DOKK Library

Introduction to Linux for Users and Administrators

Authors tuxcademy

License CC-BY-SA-4.0

                                                                    Version 4.0

Introduction to Linux
for Users and Administrators

                    $ echo tux
                    $ ls
                    $ /bin/su -

tuxcademy – Linux and Open Source learning materials for everyone
This training manual is designed to correspond to the objectives of the LPI-101 (LPIC-1, version
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      Introduction to Linux       for Users and Administrators
      Revision: grd1:62f570f98f89998d:2015-08-04
                grd1:be27bba8095b329b:2015-08-04 1–11, B–C

      © 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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      This document is published under the “Creative Commons-BY-SA 4.0 Interna-
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      Further information and the full legal license grant may be found at sa/4.0/

      Authors: Tobias Elsner, Anselm Lingnau
      Technical Editor: Anselm Lingnau ⟨ ⟩
      English Translation: Anselm Lingnau
      Typeset in Palatino, Optima and DejaVu Sans Mono
                                                                                                     $ echo tux
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1 Introduction                                                                                  13
1.1 What is Linux? . . . . . . . . . . .                .   .   .   .   .   .   .   .   .   .   14
1.2 Linux History . . . . . . . . . . .                 .   .   .   .   .   .   .   .   .   .   14
1.3 Free Software, “Open Source” and the GPL            .   .   .   .   .   .   .   .   .   .   16
1.4 Linux—The Kernel . . . . . . . . .                  .   .   .   .   .   .   .   .   .   .   19
1.5 Linux Properties . . . . . . . . . .                .   .   .   .   .   .   .   .   .   .   21
1.6 Linux Distributions . . . . . . . . .               .   .   .   .   .   .   .   .   .   .   24

2 Using the Linux System                                      29
2.1 Logging In and Out . . . . . . . . . . . . . . . . . . . 30
2.2 Switching On and Off . . . . . . . . . . . . . . . . . . 32
2.3 The System Administrator. . . . . . . . . . . . . . . . . 32

3 Who’s Afraid Of The Big Bad Shell?                                                            35
3.1 Why? . . . . . . . . . . .              .   .   .   .   .   .   .   .   .   .   .   .   .   36
   3.1.1 What Is The Shell? . . . .         .   .   .   .   .   .   .   .   .   .   .   .   .   36
3.2 Commands . . . . . . . . .              .   .   .   .   .   .   .   .   .   .   .   .   .   37
   3.2.1 Why Commands?. . . . .             .   .   .   .   .   .   .   .   .   .   .   .   .   37
   3.2.2 Command Structure. . . .           .   .   .   .   .   .   .   .   .   .   .   .   .   38
   3.2.3 Command Types . . . . .            .   .   .   .   .   .   .   .   .   .   .   .   .   39
   3.2.4 Even More Rules . . . . .          .   .   .   .   .   .   .   .   .   .   .   .   .   39

4 Getting Help                                                                                  41
4.1 Self-Help . . . . . . . . .         . . .       .   .   .   .   .   .   .   .   .   .   .   42
4.2 The help Command and the --help     Option      .   .   .   .   .   .   .   .   .   .   .   42
4.3 The On-Line Manual . . . .          . . .       .   .   .   .   .   .   .   .   .   .   .   42
   4.3.1 Overview . . . . . .           . . .       .   .   .   .   .   .   .   .   .   .   .   42
   4.3.2 Structure . . . . . . .        . . .       .   .   .   .   .   .   .   .   .   .   .   43
   4.3.3 Chapters . . . . . . .         . . .       .   .   .   .   .   .   .   .   .   .   .   44
   4.3.4 Displaying Manual Pages .      . . .       .   .   .   .   .   .   .   .   .   .   .   44
4.4 Info Pages . . . . . . . .          . . .       .   .   .   .   .   .   .   .   .   .   .   45
4.5 HOWTOs. . . . . . . . .             . . .       .   .   .   .   .   .   .   .   .   .   .   46
4.6 Further Information Sources . .     . . .       .   .   .   .   .   .   .   .   .   .   .   46

5 Editors: vi and emacs                                                                         49
5.1 Editors. . . . . . . .      .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   50
5.2 The Standard—vi . . . .     .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   50
   5.2.1 Overview . . . .       .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   50
   5.2.2 Basic Functions . .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   51
   5.2.3 Extended Commands      .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   54
5.3 The Challenger—Emacs .      .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   56
   5.3.1 Overview . . . .       .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   56
   5.3.2 Basic Functions . .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   57
   5.3.3 Extended Functions .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   59
5.4 Other Editors . . . . .     .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   61
4                                                                                            Contents

    6 Files: Care and Feeding                                                                        63
    6.1 File and Path Names . . . . . . . . . . . . .                        .   .   .   .   .   .   64
       6.1.1 File Names . . . . . . . . . . . . . .                          .   .   .   .   .   .   64
       6.1.2 Directories . . . . . . . . . . . . . .                         .   .   .   .   .   .   65
       6.1.3 Absolute and Relative Path Names . . . . .                      .   .   .   .   .   .   66
    6.2 Directory Commands . . . . . . . . . . . .                           .   .   .   .   .   .   67
       6.2.1 The Current Directory: cd & Co. . . . . . .                     .   .   .   .   .   .   67
       6.2.2 Listing Files and Directories—ls . . . . . .                    .   .   .   .   .   .   68
       6.2.3 Creating and Deleting Directories: mkdir and rmdir              .   .   .   .   .   .   69
    6.3 File Search Patterns . . . . . . . . . . . . .                       .   .   .   .   .   .   70
       6.3.1 Simple Search Patterns . . . . . . . . . .                      .   .   .   .   .   .   70
       6.3.2 Character Classes . . . . . . . . . . . .                       .   .   .   .   .   .   72
       6.3.3 Braces . . . . . . . . . . . . . . . .                          .   .   .   .   .   .   73
    6.4 Handling Files . . . . . . . . . . . . . . .                         .   .   .   .   .   .   74
       6.4.1 Copying, Moving and Deleting—cp and Friends.                    .   .   .   .   .   .   74
       6.4.2 Linking Files—ln and ln -s . . . . . . . .                      .   .   .   .   .   .   76
       6.4.3 Displaying File Content—more and less . . . .                   .   .   .   .   .   .   80
       6.4.4 Searching Files—find . . . . . . . . . .                        .   .   .   .   .   .   81
       6.4.5 Finding Files Quickly—locate and slocate . . .                  .   .   .   .   .   .   84

    7 Regular Expressions                                            87
    7.1 Regular Expressions: The Basics . . . . . . . . . . . . . . 88
       7.1.1 Regular Expressions: Extras . . . . . . . . . . . . . . 88
    7.2 Searching Files for Text—grep . . . . . . . . . . . . . . . 89

    8 Standard I/O and Filter Commands                                                                93
    8.1 I/O Redirection and Command Pipelines . . . .                    .   .   .   .   .   .   .    94
       8.1.1 Standard Channels . . . . . . . . . .                       .   .   .   .   .   .   .    94
       8.1.2 Redirecting Standard Channels . . . . . .                   .   .   .   .   .   .   .    95
       8.1.3 Command Pipelines . . . . . . . . . .                       .   .   .   .   .   .   .    98
    8.2 Filter Commands . . . . . . . . . . . . .                        .   .   .   .   .   .   .    99
    8.3 Reading and Writing Files . . . . . . . . . .                    .   .   .   .   .   .   .   100
       8.3.1 Outputting and Concatenating Text Files—cat                 .   .   .   .   .   .   .   100
       8.3.2 Beginning and End—head and tail . . . . .                   .   .   .   .   .   .   .   100
    8.4 Data Management . . . . . . . . . . . .                          .   .   .   .   .   .   .   101
       8.4.1 Sorted Files—sort and uniq . . . . . . .                    .   .   .   .   .   .   .   101
       8.4.2 Columns and Fields—cut , paste etc. . . . .                 .   .   .   .   .   .   .   106

    9 More About The Shell                                                                           111
    9.1 Simple Commands: sleep , echo , and date .      .    .   .   .   .   .   .   .   .   .   .   112
    9.2 Shell Variables and The Environment. .          .    .   .   .   .   .   .   .   .   .   .   113
    9.3 Command Types—Reloaded . . . . .                .    .   .   .   .   .   .   .   .   .   .   115
    9.4 The Shell As A Convenient Tool. . . .           .    .   .   .   .   .   .   .   .   .   .   116
    9.5 Commands From A File . . . . . .                .    .   .   .   .   .   .   .   .   .   .   119
    9.6 The Shell As A Programming Language.            .    .   .   .   .   .   .   .   .   .   .   120

    10 The File System                                                                               125
    10.1 Terms . . . . . . . . . .          .   .   .   .    .   .   .   .   .   .   .   .   .   .   126
    10.2 File Types. . . . . . . . .        .   .   .   .    .   .   .   .   .   .   .   .   .   .   126
    10.3 The Linux Directory Tree . . .     .   .   .   .    .   .   .   .   .   .   .   .   .   .   127
    10.4 Directory Tree and File Systems.   .   .   .   .    .   .   .   .   .   .   .   .   .   .   135

    11 Archiving and Compressing Files                                                               137
    11.1 Archival and Compression . . . . . .                . . . .         .   .   .   .   .   .   138
    11.2 Archiving Files Using tar . . . . . . .             . . . .         .   .   .   .   .   .   139
    11.3 Compressing Files with gzip . . . . . .             . . . .         .   .   .   .   .   .   142
    11.4 Compressing Files with bzip2 . . . . . .            . . . .         .   .   .   .   .   .   143
    11.5 Archiving and Compressing Files Using zip          and unzip .      .   .   .   .   .   .   144

A Sample Solutions                                                149

B Example Files                                                   159

C LPIC-1 Certification                                             163
C.1 Overview. . . . . . . . . . . . . . . . . . . . . . .         163
C.2 Exam LPI-101 . . . . . . . . . . . . . . . . . . . . .        163
C.3 LPI Objectives In This Manual . . . . . . . . . . . . . . .   164

D Command Index                                                   169

Index                                                             173
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List of Tables

 4.1   Manual page sections . . . . . . . . . . . . . . . . . . . . . . . . . . .                                             43
 4.2   Manual Page Topics . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                             44

 5.1   Insert-mode commands for vi . . . . .              .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   52
 5.2   Cursor positioning commands in vi . .              .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   53
 5.3   Editing commands in vi . . . . . . . .             .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   54
 5.4   Replacement commands in vi . . . . .               .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   54
 5.5   ex commands in vi . . . . . . . . . . . .          .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   56
 5.6   Possible buffer states in emacs . . . . . .         .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   57
 5.7   Cursor movement commands in emacs                  .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   59
 5.8   Deletion commands in emacs . . . . . .             .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   59
 5.9   Text-correcting commands in emacs . .              .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   60

 6.1   Some file type designations in ls       .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   68
 6.2   Some ls options . . . . . . . . . .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   68
 6.3   Options for cp . . . . . . . . . . .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   74
 6.4   Keyboard commands for more . .         .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   80
 6.5   Keyboard commands for less . .         .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   81
 6.6   Test conditions for find . . . . . .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   82
 6.7   Logical operators for find . . . . .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   83

 7.1   Regular expression support . . . . . . . . . . . . . . . . . . . . . . .                                               90
 7.2   Options for grep (selected) . . . . . . . . . . . . . . . . . . . . . . . .                                            90

 8.1   Standard channels on Linux . . . . . . . . . . . . . . . . . . . . . . . 95
 8.2   Options for cat (selection) . . . . . . . . . . . . . . . . . . . . . . . . 100
 8.3   Options for sort (selection) . . . . . . . . . . . . . . . . . . . . . . . . 104

 9.1   Important Shell Variables . . . . . . . . . . . . . . . . . . . . . . . . 114
 9.2   Key Strokes within bash . . . . . . . . . . . . . . . . . . . . . . . . . . 118

 10.1 Linux file types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
 10.2 Directory division according to the FHS . . . . . . . . . . . . . . . . 134
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List of Figures

 1.1   Ken Thompson and Dennis Ritchie with a PDP-11 . . . . . . . . . .             15
 1.2   Linux development . . . . . . . . . . . . . . . . . . . . . . . . . . . .     16
 1.3   Organizational structure of the Debian project . . . . . . . . . . . .        25

 2.1   The login screens of some common Linux distributions . . . . . . .            30
 2.2   Running programs as a different user in KDE . . . . . . . . . . . . .          33

 4.1   A manual page . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     44

 5.1   vi ’s
          modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    52
 5.2   The emacs launch screen . . . . . . . . . . . . . . . . . . . . . . . . . .   58

 8.1   Standard channels on Linux . . . . . . . . . . . . . . . . . . . . . . .      94
 8.2   The tee command . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     98

 10.1 Content of the root directory (SUSE) . . . . . . . . . . . . . . . . . . 128
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This manual is an introduction to the use of Linux. Participants will not just learn
how to operate the system according to instructions, but will receive solid funda-
mental knowledge about Linux, to serve as a basis for further study.
   The course is directed towards computer users with little or no previous expo-
sure to Linux. Knowledge of other operating systems is helpful but not a prereq-
uisite. Ambitious users as well as system administrators will be learning how to
make the most of the Linux operating system.
   Having studied this manual, participants will be able to use the Linux oper-
ating system on an elementary basis. They will be able to work on the Linux
command line and be familiar with the most important tools. Completion of this
course or equivalent knowledge is necessary for more advanced Linux courses
and for 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 “
 ”. 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 are emphasized using “marginal notes” so they can be eas- Important concepts
12                                                                                         Preface

     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 C for further information.
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1.1     What is Linux? . . . . . . . . . . .        .   .   .   .   .   .   .   .   .   .   14
1.2     Linux History . . . . . . . . . . .         .   .   .   .   .   .   .   .   .   .   14
1.3     Free Software, “Open Source” and the GPL    .   .   .   .   .   .   .   .   .   .   16
1.4     Linux—The Kernel . . . . . . . . .          .   .   .   .   .   .   .   .   .   .   19
1.5     Linux Properties . . . . . . . . . .        .   .   .   .   .   .   .   .   .   .   21
1.6     Linux Distributions . . . . . . . . .       .   .   .   .   .   .   .   .   .   .   24

      • Knowing about Linux, its properties and its history
      • Differentiating between the Linux kernel and Linux distributions
      • Understanding the terms “GPL”, “free software”, and “open-source soft-

      • Knowledge of other operating systems is useful to appreciate similarities
        and differences

grd1-einfuehrung.tex   (be27bba8095b329b )
14                                                                                                      1 Introduction

                         1.1      What is Linux?
                         Linux is an operating system. As such, it manages a computer’s basic function-
                         ality. Application programs build on the operating system. It forms the interface
                         between the hardware and application programs as well as the interface between
                         the hardware and people (users). Without an operating system, the computer is
                         unable to “understand” or process our input.
                             Various operating systems differ in the way they go about these tasks. The
                         functions and operation of Linux are inspired by the Unix operating system.

