the notion of different users.)

                         The second reason for distinguishing between different users follows from the
                     fact that various aspects of the system should not be visible, much less change-
                     able, without special privileges. Therefore Linux manages a separate user iden-
                     tity (root ) for the system administrator, which makes it possible to keep informa-
                     tion such as users’ passwords hidden from “common” users. The bane of older
                     Windows systems—programs obtained by e-mail or indiscriminate web surfing
                     that then wreak havoc on the entire system—will not plague you on Linux, since
                     anything you can execute as a common user will not be in a position to wreak
                     system-wide havoc.

                     A Unfortunately this is not entirely correct: Every now and then a bug comes
                       to light that enables a “normal user” to do things otherwise restricted to
                       administrators. This sort of error is extremely nasty and usually corrected
                       very quickly after having been found, but there is a considerable chance that
                       such a bug has remained undetected in the system for an extended period
                       of time. Therefore, on Linux (as on all other operating systems) you should
                       strive to run the most current version of critical system parts like the kernel
                       that your distributor supports.

                     A Even the fact that Linux safeguards the system configuration from unau-
                       thorised access by normal users should not entice you to shut down your
                       brain. We do give you some advice (such as not to log in to the graphical
                       user interface as root ), but you should keep thinking along. E-mail messages
                       asking you to view web site 𝑋 and enter your credit card number and PIN
                       there can reach you even on Linux, and you should disregard them in the
                       same way as everywhere else.

     user accounts      Linux distinguishes between different users by means of different user ac-
                     counts. The common distributions typically create two user accounts during
                     installation, namely root for administrative tasks and another account for a “nor-
                     mal” user. You (as the administrator) may add more accounts later, or, on a client
                     PC in a larger network, they may show up automatically from a user account
                     database stored elsewhere.

                     B Linux distinguishes between user accounts, not users. For example, no one
                       keeps you from using a separate user account for reading e-mail and surfing
                       the web, if you want to be 100% sure that things you download from the
2.1 Basics                                                                                          23

        Net have no access to your important data (which might otherwise happen
        in spite of the user/administrator divide). With a little cunning you can
        even display a browser and e-mail program running under your “surfing
        account” among your “normal” programs1 .

    Under Linux, every user account is assigned a unique number, the so-called
user ID (or UID, for short). Every user account also features a textual user name UID
(such as root or joe ) which is easier to remember for humans. In most places where user name
it counts—e. g., when logging in, or in a list of files and their owners—Linux will
use the textual name whenever possible.

B The Linux kernel does not know anything about textual user names; process
  data and the ownership data in the filesystem use the UID exclusively. This
  may lead to difficulties if a user is deleted while he still owns files on the
  system, and the UID is reassigned to a different user. That user “inherits”
  the previous UID owner’s files.

B There is no technical problem with assigning the same (numerical) UID to
  different user names. These users have equal access to all files owned by that
  UID, but every user can have his own password. You should not actually
  use this (or if you do, use it only with great circumspection).

2.1.2     Users and Groups
To work with a Linux computer you need to log in first. This allows the system
to recognise you and to assign you the correct access rights (of which more later).
Everything you do during your session (from logging in to logging out) happens
under your user account. In addition, every user has a home directory, where home directory
only they can store and manage their own files, and where other users often have
no read permission and very emphatically no write permission. (Only the system
administrator—root —may read and write all files.)

A Depending on which Linux distribution you use (cue: Ubuntu) it may be
  possible that you do not have to log into the system explicitly. This is be-
  cause the computer “knows” that it will usually be you and simply assumes
  that this is going to be the case. You are trading security for convenience; this
  particular deal probably makes sense only where you can stipulate with rea-
  sonable certainty that nobody except you will switch on your computer—
  and hence should be restricted by rights to the computer in your single-
  person household without a cleaner. We told you so.

   Several users who want to share access to certain system resources or files can
form a group. Linux identifies group members either fixedly by name or tran- group
siently by a login procedure similar to that for users. Groups have no “home di-
rectories” like users do, but as the administrator you can of course create arbitrary
directories meant for certain groups and having appropriate access rights.
   Groups, too, are identified internally using numerical identifiers (“group IDs”
or GIDs).

B Group names relate to GIDs as user names to UIDs: The Linux kernel only
  knows about the former and stores only the former in process data or the
  file system.

   Every user belongs to a primary group and possibly several secondary or addi-
tional groups. In a corporate setting it would, for example, be possible to introduce
project-specific groups and to assign the people collaborating on those projects
to the appropriate group in order to allow them to manage common data in a
directory only accessible to group members.
   1 Which of course is slightly more dangerous again, since programs runninig on the same screen

can communicate with one another
24                                                                                      2 User Administration

               For the purposes of access control, all groups carry equivalent weight—every
            user always enjoys all rights deriving from all the groups that he is a member of.
            The only difference between the primary and secondary groups is that files newly
            created by a user are usually2 assigned to his primary group.

            B Up to (and including) version 2.4 of the Linux kernel, a user could be a mem-
              ber of at most 32 additional groups; since Linux 2.6 the number of secondary
              groups is unlimited.

               You can find out a user account’s UID, the primary and secondary groups and
            the corresponding GIDs by means of the id program:

            $ id
            uid=1000(joe) gid=1000(joe) groups=24(cdrom),29(audio),44(video), 
            $ id root
            uid=0(root) gid=0(root) groups=0(root)

            B With the options -u , -g , and -G , id lets itself be persuaded to output just the
              account’s UID, the GID of the primary group, or the GIDs of the secondary
              groups. (These options cannot be combined.) With the additional option -n
              you get names instead of numbers:

                     $ id -G
                     1000 24 29 44
                     $ id -Gn
                     joe cdrom audio video

            B The groups command yields the same result as the ”‘id -Gn ”’ command.
     last     You can use the last command to find who logged into your computer and
            when (and, in the case of logins via the network, from where):

            $ last
            joe        pts/1          pcjoe.example.c     Wed   Feb   29   10:51   still logged in
            bigboss    pts/0          pc01.example.c      Wed   Feb   29   08:44   still logged in
            joe        pts/2          pcjoe.example.c     Wed   Feb   29   01:17 - 08:44 (07:27)
            sue        pts/0          :0                  Tue   Feb   28   17:28 - 18:11 (00:43)
            reboot     system boot 3.2.0-1-amd64          Fri Feb      3 17:43 - 13:25 (4+19:42)

            For network-based sessions, the third column specifies the name of the ssh client
            computer. “:0 ” denotes the graphical screen (the first X server, to be exact—there
            might be more than one).

            B Do also note the reboot entry, which tells you that the computer was started.
              The third column contains the version number of the Linux operating sys-
              tem kernel as provided by “uname -r ”.

               With a user name, last provides information about a particular user:

            $ last
            joe        pts/1          pcjoe.example.c     Wed Feb 29 10:51   still logged in
            joe        pts/2          pcjoe.example.c     Wed Feb 29 01:17 - 08:44 (07:27)
                2 The exception occurs where the owner of a directory has decreed that new files and subdirectories

            within this directory are to be assigned to the same group as the directory itself. We mention this
            strictly for completeness.
2.2 User and Group Information                                                                              25

B You might be bothered (and rightfully so!) by the fact that this somewhat
  sensitive information is apparently made available on a casual basis to arbi-
  trary system users. If you (as the administrator) want to protect your users’
  privacy somewhat better than you Linux distribution does by default, you
  can use the
        # chmod o-r /var/log/wtmp

        command to remove general read permissions from the file that last con-
        sults for the telltale data. Users without administrator privileges then get to
        see something like

        $ last
        last: /var/log/wtmp: Permission denied

2.1.3     People and Pseudo-Users
Besides “natural” persons—the system’s human users—the user and group con-
cept is also used to allocate access rights to certain parts of the system. This means
that, in addition to the personal accounts of the “real” users like you, there are fur-
ther accounts that do not correspond to actual human users but are assigned to pseudo-users
administrative functions internally. They define functional “roles” with their own
accounts and groups.
   After installing Linux, you will find several such pseudo-users and groups in
the /etc/passwd and /etc/group files. The most important role is that of the root user
(which you know) and its eponymous group. The UID and GID of root are 0 (zero).

B root ’s privileges are tied to UID 0; GID 0 does not confer any additional
  access privileges.

    Further pseudo-users belong to certain software systems (e. g., news for Usenet
news using INN, or postfix for the Postfix mail server) or certain components or
devices (such as printers, tape or floppy drives). You can access these accounts,
if necessary, like other user accounts via the su command. These pseudo-users pseudo-users for privileges
are helpful as file or directory owners, in order to fit the access rights tied to file
ownership to special requirements without having to use the root account. The
same appkies to groups; the members of the disk group, for example, have block-
level access to the system’s disks.

C 2.1 [1] How does the operating system kernel differentiate between various
  users and groups?

C 2.2 [2] What happens if a UID is assigned to two different user names? Is
  that allowed?

C 2.3 [1] What is a pseudo-user? Give examples!

C 2.4 [2] (On the second reading.) Is it acceptable to assign a user to group
  disk who you would not want to trust with the root password? Why (not)?

2.2      User and Group Information
2.2.1     The /etc/passwd File
The /etc/passwd file is the system user database. There is an entry in this file for
every user on the system—a line consisting of attributes like the Linux user name,
26                                                                               2 User Administration

                  “real” name, etc. After the system is first installed, the file contains entries for
                  most pseudo-users.
                     The entries in /etc/passwd have the following format:

                   ⟨user name⟩: ⟨password⟩: ⟨UID⟩: ⟨GID⟩: ⟨GECOS⟩: ⟨home directory⟩: ⟨shell⟩

                  ⟨user name⟩ This name should consist of lowercase letters and digits; the first char-
                         acter should be a letter. Unix systems often consider only the first eight
                         characters—Linux does not have this limitation but in heterogeneous net-
                         works you should take it into account.

                       A Resist the temptation to use umlauts, punctuation and similar special
                         characters in user names, even if the system lets you do so—not all
                         tools that create new user accounts are picky, and you could of course
                         edit /etc/passwd by hand. What seems to work splendidly at first glance
                         may lead to problems elsewhere later.

                        B You should also stay away from user names consisting of only upper-
                          case letters or only digits. The former may give their owners trouble
                          logging in (see exercise 2.6), the latter can lead to confusion, especially
                          if the numerical user name does not equal the account’s numerical
                          UID. Commands such as ”‘ls -l ”’ will display the UID if there is no
                          corresponding entry for it in /etc/passwd , and it is not exactly straight-
                          forward to tell UIDs from purely numerical user names in ls output.

                  ⟨password⟩ Traditionally, this field contains the user’s encrypted password. Today,
                        most Linux distributions use “shadow passwords”; instead of storing the
                        password in the publically readable /etc/passwd file, it is stored in /etc/shadow
                        which can only be accessed by the administrator and some privileged pro-
                        grams. In /etc/passwd , a “x ” calls attention to this circumstance. Every user
                        can avail himself of the passwd program to change his password.
                  ⟨UID⟩ The numerical user identifier—a number between 0 and 232 − 1. By con-
                       vention, UIDs from 0 to 99 (inclusive) are reserved for the system, UIDs
                       from 100 to 499 are for use by software packages if they need pseudo-user
                       accounts. With most popular distributions, “real” users’ UIDs start from
                       500 (or 1000).
                        Precisely because the system differentiates between users not by name but
                        by UID, the kernel treats two accounts as completely identical if they con-
                        tain different user names but the same UID—at least as far as the access
                        privileges are concerned. Commands that display a user name (e. g., ”‘ls
                        -l ”’ or id ) show the one used when the user logged in.

     primary group ⟨GID⟩ The GID of the user’s primary group after logging in.

                             The Novell/SUSE distributions (among others) assign a single group
                             such as users as the shared primary group of all users. This method is
                             quite established as well as easy to understand.

                             Many distributions, such as those by Red Hat or Debian GNU/Linux,
                             create a new group whenever a new account is created, with the GID
                             equalling the account’s UID. The idea behind this is to allow more
                             sophisticated assignments of rights than with the approach that puts
                             all users into the same group users . Consider the following situation:
                             Jim (user name jim ) is the personal assistant of CEO Sue (user name
                             sue ). In this capacity he sometimes needs to access files stored inside
                             Sue’s home directory that other users should not be able to get at. The
                             method used by Red Hat, Debian & co., “one group per user”, makes it
                             straightforward to put user jim into group sue and to arrange for Sue’s
2.2 User and Group Information                                                                      27

           files to be readable for all group members (the default case) but not oth-
           ers. With the “one group for everyone” approach it would have been
           necessary to introduce a new group completely from scratch, and to
           reconfigure the jim and sue accounts accordingly.

      By virtue of the assignment in /etc/passwd , every user must be a member of
      at least one group.

      B The user’s secondary groups (if applicable) are determined from en-
        tries in the /etc/group file.

⟨GECOS⟩ This is the comment field, also known as the “GECOS field”.

      B GECOS stands for “General Electric Comprehensive Operating Sys-
        tem” and has nothing whatever to do with Linux, except that in the
        early days of Unix this field was added to /etc/passwd in order to keep
        compatibility data for a GECOS remote job entry service.

      This field contains various bits of information about the user, in particular
      his “real” name and optional data such as the office number or telephone
      number. This information is used by programs such as mail or finger . The
      full name is often included in the sender’s address by news and mail soft-

      B Theoretically there is a program called chfn that lets you (as a user)
        change the content of your GECOS field. Whether that works in any
        particular case is a different question, since at least in a corporate set-
        ting one does not necessarily want to allow people to change their
        names at a whim.

⟨home directory⟩ This directory is that user’s personal area for storing his own files.
      A newly created home directory is by no means empty, since a new user
      normally receives a number of “profile” files as his basic equipment. When
      a user logs in, his shell uses his home directory as its current directory, i. e.,
      immediately after logging in the user is deposited there.
⟨shell⟩ The name of the program to be started by login after successful authentication—
       this is usually a shell. The seventh field extends through the end of the line.

      B The user can change this entry by means of the chsh program. The
        eligible programs (shells) are listed in the /etc/shells file. If a user is
        not supposed to have an interactive shell, an arbitrary program, with
        arguments, can be entered here (a common candidate is /bin/true ). This
        field may also remain empty, in which case the standard shell /bin/sh
        will be started.

      B If you log in to a graphical environment, various programs will be
        started on your behalf, but not necessarily an interactive shell. The
        shell entry in /etc/passwd comes into its own, however, when you in-
        voke a terminal emulator such as xterm or konsole , since these programs
        usually check it to identify your preferred shell.

Some of the fields shown here may be empty. Absolutely necessary are only the
user name, UID, GID and home directory. For most user accounts, all the fields
will be filled in, but pseudo-users might use only part of the fields.
   The home directories are usually located below /home and take their name from home directories
their owner’s user name. In general this is a fairly sensible convention which
makes a given user’s home directory easy to find. In theory, a home directory
might be placed anywhere in the file system under a completely arbitrary name.

B On large systems it is common to introduce one or more additional levels
  of directories between /home and the “user name” directory, such as
28                                                                         2 User Administration

                    /home/hr/joe                                      Joe from Human Resources
                    /home/devel/sue                                        Sue from Development
                    /home/exec/bob                                                  Bob the CEO

                    There are several reasons for this. On the one hand this makes it easier to
                    keep one department’s home directory on a server within that department,
                    while still making it available to other client computers. On the other hand,
                    Unix (and some Linux) file systems used to be slow dealing with directories
                    containing very many files, which would have had an unfortunate impact
                    on a /home with several thousand entries. However, with current Linux file
                    systems (ext3 with dir_index and similar) this is no longer an issue.

               Note that as an administrator you should not really be editing /etc/passwd by
      tools hand. There is a number of programs that will help you create and maintain user

             B In principle it is also possible to store the user database elsewhere than in
               /etc/passwd . On systems with very many users (thousands), storing user
               data in a relational database is preferable, while in heterogeneous networks
               a shared multi-platform user database, e. g., based on an LDAP directory,
               might recommend itself. The details of this, however, are beyond the scope
               of this course.

            2.2.2     The /etc/shadow File
            For security, nearly all current Linux distributions store encrypted user passwords
            in the /etc/shadow file (“shadow passwords”). This file is unreadable for normal
            users; only root may write to it, while members of the shadow group may read it in
            addition to root . If you try to display the file as a normal user an error occurs.

             B Use of /etc/shadow is not mandatory but highly recommended. However
               there may be system configurations where the additional security afforded
               by shadow passwords is nullified, for example if NIS is used to export user
               data to other hosts (especially in heterogeneous Unix environments).

     format Again, this file contains one line for each user, with the following format:

             ⟨user name⟩: ⟨password⟩: ⟨change⟩: ⟨min⟩: ⟨max⟩
              : ⟨warn⟩: ⟨grace⟩: ⟨lock⟩: ⟨reserved⟩

            For example:


            Here is the meaning of the individual fields:
            ⟨user name⟩ This must correspond to an entry in the /etc/passwd file. This field
                   “joins” the two files.
            ⟨password⟩ The user’s encrypted password. An empty field generally means that
                  the user can log in without a password. An asterisk or an exclamation point
                  prevent the user in question from logging in. It is common to lock user’s ac-
                  counts without deleting them entirely by placing an asterisk or exclamation
                  point at the beginning of the corresponding password.
            ⟨change⟩ The date of the last password change, in days since 1 January 1970.
2.2 User and Group Information                                                                               29

⟨min⟩ The minimal number of days that must have passed since the last password
      change before the password may be changed again.
⟨max⟩ The maximal number of days that a password remains valid without hav-
     ing to be changed. After this time has elapsed the user must change his
⟨warn⟩ The number of days before the expiry of the ⟨max⟩ period that the user will
     be warned about having to change his password. Generally, the warning
     appears when logging in.
⟨grace⟩ The number of days, counting from the expiry of the ⟨max⟩ period, after
      which the account will be locked if the user does not change his password.
      (During the time from the expiry of the ⟨max⟩ period and the expiry of this
      grace period the user may log in but must immediately change his pass-
⟨lock⟩ The date on which the account will be definitively locked, again in days
       since 1 January 1970.
Some brief remarks concerning password encryption are in order. You might password encryption
think that if passwords are encrypted they can also be decrypted again. This would
open all of the system’s accounts to a clever cracker who manages to obtain a copy
of /etc/shadow . However, in reality this is not the case, since password “encryption”
is a one-way street. It is impossible to recover the decrypted representation of a
Linux password from the “encrypted” form because the method used for encryp-
tion prevents this. The only way to “crack” the encryption is by encrypting likely
passwords and checking whether they match what is in /etc/shadow .

B Let’s assume you select the characters of your password from the 95 vis-
  ible ASCII characters (uppercase and lowercase letters are distinguished).
  This means that there are 95 different one-character passwords, 952 = 9025
  two-character passwords, and so on. With eight characters you are already
  up to 6.6 quadrillion (6.6 ⋅ 1015 ) possibilities. Stipulating that you can trial-
  encrypt 10 million passwords per second (not entirely unrealistic on current
  hardware), this means you would require approximately 21 years to work
  through all possible passwords. If you are in the fortunate position of own-
  ing a modern graphics card, another acceleration by a factor of 50–100 is
  quite feasible, which makes that about two months. And then of course
  there are handy services like Amazon’s EC2, which will provide you (or
  random crackers) with almost arbitrary CPU power, or the friendly neigh-
  bourhood Russian bot net … so don’t feel too safe.

B There are a few other problems. The traditional method (usually called
  “crypt” or “DES”—the latter because it is based on, but not identical to, the
  eponymous encryption method3 ) should no longer be used if you can avoid
  it. It has the unpleasant property of only looking at the first eight characters
  of the entered password, and clever crackers can nowadays buy enough disk
  space to build a pre-encrypted cache of the 50 million (or so) most common
  passwords. To “crack” a password they only need to search their cache for
  the encrypted password, which can be done extremely quickly, and read off
  the corresponding clear-text password.

B To make things even more laborious, when a newly entered password is
  encrypted the system traditionally adds a random element (the so-called
    3 If you must know exactly: The clear-text password is used as the key (!) to encrypt a constant

string (typically a sequence of zero bytes). A DES key is 56 bits, which just happens to be 8 characters
of 7 bits each (as the leftmost bit in each character is ignored). This process is repeated for a total of
25 rounds, with the previous round’s output serving as the new input. Strictly speaking the encryption
scheme used isn’t quite DES but changed in a few places, to make it less feasible to construct a special
password-cracking computer from commercially available DES encryption chips.
30                                                                                2 User Administration

                           “salt”) which selects one of 4096 different possibilities for the encrypted
                           password. The main purpose of the salt is to avoid random hits result-
                           ing from user 𝑋, for some reason or other, getting a peek at the content
                           of /etc/shadow and noting that his encrypted password looks just like that
                           of user 𝑌 (hence letting him log into user 𝑌’s account using his own clear-
                           text password). For a pleasant side effect, the disk space required for the
                           cracker’s pre-encrypted dictionary from the previous paragraph is blown
                           up by a factor of 4096.

                    B Nowadays, password encryption is commonly based on the MD5 algorithm,
                      allows for passwords of arbitrary length and uses a 48-bit salt instead of
                      the traditional 12 bits. Kindly enough, the encryption works much more
                      slowly than “crypt”, which is irrelevant for the usual purpose (checking a
                      password upon login—you can still encrypt several hundred passwords per
                      second) but does encumber clever crackers to a certain extent. (You should
                      not let yourself be bothered by the fact that cryptographers poo-poo the
                      MD5 scheme as such due to its insecurity. As far as password encryption is
                      concerned, this is fairly meaningless.)

                   A You should not expect too much of the various password administration pa-
                     rameters. They are being used by the text console login process, but whether
                     other parts of the system (such as the graphical login screen) pay them any
                     notice depends on your setup. Nor is there usually an advantage in forc-
                     ing new passwords on users at short intervals—this usually results in a se-
                     quence like bob1 , bob2 , bob3 , …, or users alternate between two passwords.
                     A minimal interval that must pass before a user is allowed to change their
                     password again is outright dangerous, since it may give a cracker a “win-
                     dow” for illicit access even though the user knows their password has been

                       The problem you need to cope with as a system administrator is usually not
                   people trying to crack your system’s passwords by “brute force”. It is much more
                   promising, as a rule, to use “social engineering”. To guess your password, the
                   clever cracker does not start at a , b , and so on, but with your spouse’s first name,
                   your kids’ first names, your car’s plate number, your dog’s birthday et cetera. (We
                   do not in any way mean to imply that you would use such a stupid password. No,
                   no, not you by any means. However, we are not quite so positive about your boss
                   …) And then there is of course the time-honoured phone call approach: “Hi, this
                   is the IT department. We’re doing a security systems test and urgently require
                   your user name and password.”
                       There are diverse ways of making Linux passwords more secure. Apart from
                   the improved encryption scheme mentioned above, which by now is used by de-
                   fault by most Linux distributions, these include complaining about (too) weak
                   passwords when they are first set up, or proactively running software that will
                   try to identify weak encrypted passwords, just like clever crackers would (Cau-
                   tion: Do this in your workplace only with written (!) pre-approval from your
                   boss!). Other methods avoid passwords completely in favour of constantly chang-
                   ing magic numbers (as in SecurID) or smart cards. All of this is beyond the scope
                   of this manual, and therefore we refer you to the Linup Front manual Linux Secu-

                   2.2.3     The /etc/group File
     group database By default, Linux keeps group information in the /etc/group file. This file contains
                   one-line entry for each group in the system, which like the entries in /etc/passwd
                   consists of fields separated by colons (: ). More precisely, /etc/group contains four
                   fields per line.

                    ⟨group name⟩: ⟨password⟩: ⟨GID⟩: ⟨members⟩
2.2 User and Group Information                                                                                    31

   Their meaning is as follows:
⟨group name⟩ The name of the group, for use in directory listings, etc.
⟨password⟩ An optional password for this group. This lets users who are not mem-
      bers of the group via /etc/shadow or /etc/group assume membership of the
      group using newgrp . A “* ” as an invalid character prevents normal users
      from changing to the group in question. A “x ” refers to the separate pass-
      word file /etc/gshadow .
⟨GID⟩ The group’s numerical group identifier.

⟨Members⟩ A comma-separated list of user names. This list contains all users who
    have this group as a secondary group, i. e., who are members of this group
    but have a different value in the GID field of their /etc/passwd entry. (Users
    with this group as their primary group may also be listed here but that is

   A /etc/group file could, for example, look like this:


The entries for the root and bin groups are entries for administrative groups, sim-       administrative groups
ilar to the system’s pseudo-user accounts. Many files are assigned to groups like
this. The other groups contain user accounts.
    Like UIDs, GIDs are counted from a specific value, typically 100. For a valid          GID values
entry, at least the first and third field (group name and GID) must be filled in.
Such an entry assigns a GID (which might occur in a user’s primary GID field in
/etc/passwd ) a textual name.
    The password and/or membership fields must only be filled in for groups that
are assigned to users as secondary groups. The users listed in the membership             membership list
list are not asked for a password when they want to change GIDs using the new-
grp command. If an encrypted password is given, users without an entry in the             group password
membership list can authenticate using the password to assume membership of
the group.

B In practice, group passwords are hardly if ever used, as the administrative
  overhead barely justifies the benefits to be derived from them. On the one
  hand it is more convenient to assign the group directly to the users in ques-
  tion (since, from version 2.6 of the Linux kernel on, there is no limit to the
  number of secondary groups a user can join), and on the other hand a single
  password that must be known by all group members does not exactly make
  for bullet-proof security.

B If you want to be safe, ensure that there is an asterisk (“* ”) in every group
  password slot.

2.2.4    The /etc/gshadow File
As for the user database, there is a shadow password extension for the group
database. The group passwords, which would otherwise be encrypted but read-
able for anyone in /etc/group (similar to /etc/passwd ), are stored in the separate file
/etc/gshadow . This also contains additional information about the group, for ex-
ample the names of the group administrators who are entitled to add or remove
members from the group.
32                                                                                            2 User Administration

                              2.2.5     The getent Command
                              Of course you can read and process the /etc/passwd , /etc/shadow , and /etc/group files,
                              like all other text files, using programs such as cat , less or grep (OK, OK, you need
                              to be root to get at /etc/shadow ). There are, however, some practical problems:
                                  • You may not be able to see the whole truth: Your user database (or parts of
                                    it) might be stored on an LDAP server, SQL database, or a Windows domain
                                    controller, and there really may not be much of interest in /etc/passwd .
                                  • If you want to look for a specific user’s entry, it is slightly inconvenient to
                                    type this using grep if you want to avoid “false positives”.
                              The getent command makes it possible to query the various databases for user and
                              group information directly. With
                               $ getent passwd

                              you will be shown something that looks like /etc/passwd , but has been assembled
                              from all sources of user information that are currently configured on your com-
                              puter. With
                               $ getent passwd hugo

                              you can obtain user hugo ’s entry, no matter where it is actually stored. Instead
                              of passwd , you may also specify shadow , group , or gshadow to consult the respective
                              database. (Naturally, even with getent you can only access shadow and gshadow as
                              user root .)

                               B The term “database” is understood as “totality of all sources from where
                                 the C library can obtain information on that topic (such as users)”. If you
                                 want to know exactly where that information comes from (or might come
                                 from), then read nsswitch.conf (5) and examine the /etc/nsswitch.conf file on
                                 your system.

                               B You may also specify several user or group names. In that case, information
                                 on all the named users or groups will be output:

                                      $ getent passwd hugo susie fritz

                               C 2.5 [1] Which value will you find in the second column of the /etc/passwd
                                 file? Why do you find that value there?

                               C 2.6 [2] Switch to a text console (using, e. g., Alt + F1 ) and try logging in but
                                 enter your user name in uppercase letters. What happens?

                               C 2.7 [2] How can you check that there is an entry in the shadow database for
                                 every entry in the passwd database? (pwconv only considers the /etc/passwd and
                                 /etc/shadow files, and also rewrites the /etc/shadow file, which we don’t want.

                              2.3      Managing User Accounts and Group Information
                               After a new Linux distribution has been installed, there is often just the root ac-
                               count for the system administrator and the pseudo-users’ accounts. Any other
                               user accounts must be created first (and most distributions today will gently but
                               firmly nudge the installing person to create at least one “normal” user account).
                                  As the administrator, it is your job to create and manage the accounts for all
     tools for user management required users (real and pseudo). To facilitate this, Linux comes with several tools
                               for user management. With them, this is mostly a straightforward task, but it is
                               important that you understand the background.
2.3 Managing User Accounts and Group Information                                                     33

2.3.1      Creating User Accounts
The procedure for creating a new user account is always the same (in principle)
and consists of the following steps:
     1. You must create entries in the /etc/passwd (and possibly /etc/shadow ) files.

     2. If necessary, an entry (or several) in the /etc/group file is necessary.
     3. You must create the home directory, copy a basic set of files into it, and
        transfer ownership of the lot to the new user.
     4. If necessary, you must enter the user in further databases, e. g., for disk quo-
        tas (section 7.4), database access privilege tables and special applications.

All files involved in adding a new account are plain text files. You can perform
each step manually using a text editor. However, as this is a job that is as tedious
as it is elaborate, it behooves you to let the system help you, by means of the useradd
    In the simplest case, you pass useradd merely the new user’s user name. Op-            useradd
tionally, you can enter various other user parameters; for unspecified parameters
(typically the UID), “reasonable” default values will be chosen automatically. On
request, the user’s home directory will be created and endowed with a basic set of
files that the program takes from the /etc/skel directory. The useradd command’s
syntax is:

useradd    [⟨options⟩] ⟨user name⟩

The following options (among others) are available:
-c   ⟨comment⟩ GECOS field entry
-d   ⟨home directory⟩ If this option is missing, /home/ ⟨user name⟩ is assumed

-e   ⟨date⟩ On this date the account will be deactivated automatically (format
-g   ⟨group⟩ The new user’s primary group (name or GID). This group must exist.
-G   ⟨group⟩[,⟨group⟩]… Supplementary groups (names or GIDs). These groups
         must also exist.
-s   ⟨shell⟩ The new user’s login shell
-u   ⟨UID⟩ The new user’s numerical UID. This UID must not be already in use,
        unless the “-o ” option is given

-m   Creates the home directory and copies the basic set of files to it. These files
       come from /etc/skel , unless a different directory was named using “-k
For instance, the

# useradd -c "Joe Smith" -m -d /home/joe -g devel \
>    -k /etc/skel.devel

command creates an account by the name of joe for a user called Joe Smith, and
assigns it to the devel group. joe ’s home directory is created as /home/joe , and the
files from /etc/skel.devel are being copied into it.

B With the -D option (on SUSE distributions, --show-defaults ) you may set de-
  fault values for some of the properties of new user accounts. Without addi-
  tional options, the default values are displayed:
34                                                                              2 User Administration

                      # useradd -D

                      You can change these values using the -g , -b , -f , -e , and -s options, respec-

                      # useradd -D -s /usr/bin/zsh                              zsh   as the default shell

                      The final two values in the list cannot be changed.

               B useradd is a fairly low-level tool. In real life, you as an experienced adminis-
                 trator will likely not be adding new user accounts by means of useradd , but
                 through a shell script that incorporates your local policies (just so you don’t
                 have to remember them all the time). Unfortunately you will have to come
                 up with this shell script by yourself—at least unless you are using Debian
                 GNU/Linux or one of its derivatives (see below).
                 Watch out: Even though every serious Linux distribution comes with a program
              called useradd , the implementations differ in their details.
                      The Red Hat distributions include a fairly run-of-the-mill version of useradd ,
                      without bells and whistles, which provides the features discussed above.
                      The SUSE distributions’ useradd is geared towards optionally adding users to
                      a LDAP directory rather than the /etc/passwd file. (This is why the -D option
                      cannot be used to query or set default values like it can elsewhere—it is
                      already spoken for to do LDAPy things.) The details are beyond the scope
                      of this manual.
                      On Debian GNU/Linux and Ubuntu, useradd does exist but the recom-
                      mended method to create new user accounts is a program called adduser
                      (thankfully this is not confusing). The advantage of adduser is that it plays
                      according to Debian GNU Linux’s rules, and furthermore makes it possible
                      to execute arbitrary other actions for a new account besides creating the
                      actual account. For example, one might create a directory in a web server’s
                      document tree so that the new user (and nobody else) can publish files
                      there, or the user could automatically be authorised to access a database
                      server. You can find the details in adduser (8) and adduser.conf (5).
                 After it has been created using useradd , the new account is not yet accessible;
     password the system administrator must first set up a password. We shall be explaining this

              2.3.2     The passwd Command
              The passwd command is used to set up passwords for users. If you are logged in as
              root , then

               # passwd joe

              asks for a new password for user john (You must enter it twice as it will not be
              echoed to the screen).
                 The passwd command is also available to normal users, to let them change their
              own passwords (changing other users’ passwords is root ’s prerogative):
2.3 Managing User Accounts and Group Information                                               35

$ passwd
Changing password for joe.
(current) UNIX password: secret123
Enter new UNIX password: 321terces
Retype new UNIX password: 321terces
passwd: password updated successfully

Normal users must enter their own password correctly once before being allowed
to set a new one. This is supposed to make life difficult for practical jokers that
play around on your computer if you had to step out very urgently and didn’t
have time to engage the screen lock.
   On the side, passwd serves to manage various settings in /etc/shadow . For exam-
ple, you can look at a user’s “password state” by calling the passwd command with
the -S option:

# passwd -S bob
bob LK 10/15/99 0 99999 7 0

The first field in the output is (once more) the user name, followed by the password
state: “PS ” or “P ” if a password is set, “LK ” or “L ” for a locked account, and “NP ” for
an account with no password at all. The other fields are, respectively, the date of
the last password change, the minimum and maximum interval for changing the
password, the expiry warning interval and the “grace period” before the account
is locked completely after the password has expired. (See also Section 2.2.2.)
    You can change some of these settings by means of passwd options. Here are a
few examples:

#   passwd    -l    joe                                                  Lock the account
#   passwd    -u    joe                                               Unlock the account
#   passwd    -n    7 joe                          Password change at most every 7 days
#   passwd    -x    30 joe                        Password change at least every 30 days
#   passwd    -w    3 joe                     3 days grace period before password expires

E Locking and unlocking accounts by means of -l and -u works by putting
  a “! ” in front of the encrypted password in /etc/shadow . Since “! ” cannot
  result from password encryption, it is impossible to enter something upon
  login that matches the “encrypted password” in the user database—hence
  access via the usual login procedure is prevented. Once the “! ” is removed,
  the original password is back in force. (Astute, innit?) However, you should
  keep in mind that users may be able to gain access to the system by other
  means that do not refer to the encrypted password in the user database,
  such as the secure shell with public-key authentication.

    Changing the remaining settings in /etc/shadow requires the chage command:

#   chage    -E    2009-12-01 joe                         Lock account from 1 Dec 2009
#   chage    -E    -1 joe                                             Cancel expiry date
#   chage    -I    7 joe                       Grace period 1 week from password expiry
#   chage    -m    7 joe                                            Like passwd -n (Grr.)
#   chage    -M    7 joe                                       Like passwd -x (Grr, grr.)
#   chage    -W    3 joe                                   Like passwd -w (Grr, grr, grr.)

(chage can change all settings that passwd can change, and then some.)

B If you cannot remember the option names, invoke chage with the name of
  a user account only. The program will present you with a sequence of the
  current values to change or confirm.
36                                                                                  2 User Administration

                        You cannot retrieve a clear-text password even if you are the administrator.
                     Even checking /etc/shadow doesn’t help, since this file stores all passwords already
                     encrypted. If a user forgets their password, it is usually sufficient to reset their
                     password using the passwd command.

                     B Should you have forgotten the root password and not be logged in as root by
                       any chance, your last option is to boot Linux to a shell, or boot from a rescue
                       disk or CD. (See Chapter 8.) After that, you can use an editor to clear the
                       ⟨password⟩ field of the root entry in /etc/passwd .

                     C 2.8 [3] Change user joe ’s password. How does the /etc/shadow file change?
                       Query that account’s password state.

                     C 2.9 [!2] The user dumbo has forgotten his password. How can you help him?

                     C 2.10 [!3] Adjust the settings for user joe ’s password such that he can change
                       his password after at least a week, and must change it after at most two
                       weeks. There should be a warning two days before the two weeks are up.
                       Check the settings afterwards.

                     2.3.3    Deleting User Accounts
                     To delete a user account, you need to remove the user’s entries from /etc/passwd and
                     /etc/shadow , delete all references to that user in /etc/group , and remove the user’s
                     home directory as well as all other files created or owned by that user. If the
                     user has, e. g., a mail box for incoming messages in /var/mail , that should also be
           userdel      Again there is a suitable command to automate these steps. The userdel com-
                     mand removes a user account completely. Its syntax:

                     userdel [-r ]   ⟨user name⟩

                     The -r option ensures that the user’s home directory (including its content) and
                     his mail box in /var/mail will be removed; other files belonging to the user—e. g.,
                     crontab files—must be delete manually. A quick way to locate and remove files
                     belonging to a certain user is the

                     find / -uid     ⟨UID⟩ -delete

                     command. Without the -r option, only the user information is removed from the
                     user database; the home directory remains in place.

                     2.3.4    Changing User Accounts and Group Assignment
                     User accounts and group assignments are traditionally changed by editing the
                     /etc/passwd and /etc/group files. However, many systems contain commands like
                     usermod and groupmod for the same purpose, and you should prefer these since they
                     are safer and—mostly—more convenient to use.
           usermod      The usermod program accepts mostly the same options as useradd , but changes
                     existing user accounts instead of creating new ones. For example, with

                     usermod -g    ⟨group⟩ ⟨user name⟩

                     you could change a user’s primary group.
     Changing UIDs       Caution! If you want to change an existing user account’s UID, you could edit
                     the ⟨UID⟩ field in /etc/passwd directly. However, you should at the same time trans-
                     fer that user’s files to the new UID using chown : “chown -R tux /home/tux ” re-confers
2.3 Managing User Accounts and Group Information                                                    37

ownership of all files below user tux ’s home directory to user tux , after you have
changed the UID for that account. If “ls -l ” displays a numerical UID instead of
a textual name, this implies that there is no user name for the UID of these files.
You can fix this using chown .

2.3.5    Changing User Information Directly—vipw
The vipw command invokes an editor (vi or a different one) to edit /etc/passwd di-
rectly. At the same time, the file in question is locked in order to keep other users
from simultaneously changing the file using, e. g., passwd (which changes would
be lost). With the -s option, /etc/shadow can be edited.

B The actual editor that is invoked is determined by the value of the VISUAL
  environment variable, alternatively that of the EDITOR environment variable;
  if neither exists, vi will be launched.

C 2.11 [!2] Create a user called test . Change to the test account and create a
  few files using touch , including a few in a different directory than the home
  directory (say, /tmp ). Change back to root and change test ’s UID. What do
  you see when listing user test ’s files?

C 2.12 [!2] Create a user called test1 using your distribution’s graphical tool (if
  available), test2 by means of the useradd command, and another, test3 , man-
  ually. Look at the configuration files. Can you work without problems using
  any of these three accounts? Create a file using each of the new accounts.

C 2.13 [!2] Delete user test2 ’s account and ensure that there are no files left on
  the system that belong to that user.

C 2.14 [2] Change user test1 ’s UID. What else do you need to do?

C 2.15 [2] Change user test1 ’s home directory from /home/test1 to /home/user/
  test1 .

2.3.6    Creating, Changing and Deleting Groups
Like user accounts, you can create groups using any of several methods. The
“manual” method is much less tedious here than when creating new user ac-
counts: Since groups do not have home directories, it is usually sufficient to edit
the /etc/group file using any text editor, and to add a suitable new line (see be-
low for vigr ). When group passwords are used, another entry must be added to
/etc/gshadow .
    Incidentally, there is nothing wrong with creating directories for groups.
Group members can place the fruits of their collective labour there. The approach
is similar to creating user home directories, although no basic set of configuration
files needs to be copied.
    For group management, there are, by analogy to useradd , usermod , and userdel ,
the groupadd , groupmod , and groupdel programs that you should use in favour of edit-
ing /etc/group and /etc/gshadow directly. With groupadd you can create new groups        groupadd
simply by giving the correct command parameters:

groupadd [-g   ⟨GID⟩] ⟨group name⟩

The -g option allows you to specify a given group number. As mentioned be-
fore, this is a positive integer. The values up to 99 are usually reserved for system
groups. If -g is not specified, the next free GID is used.
   You can edit existing groups with groupmod without having to write to /etc/group      groupmod
38                                                                                    2 User Administration

                         groupmod [-g   ⟨GID⟩] [-n ⟨name⟩] ⟨group name⟩

                         The “-g ⟨GID⟩” option changes the group’s GID. Unresolved file group assign-
                         ments must be adjusted manually. The “-n ⟨name⟩” option sets a new name for the
                         group without changing the GID; manual adjustments are not necessary.
                            There is also a tool to remove group entries. This is unsurprisingly called
              groupdel   groupdel :

                         groupdel   ⟨group name⟩

                         Here, too, it makes sense to check the file system and adjust “orphaned” group
                         assignments for files with the chgrp command. Users’ primary groups may not be
                         removed—the users in question must either be removed beforehand, or they must
                         be reassigned to a different primary group.
                gpasswd     The gpasswd command is mainly used to manipulate group passwords in a way
                         similar to the passwd command. The system administrator can, however, delegate
     group administrator the administration of a group’s membership list to one or more group adminis-
                         trators. Group administrators also use the gpasswd command:

                         gpasswd -a   ⟨user⟩ ⟨group⟩

                         adds the ⟨user⟩ to the ⟨group⟩, and

                         gpasswd -d   ⟨user⟩ ⟨group⟩

                         removes him again. With

                         gpasswd -A   ⟨user⟩,… ⟨group⟩

                         the system administrator can nominate users who are to serve as group adminis-

                               The SUSE distributions haven’t included gpasswd for some time. Instead
                               there are modified versions of the user and group administration tools that
                               can handle an LDAP directory.

                            As the system administrator, you can change the group database directly using
                  vigr   the vigr command. It works like vipw , by invoking an editor for “exclusive” access
                         to /etc/group . Similarly, “vigr -s ” gives you access to /etc/gshadow .

                         C 2.16 [2] What are groups needed for? Give possible examples.

                         C 2.17 [1] Can you create a directory that all members of a group can access?

                         C 2.18 [!2] Create a supplementary group test . Only user test1 should be a
                           member of that group. Set a group password. Log in as user test1 or test2
                           and try to change over to the new group.
2.3 Managing User Accounts and Group Information                                          39

Commands in this Chapter
adduser    Convenient command to create new user accounts (Debian)
                                                                       adduser (8)   34
chfn       Allows users to change the GECOS field in the user database
                                                                           chfn (1) 27
getent     Gets entries from administrative databases                   getent (1) 32
gpasswd    Allows a group administrator to change a group’s membership and up-
           date the group password                                     gpasswd (1) 38
groupadd    Adds user groups to the system group database             groupadd (8) 37
groupdel    Deletes groups from the system group database             groupdel (8) 38
groupmod    Changes group entries in the system group database groupmod (8) 37
groups     Displays the groups that a user is a member of               groups (1) 24
id         Displays a user’s UID and GIDs                                    id (1) 24
last       List recently-logged-in users                                   last (1) 24
useradd    Adds new user accounts                                      useradd (8) 33
userdel    Removes user accounts                                       userdel (8) 36
usermod    Modifies the user database                                   usermod (8) 36
vigr       Allows editing /etc/group or /etc/gshadow with “file locking”, to avoid con-
           flicts                                                           vipw (8) 38

   •   Access to the system is governed by user accounts.
   •   A user account has a numerical UID and (at least) one textual user name.
   •   Users can form groups. Groups have names and numerical GIDs.
   •   “Pseudo-users” and “pseudo-groups” serve to further refine access rights.
   •   The central user database is (normally) stored in the /etc/passwd file.
   •   The users’ encrypted passwords are stored—together with other password
       parameters—in the /etc/shadow file, which is unreadable for normal users.
   •   Group information is stored in the /etc/group and /etc/gshadow files.
   •   Passwords are managed using the passwd program.
   •   The chage program is used to manage password parameters in /etc/shadow .
   •   User information is changed using vipw or—better—using the specialised
       tools useradd , usermod , and userdel .
   •   Group information can be manipulated using the groupadd , groupmod , groupdel
       and gpasswd programs.
                                                                                             $ echo tux
                                                                                             $ ls
                                                                                             $ /bin/su -

Access Control

3.1     The Linux Access Control System . . . . . . . . .           .   .   .   .   .   42
3.2     Access Control For Files And Directories . . . . . .        .   .   .   .   .   42
      3.2.1 The Basics . . . . . . . . . . . . . . .                .   .   .   .   .   42
      3.2.2 Inspecting and Changing Access Permissions. . .         .   .   .   .   .   43
      3.2.3 Specifying File Owners and Groups—chown and chgrp       .   .   .   .   .   44
      3.2.4 The umask . . . . . . . . . . . . . . .                 .   .   .   .   .   45
3.3     Access Control Lists (ACLs) . . . . . . . . . . .           .   .   .   .   .   47
3.4     Process Ownership . . . . . . . . . . . . . .               .   .   .   .   .   47
3.5     Special Permissions for Executable Files . . . . . . .      .   .   .   .   .   47
3.6     Special Permissions for Directories . . . . . . . .         .   .   .   .   .   48
3.7     File Attributes . . . . . . . . . . . . . . . .             .   .   .   .   .   50

      •   Understanding the Linux access control/privilege mechanisms
      •   Being able to assign access permissions to files and directories
      •   Knowing about the “umask”, SUID, SGID and the “sticky bit”
      •   Knowing about file attributes in the ext file systems

      • Knowledge of Linux user and group concepts (see Chapter 2)
      • Knowledge of Linux files and directories

adm1-rechte.tex    (33e55eeadba676a3 )
42                                                                                               3 Access Control

                             3.1     The Linux Access Control System
                             Whenever several users have access to the same computer system there must be
     access control system an access control system for processes, files and directories in order to ensure that
                           user 𝐴 cannot access user 𝐵’s private files just like that. To this end, Linux imple-
                           ments the standard system of Unix privileges.
                               In the Unix tradition, every file and directory is assigned to exactly one user
       separate privileges (its owner) and one group. Every file supports separate privileges for its owner,
                           the members of the group it is assigned to (“the group”, for short), and all other
                           users (“others”). Read, write and execute privileges can be enabled individually
                           for these three sets of users. The owner may determine a file’s access privileges.
                           The group and others may only access a file if the owner confers suitable privileges
            access mode to them. The sum total of a file’s access permissions is also called its access mode.
                               In a multi-user system which stores private or group-internal data on a gen-
                           erally accessible medium, the owner of a file can keep others from reading or
           access control modifying his files by instituting suitable access control. The rights to a file can be
                           determined separately and independently for its owner, its group and the others.
                           Access permissions allow users to map the responsibilities of a group collabora-
                           tive process to the files that the group is working with.

                             3.2     Access Control For Files And Directories
                             3.2.1    The Basics
                           For each file and each directory in the system, Linux allows separate access rights
                           for each of the three classes of users—owner, members of the file’s group, others.
                           These rights include read permission, write permission, and execute permission.
          file permissions    As far as files are concerned, these permissions control approximately what
                           their names suggest: Whoever has read permission may look at the file’s content,
                           whoever has write permission is allowed to change its content. Execute permis-
                           sion is necessary to launch the file as a process.

                             B Executing a binary “machine-language program” requires only execute per-
                               mission. For files containing shell scripts or other types of “interpreted”
                               programs, you also need read permission.

     directory permissions       For directories, things look somewhat different: Read permission is required
                             to look at a directory’s content—for example, by executing the ls command. You
                             need write permission to create, delete, or rename files in the directory. “Execute”
                             permission stands for the possibility to “use” the directory in the sense that you
                             can change into it using cd , or use its name in path names referring to files farther
                             down in the directory tree.

                             B In directories where you have only read permission, you may read the file
                               names but cannot find out anything else about the files. If you have only “ex-
                               ecute permission” for a directory, you can access files as long as you know
                               their names.

                             Usually it makes little sense to assign write and execute permission to a directory
                             separately; however, it may be useful in certain special cases.

                             A It is important to emphasise that write permission on a file is completely
                               immaterial if the file is to be deleted—you need write permission to the direc-
                               tory that the file is in and nothing else! Since “deleting” a file only removes
                               a reference to the actual file information (the inode) from the directory, this
                               is purely a directory operation. The rm command does warn you if you’re
                               trying to delete a file that you do not have write permission for, but if you
                               confirm the operation and have write permission to the directory involved,
                               nothing will stand in the way of the operation’s success. (Like any other
3.2 Access Control For Files And Directories                                                                        43

        Unix-like system, Linux has no way of “deleting” a file outright; you can
        only remove all references to a file, in which case the Linux kernel decides
        on its own that no one will be able to access the file any longer, and gets rid
        of its content.)

B If you do have write permission to the file but not its directory, you cannot
  remove the file completely. You can, however, truncate it down to 0 bytes
  and thereby remove its content, even though the file itself still exists in prin-
   For each user, Linux determines the “most appropriate” access rights. For ex-
ample, if the members of a file’s group do not have read permission for the file
but “others” do, then the group members may not read the file. The (admittedly
enticing) rationale that, if all others may look at the file, then the group members,
who are in some sense also part of “all others”, should be allowed to read it as
well, does not apply.

3.2.2       Inspecting and Changing Access Permissions
You can obtain information about the rights, user and group assignment that ap- information
ply to a file using “ls -l ”:

$ ls -l
-rw-r--r--      1   joe   users    4711   Oct 4 11:11 datei.txt
drwxr-x---      2   joe   group2   4096   Oct 4 11:12 testdir

The string of characters in the first column of the table details the access permis-
sions for the owner, the file’s group, and others (the very first character is just the
file type and has nothing to do with permissions). The third column gives the
owner’s user name, and the fourth that of the file’s group.
   In the permissions string, “r ”, “w ”, and “x ” signify existing read, write, and
execute permission, respectively. If there is just a “- ” in the list, then the corre-
sponding category does not enjoy the corresponding privilege. Thus, “rw-r--r-- ”
stands for “read and write permission for the owner, but read permission only for
group members and others”.
   As the file owner, you may set access permissions for a file using the chmod com-                chmod   command
mand (from “change mode”). You can specify the three categories by means of the
abbreviations “u ” (user) for the owner (yourself), “g ” (group) for the file’s group’s
members, and “o ” (others) for everyone else. The permissions themselves are
given by the already-mentioned abbreviations “r ”, “w ”, and “x ”. Using “+ ”, “- ”,
and “= ”, you can specify whether the permissions in question should be added to
any existing permissions, “subtracted” from the existing permissions, or used to
replace whatever was set before. For example:

$   chmod   u+x file                                             Execute permission for owner
$   chmod   go+w file                              sets write permission for group and others
$   chmod   g+rw file                                sets read and write permission for group
$   chmod   g=rw,o=r file                                      sets read and write permission,
                                                           removes group execute permission;
                                                           sets just read permission for others
$ chmod a+w file                                                            equivalent to ugo+w

B In fact, permission specifications can be considerably more complex. Con-
  sult the info documentation for chmod to find out all the details.
   A file’s owner is the single user (apart from root ) who is allowed to change a
file’s or directory’s access permissions. This privilege is independent of the actual
permissions; the owner may take away all their own permissions, but that does
not keep them from giving them back later.
   The general syntax of the chmod command is
44                                                                        3 Access Control

     chmod   [⟨options⟩] ⟨permissions⟩ ⟨name⟩ …

     You can give as many file or directory names as desired. The most important
     options include:

     -R   If a directory name is given, the permissions of files and directories inside this
              directory will also be changed (and so on all the way down the tree).
     --reference= ⟨name⟩   Uses the access permissions of file ⟨name⟩. In this case no
             ⟨permissions⟩ must be given with the command.

     B You may also specify a file’s access mode “numerically” instead of “symbol-
       ically” (what we just discussed). In practice this is very common for setting
       all permissions of a file or directory at once, and works like this: The three
       permission triples are represented as a three-digit octal number—the first
       digit describes the owner’s rights, the second those of the file’s group, and
       the third those that apply to “others”. Each of these digits derives from
       the sum of the individual permissions, where read permission has value 4,
       write permission 2, and execute permission 1. Here are a few examples for
       common access modes in “ls -l ” and octal form:
             rw-r--r--   644
             r--------   400
             rwxr-xr-x   755

     B Using numerical access modes, you can only set all permissions at once—
       there is no way of setting or removing individual rights while leaving the
       others alone, like you can do with the “+ ” and “- ” operators of the symbolic
       representation. Hence, the command

             $ chmod 644 file

             is equivalent to the symbolic

             $ chmod u=rw,go=r file

     3.2.3     Specifying File Owners and Groups—chown and chgrp
     The chown command lets you set the owner and group of a file or directory. This
     command takes the desired owner’s user name and/or group name and the file
     or directory name the change should apply to. It is called like

     chown ⟨user name⟩[: ][⟨group name⟩]     ⟨name⟩ …
     chown : ⟨group name⟩ ⟨name⟩ …

     If both a user and group name are given, both are changed; if just a user name is
     given, the group remains as it was; if a user name followed by a colon is given,
     then the file is assigned to the user’s primary group. If just a group name is given
     (with the colon in front), the owner remains unchanged. For example:

     # chown joe:devel letter.txt
     # chown www-data foo.html                                           new user www-data
     # chown :devel /home/devel                                           new group devel

     B chown also supports an obsolete syntax where a dot is used in place of the
3.2 Access Control For Files And Directories                                                           45

   To “give away” files to other users or arbitrary groups you need to be root . The
main reason for this is that normal users could otherwise annoy one another if
the system uses quotas (i.e., every user can only use a certain amount of storage
   Using the chgrp command, you can change a file’s group even as a normal
user—as long as you own the file and are a member of the new group:
chgrp   ⟨group name⟩ ⟨name⟩ …

B Changing a file’s owner or group does not change the access permissions
  for the various categories.
    chown and chgrp also support the -R option to apply changes recursively to part
of the directory hierarchy.

B Of course you can also change a file’s permissions, group, and owner using
  most of the popular file browsers (such as Konqueror or Nautilus).

C 3.1 [!2] Create a new file. What is that file’s group? Use chgrp to assign the
  file to one of your secondary groups. What happens if you try to assign the
  file to a group that you are not a member of?

C 3.2 [4] Compare the mechanisms that various file browsers (like Konqueror,
  Nautilus, …) offer for setting a file’s permissions, owner, group, … Are there
  notable differences?

3.2.4     The umask
New files are usually created using the (octal) access mode 666 (read and write
permission for everyone). New directories are assigned the access mode 777.
Since this is not always what is desired, Linux offers a mechanism to remove cer-
tain rights from these access modes. This is called “umask”.

B Nobody knows exactly where this name comes from—even though there
  are a few theories that all sound fairly implausible.
   The umask is an octal number whose complement is ANDed bitwise to the
standard access mode—666 or 777—to arrive at the new file’s or directory’s actual
access mode. In other words: You can consider the umask an access mode contain- umask interpretation
ing exactly those rights that the new file should not have. Here’s an example—let
the umask be 027:
             1.                       Umask value: 027 ----w-rwx
             2.       Complement of umask value: 750 rwxr-x---
             3.            A new file’s access mode: 666 rw-rw-rw-
             4. Result (2 and 3 ANDed together): 640 rw-r-----
The third column shows the octal value, the fourth a symbolic representation. The
AND operation in step 4 can also be read off the fourth column of the second and
third lines: In the fourth line ther e is a letter in each position that had a letter in
the second and the third line—if there is just one dash (“- ”), the result will be a

B If you’d rather not bother with the complement and AND, you can simply
  imagine that each digit of the umask is subtracted from the corresponding
  digit of the actual access mode and negative results are considered as zero
  (so no “borrowing” from the place to the left). For our example—access
  mode 666 and umask 027—this means something like
                                       666 ⊖ 027 = 640,
        since 6 ⊖ 0 = 6, 6 ⊖ 4 = 2, and 6 ⊖ 7 = 0.
46                                                                                                  3 Access Control

      umask shell command       The umask is set using the umask shell command, either by invoking it di-
                             rectly or via a shell startup file—typically ~/.profile , ~/.bash_profile , or ~/.bashrc .
           process attribute The umask is a process attribute similar to the current directory or the process
                             environment, i. e., it is passed to child processes, but changes in a child process do
                             not modify the parent process’s settings.
                      syntax    The umask command takes a parameter specifying the desired umask:

                              umask [-S |⟨umask⟩]

     symbolic representation The umask may be given as an octal number or in a symbolic representation sim-
                              ilar to that used by chmod —deviously enough, the symbolic form contains the per-
                              missions that should be left (rather than those to be taken away):

                              $ umask 027                                                      … is equivalent to …
                              $ umask u=rwx,g=rx,o=

                              This means that in the symbolic form you must give the exact complement of the
                              value that you would specify in the octal form—exactly those rights that do not
                              occur in the octal specification.
                                  If you specify no value at all, the current umask is displayed. If the -S option
                              is given, the current umask is displayed in symbolic form (where, again, the re-
                              maining permissions are set):

                              $ umask
                              $ umask -S

        execute permission?      Note that you can only remove permissions using the umask. There is no way
                              of making files executable by default.
           umask and chmod       Incidentally, the umask also influences the chmod command. If you invoke chmod
                              with a “+ ” mode (e. g., “chmod +w file ”) without referring to the owner, group or oth-
                              ers, this is treated like “a+ ”, but the permissions set in the umask are not modified.
                              Consider the following example:

                              $ umask 027
                              $ touch file
                              $ chmod +x file
                              $ ls -l file
                              -rwxr-x---   1 tux      users       0 May 25 14:30 file

                              The “chmod +x ” sets execute permission for the user and group, but not the others,
                              since the umask contains the execute bit for “others”. Thus with the umask you
                              can take precautions against giving overly excessive permissions to files.

                              B Theoretically, this also works for the chmod operators “- ” and “= ”, but this
                                does not make a lot of sense in practice.

                              C 3.3 [!1] State a numerical umask that leaves the user all permissions, but
                                removes all permissions from group members and others? What is the cor-
                                responding symbolic umask?

                              C 3.4 [2] Convince yourself that the “chmod +x ” and “chmod a+x ” commands in-
                                deed differ from each other as advertised.
3.3 Access Control Lists (ACLs)                                                                          47

3.3     Access Control Lists (ACLs)
As mentioned above, Linux allows you to assign permissions for a file’s owner,
group, and all others separately. For some applications, though, this three-tier
system is too simple-minded, or the more sophisticated permission schemes of
other operating systems must be mapped to Linux. Access control lists (ACLs)
can be used for this.
   On most file systems, Linux supports “POSIX ACLs” according to IEEE 1003.1e
(draft 17) with some Linux-specific extensions. This lets you specify additional
groups and users for files and directories, who then can be assigned read, write,
and execute permissions that differ from those of the file’s group and “others”.
Other rights, such as that to assign permissions, are still restricted to a file’s owner
(or root ) and cannot be delegated even wiht ACLs. The setfacl and getfacl com-
mands are used to set and query ACLs.
   ACLs are a fairly new and rarely-used addition to Linux, and their use is subject
to certain restrictions. The kernel does oversee compliance with them, but, for
instance, not every program is able to copy ACLs along with a file’s content—you
may have to use a specially-adapted tar (star ) for backups of a file system using
ACLs. ACLs are supported by Samba, so Windows clients get to see the correct
permissions, but if you export file systems to other (proprietary) Unix systems, it
may be possible that your ACLs are ignored by Unix clients that do not support

B You can read up on ACLs on Linux on and in acl (5)
  as well as getfacl (1) and setfacl (1).

   Detailed knowledge of ACLs is not required for the LPIC-1 exams.

3.4     Process Ownership
Linux considers not only the data on a storage medium as objects that can be
owned. The processes on the system have owners, too.
   Many commands create a process in the system’s memory. During normal use,
there are always several processes that the system protects from each other. Every
process together with all data within its virtual address space is assigned to a Processes have owners
user, its owner. This is most often the user who started the process—but processes
created using administrator privileges may change their ownership, and the SUID
mechanism (section 3.5) can also have a hand in this.
   The owners of processes are displayed by the ps program if it is invoked using
the -u option.