                         1.2      Linux History
                         The history of Linux is something special in the computer world. While most other
                         operating systems are commercial products produced by companies, Linux was
                         started by a university student as a hobby project. In the meantime, hundreds of
                         professionals and enthusiasts all over the world collaborate on it—from hobbyists
                         and computer science students to operating systems experts funded by major IT
                         corporations to do Linux development. The basis for the existence of such a project
                         is the Internet: Linux developers make extensive use of services like electronic
                         mail, distributed version control, and the World Wide Web and, through these,
                         have made Linux what it is today. Hence, Linux is the result of an international
                         collaboration across national and corporate boundaries, now as then led by Linus
                         Torvalds, its original author.
                             To explain about the background of Linux, we need to digress for a bit: Unix,
                         the operating system that inspired Linux, was begun in 1969. It was developed by
     Bell Laboratories   Ken Thompson and his colleagues at Bell Laboratories (the US telecommunication
                         giant AT&T’s research institute)1 . Unix caught on rapidly especially at universi-
                         ties, because Bell Labs furnished source code and documentation at cost (due to
                         an anti-trust decree, AT&T was barred from selling software). Unix was, at first,
                         an operating system for Digital Equipment’s PDP-11 line of minicomputers, but
                         was ported to other platforms during the 1970s—a reasonably feasible endeavour,
                         since the Unix software, including the operating system kernel, was mostly writ-
                    C    ten in Dennis Ritchie’s purpose-built C programming language. Possibly most
                         important of all Unix ports was the one to the PDP-11’s successor platform, the
                 VAX     VAX, at the University of California in Berkeley, which came to be distributed as
                         “BSD” (short for Berkeley Software Distribution). By and by, various computer man-
                         ufacturers developed different Unix derivatives based either on AT&T code or on
                         BSD (e. g., Sinix by Siemens, Xenix by Microsoft (!), SunOS by Sun Microsystems,
                         HP/UX by Hewlett-Packard or AIX by IBM). Even AT&T was finally allowed to
            System V     market Unix—the commercial versions System III and (later) System V. This led to
                         a fairly incomprehensible multitude of different Unix products. A real standardi-
                         sation never happened, but it is possible to distinguish roughly between BSD-like
                         and System-V-like Unix variants. The BSD and System V lines of development
                SVR4     were mostly unified by “System V Release 4”, which exhibited the characteristics
                         of both factions.
                             The very first parts of Linux were developed in 1991 by Linus Torvalds, then
                         a 21-year-old student in Helsinki, Finland, when he tried to fathom the capabil-
                         ities of his new PC’s Intel 386 processor. After a few months, the assembly lan-
                         guage experiments had matured into a small but workable operating system ker-
                Minix    nel that could be used in a Minix system—Minix was a small Unix-like operating
                         system that computer science professor Andrew S. Tanenbaum of the Free Uni-
                         versity of Amsterdam, the Netherlands, had written for his students. Early Linux
                         had properties similar to a Unix system, but did not contain Unix source code.
                         Linus Torvalds made the program’s source code available on the Internet, and the
                            1 The name “Unix” is a pun on “Multics”, the operating system that Ken Thompson and his col-

                         leagues worked on previously. Early Unix was a lot simpler than Multics. How the name came to be
                         spelled with an “x” is no longer known.
1.2 Linux History                                                                     15

Figure 1.1: Ken Thompson (sitting) and Dennis Ritchie (standing) with a
            PDP-11, approx. 1972. (Photograph courtesy of Lucent Technologies.)

idea was eagerly taken up and developed further by many programmers. Version
0.12, issued in January, 1992, was already a stable operating system kernel. There
was—thanks to Minix—the GNU C compiler (gcc ), the bash shell, the emacs editor
and many other GNU tools. The operating system was distributed world-wide by
anonymous FTP. The number of programmers, testers and supporters grew very
rapidly. This catalysed a rate of development only dreamed of by powerful soft-
ware companies. Within months, the tiny kernel grew into a full-blown operating
system with fairly complete (if simple) Unix functionality.
   The “Linux” project is not finished even today. Linux is constantly updated
and added to by hundreds of programmers throughout the world, catering to
millions of satisfied private and commercial users. In fact it is inappropriate to
say that the system is developed “only” by students and other amateurs—many
contributors to the Linux kernel hold important posts in the computer industry
and are among the most professionally reputable system developers anywhere.
By now it is fair to claim that Linux is the operating system with the widest sup-
ported range of hardware ever, not just with respect to the platforms it is running
on (from PDAs to mainframes) but also with respect to driver support on, e. g., the
Intel PC platform. Linux also serves as a research and development platform for
new operating systems ideas in academia and industry; it is without doubt one of
the most innovative operating systems available today.

C 1.1 [4] Use the Internet to locate the famous (notorious?) discussion between
  Andrew S. Tanenbaum and Linus Torvalds, in which Tanenbaum says that,
  with something like Linux, Linus Torvalds would have failed his (Tanen-
  baum’s) operating systems course. What do you think of the controversy?

C 1.2 [2] Give the version number of the oldest version of the Linux source
  code that you can locate.
16                                                                                                                  1 Introduction

                     Linux 2.0
                     Linux 2.1
          50MiB      Linux 2.2
                     Linux 2.3
                     Linux 2.4
                     Linux 2.5
          45MiB      Linux 2.6








                     1997        1998       1999        2000      2001      2002      2003     2004   2005   2006   2007

Figure 1.2: Linux development, measured by the size of linux- *.tar.gz . Each marker corresponds to a Linux
            version. During the 10 years between Linux 2.0 and Linux 2.6.18, the size of the compressed Linux
            source code has roughly increased tenfold.

                                  1.3            Free Software, “Open Source” and the GPL
                                  From the very beginning of its development, Linux was placed under the GNU
                          GPL General Public License (GPL) promulgated by the Free Software Foundation (FSF).
      Free Software Foundation The FSF was founded by Richard M. Stallman, the author of the Emacs editor
                                  and other important programs, with the goal of making high-quality software
                  Free Software “freely” available—in the sense that users are “free” to inspect it, to change it
                              and to redistribute it with or without changes, not necessarily in the sense that
                              it does not cost anything2 . In particular, he was after a freely available Unix-like
                              operating system, hence “GNU” as a (recursive) acronym for “GNU’s Not Unix”.
                              The main tenet of the GPL is that software distributed under it may be changed
                              as well as sold at any time, but that the (possibly modified) source code must
                  Open Source always be passed along—thus Open Source—and that the recipient must receive
                              the same rights of modification and redistribution. Thus there is little point in
                              selling GPL software “per seat”, since the recipient must be allowed to copy and
                              install the software as often as wanted. (It is of course permissible to sell support
                              for the GPL software “per seat”.) New software resulting from the extension or
                              modification of GPL software must, as a “derived work”, also be placed under the
                                  Therefore, the GPL governs the distribution of software, not its use, and al-
                              lows the recipient to do things that he would not be allowed to do otherwise—for
                              example, the right to copy and distribute the software, which according to copy-
                              right law is the a priori prerogative of the copyright owner. Consequently, it differs
                              markedly from the “end user license agreements” (EULAs) of “proprietary” soft-
                              ware, whose owners try to take away a recipient’s rights to do various things. (For
                              example, some EULAs try to forbid a software recipient from talking critically—or
                                        2 The   FSF says “free as in speech, not as in beer”
1.3 Free Software, “Open Source” and the GPL                                              17

at all—about the product in public.)

B The GPL is a license, not a contract, since it is a one-sided grant of rights
  to the recipient (albeit with certain conditions attached). The recipient of
  the software does not need to “accept” the GPL explicitly. The common
  EULAs, on the other hand, are contracts, since the recipient of the software
  is supposed to waive certain rights in exchange for being allowed to use the
  software. For this reason, EULAs must be explicitly accepted. The legal
  barriers for this may be quite high—in many jurisdictions (e. g., Germany),
  any EULA restrictions must be known to the buyer before the actual sale in
  order to become part of the sales contract. Since the GPL does not in any
  way restrict a buyer’s rights (in particular as far as use of the software is
  concerned) compared to what they would have to expect when buying any
  other sort of goods, these barriers do not apply to the GPL; the additional
  rights that the buyer is conferred by the GPL are a kind of extra bonus.

B Currently two versions of the GPL are in widespread use. The newer ver-
  sion 3 (also called “GPLv3”) was published in July, 2007, and differs from the GPLv3
  older version 2 (also “GPLv2”) by more precise language dealing with ar-
  eas such as software patents, the compatibility with other free licenses, and
  the introduction of restrictions on making changes to theoretically “free”
  devices impossible by excluding them through special hardware (“Tivoisa-
  tion”, after a Linux-based personal video recorder whose kernel is impossi-
  ble to alter or exchange). In addition, GPLv3 allows its users to add further
  clauses. – Within the community, the GPLv3 was not embraced with unan-
  imous enthusiasm, and many projects, in particular the Linux kernel, have
  intentionally stayed with the simpler GPLv2. Many other projects are made
  available under the GPLv2 “or any newer version”, so you get to decide
  which version of the GPL you want to follow when distributing or modify-
  ing such software.

   Neither should you confuse GPL software with “public-domain” software. Public Domain
The latter belongs to nobody, everybody can do with it what he wants. A GPL
program’s copyright still rests with its developer or developers, and the GPL
states very clearly what one may do with the software and what one may not.

B It is considered good form among free software developers to place contri-
  butions to a project under the same license that the project is already using,
  and in fact most projects insist on this, at least for code that is supposed to
  become part of the “official” version. Indeed, some projects insist on “copy-
  right assignments”, where the code author signs the copyright over to the
  project (or a suitable organisation). The advantage of this is that, legally,
  only the project is responsible for the code and that licensing violations—
  where only the copyright owner has legal standing—are easier to prose-
  cute. A side effect that is either desired or else explicitly unwanted is that
  copyright assignment makes it easier to change the license for the complete
  project, as this is an act that only the copyright owner may perform.

B In case of the Linux operating system kernel, which explicitly does not re-
  quire copyright assignment, a licensing change is very difficult to impossible
  in practice, since the code is a patchwork of contributions from more than
  a thousand authors. The issue was discussed during the GPLv3 process,
  and there was general agreement that it would be a giant project to ascer-
  tain the copyright provenance of every single line of the Linux source code,
  and to get the authors to agree to a license change. In any case, some Linux
  developers would be violently opposed, while others are impossible to find
  or even deceased, and the code in question would have to be replaced by
  something similar with a clear copyright. At least Linus Torvalds is still in
  the GPLv2 camp, so the problem does not (yet) arise in practice.
18                                                                                               1 Introduction

         GPL and Money         The GPL does not stipulate anything about the price of the product. It is utterly
                           legal to give away copies of GPL programs, or to sell them for money, as long
                           as you provide source code or make it available upon request, and the software
                           recipient gets the GPL rights as well. Therefore, GPL software is not necessarily
                               You can find out more by reading the GPL [GPL91], which incidentally must
                           accompany every GPLlicensed product (including Linux).
     Other “free” licenses     There are other “free” software licenses which give similar rights to the soft-
                           ware recipient, for example the “BSD license” which lets appropriately licensed
                           software be included in non-free products. The GPL is considered the most thor-
                           ough of the free licenses in the sense that it tries to ensure that code, once pub-
                           lished under the GPL, remains free. Every so often, companies have tried to include
                           GPL code in their own non-free products. However, after being admonished by
                           (usually) the FSF as the copyright holder, these companies have always complied
                           with the GPL’s requirements. In various jurisdictions the GPL has been validated
                           in courts of law—for example, in the Frankfurt (Germany) Landgericht (state court),
                           a Linux kernel developer obtained a judgement against D-Link (a manufacturer of
                           network components, in this case a Linux-based NAS device) in which the latter
                           was sued for damages because they did not adhere to the GPL conditions when
                           distributing the device [GPL-Urteil06].

                          B Why does the GPL work? Some companies that thought the GPL condi-
                            tions onerous have tried to declare or have it declared it invalid. For exam-
                            ple, it was called “un-American” or “unconstitutional” in the United States;
                            in Germany, anti-trust law was used in an attempt to prove that the GPL
                            amounts to price fixing. The general idea seems to be that GPL-ed soft-
                            ware can be used by anybody if something is demonstrably wrong with the
                            GPL. All these attacks ignore one fact: Without the GPL, nobody except the
                            original author has the right to do anything with the code, since actions like
                            sharing (let alone selling) the code are the author’s prerogative. So if the
                            GPL goes away, all other interested parties are worse off than they were.

                          B A lawsuit where a software author sues a company that distributes his GPL
                            code without complying with the GPL would approximately look like this:
                                Judge What seems to be the problem?
                                Software Author Your Lordship, the defendant has distributed my soft-
                                    ware without a license.
                                Judge (to the defendant’s counsel) Is that so?
                                At this point the defendant can say “yes”, and the lawsuit is essentially over
                                (except for the verdict). They can also say “no” but then it is up to them
                                to justify why copyright law does not apply to them. This is an uncom-
                                fortable dilemma and the reason why few companies actually do this to
                                themselves—most GPL disagreements are settled out of court.

                          B If a manufacturer of proprietary software violates the GPL (e. g., by includ-
                            ing a few hundreds of lines of source code from a GPL project in their prod-
                            uct), this does not imply that all of that product’s code must now be released
                            under the terms of the GPL. It only implies that they have distributed GPL
                            code without a license. The manufacturer can solve this problem in various
                                  • They can remove the GPL code and replace it by their own code. The
                                    GPL then becomes irrelevant for their software.
                                  • They can negotiate with the GPL code’s copyright holder (if he is avail-
                                    able and willing to go along) and, for instance, agree to pay a license
                                    fee. See also the section on multiple licenses below.
                                  • They can release their entire program under the GPL voluntarily and
                                    thereby comply with the GPL’s conditions (the most unlikely method).
1.4 Linux—The Kernel                                                                                              19

      Independently of this there may be damages payable for the prior violations.
      The copyright status of the proprietary software, however, is not affected in
      any way.

   When is a software package considered “free” or “open source”? There are Freedom criteria
no definite criteria, but a widely-accepted set of rules are the Debian Free Software Debian Free Software Guidelines
Guidelines [DFSG]. The FSF summarizes its criteria as the Four Freedoms which
must hold for a free software package:

   • The freedom to use the software for any purpose (freedom 0)
   • The freedom to study how the software works, and to adapt it to one’s re-
     quirements (freedom 1)
   • The freedom to pass the software on to others, in order to help one’s neigh-
     bours (freedom 2)
   • The freedom to improve the software and publish the improvements, in or-
     der to benefit the general public (freedom 3)
Access to the source code is a prerequisite for freedoms 1 and 3. Of course, com-
mon free-software licenses such as the GPL or the BSD license conform to these
   In addition, the owner of a software package is free to distribute it under differ- Multiple licenses
ent licenses at the same time, e.g., the GPL and, alternatively, a “commercial” li-
cense that frees the recipient from the GPL restrictions such as the duty to make
available the source code for modifications. This way, private users and free soft-
ware authors can enjoy the use of a powerful programming library such as the
“Qt” graphics package (published by Qt Software—formerly Troll Tech—, a Nokia
subsidiary), while companies that do not want to make their own source code
available may “buy themselves freedom” from the GPL.

C 1.3 [!2] Which of the following statements concerning the GPL are true and
  which are false?
        1. GPL software may not be sold.
        2. GPL software may not be modified by companies in order to base their
           own products on it.
        3. The owner of a GPL software package may distribute the program un-
           der a different license as well.
        4. The GPL is invalid, because one sees the license only after having ob-
           tained the software package in question. For a license to be valid, one
           must be able to inspect it and accept it before acquiring the software.

C 1.4 [4] Some software licenses require that when a file from a software distri-
  bution is changed, it must be renamed. Is software distributed under such a
  license considered “free” according to the DFSG? Do you think this practice
  makes sense?

1.4     Linux—The Kernel
Strictly speaking, the name “Linux” only applies to the operating system “kernel”,
which performs the actual operating system tasks. It takes care of elementary
functions like memory and process management and hardware control. Applica-
tion programs must call upon the kernel to, e.g., access files on disk. The kernel
validates such requests and in doing so can enforce that nobody gets to access
20                                                                                                  1 Introduction

                         other users’ private files. In addition, the kernel ensures that all processes in the
                         system (and hence all users) get the appropriate fraction of the available CPU time.
                Versions    Of course there is not just one Linux kernel, but there are many different ver-
                         sions. Until kernel version 2.6, we distinguished stable “end-user versions” and
                         unstable “developer versions” as follows:
           stable version        • In version numbers such as 1.𝑥.𝑦 or 2.𝑥.𝑦, 𝑥 denotes a stable version if it is
                                   even. There should be no radical changes in stable versions; mistakes should
                                   be corrected, and every so often drivers for new hardware components or
                                   other very important improvements are added or “back-ported” from the
                                   development kernels.
     development version         • Versions with odd 𝑥 are development versions which are unsuitable for pro-
                                   ductive use. They may contain inadequately tested code and are mostly
                                   meant for people wanting to take active part in Linux development. Since
                                   Linux is constantly being improved, there is a constant stream of new ker-
                                   nel versions. Changes concern mostly adaptations to new hardware or the
                                   optimization of various subsystems, sometimes even completely new exten-
              kernel 2.6 The procedure has changed in kernel 2.6: Instead of starting version 2.7 for new
                          development after a brief stabilisation phase, Linus Torvalds and the other kernel
                          developers decided to keep Linux development closer to the stable versions. This
                          is supposed to avoid the divergence of developer and stable versions that grew to
                          be an enormous problem in the run-up to Linux 2.6—most notably because corpo-
                          rations like SUSE and Red Hat took great pains to backport interesting properties
                          of the developer version 2.5 to their versions of the 2.4 kernel, to an extent where,
                          for example, a SUSE 2.4.19 kernel contained many hundreds of differences to the
                          “vanilla” 2.4.19 kernel.
                              The current procedure consists of “test-driving” proposed changes and en-
                          hancements in a new kernel version which is then declared “stable” in a shorter
                          timeframe. For example, after version 2.6.37 there is a development phase during
                          which Linus Torvalds accepts enhancements and changes for the 2.6.38 version.
                          Other kernel developers (or whoever else fancies it) have access to Linus’ internal
                          development version, which, once it looks reasonable enough, is made available
        release candidate as the “release candidate” 2.6.38-rc1. This starts the stabilisation phase, where
                          this release candidate is tested by more people until it looks stable enough to be
                          declared the new version 2.6.38 by Linus Torvalds. Then follows the 2.6.39 devel-
                          opment phase and so on.

                              B In parallel to Linus Torvalds’ “official” version, Andrew Morton maintains
                 -mm   tree     a more experimental version, the so-called “-mm tree”. This is used to test
                                larger and more sweeping changes until they are mature enough to be taken
                                into the official kernel by Linus.

                              B Some other developers maintain the “stable” kernels. As such, there might
                                be kernels numbered,, …, which each contain only small
                                and straightforward changes such as fixes for grave bugs and security is-
                                sues. This gives Linux distributors the opportunity to rely on kernel ver-
                                sions maintained for longer periods of time.

             version 3.0         On 21 July 2011, Linus Torvalds officially released version 3.0 of the Linux ker-
                              nel. This was really supposed to be version 2.6.40, but he wanted to simplify the
                              version numbering scheme. “Stable” kernels based on 3.0 are accordingly num-
                              bered 3.0.1, 3.0.2, …, and the next kernels in Linus’ development series are 3.1-rc1,
                              etc. leading up to 3.1 and so forth.

                              B Linus Torvalds insists that there was no big difference in functionality be-
                                tween the 2.6.39 and 3.0 kernels—at least not more so than between any
                                two other consecutive kernels in the 2.6 series—, but that there was just a
                                renumbering. The idea of Linux’s 20th anniversary was put forward.
1.5 Linux Properties                                                                                21

   You can obtain source code for “official” kernels on the Internet from ftp. . However, only very few Linux distributors use the original kernel
sources. Distribution kernels are usually modified more or less extensively, e. g.,
by integrating additional drivers or features that are desired by the distribution
but not part of the standard kernel. The Linux kernel used in SUSE’s Linux Enter-
prise Server 8, for example, reputedly contained approximately 800 modifications
to the “vanilla” kernel source. (The changes to the Linux development process
have succeeded to an extent where the difference is not as great today.)
   Today most kernels are modular. This was not always the case; former kernels Kernel structure
consisted of a single piece of code fulfilling all necessary functions such as the
support of particular devices. If you wanted to add new hardware or make use
of a different feature like a new type of file system, you had to compile a new
kernel from sources—a very time-consuming process. To avoid this, the kernel
was endowed with the ability to integrate additional features by way of modules.
   Modules are pieces of code that can be added to the kernel dynamically (at run- Modules
time) as well as removed. Today, if you want to use a new network adapter, you do
not have to compile a new kernel but merely need to load a new kernel module.
Modern Linux distributions support automatic hardware recognition, which can hardware recognition
analyze a system’s properties and locate and configure the correct driver modules.

C 1.5 [1] What is the version number of the current stable Linux kernel? The
  current developer kernel? Which Linux kernel versions are still being sup-

1.5      Linux Properties
As a modern operating system kernel, Linux has a number of properties, some
of which are part of the “state of the art” (i. e., exhibited by similar systems in an
equivalent form) and some of which are unique to Linux.

    • Linux supports a large selection of processors and computer architectures, processors
      ranging from mobile phones (the very successful “Android” operating sys-
      tem by Google, like some other similar systems, is based on Linux) through
      PDAs and tablets, all sorts of new and old PC-like computers and server
      systems of various kinds up to the largest mainframe computers (the vast
      majority of the machines on the list of the fastest computers in the world is
      running Linux).

      B A huge advantage of Linux in the mobile arena is that, unlike Mi-
        crosoft Windows, it supports the energy-efficient and powerful ARM
        processors that most mobile devices are based upon. In 2012, Microsoft
        released an ARM-based, partially Intel-compatible, version of Win-
        dows 8 under the name of “Windows RT”, but that did not exactly
        prove popular in the market.

    • Of all currently available operating systems, Linux supports the broadest
      selection of hardware. For the very newest components there may not be hardware
      drivers available immediately, but on the other hand Linux still works with
      devices that systems like Windows have long since left behind. Thus, your
      investments in printers, scanners, graphic boards, etc. are protected opti-
    • Linux supports “preemptive multitasking”, that is, several processes are multitasking
      running—virtually or, on systems with more than one CPU, even actually—
      in parallel. These processes cannot obstruct or damage one another; the ker-
      nel ensures that every process is allotted CPU time according to its priority.
22                                                                                                   1 Introduction

                                     B This is nothing special today; when Linux was new, this was much
                                       more remarkable.

                                     On carefully configured systems this may approach real-time behaviour,
                                     and in fact there are Linux variants that are being used to control industrial
                                     plants requiring “hard” real-time ability, as in guaranteed (quick) response
                                     times to external events.
                  several users    • Linux supports several users on the same system, even at the same time
                                     (via the network or serially connected terminals, or even several screens,
                                     keyboards, and mice connected to the same computer). Different access per-
                                     missions may be assigned to each user.
                                   • Linux can effortlessly be installed alongside other operating systems on the
                                     same computer, so you can alternately start Linux or another system. By
                  virtualisation     means of “virtualisation”, a Linux system can be split into independent
                                     parts that look like separate computers from the outside and can run Linux
                                     or other operating systems. Various free or proprietary solutions are avail-
                                     able that enable this.
                     efficiency    • Linux uses the available hardware efficiently. The dual-core CPUs common
                                     today are as fully utilised as the 4096 CPU cores of a SGI Altix server. Linux
                                     does not leave working memory (RAM) unused, but uses it to cache data
                                     from disk; conversely, available working memory is used reasonably in or-
                                     der to cope with workloads that are much larger than the amount of RAM
                                     inside the computer.
     POSIX, System V and BSD       • Linux is source-code compatible with POSIX, System V and BSD and hence
                                     allows the use of nearly all Unix software available in source form.
                   file systems    • Linux not only offers powerful “native” file systems with properties such
                                     as journaling, encryption, and logical volume management, but also allows
                                     access to the file systems of various other operating systems (such as the
                                     Microsoft Windows FAT, VFAT, and NTFS file systems), either on local disks
                                     or across the network on remote servers. Linux itself can be used as a file
                                     server in Linux, Unix, or Windows networks.
                        TCP/IP     • The Linux TCP/IP stack is arguably among the most powerful in the indus-
                                     try (which is due to the fact that a large fraction of R&D in this area is done
                                     based on Linux). It supports IPv4 and IPv6 and all important options and
       graphical environments      • Linux offers powerful and elegant graphical environments for daily work
                                     and, in X11, a very popular network-transparent base graphics system. Ac-
                                     celerated 3D graphics is supported on most popular graphics cards.
      productivity applications    • All important productivity applications are available—office-type pro-
                                     grams, web browsers, programs to access electronic mail and other com-
                                     munication media, multimedia tools, development environments for a di-
                                     verse selection of programming languages, and so on. Most of this software
                                     comes with the system at no cost or can be obtained effortlessly and cheaply
                                     over the Internet. The same applies to servers for all important Internet pro-
                                     tocols as well as entertaining games.
                                The flexibility of Linux not only makes it possible to deploy the system on all
                            sorts of PC-class computers (even “old chestnuts” that do not support current
                            Windows can serve well in the kids’ room, as a file server, router, or mail server),
           embedded systems but also helps it make inroads in the “embedded systems” market, meaning com-
                            plete appliances for network infrastructure or entertainment electronics. You will,
                            for example, find Linux in the popular AVM FRITZ!Box and similar WLAN, DSL
                            or telephony devices, in various set-top boxes for digital television, in PVRs, digi-
                            tal cameras, copiers, and many other devices. Your author has seen the bottle bank
1.5 Linux Properties                                                                                        23

in the neighbourhood supermarket boot Linux. This is very often not trumpeted
all over the place, but, in addition to the power and convenience of Linux itself
the manufacturers appreciate the fact that, unlike comparable operating systems,
Linux does not require licensing fees “per unit sold”.
    Another advantage of Linux and free software is the way the community deals
with security issues. In practice, security issues are as unavoidable in free software security issues
as they are in proprietary code—at least nobody so far has written and published
a software system of interesting size that proved completely free of them in the
long run. In particular, it would be improper to claim that free software has no
security issues. The differences are more likely to be found on a philosophical
   • As a rule, a vendor of proprietary software has no interest in fixing security
     issues in their code—they will try to cover up problems and to talk down
     possible dangers for as long as they possibly can, since constantly publish-
     ing “patches” means, in the best case, terrible PR (“where there is smoke,
     there must be a fire”; the competition, which just happens not to be in the
     spotlight of scrutiny at the moment, is having a secret laugh), and, in the
     worst case, great expense and lots of hassle if exploits are around that make
     active use of the security holes. Besides, there is the usual danger of intro-
     ducing three new errors while fixing one known one, which is why fixing
     bugs in released software is normally not an econonomically viable propo-
   • A free-software publisher does not gain anything by sitting on information
     about security issues, since the source code is generally available, and ev-
     erybody can find the problems. It is, in fact, a matter of pride to fix known
     security issues as quickly as possible. The fact that the source code is pub-
     lically available also implies that third parties find it easy to audit code for
     problems that can be repaired proactively. (A common claim is that the
     availability of source code exerts a very strong attraction on crackers and
     other unsavoury vermin. In fact, these low-lifes do not appear to have major
     difficulties identifying large numbers of security issues in proprietary sys-
     tems such as Windows, whose source code is not generally available. Thus
     any difference, if it exists, must be minute indeed.)
   • Especially as far as software dealing with cryptography (the encryption and
     decryption of confidential information) is concerned, there is a strong argu-
     ment that availability of source code is an indispensable prerequisite for
     trust that a program really does what it is supposed to do, i. e., that the
     claimed encryption algorithm has been implemented completely and cor-
     rectly. Linux does have an obvious advantage here.
   Linux is used throughout the world by private and professional users— Linux in companies
companies, research establishments, universities. It plays an important role par-
ticularly as a system for web servers (Apache), mail servers (Sendmail, Postfix),
file servers (NFS, Samba), print servers (LPD, CUPS), ISDN routers, X terminals,
scientific/engineering workstations etc. Linux is an essential part of industrial IT
departments. Widespread adoption of Linux in public administration, such as the Public administration
city of Munich, also serves as a signal. In addition, many reputable IT companies Support by IT companies
such as IBM, Hewlett-Packard, Dell, Oracle, Sybase, Informix, SAP, Lotus etc. are
adapting their products to Linux or selling Linux versions already. Furthermore,
ever more computers (“netbooks”)— come with Linux or are at least tested for
Linux compability by their vendors.