# ps -u
bin      89 0.0     1.0 788        328   ?    S   13:27   0:00   rpc.portmap
test1 190 0.0       2.0 1100        28   3    S   13:27   0:00   bash
test1 613 0.0       1.3 968         24   3    S   15:05   0:00   vi XF86.tex
nobody 167 0.0      1.4 932         44   ?    S   13:27   0:00   httpd
root      1 0.0     1.0 776         16   ?    S   13:27   0:03   init [3]
root      2 0.0     0.0    0         0   ?   SW   13:27   0:00   (kflushd)

3.5     Special Permissions for Executable Files
When listing files using the “ls -l ” command, you may sometimes encounter per-
mission sets that differ from the usual rwx , such as

-rwsr-xr-x    1 root shadow    32916 Dec 11 20:47 /usr/bin/passwd
48                                                                                            3 Access Control

                          What does that mean? We have to digress here for a bit:
                            Assume that the passwd program carries the usual access mode:

                          -rwxr-xr-x   1 root shadow   32916 Dec 11 20:47 /usr/bin/passwd

                        A normal (unprivileged) user, say joe , wants to change his password and invokes
                        the passwd program. Next, he receives the message “permission denied”. What is
                        the reason? The passwd process (which uses joe ’s privileges) tries to open the /etc/
                        shadow file for writing and fails, since only root may write to that file—this cannot
                        be different since otherwise, everybody would be able to manipulate passwords
                        arbitrarily and, for example, change the root password.
               SUID bit    By means of the set-UID bit (frequently called “SUID bit”, for short) a program
                        can be caused to run not with the invoker’s privileges but those of the file owner—
                        here, root . In the case of passwd , the process executing passwd has write permission
                        to /etc/shadow , even though the invoking user, not being a system administrator,
                        generally doesn’t. It is the responsibility of the author of the passwd program to en-
                        sure that no monkey business goes on, e. g., by exploiting programming errors to
                        change arbitrary files except /etc/shadow , or entries in /etc/shadow except the pass-
                        word field of the invoking user. On Linux, by the way, the set-UID mechanism
                        works only for binary programs, not shell or other interpreter scripts.

                          B Bell Labs used to hold a patent on the SUID mechanism, which was invented
                            by Dennis Ritchie [SUID]. Originally, AT&T distributed Unix with the
                            caveat that license fees would be levied after the patent had been granted;
                            however, due to the logistical difficulties of charging hundreds of Unix in-
                            stallations small amounts of money retroactively, the patent was released
                            into the public domain.

               SGID bit      By analogy to the set-UID bit there is a SGID bit, which causes a process to be
                         executed with the program file’s group and the corresponding privileges (usually
                         to access other files assigned to that group) rather than the invoker’s group setting.
            chmod syntax     The SUID and SGID modes, like all other access modes, can be changed using
                         the chmod program, by giving symbolic permissions such as u+s (sets the SUID bit)
                         or g-s (deletes the SGID bit). You can also set these bits in octal access modes by
                         adding a fourth digit at the very left: The SUID bit has the value 4, the SGID bit
                         the value 2—thus you can assign the access mode 4755 to a file to make it readable
                         and executable to all users (the owner may also write to it) and to set the SUID bit.
               ls output     You can recognise set-UID and set-GID programs in the output of “ls -l ” by
                         the symbolic abbreviations “s ” in place of “x ” for executable files.

                          3.6    Special Permissions for Directories
                          There is another exception from the principle of assigning file ownership accord-
                          ing to the identity of its creator: a directory’s owner can decree that files created
                          in that directory should belong to the same group as the directory itself. This can
     SGID for directories be specified by setting the SGID bit on the directory. (As directories cannot be
                          executed, the SGID bit is available to be used for such things.)
                              A directory’s access permissions are not changed via the SGID bit. To create a
                          file in such a directory, a user must have write permission in the category (owner,
                          group, others) that applies to him. If, for example, a user is neither the owner of a
                          directory nor a member of the directory’s group, the directory must be writable for
                          “others” for him to be able to create files there. A file created in a SGID directory
                          then belongs to that directory’s group, even if the user is not a member of that
                          group at all.

                          B The typical application for the SGID bit on a directory is a directory that is
                            used as file storage for a “project group”. (Only) the members of the project
                            group are supposed to be able to read and write all files in the directory, and
3.6 Special Permissions for Directories                                                    49

      to create new files. This means that you need to put all users collaborating
      on the project into a project group (a secondary group will suffice):

       # groupadd project                                           Create new group
       # usermod -a -G project joe                                 joe into the group
       # usermod -a -G project sue                                             sue too

      Now you can create the directory and assign it to the new group. The owner
      and group are given all permissions, the others none; you also set the SGID
       # cd /home/project
       # chgrp project /home/project
       # chmod u=rwx,g=srwx /home/project

      Now, if user hugo creates a file in /home/project , that file should be assigned
      to group project :

       $ id
       uid=1000(joe) gid=1000(joe) groups=101(project),1000(joe)
       $ touch /tmp/joe.txt                                  Test: standard    directory
       $ ls -l /tmp/joe.txt
       -rw-r--r-- 1 joe joe 0 Jan 6 17:23 /tmp/joe.txt
       $ touch /home/project/joe.txt                                 project   directory
       $ ls -l /home/project/joe.txt
       -rw-r--r-- 1 joe project 0 Jan 6 17:24 /home/project/joe.txt

      There is just a little fly in the ointment, which you will be able to discern by
      looking closely at the final line in the example: The file does belong to the
      correct group, but other members of group project may nevertheless only
      read it. If you want all members of group project to be able to write to it as
      well, you must either apply chmod after the fact (a nuisance) or else set the
      umask such that group write permission is retained (see Exercise 3.6).

   The SGID mode only changes the system’s behaviour when new files are cre-
ated. Existing files work just the same as everywhere else. This means, for in-
stance, that a file created outside the SGID directory keeps its existing group as-
signment when moved into it (whereas on copying, the new copy would be put
into the directory’s group).
   The chgrp program works as always in SGID directories, too: the owner of a
file can assign it to any group he is a member of. Is the owner not a member of
the directory’s group, he cannot put the file into that group using chgrp —he must
create it afresh within the directory.

B It is possible to set the SUID bit on a directory—this permission does not
  signify anything, though.

  Linux supports another special mode for directories, where only a file’s owner
may delete or remove files within that directory:

drwxrwxrwt     7 root   root   1024 Apr   7 10:07 /tmp

This t mode, the “sticky bit”, can be used to counter a problem which arises when
public directories are in shared use: Write permission to a directory lets a user
delete other users’ files, regardless of their access mode and owner! For example,
the /tmp directories are common ground, and many programs create their tempo-
rary files there. To do so, all users have write permission to that directory. This
implies that any user has permission to delete files there.
50                                                                                     3 Access Control

                                     Table 3.1: The most important file attributes

                   Attribute    Meaning
                        A       atime is not updated (interesting for mobile computers)
                        a       (append-only) The file can only be appended to
                        c       The file’s content is compressed transparently (not implemented)
                        d       The file will not be backed up by dump
                        i       (immutable) The file cannot be changed at all
                        j       Write operations to the file’s content are passed through the journal
                                (ext3 only)
                        s       File data will be overwritten with zeroes on deletion (not imple-
                        S       Write operations to the file are performed “synchronously”, i. e.,
                                without buffering them internally
                        u       The file may be “undeleted” after deletion (not implemented)

                     Usually, when deleting or renaming a file, the system does not consider that
                  file’s access permissions. If the “sticky bit” is set on a directory, a file in that di-
                  rectory can subsequently be deleted only by its owner, the directory’s owner, or
                  root . The “sticky bit” can be set or removed by specifying the symbolic +t and -t
                  modes; in the octal representation it has value 1 in the same digit as SUID and

                  B The “sticky bit” derives its name from an additional meaning it used to have
                    in earlier Unix systems: At that time, programs were copied to swap space
                    in their entirety when started, and removed completely after having ter-
                    minated. Program files with the sticky bit set would be left in swap space
                    instead of being removed. This would accelerate subsequent invocations of
                    those programs since no copy would have to be done. Like most current
                    Unix systems, Linux uses demand paging, i. e., it fetches only those parts
                    of the code from the program’s executable file that are really required, and
                    does not copy anything to swap space at all; on Linux, the sticky bit never
                    had its original meaning.

                  C 3.5 [2] What does the special “s ” privilege mean? Where do you find it?
                    Can you set this privilege on a file that you created yourself?

                  C 3.6 [!1] Which umask invocation can be used to set up a umask that would, in
                    the project directory example above, allow all members of the project group
                    to read and write files in the project directory?

                  C 3.7 [2] What does the special “t ” privilege mean? Where do you find it?

                  C 3.8 [4] (For programmers.) Write a C program that invokes a suitable com-
                    mand (such as id ). Set this program SUID root (or SGID root ) and observe
                    what happens when you execute it.

                  C 3.9 [I]f you leave them alone for a few minutes with a root shell, clever users
                    might try to stash a SUID root shell somewhere in the system, in order to
                    assume administrator privileges when desired. Does that work with bash ?
                    With other shells?

                  3.7       File Attributes
     file attributes Besides the access permissions, the ext2 and ext3 file systems support further file
3.7 File Attributes                                                                                             51

attributes enabling access to special file system features. The most important file
attributes are summarised in Table 3.1.
   Most interesting are perhaps the “append-only” and “immutable” attributes,            a   and i attributes
which you can use to protect log files and configuration files from modification;
only root may set or reset these attributes, and once set they also apply to processes
running as root .

B In principle, an attacker who has gained root privileges may reset these at-
  tributes. However, attackers do not necessarily consider that they might be

   The A attribute may also be useful; you can use it on mobile computers to ensure      A   attribute
that the disk isn’t always running, in order to save power. Usually, whenever
a file is read, its “atime”—the time of last access—is updated, which of course
entails an inode write operation. Certain files are very frequently looked at in
the background, such that the disk never gets to rest, and you can help here by
judiciously applying the A attribute.

B The c , s and u attributes sound very nice in theory, but are not (yet) sup-
  ported by “normal” kernels. There are some more or less experimental en-
  hancements making use of these attributes, and in part they are still pipe

   You can set or reset attributes using the chattr command. This works rather           chattr
like chmod : A preceding “+ ” sets one or more attributes, “- ” deletes one or more
attributes, and “= ” causes the named attributes to be the only enabled ones. The
-R option, as in chmod , lets chattr operate on all files in any subdirectories passed
as arguments and their nested subdirectories. Symbolic links are ignored in the

# chattr +a /var/log/messages                                           Append only
# chattr -R +j /data/important                                    Data journaling …
# chattr -j /data/important/notso                                  … with exception

   With the lsattr command, you can review the attributes set on a file. The pro-         lsattr
gram behaves similar to “ls -l ”:

# lsattr /var/log/messages
-----a----------- /var/log/messages

Every dash stands for a possible attribute. lsattr supports various options such
as -R , -a , and -d , which generally behave like the eponymous options to ls .

C 3.10 [!2] Convince yourself that the a and i attributes work as advertised.

C 3.11 [2] Can you make all dashes disappear in the lsattr output for a given
52                                                                   3 Access Control

     Commands in this Chapter
     chattr  Sets file attributes for ext2 and ext3 file systems         chattr (1)   51
     chgrp   Sets the assigned group of a file or directory              chgrp (1)   44
     chmod   Sets access modes for files and directories                 chmod (1)   43
     chown   Sets the owner and/or assigned group of a file or directory
                                                                        chown (1)   44
     getfacl Displays ACL data                                        getfacl (1)   47
     lsattr  Displays file attributes on ext2 and ext3 file systems      lsattr (1)   51
     setfacl Enables ACL manipulation                                 setfacl (1)   47
     star    POSIX-compatible tape archive with ACL support              star (1)   47

        • Linux supports file read, write and execute permissions, where these per-
          missions can be set separately for a file’s owner, the members of the file’s
          group and “all others”.
        • The sum total of a file’s permissions is also called its access mode.
        • Every file (and directory) has an owner and a group. Access rights—read,
          write, and execute permission—are assigned to these two categories and
          “others” separately. Only the owner is allowed to set access rights.
        • Access rights do not apply to the system administrator (root ). He may read
          or write all files.
        • File permissions can be manipulated using the chmod command.
        • Using chown , the system administrator can change the user and group as-
          signment of arbitrary files.
        • Normal users can use chgrp to assign their files to different groups.
        • The umask can be used to limit the standard permissions when files and
          directories are being created.
        • The SUID and SGID bits allow the execution of programs with the privileges
          of the file owner or file group instead of those of the invoker.
        • The SGID bit on a directory causes new files in that directory to be assigned
          the directory’s group (instead of the primary group of the creating user).
        • The “sticky bit” on a directory lets only the owner (and the system admin-
          istrator) delete files.
        • The ext file systems support special additional file attributes.

     SUID Dennis M. Ritchie. “Protection of data file contents”. US patent 4,135,240.
                                                                                          $ echo tux
                                                                                          $ ls
                                                                                          $ /bin/su -

Process Management

4.1       What Is A Process? . . . . . . . . . . . . . .         .   .   .   .   .   54
4.2       Process States . . . . . . . . . . . . . . . .         .   .   .   .   .   55
4.3       Process Information—ps . . . . . . . . . . . .         .   .   .   .   .   56
4.4       Processes in a Tree—pstree . . . . . . . . . . .       .   .   .   .   .   57
4.5       Controlling Processes—kill and killall . . . . . . .   .   .   .   .   .   58
4.6       pgrep and pkill . . . . . . . . . . . . . . . .        .   .   .   .   .   59
4.7       Process Priorities—nice and renice . . . . . . . . .   .   .   .   .   .   61
4.8       Further Process Management Commands—nohup and top      .   .   .   .   .   61

      •   Knowing the Linux process concept
      •   Using the most important commands to query process information
      •   Knowing how to signal and stop processes
      •   Being able to influence process priorities

      • Linux commands

adm1-prozesse.tex   (33e55eeadba676a3 )
54                                                                                           4 Process Management

                               4.1     What Is A Process?
                               A process is, in effect, a “running program”. Processes have code that is executed,
                               and data on which the code operates, but also various attributes the operating uses
                               to manage them, such as:
           process number         • The unique process number (PID or “process identity”) serves to identify
                                    the process and can only be assigned to a single process at a time.
     parent process number        • All processes know their parent process number, or PPID. Every process can
                                    spawn others (“children”) that then contain a reference to their procreator.
                                    The only process that does not have a parent process is the “pseudo” process
                                    with PID 0, which is generated during system startup and creates the “init”
                                    process with a PID of 1, which in turn is the ancestor of all other processes
                                    in the system.

                      user        • Every process is assigned to a user and a set of groups. These are important
                    groups          to determine the access the process has to files, devices, etc. (See Section 3.4.)
                                    Besides, the user the process is assigned to may stop, terminate, or other-
                                    wise influence the process. The owner and group assignments are passed
                                    on to child processes.

                                  • The system splits the CPU time into little chunks (“time slices”), each of
                                    which lasts only for a fraction of a second. The current process is entitled to
                                    such a time slice, and afterwards the system decides which process should
                                    be allowed to execute during the next time slice. This decision is made by
                    priority        the appropriate “scheduler” based on the priority of a process.

                                     B In multi-processor systems, Linux also takes into account the particu-
                                       lar topology of the computer in question when assigning CPU time to
                                       processes—it is simple to run a process on any of the different cores
                                       of a multi-core CPU which share the same memory, while the “migra-
                                       tion” of a process to a different processor with separate memory entails
                                       a noticeable administrative overhead and is therefore less often worth-

            other attributes      • A process has other attributes—a current directory, a process environment,
                                    …—which are also passed on to child processes.
         process file system You can consult the /proc file system for this type of information. This process file
                               system is used to make available data from the system kernel which is collected at
                               run time and presented by means of directories and files. In particular, there are
                               various directories using numbers as names; every such directory corresponds to
                               one process and its name to the PID of that process. For example:

                               dr-xr-xr-x   3   root   root    0   Oct 16 11:11 1
                               dr-xr-xr-x   3   root   root    0   Oct 16 11:11 125
                               dr-xr-xr-x   3   root   root    0   Oct 16 11:11 80

                               In the directory of a process, there are various “files” containing process informa-
                               tion. Details may be found in the proc (5) man page.

                job control    B The job control available in many shells is also a form of process management—
                                 a “job” is a process whose parent process is a shell. From the corresponding
                                 shell, its jobs can be controlled using commands like jobs , bg , and fg , as well
                                 as the key combinations Ctrl + z and Ctrl + c (among others). More in-
                                 formation is available from the manual page of the shell in question, or
                                 from the Linup Front training manual, Introduction to Linux for Users and
4.2 Process States                                                                                            55

    is               runnable                     operating


           Figure 4.1: The relationship between various process states

C 4.1 [3] How can you view the environment variables of any of your pro-
  cesses? (Hint: /proc file system.)

C 4.2 [2] (For programmers.) What is the maximum possible PID? What hap-
  pens when this limit is reached? (Hint: Look for the string “PID_MAX ” in the
  files below /usr/include/linux .)

4.2     Process States
Another important property of a process is its process state. A process in mem- process state
ory waits to be executed by the CPU. This state is called “runnable”. Linux uses
pre-emptive multitasking, i. e., a scheduler distributes the available CPU time to pre-emptive multitasking
waiting processes in pieces called “time slices”. If a process is actually execut-
ing on the CPU, this state is called “operating”, and after its time slice is over the
process reverts to the “runnable” state.

B From an external point of view, Linux does not distinguish between these
  two process states; the process in question is always marked “runnable”.

    It is quite possible that a process requires further input or needs to wait for
peripheral device operations to complete; such a process cannot be assigned CPU
time, and its state is considered to be “sleeping”. Processes that have been stopped
by means of Ctrl + z using the shell’s job control facility are in state “stopped”.
Once the execution of a process is over, it terminates itself and makes a return return code
code available, which it can use to signal, for example, whether it completed suc-
cessfully or not (for a suitable definition of “success”).
    Once in a while processes appear who are marked as zombies using the “Z” zombies
state. These “living dead” usually exist only for a brief instant. A process becomes
a zombie when it finishes and dies for good once its parent process has queried
its return code. If a zombie does not disappear from the process table this means
that its parent should really have picked up the zombie’s return code but didn’t.
A zombie cannot be removed from the process table. Because the original pro-
cess no longer exists and cannot take up neither RAM nor CPU time, a zombie
has no impact on the system except for an unattractive entry in the system state.
Persistent or very numerous zombies usually indicate programming errors in the
parent process; when the parent process terminates they should do so as well.

B Zombies disappear when their parent process disappears because “or-
  phaned” processes are “adopted” by the init process. Since the init process
56                                                                  4 Process Management

             spends most of its time waiting for other processes to terminate so that it
             can collect their return code, the zombies are then disposed of fairly quickly.

     B Of course, zombies take up room in the process table that might be required
       for other processes. If that proves a problem, look at the parent process.

     C 4.3 [2] Start a xclock process in the background. In the $! shell variable you
       will find the PID of that process (it always contains the PID of the most re-
       cently launched background process). Check the state of that process by
       means of the “grep ^State: /proc/$!/status ” command. Stop the xclock by
       moving it to the foreground and stopping it using Ctrl + z . What is the
       process state now? (Alternatively, you may use any other long-running pro-
       gram in place of xclock .)

     C 4.4 [4] (When going over this manual for the second time.) Can you create
       a zombie process on purpose?

     4.3        Process Information—ps
     You would normally not access the process information in /proc directly but use
     the appropriate commands to query it.
        The ps (“process status”) command is available on every Unix-like system.
     Without any otions, all processes running on the current terminal are output. The
     resulting list contains the process number PID , the terminal TTY , the process state
     STAT , the CPU time used so far TIME and the command being executed.

     $ ps
       997   1 S    0:00 -bash
      1005   1 R    0:00 ps
     $ _

     There are two processes currently executing on the tty1 terminal: Apart from the
     bash with PID 997, which is currently sleeping (state “S ”), a ps command is executed
     using PID 1005 (state “R ”). The “operating” state mentioned above is not being
     displayed in ps output.
         The syntax of ps is fairly confusing. Besides Unix98-style options (like -l ) and
     GNU-style long options (such as --help ), it also allows BSD-style options without
     a leading dash. Here is a selection out of all possible parameters:
     a   (“all”) displays all processes with a terminal
     --forest   displays the process hierarchy
     l   (“long”) outputs extra information such as the priority
     r   (“running”) displays only runnable processes
     T   (“terminal”) displays all processes on the current terminal
     U   ⟨name⟩ (“user”) displays processes owned by user ⟨name⟩
     x   also displays processes without a terminal

     B The unusual syntax of ps derives from the fact that AT&T’s ps traditionally
       used leading dashes on options while BSD’s didn’t (and the same option
       can have quite different results in both flavours). When the big reunification
       came in System V Release 4, one could hang on to most options with their
       customary meaning.
4.4 Processes in a Tree—pstree                                                                  57

   If you give ps a PID, only information pertaining to the process in question will
be displayed (if it exists):

$ ps 1
    1 ?         Ss      0:00 init [2]

With the -C option, ps displays information about the process (or processes) based
on a particular command:

$ ps -C   konsole
    PID   TTY           TIME   CMD
   4472   ?         00:00:10   konsole
  13720   ?         00:00:00   konsole
  14045   ?         00:00:14   konsole

(Alternatively, grep would help here as well.)

C 4.5 [!2] What does the information obtainable with the ps command mean?
  Invoke ps without an option, then with the a option, and finally with the ax
  option. What does the x option do?

C 4.6 [3] The ps command allows you to determine the output format your-
  self by means of the -o option. Study the ps (1) manual page and specify a
  ps command line that will output the PID, PPID, the process state and the

4.4       Processes in a Tree—pstree
If you do not want to obtain every bit of information about a process but are rather
interested in the relationships between processes, the pstree command is helpful.      pstree
pstree displays a process tree in which the child processes are shown as depending
on their parent process. The processes are displayed by name:

$ pstree
     |           |-kblockd/0
     |           `-2*[pdflush]
     |         |-kdeinit-+-bash---bash
     |         |          |-2*[bash]
     |         |          |-bash---less
58                                                                                 4 Process Management

                   |         |          |-bash-+-pstree
                   |         |          |      `-xdvi---xdvi.bin---gs
                   |         |          `-bash---emacs---emacsserver
                   |         |-kdeinit---3*[bash]
                   |         |-kteatime
                   |         `-tclsh

           Identical processes are collected in brackets and a count and “*” are displayed.
           The most important options of pstree include:
           -p   displays PIDs along with process names
           -u   displays process owners’ user name
           -G   makes the display prettier by using terminal graphics characters—whether this
                  is in fact an improvement depends on your terminal

            B You can also obtain an approximated tree structure using “ps --forest ”. The
              tree structure is part of the COMMAND column in the output.

           4.5         Controlling Processes—kill and killall
     signals The kill command sends signals to selected processes. The desired signal can be
           specified either numerically or by name; you must also pass the process number
           in question, which you can find out using ps :

            $   kill   -15 4711                                  Send signal SIGTERM to process 4711
            $   kill   -TERM 4711                                                        Same thing
            $   kill   -SIGTERM 4711                                               Same thing again
            $   kill   -s TERM 4711                                                Same thing again
            $   kill   -s SIGTERM 4711                                             Same thing again
            $   kill   -s 15 4711                                                        Guess what

                Here are the most important signals with their numbers and meaning:
           SIGHUP   (1, “hang up”) causes the shell to terminate all of its child processes that
                    use the same controlling terminal as itself. For background processes with-
                    out a controlling terminal, this is frequently used to cause them to re-read
                    their configuration files (see below).

           SIGINT   (2, “interrupt”) Interrupts the process; equivalent to the          Ctrl   +   c   key com-
           SIGKILL   (9, “kill”) Terminates the process and cannot be ignored; the “emergency
           SIGTERM     (15, “terminate”) Default for kill and killall ; terminates the process.
           SIGCONT     (18, “continue”) Lets a process that was stopped using SIGSTOP continue.
           SIGSTOP     (19, “stop”) Stops a process temporarily.

           SIGTSTP     (20, “terminal stop”) Equivalent to the     Ctrl   +   z   key combination.

           A You shouldn’t get hung up on the signal numbers, which are not all guaran-
             teed to be the same on all Unix versions (or even Linux platforms). You’re
             usually safe as far as 1, 9, or 15 are concerned, but for everything else you
             should rather be using the names.
4.6 pgrep and pkill                                                                          59

Unless otherwise specified, the signal SIGTERM (“terminate”) will be sent, which
(usually) ends the process. Programs can be written such that they “trap” signals
(handle them internally) or ignore them altogether. Signals that a process neither
traps nor ignores usually cause it to crash hard. Some (few) signals are ignored
by default.
   The SIGKILL and SIGSTOP signals are not handled by the process but by the kernel
and hence cannot be trapped or ignored. SIGKILL terminates a process without
giving it a chance to object (as SIGTERM would), and SIGSTOP stops the process such
that it is no longer given CPU time.
   kill does not always stop processes. Background processes which provide sys-
tem services without a controlling terminal—daemons—usually reread their con- daemons
figuration files without a restart if they are sent SIGHUP (“hang up”).
   You can apply kill , like many other Linux commands, only to processes that
you actually own. Only root is not subject to this restriction.
   Sometimes a process will not even react to SIGKILL . The reason for this is ei-
ther that it is a zombie (which is already dead and cannot be killed again) or else
blocked in a system call. The latter situation occurs, for example, if a process waits
for a write or read operation on a slow device to finish.
   An alternative to the kill command is the killall command. killall acts just killall
like kill —it sends a signal to the process. The difference is that the process must
be named instead of addressed by its PID, and that all processes of the same name
are signalled. If no signal is specified, it sends SIGTERM by default (like kill ). killall
outputs a warning if there was nothing to signal to under the specified name.
   The most important options for killall include:
-i killall  will query you whether it is actually supposed to signal the process in
-l   outputs a list of all available signals.
-w   waits whether the process that was signalled actually terminates. killall
       checks every second whether the process still exists, and only terminates
       once it is gone.

A Be careful with killall if you get to use Solaris or BSD every now and then.
  On these systems, the command does exactly what its name suggests—it
  kills all processes.

C 4.7 [2] Which signals are being ignored by default? (Hint: signal (7))

4.6       pgrep   and pkill
As useful as ps and kill are, as difficult can it be sometimes to identify exactly the
right processes of interest. Of course you can look through the output of ps using
grep , but to make this “foolproof” and without allowing too many false positives
is at least inconvenient, if not tricky. Nicely enough, Kjetil Torgrim Homme has
taken this burden off us and developed the pgrep program, which enables us to
search the process list conveniently. A command like

$ pgrep -u root sshd

will, for example, list the PIDs of all sshd processes belonging to root .

B By default, pgrep restricts itself to outputting PIDs. Use the -l option to get it
  to show the command name, too. With -a it will list the full command line.

B The -d option allows you to specify a separator (the default is “\n ”):
60                                                                 4 Process Management

             $ pgrep -d, -u hugo bash

             You can obtain more detailed information on the processes by feeding the
             PIDs to ps :

             $ ps up $(pgrep -d, -u hugo bash)

             (The p option lets you give ps a comma-separated list of PIDs of interest.)
       pgrep ’s parameter is really an (extended) regular expression (consider egrep )
     which is used to examine the process names. Hence something like

     $ pgrep '^([bd]a|t?c|k|z|)sh$'

     will look for the common shells.

     B Normally pgrep considers only the process name (the first 15 characters of the
       process name, to be exact). Use the -f option to search the whole command
          You can add search criteria by means of options. Here is a small selection:
     -G   Consider only processes belonging to the given group(s). (Groups can be spec-
            ified using names or GIDs.)
     -n   Only display the newest (most recently started) of the found processes.
     -o   Only display the oldest (least recently started) of the found processes.
     -P   Consider only processes whose parent processes have one of the given PIDs.
     -t   Consider only processes whose controlling terminal is listed. (Terminal names
            should be given without the leading “/dev/ ”.)
     -u   Consider only processes with the given (effective) UIDs.

     B If you specify search criteria but no regular expression for the process name,
       all processes matching the search criteria will be listed. If you omit both you
       will get an error message.
        The pkill command behaves like pgrep , except that it does not list the found
     processes’ PIDs but sends them a signal directly (by default, SIGTERM ). As in kill
     you can specify another signal:

     # pkill -HUP syslogd

     The --signal option would also work:

     # pkill --signal HUP syslogd

     B The advantage of pkill compared to killall is that pkill can be much more

     C 4.8 [!1] Use pgrep to determine the PIDs of all processes belonging to user
       hugo . (If you don’t have a user hugo , then specify some other user instead.)

     C 4.9 [2] Use two separate terminal windows (or text consoles) to start one
       “sleep 60 ” command each. Use pkill to terminate (a) the first command
       started, (b) the second command started, (c) the command started in one
       of the two terminal windows.
4.7 Process Priorities—nice and renice                                                              61

4.7      Process Priorities—nice and renice
In a multi-tasking operating system such as Linux, CPU time must be shared
among various processes. This is the scheduler’s job. There is normally more
than one runnable process, and the scheduler must allot CPU time to runnable
processes according to certain rules. The deciding factor for this is the priority priority
of a process. The priority of a process changes dynamically according to its prior
behaviour—“interactive” processes, i. e, ones that do I/O, are favoured over those
that just consume CPU time.
   As a user (or administrator) you cannot set process priorities directly. You can
merely ask the kernel to prefer or penalise processes. The “nice value” quantifies
the degree of favouritism exhibited towards a process, and is passed along to child
   A new process’s nice value can be specified with the nice command. Its syntax nice

nice [- ⟨nice   value⟩] ⟨command⟩ ⟨parameter⟩ …

(nice is used as a “prefix” for another command).
   The possible nice values are numbers between −20 and +19. A negative nice possible nice values
value increases the priority, a positive value decreases it (the higher the value, the
“nicer” you are towards the system’s other users by giving your own processes a
lower priority). If no nice value is specified, the default value of +10 is assumed.
Only root may start processes with a negative nice value (negative nice value are
not generally nice for other users).
   The priority of a running process can be influenced using the renice command. renice
You call renice with the desired new nice value and the PID (or PIDs) of the pro-
cess(es) in question:

renice [- ⟨nice   value⟩] ⟨PID⟩ …

Again, only the system administrator may assign arbitrary nice values. Normal
users may only increase the nice value of their own processes using renice —for
example, it is impossible to revert a process started with nice value 5 back to nice
value 0, while it is absolutely all right to change its nice value to 10. (Think of a

C 4.10 [2] Try to give a process a higher priority. This may possibly not work—
  why? Check the process priority using ps .

4.8      Further Process Management Commands—nohup
         and top
When you invoke a command using nohup , that command will ignore a SIGHUP sig- Ignoring SIGHUP
nal and thus survive the demise of its parent process:

nohup   ⟨command⟩ …

The process is not automatically put into the background but must be placed there
by appending a & to the command line. If the program’s standard output is a ter-
minal and the user has not specified anything else, the program’s output together
with its standard error output will be redirected to the nohup.out file. If the current
directory is not writable for the user, the file is created in the user’s home directory
62                                                                        4 Process Management

     top      top unifies the functions of many process management commands in a single
           program. It also provides a process table which is constantly being updated. You
           can interactively execute various operations; an overview is available using h .
           For example, it is possible to sort the list according to several criteria, send signals
           to processes ( k ), or change the nice value of a process ( r ).

           Commands in this Chapter
           kill      Terminates a background process                       bash (1), kill (1)   58
           killall   Sends a signal to all processes matching the given name killall (1)        59
           nice      Starts programs with a different nice value                      nice (1)   61
           nohup     Starts a program such that it is immune to SIGHUP signals     nohup (1)    61
           pgrep     Searches processes according to their name or other criteria
                                                                                   pgrep (1)    59
           pkill     Signals to processes according to their name or other criteria
                                                                                   pkill (1)    60
           ps        Outputs process status information                                ps (1)   56
           pstree    Outputs the process tree                                     pstree (1)    57
           renice    Changes the nice value of running processes                  renice (8)    61
           top       Screen-oriented tool for process monitoring and control          top (1)   61

              • A process is a program that is being executed.
              • Besides a program text and the corresponding data, a process has attributes
                such as a process number (PID), parent process number (PPID), owner,
                groups, priority, environment, current directory, …
              • All processes derive from the init process (PID 1).
              • ps can be used to query process information.
              • The pstree command shows the process hierarchy as a tree.
              • Processes can be controlled using signals.
              • The kill and killall commands send signals to processes.
              • The nice and renice commands are used to influence process priorities.
                ulimit limits the resource usage of a process.
              • top is a convenient user interface for process management.
                                                                                                     $ echo tux
                                                                                                     $ ls
                                                                                                     $ /bin/su -


5.1  Fundamentals . . . . . . . .           .   .   .   .   .   .   .   .   .   .   .   .   .   64
5.2  Linux and PCI (Express) . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   65
   5.2.1 USB. . . . . . . . . .             .   .   .   .   .   .   .   .   .   .   .   .   .   67
5.3 Peripherals . . . . . . . . .           .   .   .   .   .   .   .   .   .   .   .   .   .   69
   5.3.1 Overview . . . . . . .             .   .   .   .   .   .   .   .   .   .   .   .   .   69
   5.3.2 Devices and Drivers . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   70
   5.3.3 The /sys Directory . . . .         .   .   .   .   .   .   .   .   .   .   .   .   .   72
   5.3.4 udev . . . . . . . . .             .   .   .   .   .   .   .   .   .   .   .   .   .   73
   5.3.5 Device Integration and D-Bus       .   .   .   .   .   .   .   .   .   .   .   .   .   74

      • Knowing the basics of Linux hardware support on PCs
      • Being familiar with terms like BIOS, UEFI, PCI, USB and others
      • Mastering the basics of dynamic peripheral support via udev

      • Basic Linux knowledge
      • A basic familiarity with PC hardware and its support by other operating
        systems is helpful

adm1-hardware.tex   (33e55eeadba676a3 )
64                                                                                                      5 Hardware

                      5.1      Fundamentals
                      Linux supports a very broad spectrum of system architectures and platforms: The
                      Linux kernel is available for all of today’s common microprocessors, and Linux
                      runs on computers from modest PDAs to the largest mainframes. Linux is also an
                      important part of many devices that one would not associate with it at first sight—
                      digital cameras and camcorders, routers, television sets and set-top boxes, GPS
                      navigation devices and many more—and that sometimes use “unusual” hard-
                      ware. The most common Linux systems by far that are used as “computers” in
                      the proper sense of the word are based on the x86 PC architecture founded by
                      IBM and Intel.

                             The x86 PC architecture by IBM and Intel is also the only one relevant to
                             LPIC-1 certification, although it should be said that architecture-specific
                             questions form a very small part of the exam today (they used to be much
                             more prevalent).

       Architecture       Computers today possess one or more processors (CPUs), memory for running
                      programs and data (RAM), and secondary storage for files (disks, SSDs). Added
                      to this are various peripheral devices for input (keyboard, mouse, graphics tablet,
                      webcam, …) and output (graphics “card”—today often part of the CPU—, audio,
                      …) and much more. These peripherals are either part of the “motherboard” or
                      are added via various interfaces (PCIe, USB, SATA, SCSI, …). Ethernet or WLAN
                      are used for networking. To be able to boot, a computer also needs “firmware” in
                      read-only memory (ROM)1

                      B On PCs, the firmware is either called “BIOS” (on older systems) or “UEFI”.
                        In former times—during the age of MS-DOS—the BIOS served as the inter-
                        face between the operating system proper (DOS) and the computer’s hard-
                        ware. Today it is only used to initialise the computer at boot and to find
                        and launch a “real” operating system. Modern operating systems tend to
                        disregard the BIOS completely.

                      B UEFI is a more modern implementation of the same idea, which does away
                        with some nasty shortcomings of the BIOS—which is after all a remnant of
                        the 1980s—and offers more extensive options, e. g., when dealing with sev-
                        eral operating systems (or operating system versions) on the same computer
                        or attempting to secure the system’s software against malicious manipula-

                      B New computers are based on UEFI, but can frequently pretend to have a tra-
                        ditional BIOS if desired, so that older operating systems can be supported.

                         While in former times extensive BIOS configuration might have been required
                      before a computer would run Linux efficiently, this is usually no longer a problem
                      today. You may only have to perform very few settings, e. g., to set the date and

     hardware clock   B You will need to decide whether the hardware clock should be set to “zone
                        time” (such as Pacific Standard Time for the west coast of North America)
                        or “universal time” (UTC). Linux—which consults the hardware clock only
                        when starting—can handle both, but must know where it is at. UTC is
                        preferable on machines running only Linux, while zone time may be useful
                        if the computer is occasionally running other alternative operating systems.

         Hard disks   B On BIOS-based computers, the BIOS must know about the hard disk the
                        system is to be booted from (at minimum). You can generally tell the BIOS
                        about the properties of the various disk drives in the system, and whether
                         1 Technically speaking, most computers today use “flash ROM”, which is rewritable using appro-

                      priate tools, instead of “true” ROM. This doesn’t change the principle.
5.2 Linux and PCI (Express)                                                                           65

      the BIOS should see them at all—these settings have no bearing on the later
      use of the disk(s) by Linux, hence it is possible to trick old-fashioned BIOS
      implementations which cannot handle modern large disk drives, simply by
      deactivating the disk drive in the BIOS. Linux can access it later, but you
      won’t be able to boot from that disk.

B Within the BIOS setup, you can enter a hard disks>geometrydisk geome- disk geometry
  try for each disk, i. e., the number of read/write heads, cylinders per head,
  and sectors per cylinder of the disk. Today’s disks have more sectors on
  the outside cylinders than on those near the centre of the platters, and no
  longer bother with the “cylinder/head/sector” or “CHS” model of address-
  ing disk sectors but simply count sectors off sequentially from 0 to …—the
  LBA mode (“linear block access”). Even so, every disk still claims a com- LBA mode
  pletely fabricated geometry matching the disk capacity in order to make
  older BIOSes and DOS happy. With modern BIOS implementations, you
  can put disks into LBA mode explicitly, but as long as the system boots us-
  ing the default values this is not required.

   The BIOS can also be used to enable or disable various types of peripherals:         peripherals

   • Assignment of serial ports from the operating system’s point of view (usu-
     ally COM1: or some such) to existing serial ports, IrDA ports, etc.
   • Support for USB, in particular USB keyboards and mice
   • Internal graphics and sound support
   • Energy and status management (APM, ACPI)
Within the scope of this course it is not feasible (and, fortunately, not necessary
for the purposes of LPIC-1) to give more detailed advice. Keep in mind that these
settings exist and that, if some device persistently refuses to get to work on Linux,
you should check what the BIOS has to say on the matter. It is possible for the
device in question to be disabled at the BIOS level, or for a BIOS-enabled device
to conflict with the device that is really wanted.

5.2     Linux and PCI (Express)
During the 30 years or more since the first IBM PC was marketed, the “internals”
of the hardware have changed radically in almost all aspects. Not least the bus
system connecting the CPU and memory with the most important peripherals
has undergone several metamorphoses since the original “ISA” bus; the current
standard is called PCI Express (PCIe) and has all but ousted its predecessors.  PCI Express

B PCIe is vaguely based on PCI and has taken over many PCI concepts (such as
  device codes for hardware detection, see below), but its electrical properties
  are completely different.

B Unlike its predecessors, PCIe is conceptually a serial bus which allows in-
  dependent point-to-point connections between different devices. Formerly,
  all devices shared a parallel bus, which put a cap on the maximum speeds
  that could be achieved.

B In the interest of efficiency, PCIe allows the transfer of data packets over
  several connections (“lanes”), but that does not take away from the general

    A major advantage of PCIe is that automatic hardware detection is possible. hardware detection
Every device connected to the bus reports a code specifying its type, manufac-
turer, and model. You can query this information using the lspci command (Fig-
ure 5.1). At the beginning of each line there is the “PCI ID” of each device, giving
its position on the PCI bus.
66                                                                              5 Hardware

     # lspci
     00:00.0 Host bridge: Intel Corp Core Processor DRAM Controller (rev 12)
     00:01.0 PCI bridge: Intel Corp Core Processor PCI Express x16 Root Port
     00:16.0 Communication controller: Intel Corporation 5 Series/3400 
       Series Chipset HECI Controller (rev 06)
     00:16.3 Serial controller: Intel Corp 5 Series/3400 Series Chipset KT 
       Controller (rev 06)
     00:19.0 Ethernet controller: Intel Corporation 82577LM Gigabit Network 
       Connection (rev 06)
     00:1a.0 USB controller: Intel Corporation 5 Series/3400 Series 
       Chipset USB2 Enhanced Host Controller (rev 06)
     00:1b.0 Audio device: Intel Corporation 5 Series/3400 Series Chipset 
     High Definition Audio (rev 06)
     01:00.0 VGA compatible controller: NVIDIA Corporation GT218M 
       [NVS 3100M] (rev a2)
     01:00.1 Audio device: NVIDIA Corporation High Definition Audio 
       Controller (rev a1)

                     Figure 5.1: Output of lspci on a typical x86-based PC

     B Linux uses the device codes to select and configure drivers for the various
       peripheral devices it detects. The kernel and the udev infrastructure collab-
       orate on this. We shall look at this in more detail later.

     B In former times you would have had to guess, but that could cause prob-
       lems including system crashes, if a driver poked the “right” wrong device.
       In cases of doubt, the system administrator (you) would have to perform
       tedious manual interventions!

        lspci   supports some interesting options: “lspci -v ” provides more verbose out-

     # lspci -v
     00:00.0 Host bridge: Intel Corp Core Processor DRAM Controller (rev 12)
             Subsystem: Hewlett-Packard Company Device 172b
             Flags: bus master, fast devsel, latency 0
             Capabilities: [e0] Vendor Specific Information: Len=0c <?>

     00:01.0 PCI bridge: Intel Corporation Core Processor PCI Express x16 
       Root Port (rev 12) (prog-if 00 [Normal decode])
             Flags: bus master, fast devsel, latency 0
             Bus: primary=00, secondary=01, subordinate=01, sec-latency=0
             I/O behind bridge: 00005000-00005fff
             Memory behind bridge: d2000000-d30fffff

     B In former times, similar information used to be available from the /proc/pci
       “file”, which is no longer provided by default by newer kernels. The official
       method of obtaining PCI data is lspci .

       “lspci -t ” gives a tree-like representation of the connections between the vari-
     ous components:

     # lspci -t
5.2 Linux and PCI (Express)                                                                        67

 |           +-00.1
 |           +-02.0
 |           +-02.1
 |           +-02.2
 |           \-02.3
             |            \-00.1

This tells you, for example, that the chipset’s “PCI bridge” (device 0000:00:01.0 )
provides the connection to the “VGA compatible controller) (device 0000:01:00.0)
and the “audio device” (0000:01:00.1 , more precisely the graphic card’s HDMI au-
dio output). The Ethernet adapter (device 0000:01:19.0 ) is also connected via PCIe.
   Finally, “lspci -n ” outputs the device codes directly, without looking up their device codes
meaning in the PCI database:

# lspci   -n
00:00.0   0600:   8086:0044   (rev   12)
00:01.0   0604:   8086:0045   (rev   12)
00:16.0   0780:   8086:3b64   (rev   06)
00:16.3   0700:   8086:3b67   (rev   06)
00:19.0   0200:   8086:10ea   (rev   06)
00:1a.0   0c03:   8086:3b3c   (rev   06)
00:1b.0   0403:   8086:3b56   (rev   06)

C 5.1 [!2] Examine the hardware structure of your system using commands
  such as “lspci -v ” or “lspci -t ”. What can you find out?

5.2.1     USB
The “Universal Serial Bus”, or USB, is nowadays used to connect practically all
external peripheral devices that do not use wireless technology (such as WLAN or
Bluetooth). The goal is a “legacy-free” computer that can do without the erstwhile
multitude of device-specific connectors for the keyboard, mouse, printer, modem,

B USB traditionally uses a “foolproof” cabling concept with different sock-
  ets for computers (“A”) and peripherals (“B”) and appropriate plugs at the
  end of the cables, in order to avoid mistakes. In addition, USB allows un-
  plugging and re-plugging of devices when powered up (even though this
  is generally not a great idea when dealing with hard disks or USB thumb-

B The newest fad (2015) are “USB C” sockets, which are identical between
  computers and peripherals and do not care which way the plug is inserted
  (which is a constant nuisance especially with USB “A” plugs—Pro tip, the
  USB logo on the plug is always supposed to point towards you). USB C con-
  nections aren’t just useful for USB, but also many other things (like powerful
  charging currents, graphics signals, and what else one might think of), but
  the system is still fairly new.
68                                                                                               5 Hardware

                                                    Table 5.1: USB standards

                         Version    since       Max speed        Devices
                           1.1      1998        1,5 MiBit/s      Keyboards, mice, modems, audio de-
                                                                 vices, …
                                                12 MiBit/s       10-MiBit ethernet, disks, …
                           2.0      2000       480 MiBit/s       disks, 100-MiBit ethernet, video …
                           3.0      2008         5 GiBit/s       disks, graphics, …
                           3.1      2013        10 GiBit/s       PCIe

                       USB is now part of all new PCs. This means that every PC contains one or more
     USB controllers USB controllers, each of which can manage up to 127 USB devices.

                    B Since PCs do not feature that many USB sockets, USB allows the tree-like
         USB hubs     connection of devices via USB hubs. You can therefore use one USB socket in
                      your computer to connect a hub with several additional sockets; the devices
                      connected to the hub will share the connection to your computer. You can
                      even connect further hubs to a hub (although this may not always be a bright
                      idea in actual practice).

                        Since its introduction in 1996, the USB standard has continuously been im-
                    proved, and newer versions pushed the maximum possible transmission speed
                    ever higher. The current incarnation (2015) is version 3.1, which basically makes
                    it possible to drive external devices at PCIe bus speeds (which is nice, e. g., for
                    external graphics cards).

                    B Newer USB implementations are, in principle, backwards-compatible. You
                      can connect USB-2.0 devices to a USB-1.1 bus and operate them at USB-1.1
                      speeds. Conversely, you can connect USB-1.1 devices to a USB-2.0 hub or
                      controller, but they won’t be any faster—but they will not slow down “real”
                      USB-2.0 devices, either.

                    B USB-3.𝑥 devices actually support USB 2.0 and USB 3.𝑥 simultaneously on
                      separate leads in the cable and separate contacts in the connectors. You
                      can plug a USB-2.0 plug into a USB-3.𝑥 socket on your computer, and that
                      will yield a USB-2.0 connection; conversely, you can plug a USB-3.𝑥 plug
                      into a USB-2.0 socket, which will also lead to a USB-2.0 connection. For
                      “genuine” USB 3.𝑥, your computer must support USB 3.𝑥, and you need
                      USB-3.𝑥 devices and matching cables.

       Enumeration    When a USB device is connected to the bus, it is assigned a number between 1
         descriptor and 127 as its device number, and the computer reads the device’s “descriptor”.
                    The descriptor specifies the type of device, the type of USB port it supports, and
              class so on. In particular, each device belongs to a class of devices that are handled in
                    a similar fashion by the computer—such as the “human interface devices”, i. e.,
                    devices that serve as input devices for people: keyboards, mice, joysticks, but also
                    switches, dials, phone keyboards, steering wheels, data gloves and so on. Classes
           subclass may have subclasses that narrow the type of device down even more.
                       You can check out which USB devices are connected to your system by using
                    the lsusb command. Like lspci , it outputs the device numbers and names:

                    $ lsusb
                    Bus 002 Device 002:   ID 8087:0020 Intel Corp. Integrated Rate     
                      Matching Hub
                    Bus 002 Device 001:   ID   1d6b:0002   Linux Foundation 2.0 root hub
                    Bus 001 Device 006:   ID   04f2:b15e   Chicony Electronics Co., Ltd
                    Bus 001 Device 008:   ID   046d:0807   Logitech, Inc. Webcam B500
                    Bus 001 Device 010:   ID   046a:0023   Cherry GmbH CyMotion Master Linux 
5.3 Peripherals                                                                    69

  Keyboard G230
Bus 001 Device 009: ID 046d:c52b Logitech, Inc. Unifying Receiver
Bus 001 Device 007: ID 05e3:0608 Genesys Logic, Inc. USB-2.0 4-Port HUB

Using the -v option, the program becomes noticeable more “chatty”:

# lsusb -v
Bus 002 Device 002: ID 8087:0020 Intel Corp. Integrated Rate   
  Matching Hub
Device Descriptor:
  bLength                 18
  bDescriptorType          1
  bcdUSB                2.00
  bDeviceClass             9 Hub
  bDeviceSubClass          0 Unused
  bDeviceProtocol          1 Single TT
  bMaxPacketSize0         64
  idVendor            0x8087 Intel Corp.
  idProduct           0x0020 Integrated Rate Matching Hub
  bcdDevice             0.00
  iManufacturer            0
  iProduct                 0
  iSerial                  0
  bNumConfigurations       1
  Configuration Descriptor:
    bLength                   9
    bDescriptorType           2
    wTotalLength             25
    bNumInterfaces            1
    bConfigurationValue       1
    iConfiguration            0
    bmAttributes          0xe0
      Self Powered
      Remote Wakeup
    MaxPower                  0mA
    Interface Descriptor:
      bLength                   9
      bDescriptorType           4
      bInterfaceNumber          0
      bAlternateSetting         0

B The output of “lsusb -v ” is quite extensive but also fairly unreadable. The
  usbview program presents the data rather more nicely (Figure 5.2).

B If you would like to know where lsusb gets its encyclopedic knowledge of
  USB peripherals: Take a look at the /var/lib/usbutils/usb.ids file.

5.3     Peripherals
5.3.1    Overview
The interesting question that remains is how Linux can handle almost arbitrary
peripheral devices—both those built in to the computer as well as those that can
come and go while the machine is running.
70                                                                                              5 Hardware

                                               Figure 5.2: The usbview program

                         Talking to peripheral devices is the Linux kernel’s natural business. Its job
                     is to find and initialise connected peripherals and to allow userspace programs
                     controlled access to them. This of course includes appropriate rights assignment,
                     such that only suitably privileged users can access peripherals directly.

                      B In real life, the Linux kernel likes to share the device management work
                        with userspace programs. This makes sense, because it both keeps the ker-
                        nel lean and also avoids the hard-wiring of too much functionality inside
                        the kernel if it is better placed in userspace programs, where it is easier to
                        program and customise.

                      B By way of an example: Simple printers are nowadays connected to the com-
                        puter via USB. If a new printer is connected, the Linux kernel finds out about
                        this and can use the USB manufacturer and model codes to figure out that
                        the newly recognised peripheral is a printer, and which model of which
                        manufacturer it is. The result of this initialisation process is a device file be-
                        low /dev which userspace programs can use to contact the printer.—Which
                        doesn’t mean that arbitrary user applications get to talk to the printer di-
                        rectly. Direct access to its device file is restricted, via suitable access rights, to
                        the printer subsystem (CUPS). The CUPS configuration routine can access
                        the Linux kernel’s information on the printer’s make and model, and se-
                        lect an appropriate printer driver. User applications—such as LibreOffice—
                        print documents by passing them to CUPS in a suitable format (PostScript or
                        PDF). CUPS then converts the job into a format that the printer understands,
                        and passes it via the device file to the Linux kernel, which then routes it to
                        the actual printer via USB.

                     5.3.2    Devices and Drivers
                     Linux supports the wide variety of peripheral devices available for PCs by means
     loadable modules of drivers which are usually furnished as loadable modules. These loadable mod-
                     ules can be found in subdirectories of /lib/modules and can be loaded into the ker-
                     nel manually using the modprobe command:

                      # modprobe foo

                     for example locates and loads the foo.ko . Should the desired module depend on
                     the functionality of other modules which are not currently loaded, modprobe ar-
                     ranges for them to be loaded as well.
5.3 Peripherals                                                                                                71

B Linux uses loadable modules not just to implement device drivers in the
  proper sense of the word, but also for network protocols, the packet filter
  infrastructure, and many more system features that are not required on ev-
  ery system or at all times. Modules also make kernel development easier
  and are therefore popular with programmers.

   The lsmod command gives an overview of the currently-loaded modules.

$ lsmod
Module                  Size   Used by
nfs                   213896   1
lockd                  54248   1 nfs
nfs_acl                 2912   1 nfs
sunrpc                162144   10 nfs,lockd,nfs_acl
tun                     8292   1
michael_mic             2304   4
arc4                    1824   4

You can also try to remove a loaded module using “modprobe -r ”:

# modprobe -r foo

(Whether this actually works depends on your kernel.) If the module to be re-
moved is the only one depending on another module, modprobe tries to remove the
other module, too.
   You will only rarely have to run modprobe manually, as the kernel can mostly take Automatic loading on demand
care of loading modules on demand. The basis for this is the /etc/modprobe.conf file
(or the files in the /etc/modprobe.d directory), which contains entries like

alias block-major-3-* ide_generic                                      Block device
alias char-major-10-1 psmouse                             Character-oriented device
alias net-pf-16-proto-8 scsi_transport_iscsi                      Network protocol

(and various others). These entries connect device files in /dev such as

$ ls -l /dev/hda1 /dev/psaux
brw-rw---- 1 root disk 3, 1 Jan 22 02:03 /dev/hda1
crw-rw---- 1 root root 10, 1 Jan 22 02:03 /dev/psaux

to the corresponding drivers—block-major-3-* means nothing other that for block-
oriented device files with a major device number of 3 and arbitrary minor device
number (such as /dev/hda1 in our example), the ide_generic driver module is appro-
priate, while access to /dev/psaux , a character-oriented device file with numbers
(10, 1) is managed through the driver named by char-major-10-1 , namely psmouse .
Whenever the device file in question is first accessed, the Linux kernel tries to
locate and enable the corresponding driver module.

B By now you are surely asking yourself how a driver like ide_generic can be
  read on demand from an IDE disk, if Linux requires it to talk to the disk
  in the first place. The answer to that is that this driver, when booting from
  IDE disk, is not really read from /lib/modules , but that the kernel obtained it
  earlier on from the “initial RAM disk”, which the boot loader fetched from
  disk together with the kernel itself (by means of the BIOS, hence without
  involving Linux). (More detail is in Chapter 8).

B With modprobe and the corresponding configuration files, you can do many
  other strange and wonderful things that are far beyond the scope of this
  discussion. You will find more about these topics from the Linup Front
  training manual, Linux System Adaptation.
72                                                                                                       5 Hardware

                           5.3.3    The /sys Directory
                           Up to and including Linux 2.4, the /proc directory represented the only way to
                           access details of the kernel and system configuration. However, the kernel devel-
                           opers disliked the uncontrolled growth of entries under /proc , in particular those
                           whose purpose did not have anything whatsoever to do with processes (the orig-
                           inal intent of the directory). For this reason, the kernel developers decided to
                           move, in the medium to long term, those aspects of /proc that didn’t have any-
                     sysfs thing to do with process management to a new pseudo file system, sysfs , where
                           stricter rules should apply. The sysfs is usually mounted on /sys , and is available
                           from the Linux kernel version 2.6 onwards.
           connection type     In particular, /sys/bus allows accessing devices depending on their connection
                           type (“bus”; pci , usb , scsi , …). File /sys/devices/ also allows accessing devices,
                           only the sort order is dfferent (device type, e. g., PCI bus address). This redun-
                           dancy is implemented by means of symbolic links:

                            $ ls -ld /sys/devices/pci0000:00/0000:00:07.2/usb1
                            drwxr-xr-x 6 root root 0 Jun 27 19:35 
                            $ ls -ld /sys/bus/usb/devices/usb1
                            lrwxrwxrwx 1 root root 0 Jun 27 19:35 

                           In the example you see the directory allowing access to the data of the first USB
                           connector. This is, in fact, the same directory. Such a directory contains various
                           files with information about the device in question:

                            $ ls /sys/bus/usb/devices/usb1
                            1-0:1.0              bmAttributes           devpath          remove
                            1-1                  bMaxPacketSize0        driver           serial
                            authorized           bMaxPower              ep_00            speed
                            authorized_default   bNumConfigurations     idProduct        subsystem
                            avoid_reset_quirk    bNumInterfaces         idVendor         uevent
                            bcdDevice            busnum                 manufacturer     urbnum
                            bConfigurationValue configuration           maxchild         version
                            bDeviceClass         descriptors            power
                            bDeviceProtocol      dev                    product
                            bDeviceSubClass      devnum                 quirks
                            $ cat /sys/bus/usb/devices/usb1/product
                            EHCI Host Controller

                              One disadvantage of the former kernel concept was the uncertainty about the
     assignment of interfaces assignment of interfaces to devices, such as the device files. The “numbering” of
                           devices (as in, sda , sdb , sdc , …, or eth0 , eth1 , …) was not easy to reproduce. sysfs ,
                           on the other hand, lets us assign interfaces to devices in an unambiguous fashion.
                              You will find the interfaces in the directories /sys/block (block devices) and /sys/
                           class (character devices, more or less):

                            $ ls /sys/block/
                            dm-0 dm-2 dm-4 dm-6       loop1   loop3   loop5   loop7    sr0
                            dm-1 dm-3 dm-5 loop0      loop2   loop4   loop6   sda
                            $ ls /sys/class/
                            ata_device dmi            mem              regulator       tty
                            ata_link    firmware      misc             rfkill          vc
                            ata_port    graphics      mmc_host         rtc             video4linux
                            backlight   hidraw        net              scsi_device     vtconsole
                            bdi         hwmon         pci_bus          scsi_disk       wmi
                            block       i2c-adapter   pcmcia_socket    scsi_host
                            bluetooth   ieee80211     power_supply     sound
5.3 Peripherals                                                                                 73

bsg         input        ppdev          spi_master
dma         leds         printer        thermal

An advantage: This lists only the devices that are actually on the system.
   Interfaces are assigned to block devices by means of symbolic links; there is a
file called device , like

$ ls -l /sys/block/sda/device
lrwxrwxrwx 1 root root 0 Jun 27 19:45 /sys/block/sda/device 
  -> ../../0:0:0:0

5.3.4    udev
In former times, Linux distributors used to create, as part of the installation pro-
cess, a /dev directory that was filled with device files for all the peripherals under
the sun. In the meantime, this approach is deprecated in favour of the idea of dy-
namically creating device files only for those devices that are actually available.
    The Linux infrastructure for this is called udev , short for “userspace /dev ”. It
was implemented by Greg Kroah-Hartman and Kay Sievers (among others) and
is available in its current form since kernel 2.6.13. udev consists mainly of a library
called namedev , which deals with assigning names to devices, and a daemon called
udevd which does (or delegates) the actual work.                                        udevd
    With udev , the /dev directory on disk only contains the essential entries. Fairly
soon after the system is started, a tmpfs file system (i. e., a RAM disk, see Sec-
tion 7.1.6) is mounted over /dev , in which udev places the appropriate device files.
To this end, the udevd is started, which listens to “uevents”—event reports by the uevents
kernel concerning devices that have been added or removed. Each such report is
compared to a set of rules which decides under which name the device should ap-
pear in the /dev directory managed by udev , and which can also execute additional
actions, like uploading “firmware” available as a binary file to the device in order firmware
to initialise it. These rules can be found in /etc/udev .

B There is nothing wrong with having a device appear in /dev under several
  names. Your digital camera, for example, might be assigned the name /dev/
  camera in addition to /dev/sd “whatever”. It all comes down to a question of

B udev can also deal with devices that do not correspond to device files in /dev ,
  such as network cards.

    An interesting side effect of this approach is that it was originally designed to
deal with devices that come and go at runtime—to be exact, at some point after
udevd was started (“hotplugging”). The same method, however, is also used un-
der the guise of “coldplugging” when the system is originally started, to initialise
the devices that are already available at that time. Of course the uevents the ker-
nel sends to register these devices cannot be processed before udevd is running.
Still, the kernel makes it possible to re-send these uevents on demand, and ex-
actly that happens after udevd has become available, in order to add the devices
to the dynamic /dev directory. Accordingly, there is no difference between device
initialisation using “hotplugging” and “coldplugging”.

B To allow this, there is a device called uevent in each device’s entry below /sys .
  You just need to write the string add to that file to trigger the corresponding

B udev isn’t the first attempt to include such an infrastructure in Linux. The
  devfs proposed by Richard Gooch did a similar thing but inside the kernel
  rather than outside (like udev ). devfs never gathered a large following in the
  community, since various matters of policy concerning, e. g., device naming
74                                                                                                5 Hardware

                                 had been hard-wired into devfs (and hence in the kernel) and not all devel-
                                 opers agreed with that. udev exposes its rules by means of editable configu-
                                 ration files and is therefore much more flexible. There used to be an infras-
                                 tructure called hotplug to handle dynamic registration and de-registration of
                                 devices, but this has been superseded by udev .

                         5.3.5     Device Integration and D-Bus
     application programs The final piece of the puzzle brings in application programs. How do you set
                         things up such that plugging in your digital camera will start your photo man-
                         agement program to download any new pictures? Or how does the icon for your
                         USB disk get placed on the backdrop of your graphical desktop environment?
                         One possible answer to this is “udisks”.

                         B Formerly this used an infrastructure called HAL (short for “hardware ab-
                           straction layer”). HAL, however, proved too complicated and limited and is
                           no longer being developed further.

                         B Udisks is one of the successor projects, and deals exclusively with stor-
                           age devices such as external disks and USB thumb drives; there are other
                           projects for other types of device. Large parts of HAL have also been assim-
                           ilated into udev .

                  HAL        The idea behind HAL is to aggregate information from various sources: The
                         kernel (via udev ) reports that a new device is available and which one. The device
                         itself can be asked, as can default settings for the device that the user once made
                         in their graphical environment.

                         B This aggregation is necessary because the kernel may not actually know
                           everything worthwhile about a device. Many digital cameras, for exam-
                           ple, register with the system as “hard disks”, and the information that they
                           should really be treated as cameras must come from elsewhere.

                 D-Bus      Udisks and similar project are based on D-Bus, a simple system for inter-
                         process communication that is supported by most desktop environments. Pro-
                         grams can tell D-Bus about services that they want to offer to other programs.
                         Programs can also wait for events. (It doesn’t matter if several programs wait for
                         the same event; all interested parties are notified.) D-Bus has two major applica-
                         tion areas:
                            • Communication between programs in the same graphical environment. A
                              possible application might be for your telephony program to automatically
                              mute your music player when a call comes in.
                            • Communication between the operating system and your graphical environ-
                            Udisks consists of two components, a background service (udisksd ) and a user-
                         accessible tool (udisksctl ) which can communicate with the background service.
                         The background service is accessible through the D-Bus and will be started on
                         demand whenever a program issues a D-Bus query to udisks. This way programs
                         can find out which storage devices are available as well as execute operations such
                         as mounting or unmounting devices. This includes appropriate privilege checks;
                         users may, for example, be allowed to plug USB thumb drives into their own com-
                         puters but will be denied access to USB thumb drives on other “seats”.
                            Here is an example of a udisksctl query about a USB thumb drive:

                         $ udisksctl status
                         MODEL                     REVISION SERIAL                DEVICE
                         ST9320423AS               0002SDM1 5VH5TBTC              sda
5.3 Bibliography                                                                         75

hp       DVDRAM GT30L      mP04        KZKA2LB3306          sr0
TOSHIBA TransMemory        1.00        7427EAB351F9CE110FE8E23E sdb
$ udisksctl info -b /dev/sdb
     Configuration:               []
     CryptoBackingDevice:         '/'
     Device:                      /dev/sdb
     DeviceNumber:                2064
     Drive:                       '/org/freedesktop/UDisks2/drives/ 
     HintAuto:                    true
     HintIgnore:                  false
     HintPartitionable:           true

Many of these data really derive from the udev database and may be influenced by
the administrator by way of suitable rules.

B In fairness we should also add that udisks represents a minimal solution.
  The common graphical desktop environments like KDE or GNOME include
  similar infrastructure, which also takes care of suitably representing storage
  devices in the graphical environment.

Commands in this Chapter
lsmod      Lists loaded kernel modules                              lsmod (8) 71
lspci      Displays information about devices on the PCI bus        lspci (8) 65
lsusb      Lists all devices connected to the USB                   lsusb (8) 68
modprobe    Loads kernel modules, taking dependencies into account
                                                                 modprobe (8) 70
udevd      Kernel uevent management daemon                          udevd (8) 73

   • The firmware (BIOS/UEFI) organises the early startup of a Linux system,
     but will subsequently not be used (apart from exceptional cases).
   • Typical BIOS settings relevant to Linux include, for example, the date and
     time, disk parameters, boot order, and the assignment of some system in-
     terfaces to actual peripherals.
   • The lspci command sheds light on a system’s PCI bus and the devices con-
     nected to it.

Hardware-HOWTO04 Steven Pritchard.      “Linux Hardware Compatibility
     HOWTO”, January 2004. HOWTO/

Linux-USB-Subsystem Brad Hards. “The Linux USB sub-system”.
                                       http://www.linux- guide/book1.html

PCI-HOWTO01 Michael Will. “Linux PCI-HOWTO”, June 2001.
                                                                                                  $ echo tux
                                                                                                  $ ls
                                                                                                  $ /bin/su -

Hard Disks (and Other Secondary

6.1     Fundamentals . . . . . . . . . .         .   .   .   .   .   .   .   .   .   .   .   78
6.2     Bus Systems for Mass Storage . . . .     .   .   .   .   .   .   .   .   .   .   .   78
6.3     Partitioning . . . . . . . . . . .       .   .   .   .   .   .   .   .   .   .   .   81
      6.3.1 Fundamentals . . . . . . . .         .   .   .   .   .   .   .   .   .   .   .   81
      6.3.2 The Traditional Method (MBR) . .     .   .   .   .   .   .   .   .   .   .   .   82
      6.3.3 The Modern Method (GPT) . . .        .   .   .   .   .   .   .   .   .   .   .   83
6.4     Linux and Mass Storage . . . . . .       .   .   .   .   .   .   .   .   .   .   .   84
6.5     Partitioning Disks. . . . . . . . .      .   .   .   .   .   .   .   .   .   .   .   86
      6.5.1 Fundamentals . . . . . . . .         .   .   .   .   .   .   .   .   .   .   .   86
      6.5.2 Partitioning Disks Using fdisk . .   .   .   .   .   .   .   .   .   .   .   .   88
      6.5.3 Formatting Disks using GNU parted    .   .   .   .   .   .   .   .   .   .   .   91
      6.5.4 gdisk . . . . . . . . . . .          .   .   .   .   .   .   .   .   .   .   .   92
      6.5.5 More Partitioning Tools . . . .      .   .   .   .   .   .   .   .   .   .   .   93
6.6     Loop Devices and kpartx . . . . . .      .   .   .   .   .   .   .   .   .   .   .   93
6.7     The Logical Volume Manager (LVM) . .     .   .   .   .   .   .   .   .   .   .   .   95

      • Understanding how Linux deals with secondary storage devices based on
        ATA, SATA, and SCSI.
      • Understanding MBR-based and GPT-based partitioning
      • Knowing about Linux partitioning tools and how to use them
      • Being able to use loop devices

      • Basic Linux knowledge
      • Knowledge about Linux hardware support (see chapter 5)

adm1-platten.tex   (33e55eeadba676a3 )
78                                                     6 Hard Disks (and Other Secondary Storage)

               6.1     Fundamentals
               RAM is fairly cheap these days, but even so not many computers can get by with-
               out the permanent storage of programs and data on mass storage devices. These
               include (among others):
                   • Hard disks with rotating magnetic platters
                   • “Solid-state disks” (SSDs) that look like hard disks from the computer’s
                     point of view, but use flash memory internally
                   • USB thumb drives, SD cards, CF cards, and other interchangeable media
                     based on flash memory
                   • RAID systems that aggregate hard disks and present them as one big storage
                   • SAN devices which provide “virtual” disk drives on the network
                   • File servers that offer file access at an abstract level (CIFS, NFS, …)
               In this chapter we shall explain the basics of Linux support for the first three en-
               tries in the list—hard disks, SSDs and flash-based portable media like USB thumb
               drives. RAID sstems and SAN are discussed in the Linup Front training man-
               ual, Linux Storage and File Systems, file servers are discussed in Linux Infrastructure

               6.2     Bus Systems for Mass Storage
               IDE, ATA and SATA Until not so long ago, hard disks and optical drives such
          IDE as CD-ROM and DVD readers and writers used to be connected via a “IDE con-
               troller”, of which self-respecting PCs had at least two (with two “channels” each).

                B “IDE” is really an abbreviation of “Integrated Drive Electronics”. The “inte-
                  grated drive electronics” alluded to here lets the computer see the disk as a
                  sequence of numbered blocks without having to know anything about sec-
                  tors, cylinders, and read/write heads, even though this elaborate charade
                  is still kept up between the BIOS, disk controller, and disks. However, for a
                  long time this description has applied to all hard disks, not just those with
                  the well-known “IDE” interface, which by now is officially called “ATA”,
                  short for “AT Attachment”1 .

                Computers bought new these days usually still contain IDE interfaces, but the
                method of choice to connect hard disks and optical drives today is a serial ver-
     Serial ATA sion of the ATA standard, imaginatively named “Serial ATA” (SATA, for short).
                Since SATA (i. e., approximately 2003), traditional ATA (or “IDE”) is commonly
                called “P-ATA”, an abbreviation of “parallel ATA”. The difference applies to the
                cable, which for traditional ATA is an inconvenient-to-place and electrically not
                100% fortunate 40- or 80-lead ribbon cable which can transfer 16 data bits in par-
                allel, and which links several devices at once to the controller. SATA, on the other
                hand, utilises narrow flat seven-lead cables for serial transmission, one per device
                (usually produced in a cheerful orange colour).

                B SATA cables and connectors are much more sensitive mechanically than the
                  corresponding P-ATA hardware, but they make up for that through other
                  advantages: They are less of an impediment to air flow in a PC case, cannot
                  be installed wrongly due to their different connectors on each end, which
                  furthermore cannot be plugged in the wrong way round. In addition, the il-
                  logical separation between 2.5-inch and 3.5-inch diskdrives, which required
                  different connectors for P-ATA, goes away.
                  1 Anyone   remember the IBM PC/AT?
6.2 Bus Systems for Mass Storage                                                                       79

B Interestingly, serial ATA allows considerably faster data transfers than tra-
  ditional ATA, even though with the former all bits are transferred in “single
  file” rather than 16 at a go in parallel. This is due to the electrical proper-
  ties of the interface, which uses differential transmission and a signalling
  voltage of only 0.5 V instead of 5 V. (This is why cables may be longer, too—
  1 m instead of formerly 45 cm.) Current SATA interfaces can theoretically
  transfer up to 16 GiBit/s (SATA 3.2) which due to encoding and other im-
  pediments comes out as approximately 2 MiB/s—rather more than single
  disk drives can keep up with at sustained rates, but useful for RAID sys-
  tems that access multiple disk drives at the same time, and for fast SSDs. It
  is unlikely that SATA speeds will evolve further, since the trend with SSDs
  is towards connecting them directly via PCIe2 .

B Besides the higher speed and more convenient cabling, SATA also offers
  the advantage of “hot-swapping”: It is possible to disconnect a SATA disk
  drive and connect another one in its place, without having to shut down the
  computer. This of course presupposes that the computer can do without
  the data on the drive in question—typically because it is part of a RAID-1
  or RAID-5, where the the data on the new drive can be reconstructed based
  on other drives in the system. With traditional ATA, this was impossible (or
  only possible by jumping through hoops).

B External SATA (“eSATA”) is a derivative of SATA for use with external eSATA
  drives. It has different connectors and electrical specifications, which are
  much more robust mechanically and better suited for hot-swapping. In
  the meantime, it has been almost completely ousted from the market by
  USB 3.𝑥, but can still be found in older hardware.

SCSI and SAS The “Small Computer System Interface” or SCSI (customary pro-
nunciation: “SCUZ-zy”) has served for more than 25 years to connect hard disks,
tape drives and other mass storage devices, but also peripherals such as scanners,
to “small” computers3 . SCSI buses and connectors exist in a confusing variety,
beginning with the “traditional” 8-bit bus and ranging from the fast 16-bit vari-
eties to new, even faster serial implementations (see below). They also differ in
the maximum number of devices per bus, and in physical parameters such as the
maximum cable length and the allowable distances between devices on the cable.
Nicely enough, most of the variants are compatible or can be made compatible
(possibly with loss of efficiency!). Varieties such as FireWire (IEEE-1394) or Fiber-
Channel are treated like SCSI by Linux.

B Nowadays, most work goes into the serial SCSI implementations, most no-
  tably “Serial Attached SCSI” (SAS). As with SATA, data transfer is poten-
  tially faster (at the moment, SAS is slightly slower than the fastest parallel
  SCSI version, Ultra-640 SCSI) and electrically much less intricate. In partic-
  ular, the fast parallel SCSI versions are plagued by clocking problems that
  derive from the electrical properties of the cables and termination, and that
  do not exist with SAS (where the pesky termination is no longer necessary
  at all).

B SAS and SATA are fairly closely related; the most notable differences are that
  SAS allows things like accessing a drive via several cable paths for redun-
  dancy (“multipath I/O”; SATA requires jumping through hoops for this),
  supports more extensive diagnosis and logging functions, and is based on
  a higher signalling voltage, which allows for longer cables (up to 8 m) and
  physically larger servers.
   2 SATA in a strict sense allows speeds up to 6 GiBit/s; the higher speed of SATA 3.2 is already

achieved by means of PCIe. This “SATA Express” specification defines an interface that can carry
SATA signals as well as PCIe, such that compatible devices can be connected not only to SATA Express
controllers, but also to older hosts which support “only” SATA with up to 6 GiBit/s.
   3 Nobody has ever defined the meaning of “small” in this context, but it must be something like

“can be bodily lifted by at most two people”
80                                                         6 Hard Disks (and Other Secondary Storage)

                                           Table 6.1: Different SCSI variants

                     Name                   Width    Transfer rate Devices   Explanation
                     SCSI-1                  8 bit     ≤ 5 MiB/s         8   “Ancestor”
                     SCSI-2 »Fast«           8 bit      10 MiB/s         8
                     SCSI-2 »Wide«          16 bit      20 MiB/s        16
                     SCSI-3 »Ultra«          8 bit      20 MiB/s         8
                     SCSI-3 »Ultrawide«     16 bit      40 MiB/s        16
                     Ultra2 SCSI            16 bit      80 MiB/s        16   LVD busa
                     Ultra-160 SCSIb        16 bit     160 MiB/s        16   LVD bus
                     Ultra-320 SCSIc        16 Bit     320 MiB/s        16   LVD bus
                     Ultra-640 SCSI         16 Bit     640 MiB/s        16   LVD bus

                    B SATA and SAS are compatible to an extent where you can use SATA disk
                      drives on a SAS backplane (but not vice-versa).

     Vorkommen         “Pure-bred” SCSI, as far as PCs are concerned, is found mostly in servers; work
                    stations and “consumer PCs” tend to use IDE or SATA for mass storage and USB
                    (qv. section 5.2.1) for other devices. Devices based on IDE and USB are much
                    cheaper to manufacture than SCSI-based devices—IDE disks, for example, cost
                    about a third or a fourth of the price of comparatively large SCSI disks.

                    B We do need to mention that SCSI disks are usually designed especially for
                      use in servers, and are therefore optimised for high speed and longevity.
                      SATA disks for workplace PCs do not come with the same warranties, are
                      not supposed to rival a jet fighter for noise, and should support fairly fre-
                      quent starting and stopping.

                       As a Linux administrator, you should know about SCSI even if you do not run
                    any SCSI-based systems, since from the point of view of the Linux kernel, in ad-
                    dition to SATA many USB or FireWire devices are accessed like SCSI devices and
                    use the same infrastrucure.

         SCSI ID    B Every device on a SCSI bus requires a unique “SCSI ID”. This number
                      between 0 and 7 (15 on the wide buses) is used to address the device.
                      Most “true” SCSI devices sport jumpers or a switch to select it; with Fibre-
                      Channel, USB, or SATA devices that are accessed via the SCSI infrastructure,
                      the system arranges for suitable unique SCSI IDs to be assigned.

     host adapter   B To use SCSI, a PC needs at least one host adapter (or “host”). Motherboard-
      SCSI BIOS       based and better expansion card host adapters contain a SCSI BIOS which
                      lets the system boot from a SCSI device. You can also use this to check which
                      SCSI IDs are available and which are used, and which SCSI device, if any,
                      should be used for booting.

                    B The host adapter counts as a device on the SCSI bus—apart from itself you
                      can connect 7 (or 15) other devices.

      boot order    B If your BIOS can boot from SCSI devices, you can also select in the boot order
                      whether the ATA disk C: should be preferred to any (potentially) bootable
                      SCSI devices.

                    B Most important for the correct function of a parallel SCSI system is appro-
     termination      priate termination of the SCSI bus. This can either be ensured via a special
                      plug (“terminator”) or switched on or off on individual devices. Erroneous
                      termination is the possible origin of all sorts of SCSI problems. If you do
                      experience difficulties with SCSI, always check first that termination is in
                      order. SAS does not require termination.
6.3 Partitioning                                                                       81

USB With the new fast USB variants (Section 5.2.1), few if any compromises will
be needed when connecting mass storage devices—reading and writing speeds
are bounded by the storage device, not (as with USB 1.1 and USB 2.0) by the bus.
Linux manages USB-based storage devices exactly like SCSI devices.

C 6.1 [1] How many hard disks or SSDs does your computer contain? What
  is their capacity? How are they connected to the computer (SATA, …)?

6.3     Partitioning
6.3.1    Fundamentals
Mass storage devices such as hard disks or SSDs are commonly “partitioned”, i. e.,
subdivided into several logical storage devices that the operating system can then
access independently. This does not only make it easier to use data structures that
are appropriate to the intended use—sometimes partitioning is the only way to
make a very large storage medium fully accessible, if limits within the operating
system preclude the use of the medium “as a whole” (even though this sort of
problem tends to be rare today).
   Advantages of partitioning include the following:
   • Logically separate parts of the system may be separated. For example, you
     could put your users’ data on a different partition from that used by the op-
     erating system itself. This makes it possible to reinstall the operating system
     from scratch without endangering your users’ data. Given the often rudi-
     mentary “upgrade” functionality of even current distributions this is very
     important. Furthermore, if inconsistencies occur in a file system then only
     one partition may be impacted at first.

   • The structure of the file system may be adapted to the data to be stored.
     Most file systems keep track of data by means of fixed-size “blocks”, where
     every file, no matter how small, occupies at least a single block. With a 4 KiB
     block size this implies that a 500-byte file only occupies 1/8 of its block—the
     rest goes to waste. If you know that a directory will contain mostly small
     files (cue: mail server), it may make sense to put this directory on a parti-
     tion using smaller blocks (1 or 2 KiB). This can reduce waste considerably.
     Some database servers, on the other hand, like to work on “raw” partitions
     (without any file system) that they manage themselves. An operating sys-
     tem must make that possible, too.
   • “Runaway” processes or incautious users can use up all the space available
     on a file system. At least on important server systems it makes sense to
     allow user data (including print jobs, unread e-mail, etc.) only on partitions
     that may get filled up without getting the system itself in trouble, e.g., by
     making it impossible to append information to important log files.
   There are currently two competing methods to partition hard disks for PCs.
The traditional method goes back to the 1980s when the first hard disks (with
awesome sizes like 5 or 10 MB) appeared. Recently a more modern method was
introduced; this does away with various limitations of the traditional approach,
but in some cases requires special tools.

B Hard disks are virtually always partitioned, even though at times only one
  partition will be created. With USB thumb drives, one sometimes eschews
  partitioning altogether.
82                                                                       6 Hard Disks (and Other Secondary Storage)

                                           Table 6.2: Partition types for Linux (hexadecimal)

                                        Type      Description
                                          81      Linux data
                                          82      Linux swap space
                                          86      RAID super block (old style)
                                          8E      Linux LVM
                                          E8      LUKS (encrypted partition)
                                          EE      “Protective partition” for GPT-partitioned disk
                                          FD      RAID super block with autodetection
                                          FE      Linux LVM (old style)

                       6.3.2      The Traditional Method (MBR)
                          The traditional method stores partitioning information inside the “master boot
                          record” (MBR), the first sector (number 0) of a hard disk. (Traditionally, PC hard
                          disk sectors are 512 bytes long, but see below.) The space there—64 bytes starting
      primary partitions at offset 446—is sufficient for four primary partitions. If you want to create more
     extended partition than four partitions, you must use one of these primary partitions as an extended
       logical partitions partition. An extended partition may contain further logical partitions.

                       B The details about logical partitions are not stored inside the MBR, but at the
                         start of the partition (extended or logical) in question, i. e., they are scattered
                         around the hard disk.

                       Partition entries today usually store the starting sector number of the partition on
                       the disk as well as the length of the partition in question in sectors4 . Since these
                       values are 32-bit numbers, given the common 512-byte sectors this results in a
                       maximum partition size of 2 TiB.

                       B There are hard disks available today which are larger than 2 TiB. Such disks
                         cannot be made fully accessible using MBR partitioning. One common ruse
                         consists in using disks whose sectors are 4096 bytes long instead of 512. This
                         will let you have 16-TiB disks even with MBR, but not every operating sys-
                         tem supports such “4Kn” drives (Linux from kernel 2.6.31, Windows from
                         8.1 or Server 2012).

                       B 4-KiB sectors are useful on hard disks even without considering partitions.
                         The larger sectors are more efficient for storing larger files and allow better
                         error correction. Therefore the market offers “512e” disks which use 4-KiB
                         sectors internally but pretend to the outside that they really have 512-byte
                         sectors. This means that if a single 512-byte sector needs to be rewritten, the
                         adjoining 7 sectors must be read and also rewritten (a certain, but usually
                         bearable, loss of efficiency, since data is most often written in larger chunks).
                         When partitioning, you will have to pay attention that the 4-KiB blocks that
                         Linux uses internally for hard disk access coincide with the disk’s internal
                         4-KiB sectors—if that is not the case, then to write one 4-KiB Linux block two
                         4-KiB disk sectors might have to be read and rewritten, and that would not
                         be good. (Fortunately, the partitioning tools help you watch out for this.)

                          Besides the starting address and length of the (primary) partitions, the parti-
         partition type tion table contains a partition type which loosely describe the type of data man-
                       agement structure that might appear on the partition. A selection of Linux parti-
                       tion types appears in table 6.2.
                           4 In former times, partitions used to be described in terms of the cylinder, head, and sector addresses

                       of the sectors in question, but this has been deprecated for a very long time.
6.3 Partitioning                                                                          83

6.3.3    The Modern Method (GPT)
In the late 1990s, Intel developed a new partitioning method that should do away
with the limitations of the MBR approach, namely “GUID Partition Table” or GPT.

B GPT was developed hand-in-hand with UEFI and is part of the UEFI spec-
  ification today. You can, however, use a BIOS-based Linux system to access
  GPT-partitioned disks and vice-versa.

B GPT uses 64-bit sector addresses and thus allows a maximum disk size of
  8 ZiB—zebibyte, in case you haven’t run into that prefix. 1 ZiB are 270 bytes,
  or, roughly speaking, about one million million tebibytes. This should last
  even the NSA for a while. (Disk manufactures, who prefer to talk powers of
  ten rather than powers of two, will naturally sell you an 8-ZiB disk as a 9.4
  zettabyte disk.)
   With GPT, the first sector of the disk remains reserved as a “protective MBR”
which designates the whole disk as partitioned from a MBR point of view. This
avoids problems if a GPT-partitioned disk is connected to a computer that can’t
handle GPT.
   The second sector (address 1) contains the “GPT header” which stores man-
agement information for the whole disk. Partitioning information is usually con-
tained in the third and subsequent sectors.

B The GPT header points to the partitioning information, and therefore they
  could be stored anywhere on the disk. It is, however, reasonable to place
  them immediately after the GPT header. The UEFI header stipulates a min-
  imum of 16 KiB for partitioning information (regardless of the disk’s sector

B On a disk with 512-byte sectors, with a 16 KiB space for partitioning infor-
  mation the first otherwise usable sector on the disk is the one at address 34.
  You should, however, avoid placing the disk’s first partition at that address
  because that will get you in trouble with 512e disks. The next correctly-
  aligned sector is the one at address 40.

B For safety reasons, GPT replicates the partitioning information at the end of
  the disk.
   Traditionally, partition boundaries are placed at the start of a new “track” on
the disk. Tracks, of course, are a relic from the hard disk paleolithic, since con-
temporary disks are addressed linearly (in other words, the sectors are numbered
consecutively from the start of the disk to the end)—but the idea of describing a
disk by means of a combination of a number of read/write heads, a number of
“cylinders”, and a number of sectors per “track” (a track is the concentric circle a
single head describes on a given cylinder) has continued to be used for a remark-
ably long time. Since the maximum number of sectors per track is 63, this means
that the first partition would start at block 63, and that is, of course, disastrous for
512e disks.

B Since Windows Vista it is common to have the first partition start 1 MiB after
  the start of the disk (with 512-byte sectors, at sector 2048). This isn’t a bad
  idea for Linux, either, since the ample free space between the partition table
  and the first partition can be used to store the GRUB boot loader. (The space
  between the MBR and sector 63 was quite sufficient earlier, too.)
   Partition table entries are at least 128 bytes long and, apart from 16-byte GUIDs
for the partition type and the partition itself and 8 bytes each for a starting and
ending block number, contains 8 bytes for “attributes” and 72 bytes for a partition
name. It is debatable whether 16-byte GUIDs are required for partition types, but
on the one hand the scheme is called “GUID partition table” after all, and on the
other hand this ensures that we won’t run out of partition types anytime soon. A
selection is displayed in table 6.3.
84                                            6 Hard Disks (and Other Secondary Storage)

                             GUID                     Description
              00000000-0000-0000-0000-000000000000    Unused entry
              C12A7328-F81F-11D2-BA4B-00A0C93EC93B    EFI system partition (ESP)
              21686148-6449-6E6F-744E-656564454649    BIOS boot partition
              0FC63DAF-8483-4772-8E79-3D69D8477DE4    Linux file system
              A19D880F-05FC-4D3B-A006-743F0F84911E    Linux RAID partition
              0657FD6D-A4AB-43C4-84E5-0933C84B4F4F    Linux swap space
              E6D6D379-F507-44C2-A23C-238F2A3DF928    Linux LVM
              933AC7E1-2EB4-4F13-B844-0E14E2AEF915    /home partition
              3B8F8425-20E0-4F3B-907F-1A25A76F98E8    /srv partition
              7FFEC5C9-2D00-49B7-8941-3EA10A5586B7    dm-crypt partition
              CA7D7CCB-63ED-4C53-861C-1742536059CC    LUKS partition

                     Table 6.3: Partition type GUIDs for GPT (excerpt)

     B Linux can use GPT-partitioned media. This needs the “EFI GUID Partition
       support” option enabled in the kernel, but with current distributions this
       is the case. Whether the installation procedure allows you to create GPT-
       partitioned disks is a different question, just like the question of whether
       the boot loader will be able to deal with them. But that is neither here nor

     6.4     Linux and Mass Storage
     If a mass storage device is connected to a Linux computer, the Linux kernel tries
     to locate any partitions. It then creates block-oriented device files in /dev for the
     device itself and its partitions. You can subsequently access the partitions’ device
     files and make the directory hierarchies there available as part of the computer’s
     file system.

     B A new mass storage device may have no partitions at all. In this case you
       can use suitable tools to create partitions. This will be explained later in this
       chapter. The next step after partitioning consists of generating file systems
       on the partitions. This is explained in detail in chapter 7.

         The device names for mass storage are usually /dev/sda , /dev/sdb , …, in the order
     the devices are recognised. Partitions are numbered, the /dev/sda device therefore
     contains partitions like /dev7sda1 , /dev/sda2 , … A maximum of 15 partitions per de-
     vice is possible. If /dev/sda is partitioned according to the MBR scheme, /dev/sda1
     to /dev/sda4 correspond to the primary partitions (possibly including an extended
     partition), while any logical partitions are numbered starting with /dev/sda5 (re-
     gardless of whether there are four primary partitions on the disk or fewer).

     B The “s ” in /dev/sda derives from “SCSI”. Today, almost all mass storage de-
       vices in Linux are managed as SCSI devices.

     B For P-ATA disks there is another, more specific mechanism. This accesses
       the IDE controllers inside the computer directly—the two drives connected
       to the first controller are called /dev/hda and /dev/hdb , the ones connected to
       the second /dev/hdc and /dev/hdd . (These names are used independently of
       whether the drives actually exist or not—if you have a single hard disk and
       a CD-ROM drive on your system, you do well to connect the one to one
       controller and the other to the other so they will not be in each other’s way.
       Therefore you will have /dev/hda for the disk and /dev/hdc for the CD-ROM
       drive.) Partitions on P-ATA disks are, again, called /dev/hda1 , /dev/hda2 and
       so on. In this scheme, 63 (!) partitions are allowed.
6.4 Linux and Mass Storage                                                                      85

B If you still use a computer with P-ATA disks, you will notice that in the
  vast majority of cases the SCSI infrastructure is used for those, too (note the
  /dev/sda style device names). This is useful for convenience and consistency.
  Some very few P-ATA controllers are not supported by the SCSI infrastruc-
  ture, and must use the old P-ATA specific infrastructure.

B The migration of an existing Linux system from “traditional” P-ATA drivers
  to the SCSI infrastructure should be well-considered and involve changing
  the configuration in /etc/fstab such that file systems are not mounted via
  their device files but via volume labels or UUIDs that are independent of
  the partitions’ device file names. (See section 7.2.3.)

    The Linux kernel’s mass storage subsystem uses a three-tier architecture. At architecture
the bottom there are drivers for the individual SCSI host adapters, SATA or USB
controllers and so on, then there is a generic “middle layer”, on top of which there
are drivers for the various devices (disks, tape drives, …) that you might encounter
on a SCSI bus. This includes a “generic” driver which is used to access devices
without a specialised driver such as scanners or CD-ROM recorders. (If you can
still find any of those anywhere.)

B Every SCSI host adapter supports one or more buses (“channels”). Up to
  7 (or 15) other devices can be connected to each bus, and every device can
  support several “local unit numbers” (or LUNs), such as the individual CDs LUNs
  in a CD-ROM changer (rarely used). Every SCSI device in the system can
  thus be describe by a quadrupel (⟨host⟩, ⟨channel⟩, ⟨ID⟩, ⟨LUN⟩). Usually
  (⟨host⟩, ⟨channel⟩, ⟨ID⟩) are sufficient.

B In former times you could find information on SCSI devices within the /proc/
  scsi/scsi directory. This is no longer available on current systems unless the
  kernel was compiled using “Legacy /proc/scsi support”.

B Nowadays, information about “SCSI controllers” is available in /sys/class/
  scsi_host (one directory per controller). This is unfortunately not quite as
  accessible as it used to be. You can still snoop around:

       # cd /sys/class/scsi_host/host0/device
       # ls
       power scsi_host subsystem target0:0:0       uevent
       # cd target0:0:0; ls
       0:0:0:0 power subsystem uevent
       # ls 0:0:0:0/block

      A peek into /sys/bus/scsi/devices will also be instructive:

       # ls /sys/bus/scsi/devices
       0:0:0:0 10:0:0:0 host1     host2   host4   target0:0:0   target10:0:0
       1:0:0:0 host0      host10 host3    host5   target1:0:0

   Device names such as /dev/sda , /dev/sdb , etc. have the disadvantage of not being
very illuminating. In addition, they are assigned to devices in the order of their
appearance. So if today you connect first your MP3 player and then your digital
camera, they might be assigned the device files /dev/sdb and /dev/sdc ; if tomorrow
you start with the digital camera and continue with the MP3 player, the names
might be the other way round. This is of course a nuisance. Fortunately, udev
assigns some symbolic names on top of the traditional device names. These can
be found in /dev/block :
86                                          6 Hard Disks (and Other Secondary Storage)

     # ls -l /dev/block/8:0
     lrwxrwxrwx 1 root root 6 Jul 12 14:02 /dev/block/8:0 -> ../sda
     # ls -l /dev/block/8:1
     lrwxrwxrwx 1 root root 6 Jul 12 14:02 /dev/block/8:1 -> ../sda1
     # ls -l /dev/disk/by-id/ata-ST9320423AS_5VH5TBTC
     lrwxrwxrwx 1 root root 6 Jul 12 14:02 /dev/disk/by-id/ 
       ata-ST9320423AS_5VH5TBTC -> ../../sda
     # ls -l /dev/disk/by-id/ata-ST9320423AS_5VH5TBTC-part1
     lrwxrwxrwx 1 root root 6 Jul 12 14:02 /dev/disk/by-id/ 
       ata-ST9320423AS_5VH5TBTC-part1 -> ../../sda1
     # ls -l /dev/disk/by-path/pci-0000:00:1d.0-usb- 
     lrwxrwxrwx 1 root root 6 Jul 12 14:02 /dev/disk/by-path/ 
       pci-0000:00:1d.0-usb-0:1.4:1.0-scsi-0:0:0:0 -> ../../sdb
     # ls -l /dev/disk/by-uuid/c59fbbac-9838-4f3c-830d-b47498d1cd77
     lrwxrwxrwx 1 root root 10 Jul 12 14:02 /dev/disk/by-uuid/ 
       c59fbbac-9838-4f3c-830d-b47498d1cd77 -> ../../sda1
     # ls -l /dev/disk/by-label/root
     lrwxrwxrwx 1 root root 10 Jul 12 14:02 /dev/disk/by-label/root 
      -> ../../sda1

     These device names are derived from data such as the (unique) serial number of
     the disk drive, its position on the PCIe bus, or the UUID or name of the file system,
     and are independent of the name of the actual device file.

     C 6.2 [!2] On your ssytem there are two SATA hard disks. The first disk has
       two primary and two logical partitions. The second disk has one primary
       and three logical partitions. Which are the device names for these partitions
       on Linux?

     C 6.3 [!1] Examine the /dev directory on your system. Which storage media are
       available and what are the corresponding device files called? (Also check
       /dev/block and /dev/disk .)

     C 6.4 [1] Plug a USB thumb drive into your computer. Check whether new
       device files have been added to /dev . If so, which ones?

     6.5     Partitioning Disks
     6.5.1    Fundamentals
     Before you partition the (possibly sole) disk on a Linux system, you should briefly
     consider what a suitable partitioning scheme might look like and how big the
     partitions ought to be. Changes after the fact are tedious and inconvenient at best
     and may at worst necessitate a complete re-install of the system (which would be
     exceedingly tedious and inconvenient). (See section 6.7 for an alternative, much
     less painful approach.)
        Here are a few basic suggestions for partitioning:
        • Apart from the partition with the root directory / , you should provide at
          least one spearate partition for the file system containing the /home directory.
          This lets you cleanly separate the operating system from your own data, and
          facilitates distribution upgrades or even switching from one Linux distribu-
          tion to a completely different one.
6.5 Partitioning Disks                                                                   87

      B If you follow this approach, you should probably also use symbolic
        links to move the /usr/local and /opt directories to (for example) /home/
        usr- local and /home/opt . This way, these directories, which also contain
        data provided by you, are on “your” partition and can more easily be
        included in regular backups.

   • It is absolutely possible to fit a basic Linux system into a 2 GiB partition, but,
     considering today’s (low) costs per gigabyte for hard disk storage, there is
     little point in scrimping and saving. With something like 30 GiB, you’re sure
     to be on the safe side and will have enough room for log files, downloaded
     distribution packages during a larger update, and so on.
   • On server systems, it may make sense to provide separate partitions for /tmp ,
     /var , and possibly /srv . The general idea is that arbitrary users can put data
     into these directories (besides outright files, this could include unread or
     unsent e-mail, queued print jobs, and so on). If these directories are on
     separate partitions, users cannot fill up the system in general and thereby
     create problems.
   • You should provide swap space of approximately the same size as the com-
     puter’s RAM, up to a maximum of 8 GiB or thereabouts. Much more is
     pointless, but on workstations and mobile computers you may want to avail
     yourself of the possibility to “suspend” your computer instead of shutting
     it down, in order to speed up a restart and end up exactly where you were
     before—and the infrastructures enabling this like to use the swap space to
     save the RAM content.

      B There used to be a rule of thumb saying that the swap space should be
        about twice or three times the available RAM size. This rule of thumb
        comes from traditional Unix systems, where RAM works as “cache”
        for the swap space. Linux doesn’t work that way, instead RAM and
        swap space are added—on a computer with 4 GiB of RAM and 2 GiB
        of swap space, you get to run processes to the tune of 6 GiB or so. With
        8 GiB of RAM, providing 16 to 24 GiB of swap space would be absurd.

      B You should dimension the RAM of a computer (especially a server) to
        be big enough that practically no swap space is necessary during nor-
        mal operations.; on an 8-GiB server, you won’t usually need 16 GiB of
        swap space, but a gigabyte or two to be on the safe side will certainly
        not hurt (especially considering today’s prices for disk storage). That
        way, if RAM gets tight, the computer will slow down before processes
        crash outright because they cannot get memory from the operating sys-

   • If you have several (physical) hard disks, it can be useful to spread the sys-
     tem across the available disks in order to increase the access speeed to indi-
     vidual components.

      B Traditionally, one would place the root file system (/ with the essential
        subdirectories /bin , /lib , /etc , and so on) on one disk and the /usr direc-
        tory with its subdirectories on a separate file system on another disk.
        However, the trend on Linux is decisively away from the (artificial)
        separation between /bin and /usr/bin or /lib and /usr/lib and towards
        a root file system which is created as a RAM disk on boot. Whether the
        traditional separation of / and /usr will gain us a lot in the future is up
        for debate.

      B What will certainly pay off is to spread swap space across several disks.
        Linux always uses the least-used disk for swapping.
88                                                    6 Hard Disks (and Other Secondary Storage)

                Provided that there is enough empty space on the medium, new partitions
             can be created and included (even while the system is running). This procedure
             consists of the following steps:
                  1. Backup the current boot sectors and data on the hard disk in question
                  2. Partition the disk using fdisk (or a similar program)
                  3. Possibly create file systems on the new partitions (“formatting”)
                  4. Making the new file systems accessible using mount or /etc/fstab

             B Items 3 and 4 on this list will be considered in more detail in chapter 7.
             Data and boot-sector contents can be saved using the dd program (among others).

             # dd if=/dev/sda of=/dev/st0

             will, for example, save all of the sda hard disk to magnetic tape.
                 You should be aware that the partitioning of a storage medium has nothing to
             do with the data stored on it. The partition table simply specifies where on the
             disk the Linux kernel can find the partitions and hence the file structures. Once the
             Linux kernel has located a partition, the content of the partition table is irrelevant
             until it searches again, possibly during the next system boot. This gives you—
             if you are courageous (or foolhardy)—far-reaching opportunities to tweak your
             system while it is running. For example, you can by all means enlarge partitions
             (if after the end of the partition there is unused space or a partition whose contents
             you can do without) or make them smaller (and possibly place another partition
             in the space thus freed up). As long as you exercise appropriate care this will be
             reasonably safe.

             B This should of course in no way discourage you from making appropriate
               backup copies before doing this kind of open-heart surgery.

             B In addition, the file systems on the disks must play along with such shenani-
               gans (many Linux file systems can be enlarged without loss of data, and
               some of them even be made smaller), or you must be prepared to move the
               data out of the way, generate a new file system, and then fetch the data back.

             6.5.2     Partitioning Disks Using fdisk
     fdisk   fdisk is an interactive program for manipulating disk partition tables. It can also
             create “foreign” partition types such as DOS partitions. Drives are addressed us-
             ing the corresponding device files (such as /dev/sda for the first disk).

             B fdisk confines itself to entering a partition into the partition table and setting
               the correct partition type. If you create a DOS or NTFS partition using fdisk ,
               this means just that the partition exists in the partition table, not that you can
               boot DOS or Windows NT now and write files to that partition. Before doing
               that, you must create a file system, i. e., write the appropriate management
               data structures to the disk. Using Linux-based tools you can do this for
               many but not all non-Linux file systems.

                  After invoking “fdisk ⟨device⟩”, the program comes back with a succinct prompt
             # fdisk /dev/sdb                                                 Neue   (leere) Platte
             Welcome to fdisk (util-linux 2.25.2).
             Changes will remain in memory only, until you decide to write them.
             Be careful before using the write command.

             Device does not contain a recognized partition table.
6.5 Partitioning Disks                                                                89

Created a new DOS disklabel with disk identifier 0x68d97339.

Command (m for help): _

The   m   command will display a list of the available commands.

B fdisk lets you partition hard disks according to the MBR or GPT schemes.
  It recognises an existing partition table and adjusts itself accordingly. On
  an empty (unpartitioned) disk fdisk will by default create an MBR partition
  table, but you can change this afterwards (we’ll show you how in a little

   You can create a new partition using the “n ” command:

Command (m for help): n
Partition type
   p   primary (0 primary, 0 extended, 4 free)
   e   extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-2097151, default 2048): ↩
Last sector, +sectors or +sizeK,M,G,T,P (2048-2097151, 
  default 2097151): +500M

Created a new partition 1 of type 'Linux' and of size 500 MiB.

Command (m for help): _

The p command displays the current partition table. This could look like this:

Command (m for help): p
Disk /dev/sdb: 1 GiB, 1073741824 bytes, 2097152 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x68d97339

Device      Boot Start     End Sectors   Size Id Type
/dev/sdb1         2048 1026047 1024000   500M 83 Linux

B You can change the partition type using the t command. You must select the
  desired partition and can then enter the code (as a hexadecimal number). L
  displays the whole list.

You can delete a partition you no longer want by means of the d command. When
you’re done, you can write the partition table to disk and quit the program using
w . With q , you can quit the program without rewriting the partition table.

B After storing the partition table, fdisk tries to get the Linux kernel to reread
  the new partition table; this works well with new or unused disks, but fails
  if a partition on the disk is currently in use (as a mounted file system, active
  swap space, …). This lets you repartition the disk with the / file system only
  with a system reboot. One of the rare occasions when a Linux system needs
  to be rebooted …

  Like all Linux commands, fdisk supports a number of command-line options. Options
The most important of those include:
90                                             6 Hard Disks (and Other Secondary Storage)

     -l   displays the partition table of the selected disk and then terminates the pro-
     -u   (“units”) lets you select the units of measure used when displaying partition
             tables. The default is “sectors”; if you specify “-u=cylinders ”, cylinders will
             be used instead (but there is no good reason for that today).

     B If you use fdisk in MBR mode, it tries to observe the usual conventions and
       arrange the partitions such that they work properly on 4Kn and 512e hard
       disks. You should follow the program’s suggestions wherever possible, and
       not deviate from them unless there are very compelling reasons.

        If you partition a hard disk according to the GPT standard and there is no GPT-
     style partition table on the disk yet, you can generate one using the g command
     (Warning: A possibly existing MBR partition table will be overwritten in the pro-

     Command (m for help): g
     Created a new GPT disklabel (GUID: C2A556FD-7C39-474A-B383-963E09AA7269)

     (The GUID shown here applies to the disk as a whole.) Afterwards you can use the
     n command to create partitions in the usual way, even if the dialog looks slightly
     Command (m for help): n
     Partition number (1-128, default 1): 1
     First sector (2048-2097118, default 2048): ↩
     Last sector, +sectors or +sizeK,M,G,T,P (2048-2097118, default 
       2097118): +32M

     Created a new partition 1 of type 'Linux filesystem' and of size 32 MiB.

     The partition type selection is different, too, because it is about GUIDs rather than
     two-digit hexadecimal numbers:

     Command (m for help): t
     Selected partition 1
     Partition type (type L to list all types): L
       1 EFI System                     C12A7328-F81F-11D2-BA4B-00A0C93EC93B
      14   Linux   swap                    0657FD6D-A4AB-43C4-84E5-0933C84B4F4F
      15   Linux   filesystem              0FC63DAF-8483-4772-8E79-3D69D8477DE4
      16   Linux   server data             3B8F8425-20E0-4F3B-907F-1A25A76F98E8
      17   Linux   root (x86)              44479540-F297-41B2-9AF7-D131D5F0458A
      18   Linux   root (x86-64)           4F68BCE3-E8CD-4DB1-96E7-FBCAF984B709
     Partition type (type L to list all types): _

     C 6.5 [!2] Create an empty 1 GiB file using the
             # dd if=/dev/zero of=$HOME/test.img bs=1M count=1024

             command. Use fdisk to “partition” the file according to the MBR scheme:
             Create two Linux partitions of 256 MiB and 512 MiB, respectively, and create
             a swap space partitions using the balance.

     C 6.6 [!2] Repeat the following exercise, but create a GPT partition table in-
       stead. Assume that the 512-MiB partition will contain a /home directory.
6.5 Partitioning Disks                                                                91

6.5.3    Formatting Disks using GNU parted
Another popular program for partitioning storage media is the GNU project’s
parted . Featurewise, it is roughly comparable with fdisk , but it has a few useful

B Unlike fdisk , parted does not come pre-installed with most distributions, but
  can generally be added after the fact from the distribution’s servers.

   Similar to fdisk , parted must be started with the name of the medium to be
partitioned as a parameter.

# parted /dev/sdb
GNU Parted 3.2
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) _

You can create a new partition using mkpart . This works either interactively …

(parted) mkpart
Partition name? []? Test
File system type? [ext2]? ext4
Start? 211MB
End? 316MB

… or directly when the command is invoked:

(parted) mkpart primary ext4 211MB 316MB

B You can abbreviate the commands down to an unambiguous prefix. Hence,
  mkp will work instead of mkpart (mk would collide with the mklabel command).

B The file system type will only be used to guess a partition type. You will
  still need to manually create a file system on the partition later on.

You can examine the partition table using the print command:

(parted) p
Disk /dev/sdb: 1074MB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number   Start    End     Size    File system   Name      Flags
 1       1049kB   106MB   105MB
 2       106MB    211MB   105MB
 3       211MB    316MB   105MB   ext4          primary

(parted) _

(This also shows you where the magic numbers “211MB ” and “316MB ” came from,
earlier on.)

B print has a few interesting subcommands: “print devices ” lists all available
  block devices, “print free ” displays free (unallocated) space, and “print all ”
  outputs the partition tables of all block devices.

You can get rid of unwanted partitions using rm . Use name to give a name to a
partition (only for GPT). The quit command stops the program.
92                                          6 Hard Disks (and Other Secondary Storage)

     A Important: While fdisk updates the partition table on the disk only once you
       leave the program, parted does it on an ongoing basis. This means that the
       addition or removal of a partition takes effect on the disk immediately.

        If you use parted on a new (unpartitioned) disk, you must first create a partition
     (parted) mklabel gpt

     creates a GPT-style partition table, and

     (parted) mklabel msdos

     one according to the MBR standard. There is no default value; without a partition
     table, parted will refuse to execute the mkpart command.
        If you inadvertently delete a partition that you would rather have kept, parted
     can help you find it again. You will just need to tell it approximately where on the
     disk the partition used to be:

     (parted) rm 3                                                                Oops.
     (parted) rescue 200MB 350MB
     Information: A ext4 primary partition was found at 211MB -> 316MB.
     Do you want to add it to the partition table?

     Yes/No/Cancel? yes

     For this to work, there must be a file system on the partition, because parted looks
     for data blocks that appear to be the start of a file system.
        In addition to the interactive mode, parted allows you to pass commands im-
     mediately on the Linux command line, like this:

     # parted /dev/sdb mkpart primary ext4 316MB 421MB
     Information: You may need to update /etc/fstab.

     C 6.7 [!2] Repeat exercise 6.5 using parted rather than fdisk , for the MBR as well
       as the GPT scheme.

     C 6.8 [2] (If you have worked through chapter 7 already.) Generate Linux file
       systems on the partitions on the “disk” from the earlier exercises. Remove
       these partitions. Can you restore them using parted ’s rescue command?

     6.5.4    gdisk
     The gdisk program specialises in GPT-partitioned disks and can do a few useful
     things the previously explained programs can’t. You may however have to install
     it specially.
         The elementary functions of gdisk correspond to those of fdisk and parted , and
     we will not reiterate those (read the documentation and do a few experiments). A
     few features of gdisk , however, are of independent interest:
        • You can use gdisk to convert an MBR-partitioned medium to a GPT-partitioned
          medium. This presupposes that there is enough space at the start and the
          end of the medium for GPT partition tables. With media where, according
          to current conventions, the first partition starts at sector 2048, the former is
          not a problem, but the latter may be. You will possibly have to ensure that
          the last 33 sectors on the medium are not assigned to a partition.
6.6 Loop Devices and kpartx                                                                       93

        For the conversion it is usually sufficient to start gdisk with the device file
        name of the medium in question as a parameter. You will either receive
        a warning that no GPT partition table was found and disk used the MPT
        partition table instead (at this point you can quit the program using w and
        you’re done), or that an intact MBR, but a damaged GPT partition table was
        found (then you tell gdisk to follow the MBR, and can then quit the program
        using w and you’re done).
   • The other direction is also possible. To do this, you must use the r command
     in gdisk to change to the menu for “recovery/transformation commands”,
     and select the g command there (“convert GPT into MBR and exit”). After-
     wards you can quit the program using w and convert the storage medium
     this way.

C 6.9 [!2] Repeat exercise 6.5 using gdisk rather than fdisk and generate a GPT
  partition table.

C 6.10 [2] Create (e. g., using fdisk ) an MBR-partitioned “disk” and use gdisk
  to convert it to GPT partitioning. Make sure that a correct “protective MBR”
  was created.

6.5.5     More Partitioning Tools
Most distributions come with alternative ways of partitioning disks. Most of them distributions
offer the cfdisk program as an alternative to fdisk . This is screen-oriented and thus
somewhat more convenient to use. Even easier to use are graphical programs,
such as SUSE’s YaST or “DiskDruid” on Red Hat distributions.

B Also worth mentioning is sfdisk , a completely non-interactive partitioning
  program. sfdisk takes partitioning information from an input file and is
  therefore useful for unattended partitioning, e. g., as part of an automated
  installation procedure. Besides, you can use sfdisk to create a backup copy
  of your partitioning information and either print it as a table or else store it
  on a disk or CD as a file. If the worst happens, this copy can then be restored
  using sfdisk .

B sfdisk only works for MBR-partitioned disks. There is a corresponding pro-
  gram called sgdisk which does an equivalent job for GPT-partitioned disks.
  However, sfdisk and sgdisk are not compatible—their option structures are
  completely different.

6.6      Loop Devices and kpartx
Linux has the useful property of being able to treat files like storage media. This
means that if you have a file you can partition it, generate file systems, and gener-
ally treat the “partitions” on that file as if they were partitions on a “real” hard
disk. In real life, this can be useful if you want to access CD-ROMs or DVDs
without having a suitable drive in your computer (it is also faster). For learn-
ing purposes, it means that you can perform various experiments without having
to obtain extra hard disks or mess with your computer.
   A CD-ROM image can be created straightforwardly from an existing CD-ROM CD-ROM image
using dd :

# dd if=/dev/cdrom of=cdrom.iso bs=1M

You can subsequently make the image directly accessible:
94                                           6 Hard Disks (and Other Secondary Storage)

     # mount -o loop,ro cdrom.iso /mnt

     In this example, the content of the CD-ROM will appear within the /mnt directory.
         You can also use the dd command to create an empty file:

     # dd if=/dev/zero of=disk.img bs=1M count=1024

     That file can then be “partitioned” using one of the common partitioning tools:

     # fdisk disk.img

     Before you can do anything with the result, you will have to ensure that there are
     device files available for the partitions (unlike with “real” storage media, this is
     not automatic for simulated storage media in files). To do so, you will first need a
     device file for the file as a whole. This—a so-called “loop device”—can be created
     using the losetup command:

     # losetup -f disk.img
     # losetup -a
     /dev/loop0: [2050]:93 (/tmp/disk.img)

     losetup uses device file names of the form “/dev/loop 𝑛”. The “-f ” option makes the
     program search for the first free name. “losetup -a ” outputs a list of the currently
     active loop devices.
        Once you have assigned your disk image to a loop device, you can create device
     files for its partitions. This is done using the kpartx command.

     B You may have to install kpartx first. On Debian and Ubuntu, the package is
       called kpartx .
     The command to create device files for the partitions on /dev/loop0 is

     # kpartx -av /dev/loop0
     add map loop0p1 (254:0): 0 20480 linear /dev/loop0 2048
     add map loop0p2 (254:1): 0 102400 linear /dev/loop0 22528

     (without the “-v ” command, kpartx keeps quiet). The device files then appear in
     the /dev/mapper directory:

     # ls /dev/mapper
     control loop0p1      loop0p2

     Now nothing prevents you from, e. g., creating file systems on these “partitions”
     and mounting them into your computer’s directory structure. See chapter 7.
        When you no longer need the device files for the partitions, you can remove
     them again using the

     # kpartx -dv /dev/loop0
     del devmap : loop0p2
     del devmap : loop0p1

     command. An unused loop device can be released using

     # losetup -d /dev/loop0

     B The
               # losetup -D

           command releases all loop devices.
6.7 The Logical Volume Manager (LVM)                                                      95

C 6.11 [!2] Use the test “disk” from exercise 6.5. Assign it a loop device using
  losetup and make its partitions accessible with kpartx . Convince yourself that
  the correct device files show up in /dev/mapper . Then release the partitions
  and the loop device again.

6.7     The Logical Volume Manager (LVM)
Partitioning a disk and creating file systems on it seems like a simple and obvious
thing to do. On the other hand, you are committing yourself: The partition scheme
of a hard disk can only be changed with great difficulty and, if the disk in question
contains the root file system, may even involve the use of a “rescue system”. In
addition, there is no compelling reason why you should be constrained in your
system architecture by trivialities such as the limited capacity of hard disks and
the fact that file system can be at most as large as the partitions they are sitting on.
    One method to transcend these limitations is the use of the “Logical Volume
Manager” (LVM). LVM provides an abstraction layer between disks (and disk par-
titions) and file systems—instead of creating file systems directly on partitions,
you can contribute partitions (or whole disks) to a “pool” of disk space and then
allocate space from that pool to create file systems. Single file systems can freely
use space which is located on more than one physical disk.
    In LVM terminology, disks and disk partitions are considered “physical vol-
umes” (PV) which are made part of a “volume group” (VG). There may be more
than one VG on the same computer. The space within a VG is available for cre-
ating “logical volumes” (LV), which can then hold arbitrary file systems or swap

B When creating LVs, you can cause their storage space to be spread deviously
  across several physical disks (“striping”) or multiple copies of their data to
  be stored in several places within the VG at the same time (“mirroring”).
  The former is supposed to decrease retrieval time (even if there is a danger
  of losing data whenever any of the disks in the volume group fail), the latter
  is supposed to reduce the risk of losing data (even if you are paying for this
  with increased access times). In real life, you will probably prefer to rely on
  (hardware or software) RAID instead of using LVM’s features.
   One of the nicer properties of LVM is that LVs can be changed in size while the
system is running. If a file system is running out of space, you can first enlarge
the underlying LV (as long as your VG has unused space available—otherwise you
would first need to install another disk and add it to the VG). Afterwards you can
enlarge the file system on the LV in question.

B This presumes that the file system in question enables size changes after the
  fact. With the popular file systems, e. g., ext3 or ext4 , this is the case. They
  even allow their size to be increased while the file system is mounted. (You
  will need to unmount the file system to reduce the size.)

B If you use a file system that does not let itself be enlarged, you will have
  to bite the bullet, copy the data elsewhere, recreate the file system with the
  new size, and copy the data back.
   If a disk within your VG should start acting up, you can migrate the LVs from
that disk to another within the VG (if you still have or can make enough space).
After that, you can withdraw the flaky disk from the VG, install a new disk, add
that to the VG and migrate the LVs back.

B You can do that, too, while the system is running and with your users none
  the wiser—at least as long as you have invested enough loose change into
  making your hard disks “hot-swappable”.
96                                                        6 Hard Disks (and Other Secondary Storage)

     “snapshots”      Also nice are “snapshots”, which you can use for backup copies without hav-
                   ing to take your system offline for hours (which would otherwise be necessary
                   to ensure that nothing changes while the backup is being performed). You can
                   “freeze” the current state of an LV on another (new) LV—which takes a couple of
                   seconds at most—and then make a copy of that new LV in your own time while
                   normal operations continue on the old LV.

                   B The “snapshot” LV only needs to be big enough to hold the amount of
                     changes to the original LV you expect while the backup is being made (plus
                     a sensible safety margin), since only the changes are being stored inside the
                     new LV. Hence, nobody prevents you from making a snapshot of your 10 TB
                     file system even if you don’t have another 10 TB of free disk space: If you
                     only expect 10 GB of data to be changed while you’re writing the copy to
                     tape, a snapshot LV of 20–30 GB should be fairly safe.

                   B As a matter of fact it is now possible to create writable snapshots. This is
                     useful, for example, if you are working with “virtual machines” that share
                     a basic OS installation but differ in details. Writable snapshots make it pos-
                     sible to make the basic installation in one LV for all virtual machines and
                     then store the configuration specific to each virtual machine in one LV with
                     a writable snapshot each. (You shouldn’t overstretch this approach, though;
                     if you change the LV with the basic installation the virtual machines won’t

                      On Linux, LVM is a special application of the “device mapper”, a system com-
                   ponent enabling the flexible use of block devices. The device mapper also pro-
                   vides other useful features such as encrypted disks or space-saving storage provi-
                   sioning for “virtual servers”. Unfortunately we do not have room in this training
                   manual to do LVM and the device mapper justice, and refer you to the manual,
                   Linux Storage and File Systems (STOR).

                   Commands in this Chapter
                   cfdisk    Character-screen based disk partitioner                    cfdisk (8) 93
                   gdisk     Partitioning tool for GPT disks                             gdisk (8) 92
                   kpartx    Creates block device maps from partition tables            kpartx (8) 94
                   losetup   Creates and maintains loop devices                        losetup (8) 94
                   sfdisk    Non-interactive hard disk partitioner                      sfdisk (8) 93
                   sgdisk    Non-interactive hard disk partitioning tool for GPT disks sgdisk (8) 93
6.7 Bibliography                                                                            97

   • Linux supports all notable types of mass storage device—magnetic hard
     disks (SATA, P-ATA, SCSI, SAS, Fibre Channel, USB, …), SSDs, USB thumb
     drives, SD cards, …
   • Storage media such as hard disks may be partitioned. Partitions allow the
     independent management of parts of a hard disk, e. g., with different file
     systems or operating systems.
   • Linux can deal with storage media partitioned according to the MBR and
     GPT schemes.
   • Linux manages most storage media like SCSI devices. There is an older
     infrastructure for P-ATA disks which is only rarely used.
   • Linux offers various tools for partitioning such as fdisk , parted , gdisk , cfdisk ,
     or sfdisk . Various distributions also provide their own tools.
   • Loop devices make block-oriented devices from files. Partitions on loop de-
     vices can be made accessible using kpartx .
   • The Logical Volume Manager (LVM) decouples physical storage space on
     media from logical storage structures. It enables the flexible management
     of mass storage, e. g., to create file systems which are larger than a single
     physical storage medium. Snapshots help create backup copies and provi-
     sion storage space for virtual machines.