C 1.6 [4] Imagine you are responsible for IT in a small company (20–30 em-
  ployees). In the office there are approximately 20 desktop PCs and two
  servers (a file and printer server and a mail and Web proxy server). So far
  everything runs on Windows. Consider the following scenarios:
24                                                                                             1 Introduction

                                   • The file and printer server is replaced by a Linux server using Samba
                                     (a Linux/Unix-based server for Windows clients).
                                   • The mail and proxy server is replaced by a Linux server.
                                   • The twenty office desktop PCs are replaced by Linux machines.

                                 Comment on the different scenarios and draw up short lists of their advan-
                                 tages and disadvantages.

                           1.6    Linux Distributions
                         Linux in the proper sense of the word only consists of the operating system ker-
                         nel. To accomplish useful work, a multitude of system and application programs,
           Distributions libraries, documentation etc. is necessary. “Distributions” are nothing but up-to-
                         date selections of these together with special programs (usually tools for instal-
                         lation and maintenance) provided by companies or other organisations, possibly
                         together with other services such as support, documentation, or updates. Distri-
                         butions differ mostly in the selection of software they offer, their administration
                         tools, extra services, and price.

     Red Hat and Fedora          “Fedora” is a freely available Linux distribution developed under the guid-
                                 ance of the US-based company, Red Hat. It is the successor of the “Red Hat
                                 Linux” distribution; Red Hat has withdrawn from the private end-user mar-
                                 ket and aims their “Red Hat” branded distributions at corporate customers.
                                 Red Hat was founded in 1993 and became a publically-traded corporation
                                 in August, 1999; the first Red Hat Linux was issued in 1994, the last (ver-
                                 sion 9) in late April, 2004. “Red Hat Enterprise Linux” (RHEL), the current
                                 product, appeared for the first time in March, 2002. Fedora, as mentioned, is
                                 a freely available offering and serves as a development platform for RHEL;
                                 it is, in effect, the successor of Red Hat Linux. Red Hat only makes Fedora
                                 available for download; while Red Hat Linux was sold as a “boxed set” with
                                 CDs and manuals, Red Hat now leaves this to third-party vendors.

                  SUSE           The SUSE company was founded 1992 under the name “Gesellschaft für
                                 Software und Systementwicklung” as a Unix consultancy and accordingly
                                 abbreviated itself as “S.u.S.E.” One of its products was a version of Patrick
                                 Volkerding’s Linux distribution, Slackware, that was adapted to the Ger-
                                 man market. (Slackware, in turn, derived from the first complete Linux
                                 distribution, “Softlanding Linux System” or SLS.) S.u.S.E. Linux 1.0 came
                                 out in 1994 and slowly differentiated from Slackware, for example by taking
                                 on Red Hat features such as the RPM package manager or the /etc/ sysconfig
                                 file. The first version of S.u.S.E. Linux that no longer looked like Slackware
                                 was version 4.2 of 1996. SuSE (the dots were dropped at some point) soon
                                 gained market leadership in German-speaking Europe and published SuSE
                                 Linux in a “box” in two flavours, “Personal” and “Professional”; the latter
                                 was markedly more expensive and contained more server software. Like
                                 Red Hat, SuSE offered an enterprise-grade Linux distribution called “SuSE
                                 Linux Enterprise Server” (SLES), with some derivatives like a specialised
                                 server for mail and groupware (“SuSE Linux OpenExchange Server” or
                                 SLOX). In addition, SuSE endeavoured to make their distribution available
                                 on IBM’s mid-range and mainframe computers.
         Novell takeover         In November 2003, the US software company Novell announced their in-
                                 tention of taking over SuSE for 210 million dollars; the deal was concluded
                                 in January 2004. (The “U” went uppercase on that occasion). Like Red Hat,
                                 SUSE has by now taken the step to open the “private customer” distribution
                                 and make it freely available as “openSUSE” (earlier versions appeared for
                                 public download only after a delay of several months). Unlike Red Hat,
1.6 Linux Distributions                                                                                                            25

                                                                       elect                               Developers

                                                    Project leader
                          appoints                                                       appoints

                 Technical committee                                                  Project secretary

                                       Delegates            approve

                 Release team                                                             Maintainers / porters

                 FTP masters                                                                       apply
                                         DAM                NM team / advocates                                   applicants

                 Security team                 Policy group                           Documentation / i18n teams

                 Press contacts             Web/list/...masters                               Quality assurance

                 Administrators                    CD team                                          etc.

                      etc.                           etc.                                  s
                                                                                 ol unt
                                                                               V                             Software in the
                                                                                                             Public Interest

           Figure 1.3: Organizational structure of the Debian project. (Graphic by Martin F. Krafft.)

      Novell/SUSE still offers a “boxed” version containing additional propri-
      etary software. Among others, SUSE still sells SLES and a corporate desktop
      platform called “SUSE Linux Enterprise Desktop” (SLED).
      In early 2011, Novell was acquired by Attachmate, which in turn was taken Attachmate
      over by Micro Focus in 2014. Both are companies whose main field of busi- Micro Focus
      ness is traditional mainframe computers and which so far haven not distin-
      guished themselves in the Linux and open-source arena. These maneuver-
      ings, however, have had fairly little influence on SUSE and its products.
      A particular property of SUSE distributions is “YaST”, a comprehensive YaST
      graphical administration tool.

      Unlike the two big Linux distribution companies Red Hat and Novell/SUSE,
      the Debian project is a collaboration of volunteers whose goal is to make Debian project
      available a high-quality Linux distribution called “Debian GNU/Linux”.
      The Debian project was announced on 16 August 1993 by Ian Murdock; the
      name is a contraction of his first name with that of his then-girlfriend (now
      ex-wife) Debra (and is hence pronounced “debb-ian”). By now the project
      includes more than 1000 volunteers.
      Debian is based on three documents:
        • The Debian Free Software Guidelines (DFSG) define which software the
          project considers “free”. This is important, since only DFSG-free soft-
          ware can be part of the Debian GNU/Linux distribution proper. The
          project also distributes non-free software, which is strictly separated
          from the DFSG-free software on the distribution’s servers: The latter
          is in subdirectory called main , the former in non- free . (There is an inter-
          mediate area called contrib ; this contains software that by itself would
          be DFSG-free but does not work without other, non-free, components.)
26                                                                                                 1 Introduction

                                     • The Social Contract describes the project’s goals.
                                     • The Debian Constitution describes the project’s organisation.
                     versions     At any given time there are at least three versions of Debian GNU/Linux:
                                  New or corrected versions of packages are put into the unstable branch.
                                  If, for a certain period of time, no grave errors have appeared in a pack-
                                  age, it is copied to the testing branch. Every so often the content of test-
                                  ing is “frozen”, tested very thoroughly, and finally released as stable . A
                                  frequently-voiced criticism of Debian GNU/Linux is the long timespan be-
                                  tween stable releases; many, however, consider this an advantage. The De-
                                  bian project makes Debian GNU/Linux available for download only; media
                                  are available from third-party vendors.
                                  By virtue of its organisation, its freedom from commercial interests, and its
                                  clean separation between free and non-free software, Debian GNU/Linux is
           derivative projects    a sound basis for derivative projects. Some of the more popular ones include
                                  Knoppix (a “live CD” which makes it possible to test Linux on a PC without
                                  having to install it first), SkoleLinux (a version of Linux especially adapted to
                                  the requirements of schools), or commercial distributions such as Xandros.
                                  Limux, the desktop Linux variant used in the Munich city administration,
                                  is also based on Debian GNU/Linux.

                     Ubuntu       One of the most popular Debian derivatives is Ubuntu, which is offered
                                  by the British company, Canonical Ltd., founded by the South African
                                  entrepreneur Mark Shuttleworth. (“Ubuntu” is a word from the Zulu lan-
                         goal     guage and roughly means “humanity towards others”.) The goal of Ubuntu
                                  is to offer, based on Debian GNU/Linux, a current, capable, and easy-to-
                                  understand Linux which is updated at regular intervals. This is facilitated,
                                  for example, by Ubuntu being offered on only three computer architec-
                                  tures as opposed to Debian’s ten, and by restricting itself to a subset of the
                                  software offered by Debian GNU/Linux. Ubuntu is based on the unstable
                                  branch of Debian GNU/Linux and uses, for the most part, the same tools
                                  for software distribution, but Debian and Ubuntu software packages are
                                  not necessarily mutually compatible.
           Ubuntu vs. Debian      Some Ubuntu developers are also active participants in the Debian project,
                                  which ensures a certain degree of exchange. On the other hand, not all De-
                                  bian developers are enthusiastic about the shortcuts Ubuntu takes every so
                                  often in the interest of pragmatism, where Debian might look for more com-
                                  prehensive solutions even if these require more effort. In addition, Ubuntu
                                  does not appear to feel as indebted to the idea of free software as does De-
                                  bian; while all of Debian’s infrastructure tools (such as the bug management
                                  system) are available as free software, this is not always the case for those
                                  of Ubuntu.
     Ubuntu vs. SUSE/Red Hat      Ubuntu not only wants to offer an attractive desktop system, but also take
                                  on the more established systems like RHEL or SLES in the server space, by
                                  offering stable distributions with a long life cycle and good support. It is
                                  unclear how Canonical Ltd. intends to make money in the long run; for the
                                  time being the project is mostly supported out of Mark Shuttleworth’s pri-
                                  vate coffers, which are fairly well-filled since he sold his Internet certificate
                                  authority, Thawte, to Verisign …

           More distributions  In addition to these distributions there are many more, such as Mageia or Linux
                            Mint as smaller “generally useful” distributions, various “live systems” for differ-
                            ent uses from firewalls to gaming or multimedia platforms, or very compact sys-
                            tems usable as routers, firewalls, or rescue systems.
              Commonalities    Even though there is a vast number of distributions, most look fairly similar in
                            daily life. This is due, on the one hand, to the fact that they use the same basic
                            programs—for example, the command line interpreter is nearly always bash . On
1.6 Bibliography                                                                       27

the other hand, there are standards that try to counter rank growth. The “Filesys-
tem Hierarchy Standard” (FHS) or “Linux Standard Base” (LSB) must be men-

C 1.7 [2] Some Linux hardware platforms have been enumerated above. For
  which of those platforms are there actual Linux distributions available?
  (Hint: )

   • Linux is a Unix-like operating system.
   • The first version of Linux was developed by Linus Torvalds and made avail-
     able on the Internet as “free software”. Today, hundreds of developers all
     over the world contribute to updating and extending the system.
   • The GPL is the best-known “free software” license. It tries to ensure that
     the recipients of software can modify and redistribute the package, and that
     these “freedoms” are passed on to future recipients. GPL software may also
     be sold.
   • To the user, “open source” means approximately the same as “free soft-
   • There are other free licenses besides the GPL. Software may also be dis-
     tributed by the copyright owner under several licenses at the same time.
   • Linux is actually just the operating system kernel. We distinguish “stable”
     and “development kernels”; with the former, the second part of the version
     number is even and with the latter, odd. Stable kernels are meant for end
     users, while development kernels are not necessarily functional, represent-
     ing interim versions of Linux development.
   • There are numerous Linux distributions bringing together a Linux kernel
     and additional software, documentation and installation and administra-
     tion tools.

DFSG “Debian Free Software Guidelines”.

GPL-Urteil06 Landgericht Frankfurt am Main. “Urteil 2-6 0 224/06”, July 2006.

GPL91 Free Software Foundation, Inc. “GNU General Public License, Version 2”,
     June 1991.                 

LR89 Don Libes, Sandy Ressler. Life with UNIX: A Guide for Everyone. Prentice-
     Hall, 1989. ISBN 0-13-536657-7.
Rit84 Dennis M. Ritchie. “The Evolution of the Unix Time-sharing System”.
      AT&T Bell Laboratories Technical Journal, October 1984. 63(6p2):1577–93.

RT74 Dennis M. Ritchie, Ken Thompson. “The Unix Time-sharing System”. Com-
     munications of the ACM, July 1974. 17(7):365–73. The classical paper on Unix.
TD02 Linus Torvalds, David Diamond. Just For Fun: The Story of an Accidental
    Revolutionary. HarperBusiness, 2002. ISBN 0-066-62073-2.
                                                                                  $ echo tux
                                                                                  $ ls
                                                                                  $ /bin/su -

Using the Linux System

2.1     Logging In and Out . . . . . . . . . . . . . . . . . . . 30
2.2     Switching On and Off . . . . . . . . . . . . . . . . . . 32
2.3     The System Administrator. . . . . . . . . . . . . . . . . 32

      • Logging on and off the system
      • Understanding the difference between normal user accounts and the system
        administrator’s account

      • Basic knowledge of using computers is helpful

grd1-bedienung.tex   (be27bba8095b329b )
30                                                                             2 Using the Linux System

         Figure 2.1: The login screens of some common Linux distributions

                 2.1      Logging In and Out
                   The Linux system distinguishes between different users. Consequently, it may
                   be impossible to start working right after the computer has been switched on.
                   First you have to tell the computer who you are—you need to “log in” (or “on”).
                   Based on the information you provide, the system can decide what you may do
     access rights (or may not do). Of course you need access rights to the system (an “account”) –
                   the system administrator must have entered you as a valid user and assigned you
                   a user name (e. g., joe ) and a password (e. g., secret ). The password is supposed to
                   ensure that only you can use your account; you must keep it secret and should not
                   make it known to anybody else. Whoever knows your user name and password
                   can pretend to be you on the system, read (or delete) all your files, send electronic
                   mail in your name and generally get up to all kinds of shenanigans.

                  B Modern Linux distributions want to make it easy on you and allow you to
                    skip the login process on a computer that only you will be using anyway. If
                    you use such a system, you will not have to log in explicitly, but the computer
                    boots straight into your session. You should of course take advantage of this
                    only if you do not foresee that third parties have access to your computer;
                    refrain from this in particular on laptop computers or other mobile systems
                    that tend to get lost or stolen.

                 Logging in in a graphical enviroment These days it is common for Linux worksta-
                 tions to present a graphical environment (as they should), and the login process
                 takes place in a graphical environment as well. Your computer shows a dialog
2.1 Logging In and Out                                                                 31

that lets you enter your user name and password (Figure 2.1 shows some repre-
sentative examples.)

B Don’t wonder if you only see asterisks when you’re entering your password.
  This does not mean that your computer misunderstands your input, but that
  it wants to make life more difficult for people who are watching you over
  your shoulder in order to find out your password.

   After you have logged in, the computer starts a graphical session for you, in
which you have convenient access to your application programs by means of
menus and icons (small pictures on the “desktop” background). Most graphical
environments for Linux support “session management” in order to restore your
session the way it was when you finished it the time before (as far as possible,
anyway). That way you do not need to remember which programs you were
running, where their windows were placed on the screen, and which files you
had been using.

Logging out in a graphical environment If you are done with your work or want
to free the computer for another user, you need to log out. This is also important
because the session manager needs to save your current session for the next time.
How logging out works in detail depends on your graphical environment, but as
a rule there is a menu item somewhere that does everything for you. If in doubt,
consult the documentation or ask your system administrator (or knowledgeable

Logging in on a text console Unlike workstations, server systems often support
only a text console or are installed in draughty, noisy machine halls, where you
don’t want to spend more time than absolutely necessary. So you will prefer to log
into such a computer via the network. In both cases you will not see a graphical
login screen, but the computer asks you for your user name and password directly.
For example, you might simply see something like

computer login: _

(if we stipulate that the computer in question is called “computer ”). Here you must
enter your user name and finish it off with the ↩ key. The computer will con-
tinue by asking you for your password:

Password: _

Enter your password here. (This time you won’t even see asterisks—simply noth-
ing at all.) If both the user name and password were correct, the system will ac-
cept your login. It starts the command line interpreter (the shell), and you can
enter commands and invoke programs. After logging in you will be placed in
your “home directory”, where you will be able to find your files.

B If you use the “secure shell”, for example, to log in to another machine over
  the network, the user name question is usually skipped, since unless you
  specify otherwise the system will assume that your user name on the re-
  mote computer will be the same as on the computer you are initiating the
  session from. The details are beyond the scope of this manual; the secure
  shell is discussed in detail in the Linup Front training manual Linux Admin-
  istration II.

Logging out on a text console On the text console, you can log out using, for
example, the logout command:

$ logout
32                                                                                          2 Using the Linux System

                             Once you have logged out, on a text console the system once more displays the
                             start message and a login prompt for the next user. With a secure shell session,
                             you simply get another command prompt from your local computer.

                              C 2.1 [!1] Try logging in to the system. After that, log out again. (You will find
                                a user name and password in your system documentation, or—in a training
                                centre—your instructor will tell you what to use.)

                              C 2.2 [!2] What happens if you give (a) a non-existing user name, (b) a wrong
                                password? Do you notice anything unusual? What reasons could there be
                                for the system to behave as it does?

                             2.2     Switching On and Off
                             A Linux computer can usually be switched on by whoever is able to reach the
                             switch (local policy may vary). On the other hand, you should not switch off a
                             Linux machine on a whim—there might be data left in main memory that really
                             belong on disk and will be lost, or—which would be worse—the data on the hard
                             disk could get completely addled. Besides, other users might be logged in to the
                             machine via the network, be surprised by the sudden shutdown, and lose valu-
                             able work. For this reason, important computers are usually only “shut down”
                             by the system administrator. Single-user workstations, though, can usually be
                             shut down cleanly via the graphical desktop; depending on the system’s settings
                             normal user privileges may suffice, or you may have to enter the administrator’s

                              C 2.3 [2] Check whether you can shut down your system cleanly as a normal
                                (non-administrator) user, and if so, try it.

                             2.3     The System Administrator
                             As a normal user, your privileges on the system are limited. For example, you may
                             not write to certain files (most files, actually—mostly those that do not belong to
                             you) and not even read some files (e. g., the file containing the encrypted pass-
                             words of all users). However, there is a user account for system administration
                             which is not subject to these restrictions—the user “root ” may read and write all
                             files, and do various other things normal users aren’t entitled to. Having admin-
                             istrator (or “root”) rights is a privilege as well as a danger—therefore you should
                             only log on to the system as root if you actually want to exercise these rights, not
                             just to read your mail or surf the Internet.

                             A Simply pretend you are Spider-Man: “With great power comes great re-
                               sponsibility”. Even Spider-Man wears his Spandex suit only if he must …

                                   In particular, you should avoid logging in as root via the graphical user inter-
                               face, since all of the GUI will run with root ’s privileges. This is a possible security
          GUI as root : risky risk—GUIs like KDE contain lots of code which is not vetted as thoroughly for
                               security holes as the textual shell (which is, by comparison, relatively compact).
     Assuming root ’s identity Normally you can use the command “/bin/su - ” to assume root ’s identity (and thus
                               root ’s privileges). su asks for root ’s password and then starts a new shell, which
                               lets you work as if you had logged in as root directly. You can leave the shell again
                               using the exit command.
2.3 The System Administrator                                                                                    33

            Figure 2.2: Running programs as a different user in KDE

E You should get used to invoking su via its full path name—“/bin/su - ”. Oth-
  erwise, a user could trick you by calling you to her computer, getting you to
  enter “su ” in one of her windows and to input the root password. What you
  don’t realize at that moment is that the clever user wrote her own “Trojan”
  su command—which doesn’t do anything except write the password to a
  file, output the “wrong password” error message and remove itself. When
  you try again (gritting your teeth) you get the correct su —and your user
  possesses the coveted administrator’s privileges …

   You can usually tell that you actually have administrator privileges by look-
ing at the shell prompt—for root , it customarily ends with the “# ” character. (For   root ’s   shell prompt
normal users, the shell prompt usually ends in “$ ” or “> ”).

      In Ubuntu you can’t even log in as root by default. Instead, the system al-
      lows the first user created during installation to execute commands with
      administrator privileges by prefixing them with the sudo command. With

       $ sudo chown joe file.txt

      for example, he could sign over the file.txt file to user joe – an operation
      that is restricted to the system administrator.

      Recent versions of Debian GNU/Linux offer a similar arrangement to

B Incidentally, with the KDE GUI, it is very easy to start arbitrary programs          root   and KDE
  as root : Select “Run command” from the “KDE” menu (usually the entry
  at the very left of the command panel—the “Start” menu on Windows sys-
  tems), and enter a command in the dialog window. Before executing the
  command, click on the “Settings” button; an area with additional settings
  appears, where you can check “As different user” (root is helpfully set up as
  the default value). You just have to enter the root password at the bottom
  (figure 2.2).
34                                                                   2 Using the Linux System

     kdesu   B Alternatively, you can put “kdesu ” in front of the actual command in the dia-
               log window (or indeed any shell command line in a KDE session). This will
               ask you for root ’s password before starting the command with administrator

             C 2.4 [!1] Use the su command to gain administrator privileges, and change
               back to your normal account.

             C 2.5 [5] (For programmers.) Write a convincing “Trojan” su program. Use it
               to try and fool your system administrator.

             C 2.6 [2] Try to run the id program as root in a terminal session under KDE, us-
               ing “Run command …”. Check the appropriate box in the extended settings
               to do so.

             Commands in this Chapter
             exit     Quits a shell                                            bash (1) 32
             kdesu    Starts a program as a different user on KDE      KDE: help:/kdesu 33
             logout   Terminates a shell session                               bash (1) 31
             su       Starts a shell using a different user’s identity            su (1) 32
             sudo     Allows normal users to execute certain commands with administrator
                      privileges                                               sudo (8) 33

                • Before using a Linux system, you have to log in giving your user name and
                  password. After using the system, you have to log out again.
                • Normal access rights do not apply to user root , who may do (essentially)
                  everything. These privileges should be used as sparingly as possible.
                • You should not log in to the GUI as root but use (e. g.) su to assume admin-
                  istrator privileges if necessary.
                                                                                                             $ echo tux
                                                                                                             $ ls
                                                                                                             $ /bin/su -

Who’s Afraid Of The Big Bad

3.1  Why? . . . . . . . .              .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   36
   3.1.1 What Is The Shell? .          .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   36
3.2 Commands . . . . . .               .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   37
   3.2.1 Why Commands?. .              .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   37
   3.2.2 Command Structure.            .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   38
   3.2.3 Command Types . .             .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   39
   3.2.4 Even More Rules . .           .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   39

      • Appreciating the advantages of a command-line user interface
      • Working with Bourne-Again Shell (Bash) commands
      • Understanding the structure of Linux commands

      • Basic knowledge of using computers is helpful

grd1-shell1-opt.tex   (be27bba8095b329b )
36                                                                               3 Who’s Afraid Of The Big Bad Shell?

                          3.1      Why?
                          More so than other modern operating systems, Linux (like Unix) is based on the
                          idea of entering textual commands via the keyboard. This may sound antediluvial
                          to some, especially if one is used to systems like Windows, who have been trying
                          for 15 years or so to brainwash their audience into thinking that graphical user
                          interfaces are the be-all and end-all. For many people who come to Linux from
                          Windows, the comparative prominence of the command line interface is at first
                          a “culture shock” like that suffered by a 21-century person if they suddenly got
                          transported to King Arthur’s court – no cellular coverage, bad table manners, and
                          dreadful dentists!
                             However, things aren’t as bad as all that. On the one hand, nowadays there
                          are graphical interfaces even for Linux, which are equal to what Windows or Ma-
                          cOS X have to offer, or in some respects even surpass these as far as convenience
                          and power are concerned. On the other hand, graphical interfaces and the text-
                          oriented command line are not mutually exclusive, but in fact complementary
                          (according to the philosophy “the right tool for every job”).
                             At the end of the day this only means that you as a budding Linux user will
                          do well to also get used to the text-oriented user interface, known as the “shell”.
                          Of course nobody wants to prevent you from using a graphical desktop for every-
                          thing you care to do. The shell, however, is a convenient way to perform many
                          extremely powerful operations that are rather difficult to express graphically. To
                          reject the shell is like rejecting all gears except first in your car1 . Sure, you’ll get
                          there eventually even in first gear, but only comparatively slowly and with a hor-
                          rible amount of noise. So why not learn how to really floor it with Linux? And if
                          you watch closely, we’ll be able to show you another trick or two.

                          3.1.1     What Is The Shell?
                          Users cannot communicate directly with the operating system kernel. This is only
                          possible through programs accessing it via “system calls”. However, you must be
                          able to start such programs in some way. This is the task of the shell, a special user
                          program that (usually) reads commands from the keyboard and interprets them
                          (for example) as commands to be executed. Accordingly, the shell serves as an
                          “interface” to the computer that encloses the actual operating system like a shell
                          (as in “nutshell”—hence the name) and hides it from view. Of course the shell is
                          only one program among many that access the operating system.

                          B Even today’s graphical “desktops” like KDE can be considered “shells”. In-
                            stead of reading text commands via the keyboard, they read graphical com-
                            mands via the mouse—but as the text commands follow a certain “gram-
                            mar”, the mouse commands do just the same. For example, you select ob-
                            jects by clicking on them and then determine what to do with them: open-
                            ing, copying, deleting, …

                             Even the very first Unix—end-1960s vintage—had a shell. The oldest shell to
                          be found outside museums today was developed in the mid-1970s for “Unix ver-
          Bourne shell    sion 7” by Stephen L. Bourne. This so-called “Bourne shell” contains most basic
                          functions and was in very wide-spread use, but is very rarely seen in its original
                C shell   form today. Other classic Unix shells include the C shell, created at the University
                          of California in Berkeley and (very vaguely) based on the C programming lan-
             Korn shell   guage, and the largely Bourne-shell compatible, but functionally enhanced, Korn
                          shell (by David Korn, also at AT&T).
     Bourne-again shell      Standard on Linux systems is the Bourne-again shell, bash for short. It was
                          developed under the auspices of the Free Software Foundation’s GNU project by
                          Brian Fox and Chet Ramey and unifies many functions of the Korn and C shells.
                             1 This metaphor is for Europeans and other people who can manage a stick shift; our American

                          readers of course all use those wimpy automatic transmissions. It’s like they were all running Win-
3.2 Commands                                                                                                 37

B Besides the mentioned shells, there are many more. On Unix, a shell is sim- shells: normal programs
  ply an application program like all others, and you need no special privi-
  leges to write one—you simply need to adhere to the “rules of the game”
  that govern how a shell communicates with other programs.

   Shells may be invoked interactively to read user commands (normally on a “ter-
minal” of some sort). Most shells can also read commands from files containing
pre-cooked command sequences. Such files are called “shell scripts”.               shell scripts
   A shell performs the following steps:
   1. Read a command from the terminal (or the file)
   2. Validate the command
   3. Run the command directly or start the corresponding program

   4. Output the result to the screen (or elsewhere)
   5. Continue at step 1.
In addition to this standard command loop, a shell generally contains further fea-
tures such as a programming language. This includes complex command struc- programming language
tures involving loops, conditions, and variables (usually in shell scripts, less fre-
quently in interactive use). A sophisticated method for recycling recently used
commands also makes a user’s life easier.
   Shell sessions can generally be terminated using the exit command. This also Terminating shell sessions
applies to the shell that you obtained immediately after logging in.
   Although, as we mentioned, there are several different shells, we shall concen-
trate here on bash as the standard shell on most Linux distributions. The LPI exams
also refer to bash exclusively.

C 3.1 [2] Log off and on again and check the output of the “echo $0 ” command
  in the login shell. Start a new shell using the “bash ” command and enter
  “echo $0 ” again. Compare the output of the two commands. Do you notice
  anything unusual?

3.2     Commands
3.2.1   Why Commands?
A computer’s operation, no matter which operating system it is running, can be
loosely described in three steps:
   1. The computer waits for user input
   2. The user selects a command and enters it via the keyboard or mouse

   3. The computer executes the command
In a Linux system, the shell displays a “prompt”, meaning that commands can be
entered. This prompt usually consists of a user and host (computer) name, the
current directory, and a final character:

joe@red:/home > _

In this example, user joe works on computer red in the /home directory.
38                                                                           3 Who’s Afraid Of The Big Bad Shell?