SCSI-2.4-HOWTO03 Douglas Gilbert. “The Linux 2.4 SCSI subsystem HOWTO”,
      May 2003.            2.4- HOWTO/
                                                                                                            $ echo tux
                                                                                                            $ ls
                                                                                                            $ /bin/su -

File Systems: Care and Feeding

7.1  Creating a Linux File System . .         .   .   .   .   .   .   .   .   .   .   .   .   .   .   100
   7.1.1 Overview . . . . . .                 .   .   .   .   .   .   .   .   .   .   .   .   .   .   100
   7.1.2 The ext File Systems . . .           .   .   .   .   .   .   .   .   .   .   .   .   .   .   102
   7.1.3 ReiserFS . . . . . . .               .   .   .   .   .   .   .   .   .   .   .   .   .   .   110
   7.1.4 XFS . . . . . . . . .                .   .   .   .   .   .   .   .   .   .   .   .   .   .   111
   7.1.5 Btrfs . . . . . . . .                .   .   .   .   .   .   .   .   .   .   .   .   .   .   113
   7.1.6 Even More File Systems .             .   .   .   .   .   .   .   .   .   .   .   .   .   .   114
   7.1.7 Swap space . . . . . .               .   .   .   .   .   .   .   .   .   .   .   .   .   .   115
7.2 Mounting File Systems . . . .             .   .   .   .   .   .   .   .   .   .   .   .   .   .   116
   7.2.1 Basics . . . . . . . .               .   .   .   .   .   .   .   .   .   .   .   .   .   .   116
   7.2.2 The mount Command . . .              .   .   .   .   .   .   .   .   .   .   .   .   .   .   116
   7.2.3 Labels and UUIDs . . .               .   .   .   .   .   .   .   .   .   .   .   .   .   .   118
7.3 The dd Command . . . . . .                .   .   .   .   .   .   .   .   .   .   .   .   .   .   120
7.4 Disk Quotas . . . . . . . .               .   .   .   .   .   .   .   .   .   .   .   .   .   .   121
   7.4.1 Basics . . . . . . . .               .   .   .   .   .   .   .   .   .   .   .   .   .   .   121
   7.4.2 User Quotas (ext and XFS)            .   .   .   .   .   .   .   .   .   .   .   .   .   .   121
   7.4.3 Group Quotas (ext and XFS)           .   .   .   .   .   .   .   .   .   .   .   .   .   .   123

      •   Knowing the most important file systems for Linux and their properties
      •   Being able to generate file systems on partitions and storage media
      •   Knowing about file system maintenance tools
      •   Being able to manage swap space
      •   Being able to mount local file systems
      •   Knowing how to set up disk quotas for users and groups

      • Competent use of the commands to handle files and directories
      • Knowledge of mass storage on Linux and partitioning (Chapter 6)
      • Existing knowledge about the structure of PC disk storage and file systems
        is helpful

adm1-dateisysteme.tex   (33e55eeadba676a3 )
100                                                                   7 File Systems: Care and Feeding

                   7.1     Creating a Linux File System
                   7.1.1    Overview
                   After having created a new partition, you must “format” that partition, i. e., write
                   the data structures necessary to manage files and directories onto it. What these
                   data structures look like in detail depends on the “file system” in question.

                   B Unfortunately, the term “file system” is overloaded on Linux. It means, for
                           1. A method to arrange data and management information on a medium
                              (“the ext3 file system”)
                           2. Part of the file hierarchy of a Linux system which sits on a particular
                              medium or partition (“the root file system”, “the /var file system”)
                           3. All of the file hierarchy, across media boundaries

                   The file systems (meaning 1 above) common on Linux may differ considerably.
                   On the one hand, there are file systems that were developed specifically for Linux,
                   such as the “ext filesystems” or the Reiser file system, and on the other hand there
                   are file systems that originally belonged to other operating systems but that Linux
                   supports (to a greater or lesser degree) for compatibility. This includes the file
                   systems of DOS, Windows, OS X, and various Unix variants as well as “network
                   file systems” such as NFS or SMB which allow access to file servers via the local
                       Many file systems “native” to Linux are part of the tradition of file systems com-
                   mon on Unix, like the Berkeley Fast Filesystem (FFS), and their data structures are
                   based on those. However, development did not stop there; more modern influ-
                   ences are constantly being integrated in order to keep Linux current with the state
                   of the art.

                   B Btrfs (pronounced “butter eff ess”) by Chris Mason (Fusion-IO) is widely
                     considered the answer to the famous ZFS of Solaris. (The source code for
                     ZFS is available but cannot be integrated in the Linux directly, due to li-
                     censing considerations.) Its focus is on “fault tolerance, repairs and simple
                     administration”. By now it seems to be mostly usable, at least some distri-
                     butions rely on it.

      superblock      With Linux file systems it is common to have a superblock at the beginning
                   of the file system. This contains information pertaining to the file system as a
                   whole—such as when it was last mounted or unmounted, whether it was un-
                   mounted “cleanly” or because of a system crash, and so on. The superblock nor-
                   mally also points to other parts of the management data structures, like where the
                   inodes or free/occupied block lists are to be found and which parts of the medium
                   are available for data.

                   B It is usual to keep spare copies of the superblock elsewhere on the file sys-
                     tem, in case something happens to the original. This is what the ext file
                     systems do.

                   B On disk, there is usually a “boot sector” in front of the superblock, into
                     which you can theoretically put a boot loader (chapter 8). This makes it
                     possible to, e. g., install Linux on a computer alongside Windows and use
                     the Windows boot manager to start the system.

            mkfs      On Linux, file systems (meaning 2 above) are created using the mkfs command.
                   mkfs is independent of the actual file system (meaning 1) desired; it invokes the real
                   routine specific to the file system (meaning 1) in question, mkfs. ⟨file system name⟩.
                   You can select a file system type by means of the -t option—with “mkfs -t ext2 ”,
                   for example, the mkfs.ext2 program would be started (another name for mke2fs ).
7.1 Creating a Linux File System                                                                         101

    When the computer has been switched off inadvertently or crashed, you have
to consider that the file system might be in an inconsistent state (even though this inconsistent state
happens very rarely in real life, even on crashes). File system errors can occur
because write operations are cached inside the computer’s RAM and may be lost
if the system is switched off before they could be written to disk. Other errors can
come up when the system gives up the ghost in the middle of an unbuffered write
    Besides data loss, problems can include errors within the file system manage- structural errors
ment structure. These can be located and repaired using suitable programs and

   • Erroneous directory entries
   • Erroneous inode entries
   • Files that do not occur in any directory
   • Data blocks belonging to several different files

Most but not all such problems can be repaired automatically without loss of data;
generally, the file system can be brought back to a consistent state.

B On boot, the system will find out whether it has not been shut down cor-
  rectly by checking a file system’s state. During a regular shutdown, the file
  systems are unmounted and the “valid flag” in every file system’s super valid flag
  block will be set. On boot, this super block information may be used to au-
  tomatically check these possibly-erroneous file systems and repair them if
  necessary—before the system tries to mount a file system whose valid flag
  is not set, it tries to do a file system check.

B With all current Linux distributions, the system initialisation scripts exe-
  cuted by init after booting contain all necessary commands to perform a
  file system check.

   If you want to check the consistency of a file system you do not need to wait
for the next reboot. You can launch a file system check at any time. Should a file file system check
contain errors, however, it can only be repaired if it is not currently mounted. This
restriction is necessary so that the kernel and the repair program do not “collide”.
This is another argument in favour of the automatic file system checks during
   Actual consistency checks are performed using the fsck command. Like mkfs ,
depending on the type of the file system to be checked this command uses a spe-
cific sub-command called fsck. ⟨type⟩—e.g., fsck.ext2 for ext2 . fsck identifies the
required sub-command by examining the file system in question. Using the

# fsck /dev/sdb1

command, for example, you can check the file system on /dev/sdb1 .

B The simple command
       # fsck

      checks all file systems listed in /etc/fstab with a non-zero value in the sixth
      (last) column in sequence. (If several different values exist, the file systems
      are checked in ascending order.) /etc/fstab is explained in more detail in
      section 7.2.2.

B fsck supports a -t option which at first sight resembles mkfs but has a differ-
  ent meaning: A command like
102                                                                   7 File Systems: Care and Feeding

                        # fsck -t ext3

                        checks all file systems in /etc/fstab that are marked as type ext3 there.

      options        The most important options of fsck include:
                -A   (All) causes fsck to check all file systems mentioned in /etc/fstab .

                       B This obeys the checking order in the sixth column of the file. If several
                         file systems share the same value in that column, they are checked in
                         parallel if they are located on different physical disks.

                -R   With -A , the root file system is not checked (which is useful if it is already
                       mounted for writing).
                -V   Outputs verbose messages about the check run.
                -N   Displays what fsck would do without actually doing it.
                -s   Inhibits parallel checking of multiple file systems. The “fsck ” command with-
                        out any parameters is equivalent to “fsck -A -s ”.
                   Besides its own options, you can pass additional options to fsck which it will
                forward to the specific checking program. These must occur after the name of the
                file system(s) to be checked and possibly a “-- ” separator. The -a , -f , -p and -v
                options are supported by most such programs. Details may be found within the
                documentation for the respective programs. The

                # fsck /dev/sdb1 /dev/sdb2 -pv

                for example would check the file systems on the /dev/sdb1 and /dev/sdb2 partitions
                automatically, fix any errors without manual intervention and report verbosely on
                its progress.

                B At program termination, fsck passes information about the file system state
                  to the shell:

                        0    No error was found in the file system
                        1    Errors were found and corrected
                        2    Severe errors were found and corrected. The system should be rebooted
                        4    Errors were found but not corrected
                        8    An error occurred while the program was executed
                        16    Usage error (e. g., bad command line)
                        128    Error in a shared library function
                        It is conceivable to analyse these return values in an init script and deter-
                        mine how to continue with the system boot. If several file systems are being
                        checked (using the -A option), the return value of fsck is the logical OR of
                        the return values of the individual checking programs.

                7.1.2         The ext File Systems
                History and Properties The original “extended file system” for Linux was imple-
                mented in April, 1992, by Rémy Card. It was the first file system designed specifi-
                cally for Linux (although it did take a lot of inspiration from general Unix file
                systems) and did away with various limitations of the previously popular Minix
                file system.
7.1 Creating a Linux File System                                                         103

B The Minix file system had various nasty limits such as a maximum file sys-
  tem size of 64 MiB and file names of at most 14 characters. (To be fair, Minix
  was introduced when the IBM PC XT was considered a hot computer and
  64 MiB, for PCs, amounted to an unimaginably vast amount of disk storage.
  By 1990, that assumption had begun to crumble.) ext allowed file systems
  of up to 2 GiB—quite useful at the time, but naturally somewhat ridiculous

B The arrival of the ext file system marks another important improvement
  to the Linux kernel, namely the introduction of the “virtual file system
  switch”, or VFS. The VFS abstracts file system operations such as the open-
  ing and closing of files or the reading and writing of data, and as such
  enables the coexistence of different file system implementations in Linux.

A The original ext file system is no longer used today. From here on, when we
  talk about “the ext file systems”, we refer to ext2 and everything newer than
   The subsequent version, ext2 (the “second extended file system”), which was
begun by Rémy Card in January, 1993, amounted to a considerable rework of the
original “extended file system”. The development of ext2 made use of many ideas
from the BSD “Berkeley Fast Filesystem”. ext2 is still being maintained and makes
eminent sense for certain applications.

B Compared to ext , ext2 pushes various size limits—with the 4 KiB block size
  typical for Intel-based Linux systems, file systems can be 16 TiB and single
  files 2 TiB in size. Another important improvement in ext2 was the intro-
  duction of separate timestamps for the last access, last content modification
  and last inode modification, which achieved compatibility to “traditional”
  Unix in this respect.

B From the beginning, ext2 was geared towards continued development and
  improvement: Most data structures contained surplus space which was
  later used for important extensions. These include ACLs and “extended
   Since the end of the 1990s, Stephen Tweedie worked on a successor to ext2 ,
which was made part of the Linux kernel at the end of 2001 under the name of
ext3 . (That was Linux 2.4.15.) The most important differences between ext2 and
ext3 include:

   • ext3 supports Journaling.
   • ext3 allows enlarging file systems while they are mounted.
   • ext3 supports more efficient internal data structures for directories with
     many entries.
Even so it is largely compatible with ext2 . It is usually possible to access ext3 file
systems as ext2 file systems (which implies that the new features cannot be used)
and vice-versa.

B “Journaling” solves a problem that can be very tedious with the increasing
  size of file systems, namely that an unforeseen system crash makes it neces-
  sary to do a complete consistency check of the file system. The Linux kernel
  does not perform write operations immediately, but buffers the data in RAM
  and writes them to disk when that is convenient (e. g., when the read/write
  head of the disk drive is in the appropriate place). In addition, many write
  operations involve writing data to various places on the disk, e. g., one or
  more data blocks, the inode table, and the list of available blocks on the
  disk. If the power fails in the right (or wrong) moment, such an operation
  can remain only half-done—the file system is “inconsistent” in the sense
  that a data block can be assigned to a file in the inode, but not marked used
  in the free-block list. This can lead to serious problems later on.
104                                                      7 File Systems: Care and Feeding

      B A journaling file system like ext3 considers every write access to the disk
        as a “transaction” which must be performed completely or not at all. By
        definition, the file system is consistent before and after a transaction is per-
        formed. Every transaction is first written into a special area of the file sys-
        tem called the journal. If it has been entirely written, it is marked “complete”
        and, as such, it is official. The Linux kernel can do the actual write opera-
        tions later.—If the system crashes, a journaling file system does not need to
        undergo a complete file system check, which with today’s file system sizes
        could take hours or even days. Instead, the journal is considered and any
        transactions marked “complete” are transferred to the actual file system.
        Transactions not marked “complete” are thrown out.

      A Most journaling file systems use the journal to log changes to the file sys-
        tem’s “metadata”, i. e., directories, inodes, etc. For efficiency, the actual file
        data are normally not written to the journal. This means that after a crash
        and reboot you will have a consistent file system without having to spend
        hours or days on a complete consistency check. However, your file contents
        may have been scrambled—for example, a file might contain obsolete data
        blocks because the updated ones couldn’t be written before the crash. This
        problem can be mitigated by writing the data blocks to disk first and then
        the metadata to the journal, but even that is not without risk. ext3 gives
        you the choice between three operating modes—writing everything to the
        journal (mount option data=journal ), writing data blocks directly and then
        metadata to the journal (data=ordered ), or no restrictions (data=writeback ). The
        default is data=ordered .

      B Writing metadata or even file data twice—once to the journal, and then later
        to the actual file system—involves a certain loss of performance compared
        to file systems like ext2 , which ignore the problem. One approach to fix
        this consists of log-structured file systems, in which the journal makes up the
        actual file system. Within the Linux community, this approach has so far not
        prevailed. Another approach is exemplified by “copy-on-write filesystems”
        like Btrfs.

      A Using a journaling file system like ext3 does not absolve you from having to
        perform complete consistency checks every so often. Errors in a file system’s
        data structures might arise through disk hardware errors, cabling problems,
        or the dreaded cosmic rays (don’t laugh) and might otherwise remain un-
        noticed until they wreak havoc. For this reason, the ext file systems force a
        file system check every so often when the system is booted (usually when
        you can least afford it). You will see how to tweak this later in this chapter.

      A With server systems that are rarely rebooted and that you cannot simply
        take offline for a few hours or days for a prophylactic file system check, you
        may have a big problem. We shall also come back to this.
         The apex of ext file system evolution is currently represented by ext4 , which has
      been developed since 2006 under the guidance of Theodore Ts’o. This has been
      considered stable since 2008 (Kernel version 2.6.28). Like ext3 and ext2 , backward
      compatibility was an important goal: ext2 and ext3 file systems can be mounted
      as ext4 file systems and will profit from some internal improvements in ext4 . On
      the other hand, the ext4 code introduces some changes that result in file systems
      no longer being accessible as ext2 and ext3 . Here are the most important improve-
      ments in ext4 as compared to ext3 :
         • Instead of maintaining the data blocks of individual files as lists of block
           numbers, ext4 uses “extents”, i. e., groups of physically contiguous blocks
           on disk. This leads to a considerable simplification of space management
           and to greater efficiency, but makes file systems using extents incompatible
           to ext3 . It also avoids fragmentation, or the wild scattering of blocks belong-
           ing to the same file across the whole file system.
7.1 Creating a Linux File System                                                                     105

     • When data is written, actual blocks on the disk are assigned as late as pos-
       sible. This also helps prevent fragmentation.
     • User programs can advise the operating system how large a file is going
       to be. Again, this can be used to assign contiguous file space and mitigate
     • Ext4 uses checksums to safeguard the journal. This increases reliability and
       avoids some hairy problems when the journal is replayed after a system
     • Various optimisations of internal data structures increase the speed of con-
       sistency checks.
     • Timestamps now carry nanosecond resolution and roll over in 2242 (rather
       than 2038).
     • Some size limits have been increased—directories may now contain 64,000
       or more subdirectories (previously 32,000), files can be as large as 16 TiB,
       and file systems as large as 1 EiB.
In spite of these useful improvements, according to Ted Ts’o ext4 is not to be con-
sidered an innovation, but rather a stopgap until even better file systems like Btrfs
become available.
   All ext file systems include powerful tools for consistency checks and file sys-
tem repairs. This is very important for practical use.

Creating ext file systems To create a ext2 or ext3 file system, it is easiest to use the
mkfs command with a suitable -t option:

# mkfs -t ext2 /dev/sdb1                                                ext2   file system
# mkfs -t ext3 /dev/sdb1                                                ext3   file system
# mkfs -t ext4 /dev/sdb1                                                ext4   file system

After the -t option and its parameter, you can specify further parameters which
will be passed to the program performing the actual operation—in the case of the
ext file systems, the mke2fs program. (In spite of the e2 in its name, it can also create
ext3 and ext4 file systems.)

B The following commands would also work:
         # mkfs.ext2 /dev/sdb1                                          ext2   file system
         # mkfs.ext3 /dev/sdb1                                          ext3   file system
         # mkfs.ext4 /dev/sdb1                                          ext4   file system

        These are exactly the commands that mkfs would invoke. All three com-
        mands are really symbolic links referring to mke2fs ; mke2fs looks at the name
        used to call it and behaves accordingly.

B You can even call the mke2fs command directly:                                            mke2fs

         # mke2fs /dev/sdb1

        (Passing no options will get you a ext2 file system.)

   The following options for mke2fs are useful (and possibly important for the
-b   ⟨size⟩ determines the block size. Typical values are 1024, 2048, or 4096. On
         partitions of interesting size, the default is 4096.
106                                                                               7 File Systems: Care and Feeding

                           -c   checks the partition for damaged blocks and marks them as unusable.

                                   B Current hard disks can notice “bad blocks” and replace them by blocks
                                     from a “secret reserve” without the operating system even noticing (at
                                     least as long as you don’t ask the disk directly). While this is going on,
                                     “mke2fs -c ”) does not provide an advantage. The command will only
                                     find bad blocks when the secret reserve is exhausted, and at that point
                                     you would do well to replace the disk, anyway. (A completely new
                                     hard disk would at this point be a warranty case. Old chestnuts are
                                     only fit for the garbage.)

                           -i   ⟨count⟩ determines the “inode density”; an inode is created for every ⟨count⟩
                                    bytes of space on the disk. The value must be a multiple of the block size
                                    (option b ); there is no point in selecting a ⟨count⟩ that is less than the block
                                    size. The minimum value is 1024, the default is the current block size.
                           -m   ⟨percentage⟩ sets the percentage of data blocks reserved for root (default: 5%)
                           -S   causes mke2fs to rewrite just the super blocks and group descriptors and leave
                                   the inodes intact
                           -j   creates a journal and, hence, an ext3 or ext4 file system.

                                   B It is best to create an ext4 file system using one of the precooked calls
                                     like “mkfs -t ext4 ”, since mke2fs then knows what it is suppsed to do. If
                                     you must absolutely do it manually, use something like
                                           # mke2fs -j -O extents,uninit_bg,dir_index /dev/sdb1

                             The ext file systems (still) need at least one complete data block for every file, no
                             matter how small. Thus, if you create an ext file system on which you intend
                             to store many small files (cue: mail or Usenet server), you may want to select a
      internal fragmentation smaller block size in order to avoid internal fragmentation. (On the other hand,
                             disk space is really quite cheap today.)

                            B The inode density (-i option) determines how many files you can create on
                              the file system—since every file requires an inode, there can be no more
                              files than there are inodes. The default, creating an inode for every single
                              data block on the disk, is very conservative, but from the point of view of
                              the developers, the danger of not being able to create new files for lack of
                              inodes seems to be more of a problem than wasted space due to unused

                            B Various file system objects require inodes but no data blocks—such as de-
                              vice files, FIFOs or short symbolic links. Even if you create as many inodes
                              as data blocks, you can still run out of inodes before running out of data

                            B Using the mke2fs -F option, you can “format” file system objects that are not
                              block device files. For example, you can create CD-ROMs containing an ext2
                              file system by executing the command sequence

                                    #   dd if=/dev/zero of=cdrom.img bs=1M count=650
                                    #   mke2fs -F cdrom.img
                                    #   mount -o loop cdrom.img /mnt
                                    #                                                        … copy stuff to /mnt …
                                    #   umount /mnt
                                    #   cdrecord -data cdrom.img

                                   (/dev/zero is a “device” that produces arbitrarily many zero bytes.) The re-
                                   sulting CD-ROMs contain “genuine” ext2 file systems with all permissions,
                                   attributes, ACLs etc., and can be mounted using
7.1 Creating a Linux File System                                                                     107

         # mount -t ext2 -o ro /dev/scd0 /media/cdrom

        (or some such command); you should replace /dev/scd0 by the device name
        of your optical drive. (It is best to avoid using an ext3 file system here, since
        the journal would be an utter waste of space. An ext4 file system, though,
        can be created without a journal.)

Repairing ext file systems e2fsck is the consistency checker for ext file systems.          e2fsck
There are usually symbolic links such as fsck.ext2 so it can be invoked from fsck .

B Like mke2fs , e2fsck also works for ext3 and ext4 file systems.

B You can of course invoke the program directly, which might save you a little
  typing when passing options. On the other hand, you can only specify the
  name of one single partition (strictly speaking, one single block device).

The most important options for e2fsck include:                                             options

-b   ⟨number⟩ reads the super block from block ⟨number⟩ of the partition (rather
        than the first super block)

-B   ⟨size⟩ gives the size of a block group between two copies of the super block;
         with the ext file systems, backup copies of the super block are usually placed
         every 8192 blocks, on larger disks every 32768 blocks. (You can query this
         using the tune2fs command explained below; look for “blocks per group” in
         the output of “tune2fs -l ”.)

-f   forces a file system to be checked even if its super block claims that it is clean
-l   ⟨file⟩ reads the list of bad blocks from the ⟨file⟩ and marks these blocks as “used”
-c   (“check”) searches the file system for bad blocks
-p   (“preen”) causes errors to be repaired automatically with no further user in-
-v   (“verbose”) outputs information about the program’s execution status and the
        file system while the program is running
   The device file specifies the partition whose file system is to be checked. If that
partition does not contain an ext file system, the command aborts. e2fsck performs
the following steps:                                                                steps

     1. The command line arguments are checked
     2. The program checks whether the file system in question is mounted

     3. The file system is opened
     4. The super block is checked for readability
     5. The data blocks are checked for errors
     6. The super block information on inodes, blocks and sizes are compared with
        the current system state
     7. Directory entries are checked against inodes
     8. Every data block that is marked “used” is checked for existence and whether
        it is referred to exactly once by some inode

     9. The number of links within directories is checked with the inode link coun-
        ters (must match)
108                                                                            7 File Systems: Care and Feeding

                             10. The total number of blocks must equal the number of free blocks plus the
                                 number of used blocks

               exit code   B e2fsck returns an exit code with the same meaning as the standard fsck exit

                               It is impossible to list all the file system errors that e2fsck can handle. Here are
                           a few important examples:
                              • Files whose inodes are not referenced from any directory are placed in the
                                file system’s lost+found directory using the inode number as the file name
                                and can be moved elsewhere from there. This type of error can occur, e. g.,
                                if the system crashes after a file has been created but before the directory
                                entry could be written.
                              • An inode’s link counter is greater than the number of links pointing to this
                                inode from directories. e2fsck corrects the inode’s link counter.

                              • e2fsck finds free blocks that are marked used (this can occur, e. g., when the
                                system crashes after a file has been deleted but before the block count and
                                bitmaps could be updated).
                              • The total number of blocks is incorrect (free and used blocks together are
                                different from the total number of blocks).

      complicated errors      Not all errors are straightforward to repair. What to do if the super block is
                           unreadable? Then the file system can no longer be mounted, and e2fsck often fails
                           as well. You can then use a copy of the super block, one of which is included with
                           every block group on the partition. In this case you should boot a rescue system
                           and invoke fsck from there. With the -b option, e2fsck can be forced to consider a
                           particular block as the super block. The command in question then becomes, for

                           # e2fsck -f -b 8193 /dev/sda2

                           B If the file system cannot be automatically repaired using fsck , it is pos-
                             sible to modify the file system directly. However, this requires very de-
                             tailed knowledge of file system structures which is beyond the scope of
                             this course.—There are two useful tools to aid with this. First, the dumpe2fs
                             program makes visible the internal management data structures of a ext
                             file system. The interpretation of its output requires the aforementioned
                             detailed knowledge. An ext file system may be repaired using the debugfs
                             file system debugger.

                           A You should keep your hands off programs like debugfs unless you know ex-
                             actly what you are doing. While debugfs enables you to manipulate the file
                             system’s data structures on a very low level, it is easy to damage a file sys-
                             tem even more by using it injudiciously. Now that we have appropriately
                             warned you, we may tell you that

                                  # debugfs /dev/sda1

                                 will open the ext file system on /dev/sda1 for inspection (debugfs , reasonably,
                                 enables writing to the file system only if it was called with the -w option).
                                 debugfs displays a prompt; “help ” gets you a list of available commands.
                                 These are also listed in the documentation, which is in debugfs (8).
7.1 Creating a Linux File System                                                                             109

Querying and Changing ext File System Parameters If you have created a parti-
tion and put an ext file system on it, you can change some formatting parameters changing format parameters
after the fact. This is done using the tune2fs command, which should be used with
utmost caution and should never be applied on a file system mounted for writing:

tune2fs    [⟨options⟩] ⟨device⟩

The following options are important:
-c   ⟨count⟩ sets the maximum number of times the file system may be mounted
         between two routine file system checks. The default value set by mke2fs is a
         random number somewhere around 30 (so that not all file systems are pre-
         emptively checked at the same time). The value 0 means “infinitely many”.
-C   ⟨count⟩ sets the current “mount count”. You can use this to cheat fsck or (by
         setting it to a larger value than the current maximum set up using -c ) force
         a file system check during the next system boot.

-e   ⟨behaviour⟩ determines the behaviour of the system in case of errors. The fol-
         lowing possibilities exist:
        continue   Go on as normal
        remount-ro   Disallow further writing to the file system
        panic   Force a kernel panic

        In every case, a file system consistency check will be performed during the
        next reboot.
-i   ⟨interval⟩⟨unit⟩ sets the maximum time between two routine file system checks.
         ⟨interval⟩ is an integer; the ⟨unit⟩ is d for days, w for weeks and m for months.
         The value 0 means “infinitely long”.
-l   displays super block information.
-m   ⟨percent⟩ sets the percentage of data blocks reserved for root (or the user speci-
         fied using the -u option). The default value is 5%.

-L   ⟨name⟩ sets a partition name (up to 16 characters). Commands like mount and
        fsck make it possible to refer to partitions by their names rather than the
        names of their device files.
   To upgrade an existing ext3 file system to an ext4 file system, you need to exe-
cute the commands

# tune2fs -O extents,uninit_bg,dir_index /dev/sdb1
# e2fsck -fDp /dev/sdb1

(stipulating that the file system in question is on /dev/sdb1 ). Make sure to change
/etc/fstab such that the file system is mounted as ext4 later on (see section 7.2).

B Do note, though, that all existing files will still be using ext3 structures—
  improvements like extents will only be used for files created later. The
  e4defrag defragmentation tool is supposed to convert older files but is not
  quite ready yet.

B If you have the wherewithal, you should not upgrade a file system “in place”
  but instead backup its content, recreate the file system as ext4 , and the re-
  store the content. The performance of ext4 is considerably better on “native”
  ext4 file systems than on converted ext3 file systems—this can amount to a
  factor of 2.
110                                                        7 File Systems: Care and Feeding

      B If you have ext2 file systems lying around that you would like to convert into
        ext3 file systems: This is easily done by creating a journal. tune2fs will do
        that for you, too:

              # tune2fs -j /dev/sdb1

              Again, you will have to adjust /etc/fstab if necessary.

      C 7.1 [!2] Generate an ext4 file system on a suitable medium (hard disk parti-
        tion, USB thumb drive, file created using dd ).

      C 7.2 [2] Change the maximum mount count of the filesystem created in ex-
        ercise 7.1 to 30. In addition, 30% of the space available on the file system
        should be reserved for user test .

      7.1.3     ReiserFS
      Overview ReiserFS is a Linux file system meant for general use. It was developed
      by a team under the direction of Hans Reiser and debuted in Linux 2.4.1 (that was
      in 2001). This made it the first journaling file system available for Linux. ReiserFS
      also contained some other innovations that the most popular Linux file system at
      the time, ext2 , did not offer:
         • Using a special tool, ReiserFS file systems could be changed in size. Enlarge-
           ment was even possible while the file system was mounted.
         • Small files and the ends of larger files could be packed together to avoid
           “internal fragmentation” which arises in file systems like ext2 because space
           on disk is allocated based on the block size (usually 4 KiB). With ext2 and
           friends, even a 1-byte file requires a full 4-KiB block, which could be consid-
           ered wasteful (a 4097-byte file requires two data blocks, and that is almost
           as bad). With ReiserFS, several such files could share one data block.

           B There is nothing in principle that would keep the ext developers to add
             this “tail packing” feature to the ext file systems. This was discussed
             and the consensus was that by now, disk space is cheap enough that
             the added complexity would be worth the trouble.

         • Inodes aren’t pregenerated when the file system is created, but are allocated
           on demand. This avoids a pathological problem possible with the ext file
           systems, where there are blocks available in the file system but all inodes
           are occupied and no new files can be generated.

           B The ext file systems mitigate this problem by allocating one inode per
             data block per default (the inode density corresponds to the block size).
             This makes it difficult to provoke the problem.

         • ReiserFS uses trees instead of lists (like ext2 ) for its internal management
           data structures. This makes it more efficient for directories with many files.

           B Ext3 and in particular ext4 can by now do that too.
              As a matter of fact, ReiserFS uses the same tree structure not just for di-
              rectory entries, but also for inodes, file metadata and file block lists, which
              leads to a performance increase in places but to a decrease in others.

              For a long time, ReiserFS used to be the default file system for the SUSE
              distributions (and SUSE contributed to the project’s funding). Since 2006,
              Novell/SUSE has moved from ReiserFS to ext3 ; very new SLES versions use
              Btrfs for their root file system.
7.1 Creating a Linux File System                                                                        111

A In real life you should give the Reiser file system (and its designated succes-
  sor, Reiser4) a wide berth unless you need to manage older systems using
  it. This is less to do with the fact that Hans Reiser was convicted of his
  wife’s murder (which of course does not speak in his favour as a human
  being, but things like these do happen not just among Linux kernel devel-
  opers), but more with the fact that the Reiser file system does have its good
  points but is built on a fairly brittle base. For example, certain directory
  operations in ReiserFS break basic assumptions that are otherwise univer-
  sally valid for Unix-like file systems. This means, for instance, that mail
  servers storing mailboxes on a ReiserFS file system are less resilient against
  system crashes than ones using different file systems. Another grave prob-
  lem, which we will talk about briefly later on, is the existence of technical
  flaws in the file system repair program. Finally—and that may be the most
  serious problem—nobody seems to maintain the code any longer.

Creating ReiserFS file systems mkreiserfs serves to create a ReiserFS file system.     mkreiserfs
The possible specification of a logical block size is currently ignored, the size is
always 4 KiB. With dumpreiserfs you can determine information about ReiserFS          dumpreiserfs
file systems on your disk. resize_reiserfs makes it possible to change the size of     resize_reiserfs
currently-unused ReiserFS partitions. Mounted partitions may be resized using a
command like “mount -o remount,resize= ⟨block count⟩ ⟨mount point⟩”.

Consistency Checks for ReiserFS For the Reiser file system, too, there is a check- Reiser file system
ing and repair program, namely reiserfsck .
   reiserfsck performs a consistency check and tries to repair any errors found,
much like e2fsck . This program is only necessary if the file system is really dam-
aged. Should a Reiser file system merely have been unmounted uncleanly, the
kernel will automatically try to restore it according to the journal.

A reiserfsck has some serious issues. One is that when the tree structure needs
  to be reconstructed (which may happen in certain situations) it gets com-
  pletely mixed up if data files (!) contain blocks that might be misconstrued
  as another ReiserFS file system’s superblock. This will occur if you have
  an image of a ReiserFS file system in a file used as a ReiserFS-formatted
  “virtual” hard disk for a virtualisation environment such as VirtualBox or
  VMware. This effectively disqualifies the ReiserFS file system for serious
  work. You have been warned.

C 7.3 [!1] What is the command to create a Reiser file system on the first logical
  partition of the second disk?

7.1.4    XFS
The XFS file system was donated to Linux by SGI (the erstwhile Silicon Graphics, XFS
Inc.); it is the file system used by SGI’s Unix variant, IRIX, which is able to handle
very large files efficiently. All Linux distributions of consequence offer XFS sup-
port, even though few deploy it by default; you may have to install the XFS tools

B In some circles, “XFS” is the abbreviation of “X11 Font Server”. This can
  occur in distribution package names. Don’t let yourself be confused.

   You can create an XFS file system on an empty partition (or file) using the

# mkfs -t xfs /dev/sda2
112                                                      7 File Systems: Care and Feeding

      command (insert the appropriate device name). Of course, the real work is done
      by a program called mkfs.xfs . You can control it using various options; consult the
      documentation (xfs (5) and mkfs.xfs (8)).

      B If performance is your goal, you can, for example, create the journal on an-
        other (physical) storge medium by using an option like “-l logdev=/dev/sdb1,size=10000b ”.
        (The actual file system should of course not be on /dev/sdb , and the partition
        for the journal should not otherwise be used.)

         The XFS tools contain a fsck.xfs (which you can invoke using “fsck -t xfs ”), but
      this program doesn’t really do anything at all—it is merely there to give the sys-
      tem something to call when “all” file systems are to be checked (which is easier
      than putting a special exception for XFS into fsck ). In actual fact, XFS file sys-
      tems are checked automatically on mounting if they have not been unmounted
      cleanly. If you want to check the consistency of an XFS or have to repair one, use
      the xfs_repair (8) program—“xfs_repair -n ” checks whether repairs are required;
      without the option, any repairs will be performed outright.

      B In extreme cases xfs_repair may not be able to repair the file system. In such a
        situation you can use xfs_metadump to create a dump of the filesystem’s meta-
        data and send that to the developers:

             # xfs_metadump /dev/sdb1 sdb1.dump

            (The file system must not be mounted when you do this.) The dump is a
            binary file that does not contain actual file data and where all file names
            have been obfuscated. Hence there is no risk of inadvertently passing along
            confidential data.

      B A dump that has been prepared using xfs_metadump can be written back
        to a file system (on a “real” storage medium or an image in a file) using
        xfs_mdrestore . This will not include file contents as these aren’t part of the
        dump to begin with. Unless you are an XFS developer, this command will
        not be particularly interesting to you.

         The xfs_info command outputs information about a (mounted) XFS file system:

      # xfs_info /dev/sdb1
      meta-data=/dev/sdb1              isize=256    agcount=4, agsize=16384 blks
               =                       sectsz=512   attr=2, projid32bit=1
               =                       crc=0        finobt=0
      data     =                       bsize=4096   blocks=65536, imaxpct=25
               =                       sunit=0      swidth=0 blks
      naming   =version 2              bsize=4096   ascii-ci=0 ftype=0
      log      =Intern                 bsize=4096   blocks=853, version=2
               =                       sectsz=512   sunit=0 blks, lazy-count=1
      realtime =keine                  extsz=4096   blocks=0, rtextents=0

      You can see, for example, that the file system consists of 65536 blocks of 4 KiB each
      (bsize and blocks in the data section), while the journal occupies 853 4-KiB blocks
      in the same file system (Intern , bsize and blocks in the log section).

      B The same information is output by mkfs.xfs after creating a new XFS file

         You should avoid copying XFS file systems using dd (or at least proceed very
      cautiously). This is because every XFS file system contains a unique UUID, and
      programs like xfsdump (which makes backup copies) can get confused if they run
      into two independent file systems using the same UUID. To copy XFS file systems,
      use xfsdump and xfsrestore or else xfs_copy instead.
7.1 Creating a Linux File System                                                        113

7.1.5        Btrfs
Btrfs is considered the up-and-coming Linux file system for the future. It com-
bines the properties traditionally associated with a Unix-like file system with some
innovative ideas that are partly based on Solaris’s ZFS. Besides some features oth-
erwise provided by the Logical Volum Manager (LVM; Section 6.7)—such as the
creation of file systems that span several physical storage media—or provided
by the Linux kernel’s RAID support—such as the redundant storage of data on
several physical media—this includes transparent data compression, consistency
checks on data blocks by means of checksums, and various others. The “killer
feature” is probably snapshots that can provide views of different versions of files
or complete file hierarchies simultaneously.

B Btrfs is several years younger than ZFS, and its design therefore contains a
  few neat ideas that hadn’t been invented yet when ZFS was first introduced.
  ZFS is currently considered the “state of the art” in file systems, but it is to
  be expected that some time in the not-too-distant future it will be overtaken
  by Btrfs.

B Btrfs is based, in principle, on the idea of “copy on write”. This means that
  if you create a snapshot of a Btrfs file system, nothing is copied at all; the
  system only notes that a copy exists. The data is accessible both from the
  original file system and the snapshot, and as long as data is just being read,
  the file systems can share the complete storage. Once write operations hap-
  pen either in the original file system or the snapshot, only the data blocks
  being modified are copied. The data itself is stored in efficient data struc-
  tures called B-trees.

   Btrfs file systems are created with mkfs , as usual:

# mkfs -t btrfs /dev/sdb1

B You can also mention several storage media, which will all be made part
  of the new file system. Btrfs stores metadata such as directory information
  redundantly on several media; by default, data is spread out across various
  disks (“striping”) in order to accelerate access1 . You can, however, request
  other storage arrangements:

          # mkfs -t btrfs -L MyBtrfs -d raid1 /dev/sdb1 /dev/sdc1

         This example generates a Btrfs file system which encompasses the /dev/sdb1
         and /dev/sdc1 disks and is labeled “MyBtrfs”. Data is stored redundantly on
         both disks (“-d raid1 ”).

B Within Btrfs file systems you can create “subvolumes”, which serve as a type
  of partition at the file system level. Subvolumes are the units of which you
  will later be able to make snapshots. If your system uses Btrfs as its root file
  system, the command

          # btrfs subvolume create /home

         would, for instance, allow you to keep your own data within a separate sub-
         volume. Subvolumes do not take a lot of space, so you should not hesitate
         to create more of them rather than fewer—in particular, one for every direc-
         tory of which you might later want to create independent snapshots, since
         it is not possible to make directories into subvolumes after the fact.

  1 In   other words, Btrfs uses RAID-1 for metadata and RAID-0 for data.
114                                                                  7 File Systems: Care and Feeding

              B You can create a snapshot of a subvolume using
                      # btrfs subvolume snapshot /mnt/sub /mnt/sub-snap

                      The snapshot (here, /mnt/sub- snap ) is at first indistinguishable from the origi-
                      nal subvolume (here, /mnt/sub ); both contain the same files and are writable.
                      At first no extra storage space is being used—only if you change files in the
                      original or snapshot or create new ones, the system copies whatever is re-

                 Btrfs makes on-the-fly consistency checks and tries to fix problems as they are
              detected. The “btrfs scrub start ” command starts a house-cleaning operation that
              recalculates the checksums on all data and metadata on a Btrfs file system and
              repairs faulty blocks according to a different copy if required. This can, of course,
              take a long time; with “btrfs scrub status ” you can query how it is getting on, with
              “btrfs scrub cancel ” you can interrupt it, and restart it later with “btrfs scrub resume ”.
                 There is a fsck.btrfs program, but it does nothing beyond outputting a message
              that it doesn’t do anything. The program is required because something needs
              to be there to execute when all file systems are checked for consistency during
              startup. To really check or repair Btrfs file systems there is the “btrfs check ” com-
              mand. By default this does only a consistency check, and if it is invoked with the
              “--repair ” it tries to actually repair any problems it found.
                 Btrfs is very versatile and complex and we can only give you a small glimpse
              here. Consult the documentation (starting at btrfs (8)).

              C 7.4 [!1] Generate a Btrfs file system on an empty partition, using “mkfs -t
                btrfs ”.

              C 7.5 [2] Within your Btrfs file system, create a subvolume called sub0 . Create
                some files within sub0 . Then create a snapshot called snap0 . Convince your-
                self that sub0 and snap0 have the same content. Remove or change a few files
                in sub0 and snap0 , and make sure that the two subvolumes are independent
                of each other.

              7.1.6     Even More File Systems
      tmpfs   tmpfs  is a flexible implementation of a “RAM disk file system”, which stores files
              not on disk, but in the computer’s virtual memory. They can thus be accessed
              more quickly, but seldom used files can still be moved to swap space. The size of
              a tmpfs is variable up to a set limit. There is no special program for generating a
              tmpfs , but you can create it simply by mounting it: For example, the

              # mount -t tmpfs -o size=1G,mode=0700 tmpfs /scratch

           command creates a tmpfs of at most 1 GiB under the name of /scratch , which can
           only be accessed by the owner of the /scratch directory. (We shall be coming back
           to mounting file systems in section 7.2.)
               A popular file system for older Windows PCs, USB sticks, digital cameras, MP3
           players and other “storage devices” without big ideas about efficiency and flexi-
      VFAT bility is Microsoft’s venerable VFAT file system. Naturally, Linux can mount, read,
           and write media formatted thusly, and also create such file systems, for example

              # mkfs -t vfat /dev/mcblk0p1
7.1 Creating a Linux File System                                                                  115

(insert the appropriate device name again). At this point you will no longer be sur-
prised to hear that mkfs.vfat is just another name for the mkdosfs program, which
can create all sorts of MS-DOS and Windows file systems—including the file sys-
tem used by the Atari ST of blessed memory. (As there are Linux variants running
on Atari computers, this is not quite as far-fetched as it may sound.)

B mkdosfs supports various options allowing you to determine the type of file
  system created. Most of these are of no practical consequence today, and
  mkdosfs will do the Right Thing in most cases, anyway. We do not want to
  disgress into a taxonomy of FAT file system variants and restrict ourselves to
  pointing out that the main difference between FAT and VFAT is that file sys-
  tems of the latter persuasion allow file names that do not follow the older,
  strict 8 + 3 scheme. The “file allocation table”, the data structure that re-
  members which data blocks belong to which file and that gave the file sys-
  tem its name, also exists invarious flavours, of which mkdosfs selects the one
  most suitable to the medium in question—floppy disks are endowed with a
  12-bit FAT, and hard disk (partitions) or (today) USB sticks of considerable
  capacity get 32-bit FATs; in the latter case the resulting file system is called

   NTFS, the file system used by Windows NT and its successors including Win- NTFS
dows Vista, is a bit of an exasperating topic. Obviously there is considerable
interest in enabling Linux to handle NTFS partitions—everywhere but on Mi-
crosoft’s part, where so far one has not deigned to explain to the general public
how NTFS actually works. (It is well-known that NTFS is based on BSD’s “Berke-
ley Fast Filesystem”, which is reasonably well understood, but in the meantime
Microsoft butchered it into something barely recognisable.) In the Linux com-
munity there have been several attempts to provide NTFS support by trying to
understand NTFS on Windows, but complete success is still some way off. At
the moment there is a kernel-based driver with good support for reading, but
questionable support for writing, and another driver running in user space which
according to the grapevine works well for reading and writing. Finally, there are
the “ntfsprogs”, a package of tools for managing NTFS file systems, which also
allow rudimentary access to data stored on them. Further information is available
from http://www.linux- .

7.1.7    Swap space
In addition to the file system partitions, you should always create a swap parti- swap partition
tion. Linux can use this to store part of the content of system RAM; the effective
amount of working memory available to you is thus greater than the amount of
RAM in your computer.
   Before you can use a swap partition you must “format” it using the mkswap com-
# mkswap /dev/sda4

This writes some administrative data to the partition.
   When the system is started, it is necessary to “activate” a swap partition. This
corresponds to mounting a partition with a file system and is done using the swapon
# swapon /dev/sda4

The partition should subsequently be mentioned in the /proc/swaps file:

# cat /proc/swaps
Filename               Type        Size    Used   Priority
/dev/sda4              partition   2144636 380    -1
116                                                                   7 File Systems: Care and Feeding

                  After use the swap partition can be deactivated using swapoff :

                   # swapoff /dev/sda4

                   B The system usually takes care of activating and deactivating swap parti-
                     tions, as long as you put them into the /etc/fstab file. See section 7.2.2.

                     You can operate up to 32 swap partitions (up to and including kernel version
                  2.4.10: 8) in parallel; the maximum size depends on your computer’s architecture
                  and isn’t documented anywhere exactly, but “stupendously gigantic” is a reason-
                  able approximation. It used to be just a little less than 2 GiB for most Linux plat-

                   B If you have several disks, you should spread your swap space across all of
                     them, which should increase speed noticeably.

                   B Linux can prioritise swap space. This is worth doing if the disks containing
                     your swap space have different speeds, because Linux will prefer the faster
                     disks. Read up on this in swapon (8).

                   B Besides partitions, you can also use files as swap space. Since Linux 2.6 this
                     isn’t even any slower! This allows you to temporarily provide space for rare
                     humongous workloads. You must initially create a swap file as a file full of
                     zeros, for instance by using

                          # dd if=/dev/zero of=swapfile bs=1M count=256

                          before preparing it using the mkswap command and activating it with swapon .
                          (Desist from tricks using dd or cp ; a swap file may not contain “holes”.)

                   B You can find information about the currently active swap areas in the /proc/
                     swaps file.

                  7.2      Mounting File Systems
                  7.2.1     Basics
                  To access data stored on a medium (hard disk, USB stick, floppy, …), it would in
                  principle be possible to access the device files directly. This is in fact being done,
                  for example when accessing tape drives. However, the well-known file manage-
                  ment commands (cp , mv , and so on) can only access files via the directory tree.
                  To use these commands, storage media must be made part of the directory tree
                  (“mounted”) using their device files. This is done using the mount command.
                     The place in the directory tree where a file system is to be mounted is called a
      mount point mount point. This can be any directory; it does not even have to be empty, but you
                  will not be able to access the original directory content while another file system
                  is mounted “over” it.

                   B The content reappears once the file system is unmounted using umount . Even
                     so you should restrain yourself from mounting stuff on /etc and other im-
                     portant system directories …

                  7.2.2     The mount Command
                  The mount command mounts file systems into the directory tree. It can also be
                  used to display the currently mounted file systems, simply by calling it without
7.2 Mounting File Systems                                                                     117

proc        /proc           proc defaults                     0   0
/dev/sda2   /               ext3 defaults,errors=remount-ro   0   1
/dev/sda1   none            swap sw                           0   0
/dev/sda3   /home           ext3 defaults,relatime            0   1
/dev/sr0    /media/cdrom0   udf,iso9660 ro,user,exec,noauto   0   0
/dev/sdb1   /media/usb      auto user,noauto                  0   0
/dev/fd0    /media/floppy   auto user,noauto,sync             0   0

                        Figure 7.1: The /etc/fstab file (example)

$ mount
/dev/sda2 on / type ext3 (rw,relatime,errors=remount-ro)
tmpfs on /lib/init/rw type tmpfs (rw,nosuid,mode=0755)
proc on /proc type proc (rw,noexec,nosuid,nodev)
sysfs on /sys type sysfs (rw,noexec,nosuid,nodev)

  To mount a medium, for example a hard disk partition, you must specify its
device file and the desired mount point:

# mount -t ext2 /dev/sda5 /home

It is not mandatory to specify the file system type using the -t option, since the
kernel can generally figure it out for itself. If the partition is mentioned in /etc/
fstab , it is sufficient to give either the mount point or the device file:

# mount /dev/sda5                                                     One possibility …
# mount /home                                                           … and another

   Generally speaking, the /etc/fstab file describes the composition of the whole /etc/fstab
file system structure from various file systems that can be located on different
partitions, disks etc. In addition to the device names and corresponding mount
points, you can specify various options used to mount the file systems. The allow-
able options depend on the file system; many options are to be found in mount (8).
   A typical /etc/fstab file could look similar to figure 7.1. The root partition usu-
ally occupies the first line. Besides the “normal” file systems, pseudo file systems
such as devpts or proc and the swap areas are mentioned here.
   The third field describes the type of the file system in question. Entries like ext3 type
and iso9660 speak for themselves (if mount cannot decide what to do with the type
specification, it tries to delegate the job to a program called /sbin/mount. ⟨type⟩), swap
refers to swap space (which does not require mounting), and auto means that mount
should try to determine the file system’s type.

B To guess, mount utilises the content of the /etc/filesystems file, or, if that file
  does not exist, the /proc/filesystems file. (/proc/filesystems is also read if /etc/
  filesystems ends with a line containing just an asterisk.) In any case, mount
  processes only those lines that are not marked nodev . For your edification,
  here is a snippet from a typical /proc/filesystems file:

       nodev   sysfs
       nodev   rootfs
       nodev   usbfs
       nodev   nfs
118                                                                   7 File Systems: Care and Feeding


                B The kernel generates /proc/filesystems dynamically based on those file sys-
                  tems for which it actually contains drivers. /etc/filesystems is useful if you
                  want to specify an order for mount ’s guesswork that deviates from the one
                  resulting from /proc/filesystems (which you cannot influence).

                B Before mount refers to /etc/filesystems , it tries its luck with the libblkid and
                  libvolume_id libraries, both of which are (among other things) able to deter-
                  mine which type of file system exists on a medium. You can experiment
                  with these libraries using the command line programs blkid and vol_id :

                         # blkid /dev/sdb1
                         /dev/sdb1: LABEL="TESTBTRFS" UUID="d38d6bd1-66c3-49c6-b272-eabdae 
                           877368" UUID_SUB="3c093524-2a83-4af0-8290-c22f2ab44ef3" 
                           TYPE="btrfs" PARTLABEL="Linux filesystem" 

      options        The fourth field contains the options, including:
                defaults    Is not really an option, but merely a place holder for the standard options
                         (see mount (8)).
                noauto   Opposite of auto , keeps a file system from being mounted automatically
                         when the system is booted.
                user   In principle, only root can mount storage devices (normal users may only
                        use the simple mount command to display information), unless the user op-
                        tion is set. In this case, normal users may say “mount ⟨device⟩” or “mount
                        ⟨mount point⟩”; this will mount the named device on the designated mount
                        point. The user option will allow the mounting user to unmount the device
                        (root , too); there is a similar option users that allows any user to unmount
                        the device.
                sync   Write operations are not buffered in RAM but written to the medium directly.
                       The end of the write operation is only signaled to the application program
                       once the data have actually been written to the medium. This is useful for
                       floppies or USB thumb drives, which might otherwise be inadvertently re-
                       moved from the drive while unwritten data is still buffered in RAM.
                ro   This file system is mounted for reading only, not writing (opposite of rw )
                exec   Executable files on this file system may be invoked. The opposite is noexec ;
                        exec is given here because the user option implies the noexec option (among
                As you can see in the /dev/sdb entry, later options can overwrite earlier ones: user
                implies the noexec option, but the exec farther on the right of the line overwrites
                this default.

                7.2.3      Labels and UUIDs
              We showed you how to mount file systems using device names such as /dev/hda1 .
              This has the disadvantage, though, that the correspondence between device files
              and actual devices is not necessarily fixed: As soon as you remove or repartition a
              disk or add another, the correspondence may change and you will have to adjust
              the configuration in /etc/fstab . With some device types, such as USB media, you
              cannot by design rely on anything. This is where labels and UUIDs come in.
        label    A label is a piece of arbitrary text of up to 16 characters that is placed in a file
              system’s super block. If you have forgotten to assign a label when creating the
7.2 Mounting File Systems                                                                 119

file system, you can add one (or modify an existing one) at any time using e2label .
The command
# e2label /dev/sda3 home

(for example) lets you refer to /dev/sda3 as LABEL=home , e. g., using

# mount -t ext2 LABEL=home /home

The system will then search all available partitions for a file system containing this

B You can do the same using the -L option of tune2fs :
       # tune2fs -L home /dev/sda3

B The other file systems have their ways and means to set labels, too. With
  Btrfs, for example, you can either specify one when the file system is gener-
  ated (option “-L ”) or use

       # btrfs filesystem label /dev/sdb1 MYLABEL

   If you have very many disks or computers and labels do not provide the re-
quired degree of uniqueness, you can fall back to a “universally unique identifier”
or UUID. An UUID typically looks like                                              UUID

$ uuidgen

and is generated automatically and randomly when a file system is created. This
ensures that no two file systems share the same UUID. Other than that, UUIDs
are used much like labels, except that you now need to use UUID=bea6383f-22a7-
453f-8ef5-a5b895c8ccb0 (Gulp.) You can also set UUIDs by means of tune2fs , or create
completely new ones using

# tune2fs -U random /dev/hda3

This should seldom prove necessary, though, for example if you replace a disk or
have cloned a file system.

B Incidentally, you can determine a file system’s UUID using
       # tune2fs -l /dev/hda2 | grep UUID
       Filesystem UUID:              4886d1a2-a40d-4b0e-ae3c-731dd4692a77

B With other file systems (XFS, Btrfs) you can query a file system’s UUID (blkid
  is your friend) but not necessarily change it.

B The
       # lsblk -o +UUID

      command gives you an overview of all your block devices and their UUIDs.

B You can also access swap partitions using labels or UUIDs:
       # swapon -L swap1
       # swapon -U 88e5f06d-66d9-4747-bb32-e159c4b3b247
120                                                                             7 File Systems: Care and Feeding

                           You can find the UUID of a swap partition using blkid or lsblk , or check the
                           /dev/disk/by- uuid directory. If your swap partition does not have a UUID nor
                           a label, you can use mkswap to assign one.
                       You can also use labels and UUIDs in the /etc/fstab file (one might indeed claim
                    that this is the whole point of the exercise). Simply put



                    into the first field instead of the device name. Of course this also works for swap

                     C 7.6 [!2] Consider the entries in files /etc/fstab and /etc/mtab . How do they

                    7.3      The dd Command
                    dd is a command for copying files “by block”. It is used with particular preference
                    to create “images”, that is to say complete copies of file systems—for example,
                    when preparing for the complete restoration of the system in case of a catastrophic
                    disk failure.
                        dd (short for “copy and convert”2 ) reads data block by block from an input file
                    and writes it unchanged to an output file. The data’s type is of no consequence.
                    Neither does it matter to dd whether the files in question are regular files or device
                        Using dd , you can create a quickly-restorable backup copy of your system par-
                    tition as follows:
                     # dd if=/dev/sda2 of=/data/sda2.dump

                    This saves the second partition of the first SCSI disk to a file called /data/sda2.
                    dump —this file should of course be located on another disk. If your first disk is
                    damaged, you can easily and very quickly restore the original state after replacing
                    it with an identical (!) drive:

                     # dd if=/data/sda2.dump of=/dev/sda2

                      (If /dev/sda is your system disk, you must of course have booted from a rescue or
                      live system.)
                          For this to work, the new disk drive’s geometry must match that of the old one.
      partition table In addition, the new disk drive needs a partition table that is equivalent to the old
                      one. You can save the partition table using dd as well (at least for MBR-partitioned

                     # dd if=/dev/sda of=/media/floppy/mbr_sda.dump bs=512 count=1

                    Used like this, dd does not save all of the hard disk to floppy disk, but writes every-
                    thing in chunks of 512 bytes (bs=512 )—one chunk (count=1 ), to be exact. In effect, all
                    of the MBR is written to the floppy. This kills two birds with the same stone: the
                    boot loader’s stage 1 also ends up back on the hard disk after the MBR is restored:
                       2 Seriously! The dd command is inspired by a corresponding command on IBM mainframes (hence

                    the parameter syntax, which according to Unix standards is quite quaint), which was called CC (as in
                    “copy and convert”), but on Unix the cc name was already spoken for by the C compiler.
7.4 Disk Quotas                                                                                  121

# dd if=/media/floppy/mbr_sda.dump of=/dev/sda

You do not need to specify a chunk size here; the file is just written once and is
(hopefully) only 512 bytes in size.

A Caution: The MBR does not contain partitioning information for logical par-
  titions! IF you use logical partitions, you should use a program like sfdisk
  to save all of the partitioning scheme—see below.

B To save partitioning information for GPT-partitioned disks, use, for exam-
  ple, gdisk (the b command).

B dd can also be used to make the content of CD-ROMs or DVDs permanently
  accessible from hard disk. The “dd if=/dev/cdrom of=/data/cdrom1.iso ” places
  the content of the CD-ROM on disk. Since the file is an ISO image, hence
  contains a file system that the Linux kernel can interpret, it can also be
  mounted. After “mount -o loop,ro /data/cdrom.iso /mnt ” you can access the
  image’s content. You can of course make this permanent using /etc/fstab .

7.4     Disk Quotas
7.4.1   Basics
Linux makes it possible to limit the maximum number of used inodes or data
blocks per user or per group. Two limits are distinguished: The soft quota may soft quota
not be exceeded in the long term, but you can allow users an “overdraft” by setting
the hard quota to a higher value. Users may then, for a short time, occupy space hard quota
up to the hard quota, but within a certain period of time they must reduce the
used space to below the soft quota again. As soon as the hard quota is reached or
the grace period is over, further write operations will fail.

B It is sufficient to dip below the soft quota very briefly. After that you will
  again be able to use space up to the hard quota, and the grace period will
  start from the beginning.
   Quotas can be assigned per file system. You could, for example, set a limit Quota assignment
for the size of the incoming mail box in /var/mail without constraining the users’
home directories or vice-versa. Linux supports quotas on all common Linux file

B As far as quotas are concerned, XFS does its own thing—but the current
  Linux tools for quota management can also handle XFS. (Should you be
  forced to deal with a fairly old Linux, it may be possible that you need to
  use the xfs_quota command instead.)

B Btrfs has its own infrastructure for quotas, the so-called “quota groups” or
  “qgroups”. In principle, you can set up quotas for individual subvolumes,
  which will then be managed hierarchically. The file system ensures that a
  file system never uses more space than the “quota group” of its subvolume,
  that of the enclosing subvolume, etc. specify. Snapshots do not count as long
  as they aren’t changed, since snapshots do not take up additional space at
  first. Study btrfs-qgroup (8).

7.4.2   User Quotas (ext and XFS)
To set up user quotas, you must first install the quota software (included with
most distributions). Then you can mark those file systems where quotas should
be enforced by including the usrquota mount option, either ad hoc for a mounted
file system as in
122                                                                         7 File Systems: Care and Feeding

                       # mount -o remount,usrquota /home

                      or (preferably) by permanently including the option in the file system’s entry in
                      /etc/fstab :

                       /dev/hda5   /home       ext2   defaults,usrquota   0 2

                      The quota database is initialised using

                       # quotacheck -avu

                      (watch for possible warnings), after the file system has been mounted using the
                      usrquota option. The quotacheck program creates the aquota.user database file in the
                      partition’s root directory, in this case /home/aquota.user . It is advisable to execute
                      quotacheck periodically during normal system operations in order to “clean” the
                         Afterwards you should start the quota system, so that the database created
                      using quotacheck is brought up to date with every file system operation:

                       # quotaon -avu

                       Your distribution will probably contain an init script which will perform this step
                       during system boot. Accordingly, the quota system will be deactivated on shut-
                       down using “quotaoff -auv ”.
        Setting quotas    You can set quotas for various users using the edquota command. edquota starts
                       your favourite editor (according to the EDITOR environment variable) with a “tem-
                       plate” where you can fill in the soft and hard quotas for the file systems in ques-
                       # edquota -u hugo                                                     Quotas for hugo

                      The grace period in days is set using “edquota -t ”.
                        You can also refer to the quotas of a “prototype user”:

                       # edquota -p tux hugo

                      sets hugo ’s quotas to those defined for tux . This makes it easy to assign quotas to
                      newly created users without having to enter them manually using edquota .
      Querying quotas    Users can query their quota status with the quota command. “quota -q ” outputs
                      a brief message mentioning only those file systems whose soft quota has been
                      exceeded. This command is suitable for files such as ~/.profile .
                         The repquota command generates a tabular overview of the disk usage of vari-
                      ous users, together with possibly active quotas:

                       # repquota -a
                                           Block limits         File limits
                       User          used soft hard grace used soft hard grace
                       root   --   166512     0     0     19562    0    0
                       tux    --     2304 10000 12000       806 1000 2000
                       hugo   --     1192 5000 6000         389 500 1000

                      The grace column contains the remaining number of days of the grace period if
                      the soft quota has been exceeded.
7.4 Disk Quotas                                                                                  123

7.4.3   Group Quotas (ext and XFS)
You can also set up group quotas applying to all members of a group together. For group quotas
this to become effective, you must limit all groups that the users in question are
members of; otherwise they could circumvent their quota by changing groups.
   The mount option for group quotas is grpquota , and the database file is called . To enable group quotas, you must use the aforementioned com-
mands while substituting or augmenting the -u option by -g . Hence,

# quotaon -auvg

activates all types of quota (user and group) on every file system, while

$ quota -vg

displays group quotas for those groups that you are a member of.

C 7.7 [2] Set up disk quotas as described: a file system with user quotas,
  one with group quotas, and one with both. Watch out for the output of
  quotacheck , quotaon , and quota .

C 7.8 [2] Set up quotas for a user and a group (containing that user). How
  does quota ’s output change when you reach the soft quota (the hard quota)?
  Is a warning displayed?
124                                                     7 File Systems: Care and Feeding

      Commands in this Chapter
      blkid    Locates and prints block device attributes                   blkid (8) 118
      dd       “Copy and convert”, copies files or file systems block by block and does
               simple conversions                                              dd (1) 120
      debugfs File system debugger for fixing badly damaged file systems. For gurus
               only!                                                     debugfs (8) 108
      dumpe2fs Displays internal management data of the ext2 file system. For gurus
               only!                                                    dumpe2fs (8) 108
      dumpreiserfs Displays internal management data of the Reiser file system. For
               gurus only!                                         dumpreiserfs (8) 111
      e2fsck   Checks ext2 and ext3 file systems for consistency           e2fsck (8) 107
      e2label Changes the label on an ext2/3 file system                  e2label (8) 118
      edquota Tool for entering and adjusting disk quotas                edquota (8) 122
      fsck     Organises file system consistency checks                       fsck (8) 101
      lsblk    Lists available block devices                                lsblk (8) 119
      mkdosfs Creates FAT-formatted file systems                        mkfs.vfat (8) 114
      mke2fs   Creates ext2 or ext3 file systems                           mke2fs (8) 105
      mkfs     Manages file system creation                                   mkfs (8) 100
      mkfs.vfat Creates FAT-formatted file systems                      mkfs.vfat (8) 114
      mkfs.xfs Creates XFS-formatted file systems                        mkfs.xfs (8) 111
      mkreiserfs Creates Reiser file systems                          mkreiserfs (8) 111
      mkswap   Initialises a swap partition or file                        mkswap (8) 115
      mount    Includes a file system in the directory tree       mount (8), mount (2) 116
      quota    Reports on a user’s quota status                             quota (1) 122
      reiserfsck Checks a Reiser file system for consistency          reiserfsck (8) 111
      repquota Summarises filesystem usage and quota usage for many users
                                                                        repquota (8) 122
      resize_reiserfs Changes the size of a Reiser file system resize_reiserfs (8) 111
      swapoff Deactivates a swap partition or file                        swapoff (8) 115
      swapon   Activates a swap partition or file                          swapon (8) 115
      tune2fs Adjusts ext2 and ext3 file system parameters           tunefs (8) 108, 119
      vol_id   Determines file system types and reads labels and UUIDs
                                                                          vol_id (8) 118
      xfs_mdrestore Restores an XFS metadata dump to a filesystem image
                                                                  xfs_mdrestore (8) 112
      xfs_metadump Produces metadata dumps from XFS file systems
                                                                   xfs_metadump (8) 112

         • After partitioning, a file system must be created on a new partition before
           it can be used. To do so, Linux provides the mkfs command (with a number
           of file-system-specific auxiliary tools that do the actual work).
         • Improperly unmounted file systems may exhibit inconsistencies. If Linux
           notes such file systems when it boots, these will be checked automatically
           and, if possible, repaired. These checks can also be triggered manually us-
           ing programs such as fsck and e2fsck .
         • The mount command serves to integrate file systems into the directory tree.
         • With dd , partitions can be backed up at block level.
         • To enable disk quotas, you must mount the file systems accordingly, ini-
           tialise the database, and activate the quota system.
         • Quotas are specified using edquota and supervised using quota or repquota .
7.4 Bibliography                                                             125

Quota-Mini-HOWTO03 Ralf van Dooren. “Quota mini-HOWTO”, August 2003.
                                                                                                $ echo tux
                                                                                                $ ls
                                                                                                $ /bin/su -

Booting Linux

8.1  Fundamentals . . . . . . . . . . .           .   .   .   .   .   .   .   .   .   .   128
8.2  GRUB Legacy . . . . . . . . . . .            .   .   .   .   .   .   .   .   .   .   131
   8.2.1 GRUB Basics . . . . . . . . .            .   .   .   .   .   .   .   .   .   .   131
   8.2.2 GRUB Legacy Configuration . . . .         .   .   .   .   .   .   .   .   .   .   132
   8.2.3 GRUB Legacy Installation . . . . .       .   .   .   .   .   .   .   .   .   .   133
   8.2.4 GRUB 2 . . . . . . . . . . .             .   .   .   .   .   .   .   .   .   .   134
   8.2.5 Security Advice . . . . . . . .          .   .   .   .   .   .   .   .   .   .   135
8.3 Kernel Parameters . . . . . . . . .           .   .   .   .   .   .   .   .   .   .   135
8.4 System Startup Problems . . . . . . .         .   .   .   .   .   .   .   .   .   .   137
   8.4.1 Troubleshooting . . . . . . . .          .   .   .   .   .   .   .   .   .   .   137
   8.4.2 Typical Problems . . . . . . . .         .   .   .   .   .   .   .   .   .   .   137
   8.4.3 Rescue systems and Live Distributions    .   .   .   .   .   .   .   .   .   .   139

      • Knowing the GRUB Legacy and GRUB 2 boot loaders and how to configure
      • Being able to diagnose and fix system start problems

      • Basic knowledge of the PC startup procedure
      • Handling of configuration files

adm1-boot.tex   (33e55eeadba676a3 )
128                                                                                             8 Booting Linux

                         8.1     Fundamentals
                         When you switch on a Linux computer, an interesting and intricate process takes
                         place during which the computer initialises and tests itself before launching the
                         actual operating system (Linux). In this chapter, we consider this process in some
                         detail and explain how to adapt it to your requirements and to find and repair
                         problems if necessary.

                          B The word “to boot” is short for “to pull oneself up by one’s bootstraps”.
                            While, as Newton tells us, this is a physical impossibility, it is a good image
                            for what goes on, namely that the computer gets itself started from the most
                            basic beginnings.

                            Immediately after the computer is switched on, its firmware—depending on
                         the computer’s age, either the “basic input/output system” (BIOS) or “unified
                         extensible firmware interface” (UEFI) takes control. What happens next depends
                         on the firmware.

                         BIOS startup On BIOS-based systems, the BIOS searches for an operating system
                         on media like CD-ROM or hard disk, depending on the boot order specified in the
                         BIOS setup. On disks (hard or floppy), the first 512 bytes of the boot medium will
                         be read. These contain special information concerning the system start. Generally,
             boot sector this area is called the boot sector; a hard disk’s boot sector is also called the master
      master boot record boot record (MBR).

                          B We already came across the MBR when discussing the eponymous disk par-
                            titioning scheme in chapter 6. We’re now looking at the part of the MBR that
                            does not contain partitioning information.

                            The first 446 bytes of the MBR contain a minimal startup program which in
            boot loader turn is responsible for starting the operating system—the boot loader. The rest
                      is occupied by the partition table. 446 bytes are not enough for the complete boot
                      loader, but they suffice for a small program which can fetch the rest of the boot
                      loader from disk using the BIOS. In the space between the MBR and the start of
                      the first partition—at least sector 63, today more likely sector 2048 there is enough
                      room for the rest of the boot loader. (We shall come back to that topic presently.)
                          Modern boot loaders for Linux (in particular, the “Grand Unified Boot loader”
                 GRUB or GRUB) can read common Linux file systems and are therefore able to find the
                      operating system kernel on a Linux partition, load it into RAM and start it there.

          boot manager    B GRUB serves not just as a boot loader, but also as a boot manager. As such,
                            it can, according to the user’s preferences, launch various Linux kernels or
                            even other operating systems.

                          B Bootable CD-ROMs or DVDs play an important role for the installation or
                            update of Linux systems, or as the basis of “live systems” that run directly
                            from read-only media without having to be installed on disk. To boot a
                            Linux computer from CD, you must in the simplest case ensure that the
                            CD-ROM drive is ahead of the firmware’s boot order than the hard disk,
                            and start the computer while the desired CD is in the drive.

                          B In the BIOS tradition, booting off CD-ROMs follows different rules than
                            booting off hard disk (or floppy disk). The “El Torito” standard (which
                            specifies these rules) basically defines two approaches: One method is to
                            include an image of a bootable floppy disk on the CD-ROM (it may be as big
                            as 2.88 MiB), which the BIOS finds and boots; the other method is to boot
                            directly off the CD-ROM, which requires a specialised boot loader (such as
                            ISOLINUX for Linux).
8.1 Fundamentals                                                                              129

B With suitable hardware and software (usually part of the firmware today),
  a PC can boot via the network. The kernel, root file system, and everything
  else can reside on a remote server, and the computer itself can be diskless
  and hence ear-friendly. The details would be a bit too involved and are irrel-
  evant for LPIC-1 in any case; if necessary, look for keywords such as “PXE”
  or “Linux Terminal Server Project”.

UEFI boot procedure UEFI-based systems do not use boot sectors. Instead, the
UEFI firmware itself contains a boot manager which exploits information about
the desired operating system which is held in non-volatile RAM (NVRAM). Boot
loaders for the different operating systems on the computer are stored as regular
files on an “EFI system partition” (ESP), where the firmware can read and start
them. The system either finds the name of the desired boot loader in NVRAM, or
else falls back to the default name, /EFI/BOOT/BOOTX64.EFI . (The X64 here stands for
“64-bit Intel-style PC”. Theoretically, UEFI also works for 32-bit systems, but that
doesn’t mean it is a great idea.) The operating-system specific boot loader then
takes care of the rest, as in the BIOS startup procedure.

B The ESP must officially contain a FAT32 file system (there are Linux distri-
  butions that use FAT16, but that leads to problems with Windows 7, which
  requires FAT32). A size of 100 MiB is generally sufficient, but some UEFI
  implementations have trouble with FAT32 ESPs which are smaller than
  512 MiB, and the Linux mkfs command will default to FAT16 for partitions
  of up to 520 MiB. With today’s prices for hard disks, there is little reason
  not to play it safe and create an ESP of around 550 MiB.

B In principle it is possible to simply write a complete Linux kernel as BOOTX64.
  EFI on the ESP and thus manage without any boot loader at all. PC-based
  Linux distributions don’t usually do this, but this approach is interesting for
  embedded systems.

B Many UEFI-based systems also allow BIOS-style booting from MBR-parti-
  tioned disks, i. e., with a boot sector. This is called “compatibility support
  module” or CSM. Sometimes this method is used automatically if a tradi-
  tional MBR is found on the first recognised hard disk. This precludes an
  UEFI boot from an ESP on an MBR-partitioned disk and is not 100% ideo-
  logically pure.

B UEFI-based systems boot from CD-ROM by looking for a file called /EFI/
  BOOT/BOOTX64.EFI —like they would for disks. (It is feasible to produce CD-
  ROMs that boot via UEFI on UEFI-based systems and via El Torito on BIOS-
  based systems.)

   “UEFI Secure Boot” is supposed to prevent computers being infected with UEFI Secure Boot
“root kits” that usurp the startup procedure and take over the system before the
actual operating system is being started. Here the firmware refuses to start boot
loaders that have not been cryptographically signed using an appropriate key. Ap-
proved boot loaders, in turn, are responsible for only launching operating system
kernels that have been cryptographically signed using an appropriate key, and
approved operating system kernels are expected to insist on correct digital sig-
natures for dynamically loadable drivers. The goal is for the system to run only
“trusted” software, at least as far as the operating system is concerned.

B A side effect is that this way one gets to handicap or exclude potentially un-
  desirable operating systems. In principle, a company like Microsoft could
  exert pressure on the PC industry to only allow boot loaders and operating
  systems signed by Microsoft; since various anti-trust agencies would take a
  dim view to this, it is unlikely that such a step would become part of offi-
  cial company policy. It is more likely that the manufacturers of PC mother-
  boards and UEFI implementations concentrate their testing and debugging
130                                                                                 8 Booting Linux

                     efforts on the “boot Windows” application, and that Linux boot loaders will
                     be difficult or impossible to get to run simply due to inadvertent firmware

                  Linux supports UEFI Secure Boot in various ways. There is a boot loader called
          Shim “Shim” (developed by Matthew Garrett) which a distributor can have signed by
               Microsoft. UEFI starts Shim and Shim then starts another boot loader or operating
               system kernel. These can be signed or unsigned; the security envisioned by UEFI
               Secure Boot is, of course, only obtainable with the signatures. You can install your
               own keys and then sign your own (self-compiled) kernels.

                B The details for this would be carrying things too far. Consult the Linup
                  Front training manual Linux System Customisation

      PreLoader An alternative to Shim is “PreLoader” (by James Bottomley, distributed by the
               Linux Foundation). PreLoader is simpler than Shim and makes it possible to ac-
               credit a (possibly unsigned) subsequent boot loader with the system, and boot it
               later without further enquiries.

               Hard disks: MBR vs. GPT The question of which partitioning scheme a hard
               disk is using and the question of whether the computer boots via the BIOS (or
               CSM) or UEFI really don’t have a lot to do with each other. At least with Linux it
               is perfectly possible to boot a BIOS-based system from a GPT-partitioned disk or
               a UEFI-based system from an MBR-partitioned disk (the latter possibly via CSM).

                B To start a BIOS-based system from a GPT-partitioned disk it makes sense to
                  create a “BIOS boot partition” to hold that part of the boot loader that does
                  not fit into the MBR. The alternative—using the empty space between the
                  MBR and the start of the first partition—is not reliable for GPT-partitioned
                  disks, since the GPT partition table takes up at least part of this space and/
                  or the first partition might start immediately after the GPT partition table.
                  The BIOS boot partition does not need to be huge at all; 1 MiB is probably
                  amply enough.

               After the boot loader The boot loader loads the Linux operating system kernel
               and passes the control to it. With that, it is itself extraneous and can be removed
               from the system; the firmware, too, will be ignored from now on—the kernel is
               left to its own devices. In particular, it must be able to access all drivers required
               to initialise the storage medium containing the root file system, as well as that file
               system itself (the boot loader used the firmware to access the disk), typically at
               least a driver for an IDE, SATA, or SCSI controller and the file system in question.
               These drivers must be compiled into the kernel or—the preferred method today—
               will be taken from “early userspace”, which can be configured without having to
               recompile the kernel. (As soon as the root file system is available, everything is
               peachy because all drivers can be read from there.) The boot loader’s tasks also
               include reading the early-userspace data.

                B The “early userspace” used to be called an “initial RAM disk”, because the
                  data was read into memory en bloc as a (usually read-only) medium, and
                  treated by the kernel like a block-oriented disk. There used to be special
                  compressed file systems for this application. The method most commonly
                  used today stipulates that the early-userspace data is available as a cpio
                  archive which the kernel extracts directly into the disk block cache, as if
                  you had read each file in the archive directly from a (hypothetical) storage
                  medium. This makes it easier to get rid of the early userspace once it is no
                  longer required.

                B The kernel uses cpio instead of tar because cpio archives in the format used
                  by the kernel are better-standardised and easier to unpack than tar archives.
8.2 GRUB Legacy                                                                                            131

   As soon as the “early userspace” is available, a program called /init is invoked.
This is in charge of the remaining system initialisation, which includes tasks such
as the identification of the storage medium that should be made available as the
root file system, the loading of any required drivers to access that medium and the
file system (these drivers, of course, also come from early userspace), possibly the
(rudimentary) configuration of the network in case the root file system resides on
a remote file server, and so on. Subsequently, the early userspace puts the desired
root file system into place at “/ ” and transfers control to the actual init program—
today most often either System-V init (chapter 9) or systemd (chapter 10), in each
case under the name of /sbin/init . (You can juse the kernel command line option
init= to pick a different program.)

B If no early userspace exists, the operating system kernel makes the storage
  medium named on its command line using the root= option available as the
  root file system, and starts the program given by the init= option, by default
  /sbin/init .

C 8.1 [2] Whereabouts on an MBR-partitioned hard disk may a boot loader
  reside? Why?

8.2      GRUB Legacy
8.2.1      GRUB Basics
Many distributions nowadays use GRUB as their standard boot loader. It has var-
ious advantages compared to LILO, most notably the fact that it can handle the
common Linux file systems. This means that it can read the kernel directly from a
file such as /boot/vmlinuz , and is thus immune against problems that can develop if
you install a new kernel or make other changes to your system. Furthermore, on
the whole GRUB is more convenient—for example offering an interactive GRUB GRUB shell
shell featuring various commands and thus allowing changes to the boot setup
for special purposes or in case of problems.

A The GRUB shell allows access to the file system without using the usual
  access control mechanism. It should therefore never be made available to
  unauthorised people, but be protected by a password (on important com-
  puters, at least). See also Section 8.2.5.

    Right now there are two widespread versions of GRUB: The older version
(“GRUB Legacy”) is found in older Linux distributions—especially those with an
“enterprise” flavour’—, while the newer distributions tend to rely on the more
modern version GRUB 2 (section 8.2.4).
    The basic approach taken by GRUB Legacy follows the procedure outlined in
section 8.1. During a BIOS-based startup, the BIOS finds the first part (“stage 1”)
of the boot loader in the MBR of the boot disk (all 446 bytes of it). Stage 1 is able
to find the next stage based on sector lists stored inside the program (as part of
the 446 bytes) and the BIOS disk access functions1 .
    The “next stage” is usually stage 1.5, which is stored in the otherwise un-
used space immediately after the MBR and before the start of the first partition.
Stage 1.5 has rudimentary support for Linux file systems and can find GRUB’s
“stage 2” within the file system (normally below /boot/grub ). Stage 2 may be any-
where on the disk. It can read file systems, too, and it fetches its configuration
file, displays the menu, and finally loads and starts the desired operating system
(in the case of Linux, possibly including the “early userspace”).
   1 At least as long as the next stage can be found within the first 1024 “cylinders” of the disk. There

are historical reasons for this and it can, if necessary, be enforced through appropriate partitioning.
132                                                                                                8 Booting Linux

                            B Stage 1 could read stage 2 directly, but this would be subject to the same
                              restrictions as reading stage 1.5 (no file system access and only within the
                              first 1024 cylinders). This is why things aren’t usually arranged that way.

                            B GRUB can directly load and start most Unix-like operating systems for x86
                              computers, including Linux, Minix, NetBSD, GNU Hurd, Solaris, Reac-
                              tOS, Xen, and VMware ESXi2 . The relevant standard is called “multiboot”.
                              GRUB starts multiboot-incompatible systems (notably Windows) by invok-
                              ing the boot loader of the operating system in question—a procedure called
                              “chain loading”.

                               To make GRUB Legacy work with GPT-partitioned disks, you need a BIOS boot
                            partition to store its stage 1.5. There is a version of GRUB Legacy that can deal with
                            UEFI systems, but for UEFI boot you are generally better off using a different boot

                            8.2.2       GRUB Legacy Configuration
      /boot/grub/menu.lst   The main configuration file for GRUB Legacy is usually stored as /boot/grub/menu.
                            lst . It contains basic configuration as well as the settings for the operating systems
                            to be booted. This file might look as follows:

                            default 1
                            timeout 10

                            title linux
                               kernel (hd0,1)/boot/vmlinuz root=/dev/sda2
                               initrd (hd0,1)/boot/initrd
                            title failsafe
                               kernel (hd0,1)/boot/vmlinuz.bak root=/dev/sda2 apm=off acpi=off
                               initrd (hd0,1)/initrd.bak
                            title someothersystem
                               root (hd0,2)
                               chainloader +1
                            title floppy
                               root (fd0)
                               chainloader +1

                            The individual parameters have the following meaning:
                            default   Denotes the default system to be booted. Caution: GRUB counts from 0!
                                     Thus, by default, the configuration above launches the failsafe entry.
                            timeout This is how many          seconds the GRUB menu will be displayed before the
                                  default entry will be       booted.
                            title    Opens an operating system entry and specifies its name, which will be dis-
                                     played within the GRUB menu.
                            kernel    Specifies the Linux kernel to be booted. (hd0, 1)/boot/vmlinuz , for example,
                                     means that the kernel is to be found in /boot/vmlinuz on the first partition of
                                     the zeroth hard disk, thus in our example, for linux , on /dev/hda2 . Caution:
                                     The zeroth hard disk is the first hard disk in the BIOS boot order! There is
                                     no distinction between IDE and SCSI! And: GRUB starts counting at 0 …
                                     Incidentally, GRUB takes the exact mapping of the individual drives from
                                     the file.
                                     After the kernel location, arbitrary kernel parameters can be passed. This
                                     includes the boot= entry.
                              2 The   “U” in GRUB must stand for something, after all.
8.2 GRUB Legacy                                                                                           133

initrd   Denotes the location of the cpio archive used for the “early userspace”.
root   Determines the system partition for foreign operating systems. You can also
        specify media that only occasionally contain something bootable, such as
        the floppy disk drive—this will let you boot from floppy even though the
        floppy disk is disabled in the BIOS boot order.
chainloader +1   Denotes the boot loader to be loaded from the foreign system’s sys-
         tem partition. Generally this is the content of that partition’s boot loader.
makeactive    Marks the specified partition temporarily as “bootable”. Some operat-
         ing systems (not Linux) require this in order to be able to boot off the par-
         tition in question. By the way: GRUB supports a few more such directives,
         for example map , which makes it possible to fool a system into believing it
         is installed on a different hard disk (than, e. g., the often disdained second
         disk) than it actually is.

8.2.3      GRUB Legacy Installation
Here “installation” does not refer to the installation of an RPM package but the
installation of the GRUB boot sector, or stage 1 (and very likely the stage 1.5). This
is very seldom required, for example during the original installation of the system
(where the installation procedure of your distribution will do it for you).
    The installation is done using the grub command, which invokes the GRUB
shell. It is most convenient to use a “batch” file, since otherwise you would have to
start from the very beginning after an erroneous input. Some distributions (e. g.,
those by SUSE/Novell) already come with a suitable file. In this case, the instal-
lation procedure might look like

# grub --batch --device-map=/boot/grub/ < /etc/grub.inst

The --device-map option creates a file under the specified name, if none
exists already.
   The /etc/grub.inst file could have the following content:                              /etc/grub.inst

root (hd0,1)
setup (hd0)

Here, root denotes the partition containing GRUB’s “home directory” (usually
/boot/grub —the other parts of GRUB necessary for the installation will be looked
for in this directory).

A The partition you specify using root here has nothing to do with the partition
  containing your Linux distribution’s root directory, which you specify using
  root= in your Linux kernels’ menu entries. At least not necessarily. See also
  Section 8.3.
   setup installs GRUB on the specified device, here in hd0 ’s MBR. GRUB’s setup
command is a simplified version of a more general command called install , which
should work in most cases.

B Alternatively, you may use the grub-install script to install the GRUB com-            grub-install
  ponents. This comes with some distributions.
   Inside the GRUB shell it is straightforward to figure out how to specify a hard disk specification
disk in the root or kernel directives. The GRUB shell command find is useful here:
# grub

grub> find /boot/vmlinuz
134                                                                                                  8 Booting Linux

                              8.2.4     GRUB 2
      new implementation GRUB 2 is a completely new implementation of the boot loader that did not make
                              particular concessions to GRUB-Legacy compatibility. GRUB 2 was officially re-
                              leased in June 2012, even though various distributions used earlier versions by

                                      The LPIC-1 certificate requires knowledge of GRUB 2 from version 3.5 of the
                                      exam (starting on 2 July 2012).

                                 As before, GRUB 2 consists of several stages that build on each other:
                                 • Stage 1 (boot.img ) is placed inside the MBR (or a partition’s boot sector) on
                                   BIOS-based systems. It can read the first sector of stage 1.5 by means of the
                                   BIOS, and that in turn will read the remainder of stage 1.5.
                                 • Stage 1.5 (core.img ) goes either between the MBR and the first partition
                                   (on MBR-partitioned disks) or else into the BIOS boot partition (on GPT-
                                   partitioned disks). Stage 1.5 consists of a first sector which is tailored to
                                   the boot medium (disk, CD-ROM, network, …) as well as a “kernel” that
                                   provides rudimentary functionality like device and file access, processing
                                   a command line, etc., and an arbitrary list of modules.

                                    B This modular structure makes it easy to adapt stage 1.5 to size restric-

                                 • GRUB 2 no longer includes an explicit stage 2; advanced functionality will
                                   be provided by modules and loaded on demand by stage 1.5. The modules
                                   can be found in /boot/grub , and the configuration file in /boot/grub/grub.cfg .

                              B On UEFI-based systems, the boot loader sits on the ESP in a file called
                                EFI/ ⟨operating system⟩/grubx64.efi , where ⟨operating system⟩ is something
                                like debian or fedora . Have a look at the /boot/efi/EFI directory on your
                                UEFI”=based Linux system.

                              B Again, the “x64 ” in “grubx64.efi ” stands for “64-bit PC”.
         configuration file      The configuration file for GRUB 2 looks markedly different from that for GRUB
                              Legacy, and is also rather more complicated (it resembles a bash script more than
                              a GRUB Legacy configuration file). The GRUB 2 authors assume that system man-
                              agers will not create and maintain this file manually. Instead there is a command
             grub-mkconfig    called grub-mkconfig which can generate a grub.cfg file. To do so, it makes use of
                              a set of auxiliary tools (shell scripts) in /etc/grub.d , which, e. g., search /boot for
                              Linux kernels to add to the GRUB boot menu. (grub-mkconfig writes the new con-
               update-grub    figuration file to its standard output; the update-grub command calls grub-mkconfig
                              and redirects its output to /boot/grub/grub.cfg .)
                                 You should therefore not modify /boot/grub/grub.cfg directly, since your distri-
                              bution is likely to invoke update-grub after, e. g., installing a kernel update, which
                              would overwrite your changes to grub.cfg .
                                 Usually you can, for instance, add more items to the GRUB 2 boot menu by
                              editing the /etc/grub.d/40_custom file. grub-mkconfig will copy the content of this file
                              verbatim into the grub.cfg file. As an alternative, you could add your configuration
                              settings to the /boot/grub/custom.cfg file, which will be read by grub.cfg if it exists.
                                 For completeness’ sake, here is an excerpt from a typical grub.cfg file. By anal-
                              ogy to the example in Section 8.2.2, a menu entry to start Linux might look like
                              this for GRUB 2:
                              menuentry 'Linux' --class gnu-linux --class os {
                                insmod gzio
                                insmod part_msdos
                                insmod ext2
8.3 Kernel Parameters                                                                                       135

    set root='(hd0,msdos2)'
    linux /boot/vmlinuz root=/dev/hda2
    initrd /boot/initrd.img

(grub-mkconfig usually produces more complicated stuff.) Do note that the GRUB
modules for decompression (gzio ), for MS-DOS-like partitioning support (part_msdos )
and the ext2 file system must be loaded explicitly. With GRUB 2, partition num-
bering starts at 1 (it used to be 0 for GRUB Legacy), so (hd0,msdos2) refers to the
second MS-DOS partition on the first hard disk. Instead of kernel , linux is used to
start a Linux kernel.

8.2.5     Security Advice
The GRUB shell offers many features, in particular access to the file system with-
out the root password! Even entering boot parameters may prove dangerous since boot parameters
it is easy to boot Linux directly into a root shell. GRUB makes it possible to close
these loopholes by requiring a password.                                               password
    For GRUB Legacy, the password is set in the menu.lst file. Here, the entry
“password --md5 ⟨encrypted password⟩” must be added to the global section. You
can obtain the encrypted password via the grub-md5-crypt command (or “md5crypt ”
within the GRUB shell) and then use, e. g., the GUI to “copy and paste” it to the file.
Afterwards, the password will need to be input whenever something is changed
interactively in the GRUB menu.

B You can also prevent particular systems from being booted by adding the
  lock option to the appropriate specific section within menu.lst . GRUB will
  query for the password when that system is to be booted. All other systems
  can still be started without a password.

C 8.2 [2] Which file contains your boot loader’s configuration? Create a new
  entry that will launch another operating system. Make a backup copy of the
  file first.

C 8.3 [!3] Prevent a normal user from circumventing init and booting directly
  into a shell. How do you generate a password request when a particular
  operating system is to be booted?

8.3      Kernel Parameters
Linux can accept a command line from the boot loader and evaluate it during the
kernel start procedure. The parameters on this command line can configure de-
vice drivers and change various kernel options. This mechanism for Linux kernel Linux kernel runtime configura-
runtime configuration is particularly helpful with the generic kernels on Linux tion
distribution boot disks, when a system with problematic hardware needs to be
booted. To do this, LILO supports the append= …option, while GRUB lets you ap-
pend parameters to the kernel specification.
   Alternatively, you can enter parameters interactively as the system is being
booted. You may have to grab GRUB’s attention quickly enough (e. g., by press-
ing a cursor or shift key while the boot menu or splash screen is displayed). Af-
terwards you can navigate to the desired menu entry and type e . GRUB then
presents you with the desired entry, which you can edit to your heart’s content
before continuing the boot operation.
   There are various types of parameters. The first group overwrites hardcoded
defaults, such as root or rw . Another group of parameters serves to configure de- configuring device drivers
136                                                                                                8 Booting Linux

                         vice drivers. If one of these parameters occurs on the command line, the initial-
                         isation function for the device driver in question is called with the arguments
                         specified there rather than the built-in default values.

                         B Nowadays most Linux distributions use modular kernels that have only
                           very few device drivers built in. Modular device drivers cannot be con-
                           figured from the kernel command line.

                         B During booting, if there are problems with a device driver that is built into
                           the kernel, you can usually disable this driver by specifying the number 0
                           as the parameter for the corresponding boot parameter.

      general settings       Finally, there are parameters governing general settings. These include, e. g.,
                         init or reserve .We shall be discussing some typical parameters from the multitude
                         of possible settings. Further parameters can be found within the kernel sources’
                         documentation area. Specific details for particular hardware must be researched
                         in the manual or on the Internet.
                         ro   This causes the kernel to mount the root partition read-only
                         rw   This causes the kernel to mount the root partition with writing enabled, even if
                                the kernel executable or the boot loader configuration file specify otherwise
                         init= ⟨program⟩     Runs ⟨program⟩ (e. g., /bin/bash ) instead of the customary /sbin/init
                         ⟨runlevel⟩ Boots into runlevel ⟨runlevel⟩, where ⟨runlevel⟩ is generally a number
                               between 1 and 5. Otherwise the initial runlevel is taken from /etc/inittab .
                               (Irrelevant for computers running systemd.)
                         single   Boots to single-user mode.
                         maxcpus= ⟨number⟩   On a multi-processor (or, nowadays, multi-core) system, use
                                  only as many CPUs as specified. This is useful for troubleshooting or per-
                                  formance measurements.
                         mem= ⟨size⟩  Specifies the amount of memory to be used. On the one hand, this is
                                  useful if the kernel cannot recognise the correct size by itself (fairly unlikely
                                  these days) or you want to check how the system behaves with little mem-
                                  ory. The ⟨size⟩ is a number, optionally followed by a unit (“TokenG” for
                                  gibibytes, “M ” for mebibytes, or “K ” for kibibytes).

                                A A typical mistake is something like mem=512 . Linux is thrifty about sys-
                                  tem resources, but even it can’t quite squeeze itself into 512 bytes (!) of

                         panic= ⟨seconds⟩  Causes an automatic reboot after ⟨seconds⟩ in case of a catastrophic
                                  system crash (called a “kernel panic” in the patois, Edsger Dijkstra’s dictum,
                                  “The use of anthropomorphic terminology when dealing with computing
                                  systems is a symptom of professional immaturity”, notwithstanding).
                         hd 𝑥=noprobe  Causes the kernel to ignore the disk-like device /dev/hd 𝑥 (IDE disk, CD-
                                  ROM, …) completely. It is not sufficient to disable the device in the BIOS, as
                                  Linux will find and access it even so.
                         noapic    and similar parameters like nousb , apm=off , and acpi=off tell Linux not to use
                                  certain kernel features. These options can help getting Linux to run at all
                                  on unusual computers, in order to analyse problems in these areas more
                                  thoroughly and sort them out.
                         A complete list of all parameters available on the kernel command line is given in
                         the file Documentation/kernel- parameters.txt , which is part of the Linux source code.
                         (However, before you install kernel sources just to get at this file, you should prob-
                         ably look for it on the Internet.)
8.4 System Startup Problems                                                                                              137

B Incidentally, if the kernel notices command-line options that do not corre-
  spond to kernel parameters, it passes them to the init process as environ-              init   environment variables
  ment variables.

8.4      System Startup Problems
8.4.1     Troubleshooting
Usually things are simple: You switch on the computer, stroll over to the coffee
machine (or not—see Section 9.1), and when you come back you are greeted by
the graphical login screen. But what to do if things don’t work out that way?
   The diagnosis of system startup problems sometimes isn’t all that easy—all
sorts of messages zoom by on the screen or (with some distributions) are not dis-
played at all, but hidden behind a nice little picture. The system logging service
(syslogd ) is also started only after a while. Fortunately, though, Linux does not
leave you out in the cold if you want to look at the kernel boot messages at leisure.
   For logging purposes, the system startup process can be divided into two
phases. The “early” phase begins with the first signs of life of the kernel and
continues up to the instant where the system logging service is activated. The
“late” phase begins just then and finishes in principle when the computer is shut
   The kernel writes early-phase messages into an internal buffer that can be dis-
played using the dmesg command. Various distributions arrange for these messages
to be passed on to the system logging service as soon as possible so they will show
up in the “official” log.
   The system logging service, which we are not going to discuss in detail here,
runs during the “late” phase. It will be covered in the Linup Front training man-
ual, Linux Administration II (and the LPI-102 exam). For now it will be sufficient
to know that most distribution place most messages sent to the system logging
service into the /var/log/messages file. This is also where messages from the boot
process end up if they have been sent after the logging service was started.

        On Debian GNU/Linux, /var/log/messages contains only part of the system
        messages, namely anything that isn’t a grave error message. If you would
        like to see everything you must look at /var/log/syslog —this contains all mes-
        sages except (for privacy reasons) those dealing with authentication. The
        “early phase” kernel messages, too, incidentally.

B Theoretically, messages sent after init was started but before the system log-
  ging service was launched might get lost. This is why the system logging
  service is usually among the first services started after init .

8.4.2     Typical Problems
Here are some of the typical snags you might encounter on booting:
The computer does not budge at all If your computer does nothing at all, it
     probably suffers from a hardware problem. (If you’re diagnosing such a
     case by telephone, then do ask the obvious questions such as “Is the power
     cable plugged into the wall socket?”—perhaps the cleaning squad was des-
     perate to find a place to plug in their vacuum cleaner—, and “Is the power
     switch at the back of the case switched to On?”. Sometimes the simple
     things will do.) The same is likely when it just beeps or flashes its LEDs
     rhythmically but does not appear to actually start booting.

      B The beeps or flashes can allow the initiated to come up with a rough di-
        agnosis of the problem. Details of hardware troubleshooting, though,
        are beyond the scope of this manual.
138                                                                      8 Booting Linux

      Things go wrong before the boot loader starts The firmware performs various
           self-tests and outputs error messages to the screen if things are wrong (such
           as malfunctioning RAM chips). We shall not discuss how to fix these prob-
           lems. If everything works fine, your computer ought to identify the boot
           disk and launch the boot loader.

      The boot loader does not finish This could be because the operating system can-
           not find it (e. g., because the drive it resides on does not show up in the
           firmware boot order) or it is damaged. In the former case you should ensure
           that your firmware does the Right Thing (not our topic). In the latter case
           you should receive at least a rudimentary error message, which together
           with the boot loader’s documentation should allow you to come up with an

           B GRUB as a civilised piece of software produces clear-text error mes-
             sages which are explained in more detail in the GRUB info documen-

            The cure for most of the fundamental (as opposed to configuration-related)
            boot loader problems, if they cannot be obviously reduced to disk or BIOS
            errors, consist of booting the system from CD-ROM—the distribution’s
            “rescue system” or a “live distribution” such as Knoppix recommend
            themselves—and to re-install the boot loader.

           B The same applies to problems like a ruined partition table in the MBR.
             Should you ever accidentally overwrite your MBR, you can restore a
             backup (you do have one, don’t you?) using dd or re-instate the par-
             titioning using sfdisk (you do have a printout of your partition table
             stashed away somewhere, don’t you?) and rewrite the boot loader.

           B In case of the ultimate worst-case partition table scenario, there are
             programs which will search the whole disk looking for places that look
             like file system superblocks, and (help) recover the partition scheme
             that way. We’re keeping our fingers crossed on your behalf that you
             will never need to run such a program.

      The kernel doesn’t start Once the boot loader has done its thing the kernel
           should at least start (which generally leads to some activity on the screen).
           Distribution kernels are generic enough to run on most PCs, but there may
           still be problems, e. g., if you have a computer with extremely modern hard-
           ware which the kernel doesn’t yet support (which is fatal if, for example, a
           driver for the disk controller is missing) or you have messed with the initial
           RAM disk (Shame, if you didn’t know what you were doing!). It may be
           possible to reconfigure the BIOS (e. g., by switching a SATA disk controller
           into a “traditional” IDE-compatible mode) or to deactivate certain parts of
           the kernel (see Section 8.3) in order to get the computer to boot. It makes
           sense to have another computer around so you can search the Internet for
           help and explanations.

           B If you are fooling around with the kernel or want to install a new ver-
             sion of your distribution kernel, do take care to have a known-working
             kernel around. If you always have a working kernel in your boot
             loader menu, you can save yourself from the tedious job of slinging
             CDs about.

      Other problems Once the kernel has finished its initialisations, it hands control
           off to the “init” process. You will find out more about this in Chapter 9.
           However, you should be out of the woods by then.
8.4 System Startup Problems                                                             139

8.4.3     Rescue systems and Live Distributions
As a system administrator, you should always keep a “rescue system” for your
distribution handy, since usually you need it exactly when you are least in a posi-
tion to obtain it quickly. (This applies in particular if your Linux machine is your
only computer.) A rescue system is a pared-down version of your distribution
which you can launch from a CD or DVD (formerly a floppy disk or disks) and
which runs in a RAM disk.

B Should your Linux distribution not come with a separate rescue system on
  floppy disk or CD, then get a “live distribution” such as Knoppix. Live dis-
  tributions are started from CD (or DVD) and work on your computer with-
  out needing to be installed first. You can find Knoppix as an ISO image on or, every so often, as a freebie with computer maga-

   The advantage of rescue systems and live distributions consists in the fact that
they work without involving your hard disk. Thus you can do things like fsck
your root file system, which are forbidden while your system is running from
hard disk. Here are a few problems and their solutions:
Hosed the kernel? Boot the rescue system and re-install the corresponding pack-
    age. In the simplest case, you can enter your installed system’s root file from
    the rescue system like so:

        # mount -o rw /dev/sda1 /mnt                         Device name may differ
        # chroot /mnt
        # _                              We are now seeing the installed distribution

        After this you can activate the network interface or copy a kernel package
        from a USB key or CD-ROM and install it using the package management
        tool of your distribution.

Forgot the root password? Boot the rescue system and change to the installed dis-
     tribution as above. Then do
        # passwd

        (You could of course fix this problem without a rescue system by restarting
        your system with “init=/bin/bash rw ” as a kernel parameter.)

B Live distributions such as Knoppix are also useful to check in the computer
  store whether Linux supports the hardware of the computer you have been
  drooling over for a while already. If Knoppix recognises a piece of hardware,
  you can as a rule get it to run with other Linux distributions too. If Knoppix
  does not recognise a piece of hardware, this may not be a grave problem
  (there might be a driver for it somewhere on the Internet that Knoppix does
  not know about) but you will at least be warned.

B If there is a matching live version of your distribution—with Ubuntu, for
  example, the live CD and the installation CD are identical—, things are es-
  pecially convenient, since the live distribution will typically recognise the
  same hardware that the installable distribution does.
140                                                                       8 Booting Linux

      Commands in this Chapter
      dmesg    Outputs the content of the kernel message buffer             dmesg (8) 137
      grub-md5-crypt Determines MD5-encrypted passwords for       GRUB Legacy
                                                                  grub-md5-crypt (8) 135

         • A boot loader is a program that can load and start an operating system.
         • A boot manager is a boot lader that lets the user pick one of several operating
           systems or operating system installations.
         • GRUB is a powerful boot manager with special properties—such as the pos-
           sibility of accessing arbitrary files and a built-in command shell.
         • The GRUB shell helps to install GRUB as well as to configure individual boot
                                                                                                             $ echo tux
                                                                                                             $ ls
                                                                                                             $ /bin/su -

System-V Init and the Init Process

9.1       The Init Process . . . .     .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   142
9.2       System-V Init . . . . .      .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   142
9.3       Upstart . . . . . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   148
9.4       Shutting Down the System     .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   150

      •   Understanding the System-V Init infrastructure
      •   Knowing /etc/inittab structure and syntax
      •   Understanding runlevels and init scripts
      •   Being able to shut down or restart the system orderly

      • Basic Linux system administration knowledge
      • Knowledge of system start procedures (Chapter 8)

adm1-init.tex    (33e55eeadba676a3 )
142                                                                   9 System-V Init and the Init Process

                     9.1      The Init Process
                     After the firmware, the boot loader, the operating system kernel and (possibly)
                     the early userspace have done their thing, the “init process” takes over the reins.
                     Its job is to finish system initialisation and supervise the ongoing operation of the
        /sbin/init   system. For this, Linux locates and starts a program called /sbin/init .

                     B The init process always has process ID 1. If there is an early userspace, it in-
                       herits this from the process that was created to run /init , and subsequently
                       goes on to replace its program text by that of the init process.

                     B Incidentally, the init process enjoys a special privilege: it is the only pro-
                       cess that cannot be aborted using “kill -9 ”. (It can decide to shuffle off this
                       mortal coil of its own accord, though.)

                     B If the init process really quits, the kernel keeps running. There are purists
                       who start a program as the init process that will set up packet filtering rules
                       and then exit. Such a computer makes a practically impregnable firewall,
                       but is somewhat inconvenient to reconfigure without a rescue system …

                     B You can tell the Linux kernel to execute a different program as the init pro-
                       cess by specifying an option like “init=/sbin/myinit ” on boot. There are no
                       special properties that this program must have, but you should remember
                       that, if it ever finishes, you do not get another one without a reboot.

                     9.2      System-V Init
                     Basics   The traditional infrastructure that most Linux distributions used to use
      System-V init is called “System-V init” (pronounced “sys-five init”). The “V” is a roman nu-
                     meral 5, and it takes its name from the fact that it mostly follows the example of
                     Unix System V, where something very similar showed up for the first time. That
                     was during the 1980s.

                     B For some time there was the suspicion that an infrastructure designed ap-
                       proximately 30 years ago was no longer up to today’s demands on a Linux
                       computer’s init system. (Just as a reminder: When System-V init was new,
                       the typical Unix system was a VAX with 30 serial terminals.) Modern com-
                       puters must, for example, be able to deal with frequent changes of hard-
                       ware (cue USB), and that is something that System-V init finds relatively
                       difficult to handle. Hence there were several suggestions for alternatives
                       to System-V init. One of these—systemd by Lennart Poettering and Kay
                       Sievers—seems to have won out and is the current or upcoming standard of
                       practically all Linux distributions of importance (we discuss it in more detail
                       in chapter 10). Another is Upstart by Scott James Remnant (see section 9.3).

         runlevels      One of the characteristic features of System-V init are runlevels, which describe
                     the system’s state and the services that it offers. Furthermore, the init process en-
                     sures that users can log in on virtual consoles, directly-connected serial terminals,
                     etc., and manages system access via modems if applicable. All of this is configured
                     by means of the /etc/inittab file
      /etc/inittab      The syntax of /etc/inittab (Figure 9.1), like that of many other Linux configu-
                     ration files, is somewhat idiosyncratic (even if it is really AT&T’s fault). All lines
                     that are not either empty or comments— starting with “# ” as usual—consist of
                     four fields separated by colons:
                     Label The first field’s purpose is to identify the line uniquely. You may pick an
                          arbitrary combination of up to four characters. (Do yourself a favour and
                          stick with letters and digits.) The label is not used for anything else.
9.2 System-V Init                                                      143

# Standard runlevel

# First script to be executed

# runlevels
l0:0:wait:/etc/init.d/rc 0
l1:1:wait:/etc/init.d/rc 1
l2:2:wait:/etc/init.d/rc 2
l3:3:wait:/etc/init.d/rc 3
#l4:4:wait:/etc/init.d/rc 4
l5:5:wait:/etc/init.d/rc 5
l6:6:wait:/etc/init.d/rc 6

ls:S:wait:/etc/init.d/rc S

# Ctrl-Alt-Del
ca::ctrlaltdel:/sbin/shutdown -r -t 4 now

# Terminals
1:2345:respawn:/sbin/mingetty   --noclear tty1
2:2345:respawn:/sbin/mingetty   tty2
3:2345:respawn:/sbin/mingetty   tty3
4:2345:respawn:/sbin/mingetty   tty4
5:2345:respawn:/sbin/mingetty   tty5
6:2345:respawn:/sbin/mingetty   tty6

# Serial terminal
# S0:12345:respawn:/sbin/agetty -L 9600 ttyS0 vt102

# Modem
# mo:235:respawn:/usr/sbin/mgetty -s 38400 modem

                    Figure 9.1: A typical /etc/inittab file (excerpt)
144                                                       9 System-V Init and the Init Process

           B This is not 100% true for lines describing terminals, where according
             to convention the label corresponds to the name of the device file in
             question, but without the “tty ” at the beginning, hence 1 for tty1 or S0
             for ttyS0 . Nobody knows exactly why.

      Runlevels The runlevels this line applies to. We haven’t yet explained in detail
           how runlevels work, so excuse us for the moment for limiting ourselves to
           telling you that they are usually named with digits and the line in question
           will be considered in all runlevels whose digit appears in this field.

           B In addition to the runlevels with digits as names there is one called “S ”.
             More details follow below.

      Action The third field specifies how to handle the line. The most important pos-
           sibilities include
            respawn    The process described by this line will immediately be started again
                    once it has finished. Typically this is used for terminals which, after the
                    current user is done with their session, should be presented brand-new
                    to the next user.
            wait    The process described by this line is executed once when the system
                    changes to the runlevel in question, and init waits for it to finish.
            bootwait    The process described by this line will be executed once during
                    system startup. init waits for it to finish. The runlevel field on this line
                    will be ignored.
            initdefault   The runlevel field of this line specifies which runlevel the system
                    shoud try to reach after booting.
                   B With LSB-compliant distributions, this field usually says “5 ” if the
                     system should accept logins on the graphical screen, otherwise
                     “3 ”. See below for details.
                   B If this entry (or the whole file /etc/inittab ) is missing, you will need
                     to state a run level on the console.
            ctrlaltdel    Specifies what the system should do if the init process is being
                    sent a SIGINT —which usually happens if anyone presses the Ctrl + Alt
                    + Del combination. Normally this turns out to be some kind of shutdown
                    (see Section 9.4).

           B There are a few other actions. powerwait , powerfail , powerokwait , and
             powerfailnow , for example, are used to interface System-V init with
             UPSs. The details are in the documentation (init (8) and inittab (5)).

      Command The fourth field describes the command to be executed. It extends to
          the end of the line and you can put whatever you like.
         If you have made changes to /etc/inittab , these do not immediately take effect.
      You must execute the “telinit q ” command first in order to get init to reread the
      configuration file.

      The Boot Script With System-V init, the init process starts a shell script, the boot
      script, typically /etc/init.d/boot (Novell/SUSE), /etc/rc.d/init.d/boot (Red Hat), or
      /etc/init.d/rcS (Debian). (The exact name occurs in /etc/inittab ; look for an entry
      whose action is bootwait .)
         The boot script performs tasks such as checking and possibly correcting the
      file systems listed in /etc/fstab , initialising the system name and Linux clock, and
      other important prerequisites for stable system operation. Next, kernel modules
      will be loaded if required, file systems mounted and so on. The specific actions
      and their exact order depend on the Linux distribution in use.
9.2 System-V Init                                                                                          145

B Today, boot usually confines itself to executing the files in a directory such as
  /etc/init.d/boot.d (SUSE) in turn. The files are executed in the order of their
  names. You can put additional files in this directory in order to execute
  custom code during system initialisation.

C 9.1 [2] Can you find out where your distribution keeps the scripts that the
  boot script executes?

C 9.2 [!2] Name a few typical tasks performed by the boot script. In which
  order should these be executed?

Runlevels After executing the boot script, the init process attempts to place the
system in one of the various runlevels. Exactly which one is given by /etc/inittab runlevels
or determined at the system’s boot prompt and passed through to init by the
   The various runlevels and their meaning have by now been standardised across standardised runlevels
most distributions roughly as follows:
1 Single-user mode with no networking

2 Multi-user mode with no network servers
3 Multi-user mode with network servers
4 Unused; may be configured individually if required

5 As runlevel 3, but with GUI login
6 Reboot
0 System halt

B The system runs through the S (or s ) runlevel during startup, before it
  changes over to one out of runlevels 2 to 5. If you put the system into
  runlevel 1 you will finally end up in runlevel S .

When the system is started, the preferred runlevels are 3 or 5—runlevel 5 is typical
for workstations running a GUI, while runlevel 3 makes sense for server systems
that may not even contain a video interface. In runlevel 3 you can always start a
GUI afterwards or redirect graphics output to another computer by logging into
your server from that machine over the network.

      These predefined runlevels derive from the LSB standard. Not all distribu-
      tions actually enforce them; Debian GNU/Linux, for example, mostly leaves
      runlevel assignment to the local administrator.

B You may use runlevels 7 to 9 but you will have to configure them yourself.
   During system operation, the runlevel can be changed using the telinit com-         telinit   command
mand. This command can only be executed as root : “telinit 5 ” changes imme-
diately to runlevel ⟨runlevel⟩. All currently running services that are no longer
required in the new runlevel will be stopped, while non-running services that are
required in the new runlevel will be started.

B You may use init in place of telinit (the latter is just a symbolic link to the
  former, anyway). The program checks its PID when it starts, and if it is not 1,
  it behaves like telinit , else init .

   The runlevel command displays the previous and current runlevel:                    runlevel
146                                                                             9 System-V Init and the Init Process

                           # runlevel
                           N 5

                          Here the system is currently in runlevel 5, which, as the value “N ” for the “previous
                          runlevel” suggests, was entered right after the system start. Output such as “5 3 ”
                          would mean that the last runlevel change consisted of bringing the system from
                          runlevel 5 to runlevel 3.

                           B We have concealed some more runlevels from you, namely the “on-demand
                             runlevels” A , B , and C . You may make entries in /etc/inittab which are meant
                             for any of these three runlevels and use the ondemand action, such as


                                 If you say something like

                                  # telinit A

                                 these entries are executed, but the actual runlevel does not change: If you
                                 were in runlevel 3 before the telinit command, you will still be there when
                                 it finishes. a , b , and c are synonyms for A , B , and C.

                           C 9.3 [!2] Display the current runlevel. What exactly is being output? Change
                             to runlevel 2. Check the current runlevel again.

                           C 9.4 [2] Try the on-demand runlevels: Add a line to /etc/inittab which ap-
                             plies to, e. g., runlevel A . Make init reread the inittab file. Then enter the
                             »telinit A « command.

                             Init Scripts The services available in the various runlevels are started and
                             stopped using the scripts in the /etc/init.d (Debian, Ubuntu, SUSE) or /etc/
                             rc.d/init.d (Red Hat) directories. These scripts are executed when changing from
                             one runlevel to another, but may also be invoked manually. You may also add
                init scripts your own scripts. All these scripts are collectively called init scripts.
                                 The init scripts>parametersinit scripts usually support parameters such asstart ,
                             stop , status , restart , or reload , which you can use to start, stop, …, the correspond-
                             ing services. The “/etc/init.d/network restart ” command might conceivably deac-
                             tivate a system’s network cards and restart them with an updated configuration.
                                 Of course you do not need to start all services manually when the system is
      runlevel directories started or you want to switch runlevels. For each runlevel 𝑟 there is a rc 𝑟.d di-
                             rectory in /etc (Debian and Ubuntu), /etc/rc.d (Red Hat), or /etc/init.d (SUSE).
                             The services for each runlevel and the runlevel transitions are defined in terms of
                             these directories, which contain symbolic links to the scripts in the init.d direc-
                             tory. These links are used by a script, typically called /etc/init.d/rc , to start and
                             stop services when a runlevel is entered or exited.
                                 This is done according to the names of the links, in order to determine the start-
                             ing and stopping order of the services. There are dependencies between various
                             services—there would not be much point in starting network services such as the
                             Samba or web servers before the system’s basic networking support has been ac-
       Activating services tivated. The services for a runlevel are activated by calling all symbolic links in
                             its directory that start with the letter “S ”, in lexicographical order with the start
                             parameter. Since the link names contain a two-digit number after the “S ”, you
                             can predetermine the order of invocation by carefully choosing these numbers.
                             Accordingly, to deactivate the services within a runlevel, all the symbolic links
                             starting with the letter “K ” are called in lexicographical order with the stop pa-
9.2 System-V Init                                                                                            147

   If a running service is also supposed to run in the new run level, an extraneous
restart can be avoided. Therefore, before invoking a K link, the rc script checks
whether there is an S link for the same service in the new runlevel’s directory. If
so, the stopping and immediate restart are skipped.

      Debian GNU/Linux takes a different approach: Whenever a new runlevel
      𝑟 is entered, all symbolic links in the new directory (/etc/rc 𝑟.d ) are executed.
      Links beginning with a “K ” are passed stop and links beginning with a “S ”
      are passed start as the parameter.

   To configure services in a runlevel or to create a new runlevel, you can in princi- Configuring services
ple manipulate the symbolic links directly. However, most distributions deprecate

      The Red Hat distributions use a program called chkconfig to configure run-
      levels. “chkconfig quota 35 ”, for example, inserts the quota service not in run-
      level 35, but runlevels 3 and 5. “chkconfig -l ” gives a convenient overview
      of the configured runlevels.

      The SUSE distributions use a program called insserv to order the services
      in each runlevel. It uses information contained in the init scripts to calcu-
      late a sequence for starting and stopping the services in each runlevel that
      takes the dependencies into account. In addition, YaST2 offers a graphical
      “runlevel editor”, and there is a chkconfig program which however is just a
      front-end for insserv .

      Nor do you have to create links by hand on Debian GNU/Linux—you
      may use the update-rc.d program. However, manual intervention is still
      allowed—update-rc.d ’s purpose is really to allow Debian packages to inte-
      grate their init scripts into the boot sequence. With the

       # update-rc.d mypackage defaults

      command, the /etc/init.d/mypackage script will be started in runlevels 2, 3, 4,
      and 5 and stopped in runlevels 0, 1 and 6. You can change this behaviour by
      means of options. If you do not specify otherwise, update-rc.d uses the se-
      quence number 20 to calculate the position of the service—contrary to SUSE
      and Red Hat, this is not automated.—The insserv command is available on
      Debian GNU/Linux as an optional package; if it is installed, it can man-
      age at least those init scripts that do contain the necessary metadata like it
      would on the SUSE distributions. However, this has not been implemented

C 9.5 [!2] What do you have to do to make the syslog service reread its config-

C 9.6 [1] How can you conveniently review the current runlevel configura-

C 9.7 [!2] Remove the cron service from runlevel 2.

Single-User Mode In single-user mode (runlevel S ), only the system administra- single-user mode
tor may work on the system console. There is no way of changing to other virtual
consoles. The single-user mode is normally used for administrative work, espe-
cially if the file system needs to be repaired or the quota system set up.
148                                                    9 System-V Init and the Init Process

      B You can mount the root file system read-only on booting, by passing the S
        option on the kernel command line. If you boot the system to single-user
        mode, you can also disable writing to the root file system “on the fly”, using
        the remount and ro mount options: “mount -o remount,ro / ” remounts the root
        partition read-only; “mount -o remount,rw / ” undoes it again.

      B To remount a file system “read-only” while the system is running, no pro-
        cess may have opened a file on the file system for writing. This means that
        all such programs must be terminated using kill . These are likely to be
        daemons such as syslogd or cron .

        It depends on your distribution whether or not you get to leave single-user
      mode, and how.

            To leave single-user mode, Debian GNU/Linux recommends a reboot
            rather than something like »telinit 2 «. This is because entering single-
            user mode kills all processes that are not required in signle-user mode.
            This removes some essential background processes that were started when
            the system passed through runlevel S during boot, which is why it is unwise
            to change from runlevel S to a multi-user runlevel.

      C 9.8 [!1] Put the system into single-user mode (Hint: telinit ). What do you
        need to do to actually enter single-user mode?

      C 9.9 [1] Convince yourself that you really are the single user on the system
        while single-user mode is active, and that no background processes are run-

      9.3    Upstart
      While System-V init traditionally stipulates a “synchronous” approach—the init
      system changes its state only through explicit user action, and the steps taken
      during a state change, like init scripts, are performed in sequence—, Upstart uses
      an “event-based” philosophy. This means that the system is supposed to react to
      external events (like plugging in an USB device). This happens “asynchronously”.
      Starting and stopping services creates new events, so that—and that is one of the
      most important differences between System-V init and Upstart—a service can be
      restarted automatically if it crashes unexpectedly. (System-V init, on the other
      hand, wouldn’t be bothered at all.)
         Upstart has been deliberately designed to be compatible with System-V init, at
      least to a point where init scripts for services can be reused without changes.

            Upstart was developed by Scott James Remnant, at the time an employee of
            Canonical (the company behind Ubuntu) and accordingly debuted in that
            distributon. Since Ubuntu 6.10 (“Edgy Eft”) it is the standard init system on
            Ubuntu, although it used to be run in a System-V compatible mode at first;
            since Ubuntu 9.10 (“Karmic Koala”) it is running in “native” mode.

            It turns out that Ubuntu is currently in the process of switching over to sys-
            temd (see chapter 10).

            Since version 3.5 of the LPIC-1 certificate exams (as of 2 July 2012) you are
            expected to know that Upstart exists and what its major properties are. Con-
            figuration and operational details are not required.
9.3 Upstart                                                                               149

# rsyslog - system logging daemon
# rsyslog is an enhanced multi-threaded replacement for the traditional
# syslog daemon, logging messages from applications

description      "system logging daemon"

start on filesystem
stop on runlevel [06]

expect fork

exec rsyslogd -c4

               Figure 9.2: Upstart configuration file for job rsyslog

B Upstart is also purported to accelerate the boot process by being able to
  initialise servides in parallel. In actual practice this isn’t the case, as the
  limiting factor during booting is, for the most part, the speed with which
  blocks of data can be moved from disk to RAM. At the Linux Plumbers
  Conference 2008, Arjan van de Ven and Auke Kok demonstrated that it is
  possible to boot an Asus EeePC all the way to a usable desktop (i. e., not a
  Windows-like desktop with a churning hard disk in the background) within
  5 seconds. This work was based on System-V init rather than Upstart.