                             3.2.2    Command Structure
                             A command is essentially a sequence of characters which is ends with a press
                             of the ↩ key and is subsequently evaluated by the shell. Many commands are
                             vaguely inspired by the English language and form part of a dedicated “command
                   syntax    language”. Commands in this language must follow certain rules, a “syntax”, for
                             the shell to be able to interpret them.
                    words       To interpret a command line, the shell first tries to divide the line into words.
     First word: command     Just like in real life, words are separated by spaces. The first word on a line is usu-
                parameters   ally the actual command. All other words on the line are parameters that explain
                             what is wanted in more detail.

                             A DOS and Windows users may be tripped up here by the fact that the shell
                               distinguishes between uppercase and lowercase letters. Linux commands
                               are usually spelled in lowercase letters only (exceptions prove the rule) and
                               not understood otherwise. See also section 3.2.4.

                             B When dividing a command into words, one space character is as good as
                               many – the difference does not matter to the shell. In fact, the shell does
                               not even insist on spaces; tabulator characters are also allowed, which is
                               however mostly of importance when reading commands from files, since
                               the shell will not let you enter tab character directly (not without jumping
                               through hoops, anyway).

                             B You may even use the line terminator ( ↩ ) to distribute a long command
                               across several input lines, but you must put a “Token\” immediately in front
                               of it so the shell will not consider your command finished already.
                                A command’s parameters can be roughly divided into two types:
                  options       • Parameters starting with a dash (“- ”) are called options. These are usually,
                                  er, optional—the details depend on the command in question. Figuratively
                                  spoken they are “switches” that allow certain aspects of the command to
                                  be switched on or off. If you want to pass several options to a command,
                                  they can (often) be accumulated behind a single dash, i. e., the options se-
                                  quence “-a -l -F ” corresponds to “-alF ”. Many programs have more options
                                  than can be conveniently mapped to single characters, or support “long op-
                                  tions” for readability (frequently in addition to equivalent single-character
                                  options). Long options most often start with two dashes and cannot be ac-
                                  cumulated: “foo --bar --baz ”.
               arguments        • Parameters with no leading dash are called arguments. These are often the
                                  names of files that the command should process.
       command structure        The general command structure can be displayed as follows:
                                • Command—“What to do?”
                                • Options—“How to do it?”
                                • Arguments—“What to do it with?”
                             Usually the options follow the command and precede the arguments. However,
                             not all commands insist on this—with some, arguments and options can be mixed
                             arbitrarily, and they behave as if all options came immediately after the command.
                             With others, options are taken into account only when they are encountered while
                             the command line is processed in sequence.

                             A The command structure of current Unix systems (including Linux) has
                               grown organically over a period of almost 40 years and thus exhibits vari-
                               ous inconsistencies and small surprises. We too believe that there ought to
                               be a thorough clean-up, but 30 years’ worth of shell scripts are difficult to
                               ignore completely … Therefore be prepared to get used to little weirdnesses
                               every so often.
3.2 Commands                                                                                          39

3.2.3     Command Types
In shells, there are essentially two kinds of commands:
Internal commands These commands are made available by the shell itself. The
      Bourne-again shell contains approximately 30 such commands, which can
      be executed very quickly. Some commands (such as exit or cd ) alter the state
      of the shell itself and thus cannot be provided externally.
External commands The shell does not execute these commands by itself but
     launches executable files, which within the file system are usually found
     in directories like /bin or /usr/bin . As a user, you can provide your own pro-
     grams, which the shell will execute like all other external commands.
   You can use the type command to find out the type of a command. If you pass External or internal?
a command name as the argument, it outputs the type of command or the corre-
sponding file name, such as

$ type echo
echo is a shell builtin
$ type date
date is /bin/date

(echo is an interesting command which simply outputs its parameters:

$ echo Thou hast it now, king, Cawdor, Glamis, all
Thou hast it now, king, Cawdor, Glamis, all

datedisplays the current date and time, possibly adjusted to the current time zone
and language setup:

$ date
Mon May   7 15:32:03 CEST 2012

You will find out more about echo and date in Chapter 9.)
  You can obtain help for internal Bash commands via the help command:                 help

$ help type
type: type [-afptP] name [name ...]
  For each NAME, indicate how it would be interpreted if used as a
  command name.

  If the -t option is used, `type' outputs a single word which is one of
  `alias', `keyword', `function', `builtin', `file' or `', if NAME is an

C 3.2 [2] With bash, which of the following programs are provided externally
  and which are implemented within the shell itself: alias , echo , rm , test ?

3.2.4     Even More Rules
As mentioned above, the shell distinguishes between uppercase and lowercase
letters when commands are input. This does not apply to commands only, but
consequentially to options and parameters (usually file names) as well.
    Furthermore, you should be aware that the shell treats certain characters in the
input specially. Most importantly, the already-mentioned space character is used space character
to separate words on teh command line. Other characters with a special meaning
40                                                                       3 Who’s Afraid Of The Big Bad Shell?


                         If you want to use any of these characters without the shell interpreting according
     “Escaping” characters to its the special meaning, you need to “escape” it. You can use the backslash “\ ”
                         to escape a single special character or else single or double quotes (' …' , " …" ) to
                         excape several special characters. For example:

                          $ touch 'New File'

                         Due to the quotes this command applies to a single file called New File . Without
                         quotes, two files called New and File would have been involved.

                          B We can’t explain all the other special characters here. Most of them will
                            show up elsewhere in this manual – or else check the Bash documentation.

                         Commands in this Chapter
                         bash     The “Bourne-Again-Shell”, an interactive command interpreter
                                                                                                  bash (1)   36
                         date     Displays the date and time                                      date (1)   39
                         echo     Writes all its parameters to standard output, separated by spaces
                                                                                        bash (1), echo (1)   39
                         help     Displays on-line help for bash commands                         bash (1)   39
                         type     Determines the type of command (internal, external, alias) bash (1)        39

                             • The shell reads user commands and executes them.
                             • Most shells have programming language features and support shell scripts
                               containing pre-cooked command sequences.
                             • Commands may have options and arguments. Options determine how the
                               command operates, and arguments determine what it operates on.
                             • Shells differentiate between internal commands, which are implemented in
                               the shell itself, and external commands, which correspond to executable files
                               that are started in separate processes.
                                                                                                   $ echo tux
                                                                                                   $ ls
                                                                                                   $ /bin/su -

Getting Help

4.1  Self-Help . . . . . . . . .         . . .    .   .   .   .   .   .   .   .   .   .   .   42
4.2  The help Command and the --help     Option   .   .   .   .   .   .   .   .   .   .   .   42
4.3  The On-Line Manual . . . .          . . .    .   .   .   .   .   .   .   .   .   .   .   42
   4.3.1 Overview . . . . . .            . . .    .   .   .   .   .   .   .   .   .   .   .   42
   4.3.2 Structure . . . . . . .         . . .    .   .   .   .   .   .   .   .   .   .   .   43
   4.3.3 Chapters . . . . . . .          . . .    .   .   .   .   .   .   .   .   .   .   .   44
   4.3.4 Displaying Manual Pages .       . . .    .   .   .   .   .   .   .   .   .   .   .   44
4.4 Info Pages . . . . . . . .           . . .    .   .   .   .   .   .   .   .   .   .   .   45
4.5 HOWTOs. . . . . . . . .              . . .    .   .   .   .   .   .   .   .   .   .   .   46
4.6 Further Information Sources . .      . . .    .   .   .   .   .   .   .   .   .   .   .   46

      • Being able to handle manual and info pages
      • Knowing about and finding HOWTOs
      • Being familiar with the most important other information sources

      • Linux Overview
      • Basic command-line Linux usage (e. g., from the previous chapters)

grd1-hilfe.tex   (be27bba8095b329b )
42                                                                                           4 Getting Help

                           4.1     Self-Help
                           Linux is a powerful and intricate system, and powerful and intricate systems are,
                           as a rule, complex. Documentation is an important tool to manage this complex-
                           ity, and many (unfortunately not all) aspects of Linux are documented very exten-
                           sively. This chapter describes some methods to access this documentation.

                           B “Help” on Linux in many cases means “self-help”. The culture of free soft-
                             ware implies not unnecessarily imposing on the time and goodwill of other
                             people who are spending their free time in the community by asking things
                             that are obviously explained in the first few paragraphs of the manual. As
                             a Linux user, you do well to have at least an overview of the available doc-
                             umentation and the ways of obtaining help in cases of emergency. If you
                             do your homework, you will usually experience that people will help you
                             out of your predicament, but any tolerance towards lazy individuals who
                             expect others to tie themselves in knots on their behalf, on their own time,
                             is not necessarily very pronounced.

                           B If you would like to have somebody listen around the clock, seven days a
                             week, to your not-so-well-researched questions and problems, you will have
                             to take advantage of one of the numerous “commercial” support offerings.
                             These are available for all common distributions and are offered either by
                             the distribution vendor themselves or else by third parties. Compare the
                             different service vendors and pick one whose service level agreements and
                             pricing suit you.

                           4.2     The help Command and the --help Option
     Internal bash commands In bash , internal commands are described in more detail by the help command,
                           giving the command name in question as an argument:

                           $ help exit
                           exit: exit [n]
                               Exit the shell with a status of N.
                               If N is omitted, the exit status
                               is that of the last command executed.
                           $ _

                           B More detailed explanations are available from the shell’s manual page and
                             info documentation. These information sources will be covered later in this

                             Many external commands (programs) support a --help option instead. Most
                           commands display a brief listing of their parameters and syntax.

                           B Not every command reacts to --help ; frequently the option is called -h or -? ,
                             or help will be output if you specify any invalid option or otherwise illegal
                             command line. Unfortunately there is no universal convention.

                           4.3     The On-Line Manual
                           4.3.1   Overview
                           Nearly every command-line program comes with a “manual page” (or “man
                           page”), as do many configuration files, system calls etc. These texts are generally
             Command man   installed with the software, and can be perused with the “man ⟨name⟩” command.
4.3 The On-Line Manual                                                                                 43

                         Table 4.1: Manual page sections

          Section       Content
        NAME            Command name and brief description
       SYNOPSIS         Description of the command syntax
     DESCRIPTION        Verbose description of the command’s effects
       OPTIONS          Available options
     ARGUMENTS          Available Arguments
         FILES          Auxiliary files
      EXAMPLES          Sample command lines
       SEE ALSO         Cross-references to related topics
     DIAGNOSTICS        Error and warning messages
      COPYRIGHT         Authors of the command
         BUGS           Known limitations of the command

Here, ⟨name⟩ is the command or file name that you would like explained. “man
bash ”, for example, produces a list of the aforementioned internal shell commands.
   However, the manual pages have some disadvantages: Many of them are only
available in English; there are sets of translations for different languages which are
often incomplete. Besides, the explanations are frequently very complex. Every
single word can be important, which does not make the documentation accessi-
ble to beginners. In addition, especially with longer documents the structure can
be obscure. Even so, the value of this documentation cannot be underestimated.
Instead of deluging the user with a large amount of paper, the on-line manual is
always available with the system.

B Many Linux distributions pursue the philosophy that there should be a
  manual page for every command that can be invoked on the command line.
  This does not apply to the same extent to programs belonging to the graph-
  ical desktop environments KDE and GNOME, many of which not only do
  not come with a manual page at all, but which are also very badly docu-
  mented even inside the graphical environment itself. The fact that many of
  these programs have been contributed by volunteers is only a weak excuse.

4.3.2    Structure
The structure of the man pages loosely follows the outline given in table 4.1, even Man page outline
though not every manual page contains every section mentioned there. In partic-
ular, the EXAMPLES are frequently given short shrift.

B The BUGS heading is often misunderstood: Read bugs within the imple-
  mentation get fixed, of course; what is documented here are usually restric-
  tions which follow from the approach the command takes, which are not able
  to be lifted with reasonable effort, and which you as a user ought to know
  about. For example, the documentation for the grep command points out
  that various constructs in the regular expression to be located may lead to
  the grep process using very much memory. This is a consequence of the way
  grep implements searching and not a trivial, easily fixed error.

    Man pages are written in a special input format which can be processed for text
display or printing by a program called groff . Source code for the manual pages is
stored in the /usr/share/man directory in subdirectories called man 𝑛, where 𝑛 is one
of the chapter numbers from table 4.2.

B You can integrate man pages from additional directories by setting the MAN-
  PATH environment variable, which contains the directories which will be
  searched by man , in order. The manpath command gives hints for setting up
44                                                                                       4 Getting Help

                                              Table 4.2: Manual Page Topics

                        No.      Topic
                           1     User commands
                           2     System calls
                           3     C language library functions
                           4     Device files and drivers
                           5     Configuration files and file formats
                           6     Games
                           7     Miscellaneous (e. g. groff macros, ASCII tables, …)
                           8     Administrator commands
                           9     Kernel functions
                           n     »New« commands

                   4.3.3       Chapters
        Chapters Every manual page belongs to a “chapter” of the conceptual “manual” (table 4.2).
                   Chapters 1, 5 and 8 are most important. You can give a chapter number on the
                   man command line to narrow the search. For example, “man 1 crontab ” displays the
                   man page for the crontab command, while “man 5 crontab ” explains the format of
                   crontab files. When referring to man pages, it is customary to append the chap-
                   ter number in parentheses; we differentiate accordingly between crontab (1), the
                   crontab command manual, and crontab (5), the description of the file format.
          man -a      With the -a option, man displays all man pages matching the given name; with-
                   out this option, only the first page found (generally from chapter 1) will be dis-

                   4.3.4       Displaying Manual Pages
                   The program actually used to display man pages on a text terminal is usually
                   less , which will be discussed in more detail later on. At this stage it is important
                   to know that you can use the cursor keys ↑ and ↓ to navigate within a man
                   page. You can search for keywords inside the text by pressing / —after entering
                   the word and pressing the return key, the cursor jumps to the next occurrence of
                   the word (if it does occur at all). Once you are happy, you can quit the display
                   using q to return to the shell.

                   B Using the KDE web browser, Konqueror, it is convenient to obtain nicely for-
                     matted man pages. Simply enter the URL “man:/ ⟨name⟩” (or even “# ⟨name⟩”)

     Figure 4.1: A manual page in a text terminal (left) and in Konqueror (right)
4.4 Info Pages                                                                                       45

      in the browser’s address line. This also works on the KDE command line
      (figure 2.2).
   Before rummaging aimlessly through innumerable man pages, it is often sen-
sible to try to access general information about a topic via apropos . This command Keyword search
works just like “man -k ”; both search the “NAME” sections of all man pages for
a keyword given on the command line. The output is a list of all manual pages
containing the keyword in their name or description.
   A related command is whatis . This also searches all manual pages, but for a whatis
command (file, …) name rather than a keyword—in other words, the part of the
“NAME” section to the left of the dash. This displays a brief description of the
desired command, system call, etc.; in particular the second part of the “NAME”
section of the manual page(s) in question. whatis is equivalent to “man -f ”.