   Upstart configuration is based on the idea of “Jobs” that take on the role of Jobs
init scripts (although init scripts, as we mentioned, are also supported). Upstart
distinguishes “tasks”—jobs that run for a limited time and then shut themselves
down—and “services”—jobs that run permanently “in the background”.

B Tasks can be long-running, too. The main criterion is that services—think
  of a mail, database, or web server—do not terminate of their own accord
  while tasks do.

Jobs are configured using files within the /etc/init directory. The names of these
files derive from the job name and the “.conf ” suffix. See figure 9.2 for an example.
    One of the main objectives of Upstart is to avoid the large amounts of template-
like code typical for most System-V init scripts. Accordingly, the Upstart configu-
ration file confines itself to stating how the service is to be started (“exec rsyslogd
-c4 ”). In addition, it specifies that the service is to be restarted in case it crashes
(“respawn ”) and how Upstart can find out which process to track (“expect fork ” says
that the rsyslog process puts itself into the background by creating a child process
and then exiting—Upstart must then watch out for that child process).—Compare
this to /etc/init.d/syslogd (or similar) on a typical Linux based on System-V init.
    While with “classic” System-V init the system administrator assigns a “global”
order in which the init scripts for a particular runlevel are to be executed, with
Upstart the jobs decide “locally” where they want to place themselves within a
network of dependencies. The “start on …” and “stop on …” lines stipulate events
that lead to the job being started or stopped. In our example, rsyslog is started as
soon as the file system is available, and stopped when the system transitions to
the “runlevels” 0 (halt) or 6 (reboot). System-V init’s runlevel directories with
symbolic links are no longer required.

B Upstart supports runlevels mostly for compatibility with Unix tradition and
  to ease the migration of System-V init based systems to Upstart. They are
  not required in principle, but at the moment are still necessary to shut down
  the system (!).
150                                                   9 System-V Init and the Init Process

      B Newer implementations of System-V init also try to provide dependencies
        between services in the sense that init script 𝑋 is always executed after init
        script 𝑌 and so on. (This amounts to a scheme for automatic assignment
        of the priority numbers within the runlevel directories.) This is done using
        metadata contained in standardised comments at the beginning of the init
        scripts. The facilities that this approach provides do fall short of those of
        Upstart, though.
         On system boot, Upstart creates the startup event as soon as its own initialisa-
      tion is complete. This makes it possible to execute other jobs. The complete boot
      sequence derives from the startup event and from events being created through
      the execution of further jobs and expected by others.

      B For example, on Ubuntu 10.04 the startup event invokes the mountall task
        which makes the file systems available. Once that is finished, the filesystem
        event is created (among others), which in turn triggers the start of the rsyslog
        service from Figure 9.2.
         With Upstart, the initctl command is used to interact with the init process:
      # initctl list                                        Which jobs are running now?
      alsa-mixer-save stop/waiting
      avahi-daemon start/running, process 578
      mountall-net stop/waiting
      rc stop/waiting
      rsyslog start/running, process 549
      # initctl stop rsyslog                                                   Stop a job
      rsyslog stop/waiting
      # initctl status rsyslog                                        What is its status?
      rsyslog stop/waiting
      # initctl start rsyslog                                               Restart a job
      rsyslog start/running, process 2418
      # initctl restart rsyslog                                            Stop and start
      rsyslog start/running, process 2432

      B The “initctl stop ”, “initctl start ”, “initctl status ”, and “initctl stop ” can
        be abbreviated to “stop ”, “start ”, ….

      9.4     Shutting Down the System
      A Linux computer should not simply be powered off, as that could lead to data
      loss—possibly there are data in RAM that ought to be written to disk but are still
      waiting for the proper moment in time. Besides, there might be users logged in on
      the machine via the network, and it would be bad form to surprise them with an
      unscheduled system halt or restart. The same applies to users taking advantage
      of services that the computer offers on the Net.

      B It is seldom necessary to shut down a Linux machine that should really run
        continuously. You can install or remove software with impunity and also re-
        configure the system fairly radically without having to restart the operating
        system. The only cases where this is really necessary include kernel changes
        (such as security updates) or adding new or replacing defective hardware
        inside the computer case.

      B The first case (kernel changes) is being worked on. The kexec infrastructure
        makes it possible to load a second kernel into memory and jump into it
        directly (without the detour via a system reboot). Thus it is quite possible
        that in the future you will always be able to run the newest kernel without
        actually having to reboot your machine.
9.4 Shutting Down the System                                                                        151

B With the correct kind of (expensive) hardware you can also mostly sort out
  the second case: Appropriate server systems allow you to swap CPUs, RAM
  modules, and disks in and out while the computer is running.

  There are numerous ways of shutting down or rebooting the system:
   • By valiantly pushing the system’s on/off switch. If you keep it pressed until on/off switch
     the computer is audibly shutting down the system will be switched off. You
     should only do this in cases of acute panic (fire in the machine hall or a
     sudden water influx).
   • Using the shutdown command. This is the cleanest method of shutting down          shutdown
     or rebooting.
   • For System-V init: The “telinit 0 ” command can be used to switch to run-
     level 0. This is equivalent to a shutdown.
   • Using the halt command. This is really a direct order to the kernel to halt the
     system, but many distributions arrange for halt to call shutdown if the system
     is not in runlevels 0 or 6 already.

     B There is a reboot command for reboots, which like halt usually relies on        reboot
       shutdown . (In fact, halt and reboot are really the same program.)

The commands are all restricted to the system administrator.

B The key combination Ctrl + Alt + Del may also work if it is configured ap-
  propriately in /etc/inittab (see Section 9.1).

B Graphical display managers often offer an option to shut down or reboot
  the system. You may have to configure whether the root password must be
  entered or not.

B Finally, modern PCs may interpret a (short) press on the on/off switch as
  “Please shut down cleanly” rather than “Please crash right now”.

   Normally you will be using the second option, the shutdown command. It en-
sures that all logged-in users are made aware of the impending system halt, pre-
vents new logins, and, according to its option, performs any requisite actions to
shut down the system:

# shutdown -h +10

for example will shut down the system in ten minutes’ time. With the -r option,
the system will be restarted. With no option, the system will go to single-user
mode after the delay has elapsed.

B You may also give the time of shutdown/reboot as an absolute time:
      # shutdown -h 12:00                                               High Noon

B For shutdown , the now keyword is a synonym of “+0 ”—immediate action. Do
  it only if you are sure that nobody else is using the system.

  Here is exactly what happens when the shutdown command is given:
   1. All users receive a broadcast message saying that the system will be shut broadcast message
      down, when, and why.
   2. The shutdown command automatically creates the /etc/nologin file, which is
      checked by login (or, more precisely, the PAM infrastructure); its existence
      prevents new user logins (except for root ).
152                                                            9 System-V Init and the Init Process

                  B For consolation, users that the system turns away are being shown the
                    content of the /etc/nologin file.

                   The file is usually removed automatically when the system starts up again.
                3. The system changes to runlevel 0 or 6. All services will be terminated by
                   means of their init scripts (more exactly, all services that do not occur in
                   runlevels 0 or 6, which is usually all of them).
                4. All still-running processes are first sent SIGTERM . They may intercept this sig-
                   nal and clean up after themselves before terminating.
                5. Shortly afterwards, all processes that still exist are forcibly terminated by
                   SIGKILL .

                6. The file systems are unmounted and the swap spaces are deactivated.
                7. Finally, all system activities are finished. Then either a warm start is initi-
                   ated or the computer shut off using APM or ACPI. If that doesn’t work, the
                   message “System halted ” is displayed on the console. At that point you can
                   hit the switch yourself.

             B You may pass some text to shutdown after the shut-down delay time, which
               is displayed to any logged-in users:

                    # shutdown -h 12:00 '
                    System halt for hardware upgrade.
                    Sorry for the inconvenience!

             B If you have executed shutdown and then change your mind after all, you can
               cancel a pending shutdown or reboot using

                    # shutdown -c "No shutdown after all"

                   (of course you may compose your own explanatory message).

                By the way: The mechanism that shutdown uses to notify users of an impending
      wall   system halt (or similar) is available for your use. The command is called wall (short
             for “write to all”):

             $ wall "Cake in the break room at 3pm!"

             will produce a message of the form

             Broadcast message from hugo@red (pts/1) (Sat Jul 18 00:35:03 2015):

             Cake in the break room at 3pm!

             on the terminals of all logged-in users.

             B If you send the message as a normal user, it will be received by all users who
               haven’t blocked their terminal for such messages using “mesg n ”. If you want
               to reach those users, too, you must send the message as root .

             B Even if you’re not logged in on a text terminal but are instead using a graphi-
               cal environment: Today’s desktop environments will pick up such messages
               and show them in an extra window (or something; that will depend on the
               desktop environment).

             B If you’re root and the parameter of wall looks like the name of an existing
               file, that file will be read and its content sent as the message:
9.4 Shutting Down the System                                                              153

       # echo "Cake in the break room at 3pm!" >cake.txt
       # wall cake.txt

      You don’t get to do this as an ordinary user, but you can still pass the mes-
      sage on wall ’s standard input. (You can do that as root , too, of course.) Don’t
      use this for War and Peace.

B If you’re root , you can suppress the header line “Broadcast message …” using
  the -n option (short for --nobanner ).

C 9.10 [!2] Shut down your system 15 minutes from now and tell your users
  that this is simply a test. How do you prevent the actual shutdown (so that
  it really is simply a test)?

B What happens if you (as root ) pass wall the name of a non-existent file as its

C 9.11 [2] wall is really a special case of the write command, which you can use
  to “chat” with other users of the same computer in an unspeakably primitive
  fashion. Try write , in the easiest case between two different users in different
  windows or consoles. (write was a lot more interesting back when one had
  a VAX with 30 terminals.)

Commands in this Chapter
chkconfig   Starts or shuts down system services (SUSE, Red Hat)
                                                                 chkconfig (8) 147
halt     Halts the system                                             halt (8) 151
initctl  Supervisory tool for Upstart                              initctl (8) 150
insserv  Activates or deactivates init scripts (SUSE)              insserv (8) 147
reboot   Restarts the computer                                      reboot (8) 151
runlevel  Displays the previous and current run level             runlevel (8) 145
shutdown  Shuts the system down or reboots it, with a delay and warnings for
         logged-in users                                          shutdown (8) 151
update-rc.d Installs and removes System-V style init script links (Debian)
                                                               update-rc.d (8) 147

   • After starting, the kernel initialises the system and then hands off control to
     the /sbin/init program as the first userspace process.
   • The init process controls the system and takes care, in particular, of acti-
     vating background services and managing terminals, virtual consoles, and
   • The system distinguishes various “runlevels” (operating states) that are de-
     fined through different sets of running services.
   • A single-user mode is available for large or intrusive administrative tasks.
   • The shutdown command is a convenient way of shutting down or rebooting
     the system (and it’s friendly towards other users, too).
   • You can use the wall command to send a message to all logged-in users.
   • Linux systems seldom need to be rebooted—actually only when a new op-
     erating system kernel or new hardware has been installed.
                                                                                                               $ echo tux
                                                                                                               $ ls
                                                                                                               $ /bin/su -


10.1   Overview. . . . . .          .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   156
10.2   Unit Files . . . . . .       .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   157
10.3   Unit Types . . . . .         .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   161
10.4   Dependencies . . . .         .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   162
10.5   Targets. . . . . . .         .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   164
10.6   The systemctl Command        .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   166
10.7   Installing Units. . . .      .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   169

   • Understanding the systemd infrastructure
   • Knowing the structure of unit files
   • Understanding and being able to configure targets

   • Knowledge of Linux system administration
   • Knowledge of system start procedures (Chapter 8)
   • Knowledge about System-V init (Chapter 9)

adm1-systemd.tex   (33e55eeadba676a3 )
156                                                                                          10 Systemd

                   10.1      Overview
                   Systemd, by Lennart Poettering and Kay Sievers, is another alternative to the old-
                   fashioned System-V init system. Like Upstart, systemd transcends the rigid lim-
                   itations of System-V init, but implements various concepts for the activation and
                   control of services with much greater rigour than Upstart.

                    B Systemd is considered the future standard init system by all mainstream
                      distributions. On many of them—such as Debian, Fedora, RHEL, CentOS,
                      openSUSE, and SLES—it is now provided by default. Even Ubuntu, origi-
                      nally the main instigator of Upstart, has by now declared for systemd.
                       While System-V init and Upstart use explicit dependencies among services—
                   for instance, services using the system log service can only be started once that
      dependencies service is running—, systemd turns the dependencies around. A service requiring
                   the system log service doesn’t do this because the log service needs to be running,
                   but because it itself wants to send messages to the system log. This means it must
                   access the communication channel that the system log service provides. Hence it
                   is sufficient if systemd itself creates that communication channel and passes it to
                   the system log service once that becomes available—the service wanting to send
                   messages to the log will wait until its messages can actually be accepted. Hence,
                   systemd can in principle create all communication channels first and then start
                   all services simultaneously without regard to any dependencies whatsoever. The
                   dependencies will sort themselves out without any explicit configuration.

                    B This approach also works when the system is running: If a service is ac-
                      cessed that isn’t currently running, systemd can start it on demand.

                    B The same approach can in principle also be used for file systems: If a service
                      wants to open a file on a file system that is currently unavailable, the access
                      is suspended until the file system can actually be accessed.
              units    Systemd uses “units” as an abstraction for parts of the system to be managed
            targets such as services, communication channels, or devices. “Targets” replace SysV
                   init’s runlevels and are used to collect related units. For example, there is a target
          that corresponds to the traditional runlevel 3. Targets can depend
                   on the availability of devices—for instance, a could be requested
                   when a USB Bluetooth adapter is plugged in, and it could launch the requisite
                   software. (System-V init starts the Bluetooth software as soon as it is configured,
                   irrespective of whether Bluetooth hardware is actually available.)
                      In addition, systemd offers more interesting properties that System-V init and
                   Upstart cannot match, including:
                       • Systemd supports service activation “on demand”, not just depending on
                         hardware that is recognised (as in the Bluetooth example above), but also
                         via network connections, D-Bus requests or the availability of certain paths
                         within the file system.
                       • Systemd allows very fine-grained control of the services it launches, con-
                         cerning, e. g., the process environment, resource limits, etc. This includes
                         security improvements, e. g., providing only a limited view on the file sys-
                         tem for certain services, or providing services with a private /tmp directory
                         or networking environment.

                         B With SysV init this can be handled on a case-by-case basis within the
                           init scripts, but by comparison this is very primitive and tedious.

                       • Systemd uses the Linux kernel’s cgroups mechanism to ensure, e. g., that
                         stopping a service actually stops all related processes.
                       • If desired, systemd handles services’ logging output; the services only need
                         to write messages to standard output.
10.2 Unit Files                                                                              157

   • Systemd makes configuration maintenance easier, by cleanly separating dis-
     tribution default and local customisations.
   • Systemd contains a number of tools in C that handle system initialisation
     and do approximately what distribution-specific “runlevel S” scripts would
     otherwise do. Using them can speed up the boot process considerably and
     also improves cross-distribution standardisation.
Systemd is designed to offer maximum compatibility with System-V init and other
“traditions”. For instance, it supports the init scripts of System-V init if no na-
tive configuration file is available for a service, or it takes the file systems to be
mounted on startup from the /etc/fstab file.
    You can use the systemctl command to interact with a running systemd, e. g.,
to start or stop services explicitly:

# systemctl status rsyslog.service
● rsyslog.service - System Logging Service
   Loaded: loaded (/lib/systemd/system/rsyslog.service; enabled)
   Active: active (running) since Do 2015-07-16 15:20:38 CEST; 
     3h 12min ago
     Docs: man:rsyslogd(8)
 Main PID: 497 (rsyslogd)
   CGroup: /system.slice/rsyslog.service
           └─497 /usr/sbin/rsyslogd -n
# systemctl stop rsyslog.service
Warning: Stopping rsyslog.service, but it can still be activated by:
# systemctl start rsyslog.service

Systemd calls such change requests for the system state “jobs”, and puts them into
a queue.

B Systemd considers status change requests “transactions”. If a unit is being transactions
  started or stopped, it (and any units that depend on it) are put into a tempo-
  rary transaction. Then systemd checks that the transaction is consistent—in
  particular, that no circular dependencies exist. If that isn’t the case, systemd
  tries to repair the transaction by removing jobs that are not essential in order
  to break the cycle(s). Non-essential jobs that would lead to running services
  being stopped are also removed. Finally, systemd checks whether the jobs
  within the transaction would conflict with other jobs that are already in the
  queue, and refuses the transaction if that is the case. Only if the transaction
  is consistent and the minimisation of its impact on the system is complete,
  will its jobs be entered into the queue.

C 10.1 [!1] Use “systemctl status ” to get a picture of the units that are active on
  your computer. Check the detailed status of some interesting-looking units.

10.2      Unit Files
One of the more important advantages of systemd is that it uses a unified file
format for all configuration files—no matter whether they are about services to
be started, devices, communication channels, file systems, or other artefacts that
systemd manages.
158                                                                                                10 Systemd

               B This is in stark contrast to the traditional infrastructure based on System-V
                 init, where almost every functionality is configured in a different way: per-
                 manently running background services in /etc/inittab , runlevels and their
                 services via init scripts, file systems in /etc/fstab , services that are run on
                 demand in /etc/inetd.conf , … Every single such file is syntactically different
                 from all others, while with systemd, only the details of the possible (and
                 sensible) configuration settings differ—the basic file format is always the

                 A very important observation is: Unit files are “declarative”. This means that
              they simply describe what the desired configuration looks like—unlike System V
              init’s init scripts, which contain executable code that tries to achieve the desired

               B Init scripts usually consider of huge amounts of boilerplate code which de-
                 pends on the distribution in question, but which you still need to read and
                 understand line-per-line if there is a problem or you want to do something
                 unusual. For somewhat more complex background services, init scripts of a
                 hundred lines or more are not unusual. Unit files for systemd, though, usu-
                 ally get by with a dozen lines or two, and these lines are generally pretty
                 straightforward to understand.

               B Of course unit files occasionally contain shell commands, for example to
                 explain how a specific service should be started or stopped. These, however,
                 are generally fairly obvious one-liners.

              Syntax The basic syntax of unit files is explained in systemd.unit (5). You can find
              an example for a unit file in figure 10.1. A typical characteristic is the subdivision
              into sections that start with a title in square brackets1 . All unit files (no matter
              what they are supposed to do) can include [Unit] and [Install] sections (see be-
              low). Besides, there are sections that are specific to the purpose of the unit.
                 As usual, blank lines and comment lines are ignored. Comment lines can start
              with a # or ; . Over-long lines can be wrapped with a \ at the end of the line,
              which will be replaced by a space character when the file is read. Uppercase and
              lowercase letters are important!
                 Lines which are not section headers, empty lines, nor comment lines contain
      options “options” according to a “⟨name⟩ = ⟨value⟩” pattern. Various options may occur
              several times, and systemd’s handling of that depends on the option: Multiple
              options often form a list; if you specify an empty value, all earlier settings will be
              ignored. (If that is the case, the documentation will say so.)

               B Options that are not listed in the documentation will be flagged with a warn-
                 ing by systemd and otherwise ignored. If a section or option name starts
                 with “X- ”, it is ignored completely (options in an “X- ” section do not need
                 their own “X- ” prefix).

               B Yes/no settings in unit files can be given in a variety of ways. 1 , true , yes ,
                 and on stand for “yes”, 0 , false , no , and off for “no”.

               B Times can also be specified in various ways. Simple integers will be inter-
                 preted as seconds2 . If you append a unit, then that unit applies (allowed
                 units include us , ms , s , min , h , d , w in increasing sequence from microseconds
                 to weeks—see systemd.time (7)). You can concatenate several time specifica-
                 tions with units (as in “10min 30s ”), and these times will be added (here,
                 630 seconds).
                1 The syntax is inspired by the .desktop files of the “XDG Desktop Entry Specification” [XDG-DS14],

              which in turn have been inspired by the INI files of Microsoft Windows.
                2 Most of the time, anyway—there are (documented) exceptions.
10.2 Unit Files                                                         159

#   This file is part of systemd.
#   systemd is free software; you can redistribute it and/or modify
#   it under the terms of the GNU Lesser General Public License as
#   published by the Free Software Foundation; either version 2.1
#   of the License, or (at your option) any later version.

Description=Console Getty
After=systemd-user-sessions.service plymouth-quit-wait.service

ExecStart=-/sbin/agetty --noclear --keep-baud console   
  115200,38400,9600 $TERM


              Figure 10.1: A systemd unit file: console- getty.service
160                                                                                                  10 Systemd

                            Searching and finding settings Systemd tries to locate unit files along a list of
                            directories that is hard-coded in the program. Directories nearer the front of the
                            list have precedence over directories nearer the end.

                            B The details are system-dependent, at least to a certain degree. The usual list
                              is normally something like

                                   /etc/systemd/system                                        Local configuration
                                   /run/systemd/system                         Dynamically generated unit files
                                   /lib/systemd/system                        Unit files for distribution packages

      Local customisation       Systemd offers various clever methods for customising settings without having
                            to change the unit files generally provided by your distribution—which would be
                            inconvenient if the distribution updates the unit files. Imagine you want to change
                            a few settings in the example.service file:
                               • You can copy the distribution’s example.service file from /lib/systemd/system
                                 to /etc/systemd/system and make any desired customisations. The unit file
                                 furnished by the distribution will then not be considered at all.
                               • You can create a directory /etc/systemd/system/example.service.d containing a
                                 file—for example, local.conf . The settings in that file override settings with
                                 the same name in /lib/systemd/system/example.service , but any settings not
                                 mentioned in local.conf stay intact.

                                  B Take care to include any required section titles in local.conf , such that
                                    the options can be identified correctly.

                                  B Nobody keeps you from putting several files into /etc/systemd/system/
                                    example.service.d . The only prerequisite is that file names must end in
                                    .conf . Systemd makes no stipulation about the order in which these
                                    files are read—it is best to ensure that every option occurs in just one
                                    single file.

                           Template unit files Sometimes several services can use the same or a very similar
                           unit file. In this case it is convenient not to have to maintain several copies of
                           the same unit file. Consider, for example, the terminal definition lines in /etc/
                           inittab —it would be good not to have to have one unit file per terminal.
                               Systemd supports this by means of unit files with names like example@.service .
             Instantiation You could, for example, have a file called getty@.service and then configure a vir-
                           tual console on /dev/tty2 simply by creating a symbolic link from getty@tty2.service
                           to getty@.service . When this console is to be activated, systemd reads the getty@
                           .service file and replaces the %I key, wherever it finds it, by whatever comes be-
                           tween @ and . in the name of the unit file, i. e., tty2 . The result of that replacement
                           is then put into force as the configuration.

                            B In fact, systemd replaces not just %I but also some other sequences (and that
                              not just in template unit files). The details may be found in systemd.unit (5),
                              in the “Specifiers” section.

                            Basic settings All unit files may contain the [Unit] and [Install] sections. The
                            former contains general information about the unit, the latter provides details for
                            its installation (for example, to introduce explicit dependencies—which we shall
                            discuss later).
                                Here are some of the more important options from the [Unit] section (the com-
                            plete list is in systemd.unit (5)):
                            Description A description of the unit (as free text). Will be used to make user in-
                                  terfaces more friendly.
10.3 Unit Types                                                                            161

Documentation    A space-separated list of URLs containing documentation for the
         unit. The allowed protocol schemes include http: , https: , file: , info: , and
         man: (the latter three refer to locally-installed documentation). An empty
         value clears the list.
OnFailure    A space-separated list of other units which will be activated if this unit
         transitions into the failed state.
SourcePath    The path name of a configuration file from which this unit file has been
         generated. This is useful for tools that create unit files for systemd from
         external configuration files.
ConditionPathExists     Checks whether there is a file (or directory) under the given
         absolute path name. If not, the unit will be classed as failed . If there is a
         ! in front of the path name, then a file (or directory) with that name must
         not exist. (There are loads of other “Condition …” tests—for example, you can
         have the execution of units depend on whether the system has a particular
         computer architecture, is running in a virtual environment, is running on
         AC or battery power or on a computer with a particular name, and so on.
         Read up in systemd.unit (5).)

C 10.2 [!2] Browse the unit files of your system under /lib/systemd/system (or
  /usr/lib/systemd/system , depending on the distribution). How many different
  Condition … options can you find?

10.3         Unit Types
Systemd supports a wide variety of “units”, or system components that it can
manage. These are easy to tell apart by the extensions of the names of the corre-
sponding unit files. As mentioned in Section 10.2, all units share the same basic
file format. Here is a list of the most important unit types:
.service    A process on the computer that is executed and managed by systemd.
         This includes both background services that stay active for a long time (pos-
         sibly until the system is shut down), and processes that are only executed
         once (for example when the system is booting).

         B When a service is invoked by name (such as example ) but no correspond-
           ing unit file (here, example.service ) can be found, systemd looks for a
           System-V init script with the same name and generates a service unit
           for that on the fly. (The compatibility is fairly wide-ranging but not
           100% complete.)

.socket    A TCP/IP or local socket, i. e., a communication end point that client pro-
         grams can use to contact a server. Systemd uses socket units to activate
         background services on demand.

         B Socket units always come with a corresponding service unit which will
           be started when systemd notes activity on the socket in question.

.mount    A “mount point” on the system, i. e., a directory where a file system should
         be mounted.

         B The names of these units are derived from the path name by means
           of replacing all slashes (“/ ”) with hyphens (“- ”) and all other non-
           alphanumeric (as per ASCII) characters with a hexadecimal replace-
           ment such as \x2d (“. ” is only converted if it is the first charac-
           ter of a path name). The name of the root directory (“/ ”) becomes
162                                                                                10 Systemd

                    “- ”, but slashes at the start or end of all other names are removed.
                    The directory name /home/lost+found , for instance, becomes home- lost\
                    textbackslash x2bfound .

               B You can try this replacement using the “systemd-escape -p ” command:
                     $ systemd-escape -p /home/lost+found
                     $ systemd-escape -pu home-lost\\x2bfound

                    The “-p ” option marks the parameter as a path name. The “-u ” option
                    undoes the replacement.

      .automount    Declares that a mount point should be mounted on demand (instead
               of prophylactically when the system is booted). The names of these units
               result from the same path name transformation. The details of mounting
               must be described by a corresponding mount unit.
      .swap    Describes swap space on the system. The names of these units result from
               the path name transformation applied to the device or file name in question.
      .target    A “target”, or synchronisation point for other units during system boot
               or when transitioning into other system states. Vaguely similar to System-V
               init’s runlevels. See section 10.5.
      .path    Observes a file or a directory and starts another unit (by default, a service
               unit of the same name) when, e. g., changes to the file have been noticed or
               a file has been added to an otherwise empty directory.

      .timer    Starts another unit (by default, a service unit of the same name) at a cer-
               tain point in time or repeatedly at certain intervals. This makes systemd a
               replacement for cron and at .
      (There are a few other unit types, but to explain all of them here would be carrying
      things too far.)

      C 10.3 [!2] Look for examples for all of these units on your system. Examine
        the unit files. If necessary, consult the manpages for the various types.

      10.4         Dependencies
      As we have mentioned before, systemd can mostly get by without explicit depen-
      dencies because it is able to exploit implicit dependencies (e. g., on communication
      channels). Even so, it is sometimes necessary to specify explicit dependencies.
      Various options in the [Unit] section of a service file (e.g., example.service ) allow
      you to do just that. For example:
      Requires    Specifies a list of other units. If the current unit is activated, the listed
               units are also activated. If one of the listed units is deactivated or its acti-
               vation fails, then the current unit will also be deactivated. (In other words,
               the current unit “depends on the listed units”.)

               B The Requires dependencies have nothing to do with the order in which
                 the units are started or stopped—you will have to configure that sepa-
                 rately with After or Before . If no explicit order has been specified, sys-
                 temd will start all units at the same time.
10.4 Dependencies                                                                              163

         B You can specify these dependencies without changing the unit file, by
           creating a directory called /etc/systemd/system/example.service.requires
           and adding symbolic links to the desired unit files to it. A directory
               # ls -l /etc/systemd/system/example.service.requires
               lrwxrwxrwx 1 root root 34 Jul 17 15:56 ->    
               lrwxrwxrwx 1 root root 34 Jul 17 15:57 syslog.service ->    

              corresponds to the setting
               Requires = syslog.service

              in example.service .

Wants    A weaker form of Requires . This means that the listed units will be started to-
         gether with the current unit, but if their activation fails this has no influence
         on the process as a whole. This is the recommended method of making the
         start of one unit depend on the start of another one.

         B Here, too, you can specify the dependencies “externally” by creating a
           directory called example.service.wants .

Conflicts    The reverse of Requires —the units listed here will be stopped when the
         current unit is started, and vice versa.

         B Like Requires , Conflicts makes no stipulation to the order in which units
           are started or stopped.

         B If a unit 𝑈 conflicts with another unit 𝑉 and both are to be started at
           the same time, this operation fails if both units are an essential part of
           the operation. If one (or both) units are not essential parts of the op-
           eration, the operation is modified: If only one unit is not mandatory,
           that one will not be started, if both are not mandatory, the one men-
           tioned in Conflicts will be started and the one whose unit file contains
           the Conflicts option will be stopped.

Before    (and After ) These lists of units determine the starting order. If example.
         service contains the “Before=example2.service ” option and both units are be-
         ing started, the start of example2.service will be delayed until example.service
         has been started. After is the converse of Before , i. e., if example2.service con-
         tains the option “After=example.service ” and both units are being started, the
         same effect results—example2.service will be delayed.

         B Notably, this has nothing to do with the dependencies in Requires and
           Conflicts . It is common, for example, to list units in both Requires and
           After . This means that the listed unit will be started before the one
           whose unit file contains these settings.

         When deactivating units, the reverse order is observed. If a unit with a Before
         or After dependency on another unit is deactivated, while the other is being
         started, then the deactivation takes place before the activation no matter in
         which direction the dependency is pointing. If there is no Before or After
         dependency between two units, they will be started or stopped simultane-
164                                                                           10 Systemd

                      Table 10.1: Common targets for systemd (selection)

       Target                  Description            Basic system startup is finished (file systems, swap
                               space, sockets, timers etc.)     Is executed when Ctrl + Alt + Del was pressed. Often
                               the same as .          Target which systemd attempts to reach on sys-
                               tem startup.        Usually either or
                      .        Starts a shell on the system console.         For emer-
                               gencies.     Is usually activated by means of the
                               “ ” on the kernel command
                               line.            Activates the statically-defined getty instances (for ter-
                               minals). Corresponds to the getty lines in /etc/inittab
                               on System-V init.        Establishes a graphical login prompt. Depends on
                      .             Stops the system (without powering it down).       Establishes a multi-user system without a graphical lo-
                               gin prompt. Used by .   Serves as a dependency for units that require network
                               services (not ones that provide network services), such
                               as mount units for remote file systems. How exactly the
                               system determines whether the network is available de-
                               pends on the method for network configuration.         Stops the system and powers it down.           Restarts the system.           Performs basic system initialisation and then starts a

      C 10.4 [!1] What advantage do we expect from being able to configure depen-
        dencies via symbolic links in directories like example.service.requires instead
        of the example.service unit file?

      C 10.5 [2] Check your system configuration for examples of Requires , Wants and
        Conflicts dependencies, with or without corresponding Before and After de-

      10.5      Targets
      Targets in systemd are roughly similar to runlevels in System-V init: a possibil-
      ity of conveniently describing a set of services. While System-V init allows only
      a relatively small number of runlevels and their configuration is fairly involved,
      systemd makes it possible to define various targets very easily.
         Unit files for targets have no special options (the standard set of options for
      [Unit] and [Install] should be enough). Targets are only used to aggregate other
      units via dependencies or create standardised names for synchronisation points in
      dependencies ( , for example, can be used to start units depending
      on local file systems only once these are actually available). An overview of the
      most important targets is in Table 10.1.
         In the interest of backwards compatibility to System-V init, systemd defines a
      number of targets that correspond to the classical runlevels. Consider table 10.2.
10.5 Targets                                                                                                165

                   Table 10.2: Compatibility targets for System-V init

                  Ziele              Äquivalent

   You can set the default target which systemd will attempt to reach on system default target
boot by creating a symbolic link from /etc/systemd/system/ to the de-
sired target’s unit file:

# cd /etc/systemd/system
# ln -sf /lib/systemd/system/

(This is the moral equivalent to the initdefault line in the /etc/inittab file of System-
V init.) A more convenient method is the “systemctl set-default ” command:

# systemctl get-default
# systemctl set-default graphical
Removed symlink /etc/systemd/system/
Created symlink from /etc/systemd/system/ to    
# systemctl get-default

(As you can see, that doesn’t do anything other than tweak the symbolic link,
   To activate a specific target (like changing to a specific runlevel on System-V Activate specific target
init), use the “systemctl isolate ” command:

# systemctl isolate multi-user

(“File*.target” will be appended to the parameter if necessary). This command
starts all units that the target depends upon and stops all other units.

B “systemctl isolate ” works only for units in whose [Unit] sections the “AllowIsolate ”
  option is switched on.

  To stop the system or to change to the rescue mode (System-V init aficionados
would call this “single-user mode”) there are the shortcuts

#   systemctl   rescue
#   systemctl   halt
#   systemctl   poweroff                               Like halt , but with power-down
#   systemctl   reboot

These commands correspond roughly to their equivalents using “systemctl isolate ”,
but also output a warning to logged-in users. You can (and should!) of course
keep using the shutdown command.
   You can return to the default operating state using

# systemctl default
166                                                                                                            10 Systemd

                                 C 10.6 [!2] Which other services does the depend on? Do
                                   these units depend on other units in turn?

                                 C 10.7 [2] Use “systemctl isolate ” to change your system to the rescue (single-
                                   user) mode, and “systemctl default ” to come back to the standard mode.
                                   (Hint: Do this from a text console.)

                                 C 10.8 [2] Restart your system using “systemctl reboot ” and then once again
                                   with shutdown . Consider the difference.

                                 10.6        The systemctl Command
                                 The systemctl command is used to control systemd. We have already seen a few
                                 applications, and here is a more systematic list. This is, however, still only a small
                                 excerpt of the complete description.
                                    The general structure of systemctl invocations is

                                 # systemctl   ⟨subcommand⟩ ⟨parameters⟩ …

                                 systemctlsupports a fairly large zoo of subcommands. The allowable parameters
                                 (and options) depend on the subcommand in question.

      unit names as parameters   B Often unit names are expected as parameters. These can be specified
                                   either with a file name extension (like, e. g., example.service ) or without
                                   (example ). In the latter case, systemd appends an extension that it considers
                                   appropriate—with the start command, for example, “.service ”, with the
                                   isolate command on the other hand, “.target ”.

                                 Commands for units        The following commands deal with units and their man-
                                 list-units Displays the units systemd knows about. You may specify a unit type
                                       (service , socket , …) or a comma-separated list of unit types using the -t op-
                                         tion, in order to confine the output to units of the type(s) in question. You
                                         can also pass a shell search pattern in order to look for specific units:

                                          # systemctl list-units "ssh*"
                                          UNIT        LOAD   ACTIVE SUB     DESCRIPTION
                                          ssh.service loaded active running OpenBSD Secure Shell server

                                          LOAD   = Reflects whether the unit definition was properly loaded.
                                          ACTIVE = The high-level unit activation state, i.e. generalization
                                                   of SUB.
                                          SUB    = The low-level unit activation state, values depend on
                                                   unit type.

                                          1 loaded units listed. Pass --all to see loaded but inactive units,
                                          too. To show all installed unit files use 'systemctl

                                         B As usual, quotes are a great idea here, so the shell will not vandalise
                                           the search patterns that are meant for systemd.

                                 start   Starts one or more units mentioned as parameters.
10.6 The systemctl Command                                                                167

         B You can use shell search patterns here, too. The search patterns only
           work for units that systemd knows about; inactive units that are not
           in a failed state will not be searched, nor will units instantiated from
           templates whose exact names are not known before the instantiation.
           You should not overtax the search patterns.

stop   Stops one or more units mentioned as parameters (again with search pat-
reload    Reloads the configuration for the units mentioned as parameters (if the pro-
         grams underlying these units go along). Search patterns are allowed.

         B This concerns the configuration of the background services them-
           selves, not the configuration of the services from systemd’s point of
           view. If you want systemd to reload its own configuration with respect
           to the background services, you must use the “systemctl daemon-reload ”

         B What exactly happens on a “systemctl reload ” depends on the back-
           ground service in question (it usually involves a SIGHUP ). You can con-
           figure this in the unit file for the service.

restart     Restarts the units mentioned as parameters (search patterns are allowed).
         If a unit doesn’t yet run, it is simply started.
try-restart    Like restart , but units that don’t run are not started.
reload-or-restart    (and reload-or-try-restart ) Reloads the configuration of the
         named units (as per reload ), if the units allow this, or restarts them (as
         per restart or try-restart ) if they don’t.

         B Instead of reload-or-try-restart you can say force-reload for convenience
           (this is at least somewhat shorter).

isolate   The unit in question is started (including its dependencies) and all other
         units are stopped. Corresponds to a runlevel change on System-V init.
kill   Sends a signal to one or several processes of the unit. You can use the
        --kill-who option to specify which process is targeted. (The options include
        main , control , and all —the latter is the default—, and main and control are
        explained in more detail in systemctl (1).) Using the --signal option (-s for
        short) you can determine which signal is to be sent.
status    Displays the current status of the named unit(s), followed by its most recent
         log entries. If you do not name any units, you will get an overview of all
         units (possibly restricted to particular types using the -t option).

         B The log excerpt is usually abridged to 10 lines, and long lines will be
           shortened. You can change this using the --lines (-n ) and --full (-l )

         B “status ” is used for human consumption. If you want output that is
           easy to process by other programs, use “systemctl show ”.

cat    Displays the configuration file(s) for one or more units (including fragments
        in configuration directories specifically for that unit). Comments with the
        file names in question are inserted for clarity.
help   Displays documentation (such as man pages) for the unit(s) in question: For

          $ systemctl help syslog
168                                                                               10 Systemd

            invokes the manual page for the system log service, regardless of which
            program actually provides the log service.

      B With most distributions, commands like
             #   service   example   start
             #   service   example   stop
             #   service   example   restart
             #   service   example   reload

            work independently of whether the system uses systemd or System-V init.
         In the next section, there are a few commands that deal with installing and
      deinstalling units.

      Other commands Here are a few commands that do not specifically deal with
      particular units (or groups of units).
      daemon-reload This command causes systemd to reload its configuration. This in-
            cludes regenerating unit files that have been created at runtime from other
            configuration files on the system, and reconstructing the dependency tree.

            B Communication channels that systemd manages on behalf of back-
              ground services will persist across the reload.
      daemon-reexec Restarts the systemd program. This saves systemd’s internal state and
            restores it later.

            B This command is mostly useful if a new version of systemd has been
              installed (or for debugging systemd . Here, too, communication channels
              that systemd manages on behalf of background services will persist
              across the reload.
      is-system-running     Outputs the current state of the system. Possible answers in-
            initializing The system is in the early boot stage (the , ,
                 or targets have not yet been reached).
            starting  The system is in the late boot stage (there are still jobs in the queue).
            running The system is running normally.
            degraded The system is running normally, but one or more units are in a
                 failed state.
            maintenance One of the or targets are active.
            stopping The system is being shut down.

      C 10.9 [!2] Use systemctl to stop, start, and restart a suitably innocuous service
        (such as cups.service ) and to reload its configuration.

      C 10.10 [2] The runlevel command of System-V init outputs the system’s cur-
        rent runlevel. What would be an approximate equivalent for systemd?

      C 10.11 [1] What is the advantage of
             # systemctl kill example.service

             # killall example

            (or “pkill example ”)?
10.7 Installing Units                                                                                  169

10.7      Installing Units
To make a new background service available using systemd, you need a unit file,
for example example.service . (Thanks to backwards compatibility, a System-V init
script would also do, but we won’t go there just now.) You need to place this in a
suitable file (we recommend /etc/systemd/system . Next, it should appear when you
invoke “systemctl list-unit-files ”:

# systemctl list-unit-files
UNIT FILE                                  STATE
proc-sys-fs-binfmt_misc.automount          static
org.freedesktop.hostname1.busname          static
org.freedesktop.locale1.busname            static

example.service                            disabled

The disabled state means that the unit is available in principle, but is not being
started automatically.
   You can “activate” the unit, or mark it to be started when needed (e. g., during Activating units
system startup or if a certain communication channel is being accessed), by issuing
the “systemctl enable ” command:

# systemctl enable example
Created symlink from /etc/systemd/system/ 
 example.service to /etc/systemd/system/example.service.

The command output tells you what happens here: A symbolic link to the ser-
vice’s unit file from the /etc/systemd/system/multi- directory en-
sures that the unit will be started as a dependency of the .

B You may ask yourself how systemd knows that the example unit should be
  integrated in the (and not some other target). The answer
  to that is: The example.service file has a section saying


After an enable , systemd does the equivalent of a “systemctl daemon-reload ”. How-
ever, no units will be started (or stopped).

B You could just as well create the symbolic links by hand. You would, how-
  ever, have to take care of the “systemctl daemon-reload ” yourself, too.

B If you want the unit to be started immediately, you can either give the
       # systemctl start example

      command immediately afterwards, or you invoke “systemctl enable ” with the
      --now option.

B You can start a unit directly (using “systemctl start ”) without first activating
  it with “systemctl enable ”. The former actually starts the service, while the
  latter only arranges for it to be started at an appropriate moment (e. g., when
  the system is booted, or a specific piece of hardware is connected).

   You can deactivate a unit again with “systemctl disable ”. As with enable , sys-
temd does an implicit daemon-reload .
170                                                                                              10 Systemd

                       B Here, too, the unit will not be stopped if it is currently running. (You are
                         just preventing it from being activated later on.) Use the --now option or an
                         explicit “systemctl stop ”.

                       B The “systemctl reenable ” command is equivalent to a “systemctl disable ” im-
                         mediately followed by a “systemctl enable ” for the units in question. This lets
                         you do a “factory reset” of units.

      Masking a unit      The “systemctl mask ” command lets you “mask” a unit. This means to block it
                       completely. This will not only prevent it from starting automatically, but will also
                       keep it from being started by hand. “systemctl unmask ” reverts that operation.

                       B Systemd implements this by linking the name of the unit file in /etc/systemd/
                         system symbolically to /dev/null . Thus, eponymous files in directories that
                         systemd considers later (like /lib/systemd/system ) will be completely ignored.

                       C 10.12 [!2] What happens if you execute “systemctl disable cups ”? (Watch the
                         commands being output.) Reactivate the service again.

                       C 10.13 [2] How can you “mask” units whose unit files are in /etc/systemd/
                         system ?

                       Commands in this Chapter
                       systemctl   Main control utility for systemd                  systemctl (1)   157, 166

                          • Systemd is a modern alternative to System-V init.
                          • “Units” are system components managed by systemd. They are configured
                            using unit files.
                          • Unit files bear a vague resemblance to Microsoft Windows INI files.
                          • Systemd supports flexible mechanisms for local configuration and the au-
                            tomatic creation of similar unit files from “templates”.
                          • Systemd lets you manage a multitude of different units—services, mount
                            points, timers, …
                          • Dependencies between units can be expressed in various ways.
                          • “Targets” are units that vaguely correspond to System-V init’s runlevels.
                            They are used to group related services and for synchronisation.
                          • You can use the systemctl command to control systemd.
                          • Systemd contains powerful tools to install and deinstall units.

                       systemd “systemd System and Service Manager”.

                       XDG-DS14 Preston Brown, Jonathan Blandford, Owen Taylor, et al. “Desktop
                           Entry Specification”, April 2014.
                                       entry- spec/latest/
                                                                                                   $ echo tux
                                                                                                   $ ls
                                                                                                   $ /bin/su -

Dynamic (AKA Shared) Libraries

11.1   Compiling and Installing Software . . .       .   .   .   .   .   .   .   .   .   .   172
11.2   Dynamic Libraries In Practice . . . . .       .   .   .   .   .   .   .   .   .   .   174
11.3   Installing and Locating Dynamic Libraries .   .   .   .   .   .   .   .   .   .   .   176
11.4   Dynamic Library Versioning . . . . . .        .   .   .   .   .   .   .   .   .   .   177

   • Being able to identify shared libraries
   • Knowing where shared libraries are usually stored
   • Being able to manage shared libraries

   • Solid knowledge of the Linux command line
   • Programming experience is useful but not essential

adm1-biblio.tex   (33e55eeadba676a3 )
172                                                                      11 Dynamic (AKA Shared) Libraries

                       11.1      Compiling and Installing Software
                         An important principle of programming is not to reinvent the wheel continually.
                         Most programs share a lot of code—from basic functions like handling files and
                         strings to more specialised stuff like dealing with date or time data. Neither does it
                         make sense to write this code from scratch over and over again, nor is it reasonable
                         to integrate it bodily into every program. The former because it is a lot of tedious
                         work and the same mistakes would have to be corrected again and again; the latter
                         because it takes a lot of space on disk and is extremely inconvenient if a mistake
                         does need to be corrected.
                             The answer to the first objection—the code should not have to be rewritten
      software libraries over and over again—is software libraries. They serve to collect often-required
                         functionality, to endow it with a standardised and well-documented interface, and
                         to make it available to other programs. For example, an essential ingredient for a
                         functioning Linux system is libc , which bundles all sorts of vital functionality for
                         programs written in the C programming language.

                       B Practically all programs on a Linux system are written in C, if only indi-
                         rectly: Many are written in native C, some are written in languages derived
                         from C such as C++ which usually also use libc , and even shell scripts and
                         programs in languages such as awk , Python, and Perl use the shell or the awk ,
                         python , and perl programs—all of which are written in C …

                       In addition to libc , a typical Linux system contains a few hundred or thousand
                       other libraries that cater for requirements ranging from support for mathematical
                       functions or various data formats to providing complete graphical interfaces.
                          In the first instance, libraries are important when programming. In the sim-
                linker plest case, a “linker” is used to combine a program’s “object code”—that is, the
                       output of the compiler, which translates a program written, say, in C to instruc-
                       tions executable on the CPU—with the object code of the libraries used by this
                       program to yield an “executable program” that you can then start.

                       B Consider, by way of illustration, the following tiny C program:
                              /* sqrttest.c */
                              #include <math.h>
                              #include <stdio.h>
                              int main(void)
                                printf("%g\n", sin(3.141592654/2.0));
                                return 0;

                             (It serves to answer the nagging question about the value of sin(𝜋/2).) The C
                             compiler turns this into an assembly language program that starts approx-
                             imately like

                              ; sqrttest.s
                                      .file    "sqrttest.c"
                                      .section         .rodata
                                      .string "%g\n"
                                      .align 8
                                      .long    1414677840
                                      .long    1073291771
                              .globl main
                                      .type    main, @function
11.1 Compiling and Installing Software                                                                          173

                      leal        4(%esp), %ecx
                      andl        $-16, %esp
                      pushl       -4(%ecx)
                      pushl       %ebp

      and the assembler, in turn, produces an object file which, displayed “read-
      ably”, looks like

       7f   45   4c    46   01   01   01   00   00   00   00   00   00   00   00   00
       01   00   03    00   01   00   00   00   00   00   00   00   00   00   00   00
       14   01   00    00   00   00   00   00   34   00   00   00   00   00   28   00
       0b   00   08    00   8d   4c   24   04   83   e4   f0   ff   71   fc   55   89
       e5   51   83    ec   14   dd   05   08   00   00   00   dd   1c   24   e8   fc
       ff   ff   ff    dd   5c   24   04   c7   04   24   00   00   00   00   e8   fc

      (952 bytes in total, if you must know). This file then needs to be put together
      with the libraries libc (for the printf() function) and libm (for the sin() func-
      tion) to create an executable program, which is the linker’s job. Fortunately
      all of this can be done using a single command such as

       $ gcc -static sqrttest.c -lm -o sqrttest-static

      On the author’s system, the resulting program weighs in at a mere 560,608 bytes.

   In this case, the program and libraries form a permanent combinaton. The
libraries spare us the tedious repetition of code that is useful in several programs, Advantages of libraries
but if a problem is found in a library, we need to re-link all programs using that
library. The fact that a trivial program is half a megabyte long is also vaguely less
than satisfying.
   Here is where dynamic libraries come in. The basic idea behind them is that dynamic libraries
the actual program and libraries are not combined permanently during linking,
but are only put together when the program is actually executed. Thus, program
and libraries remain independent of each other. This results in the following ad-
   • As long as a library’s interface stays the same (i. e., the “signatures” of
     the functions concerned—the number and types of their arguments and
     results—do not change), the library can easily be replaced without having
     to re-link all programs using it. When a program is started, it automatically
     fetches the new library version.
   • If 𝑛 programs are using the library, it does not require 𝑛 times the space on
     disk (as part of each of those programs). One file is enough.

   • In fact it usually suffices to have the library in memory just once. All pro-
     cesses can share one copy. Another saving! This is why we speak of “shared
Incidentally, you’re paying for this with time: On the one hand it takes a bit longer
to launch a program using dynamic libraries, since the connection between the
actual program and its libraries needs to be made at runtime instead of when the
program is compiled to an executable file. On the other hand—depending on the
processor and programming model—it may be the case that dynamic libraries
carry a runtime penalty as opposed to “static”, i. e., connected-at-compile-time,
libraries. This penalty usually amounts to a low single-digit percentage and is
more than outweighed by the advantages mentioned above.
174                                                                               11 Dynamic (AKA Shared) Libraries

                              B Our example from above could be linked against dynamic libraries using
                                       $ gcc sqrttest.c -lm -o sqrttest-dynamic

                                       The result is all of 6510 bytes long—a marked improvement from more than

                              B Perhaps you would think that disk space is so cheap nowadays that half a
                                megabyte here or there is not worth losing sleep over. Dynamic libraries
                                were introduced to Unix in the 1980s, when disk space was still measured
                                in megabytes rather than terabytes; libraries weren’t quite as big as they
                                are today, but one used to notice the difference rather more in these days.
                                Anyway, the other advantages are really even more important.

       Rehabilitation of stat-  To rehabilitate statically linked programs we should mention that they also
      ically linked programs have their place. On the one hand, they work fairly reliably even is the system
                              is all but hosed otherwise—should you ever manage to somehow junk your dy-
                              namic libc , you will be glad to have a statically-linked version of busybox or sash
                              on your system, since bash and its friends such as cp or tar will be no good for re-
                              pairs. (The alternative would be to boot the rescue system, but that is of course a
                              cop-out for wimps.) On the other hand, statically linked programs are huge and
                              inconvenient, but you know what you have1 – so if you want to distribute a com-
                              plex program in executable form, linking statically makes you independent of the
                              libraries available on the systems the program is supposed to run on later. (This
                              can of course imply different problems, but an exhausting discussion would be
                              beyond the scope of this manual.)

                       LSB    B A large part of the exertions of the LSB (Linux Standard Base), an agreement
                                between various distributors as to which environment should be available
                                for software distributed in executable form by third-party vendors (think
                                SAP, Oracle and games), concerns itself with prescribing the version and
                                content of important dynamic libraries.

                              11.2        Dynamic Libraries In Practice
                              As we said earlier, most executable programs on a typical Linux system are linked
                              dynamically. You can make sure using the file command:

                              $ file sqrttest-dynamic
                              sqrttest-dynamic: ELF 32-bit LSB executable, Intel 80386, 
                                version 1 (SYSV), dynamically linked (uses shared libs), 
                                for GNU/Linux 2.6.8, not stripped

                              By way of comparison:

                              $ file sqrttest-static
                              sqrttest-static: ELF 32-bit LSB executable, Intel 80386, 
                                version 1 (SYSV), statically linked, for GNU/Linux 2.6.8, 
                                not stripped

                              B The “not stripped ” refers to the fact that our small (?) C program still con-
                                tains information supposed to make debugging easier. You can slim it down
                                to just below 500 kB by executing “strip sqrttest- static ”.

         Required libraries         You can find out in detail which libraries sqrttest- dynamic requires by using the
                              ldd   command:
                                1 Apologies   to the Persil man.
11.2 Dynamic Libraries In Practice                                                                      175

$ ldd sqrttest-dynamic => (0xb7f51000) => /lib/i686/cmov/ (0xb7f0b000) => /lib/i686/cmov/ (0xb7db0000)
        /lib/ (0xb7f52000)

Its output tells you the names of the libraries as well as the names of the files
corresponding to them—if possible (otherwise the file name is empty).

B We have already discussed libc and libm . /lib/ld- is the “dynamic
  linker”, the program that causes the actual program to be connected to its
  dynamic libraries when it is started (we shall come back to this presently).

B So what is , and what is it good for? This question is great
  for finding out how much your buddies really know about Linux. The most
  obvious observation is that ldd didn’t find a file corresponding to this li-
  brary. Thus by rights the program should not even run. If you look you
  will even find that there is no file on the system by this name at all. However,
  this doesn’t matter, as the kernel takes care of making this “virtual dynamic
  library” available to all programs. is used to speed up sys-
  tem calls on modern x86 processors (which, in 2009, basically means “all of
  them”). The details are too unsavoury to explain in detail; read [Pet08] but
  not immediately before or after a meal.

B On Intel- and AMD-based 64-bit systems, the “library”
  serves the same purpose as on 32-bit systems. Don’t let
  yourself become confused.

   Typical places where Linux distributors put libraries (static or dynamic ones) Libraries: where?
include the /lib and /usr/lib directories. As usual, the former is mostly meant
for libraries that need to be available immediately after the system is booted, and
the latter caters for those that are only needed after all file systems have been
   How do you recognise a dynamic library, anyway? By its name—the names of Recognising dynamic libraries
dynamic libraries usually end in .so , usually followed by a version number. (The
names of static libraries end in .a , and a version number does not apply there.) If
you want to be anal-retentive, you can can also trot out file :

$ file /lib/
/lib/ ELF 32-bit LSB shared object, Intel 80386, 
  version 1 (SYSV), dynamically linked (uses shared libs), 
  for GNU/Linux 2.6.8, stripped

Look closely—we have

ELF 32-bit LSB shared object

instead of
ELF 32-bit LSB executable

—a subtle difference!

B With static libraries, the following happens:
      $ file /usr/lib/libc.a
      /usr/lib/libc.a: current ar archive
176                                                                                  11 Dynamic (AKA Shared) Libraries

                                          It does take a little experience to conclude “library!” from this—take a look
                                          at ar (1).

 File names for dynamic libraries      In “real life”, the file name of a dynamic library could look approximately like
                                    $ ls -l /usr/lib/libcurl.*
                                    lrwxrwxrwx root root     16 Jan 22 01:23 /usr/lib/ 
                                    -rw-r--r-- root root 271364 Dec 27 14:33 /usr/lib/

                                    The symbolic link is used to relate actual programs and the library. A program
                                    actually asks for a specific version of a library—in our example, . It
                                    depends on the system whether this amounts to /usr/lib/ or /usr/
                                    lib/ , whichever is installed. (The basic assumption is that all files
                                    purporting to be implement the same interface.) More information
                                    about versioning dynamic libraries is in Section 11.4.

                                    B Incidentally, dynamic libraries may depend on other dynamic libraries, too.
                                      You may find out about this using ldd (of course):

                                           $ ldd /usr/lib/
                                          => (0xb7f6f000)
                                          => /usr/lib/ (0xb7edb000)
                                          => /usr/lib/ (0xb7eba000)
                                          => /usr/lib/ (0xb7eac000)

                                          (What you get to see if you apply ldd to a program is the transitive closure
                                          over all dynamic libraries that the program together with all of its dynamic
                                          libraries depends on.)

                                    C 11.1 [!1] Find a statically-linked executable on your system (apart from
                                      sqrttest- static ).

                                    C 11.2 [1] What does ldd say if you apply it to a statically-linked program?

                                    C 11.3 [2] (For shell programmers.) Which program in /usr/bin on your system
                                      is linked against the greatest number of dynamic libraries?

                                    11.3      Installing and Locating Dynamic Libraries
                  dynamic linker When you start a program, the dynamic linker must find and load the required
                                   libraries. However, the program itself contains only a library’s name, not the name
                                   of the file it is to be found in. To avoid having the dynamic linker search all of the
             File/etc/ file system (or even only the /lib and /usr/lib directories), the File/etc/
                                   file contains an “index” of all known dynamic libraries. The dynamic linker finds
                                   only those libraries that occur in this index.
                          ldconfig     The index is created using the ldconfig program, which searches all directories
                  /etc/ listed in /etc/ as well as the two standard directories /lib and /usr/lib for
                                   libraries. All found libraries are entered into the index. In addition, ldconfig takes
                                   care of creating the “main version number” symbolic links for library files.

                                    B /lib and /usr/lib are always searched for libraries, no matter what /etc/ld.
                                      so.conf says.

                   index content       You can inspect the current state of the index using
11.4 Dynamic Library Versioning                                                                                         177

# ldconfig -p
909 libs found in cache `/etc/' (libc6) => /opt/gnome/lib/ (libc6) => /lib/ (libc6) => /usr/lib/

This does not generate a new index, but only outputs the content of in
a readable format. If a library does not show up here, it cannot be found by the
linker, either.
   To add a new dynamic library from the standard directories—besides /lib and New dynamic libraries
/usr/lib , this typically includes /usr/local/lib —, then a simple call to

# ldconfig

should suffice to update /etc/ . If your library is not located in one of
the directories mentioned in /etc/ , you have to add the directory to that
file first.

B Usually the package installation tool of your distribution should take care of
  invoking ldconfig when packages including dynamic libraries are installed.
  Therefore, the previous paragraph applies mostly if you want to install li-
  braries that you wrote yourself, or ones in software packages that you com-
  piled from source code.

   If you lack the administrator rights necessary to call ldconfig you can still use
your own dynamic libraries. On top of the cache in /etc/ , the dynamic
linker also searches the directories enumerated in the LD_LIBRARY_PATH environment                    LD_LIBRARY_PATH
variable. Its syntax corresponds to that of the PATH variable—directory names are
separated by colons:

$ export LD_LIBRARY_PATH=$HOME/lib:/opt/foo/lib

B The dynamic linker searches the content of the directories listed in LD_LIBRARY_PATH
  (if set) first, then the content of /etc/ , and then the content of
  the /lib and /usr/lib directories (in that order—this is just a safety net in
  case /etc/ does not exist). Libraries in a directory mentioned in
  LD_LIBRARY_PATH whose names are the same as “official” libraries are thus
  found first.

B The documentation of the dynamic linker ( (8)) contains many in-
  teresting and thrilling options that allow you to do strange and wonderful

C 11.4 [!2] The directories in LD_LIBRARY_PATH are not considered when the pro-
  gram to be started uses the Set-UID or Set-GID mechanism (see Section 3.5).
  Why is that?

11.4       Dynamic Library Versioning
Users of Microsoft Windows know the problem called “DLL hell”2 : Many soft-
ware packages come with their own versions of important system libraries and do
   2 DLLs, or “dynamically loadable libraries”, are the Windows equivalent to the dynamic libraries

of Linux.
178                                                                                       11 Dynamic (AKA Shared) Libraries

                             not shy away from recklessly supplanting pre-existing versions of those libraries
                             with their own. If, of course, the supplanted version happens to be exactly the
                             one that another software package brought in, chaos is not far away.
        third-party software    Under Linux this problem is much less grave, since much less third-party
                             software is delivered in executable form to begin with. You receive binary soft-
                             ware mostly from your distributor, and other things are compiled from source
                             code (making use of pre-installed libraries as much as possible). Problems are
                             more likely to result from the fact that the Linux software landscape evolves fairly
                             quickly, which of course also applies to libraries.
            version numbers     For this reason, dynamic libraries are assigned version numbers that typically
                             contain up to three parts. For example, earlier on we talked about libcurl.4.1.0 .
                             These version numbers work as follows:
      major version number        • The first number after the library name is the “major version number”. It
                                    should be incremented whenever the programming interface offered by the
                                    library to other programs changes in an incompatible fashion. For example,
                                    a function that used to accept two arguments might suddenly require three,
                                    or an argument that used to be an integer might turn into a pointer to a

                                     B It goes without saying that a responsible developer should try to avoid
                                       such changes whenever possible, as they imply that the programs us-
                                       ing the library need to be adapted.

                                  • The second number (after the second dot) is the “minor version number”.
                                    It is incremented when the programming interface is extended without
                                    changes to existing calls. For example, a completely new function might be
                                    added. Older programs can still use the library because everything they
                                    require is still available in an identical fashion; of course, newer programs
                                    that do use the newly added calls will not work with older versions of the
                patch level       • The third number (at the end) is the “patch level”. It is incremented when
                                    there are changes to the library implementation that do not affect the inter-
                                    face. For example, a bug in the implementation of a function might be fixed.
                                    The fact that the library is dynamically linked makes it possible to simply
                                    replace the defective library by the corrected one—all subsequently-started
                                    programs automatically pick up the new version.

                                     B Replacing a defective library by a new one has no impact on programs
                 daemons               already running with the defective library—such as daemons. When in
                                       doubt, such programs must be restarted manually to benefit from the

                                     B The sufferers in this improvement process are, of course, programs that
                                       rely on the presence of the implementation error. It sometimes happens
                                       that knowledge of a bug gets passed around the developer commu-
                                       nity, and programmers either exploit it (if it has some beneficial side
                                       effect—which happens) or work around it in a way that does not take
                                       into account the fact that at some point the error might no longer be
                                       present3 . In such a situation, a correction amounts to an incompati-
                                       ble change and should, in the worst case, cause even the major version
                                       number to be incremented.
                                  3 A long time ago, your author had to contend with a C compiler that would calculate the reciprocal

                              of the actual result when performing floating-point divisions. (Don’t ask.) In a software package writ-
                              ten by a colleague, there occured exactly one such division in a strategic place, where (uncommented
                              and long forgotten) the dividend and divisor had been swapped to accommodate this error. Ferreting
                              this out as the reason for a grisly sequence of other faults, after updating to a fixed C compiler, took
                              us a few days.
11.4 Dynamic Library Versioning                                                                               179

Programs always require a certain major number of a library, such as ,
which is mapped to a concrete file, such as /usr/lib/ , by means of
the symbolic link.
   Hence it is quite feasible for a system to simultaneously contain programs re- multiple versions simultaneously
quiring multiple (major) versions of the same library. You can simply leave the
various library files installed and put the burden of finding the correct one for
each program on the dynamic linker.

B The LSB standard prescribes certain major versions of common libraries
  and, in each case, standardises a programming interface for the library in
  question. LSB-conforming distributions must offer these libraries for the
  benefit of LSB executables, but these libraries do not need to be the ones
  that all of the rest of the system is using. Hence it is quite possible to make
  a distribution conform to LSB from the point of view of library support by
  placing a set of the required libraries in a different directory, and to foist
  this on LSB executables instead of the usual system directories for libraries
  through dynamic linker trickery (such as LD_LIBRARY_PATH ).

B The same strategy is also useful for “normal” third-party software that does
  not rely on LSB. As long as the kernel-libc interface does not change incom-
  patibly (which, fortunately, the kernel developers diligently try to avoid),
  a software package can provide its own libc (plus, presumably, all sorts of
  other libraries) and so try to become independent of the library support
  offered by the underlying Linux system. Whether this is always a good idea
  in real life remains an open question—after all, as a software vendor one
  may have to support independent bug fixes to the libraries and distribute
  them to all customers—, but it is a fact that on Linux there is no direct equiv-
  alent to “DLL hell”. Score one for the penguin!

Commands in this Chapter
busybox    A shell that already contains variants of many Unix tools
                                                                    busybox (1)   174
file       Guesses the type of a file’s content, according to rules     file (1)   174
ldconfig    Builds the dynamic library cache                       ldconfig (8)   176
ldd        Displays the dynamic libraries used by a program             ldd (1)   174
sash       “Stand-Alone Shell” with built-in commands, for troubleshooting
                                                                       sash (8)   174
strip      Removes symbol tables from object files                     strip (1)   174

   • Libraries offer frequently-needed functionality in standardised form to sev-
     eral programs.
   • Dynamic libraries save memory and disk space and facilitate program main-
   • file lets you find out whether a program is dynamically linked, and identify
     shared libraries as such.
   • ldd outputs the names of dynamic libraries that a program (or a dynamic
     library) uses.
   • Dynamic libraries are searched in /lib , /usr/lib , and the directories men-
     tioned in /etc/ . The ldconfig command constructs an index.
   • Several versions of the same library may be installed at the same time.
180                                                  11 Dynamic (AKA Shared) Libraries

      Pet08 Johan Petersson. “What is”, August 2008.
                                                                                            $ echo tux
                                                                                            $ ls
                                                                                            $ /bin/su -

Software Package Management
Using Debian Tools

12.1 Overview. . . . . . . . . . . . . . . .              .   .   .   .   .   .   .   182
12.2 The Basis: dpkg . . . . . . . . . . . . . .          .   .   .   .   .   .   .   182
    12.2.1 Debian Packages . . . . . . . . . . .          .   .   .   .   .   .   .   182
    12.2.2 Package Installation . . . . . . . . . .       .   .   .   .   .   .   .   183
    12.2.3 Deleting Packages . . . . . . . . . .          .   .   .   .   .   .   .   184
    12.2.4 Debian Packages and Source Code . . . .        .   .   .   .   .   .   .   185
    12.2.5 Package Information. . . . . . . . . .         .   .   .   .   .   .   .   185
    12.2.6 Package Verification . . . . . . . . . .        .   .   .   .   .   .   .   188
12.3 Debian Package Management: The Next Generation       .   .   .   .   .   .   .   189
    12.3.1 APT . . . . . . . . . . . . . . .              .   .   .   .   .   .   .   189
    12.3.2 Package Installation Using apt-get . . . . .   .   .   .   .   .   .   .   189
    12.3.3 Information About Packages . . . . . . .       .   .   .   .   .   .   .   191
    12.3.4 aptitude . . . . . . . . . . . . . .           .   .   .   .   .   .   .   192
12.4 Debian Package Integrity . . . . . . . . . .         .   .   .   .   .   .   .   194
12.5 The debconf Infrastructure . . . . . . . . .         .   .   .   .   .   .   .   195
12.6 alien : Software From Different Worlds . . . . .      .   .   .   .   .   .   .   196

   •   Knowing the basics of Debian packaging tools
   •   Being able to use dpkg for package management
   •   Being able to use apt-get , apt-cache , and aptitude
   •   Being aware of the principles of Debian package integrity
   •   Knowing how to convert RPM packages to Debian packages using alien

   • Knowledge of Linux system administration
   • Experience with Debian GNU/Linux or a Debian GNU/Linux derivative is

adm1-deb.tex   (33e55eeadba676a3 )
182                                               12 Software Package Management Using Debian Tools

                    12.1      Overview
                    Software packages in Debian GNU/Linux and derived distributions such as
                    Ubuntu, Knoppix, Xandros, or Sidux are maintained using the dpkg tools. It
                    serves to install software packages, to manage dependencies, to catalog installed
                    software, to control updates to software packages, and to de-install packages that
                    are no longer required. Program such as aptitude serve as front-ends to dpkg , al-
                    lowing the convenient selection of software packages. The debconf infrastructure
                    is used to configure packages upon installation.

                     B The Debian and Red Hat package management systems were developed at
                       about the same time and have different strengths and weaknesses. As usual
                       in the free software community, the religious wars around dpkg and rpm have
                       not led to one of the competitors carrying the day. With the increasing pop-
                       ularity of Debian derivatives—most notably Ubuntu—this remains unlikely
                       for the foreseeable future, too.

                     B The LSB standard for a basic Linux infrastructure that third-party vendors
                       can port their software to does prescribe a restricted version of RPM as its
                       package format. However, this does not imply that a LSB-compliant Linux
                       distribution must be RPM-based from the ground up, but only that it must
                       be able to install software packages from third-party vendors that conform
                       to the LSB flavour of RPM.

                          For Debian GNU/Linux, this is of course a piece of cake. The reason
                          why Debian GNU/Linux is not officially touted as “LSB-compliant” is be-
                          cause LSB is run by an industry consortium of which Debian, being a non-
                          commercial project, is not a member. The description for the lsb package
                          on Debian GNU/Linux states:
                               The intent of this package is to provide a best current practice way
                               of installing and running LSB packages on Debian GNU/Linux.
                               Its presence does not imply that Debian fully complies with the
                               Linux Standard Base, and should not be construed as a statement
                               that Debian is LSB-compliant.

                          While its title talks about “Debian tools”, everything in this chapter also
                          applies to Ubuntu, since Ubuntu takes substantial parts of its infrastructure
                          from Debian GNU/Linux. We shall be pointing out significant differences
                          that do exist.

                    12.2      The Basis: dpkg
                    12.2.1     Debian Packages
           packages Within the Debian infrastructure, the software on the system is divided into pack-
      package names ages. Packages have names that indicate the software contained within and their
                    version. The

                    file, for example, contains the hello program’s 2.8 version; in particular this is the
                    second release of this package within the distribution (for a future 2.9 package
                    the count would start over at 1). Packages like apt which have been specifically
                    developed for Debian do not include a “Debian release number”. The amd64 indi-
                    cates that the package contains architecture-specific parts for Intel and AMD x86
                    processors (and compatibles) in 64-bit mode—32-bit packages use i386 , and pack-
                    ages that contain only documentation or architecture-independent scripts use all
12.2 The Basis: dpkg                                                                                       183

B A Debian package is an archive created using the ar program and generally package structure
  contains three components:

       $ ar t hello_2.8-2_amd64.deb

      The debian- binary file contains the version number of the package format
      (currently 2.0 ). In control.tar.gz there are Debian-specific scripts and control
      files, and data.tar.gz contains the actual package files. During installation, installation
      control.tar.gz is unpacked first, so that a possible preinst script can be exe-
      cuted prior to unpacking the actual package files. After this, data.tar.gz will
      be unpacked, and the package will be configured if necessary by executing
      the postinst script from control.tar.gz .

C 12.1 [2] Obtain an arbitrary Debian package (such as hello ) and take it apart
  using ar and tar . Can you find the installation scripts? Which information
  is contained in control.tar.gz , and which is in data.tar.gz ?

12.2.2    Package Installation
You can easily install a locally-available Debian package using the

# dpkg --install hello_2.8-2_amd64.deb

command, where --install can be abbreviated to -i . With --unpack and --configure
(-a ), the unpacking and configuration steps can also be executed separately.

      In real life, the short option names such as -i are convenient. However, if
      you intend to pass the LPI-101 exam, you should be sure to learn the long
      option names as well, since these, vexatingly, occur in the exam questions.
      In the case of -i and --install , it is probably straightforward to come up with
      the correspondence; with -a and --configure , this is already somewhat less

B Options for dpkg can be given on the command line or else placed in the
  /etc/dpkg/dpkg.cfg file. In this file, the dashes at the start of the option names       dpkg.cfg
  must be omitted.

   If a package is installed using “dpkg --install ”, even though an earlier version Upgrade
already exists on the system, the older version is deinstalled before configuring
the new one. If an error occurs during installation, the old version can be restored
in many cases.
   There are various reasons that might prevent a successful package installation, installation problems
   • The package requires one or more other packages that either have not yet
     been installed, or that are not included in the same installation operation.
     The corresponding check can be disabled using the --force-depends option—
     but this can severely mess up the system.
   • An earlier version of the package is installed and set to hold (e. g., using
     aptitude ). This prevents newer versions of the package from being installed.

   • The package tries to unpack a file that already exists on the system and be-
     longs to a different package, unless the current package is explicitly labeled
     as “replacing” that package, or the --force-overwrite option was specified.
184                                                            12 Software Package Management Using Debian Tools

               conflicts Some packages conflict with each other (see the possibilities for package depen-
                         dencies on page 187). For example, only one mail transport program may be in-
                         stalled at one time; if you want to install, e. g., Postfix, Exim (the Debian default
                         MTA) must be removed at the same time. dpkg manages this if certain conditions
                         are fulfilled.
        Virtual packages     Sometimes packages do not depend on a particular other package but on a
                         “virtual” package describing a feature that can, in principle, be provided by any
                         of several other packages, such as mail-transport-agent , which is provided by pack-
                         ages like postfix , exim , or sendmail . In this case it is possible to replace, say, Exim by
                         Postfix in spite of dependencies, as a program providing the “virtual” functional-
                         ity will be available at all times.

                          C 12.2 [1] Download a Debian package—say, hello —from (or
                            any other Debian mirror) and install it using dpkg . (If you use anything else,
                            such as apt-get —see next section—, you’re cheating!) You can find Debian
                            packages on the server reasonably conveniently given their names, by look-
                            ing in pool/main for a subdirectory corresponding to the first character of
                            the name, and in there looking for a subdirectory whose name is that of the
                            package1 , in our case pool/main/h/hello . Exception: Since very many package
                            names start with lib , a package like libfoobar ends up in pool/main/libf .

                          C 12.3 [2] Locate the current list of virtual packages in Debian GNU/Linux.
                            Where did you find it? When did the last update take place?

                         12.2.3         Deleting Packages
                         A package is removed using the

                          # dpkg --remove hello

                         command (“dpkg -r ”, for short). Its configuration files (all the files listed in the
                         conffiles file within control.tar.gz ), though, are kept around in order to facilitate
                         a subsequent reinstallation of the package. The

                          # dpkg --purge hello

                         (or “dpkg -P ”) command removes the package including its configuration files.

                          B The “configuration files” of a Debian package are all files in the package
                            whose names occur in the conffiles file in control.tar.gz . (Look at /var/lib/
                            dpkg/info/ ⟨package name⟩.conffiles .)

      Removal problems       Package removal does not necessarily work, either. Possible obstacles include:
                             • The package is required by one or more other packages that are not about
                               to be removed as well.
                             • The package is marked “essential” (to system functionality). The shell, for
                               example, cannot simply be removed since some important scripts could no
                               longer be executed.
                         Here, too, the relevant checks can be disabled using suitable --force- … options (at
                         your own risk).

                          C 12.4 [1] Remove the package you installed during Exercise 12.2. Make sure
                            that its configuration files are removed as well.
                            1 The   source code package, really, which may differ. So don’t get too fancy.
12.2 The Basis: dpkg                                                                                        185

12.2.4     Debian Packages and Source Code
When dealing with source code, a basic principle of the Debian project is to distin-
guish clearly between the original source code and any Debian-specific changes.
Accordingly, all changes are placed in a separate archive. In addition to the
Debian-specific control files, these include more or less extensive fixes and cus-
tomisations to the software itself. For each package version, there is also a “source source control file
control file” (using the .dsc suffix) containing checksums for the original archive
and the changes file, which will be digitally signed by the Debian maintainer in
charge of the package:

$ ls hello*
-rw-r--r-- 1 anselm anselm   6540 Jun 7 13:18 hello_2.8-2.debian.tar.gz
-rw-r--r-- 1 anselm anselm   1287 Jun 7 13:18 hello_2.8-2.dsc
-rw-r--r-- 1 anselm anselm 697483 Mai 27 23:47 hello_2.8.orig.tar.gz

You can also see that the original source code archive does not change for all of the
2.8 version of the program (it does not contain a Debian release number). Every
new version of the Debian package of hello ’s 2.8 version comes with new .dsc
and .debian.tar.gz files. The latter contains all the changes relative to the original
archive (rather than the hello_2.8-2 package).

B In former times, the Debian project used a less complicated structure where
  there was one single file (created with diff ) containing all Debian-specific
  changes—in our example, hypothetically hello_2.8- 2.diff.gz . This approach
  is still supported, and you may find this structure with older packages that
  have not been changed to use the new method instead. The new struc-
  ture does have the advantage that different changes—like the introduction
  of the Debian-specific control files and any changes to the actual original
  code—can be more cleanly separated, which greatly simplifies maintaining
  the package within the Debian project.

    The dpkg-source command is used to reconstruct a package’s source code from         dpkg-source
the original archive and the Debian changes such that you can recompile your
own version of the Debian package. To do so, it must be invoked with the name
of the source control file as an argument:

$ dpkg-source -x hello_2.8-2.dsc

The original archive and the .debian.tar.gz or .diff.gz file must reside in the same
directory as the source control file. dpkg-source also places the unpacked source
code there.

B dpkg-source is also used when generating source archives and Debian change Preparing Debian packages
  files during Debian package preparation. However, this topic is beyond the
  scope of the LPIC-1 certification.

C 12.5 [1] Obtain the source code for the package you installed during Exer-
  cise 12.2, and unpack it. Take a look at the debian subdirectory of the result-
  ing directory.

12.2.5     Package Information
You can obtain a list of installed packages using “dpkg --list ” (-l , for short):      package list

$ dpkg --list
| Status=Not/Installed/Config-files/Unpacked/Failed-config/H
186                                                              12 Software Package Management Using Debian Tools

                             |/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,
                             ||/ Name           Version        Description
                             ii a2ps            4.13b+cvs.2003 GNU a2ps - 'Anything to Po
                             ii aalib1          1.4p5-19       ascii art library
                             ii abcm2ps         4.0.7-1        Translates ABC music descr
                             ii abcmidi         20030521-1     A converter from ABC to MI

                             (truncated on the right for space reasons) This list can be narrowed down using
      shell search patterns shell search patterns:

                             $ dpkg -l lib*-tcl
                             | Status=Not/Installed/Config-files/Unpacked/Failed-config/H
                             |/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,
                             ||/ Name           Version        Description
                             pn libdb3-tcl      <none>         (no description available)
                             un libdb4.0-tcl    <none>         (no description available)
                             un libdb4.1-tcl    <none>         (no description available)
                             un libmetakit-tcl <none>          (no description available)
                             ii libsqlite-tcl 2.8.9-1          SQLite TCL bindings
                             rc libsqlite0-tcl 2.6.1-2         SQLite TCL bindings

                          The packages with version “<none> ” are part of the distribution but are either not
                          installed on the current system (status un ) or have been removed (status pn ).
           package status    You can find out about an individual package’s status with the --status (-s )
                             $ dpkg --status hello
                             Package: hello
                             Status: install ok installed
                             Priority: optional
                             Section: devel
                             Installed-Size: 553
                             Maintainer: Santiago Vila <>
                             Architecture: amd64
                             Version: 2.8-2
                             Depends: libc6 (>= 2.4), dpkg (>= 1.15.4) | install-info
                             Description: The classic greeting, and a good example
                              The GNU hello program produces a familiar, friendly greeting. It
                               allows non-programmers to use a classic computer science tool which
                               would otherwise be unavailable to them.
                               Seriously, though: this is an example of how to do a Debian package.
                               It is the Debian version of the GNU Project's `hello world' program
                               (which is itself an example for the GNU Project).

                             Besides the package name (Package: ), its output includes information about the
                             package’s status and priority (from required via important , standard and optional
                             down to extra ) and its approximate area of interest (Section: ). The Maintainer: is
                             the person who is in charge of the package on behalf of the Debian project.
                Priorities   B Packages of priority required are necessary for proper operation of the sys-
                               tem (usually because dpkg depends on them). The important priority encom-
                               passes packages one would expect to be available on a Unix-like system2 .
                                2 The definition is something like “A package is important if, should it be missing, experienced Unix

                             users would shake their heads and go “WTF?”.
12.2 The Basis: dpkg                                                                                    187

      standard  adds those packages that make sense for a net but not overly re-
      strictive system running in text mode—this priority describes what you get
      if you install Debian GNU/Linux without selecting any additional pack-
      ages. The optional priority applies to everything you might want to install
      if you do not look too closely and have no particular requirements. This
      includes things like the X11 GUI and a whole load of applications (such as
      TEX). There should be no conflicts within optional . Finally, extra is used for
      all packages that conflict with packages of other priorities, or that are only
      useful for specialised applications.

B Packages may not depend on packages of lower priority. For this to hold in
  all cases, the priorities of some packages have deliberately been tweaked.

   An important area of information are the package dependencies, of which package dependencies
there are several types:
Depends The named packages must be configured for the package to be able to be
      configured. As in the preceding example, specific versions of the packages
      may be called for.
Pre-Depends The named packages must be completely installed before installation
      of the package can even begin. This type of dependency is used if, for exam-
      ple, the package’s installation scripts absolutely require software from the
      other package.

RecommendsA non-absolute but very obvious dependency. You would nearly al-
      ways install the named packages alongside this package, and only refrain
      from doing so in very unusual circumstances.
Suggests The named packages are useful in connection with the package but not
EnhancesLike Suggests , but in reverse—this package is useful for the named pack-
      age (or packages).
ConflictsThis package cannot be installed at the same time as the named pack-

  If a package isn’t installed locally at all, “dpkg --status ” only outputs an error

# dpkg -s xyzzy
Package `xyzzy' is not installed and no info is available.
Use dpkg --info (= dpkg-deb --info) to examine archive files,
and dpkg --contents (= dpkg-deb --contents) to list their contents.

   The --listfiles (-L ) option provides a list of files within the package:             list of files

$ dpkg --listfiles hello
188                                                             12 Software Package Management Using Debian Tools


               package search        Finally, you can use the --search (or -s ) option to find out which package (if any)
                                  claims a given file. Search patterns are allowed:

                                  $ dpkg -S bin/m*fs
                                  dosfstools: /sbin/mkdosfs
                                  cramfsprogs: /usr/sbin/mkcramfs
                                  util-linux: /sbin/mkfs.cramfs
                                  smbfs: /sbin/mount.smbfs

                                  The search may take some time, though.

                                  B If you’re looking for the package for a file that is not on your system—for
                                    example, if you plan to install that package afterwards—, you can use the
                                    search form on . This
                                    allows you to search any or all Debian distributions and architectures as
                                    well as to search for exact file name matches and file names containing cer-
                                    tain search terms.

                                  C 12.6 [3] How many packages whose names start with lib are installed on
                                    your system? How many of those packages have priority required ?

                                  12.2.6    Package Verification
 integrity of an installed package The integrity of an installed package can be checked using the debsums program
                                  (from the eponymous package):

                                  $ debsums hello
                                  /usr/share/doc/hello/changelog.Debian.gz     OK
                                  /usr/share/doc/hello/copyright               OK
                                  /usr/share/doc/hello/NEWS                    OK
                                  /usr/share/doc/hello/changelog.gz            OK
                                  /usr/share/info/                OK

                                  This compares the MD5 checksums of the individual files with the content of
                                  the corresponding file in /var/lib/dpkg/info (here, hello.md5sums ). If an actual file’s
                                  checksum does not match the set value, FAILED is displayed in place of OK .

                                  B debsums can uncover “inadvertent” changes to a package’s files, but does not
      protection from intruders     provide protection from intruders who maliciously modify files. After all, a
                                    cracker could place the checksum of a modified file in its package’s md5sums
                                    list. Neither does this method help agains “Trojan” packages that hide ma-
                                    licious code behind an innocuous facade. We shall be coming back to the
                                    “integrity of packages” topic in Section 12.4.

                                  C 12.7 [!2] Change a file in an installed Debian package. (Look for a not-so-
                                    important one, like that from Exercise 12.2.) You could, for example, append
                                    a few lines of text to the README.Debian file (be root ). Check the integrity of the
                                    package’s files using debsums .
12.3 Debian Package Management: The Next Generation                                                            189

12.3      Debian Package Management: The Next Genera-
12.3.1    APT
dpkg  is a powerful tool, but still somewhat restricted in its potential. For example,
it is a bit aggravating that it will notice unfilled dependencies between packages,
but then just throw in the towel instead of contributing constructively to a solution
of the problem. Furthermore, while it is nice be able to install locally-available
packages, one would wish for convenient access to FTP or web servers offering

B The dselect program, which in the early days of Debian served as an inter-
  active front-end to package management, is officially deprecated today—its
  inconvenience was proverbial, even though reading the manual did help as
  a rule.

   Quite early in the history of the Debian project (by today’s standards), the De-
bian community started developing APT, the “Advanced Packaging Tool”. This
project, in its significance as in its ultimate pointlessness, is comparable to the
quest of the Knights of the Round Table for the Holy Grail, but, like the Grail
quest, APT development led to many noble deeds along the way. Although few
dragons were slain and damsels freed from distress, the APT developers produced
very important and powerful “partial solutions” whose convenience and feature
set remains unequalled (which is why some RPM-based distributions have begun
to ad-“apt” them for their purposes).

12.3.2    Package Installation Using apt-get
The first of these tools is apt-get , which represents a sort of intelligent superstruc-
ture for dpkg . It does not offer an interactive interface for package selection, but
could initially be used as a back-end for dselect , to install packages selected in
dselect . Today it is mostly useful on the command line. The most important prop-
erties of apt-get include the following:
   • apt-get can manage a set of installation sources simultaneously. For exam- Several installation sources
     ple, it is possible to use a “stable” Debian distribution on CD-ROM in par-
     allel to a HTTP-based server containing security updates. Packages are nor-
     mally installed from CD-ROM; only if the HTTP server offers a more current
     version of a package will it be fetched from the network. Certain packages
     can be requested from certain sources; you can, for example, use a stable
     Debian distribution for the most part but take some packages from a newer
     “unstable” distribution.
   • It is possible to update all of the distribution at once (using “apt-get dist-upgradeUpgrades
     with dependencies being resolved even in the face of package renamings
     and removals.
   • A multitude of auxiliary tools allows, e. g., setting up caching proxy servers auxiliary tools
     for Debian packages (apt-proxy ), installing packages on systems that are not
     connected to the Internet (apt-zip ), or retrieving a list of bugs for a pack-
     age before actually installing it (apt-listbugs ). With apt-build , you can com-
     pile packages with specific optimisations for your system and create a local
     package repository containing such packages.
   Package sources for apt-get are declared in /etc/apt/sources.list :                    package sources

deb stable main
deb stable/updates main
deb-src main
190                                                     12 Software Package Management Using Debian Tools

                          Binary packages will be fetched from , as will the corre-
                          sponding source code. In addition, the server is accessed, on
                          which the Debian project places updated package version that fix security bugs.
      operating procedure    The standard operating procedure using apt-get is as follows: First you update
                          the local package availability database:

                          # apt-get update

                         This consults all package sources and integrates the results into a common pack-
                         age list. You can install packages using “apt-get install ”:

                          # apt-get install hello
                          Reading Package Lists... Done
                          Building Dependency Tree... Done
                          The following NEW packages will be installed:
                          0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
                          Need to get 68.7kB of archives.
                          After unpacking 566kB of additional disk space will be used.

                         This will also install or upgrade all packages mentioned in Depends: dependencies,
                         as well as any packages that these packages depend upon, and so on.
                            You may also install several packages at the same time:

                          # apt-get install hello python

                         or install some packages and install others simultaneously: The

                          # apt-get install hello- python python-django+

                           command would remove the hello package and install the python and python-django
                           packages (including their dependencies). The “+ ” is not mandatory but allowed.
                           With “apt-get remove ” you can remove packages directly.
            simple update     The “apt-get upgrade ” installs the newest available versions of all packages in-
                           stalled on the system. This will not remove installed packages nor install new
                           packages; packages that cannot be updated without such actions (because depen-
                           dencies have changed) remain at their present state.
      “intelligent” update    The “apt-get dist-upgrade ” command enables an “intelligent” conflict resolution
                           scheme which tries to resolve changed dependencies by judiciously removing and
                           installing packages. This prefers more important packages (according to their pri-
                           ority) over less important ones.
               source code    You can fetch a package’s source code using the “apt-get source ” command:

                          # apt-get source hello

                         This also works if the binary package is one of several that have been created from
                         a (differently named) source package.

                          B The apt programs are usually configured by means of the /etc/apt/apt.conf
                            file. This includes options for apt-get , apt-cache , and other commands from
                            the apt bunch.

                          C 12.8 [!1] Use apt-get to install the hello package and then remove it again.

                          C 12.9 [1] Download the source code for the hello package using apt-get .
12.3 Debian Package Management: The Next Generation                                                   191

12.3.3    Information About Packages
Another useful program is apt-cache , which searches apt-get ’s package sources:          apt-cache

$ apt-cache search hello                                 hello   in name or description

grhino-data - othello/reversi boardgame - data-files
gtkboard - many board games in one program
hello - The classic greeting, and a good example
hello-debhelper - The classic greeting, and a good example
jester - board game similar to Othello

$ apt-cache show hello
Package: hello
Version: 2.8-2
Installed-Size: 553
Maintainer: Santiago Vila <>
Architecture: amd64

The output of “apt-cache show ” mostly corresponds to that of “dpkg --status ”, except
that it works for all packages in a package source, no matter whether they are
installed locally, while dpkg only deals with packages that are actually installed.
    There are also a few other interesting apt-cache subcommands: depends displays
all the dependencies of a package as well as the names of packages fulfilling that

$ apt-cache depends hello
  Depends: libc6
 |Depends: dpkg
  Depends: install-info

B The vertical bar in the second dependency line indicates that the depen-
  dency in this line or the one in the following line must be fulfilled. In this
  example, the dpkg package or the install-info package must be installed (or

Conversely, rdepends lists the names of all packages depending on the named pack-

$ apt-cache rdepends python
Reverse Depends:

B If a package occurs several times in the list, this is probably because the
  original package specified it several times, typically with version numbers.
  The python-apt package, for example, contains among other things

       … python (>= 2.6.6-7~), python (<< 2.8), …

      to signal that it will only work with particular versions of the Debian Python
192                                                   12 Software Package Management Using Debian Tools

                          stats   provides an overview of the content of the package cache:

                       $ apt-cache stats
                       Total package names: 33365 (1335k)                           All packages in the cache
                         Normal packages: 25672                                     Packages that really exist
                         Pure virtual packages: 757                            Placeholders for functionality
                         Single virtual packages: 1885                              Just one implementation
                         Mixed virtual packages: 267                                Several implementations
                         Missing: 4784                      Packages   in dependencies that no (longer?) exist
                       Total distinct versions: 28955 (1506k)                   Package versions in the cache
                       Total distinct descriptions: 28955 (695k)
                       Total dependencies: 182689 (5115k)                   Number of pairwise relationships
                       Total ver/file relations: 31273 (500k)
                       Total Desc/File relations: 28955 (463k)
                       Total Provides mappings: 5747 (115k)
                       Total globbed strings: 100 (1148)
                       Total dependency version space: 756k
                       Total slack space: 73.5k
                       Total space accounted for: 8646k

                       C 12.10 [2] How can you find all packages that must be installed for a particu-
                         lar package to work? (Compare the output of “apt-cache depends x11-apps ” to
                         that of “apt-cache depends libxt6 ”.)

                       12.3.4      aptitude
                       The program aptitude does package selection and management and has taken over
                       the old dselect ’s rôle in Debian GNU/Linux. On the console or inside a terminal
                       emulator, it features an interactive user interface with menus and dialogs, but also
                       provides command-line options that are roughly compatible to those of apt-get .
                       Since Debian 4.0 (popularly called “etch”), aptitude is the recommended program
                       for package installation and updates.

                       B Newer versions of aptitude include a GTK+-based user interface that can be
                         installed optionally.

      improvements        Compared to apt-get and dselect , aptitude offers various improvements, includ-
                          • It does not necessarily need to be invoked as root , but asks for the root pass-
                            word before actions requiring administrator privileges are performed.
                          • It can remember which packages have been installed to fulfil dependencies,
                            and remove these automatically if all packages depending on them have
                            been removed. (In the meantime apt-get has learned to do that, too; see
                            apt-get (8), the autoremove command.)

                          • With aptitude , you have interactive access to all versions of a package avail-
                            able from various package sources, not just the most up-to-date one.
      interactive UI       The aptitude command invokes the interactive UI (Figure 12.1). Near the top
                       of the console (or terminal, as the case may be) you see a “menu bar”, below that
                       there is a line with a short help message and a line with the program’s version
                       number. The remainder of the screen is split in two parts: The upper half displays
                       an overview of the various types of package (updated, new, installed, and so on),
                       the lower half is used for explanatory messages.
                           With the ↑ and ↓ keys you can navigate in the package list. Lines starting
                       wiht --- represent the headings of “subtrees” of the package list, and ↩ can be
12.3 Debian Package Management: The Next Generation                                                         193

                         Figure 12.1: The aptitude program

used to “open” the next level of such a subtree. (You can open all of the subtree
by typing ] .) / gives you a window that lets you enter a search term (or reg-
ular expression) for a package name. When scrolling through the package lists,
explanations for the packages encountered are displayed in the lower part of the
screen, and you can scroll these up or down using the a and z keys. The i key
lets you change from the explanatory text to a representation of the dependencies.
    If the cursor bar sits on a package’s line, you can select it for installation or
updating using the + key, or mark it for deletion using - . If you want to remove
it completely (as in “dpkg --purge ”), use _ . = sets a package’s status to “hold”,
which means that it will no longer be automatically upgraded.
    With u , you can update the package lists (like “apt-get update ”) and then check
the “Updated Packages” subtree to find which packages aptitude would update.
Using U , you can mark all of these packages for actual updating. The “New
Packages” subtree displays those packages added since the last update; f empties
this list and places these packages among the “normal” lists. The Ctrl + t key
combination opens the menu bar, in which you can move using the arrow keys
and select a function using ↩ .
    The g command starts the actual installation, update, or package removal. At
first it shows an overview of the planned actions, which you may revise using the
usual commands. Another g starts the actual work: First all required new pack-
ages are fetched, then aptitude calls dpkg to atually install or remove the desired

B Contrary to popular perception, aptitude is not really a front-end to apt-get ,
  but does by itself whatever apt-get would otherwise do.

   If conflicts occur, aptitude offers solution strategies by way of suitable proposals solution strategies
for installations, updates, or package removals, from which you can pick the one
that is most appropriate.

B In its default configuration, aptitude automatically installs even those pack-
  ages that a package marks Recommended: . This is not always what is wanted
  and can be switched off from the “Options” menu.

      You can install and use aptitude on Ubuntu, but it is not the recommended
      program. For this reason, it does not agree 100% with the graphical tools
194                                                         12 Software Package Management Using Debian Tools

                                    proposed for package management by Ubuntu—so you should either do
                                    everything like Ubuntu recommends, or else do everything using aptitude .

                              12.4      Debian Package Integrity
                                The debsums program is used to check the integrity of the files in a single package
                                (Section 12.2.6). This is nice but does not ensure that an attacker has not manip-
                                ulated both the files in the package and the .md5sums file containing the original
 integrity of complete packages checksums. The question remains: How does the Debian project ensure the in-
                                tegrity of complete packages (and, based on this, the integrity of the whole distri-
                                bution)? This works as follows:
                                 • Every Debian package is cryptographically signed by a Debian developer.
                                   This means that the receipient of the package can use the developer’s public
                                   key to verify that they received the package in the state it was in when the
                                   developer released it.

                                    B The Debian developer who signed the package must not necessar-
                                      ily have been the person who assembled it. In principle, every De-
                                      bian developer may sign and release any package in Debian (a “non-
                                      maintainer upload”), and this is being done in practice for timely
                                      updates fixing critical security holes and to adopt “orphaned” pack-
                                      ages. Furthermore, there are numerous people who help with Debian
                                      GNU/Linux and, even though they are not formally Debian develop-
                                      ers (or whose applications for developer status are pending), maintain
                                      packages. These people cannot by themselves release packages, but
                                      must do this via a “sponsor” who must be a Debian developer. The
                                      sponsor assumes the responsibility that the package is reasonable.

                                   A You should not overestimate the security gained through digital sig-
                                     natures: A developer’s signature does not guarantee that there is no
                                     malicious code in a package, but only that the developer signed the
                                     package. Theoretically it is possible for a cracker to pass the Debian
                                     developer accreditation procedure and be able to release official pack-
                                     ages into the distribution—whose control scripts most users will exe-
                                     cute uncritically as root . Most other Linux distributions share the same

                                 • The Debian infrastructure only accepts packages for publication that have
                                   been signed by a Debian developer.
                                 • On the Debian server (and all servers mirroring Debian GNU/Linux) there
                                   is a file (or several) called Packages.gz for each current Debian distribution.
                                   This file contains the MD5 checksums of all packages in the distribution, as
                                   they are on the server; since the server only accepts packages from accred-
                                   ited developers, these are authentic.
                                 • For each current Debian distribution on the server there is a file called
                                   Release , which contains the MD5 checksums of the Packages.gz file(s) in-
                                   volved. This file is cryptographically signed (the signature is in a file called
                                   Release.gpg ).

                              With this chain of checksums and signatures, the integrity of packages in the dis-
                              tribution can be checked:

                                 • A new package is downloaded and its MD5 checksum is determined.
                                 • We check whether the signature of the Release file is correct, and, if so, read
                                   the MD5 checksum of Packages.gz from that file.
12.5 The debconf Infrastructure                                                         195

   • With that checksum, we verify the integrity of the actual Packages.gz file.
   • The MD5 checksum of the package given in Packages.gz must match that of
     the downloaded file.
If the MD5 checksum of the downloaded file does not match the “nominal value”
from Packages.gz , the administrator is made aware of this fact and the package is
not installed (just yet, anyway).

B It is possible to configure a Debian system such that it only installs packages
  that can be verified in this way. (Usually all you get is warnings which can be
  overridden manually.) With this, you can construct an infrastructure where
  only packages from a considered-safe-and-sensible “subdistribution” can
  be installed. These packages may be from Debian GNU/Linux or else have
  been made available locally.

B The APT infrastructure only trusts package sources for which a public
  GnuPG key has been placed in the /etc/apt/trusted.gpg . The apt-key pro-
  gram is used to maintain this file.

B The current public keys for Debian package sources are contained in the
  debian-archive-keyring package and can be renewed by updating this file.
  (Debian rotates the keys on a yearly basis).

    You can find out more about managing signed packages in Debian in [F+ 07,
chapter 7]. We explain GnuPG in the Linup Front training manual Linux Admin-
istration II.

12.5      The debconf Infrastructure
Sometimes questions come up during the installation of software packages. For
example, if you are installing a mail server package, it is important to know, in or-
der to generate an appropriate configuration file, whether the computer in ques-
tion is connected directly to the Internet, whether it is part of a LAN with its own
dedicated mail server, or whether it uses a dial-up connection to access the net. It
is also necessary to know the domain the computer is to use for its messages and
so on.
    The debconf mechanism is designed to collect this information and to store
it for future use. It is basically a database for system-wide configuration set-
tings, which can, for example, be accessed by the installation scripts of a package.
To manipulate the database, debconf supports modular user interfaces covering
all tastes from very simple textual prompts to text-oriented dialogs and various
graphical desktop applications such as KDE and GNOME. There are also inter-
faces to popular programming languages like Python.
    You can redo the initial debconf-based configuration of a software package at
any time by giving a command like

# dpkg-reconfigure my-package

dpkg-reconfigurerepeats the questions asked during the original installation pro-
cess of the package, using the pre-set user interface.

B You can select another user interface on an ad-hoc basis by means of the
  --frontend (or -f ) option. The possible names are given in debconf (7) if you
  have installed the debconf-doc package. The default is dialog .

B To change the user interface temporarily if you are not calling dpkg-reconfigure
  directly, use the DEBCONF_FRONTEND environment variable:

       # DEBCONF_FRONTEND=noninteractive aptitude upgrade
196                                   12 Software Package Management Using Debian Tools

        With dpkg-reconfigure , you can also control the level of detail of the questions
      you will be asked. Use the --priority (or -p ) option followed by a priority. The
      possible priorities are (in descending order):
      critical   Questions you absolutely must answer lest terrible things happen.

      high   Questions without a sensible default—your opinion counts.
      medium   Questions with a sensible default.
      low    Trivial questions with a default that works most of the time.

      If you say something like

      # dpkg-reconfigure --priority=medium my-package

      you will be asked all priority critical , high , and medium questions; any priority low
      questions will be skipped.

      B For ad-hoc changes if debconf is called indirectly, there is also the DEBCONF_PRIORITY
        environment variable.

         The debconf infrastructure is fairly complex but useful. For example, it is pos-
      sible to put the answers into an LDAP database that is accessible to all computers
      on a network. You can thus install a large number of machines without manual
      intervention. To explain this in detail would, however, be beyond the scope of this

      C 12.11 [1] How can you change the pre-set user interface for debconf on a
        permanent basis?

      12.6        alien :   Software From Different Worlds
      Many software packages are only available in the popular RPM format. Commer-
      cial packages in particular are more likely to be offered for the Red Hat or SUSE
      distributions, even though nothing would prevent anyone from trying the soft-
      ware on Debian GNU/Linux (serious use may be precluded by the loss of man-
      ufacturer support if a non-approved platform is used). You cannot install RPM
      packages on a Debian system directly, but the alien program makes it possible
      to convert the package formats of various Linux distributions—besides RPM also
      the Stampede and Slackware formats (not that these are desperately required)—to
      the Debian package format (and vice-versa).
          Important: While alien will let you convert packages from one format to another,
      there is no guarantee whatsoever that the resulting package will be useful in any
      way. On the one hand, the programs in the package may depend on libraries
      that are not available (or not available in the appropriate version) in the target
      distribution—since alien does not deal with dependencies, you must sort out any
      problems of this type manually. On the other hand, it is quite possible that the
      package integrates itself into the source distribution in a way that is impossible or
      difficult to replicate on the target distribution.
          As a matter of principle, the farther “down” a package sits in the system the
      smaller the probability that alien will do what you want. With packages that con-
      sist of a few executable programs without bizarre library dependencies, the cor-
      responding manual pages, and a few example files, chances are good for alien
      to do the Right Thing. With system services that must integrate into the system
      boot sequence, things may well look different. And you should not even think of
      replacing libc …
12.6 Bibliography                                                                         197

B alien is used, in particular, to convert LSB-compliant software packages for
  installation on a Debian GNU/Linux system—LSB specifies RPM as the
  software distribution package format.

   After these introductory remarks, we’ll show you quickly how to use alien to
convert a RPM package to a Debian package:

# alien --to-deb paket.rpm

(Where --to-deb represents the default case and may be left out.) The reverse is
possible using

# alien --to-rpm paket.deb

To assemble and disassemble RPM files, the rpm program must be installed (which
is available as a package for Debian GNU/Linux); to assemble deb packages you
need a few pertinent Debian packages which are listed in alien (1p). (As mentioned
in Section 12.2, you can take deb packages to bits on almost all Linux systems using
“on-board tools” such as tar , gzip , and ar .)

Commands in this Chapter
alien    Converts various software packaging formats             alien (1) 196
apt-get  Powerful command-line tool for Debian GNU/Linux package manage-
         ment                                                  apt-get (8) 189
aptitude Convenient package installation and maintenance tool (Debian)
                                                              aptitude (8) 192
dpkg     Debian GNU/Linux package management tool                 dpkg (8) 182
dpkg-reconfigure Reconfigures an already-installed Debian package
                                                      dpkg-reconfigure (8) 195

F+ 07 Javier Fernández-Sanguino Peña, et al. “Securing Debian Manual”, 2007.
                    debian- howto/
                                                                                          $ echo tux
                                                                                          $ ls
                                                                                          $ /bin/su -

Package Management with RPM
and YUM

13.1 Introduction. . . . . . . . . . . . . . .          .   .   .   .   .   .   .   200
13.2 Package Management Using rpm . . . . . . . .       .   .   .   .   .   .   .   201
    13.2.1 Installation and Update . . . . . . . .      .   .   .   .   .   .   .   201
    13.2.2 Deinstalling Packages . . . . . . . . .      .   .   .   .   .   .   .   201
    13.2.3 Database and Package Queries . . . . . .     .   .   .   .   .   .   .   202
    13.2.4 Package Verification . . . . . . . . . .      .   .   .   .   .   .   .   204
    13.2.5 The rpm2cpio Program . . . . . . . . .       .   .   .   .   .   .   .   204
13.3 YUM . . . . . . . . . . . . . . . . .              .   .   .   .   .   .   .   205
    13.3.1 Overview . . . . . . . . . . . . .           .   .   .   .   .   .   .   205
    13.3.2 Package Repositories . . . . . . . . .       .   .   .   .   .   .   .   205
    13.3.3 Installing and Removing Packages Using YUM   .   .   .   .   .   .   .   206
    13.3.4 Information About Packages . . . . . . .     .   .   .   .   .   .   .   208
    13.3.5 Downloading Packages. . . . . . . . .        .   .   .   .   .   .   .   210

   • Knowing the basics of RPM and related tools
   • Being able to use rpm for package management
   • Being able to use YUM

   • Knowledge of Linux system administration
   • Experience with an RPM-based Linux distribution is helpful

adm1-rpm.tex   (33e55eeadba676a3 )
200                                                       13 Package Management with RPM and YUM

                   13.1      Introduction
                   The “Red Hat Package Manager” (RPM, for short) is a tool for managing software
                   packages. It supports the straightforward installation and deinstallation of pack-
                   ages while ensuring that different packages do not conflict and that dependencies
                   between the packages are taken into account. In addition, RPM allows you to
                   specify package queries and to ensure the integrity of packages.
                      RPM’s core is a database. Software packages add themselves when they are
                   installed and remove themselves again when they are deinstalled. To allow this,
                   software packages must be provided in a specific format, i. e., as RPM packages.
                      The RPM package format is used used by many distributions (including those
                   by Red Hat, Novell/SUSE, TurboLinux, and Mandriva). An arbitrary RPM pack-
                   age, though, cannot generally be installed on any RPM-based distribution without
                   forethought: Since the RPM package contains compiled software, it must fit the
                   processor architecture in use; since file system structure, the type of service con-
                   trol (init scripts, etc.), and the internal description of dependencies differ between
                   distributions or even between different versions of the same distribution, careless
                   installation across distribution may cause problems.

                   B RPM was originally developed by Red Hat and was accordingly called “Red
                     Hat Package Manager” at first. Since various other distributions have taken
                     to using the program, it has been renamed to “RPM Package Manager”.

                   B At the moment there is a certain controversy as to who is in charge of further
                     development of this critical piece of infrastructure. After a long hiatus, dur-
                     ing which nobody really bothered to put out a canonical version, some Fe-
                     dora developers tried in 2006/7 to restart RPM development as an officially
                     distribution-neutral project (this project is now led by Panu Matilainen of
                     Red Hat, with developers affiliated with some other RPM-using distribu-
                     tions in support). Independently, Jeff Johnson, the last official RPM devel-
                     oper at Red Hat (who is no longer with the company), is putting work into
                     RPM and claims that his code represents “the official code base”—although
                     no Linux distribution seems to pay attention.

       file name      An RPM package has a compound file name, for example


                  which usually consists of the package name (openssh-3.5p1-107 ), the architecture
                  (i586 ) and the .rpm suffix. The package name is used to identify the package inter-
                  nally once it has been installed. It contains the name of the software (openssh ) and
                  the software version as assigned by its original developers (3.5p1 ) followed by a
                  release number (107 ) assigned by the package builder (the distributor).
                     The “RPM Package Manager” is invoked using the rpm command, followed by
      basic mode a basic mode. The most important modes will be discussed presently, excepting
                  the modes for initialising the RPM database and constructing and signing RPM
                  packages, which are outside the scope of this course.
          options    There is a number of global options as well as supplementary, mode-specific
                  options. Since some modes and supplementary options are identical, the mode
                  must (unlike with tar ) be specified first.
                     Global options include -v and -vv , which increase the “verbosity” of RPM’s

                   B RPM’s configuration is stored within the /usr/lib/rpm directory; local or in-
                     dividual customisations are made within the /etc/rpmrc or ~/.rpmrc files, but
                     should not be necessary for normal operations.
13.2 Package Management Using rpm                                                                     201

13.2      Package Management Using rpm
13.2.1    Installation and Update
An RPM package is installed in -i mode, followed by the package file’s path name,
such as
# rpm -i /tmp/openssh-3.5p1-107.i586.rpm

You may also specify the path as an HTTP or FTP URL in order to install package
files that reside on remote serers. It is permissible to specify several packages at
once, as in “rpm -i /tmp/*.rpm ”.
   Additionally, there are the two related modes -U (“upgrade”) and -F (“freshen”).
The former removes any older versions of a package as well as installing the new
one, while the latter installs the package only if an earlier version is already
installed (which is subsequently removed).
   All three modes support a number of options, which must absolutely be men-
tioned after the mode. Besides -h (“hash mark”, for a progress bar) there is --test ,
which prevents the actual installation and only checks for possible conflicts.
   When a conflict occurs, the package in question is not installed. Conflicts arise
   • an already-installed package is to be installed again,
   • a package is to be installed even though it is already installed in a different
     version (mode -i ) or a more current version (mode -U ),
   • the installation would overwrite a file belonging to a different package,
   • a package requires a different package which is not already installed or
     about to be installed.
If the installation fails for any of these reasons, you can force it to be performed
through options. For example, the --nodeps option disables the dependency check.
    Further options can influence the installation itself (rather than just the security
checks). For example, you can move packages to different directories on installa-
tion. This is the only way to install, e. g., Apache 1.3 and Apache 2.0 at the same
time, since usually both of them would claim /sbin/http for themselves: one of the
two must move to /usr/local .

13.2.2    Deinstalling Packages
Packages can be deinstalled using -e (“erase”) mode, e. g.,

# rpm -e openssh-3.5p1-107

Note that you need to specify the internal package name rather than the package
file path, since RPM does not remember the latter. (The next section will tell you
how to find out the package name.) You can also abbreviate the package name as
long as it stays unique. If there is no other openssh package, you might also remove
it by

# rpm -e openssh

Again, RPM takes care not to remove packages that other packages depend upon.
   The --test and --nodeps options have the same meaning as upon installation;
they must also appear after the mode.
   When deinstalling, all of the package’s installed files will be removed unless
they are configuration files that you have changed. These will not be removed but configuration files
merely renamed by appending the .rpmsave suffix. (The RPM package determines
which of its files will be considered configuration files.)
202                                                13 Package Management with RPM and YUM

      13.2.3      Database and Package Queries
      The “RPM Package Manager” becomes even more useful if you do not just con-
      sider it a package installation and removal tool, but also an information source.
      The mode for this is -q (“query”), and you can specify in more detail what kind of
      information you would like to obtain and from which package.

      Specifying the Package Without further options, rpm expects an internal package
      name, which may be abbreviated, and it outputs the full package name:

      $ rpm -q openssh

      This makes it easy to determine how current your system is. You can also find the
      package claiming a file, using the -f option:

      $ rpm -qf /usr/bin/ssh

      This lets you relate unknown files to a package. As a third possibility, you can
      obtain a list of all installed packages with the -a option:

      $ rpm -qa

      This list may of course be processed further, as in the following example:1

      $ rpm -qa | grep cups

         Finally, RPM allows you to query a non-installed package. Use -p followed by
      the package file’s name:

      $ rpm -qp /tmp/openssh-3.5p1-107.i586.rpm

      This does not look too spectacular, since the internal package name was already
      part of the package file name. But the file name might have been changed until it
      no longer had anything to do with the actual package name, and secondly, there
      are other questions that you might want to ask.

      Specifying the Query If you are not just interested in the package name, you can
      extend your query. Every extension may be combined with every way of specify-
      ing a packet. Via

      $ rpm -qi openssh

      you can obtain detailed information (-i ) about the package; while -l provides a
      list of all files belonging to the package, together with -v it forms an equivalent to
      the ls -l command:
        1 The naming and arrangement of packages is the package preparer’s concern; differences may occur

      depending on the distribution and version.
13.2 Package Management Using rpm                                                                        203

$ rpm -qlf /bin/bash

$ rpm -qlvf /bin/bash
-rwxr-xr-x root root           491992 Mar 14     2003 /bin/bash
lrwxrwxrwx root root                4 Mar 14     2003 /bin/sh -> bash

It is important to note that the files listed for a package are only those that show
up in the RPM database, namely those that a package brought along when it was
installed. This does not include files that were generated during installation (by
the package’s installation scripts) or during system operation (log files, etc.).
    We have already seen that RPM treats configuration files specially (when de-
installing). The second class of special files are documentation files; these can be
omitted from installation. The -c and -d options of query mode behave like -l , but
they confine themselves to configuration and documentation files, respectively.

Advanced Queries The following details are not relevant for LPIC-1, but they
will improve your understanding of RPM’s concepts and database structure.
   Dependencies between packages can belong to various types. For example, a dependencies
package may simply require a shell, i. e., /bin/sh . By means of

$ rpm -qf /bin/sh

it is straightforward to find out that this file is provided by the bash package (the
same can be done for non-installed packages).
    Things are different, e. g., for the SAINT package, a security analysis tool which
requires a web browser. Every specific dependency on a particular web browser
would be unduly limiting. For this reason, RPM lets packages provide or depend
upon “capabilities”. In our example, SAINT requires the abstract capability “web
browser”. The files and capabilities that a package requires can be queried using
the --requires option:2

$ rpm -q --requires saint

The packages providing these capabilities can be found using the --whatprovides

$ rpm -q --whatprovides web_browser

For SAINT, you need just one of these packages.
   In the same manner, the --provides and --whatrequires options allow you to
query the services (or files, with the -l option) that a package offers, and a ser-
vice’s consumers.
   2 Here, again, the assignment and naming of capabilities is up to the package preparer; it may thus

differ between distributions and versions.
204                                          13 Package Management with RPM and YUM

      13.2.4     Package Verification
      Pre-Installation Checks Two things may happen to a package which might pre-
      clude its installation: It may have been damaged during the download, i. e., the
      package is erroneous. Or the package is not what it pretends to be, i. e., it has
      been falsified—for example, because some malicious person tries to pass a “Tro-
      jan” package off as the original.
         RPM safeguards you against both scenarios: with

      $ rpm --checksig /tmp/openssh-3.5p1-107.i586.rpm
      /tmp/openssh-3.5p1-107.i586.rpm: md5 gpg OK

      an MD5 checksum of the package is compared to the checksum contained within
      itself, which guarantees the proper transmission of the package. In addition, the
      signature within the package, which was created using the private PGP or GPG
      key of the package preparer, is checked using the package preparer’s public key.
      This guarantees that the correct package has arrived.
          Should the MD5 checksum be correct but not the signature, the output looks
      correspondingly different:

      $ rpm --checksig /tmp/openssh-3.5p1-107.i586.rpm
      /tmp/openssh-3.5p1-107.i586.rpm: md5 GPG NOT OK

         Of course your distributor’s public key must be available on your system for
      the signature checks.

      Post-Installation Verification RPM lets you compare certain values within the
      RPM database to the file system. This is done by means of the -V (“verify”) mode;
      instead of one or more internal package names, this mode can use all specifica-
      tions made available for the query mode.

      # rpm -V   openssh
      .......T   c /etc/init.d/sshd
      S.5....T   c /etc/pam.d/sshd
      S.5....T   c /etc/ssh/ssh_config
      SM5....T   c /etc/ssh/sshd_config
      .M......     /usr/bin/ssh

      This output contains all files for which at least one “required” value from the
      database differs from the “actual” value within the file system: a “. ” signifies
      agreement, while a letter indicates a deviation. The following checks are per-
      formed: access mode and file type (M ), owner and group (U , G ); for symbolic links,
      the path of the referenced file (L ); for device files, major and minor device num-
      bers (D ); for plain files the size (S ), modification time (T ), and content (5 ). Since
      configuration files are unlikely to remain in their original state, they are labeled
      with a c .
         Even though the verification of installed packages using RPM cannot replace
      an “intrusion detection system” (why should an intruder not modify the RPM
      database as well?), it can be useful to limit the damage, e. g., after a hard disk

      13.2.5     The rpm2cpio Program
      RPM packages are essentially cpio archives with a prepended “header”. You can
      use this fact to extract individual files from an RPM package without having to
      install the package first. Simply convert the RPM package to a cpio archive using
      the rpm2cpio program, and feed the archive into cpio . Since rpm2cpio works as a filter,
      you can easily connect the two programs using a pipe:
13.3 YUM                                                                                          205

$ rpm2cpio hello-2.4-1.fc10.i386.rpm \
> | cpio -idv ./usr/share/man/man1/hello.1.gz
387 blocks
$ zcat usr/share/man/man1/hello.1.gz | head
.\" DO NOT MODIFY THIS FILE! It was generated by help2man 1.35.
.TH HELLO "1" "December 2008" "hello 2.4" "User Commands"
hello \- friendly greeting program

C 13.1 [2] Use rpm2cpio and cpio to display the list of files contained in an RPM

13.3      YUM
13.3.1     Overview
The rpm program is useful but does have its limits. As a basic tool it can install pack-
ages that are available as files or URLs, but, for example, does not help with lo-
cating appropriate, possibly installable packages. Many RPM-based distributions
use YUM (short for “Yellow Dog Updater, Modified”, after the distribution for
which the program was originally developed) to enable access to package sources package sources
(repositories) available on the Internet or on CD-ROM.

B In RPM-based distributions, YUM takes up approximately the same “eco-
  logical niche” occupied by apt-get in Debian GNU/Linux and its deriva-

B YUM is usually controlled via the command line, but the “yum shell ” com-
  mand starts a “YUM shell” where you can enter multiple YUM commands

13.3.2     Package Repositories
YUM introduces the concept of package repositories. A package repository is a set of
RPM packages that is available via the network and allows the installation of pack-
ages with YUM. The “yum repolist ” command outputs a list of configured package

$ yum repolist
Loaded plugins: refresh-packagekit
repo id         repo name                    status
fedora          Fedora 10 - i386             enabled: 11416
updates         Fedora 10 - i386 - Updates   enabled: 3324
repolist: 14740

“yum repolist disabled ” yields a list of known but disabled repositories:

$ yum repolist disabled
Loaded plugins: refresh-packagekit
repo id           repo name                        status
fedora-debuginfo Fedora 10 - i386 - Debug          disabled
fedora-source     Fedora 10 - Source               disabled
206                                          13 Package Management with RPM and YUM

      rawhide           Fedora - Rawhide - Development disabled

      To enable a repository, you need to give the --enablerepo= option, followed by the
      “repo ID” from the list. This only works in connection with a “genuine” yum com-
      mand; repolist is fairly innocuos:

      $ yum --enablerepo=rawhide repolist
      Loaded plugins: refresh-packagekit
      rawhide                                        |   3.4 kB     00:00
      rawhide/primary_db                             |   7.2 MB     00:14
      repo id         repo name                          status
      fedora          Fedora 10 - i386                   enabled:   11416
      rawhide         Fedora - Rawhide - Development     enabled:   12410
      updates         Fedora 10 - i386 - Updates         enabled:    3324
      repolist: 27150

      You can disable a repository using the --disablerepo option.

      B Repositories are most conveniently made known to YUM by means of con-
        figuration files in the /etc/yum.repos.d directory. (You could also enter them
        into /etc/yum.conf directly, but this is more inconvenient to manage.)

      B YUM keeps itself current as far as the content of repositories is concerned.
        There is no equivalent to the Debian tools’ “apt-get update ”.

      13.3.3    Installing and Removing Packages Using YUM
      To install a new package using YUM, you merely need to know its name. YUM
      checks whether the active repositories contain an appropriately-named package,
      resolves any dependencies the package may have, downloads the package and
      possibly other packages that it depends upon, and installs all of them:

      # yum install hello
      Setting up Install Process
      Parsing package install arguments
      Resolving Dependencies
      --> Running transaction check
      ---> Package hello.i386 0:2.4-1.fc10 set to be updated
      --> Finished Dependency Resolution

      Dependencies Resolved

       Package       Arch      Version         Repository     Size
       hello         i386      2.4-1.fc10      updates        68 k

      Transaction Sum
      Install      1 Package(s)
      Update       0 Package(s)
      Remove       0 Package(s)

      Total download size: 68 k
      Is this ok [y/N]: y
      Downloading Packages:
      hello-2.4-1.fc10.i386.rpm                      |    68 kB       00:00
13.3 YUM                                                                             207

====================== Entering rpm code =======================
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing     : hello                                    1/1
====================== Leaving rpm code ========================

  hello.i386 0:2.4-1.fc10


B YUM accepts not just simple package names, but also package names with
  architecture specifications, version numbers, and release numbers. Check
  yum (8) to find the allowable formats.