C 4.1 [!1] View the manual page for the ls command. Use the text-based man
  command and—if available—the Konqueror browser.

C 4.2 [2] Which manual pages on your system deal (at least according to their
  “NAME” sections) with processes?

C 4.3 [5] (Advanced.) Use a text editor to write a manual page for a hypotheti-
  cal command. Read the man (7) man page beforehand. Check the appearance
  of the man page on the screen (using “groff -Tascii -man ⟨file⟩ | less ”) and
  as printed output (using something like “groff -Tps -man ⟨file⟩ | gv - ”).

4.4     Info Pages
For some commands—often more complicated ones—there are so-called “info
pages” instead of (or in addition to) the more usual man pages. These are usu-
ally more extensive and based on the principles of hypertext, similar to the World hypertext
Wide Web.

B The idea of info pages originated with the GNU project; they are therefore
  most frequently found with software published by the FSF or otherwise be-
  longing to the GNU project. Originally there was supposed to be only info
  documentation for the “GNU system”; however, since GNU also takes on
  board lots of software not created under the auspices of the FSF, and GNU
  tools are being used on systems pursuing a more conservative approach,
  the FSF has relented in many cases.
   Analogously to man pages, info pages are displayed using the “info ⟨command⟩”
command (the package containing the info program may have to be installed
explicitly). Furthermore, info pages can be viewed using the emacs editor or dis-
played in the KDE web browser, Konqueror, via URLs like “info:/ ⟨command⟩”.

B One advantage of info pages is that, like man pages, they are written in
  a source format which can conveniently be processed either for on-screen
  display or for printing manuals using PostScript or PDF. Instead of groff ,
  the TEX typesetting program is used to prepare output for printing.

C 4.4 [!1] Look at the info page for the ls program. Try the text-based info
  browser and, if available, the Konqueror browser.

C 4.5 [2] Info files use a crude (?) form of hypertext, similar to HTML files on
  the World Wide Web. Why aren’t info files written in HTML to begin with?
46                                                                                               4 Getting Help

                            4.5     HOWTOs
                            Both manual and info pages share the problem that the user must basically know
                            the name of the program to use. Even searching with apropos is frequently nothing
                            but a game of chance. Besides, not every problem can be solved using one sin-
         Problem-oriented   gle command. Accordingly, there is “problem-oriented” rather than “command-
           documentation    oriented” documentation is often called for. The HOWTOs are designed to help
                            with this.
                                HOWTOs are more extensive documents that do not restrict themselves to sin-
                            gle commands in isolation, but try to explain complete approaches to solving
                            problems. For example, there is a “DSL HOWTO” detailing ways to connect a
                            Linux system to the Internet via DSL, or an “Astronomy HOWTO” discussing as-
                            tronomy software for Linux. Many HOWTOs are available in languages other
                            than English, even though the translations often lag behind the English-language
        HOWTO packages          Most Linux distributions furnish the HOWTOs (or significant subsets) as pack-
                            ages to be installed locally. They end up in a distribution-specific directory—/usr/
                            share/doc/howto for SUSE distributions, /usr/share/doc/HOWTO for Debian GNU/Linux—
     HOWTOs on the Web      , typically either als plain text or else HTML files. Current versions of all HOWTOs
                            and other formats such as PostScript or PDF can be found on the Web on the site
                            of the “Linux Documentation Project” ( ) which also offers other
                            Linux documentation.

                            4.6     Further Information Sources
     Additional information You will find additional documentation and example files for (nearly) every in-
                        stalled software package under /usr/share/doc or /usr/share/doc/packages (depend-
                        ing on your distribution). Many GUI applications (such as those from the KDE or
                        GNOME packages) offer “help” menus. Besides, many distributions offer special-
                        ized “help centers” that make it convenient to access much of the documentation
                        on the system.
                           Independently of the local system, there is a lot of documentation available on
                  WWW the Internet, among other places on the WWW and in USENET archives.
                 USENET    Some of the more interesting web sites for Linux include:

                                              The “Linux Documentation Project”, which is in charge of
                                  man pages and HOWTOs (among other things).
                     A general “portal” for Linux enthusiasts.
                   “free-form text information database for everything
                                  pertaining to Linux” (in German).
                    Linux Weekly News—probably the best web site for Linux news of
                                  all sorts. Besides a daily overview of the newest developments, products,
                                  security holes, Linux advocacy in the press, etc., on Thursdays there is an
                                  extensive on-line magazine with well-researched background reports about
                                  the preceding week’s events. The daily news are freely available, while the
                                  weekly issues must be paid for (various pricing levels starting at US-$ 5 per
                                  month). One week after their first appearance, the weekly issues are made
                                  available for free as well.
                    This site publishes announcements of new (predominantly
                                  free) software packages, which are often available for Linux. In addition to
                                  this there is a database allowing queries for interesting projects or software
                                                                  A site collecting “headlines” from other in-
                            http://www.linux- knowledge-
                                  teresting Linux sites, including LWN and Freshmeat.
4.6 Further Information Sources                                                                       47

   If there is nothing to be found on the Web or in Usenet archives, it is possible to
ask questions in mailing lists or Usenet groups. In this case you should note that
many users of these forums consider it very bad form to ask questions answered
already in the documentation or in a “FAQ” (frequently answered questions) re-
source. Try to prepare a detailed description of your problem, giving relevant
excerpts of log files, since a complex problem like yours is difficult to diagnose at
a distance (and you will surely be able to solve non-complex problems by your-

B A news archive is accessible on (formerly De-

B Interesting news groups for Linux can be found in the English-language
  comp.os.linux.* or the German-language de.comp.os.unix.linux.* hierarchies.
  Many Unix groups are appropriate for Linux topics; a question about the
  shell should be asked in a group dedicated to shell programming rather
  than a Linux group, since shells are usually not specific to Linux.

B Linux-oriented mailing lists can be found, for example, at majordomo@vger. . You should send an e-mail message including “subscribe LIST ” to
  this address in order to subscribe to a list called LIST. A commented list of
  all available mailing lists on the system may be found at http://vger.kernel.
  org/vger- lists.html .

B An established strategy for dealing with seemingly inexplicable problems is
  to search for the error message in question using Google (or another search search engine
  engine you trust). If you do not obtain a helpful result outright, leave out
  those parts of your query that depend on your specific situation (such as
  domain names that only exist on your system). The advantage is that Google
  indexes not just the common web pages, but also many mailing list archives,
  and chances are that you will encounter a dialogue where somebody else
  had a problem very like yours.

    Incidentally, the great advantage of open-source software is not only the large
amount of documentation, but also the fact that most documentation is restricted Free documentation
as little as the software itself. This facilitates collaboration between software
developers and documentation authors, and the translation of documentation
into different languages is easier. In fact, there is ample opportunity for non-
programmers to help with free software projects, e. g., by writing good documen-
tation. The free-software scene should try to give the same respect to documen-
tation authors that it does to programmers—a paradigm shift that has begun but
is by no means finished yet.

Commands in this Chapter
apropos Shows all manual pages whose NAME sections contain a given keyword
                                                                apropos (1) 45
groff   Sophisticated typesetting program                     groff (1) 43, 45
help    Displays on-line help for bash commands                    bash (1) 42
info    Displays GNU Info pages on a character-based terminal      info (1) 45
less    Displays texts (such as manual pages) by page              less (1) 44
man     Displays system manual pages                                 man (1) 42
manpath Determines the search path for system manual pages      manpath (1) 43
whatis  Locates manual pages with a given keyword in its description
                                                                 whatis (1) 45
48                                                                 4 Getting Help

      • “help ⟨command⟩” explains internal bash commands. Many external com-
        mands support a --help option.
      • Most programs come with manual pages that can be perused using man .
        apropos searches all manual pages for keywords, whatis looks for manual
        page names.
      • For some programs, info pages are an alternative to manual pages.
      • HOWTOs form a problem-oriented kind of documentation.
      • There is a multitude of interesting Linux resources on the World Wide Web
        and USENET.
                                                                                                               $ echo tux
                                                                                                               $ ls
                                                                                                               $ /bin/su -

Editors: vi and emacs

5.1  Editors. . . . . . . .               .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   50
5.2  The Standard—vi . . . .              .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   50
   5.2.1 Overview . . . .                 .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   50
   5.2.2 Basic Functions . .              .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   51
   5.2.3 Extended Commands                .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   54
5.3 The Challenger—Emacs .                .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   56
   5.3.1 Overview . . . .                 .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   56
   5.3.2 Basic Functions . .              .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   57
   5.3.3 Extended Functions .             .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   59
5.4 Other Editors . . . . .               .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   61

      • Becoming familiar with the vi and emacs editors
      • Being able to create and change text files

      • Basic shell operation (qv. chapter 2)

grd1-editoren.tex   (be27bba8095b329b )
50                                                                                                5 Editors: vi and emacs

                                     5.1     Editors
                                   Most operating systems offer tools to create and change text documents. Such
                                   programs are commonly called “editors” (from the Latin “edire”, “to work on”).
                                      Generally, text editors offer functions considerably exceeding simple text input
                                   and character-based editing. Good editors allow users to remove, copy or insert
                                   whole words or lines. For long files, it is helpful to be able to search for partic-
                                   ular sequences of characters. By extension, “search and replace” commands can
                                   make tedious tasks like “replace every x by a u ” considerably easier. Many editors
                                   contain even more powerful features for text processing.
     Difference to word processors    In contrast to widespread “word processors” such as Writer or
                                   Microsoft Word, text editors usually do not offer markup elements such as various
                                   fonts (Times, Helvetica, Courier, …), type attributes (boldface, italic, underlined,
                                   …), typographical features (justified type, …) and so on—they are predominantly
                                   intended for the creation and editing of pure text files, where these things would
                                   really be a nuisance.

                                     B Of course there is nothing wrong with using a text editor to prepare input
                                       files for typesetting systems such as groff or LATEX that offer all these typo-
                                       graphic features. However, chances are you won’t see much of these in your
                                       original input—which can really be an advantage: After all, much of the ty-
                                       pography serves as a distraction when writing, and authors are tempted to
                                       fiddle with a document’s appearance while inputting text, rather than con-
                                       centrating on its content.

               Syntax highlighting   B Most text editors today support syntax highlighting, that is, identifying cer-
                                       tain elements of a program text (comments, variable names, reserved words,
                                       strings) by colours or special fonts. This does look spiffy, even though the
                                       question of whether it really helps with programming has not yet been an-
                                       swered through suitable psychological studies.

                                        In the rest of the chapter we shall introduce two common Linux editors. How-
                                     ever, we shall restrict ourselves to the most basic functionality; it would be easy
                                     to conduct multi-day training courses for each of the two. As with the shells, the
                                     choice of text editor is up to a user’s own personal preference.

                                     C 5.1 [2] Which text editors are installed on your system? How can you find

                                     5.2     The Standard—vi
                                     5.2.1   Overview
                                   The only text editor that is probably part of every Linux system is called vi (from
                                   “visual”, not Roman 6—usually pronounced “vee-i”). For practical reasons, this
                vi : today a clone usually doesn’t mean the original vi (which was part of BSD and is decidedly long
                                   in the tooth today), but more modern derivatives such as vim (from “vi improved”)
                                   or elvis ; these editors are, however, sufficiently close to the original vi , to be all
                                   lumped together.
                                       vi , originally developed by Bill Joy for BSD, was one of the first “screen-
                                   oriented” editors in widespread use for Unix. This means that it allowed users to
                                   use the whole screen for editing rather than let them edit just one line at a time.
                                   This is today considered a triviality, but used to be an innovation—which is not to
                                   say that earlier programmers were too stupid to figure it out, but that text termi-
                                   nals allowing free access to arbitrary points on the screen (a mandatory feature for
                                   programs like vi ) had only just become affordable. Out of consideration for older
5.2 The Standard—vi                                                                                    51

systems using teletypes or “glass ttys” (terminals that could only add material at
the bottom of the screen), vi also supports a line-oriented editor under the name
of ex .
   Even with the advanced terminals of that time, one could not rely on the
availability of keyboards with special keys for cursor positioning or advanced Keyboard restrictions
functions—today’s standard PC keyboards would have been considered luxuri-
ous, if not overloaded. This justifies vi ’s unusual concepts of operation, which
today could rightly be considered antediluvian. It cannot be taken amiss if peo-
ple reject vi because of this. In spite of this, having rudimentary knowledge of
vi cannot possibly hurt, even if you select a different text editor for your daily
work—which you should by all means do if vi does not agree with you. It is not
as if there was no choice of alternatives, and we shall not get into childish games
such as “Whoever does not use vi is not a proper Linux user”. Today’s graphical
desktops such as KDE do contain very nice and powerful text editors.

B There is, in fact, an editor which is even cruder than vi —the ed program.
  The title “the only editor that is guaranteed to be available on any Unix sys-
  tem” rightfully belongs to ed instead of vi , but ed as a pure line editor with
  a teletype-style user interface is too basic for even hardcore Unix advocates.
  (ed can be roughly compared with the DOS program, EDLIN ; ed , however, is
  vastly more powerful than the Redmond offering.) The reason why ed is still
  available in spite of the existence of dozens of more convenient text editors
  is unobvious, but very Unix-like: ed accepts commands on its standard input
  and can therefore be used in shell scripts to change files programmatically.
  ed allows editing operations that apply to the whole file at once and is, thus,
  more powerful than its colleague, the “stream editor” sed , which copies its
  standard input to its standard output with certain modifications; normally
  one would use sed and revert to ed for exceptional cases, but ed is still useful
  every so often.

5.2.2   Basic Functions
The Buffer Concept vi works in terms of so-called buffers. If you invoke vi with buffers
a file name as an argument, the content of that file will be read into a buffer. If no
file exists by that name, an empty buffer is created.
   All the modifications made with the editor are only applied inside the buffer.
To make these modifications permanent, the buffer content must be explicitly
written back to the file. If you really want to discard the modifications, simply
leave vi without storing the buffer content—the file on the storage medium will
remain unchanged.
   In addition to a file name as an argument, you can pass options to vi as usual.
Refer to the documentation for the details.