   Removing packages is just as simple:

# yum remove hello

This will also remove packages that this package depends upon—as long as these
are not required by another installed package, anyway.

B Instead of “yum remove ” you can also say “yum erase ”—the two are equivalent.
   You can update packages using “yum update ”:

# yum update hello

checks whether a newer version of the package is available and installs that if
this is the case. YUM takes care that all dependencies are resolved. “yum update ”
without a package name attempts to update all installed packages.

B When the --obsoletes is specified (the default case), yum tries to handle the
  case where one package has been replaced by another (of a different name).
  This makes full upgrades of the whole distribution easier to perform.

B “yum upgrade ” is the same as “yum update --obsoletes ”—but saves some typing
  in the case that you have switched off the obsoletes option in the configura-

B YUM supports the idea of “package groups”, i. e., packages that together
  are useful for a certain task. The available package groups can be displayed
  using “yum grouplist ”:

      $ yum grouplist
      Loaded plugins: refresh-packagekit
      Setting up Group Process
      Installed Groups:
         Administration Tools
         Authoring and Publishing
         Dial-up Networking Support

B If you want to know which packages a group consists of, use “yum groupinfo ”:
208                                           13 Package Management with RPM and YUM

             $ yum groupinfo 'Printing Support'
             Loaded plugins: refresh-packagekit
             Setting up Group Process

             Group: Printing Support
              Description: Install these tools to enable the system to 
               print or act as a print server.
              Mandatory Packages:
              Default Packages:

            A group is considered “installed” if all its “mandatory” packages are in-
            stalled. Besides these there are “default packages” and “optional packages”.

      B “yum groupinstall ” lets you install the packages of a group. The configuration
        option group_package_types determines which class package will actually be
        installed—usually the “mandatory” and the “default packages”.

      B “yum groupremove ” removes all packages of a group, without taking into ac-
        count package classes (group_package_types is ignored). Note that packages
        can belong to more than one group at the same time, so they may be miss-
        ing from group 𝑋 after having been removed along with group 𝑌.

      13.3.4     Information About Packages
      The “yum list ” command is available to find out which packages exist:

      $ yum list gcc
      Loaded plugins: refresh-packagekit
      Installed Packages
      gcc.i386              4.3.2-7                 installed

      You can also give a search pattern (it is best to put it inside quotes so the shell will
      not mess with it):

      $ yum list "gcc*"
      Loaded plugins: refresh-packagekit
      Installed Packages
      gcc.i386              4.3.2-7                 installed
      gcc-c++.i386          4.3.2-7                 installed
      Availabe Packages
      gcc-gfortran.i386     4.3.2-7                 fedora
      gcc-gnat.i386         4.3.2-7                 fedora

      The “installed packages” are installed on the local system, while the “available
      packages” can be fetched from repositories. The repository offering the package
      is displayed on the far right.
          To restrict the search to locally installed, or uninstalled but available, packages,
      you can use “yum list installed ” or “yum list available ”:

      $ yum list installed "gcc*"
      Loaded plugins: refresh-packagekit
13.3 YUM                                                                                 209

Installed Packages
gcc.i386              4.3.2-7                installed
gcc-c++.i386          4.3.2-7                installed
$ yum list available "gcc*"
Loaded plugins: refresh-packagekit
Available Packages
gcc-gfortran.i386     4.3.2-7                fedora
gcc-gnat.i386         4.3.2-7                fedora

B “yum list updates ” lists the packages that are installed and for which updates
  are available, while “yum list recent ” lists the packages that have “recently”
  arrived in a repository. “yum list extras ” points out packages that are in-
  stalled locally but are not available from any repository.

   To find out more about a package, use “yum info ”:

$ yum info hello
Loaded plugins: refresh-packagekit
Installed Packages
Name       : hello
Arch       : i386
Version    : 2.4
Release    : 1.fc10
Size       : 186 k
Repo       : installed
Summary    : Prints a Familiar, Friendly Greeting
URL        :
License    : GPLv3+ and GFDL and BSD and Public Domain
Description: Hello prints a friendly greeting. It also serves as a
           : sample GNU package, showing practices that may be
           : useful for GNU projects.

The advantage over “rpm -qi ” is that “yum info ” also works for packages that are not
installed locally but are available from a repository.

B You con otherwise use “yum info ” like “yum list”—“yum info installed ”, for ex-
  ample, displays detailed information about all installed packages.

   Using “yum search ”, you can search for all packages in whose name or descrip-
tion a given string occurs:

$ yum search mysql
Loaded plugins: refresh-packagekit
============================ Matched: mysql ========================
dovecot-mysql.i386 : MySQL backend for dovecot
koffice-kexi-driver-mysql.i386 : Mysql-driver for kexi
libgda-mysql.i386 : MySQL provider for libgda

Unfortunately, the resulting list is unsorted and a little difficult to read. yum uses
boldface to emphasise the places where the search string occurs.

B You can examine a package’s dependencies using “yum deplist ”:
       $ yum deplist gcc
       Loaded plugins: refresh-packagekit
       Finding dependencies:
       package: gcc.i386 4.3.2-7
210                                          13 Package Management with RPM and YUM

               dependency: binutils >=
                provider: binutils.i386
                provider: glibc.i386 2.9-2

      13.3.5    Downloading Packages
      If you want to download a package from a repository but do not want to install it
      outright, you can use the yumdownloader program. A command like

      $ yumdownloader --destdir /tmp hello

      searches the repositories for the hello package just like YUM would and down-
      loads the corresponding file to the /tmp directory.
         The --resolve option causes dependencies to be resolved and any other missing
      packages to be downloaded as well—but only those that are not installed on the
      local system.

      B With the --urls option, nothing is downloaded at all, but yumdownloader out-
        puts the URLs of the packages it would otherwise have downloaded.

      B With the --source option, yumdownloader downloads source RPMs instead of
        binary RPMs.

      Commands in this Chapter
      cpio     File archive manager                                     cpio (1) 204
      rpm      Package management tool used by various Linux distributions (Red Hat,
               SUSE, …)                                                  rpm (8) 200
      rpm2cpio Converts RPM packages to cpio archives               rpm2cpio (1) 204
      yum      Convenient RPM package maintenance tool                   yum (8) 205

         • RPM is a system for Linux software package management which is used by
           various distributions such as those by Red Hat and Novell/SUSE.
         • YUM is a front-end for rpm that gives access to package repositories over the
                                                                                          $ echo tux
                                                                                          $ ls
                                                                                          $ /bin/su -

Sample Solutions
This appendix contains sample solutions for selected exercises.

1.1 Access control applies to normal users but not the administrator. root may do
anything! The root account should only be used to execute commands that really
require root ’s privileges, e. g., to partition the disk, to create file systems, to add
user accounts, or to change system configuration files. All other actions should be
performed from an unprivileged account. This includes invoking administration
commands to gather information (where possible) and unpacking tar archives.

1.2 As root you may do anything, therefore it is very easy to damage the system,
e. g., through inadvertently mistyped commands.

1.3 This question aims at a comparison to other operating systems. Depend-
ing on the system in question, there are no access controls at all (DOS, Win-
dows 95/98) or different methods for access control (Windows NT/2000/XP or
Windows Vista). Accordingly, the former do not support administrator access (as
opposed to normal user access), while the latter even allow the creation of several
administrator accounts.

1.4 Basically you can log in as root , or create a UID 0 shell using su . The latter
method is better, e. g., because the change including the former UID is logged.

1.5 The shell prompt often looks different. In addition, the id command may

1.6 You can either log in directly or su . For frequent changes, it is a good idea to
log in on two consoles at the same time, and obtain a root shell using su on one.
Alternatively, you could open several terminal windows on a GUI.

1.7   You will generally find the log entry in /var/log/messages .

1.10 The obvious advantage is that administration is possible from anywhere, if
necessary by using an internet-enabled cell phone on the beach, and without hav-
ing to have access to specialised hardware or software. The obvious disadvantage
is that you need to secure access to the administration tool very carefully, in order
to prevent unbidden guests to “misconfigure” your system (or worse). This may
imply that (the obvious advantage notwithstanding) you may be able to provide
the administration tool only from within the local network, or that you should
212                                                                           A Sample Solutions

      secure access to it using strong cryptography (e. g., SSL with client certificates). If
      you consider deploying Webmin in your company, you should discuss the possi-
      bility of external access very carefully with the appropriate decision makers and/or
      corporate data protection officers in order to avoid extremely dire consequences
      that could hit you if problems appear. Consider yourself warned.

      2.1       By their respective numerical UIDs and GIDs.

      2.2 This works but is not necessarily a good idea. As far as the system is con-
      cerned, the two are a single user, i. e., all files and processes with that UID belong
      to both user names.

      2.3 A pseudo-user’s UID is used by programs in order to obtain particular well-
      defined access rights.

      2.4 Whoever is in group disk has block-level read and write permission to the
      system’s disks. With knowledge of the file system structure it is easy to make
      a copy of /bin/sh into a SUID root shell (section 3.5) by changing the file system
      metadata directly on disk. Thus, group disk membership is tantamount to root
      privileges; you should put nobody into the disk group whom you would not want
      to tell the root password outright.

      2.5 You will usually find an “x ”. This is a hint that the password that would
      usually be stored there is indeed stored in another file, namely /etc/shadow , which
      unlike the former file is readable only for root .

      2.6       There are basically two possibilities:
            1. Nothing. In this case the system should turn you away after you entered
               your password, since no user account corresponds to the all-uppercase user
            2. From now on, the system talks to you in uppercase letters only. In this case
               your Linux system assumes that you are sitting in front of an absolutely
               antediluvial terminal (1970s vintage or so) that does not support lowercase
               letters, and kindly switches its processing of input and output data such that
               uppercase letters in the input are interpreted as lowercase, and lowercase
               letters in the output are displayed as uppercase. Today this is of limited
               benefit (except if you work in a computer museum), and you should log out
               as quickly again as possible before your head explodes. Since this behaviour
               is so atavistic, not every Linux distribution goes along with it, though.

      2.7       Use getent , cut , and sort to generate lists of user names for the databases, and
      comm     to compare the two lists.

      2.8  Use the passwd command if you’re logged in as user joe , or “passwd joe ” as
      root .
           In joe ’s entry in the /etc/shadow file there should be a different value in the
      second field, and the date of the last password change (field 3) should show the
      current date (in what unit?)

      2.9 As root , you set a new password for him using “passwd dumbo ”, as you cannot
      retrieve his old one even though you are the administrator.

      2.10 Use the command “passwd -n 7 -x 14 -w 2 joe ”. You can verify the settings
      using “passwd -S joe ”.
A Sample Solutions                                                                        213

2.11 Use the useradd command to create the user, “usermod -u ” to modify the UID.
Instead of a user name, the files should display a UID as their owner, since no user
name is known for that UID …

2.12 For each of the three user accounts there should be one line in /etc/passwd
and one in /etc/shadow . To work with the accounts, you do not necessarily need a
password (you can use su as root ), but if you want to login you do. You can create
a file without a home directory by placing it in /tmp (in case you forgot—a home
directory for a new user would however be a good thing).

2.13 Use the userdel command to delete the account. To remove the files, use the
“find / -uid ⟨UID⟩ -delete ” command.

2.14 If you use “usermod -u ”, you must reassign the user’s file to the new UID,
for example by means of “find / -uid ⟨UID⟩ -exec chown test1 {} \; ” or (more effi-
ciently) “chown -R --from= ⟨UID⟩ test1 / ”. In each case, ⟨UID⟩ is the (numerical) for-
mer UID.

2.15    You can either edit /etc/passwd using vipw or else call usermod .

2.16 Groups make it possible to give specific privileges to groups [sic!] of users.
You could, for example, add all HR employees to a single group and assign that
group a working directory on a file server. Besides, groups can help organise
access rights to certain peripherals (e. g., by means of the groups disk , audio , or
video ).

2.17 Use the “mkdir ⟨directory⟩” command to create the directory and “chgrp
⟨groupname⟩ ⟨directory⟩” to assign that directory to the group. You should also set
the SGID bit to ensure that newly created files belong to the group as well.

2.18    Use the following commands:

# groupadd test
# gpasswd -a test1 test
Adding user test1 to group test
# gpasswd -a test2 test
Adding user test2 to group test
# gpasswd test
Changing the password for group test
New Password:x9q.Rt/y
Re-enter new password:x9q.Rt/y

To change groups, use the “newgrp test ” command. You will be asked for the pass-
word only if you are not a member of the group in question.

3.1 A new file is assigned to your current primary group. You can’t assign a file
to a group that you are not a member of—unless you are root .

3.3    077 and ”‘u=rwx,go= ”’, respectively.

3.5 This is the SUID or SGID bit. The bits cause a process to assume the
UID/GID of the executable file rather than that of the executing user. You can
see the bits using “ls -l ”. Of course you may change all the permissions on your
own files. However, at least the SUID bit only makes sense on binary executable
files, not shell scripts and the like.
214                                                                     A Sample Solutions

      3.6   One of the two following (equivalent) commands will serve:

      $ umask 007
      $ umask -S u=rwx,g=rwx

      You may perhaps ask yourself why this umask contains x bits. They are indeed
      irrelevant for files, as files are not created executable by default. However it might
      be the case that subdirectories are desired in the project directory, and it makes
      sense to endow these with permissions that allow them to be used reasonably.

      3.7 The so-called “sticky bit” on a directory implies that only the owner of a file
      (or the owner of the directory) may delete or rename it. You will find it, e. g., on
      the /tmp directory.

      3.9 This doesn’t work with the bash shell (at least not without further trickery).
      We can’t speak for other shells here.

      3.11 You cannot do this with chattr alone, since various attributes can be dis-
      played with lsattr but not set with chattr . Read up on the details in chattr (1).—In
      addition, some attributes are only defined for “plain” files while others are only
      defined for directories; you will, for example, find it difficult to make the D and
      E attributes visible for the same “file system object” at the same time. (The E at-
      tribute is to do with transparent compression, which cannot be used on directo-
      ries, while D only applies to directories—write operations to such directories will
      be performed synchronously.)

      4.1 In the directory of a process below /proc there is a file called environ which
      contains the environment variables of that process. You can output this file using
      cat . The only blemish is that the variables in this file are separated using zero
      bytes, which looks messy on the screen; for convenience, you might use something
      like “tr "\0" "\n" </proc/4711/environ ” to display the environment.

      4.2   Funnily enough, the limit is not documented in any obvious placees. In
      /usr/include/linux/threads.h  on a Linux 2.6 kernel, the constant PID_MAX_LIMIT is de-
      fined with a value of 32768; this is the lowest value that will by default not be
      assigned to processes. You can query the actual value in /proc/sys/kernel/pid_max
      (or even change it—the maximum for 32-bit platforms is actually 32768, while on
      64-bit systems you may set an arbitrary value of up to 222 , which is approximately
      4 million).
         The PIDs assigned to processes rise monotonically at first. When the above-
      mentioned limit is reached, assignment starts again with lower PIDs, where PIDs
      that are still being used by processes are of course not given again to others. Many
      low PIDs are assigned to long-running daemons during the boot process, and for
      this reason after the limit has been reached, the search for unused PIDs starts again
      not at PID 1 but at PID 300. (The details are in the kernel/pid_namespace.c file within
      the Linux source code.)

      4.4 As we said, zombies arise when the parent process does not pick up the
      return code of a child process. Thus, to create a zombie you must start a child
      process and then prevent the parent process from picking up its return code, for
      example by stopping it by means of a signal. Try something like

      $ sh
      $ echo $$                                                              In the subshell
      $ sleep 20
                                                                     In a different window:
A Sample Solutions                                                                     215

$ kill -STOP 12345
$ ps u | grep sleep
joe 12346 0.0 0.0 3612               456 pts/2     Z 18:19 0:00 sleep 20

4.5    Consult ps (1).

4.6    Try

$ ps -o pid,ppid,state,cmd

4.7 Usually SIGCHLD (“child process finished”—sometimes called SIGCLD ), SIGURG
(urgent data was received on a network connection) and SIGWINCH (the size of the
window for a text-based program was changed). These three events are so inane
that the process should not be terminated on their account.

4.8    Something like

$ pgrep -u hugo

should suffice.

4.10 Use, e. g., the “renice -10 ⟨PID⟩” command. You can only specify negative
nice values as root .

6.2    sda1 , sda2 , sda5 , sda6 ,   and sdb1 , sdb5 , sdb6 , sdb7 .

7.2    Use tune2fs with the -c , -u and -m options.

7.3    mkreiserfs /dev/sdb5

7.6 /etc/fstab contains all frequently-used file systems and their mount points,
while /etc/mtab contains those file systems that are actually mounted at the mo-

8.1 The boot loader can be placed inside the MBR, in another (partition) boot
sector, or a file on disk. In the two latter cases, you will need another boot loader
that can “chain load” the Linux boot loader. Be that as it may, for a Linux system
you absolutely need a boot loader that can boot a Linux kernel, such as GRUB
(there are others).

8.3 Assign a password preventing the unauthorised entry of kernel parameters.
With GRUB Legacy, e. g., using

password --md5      ⟨encrypted keyword⟩

lock   helps with the password request for a specific operating system.

9.3 You can display the previous and current runlevel using runlevel . If the pre-
vious runlevel is “N ” that means “none”—the system started into the current run-
level. To change, say “init 2 ”, then “runlevel ” again to check.
216                                                                      A Sample Solutions

      9.4    A possible entry for the inittab file might be
      aa:A:ondemand:/bin/date >/tmp/runlevel-a.txt

      This entry should write the current time to the mentioned file if you activate it
      using “telinit A ”. Don’t forget the “telinit q ” to make init reread its configuration

      9.5    Call the syslog init script with the restart or reload parameters.

      9.6    For example, by using “chkconfig -l ” (on a SUSE or Red Hat system).

      9.7 It is tempting just to remove the symbolic links from the runlevel directory
      in question. However, depending on the distribution, they may reappear after the
      next automated change. So if your distribution uses a tool like chkconfig or insserv
      you had better use that.

      9.8    You should be prepared for the system asking for the root password.

      9.10    Use the
      # shutdown -h +15 'This is just a test'

      command; everything that you pass to shutdown after the delay will be sent to your
      users as a broadcast message. To cancel the shutdown, you can either interrupt
      the program using the Ctrl + c key combination (if you started shutdown in the
      foreground), or give the “shutdown -c ” command.

      9.10    The file name will be sent as the message.

      10.4 The unit file does not need to be modified in order to express dependencies.
      This makes the automatic installation and, in particular, deinstallation of units
      as part of software packages easier (e. g., in the context of a distribution-specific
      package management tool) and allows the seamless updating of unit files by a

      10.10 There is no exact equivalent because systemd does not use the runlevel
      concept. You can, however, display all currently active targets:
      # systemctl list-units -t target

      10.11 “systemctl kill ” guarantees that the signal will only be sent to processes
      belonging to the unit in question. The other two commands send the signal to all
      processes whose name happens to be example .

      10.13 You can’t (“systemctl mask ” outputs an error message). You must deactivate
      the service and then remove, move, or rename the unit file.

      11.3    You can find out using a shell script like
      for f in /usr/bin/*
          printf "%4u %s\n" $(ldd $f 2>/dev/null | wc -l) $f
      done | sort -nr | head

      (the 2>/dev/null suppresses the error message that ldd outputs if you feed it a shell
      script rather than an executable program.) On the author’s system, mplayer is at
      the top of the list, sporting an impressive 118 libraries.
A Sample Solutions                                                                    217

11.4 This would be a major security hole, since you could attempt to use a pri-
vate directory in LD_LIBRARY_PATH to sneak a “Trojan horse” version of a common
library into the program. For example, you can assume with near certainty that
a program such as passwd will call a C function like open() . If you should succeed
in arranging for this function to be called not from libc but your own library, you
would essentially be able to do whatever you wanted using root privileges. This
of course is not desirable (in the greater scheme of things, anyway).

12.11    Try

# dpkg-reconfigure debconf

13.1    This is most easily done using something like

$ rpm2cpio   ⟨package⟩ | cpio -t
                                                                                                   $ echo tux
                                                                                                   $ ls
                                                                                                   $ /bin/su -

LPIC-1 Certification

B.1     Overview
The Linux Professional Institute (LPI) is a vendor-independent non-profit organi-
zation dedicated to furthering the professional use of Linux. One aspect of the
LPI’s work concerns the creation and delivery of distribution-independent certi-
fication exams, for example for Linux professionals. These exams are available
world-wide and enjoy considerable respect among Linux professionals and em-
   Through LPIC-1 certification you can demonstrate basic Linux skills, as re-
quired, e. g., for system administrators, developers, consultants, or user support
professionals. The certification is targeted towards Linux users with 1 to 3 years
of experience and consists of two exams, LPI-101 and LPI-102. These are offered
as computer-based multiple-choice and fill-in-the-blanks tests in all Pearson VUE
and Thomson Prometric test centres. On its web pages at , the
LPI publishes objectives outlining the content of the exams.                          objectives
   This training manual is part of Linup Front GmbH’s curriculum for preparation
of the LPI-101 exam and covers part of the official examination objectives. Refer
to the tables below for details. An important observation in this context is that
the LPIC-1 objectives are not suitable or intended to serve as a didactic outline for
an introductory course for Linux. For this reason, our curriculum is not strictly
geared towards the exams or objectives as in “Take classes 𝑥 and 𝑦, sit exam 𝑝,
then take classes 𝑎 and 𝑏 and sit exam 𝑞.” This approach leads many prospective
students to the assumption that, being complete Linux novices, they could book
𝑛 days of training and then be prepared for the LPIC-1 exams. Experience shows
that this does not work in practice, since the LPI exams are deviously constructed
such that intensive courses and exam-centred “swotting” do not really help.
   Accordingly, our curriculum is meant to give you a solid basic knowledge of
Linux by means of a didactically reasonable course structure, and to enable you as
a participant to work independently with the system. LPIC-1 certification is not a
primary goal or a goal in itself, but a natural consequence of your newly-obtained
knowledge and experience.

B.2     Exam LPI-101
The following table displays the objectives for the LPI-101 exam and the materials
covering these objectives. The numbers in the columns for the individual manuals
refer to the chapters containing the material in question.

adm1-objs-101.tex   (33e55eeadba676a3 )
220                                                                                     B LPIC-1 Certification

  No     Wt   Title                                                                         GRD1       ADM1
 101.1   2    Determine and configure hardware settings                                         –         5–6
 101.2   3    Boot the system                                                                  –        8–10
 101.3   3    Change runlevels/boot targets and shutdown or reboot system                      –        9–10
 102.1   2    Design hard disk layout                                                          –          6
 102.2   2    Install a boot manager                                                           –          8
 102.3   1    Manage shared libraries                                                          –          11
 102.4   3    Use Debian package management                                                    –          12
 102.5   3    Use RPM and YUM package management                                               –          13
 103.1   4    Work on the command line                                                        3–4         –
 103.2   3    Process text streams using filters                                                8          –
 103.3   4    Perform basic file management                                                   6, 11       7.3
 103.4   4    Use streams, pipes and redirects                                                 8          –
 103.5   4    Create, monitor and kill processes                                               –          4
 103.6   2    Modify process execution priorities                                              –          4
 103.7   2    Search text files using regular expressions                                      7–8         –
 103.8   3    Perform basic file editing operations using vi                                   5, 7        –
 104.1   2    Create partitions and filesystems                                                 –         6–7
 104.2   2    Maintain the integrity of filesystems                                             –          7
 104.3   3    Control mounting and unmounting of filesystems                                    –          7
 104.4   1    Manage disk quotas                                                               –         7.4
 104.5   3    Manage file permissions and ownership                                             –          3
 104.6   2    Create and change hard and symbolic links                                        6          –
 104.7   2    Find system files and place files in the correct location                        6, 10        –

                            B.3     Exam LPI-102
                            The following table displays the objectives for the LPI-102 exam and the materials
                            covering these objectives. The numbers in the columns for the individual manuals
                            refer to the chapters containing the material in question.

  No     Wt   Title                                                             ADM1      GRD2        ADM2
 105.1   4    Customize and use the shell environment                              –       1–2             –
 105.2   4    Customize or write simple scripts                                    –       2–5             –
 105.3   2    SQL data management                                                  –         8             –
 106.1   2    Install and configure X11                                             –        11             –
 106.2   1    Setup a display manager                                              –        11             –
 106.3   1    Accessibility                                                        –        12             –
 107.1   5    Manage user and group accounts and related system files               2         –             –
 107.2   4    Automate system administration tasks by scheduling jobs              –         9             –
 107.3   3    Localisation and internationalisation                                –        10             –
 108.1   3    Maintain system time                                                 –         –             8
 108.2   3    System logging                                                       –         –           1–2
 108.3   3    Mail Transfer Agent (MTA) basics                                     –         –            11
 108.4   2    Manage printers and printing                                         –         –             9
 109.1   4    Fundamentals of internet protocols                                   –         –           3–4
 109.2   4    Basic network configuration                                           –         –         4–5, 7
 109.3   4    Basic network troubleshooting                                        –         –         4–5, 7
 109.4   2    Configure client side DNS                                             –         –             4
 110.1   3    Perform security administration tasks                                2         –        4–5, 13
 110.2   3    Setup host security                                                  2         –       4, 6–7, 13
 110.3   3    Securing data with encryption                                        –         –         10, 12
B LPIC-1 Certification                                                                        221

B.4      LPI Objectives In This Manual
101.1     Determine and configure hardware settings
Weight        2
Description Candidates should be able to determine and configure fundamen-
tal system hardware.
Key Knowledge Areas

   •   Enable and disable integrated peripherals
   •   Configure systems with or without external peripherals such as keyboards
   •   Differentiate between the various types of mass storage devices
   •   Know the differences between coldplug and hotplug devices
   •   Determine hardware resources for devices
   •   Tools and utilities to list various hardware information (e.g. lsusb , lspci , etc.)
   •   Tools and utilities to manipulate USB devices
   •   Conceptual understanding of sysfs, udev, dbus

The following is a partial list of the used files, terms and utilities:

   •   /sys/
   •   /proc/
   •   /dev/
   •   modprobe
   •   lsmod
   •   lspci
   •   lsusb

101.2     Boot the system
Weight      3
Description Candidates should be able to guide the system through the booting
Key Knowledge Areas

   • Provide common commands to the boot loader and options to the kernel at
     boot time
   • Demonstrate knowledge of the boot sequence from BIOS to boot completion
   • Understanding of SysVinit and systemd
   • Awareness of Upstart
   • Check boot events in the log files

The following is a partial list of the used files, terms and utilities:

   •   dmesg
   •   BIOS
   •   bootloader
   •   kernel
   •   initramfs
   •   init
   •   SysVinit
   •   systemd

101.3     Change runlevels/boot targets and shutdown or reboot sys-
Weight            3
222                                                                   B LPIC-1 Certification

      Description Candidates should be able to manage the SysVinit runlevel or sys-
      temd boot target of the system. This objective includes changing to single user
      mode, shutdown or rebooting the system. Candidates should be able to alert users
      before switching runlevels/boot targets and properly terminate processes. This
      objective also includes setting the default SysVinit runlevel or systemd boot target.
      It also includes awareness of Upstart as an alternative to SysVinit or systemd.
      Key Knowledge Areas

         • Set the default runlevel or boot target
         • Change between runlevels/boot targets including single user mode
         • Shutdown and reboot from the command line
         • Alert users before switching runlevels/boot targets or other major system
         • Properly terminate processes

      The following is a partial list of the used files, terms and utilities:

         •   /etc/inittab
         •   shutdown
         •   init
         •   /etc/init.d/
         •   telinit
         •   systemd
         •   systemctl
         •   /etc/systemd/
         •   /usr/lib/systemd/
         •   wall

      102.1     Design hard disk layout
      Weight        2
      Description Candidates should be able to design a disk partitioning scheme for
      a Linux system.
      Key Knowledge Areas

         • Allocate filesystems and swap space to separate partitions or disks
         • Tailor the design to the intended use of the system
         • Ensure the /boot partition conforms to the hardware architecture require-
           ments for booting
         • Knowledge of basic features of LVM

      The following is a partial list of the used files, terms and utilities:

         •   / (root) filesystem
         •   /var filesystem
         •   /home filesystem
         •   /boot filesystem
         •   swap space
         •   mount points
         •   partitions

      102.2     Install a boot manager
      Weight      2
      Description Candidates should be able to select, install and configure a boot
      Key Knowledge Areas

         • Providing alternative boot locations and backup boot options
B LPIC-1 Certification                                                             223

   • Install and configure a boot loader such as GRUB Legacy
   • Perform basic configuration changes for GRUB 2
   • Interact with the boot loader

The following is a partial list of the used files, terms and utilities:

   •   menu.lst , grub.cfg   and grub.conf
   •   grub-install
   •   grub-mkconfig
   •   MBR

102.3      Manage shared libraries
Weight        1
Description Candidates should be able to determine the shared libraries that
executable programs depend on and install them when necessary.
Key Knowledge Areas

   • Identify shared libraries
   • Identify the typical locations of system libraries
   • Load shared libraries

The following is a partial list of the used files, terms and utilities:

   •   ldd
   •   ldconfig
   •   /etc/

102.4      Use Debian package management
Weight       3
Description Candidates should be able to perform package management using
the Debian package tools.
Key Knowledge Areas

   • Install, upgrade and uninstall Debian binary packages
   • Find packages containing specific files or libraries which may or may not be
   • Obtain package information like version, content, dependencies, package
     integrity and installation status (whether or not the package is installed)

The following is a partial list of the used files, terms and utilities:

   •   /etc/apt/sources.list
   •   dpkg
   •   dpkg-reconfigure
   •   apt-get
   •   apt-cache
   •   aptitude

102.5      Use RPM and YUM package management
Weight      3
Description Candidates should be able to perform package management using
RPM and YUM tools.
Key Knowledge Areas
224                                                                   B LPIC-1 Certification

         • Install, re-install, upgrade and remove packages using RPM and YUM
         • Obtain information on RPM packages such as version, status, dependencies,
           integrity and signatures
         • Determine what files a package provides, as well as find which package a
           specific file comes from

      The following is a partial list of the used files, terms and utilities:

         •   rpm
         •   rpm2cpio
         •   /etc/yum.conf
         •   /etc/yum.repos.d/
         •   yum
         •   yumdownloader

      103.3     Perform basic file management
      Weight       4
      Description Candidates should be able to use the basic Linux commands to
      manage files and directories.
      Key Knowledge Areas

         •   Copy, move and remove files and directories individually
         •   Copy multiple files and directories recursively
         •   Remove files and directories recursively
         •   Use simple and advanced wildcard specifications in commands
         •   Using find to locate and act on files based on type, size, or time
         •   Usage of tar, cpio and dd

      The following is a partial list of the used files, terms and utilities:

         •   cp
         •   find
         •   mkdir
         •   mv
         •   ls
         •   rm
         •   rmdir
         •   touch
         •   tar
         •   cpio
         •   dd
         •   file
         •   gzip
         •   gunzip
         •   bzip2
         •   xz
         •   file globbing

      103.5     Create, monitor and kill processes
      Weight      4
      Description Candidates should be able to perform basic process management.
      Key Knowledge Areas

         • Run jobs in the foreground and background
         • Signal a program to continue running after logout
         • Monitor active processes
B LPIC-1 Certification                                                          225

   • Select and sort processes for display
   • Send signals to processes
The following is a partial list of the used files, terms and utilities:
   •   &
   •   bg
   •   fg
   •   jobs
   •   kill
   •   nohup
   •   ps
   •   top
   •   free
   •   uptime
   •   pgrep
   •   pkill
   •   killall
   •   screen

103.6     Modify process execution priorities
Weight      2
Description Candidates should be able to manage process execution priorities.
Key Knowledge Areas
   • Know the default priority of a job that is created
   • Run a program with higher or lower priority than the default
   • Change the priority of a running process
The following is a partial list of the used files, terms and utilities:
   •   nice
   •   ps
   •   renice
   •   top

104.1     Create partitions and filesystems
Weight         2
Description Candidates should be able to configure disk partitions and then
create filesystems on media such as hard disks. This includes the handling of
swap partitions.
Key Knowledge Areas
   • Manage MBR partition tables
   • Use various mkfs commands to create various filesystems such as:
         – ext2/ext3/ext4
         – XFS
         – VFAT
   • Awareness of ReiserFS and Btrfs
   • Basic knowledge of gdisk and parted with GPT
The following is a partial list of the used files, terms and utilities:
   •   fdisk
   •   gdisk
   •   parted
   •   mkfs
   •   mkswap
226                                                                   B LPIC-1 Certification

      104.2     Maintain the integrity of filesystems
      Weight         2
      Description Candidates should be able to maintain a standard filesystem, as
      well as the extra data associated with a journaling filesystem.
      Key Knowledge Areas

         • Verify the integrity of filesystems
         • Monitor free space and inodes
         • Repair simple filesystem problems

      The following is a partial list of the used files, terms and utilities:

         •   du
         •   df
         •   fsck
         •   e2fsck
         •   mke2fs
         •   debugfs
         •   dumpe2fs
         •   tune2fs
         •   XFS tools (such as xfs_metadump and xfs_info )

      104.3     Control mounting and unmounting of filesystems
      Weight      3
      Description Candidates should be able to configure the mounting of a filesys-
      Key Knowledge Areas

         • Manually mount and unmount filesystems
         • Configure filesystem mounting on bootup
         • Configure user mountable removable filesystems

      The following is a partial list of the used files, terms and utilities:

         •   /etc/fstab
         •   /media/
         •   mount
         •   umount

      104.4     Manage disk quotas
      Weight      1
      Description Candidates should be able to manage disk quotas for users.
      Key Knowledge Areas:

         • Set up a disk quota for a filesystem
         • Edit, check and generate user quota reports

      The following is a partial list of the used files, terms and utilities:

         •   quota
         •   edquota
         •   repquota
         •   quotaon
B LPIC-1 Certification                                                                227

104.5     Manage file permissions and ownership
Weight        3
Description Candidates should be able to control file access through the proper
use of permissions and ownerships.
Key Knowledge Areas

   •   Manage access permissions on regular and special files as well as directories
   •   Use access modes such as suid, sgid and the sticky bit to maintain security
   •   Know how to change the file creation mask
   •   Use the group field to grant file access to group members

The following is a partial list of the used files, terms and utilities:

   •   chmod
   •   umask
   •   chown
   •   chgrp

107.1     Manage user and group accounts and related system files
Weight         5
Description Candidates should be able to add, remove, suspend and change
user accounts.
Key Knowledge Areas

   • Add, modify and remove users and groups
   • Manage user/group info in password/group databases
   • Create and manage special purpose and limited accounts

The following is a partial list of the used files, terms and utilities:

   •   /etc/passwd
   •   /etc/shadow
   •   /etc/group
   •   /etc/skel/
   •   chage
   •   getent
   •   groupadd
   •   groupdel
   •   groupmod
   •   passwd
   •   useradd
   •   userdel
   •   usermod

110.1     Perform security administration tasks
Weight        3
Description Candidates should know how to review system configuration to
ensure host security in accordance with local security policies.
Key Knowledge Areas

   •   Audit a system to find files with the suid/sgid bit set
   •   Set or change user passwords and password aging information
   •   Being able to use nmap and netstat to discover open ports on a system
   •   Set up limits on user logins, processes and memory usage
228                                                                   B LPIC-1 Certification

         • Determine which users have logged in to the system or are currently logged
         • Basic sudo configuration and usage

      The following is a partial list of the used files, terms and utilities:

         •   find
         •   passwd
         •   fuser
         •   lsof
         •   nmap
         •   chage
         •   netstat
         •   sudo
         •   /etc/sudoers
         •   su
         •   usermod
         •   ulimit
         •   who , w , last

      110.2      Setup host security
      Weight      3
      Description Candidates should know how to set up a basic level of host security.
      Key Knowledge Areas

         • Awareness of shadow passwords and how they work
         • Turn off network services not in use
         • Understand the role of TCP wrappers

      The following is a partial list of the used files, terms and utilities:

         •   /etc/nologin
         •   /etc/passwd
         •   /etc/shadow
         •   /etc/xinetd.d/
         •   /etc/xinetd.conf
         •   /etc/inetd.d/
         •   /etc/inetd.conf
         •   /etc/inittab
         •   /etc/init.d/
         •   /etc/hosts.allow
         •   /etc/hosts.deny
                                                                                      $ echo tux
                                                                                      $ ls
                                                                                      $ /bin/su -

Command Index
This appendix summarises all commands explained in the manual and points to
their documentation as well as the places in the text where the commands have
been introduced.

adduser   Convenient command to create new user accounts (Debian)
                                                                     adduser (8) 34
alien    Converts various software packaging formats                  alien (1) 196
apt-get  Powerful command-line tool for Debian GNU/Linux package manage-
         ment                                                       apt-get (8) 189
aptitude Convenient package installation and maintenance tool (Debian)
                                                                   aptitude (8) 192
blkid    Locates and prints block device attributes                   blkid (8) 118
busybox A shell that already contains variants of many Unix tools
                                                                    busybox (1) 174
cfdisk   Character-screen based disk partitioner                      cfdisk (8) 93
chattr   Sets file attributes for ext2 and ext3 file systems            chattr (1) 51
chfn     Allows users to change the GECOS field in the user database
                                                                        chfn (1) 27
chgrp    Sets the assigned group of a file or directory                 chgrp (1) 44
chkconfig Starts or shuts down system services (SUSE, Red Hat)
                                                                  chkconfig (8) 147
chmod    Sets access modes for files and directories                    chmod (1) 43
chown    Sets the owner and/or assigned group of a file or directory
                                                                       chown (1) 44
cpio     File archive manager                                          cpio (1) 204
dd       “Copy and convert”, copies files or file systems block by block and does
         simple conversions                                              dd (1) 120
debugfs File system debugger for fixing badly damaged file systems. For gurus
         only!                                                      debugfs (8) 108
dmesg    Outputs the content of the kernel message buffer              dmesg (8) 137
dpkg     Debian GNU/Linux package management tool                      dpkg (8) 182
dpkg-reconfigure Reconfigures an already-installed Debian package
                                                           dpkg-reconfigure (8) 195
dumpe2fs Displays internal management data of the ext2 file system. For gurus
         only!                                                     dumpe2fs (8) 108
dumpreiserfs Displays internal management data of the Reiser file system. For
         gurus only!                                           dumpreiserfs (8) 111
e2fsck   Checks ext2 and ext3 file systems for consistency            e2fsck (8) 107
e2label Changes the label on an ext2/3 file system                   e2label (8) 118
edquota Tool for entering and adjusting disk quotas                 edquota (8) 122
230                                                                    C Command Index

      file     Guesses the type of a file’s content, according to rules        file (1) 174
      fsck     Organises file system consistency checks                        fsck (8) 101
      gdisk    Partitioning tool for GPT disks                                gdisk (8) 92
      getent   Gets entries from administrative databases                    getent (1) 32
      getfacl  Displays ACL data                                            getfacl (1) 47
      gpasswd  Allows a group administrator to change a group’s membership and up-
               date the group password                                      gpasswd (1) 38
      groupadd Adds user groups to the system group database               groupadd (8) 37
      groupdel Deletes groups from the system group database               groupdel (8) 38
      groupmod Changes group entries in the system group database          groupmod (8) 37
      groups   Displays the groups that a user is a member of                groups (1) 24
      grub-md5-crypt Determines MD5-encrypted passwords for GRUB Legacy
                                                                  grub-md5-crypt (8) 135
      halt     Halts the system                                               halt (8) 151
      id       Displays a user’s UID and GIDs                                     id (1) 24
      initctl Supervisory tool for Upstart                                initctl (8) 150
      insserv Activates or deactivates init scripts (SUSE)                insserv (8) 147
      kill     Terminates a background process                        bash (1), kill (1) 58
      killall Sends a signal to all processes matching the given name killall (1) 59
      kpartx   Creates block device maps from partition tables               kpartx (8) 94
      last     List recently-logged-in users                                    last (1) 24
      ldconfig Builds the dynamic library cache                          ldconfig (8) 176
      ldd      Displays the dynamic libraries used by a program                ldd (1) 174
      losetup Creates and maintains loop devices                            losetup (8) 94
      lsattr   Displays file attributes on ext2 and ext3 file systems          lsattr (1) 51
      lsblk    Lists available block devices                                 lsblk (8) 119
      lsmod    Lists loaded kernel modules                                    lsmod (8) 71
      lspci    Displays information about devices on the PCI bus              lspci (8) 65
      lsusb    Lists all devices connected to the USB                         lsusb (8) 68
      mkdosfs Creates FAT-formatted file systems                         mkfs.vfat (8) 114
      mke2fs   Creates ext2 or ext3 file systems                             mke2fs (8) 105
      mkfs     Manages file system creation                                    mkfs (8) 100
      mkfs.vfat Creates FAT-formatted file systems                       mkfs.vfat (8) 114
      mkfs.xfs Creates XFS-formatted file systems                         mkfs.xfs (8) 111
      mkreiserfs Creates Reiser file systems                            mkreiserfs (8) 111
      mkswap   Initialises a swap partition or file                          mkswap (8) 115
      modprobe Loads kernel modules, taking dependencies into account
                                                                           modprobe (8) 70
      mount    Includes a file system in the directory tree        mount (8), mount (2) 116
      nice     Starts programs with a different nice value                       nice (1) 61
      nohup    Starts a program such that it is immune to SIGHUP signals      nohup (1) 61
      pgrep    Searches processes according to their name or other criteria
                                                                              pgrep (1) 59
      pkill    Signals to processes according to their name or other criteria
                                                                              pkill (1) 60
      ps       Outputs process status information                                 ps (1) 56
      pstree   Outputs the process tree                                      pstree (1) 57
      quota    Reports on a user’s quota status                              quota (1) 122
      reboot   Restarts the computer                                        reboot (8) 151
      reiserfsck Checks a Reiser file system for consistency            reiserfsck (8) 111
      renice   Changes the nice value of running processes                   renice (8) 61
      repquota Summarises filesystem usage and quota usage for many users
                                                                         repquota (8) 122
      resize_reiserfs Changes the size of a Reiser file system resize_reiserfs (8) 111
      rpm      Package management tool used by various Linux distributions (Red Hat,
               SUSE, …)                                                        rpm (8) 200
      rpm2cpio Converts RPM packages to cpio archives                    rpm2cpio (1) 204
      runlevel Displays the previous and current run level               runlevel (8) 145
C Command Index                                                                          231

sash     “Stand-Alone Shell” with built-in commands, for troubleshooting
                                                                          sash (8) 174
setfacl Enables ACL manipulation                                       setfacl (1) 47
sfdisk   Non-interactive hard disk partitioner                           sfdisk (8) 93
sgdisk   Non-interactive hard disk partitioning tool for GPT disks sgdisk (8) 93
shutdown Shuts the system down or reboots it, with a delay and warnings for
         logged-in users                                            shutdown (8) 151
star     POSIX-compatible tape archive with ACL support                    star (1) 47
strip    Removes symbol tables from object files                         strip (1) 174
su       Starts a shell using a different user’s identity                     su (1) 16
sudo     Allows normal users to execute certain commands with administrator
         privileges                                                        sudo (8) 14
swapoff Deactivates a swap partition or file                          swapoff (8) 115
swapon   Activates a swap partition or file                            swapon (8) 115
systemctl Main control utility for systemd                   systemctl (1) 157, 166
top      Screen-oriented tool for process monitoring and control            top (1) 61
tune2fs Adjusts ext2 and ext3 file system parameters              tunefs (8) 108, 119
udevd    Kernel uevent management daemon                                  udevd (8) 73
update-rc.d Installs and removes System-V style init script links (Debian)
                                                                 update-rc.d (8) 147
useradd Adds new user accounts                                         useradd (8) 33
userdel Removes user accounts                                          userdel (8) 36
usermod Modifies the user database                                      usermod (8) 36
vigr     Allows editing /etc/group or /etc/gshadow with “file locking”, to avoid con-
         flicts                                                             vipw (8) 38
vol_id   Determines file system types and reads labels and UUIDs
                                                                      vol_id (8) 118
xfs_mdrestore Restores an XFS metadata dump to a filesystem image
                                                               xfs_mdrestore (8) 112
xfs_metadump Produces metadata dumps from XFS file systems
                                                                xfs_metadump (8) 112
yum      Convenient RPM package maintenance tool                           yum (8) 205
                                                                                         $ echo tux
                                                                                         $ ls
                                                                                         $ /bin/su -

This index points to the most important key words in this document. Particu-
larly important places for the individual key words are emphasised by bold type.
Sorting takes place according to letters only; “~/.bashrc ” is therefore placed under

/,   86                                    busybox ,   174

access mode, 42                            Cameron, Jamie, 18
adduser , 34                               Card, Rémy, 102–103
administration tools, 14                   cat , 31, 214
alien , 181, 196–197                       cc , 120
      --to-deb (option), 197               cd , 42
apt , 182, 190                             cfdisk , 93
apt-cache , 181, 190–192                   chage , 35
apt-get , 181, 184, 189–190, 192–193,      chattr , 51, 214
           205–206                               -R (option), 51
      dist-upgrade (option), 189–190       chfn , 27
      install (option), 190                chgrp , 38, 44–45, 49
      remove (option), 190                       -R (option), 45
      source (option), 190                 chkconfig , 147, 216
      upgrade (option), 190                chmod , 15, 43, 46, 48–49, 51
apt-key , 195                                    -R (option), 44
aptitude , 181–183, 192–193                      --reference= ⟨name⟩ (option), 44 , 122                         chown , 36, 44–45
aquota.user , 122                                -R (option), 45
ar , 182–183, 197                          chsh , 27
at , 162                                   comm , 212
ATA, 78                                    cp , 116, 174
awk , 172                                  cpio , 130, 132, 204–205, 210, 230
                                           cron , 147, 162
bash , 50, 56, 174,   214                  cut , 212
bg , 54
/bin/sh , 212                              D-Bus, 74
/bin/true , 27                             dd , 88, 93–94, 110, 112, 116, 120–121, 138
blkid , 118–119                            DEBCONF_FRONTEND (environment
/boot , 134                                           variable), 195
boot manager, 128                          DEBCONF_PRIORITY (environment
boot script, 144                                      variable), 196
boot sector, 128                           debsums , 188, 194
/boot/grub , 133                           debugfs , 108
/boot/grub/custom.cfg , 134                      -w (option), 108
/boot/grub/grub.cfg , 134                  definitions, 12
/boot/grub/menu.lst , 132                  demand paging, 50
Bottomley, James, 130                      /dev , 73
btrfs , 114                                /dev/block , 85
btrfs check                                /dev/mapper , 94
     --repair   (option), 114              /dev/null , 170
234                                                                                        Index

      /dev/psaux , 71                                 LD_LIBRARY_PATH ,    177, 179, 216
      /dev/scd0 , 107                                 PATH , 177
      /dev/sda , 84, 88                               VISUAL , 37
      /dev/ttyS0 , 16                            /etc , 17, 116
      /dev/zero , 106                            /etc/apt/apt.conf , 190
      device , 73                                /etc/apt/sources.list , 189
      diff , 185                                 /etc/apt/trusted.gpg , 195
      Dijkstra, Edsger, 136                      /etc/dpkg/dpkg.cfg , 183
      disk , 92                                  /etc/filesystems , 117–118
      disk cache, 100                            /etc/fstab , 85, 88, 101–102, 109,
      dmesg , 137                                           116–117, 119, 121, 144, 157,
      dpkg , 181–184, 186–189, 191, 193                     215
            -a (option), 183                     /etc/group , 25, 27, 30–31, 33, 36–38
            --configure (option), 183            /etc/grub.d , 134
            --force-depends (option), 183        /etc/grub.d/40_custom , 134
            --force-overwrite (option), 183      /etc/grub.inst , 133
            -i (option), 183                     /etc/gshadow , 31, 37–39, 231
            --install (option), 183              /etc/inetd.conf , 157
            -L (option), 187                     /etc/init , 149
            -l (option), 185                     /etc/inittab , 142, 144–146, 151, 157, 160,
            --list (option), 185                            164–165
            --listfiles (option), 187            /etc/ , 177
            -P (option), 184                     /etc/ , 176–177
            --purge (option), 193                /etc/modprobe.conf , 71
            -r (option), 184                     /etc/modprobe.d , 71
            -s (option), 186, 188                /etc/mtab , 120, 215
            --search (option), 188               /etc/nologin , 151
            --status (option), 186–187, 191      /etc/nsswitch.conf , 32
            --unpack (option), 183               /etc/passwd , 25–28, 30–34, 36–37, 213
      dpkg-reconfigure , 195                     /etc/rpmrc , 200
            -f (option), 195                     /etc/securetty , 16
            --frontend (option), 195             /etc/shadow , 26, 28–29, 31–33, 35–37, 39,
            -p (option), 195                                48, 212–213
            --priority (option), 195             /etc/shells , 27
      dpkg-source , 185                          /etc/skel , 33
      dselect , 189, 192                         /etc/sysconfig , 18
      dump , 50                                  /etc/udev , 73
      dumpe2fs , 108                             /etc/yum.conf , 206
      dumpreiserfs , 111                         /etc/yum.repos.d , 206

      e2fsck , 107–108, 111                      fdisk , 88–93
           -B (option), 107                            -l (option),   89
           -b (option), 107–108                        -u (option),   89
           -c (option), 107                      fg , 54
           -f (option), 107                      file , 174–175
           -l (option), 107                      file attributes, 50
           -p (option), 107                      finger , 27
           -v (option), 107                      fsck , 101–102, 107–109, 112, 139
      e2label , 118                                    -A (option), 102
      e4defrag , 109                                   -a (option), 102
      EDITOR (environment variable),   37, 122         -f (option), 102
      edquota , 122                                    -N (option), 102
           -g (option), 122                            -p (option), 102
           -t (option), 122                            -R (option), 102
      egrep , 60                                       -s (option), 102
      environment variable                             -t (option), 101, 112
          DEBCONF_FRONTEND , 195                       -V (option), 102
          DEBCONF_PRIORITY , 196                       -v (option), 102
          EDITOR , 37, 122                       fsck.ext2 , 107
Index                                                                          235

fsck.xfs ,   112                     initctl status , 150
                                     initctl stop , 150
Garrett, Matthew, 130                insserv , 147, 216
gdisk , 92–93, 121                   ISOLINUX , 128
getent , 31–32, 212
getfacl , 47                         jobs ,
getty , 164                          Johnson, Jeff, 200
GNOME, 195                           Journaling, 103
Gooch, Richard, 73
gpasswd , 38                         KDE, 195
      -A (option), 38                kill , 58–60, 147
      -a (option), 38                killall , 58–60
      -d (option), 38                      -i (option), 59
grep , 31–32, 57, 59                       -l (option), 59
group, 23                                  -w (option), 59
      administrative, 31             Kok, Auke, 148
      administrator, 38              konsole , 27
      password, 31, 38               kpartx , 93–94, 96
groupadd , 37                              -v (option), 94
      -g (option), 37                Kroah-Hartman, Greg, 73
groupdel , 37–38
groupmod , 36–37                     label, 118
      -g (option), 37                last , 24–25
      -n (option), 37       , 177
groups, 15                           LD_LIBRARY_PATH (environment variable),
groups , 24                                      177, 179, 216
GRUB, 128                            ldconfig , 176–177
      boot problems, 138                   -p (option), 177
grub , 133                           ldd , 174–176, 216
      --device-map (option), 133     less , 31
      lock (option), 215             /lib , 175–177
      password (option), 135         /lib/modules , 70–71
grub-install , 133                   login , 16, 27, 151
grub-md5-crypt , 135                 losetup , 94
grub-mkconfig , 134–135                    -a (option), 94
gzip , 197                                 -f (option), 94
                                     lost+found , 108
halt ,151                            ls , 26, 42–43, 51
hard disks                                 -l (option), 26, 43, 51
     geometry, 65                    lsattr , 51, 214
     SCSI, 79                              -a (option), 51
hard quota, 121                            -d (option), 51
hello , 182, 185                           -R (option), 51
/home , 27–28, 86                    LSB, 182
home directory, 23                   lsblk , 119
/home/opt , 86                       lsmod , 71
Homme, Kjetil Torgrim, 59            lspci , 65–68
                                           -n (option), 67
id ,24, 26, 50, 211                        -t (option), 66–67
      -G (option), 24                      -v (option), 66–67
      -g (option), 24                lsusb , 68–69
      -Gn (option), 24                     -v (option), 69
      -n (option), 24
      -u (option), 24                mail , 27
init , 101, 135, 137, 144–146, 215   Mason, Chris, 100
init scripts, 146, 151               master boot record, 128
      parameters, 146                Matilainen, Panu, 200
initctl , 150                        mesg , 152
initctl start , 150                  Minix, 102
236                                                                                    Index

      mkdosfs , 114–115                                   -t (option), 60
      mke2fs , 100, 105–106, 109                          -u (option), 60
            -F (option), 106                         pkill , 59–60, 168
      mkfs , 100–101, 105–106, 113–114, 129               --signal (option),  60
            -t (option), 100, 105–106, 114           Poettering, Lennart, 142, 156
      mkfs.btrfs                                     pre-emptive multitasking, 55
            -d (option), 113                         primary group, 26
            -L (option), 119                         priority, 60
      mkfs.vfat , 114                                /proc , 54, 56, 71, 214
      mkfs.xfs , 111–112                             /proc/filesystems , 117–118
            -l (option), 112                         /proc/pci , 66
      mkreiserfs , 111                               /proc/swaps , 115–116
      mkswap , 115–116, 119                          /proc/sys/kernel/pid_max , 214
      /mnt , 106                                     process state, 55
      modprobe , 70–71                               ~/.profile , 122
            -r (option), 71                          ps , 47, 56–60
      mount , 88, 109, 116–118                             a (option), 56–57
            grpquota (option), 122                         ax (option), 57
            -t (option), 117                               -C (option), 57
            usrquota (option), 121–122                     --forest (option), 56, 58
      mount point, 116                                     --help (option), 56
      mv , 116                                             -l (option), 56
                                                           -o (option), 57
      newgrp , 31                                          p (option), 60
      nice , 61                                            r (option), 56
      nohup , 61                                           T (option), 56
      nohup.out ,   61                                     U (option), 56
                                                           -u (option), 47
      objectives, 219                                      x (option), 56–57
      /opt , 86                                      pseudo-users, 25
                                                     pstree , 57–58
      Packages.gz , 194–195
                                                           -G (option), 58
      parted , 90–92
                                                           -p (option), 58
      passwd , 26, 34–35, 37–38,   47–48, 212, 216         -u (option), 58
           -l (option), 35
                                                     pwconv , 32
           -S (option), 35
                                                     Python, 195
           -u (option), 35
                                                     python , 172
      passwd -n , 35
      passwd -w , 35                                 quota , 122–123
      passwd -x , 35                                      -g (option),   122
      passwords, 23, 26, 28                               -q (option),   122
            changing, 34                             quotacheck , 122
            group —, 31, 38                          quotaoff , 122
            GRUB, 135                                quotaon
            setting up, 34                                -g (option),   122
            shadow –, 26
            shadow —, 28                             reboot , 151
      PATH (environment variable), 177               Reiser, Hans, 110
      PCI Express, 65                                reiserfsck , 111
      perl , 172                                     Release , 194
      pgrep , 59–60                                  Release.gpg , 194
            -a (option), 59                          Remnant, Scott James, 142, 148
            -d (option), 59                          renice , 61
            -f (option), 60                          repquota , 122
            -G (option), 60                          resize_reiserfs , 111
            -l (option), 59                          restart , 216
            -n (option), 60                          return code, 55
            -o (option), 60                          Ritchie, Dennis, 48
            -P (option), 60                          rm , 42
Index                                                                             237

rpm ,
    182, 197, 200, 202, 205, 209       swapon , 115–116
     -a (option), 202                  /sys , 73
     -c (option), 203                  /sys/block , 72
     -d (option), 203                  /sys/bus , 72
     -e (option), 201                  /sys/bus/scsi/devices , 85
     -F (option), 201                  /sys/class , 72
     -f (option), 202                  syslog , 147, 216
     -h (option), 201                  syslogd , 137, 147
     -i (option), 200–202              systemctl , 157, 165–170, 216
     -l (option), 202–203                    --full (option), 167
     --nodeps (option), 201                  --kill-who (option), 167
     -p (option), 202                        -l (option), 167
     --provides (option), 203                --lines (option), 167
     -q (option), 201                        -n (option), 167
     -qi (option), 209                       --now (option), 169
     --requires (option), 203                -s (option), 167
     --test (option), 201                    --signal (option), 167
     -U (option), 201                        -t (option), 166–167
     -V (option), 204                  systemd , 168
     -v (option), 200, 202             systemd-escape , 162
     -vv (option), 200                       -p (option), 162
     --whatprovides (option), 203            -u (option), 162
     --whatrequires (option), 203
rpm2cpio , 204–205                     tar , 47, 130, 174, 183,   197, 200, 211
~/.rpmrc , 200                         telinit , 144–146, 148
runlevel, 151                                q (option), 144
     changing —, 145                   termination, 80
runlevel , 145, 168, 215               /tmp , 37, 49, 87, 213
runlevels, 142                         top , 61
     configuring —, 147                 touch , 37
     meaning, 145                      Ts’o, Theodore, 104
                                       tune2fs , 107–109, 119, 215
sash , 174                                   -c (option), 215
/sbin/init ,  142                            -L (option), 119
SELinux, 14                                  -l (option), 107
setfacl , 47                                 -m (option), 215
sfdisk , 93, 121, 138                        -u (option), 215
sgdisk , 93                            Tweedie, Stephen, 103
shutdown , 15, 144, 151–152, 165–166
      -c (option), 216                 udevd , 73
      -r (option), 151                 udisksctl , 74
Sievers, Kay, 73, 142, 156             udisksd , 74
signals, 58                            UID, 23
single-user mode, 147                  umask , 46, 50
sleep , 60                                  -S (option), 46
soft quota, 121                        umount , 116
sort , 212                             uname , 24
/srv , 87                                   -r (option), 24
ssh , 24                               update-grub , 134
sshd , 59                              update-rc.d , 147
star , 47                              usbview , 69
strip , 174                            user accounts, 22
su , 16–17, 25, 211, 213               user database, 25, 28
sudo , 14, 17                               stored elsewhere, 28
super user, 14                         user name, 23
superblock, 100                        useradd , 33–34, 36–37, 212
SuSEconfig , 18                        userdel , 36–37
swap partition, 115                         -r (option), 36
swapoff , 115                          usermod , 36–37, 213
238                                              Index

      /usr/lib , 175–177
      /usr/lib/rpm , 200
      /usr/local , 86, 201
      /usr/local/lib , 177
      UUID, 119

      van de Ven, Arjan, 148
      /var , 87
      /var/lib/dpkg/info , 188
      /var/lib/usbutils/usb.ids , 69
      /var/log/messages , 17, 137, 211
      /var/log/syslog , 137
      /var/mail , 36, 121
      vi , 19, 37
      vigr , 37–38
            -s (option), 38
      vipw , 37–38, 213
            -s (option), 37
      VISUAL (environment variable),        37
      vol_id , 118

      w , 92
      wall ,   152–153
               -n (option), 153
               --nobanner (option),   153
      Webmin, 18
      write , 153

      xclock , 56
      xfs_copy , 112
      xfs_info , 112
      xfs_mdrestore , 112
      xfs_metadump , 112
      xfs_quota , 121
      xfs_repair , 112
           -n (option), 112
      xfsdump , 112
      xfsrestore , 112
      xterm , 27

      YUM, 205
      yum , 205, 207–209
           --disablerepo (option), 206
           --enablerepo= (option), 205
           --obsoletes (option), 207
      yumdownloader , 210
           --resolve (option), 210
           --source (option), 210
           --urls (option), 210

      zombies, 55
      zsh , 34