Modes As mentioned earlier, one of the characteristics of vi is its unusual man-
ner of operation. vi supports three different working “modes”:
Command mode All keyboard input consists of commands that do not appear
    on screen and mostly do not need to be finalized using the return key. Af-
    ter invoking vi , you end up in this mode. Be careful: Any key press could
    invoke a command.

Insert mode All keyboard input is considered text and displayed on the screen.
      vi behaves like a “modern” editor, albeit with restricted navigation and cor-
      rection facilities.
Command-line mode This is used to enter long commands. These usually start
    with a colon (“: ”) and are finished using the return key.

In insert mode, nearly all navigation or correction commands are disabled, which
requires frequent alternation between insert and command modes. The fact that
52                                                                                  5 Editors: vi and emacs

                                                                  Insert Mode

                                                              [Esc]                   a, i, o, ...

                          vi command
                                                              Command Mode
                                   ZZ, ...

                                                                    :                 [Return]

                                                         Command-Line Mode

                                                   Figure 5.1: vi ’s modes

                                         Table 5.1: Insert-mode commands for vi

                         Command        Result
                               a        Appends new text after the cursor
                              A         Appends new text at the end of the line
                               i        Inserts new text at the cursor position
                               I        Inserts new text at the beginning of the line
                               o        Inserts a new line below the line containing the cursor
                              O         Inserts a new line above the line containing the cursor

                    it may be difficult to find out which mode the editor is currently in (depending on
                    the vi implementation used and its configuration) does not help to make things
                    easier for beginners. An overview of vi modes may be found in figure 5.1.

                    B Consider: vi started when keyboards consisting only of the “letter block” of
                      modern keyboards were common (127 ASCII characters). There was really
                      no way around the scheme used in the program.

     command mode      After invoking vi without a file name you end up in command mode. In con-
                    trast to most other editors, direct text input is not possible. There is a cursor at the
                    top left corner of the screen above a column filled with tildes. The last line, also
                    called the “status line”, displays the current mode (maybe), the name of the file
                    currently being edited (if available) and the current cursor position.

                    B If your version of vi does not display status information, try your luck with
                       Esc :set showmode ↩ .

                       Shortened by a few lines, this looks similar to Das sieht (um einige Zeilen
                    verkürzt) etwa so aus:

5.2 The Standard—vi                                                                              53

                               Table 5.2: Cursor positioning commands in vi

      Command                     Cursor moves …
          h       or       ←      one character to the left
          l       or    →         one character to the right
          k        or      ↑      one character up
              j   or       ↓      one character down
                   0              to the beginning of the line
                   $              to the end of the line
                  w               to the next word
                   b              to the previous word
      f   ⟨character⟩             to the next ⟨character⟩ on the line
          Strg + F                to the next page (screenful)
          Strg + B                to the previous page
                  G               to the last line of the file
              ⟨n⟩      G          to line no. ⟨n⟩

Empty Buffer                                        0,0-1

    Only after a command such as a (“append”), i (“insert”), or o (“open”)
will vi change into “insert mode”. The status line displays something like “-- insert mode
INSERT -- ”, and keyboard input will be accepted as text.
    The possible commands to enter insert mode are listed in table 5.1; note that
lower-case and upper-case commands are different. To leave insert mode and go
back to command mode, press the Esc key. In command mode, enter Z Z to
write the buffer contents to disk and quit vi .
    If you would rather discard the modifications you made, you need to quit the
editor without saving the buffer contents first. Use the command : q! ↩ . The
leading colon emphasises that this is a command-line mode command.
    When : is entered in command mode, vi changes to command-line mode. command-line mode
You can recognize this by the colon appearing in front of the cursor on the bottom
line of the screen. All further keyboard input is appended to that colon, until the
command is finished with the return key ( ↩ ); vi executes the command and
reverts to command mode. In command-line mode, vi processes the line-oriented
commands of its alter ego, the ex line editor.
    There is an ex command to save an intermediate version of the buffer called :
w (“write”). Commands : x and : wq save the buffer contents and quit the editor;
both commands are therefore identical to the Z Z command.

Movement Through the Text In insert mode, newly entered characters will be put
into the current line. The return key starts a new line. You can move about the text
using cursor keys, but you can remove characters only on the current line using
 ⇐ —an inheritance of vi ’s line-oriented predecessors. More extensive navigation
is only possible in command mode (table 5.2).
    Once you have directed the cursor to the proper location, you can begin cor-
recting text in command mode.

Deleting characters The d command is used to delete characters; it is always
followed by another character that specifies exactly what to delete (table 5.3). To
make editing easier, you can prefix a repeat count to each of the listed commands. repeat count
For example; the 3 x command will delete the next three characters.
    If you have been too eager and deleted too much material, you can revert the
last change (or even all changes one after the other) using the u (“undo”) com- undo
54                                                                                     5 Editors: vi and emacs

                                                   Table 5.3: Editing commands in vi

                      Command                   Result
                                    x           Deletes the character below the cursor
                                    X           Deletes the character to the left of the cursor
                            r   ⟨char⟩          Replaces the character below the cursor by ⟨char⟩
                                d       w       Deletes from cursor to end of current word
                                d       $       Deletes from cursor to end of current line
                                d       0       Deletes from cursor to start of current line
                        d       f   ⟨char⟩      Deletes from cursor to next occurrence of ⟨char⟩ on the
                                                current line
                                d       d       Deletes current line
                                d       G       Deletes from current line to end of text
                            d       1       G   Deletes from current line to beginning of text

                                                 Table 5.4: Replacement commands in vi

                      Command                   Result
                                c       w       Replace from cursor to end of current word
                                c       $       Replace from cursor to end of current line
                                c       0       Replace from cursor to start of current line
                        c       f   ⟨char⟩      Replace from cursor to next occurrence of ⟨char⟩ on the
                                                current line
                            c       / abc       Replace from cursor to next occurrence of character se-
                                                quence abc

                mand. This is subject to appropriate configuration settings.

     Overwriting Replacing characters    The c command (“change”) serves to overwrite a selected
                part of the text. c is a “combination command” similar to d , requiring an addi-
                tional specification of what to overwrite. vi will remove that part of the text before
                changing to insert mode automatically. You can enter new material and return to
                command mode using Esc . (table 5.4).

                5.2.3           Extended Commands
                 Cutting, Copying, and Pasting Text A frequent operation in text editing is to move
                 or copy existing material elsewhere in the document. vi offers handy combination
                 commands to do this, which take specifications similar to those for the c com-
                 mand. y (“yank”) copies material to an interim buffer without changing the
                 original text, whereas d moves it to the interim buffer, i. e., it is removed from
                 its original place and only available in the interim buffer afterwards. (We have
                 introduced this as “deletion” above.)
                     Of course there is a command to re-insert (or “paste”) material from an interim
                 buffer. This is done using p (to insert after the current cursor position) or P (to
                 insert at the current cursor position).
      26 buffers     A peculiarity of vi is that there is not just one interim buffer but 26. This makes
                 it easy to paste different texts (phrases, …) to different places in the file. The in-
                 terim buffers are called “a ” through “z ” and can be invoked using a combination
                 of double quotes and buffer names. The command sequence ” c y 4 w , for
                 instance, copies the next four words to the interim buffer called c ; the command
                 sequence ” g p inserts the contents of interim buffer g after the current cursor
5.2 The Standard—vi                                                                                  55

Regular-Expression Text Search Like every good editor, vi offers well-thought-
out search commands. “Regular expressions” make it possible to locate character
sequences that fit elaborate search patterns. To start a search, enter a slash / in
command mode. This will appear on the bottom line of the terminal followed by
the cursor. Enter the search pattern and start the search using the return key. vi
will start at the current cursor position and work towards the end of the docu-
ment. To search towards the top, the search must be started using ? instead of /
. Once vi has found a matching character sequence, it stops the search and places
the cursor on the first character of the sequence. You can repeat the same search
towards the end using n (“next”) or towards the beginning using N .

B Regular expressions are explained in more detail in sections 7.1 and 7.1.1.

Searching and Replacing Since locating character sequences is often not all that is
desired. Therefore, vi also allows replacing found character sequences by others.
The following ex command can be used:

 :   [⟨start line⟩, ⟨end line⟩]s/ ⟨regexp⟩/ ⟨replacement⟩[/q ]

The parts of the command within square brackets are optional. What do the differ-
ent components of the command mean?
   ⟨Start line⟩ and ⟨end line⟩ determine the range of lines to be searched. Without range of lines
these, only the current line will be looked at! Instead of line numbers, you can
use a dot to specify the current line or a dollar sign to specify the last line—but do
not confuse the meanings of these characters with their meanings within regular
expressions (section 7.1).:


replaces the first occurrence of red on each line by blue , where the first four lines
are not considered.

replaces every occurrence of red in those lines by blue . (Watch out: Even Fred Flint-
stone will become Fblue Flintstone .)

B Instead of line numbers, “. ”, and “$ ”, vi also allows regular expressions
  within slashes as start and end markers:

        replaces red by blue only in lines located between a line starting with BEGIN

    After the command name s and a slash, you must enter the desired regular
expression. After another slash, ⟨replacement⟩ gives a character sequence by which
the original text is to be replaced.
    There is a special function for this argument: With a & character you can “ref- Back reference
erence back” to the text matched by the ⟨regexp⟩ in every actual case. That is, “
 : s/bull/& frog ” changes every bull within the search range to a bull frog—a task
which will probably give genetic engineers trouble for some time to come.

Command-line Mode Commands So far we have described some command-line
mode (or “ex mode”) commands. There are several more, all of which can be
accessed from command mode by prefixing them with a colon and finishing them
with the return key (table 5.5).
56                                                                                               5 Editors: vi and emacs

                                                        Table 5.5: ex commands in vi

                              Command                            Result
                                : w   ⟨file name⟩                 Writes the complete buffer content to the
                                                                 designated file
                                : w!   ⟨file name⟩                Writes to the file even if it is write-
                                                                 protected (if possible)
                                : e   ⟨file name⟩                 Reads the designated file into the buffer
                                : e #                            Reads the last-read file again
                                : r   ⟨file name⟩                 Inserts the content of the designated file
                                                                 after the line containing the cursor
                                : !   ⟨shell command⟩            Executes the given shell command and re-
                                                                 turns to vi afterwards
                                : r!   ⟨shell command⟩           Inserts the output of ⟨shell command⟩ after
                                                                 the line containing the cursor
                                : s/ ⟨regexp⟩/ ⟨replacement⟩     Searches for ⟨regexp⟩ and replaces by
                                : q                              Quits vi
                                : q!                             Quits vi even if the buffer contents is un-
                                : x   oder   :e wq               Saves the buffer contents and quits vi

                        C 5.2 [5] (For systems with vim , e. g., the SUSE distributions.) Find out how to
                          access the interactive vim tutorial and work through it.

                        5.3      The Challenger—Emacs
                        5.3.1     Overview
                        The emacs text editor (“editing macros”1 ) was originally developed by Richard M.
                        Stallman as an extension of the antiquated TECO editor2 . Later, Stallman wrote
                        a completely new, eponymous program that did not require TECO. According to
                        the GNU philosophy, Emacs source code is freely available. Thus the program can
                        be adapted or extended according to its users’ requirements. Emacs is available
                        on nearly every computing platform.

                        B In addition, there are dozens of editors inspired by Emacs. Usually they
                          trade functionality and extensibility for decreased space requirements. The
                          best-known of these is probably “MicroEmacs”, which has unfortunately
     Emacs derivative     been unmaintained for quite some time now. Another popular Emacs
                          derivative is, for example, jove . The Emacs epigones—or at least a few of
                          them—are not just available for Linux but all computing platforms worth
                          mentioning, including various flavours of Windows.

                           Emacs itself offers many more functions than vi and can easily be considered
                        a complete work environment, especially because it can be extended in a dialect
                           1 There are various facetious expansions, such as “Escape-Meta-Alt-Control-Shift”, due to the

                        predilection of the program for weird key combinations, or “Eight Megabytes And Constantly Swap-
                        ping”; when Emacs was new, 8 megabytes of RAM were quite a lot, so today this should probably be
                        read “Eight Hundred Megabytes”—and it cannot be too long before “Eight Thousand” will be appro-
                        priate …
                           2 TECO was famous for its completely obtuse user interface. If you find vi counter-intuitive, you

                        should not even think of looking at TECO. A favourite pastime among TECO users in the late seventies
                        was to enter one’s own first name as a TECO command and try to predict what would happen—not
                        always a straightforward task.
5.3 The Challenger—Emacs                                                                                     57

                             Table 5.6: Possible buffer states in emacs

        Display        Buffer state
             -:---     Content unchanged since last write
             -:**-     Content changed but not written
             -:%%-     Content can only be read but not modified

of the Lisp programming language. In addition to functioning as a text editor, a Diverse Functions
complete installation of Emacs allows, for example, file operations such as copy-
ing, deleting, etc., sending and receiving e-mail, calling a calendar, diary or pocket
calculator, playing Tetris or getting psychoanalyzed3 and much more. This func-
tionality and the wide availability of Emacs imply that this program is often con-
sidered the new standard editor on Linux systems4 .

         Only vi is pertinent to LPI certification. Therefore, if you are just swotting
         up for the exam and are not interested in a wider learning experience you
         may skip to the end of this section.

5.3.2         Basic Functions
Buffers Just like vi , emacs is a screen-oriented editor and uses buffers, keeping
the text being edited completely in RAM and making modifications to that copy
only. In contrast to vi , emacs can manage multiple buffers at once, therefore it is Multiple buffers
possible to edit several files at the same time and move text between them. Even
the bottom line on the screen, where commands can be input and messages shown
to the user, has its own “mini buffer”.
    Since emacs does not distinguish between different work modes, you do not have
to change to a command mode to execute editor commands. Instead, editor func-
tions are invoked using the Ctrl key in combination with other keys such as x
or c , and using another, freely definable key (usually Esc ).

 B In Emacs jargon, the Esc key’s function is called “meta”. You can also use
   one of the additional “shift-like” keys available on current PC keyboards.
   Typically, the Alt key to the left of the space bar is preconfigured accord-

Starting, Quitting and Help After launch, emacs first displays a launch screen with launch screen
some noteworthy messages. This looks roughly like figure 5.2. While the manual
page lists only the command line parameters for emacs , the Ctrl + h command
inside the editor invokes the built-in help system. This explains everything one
could possibly want for, from keyboard commands to search patterns and much
more. There is also a step-by-step tutorial that will introduce you to the basics of
emacs operation.
     The top line on the screen shows a menu bar, which however is inaccessible by menu bar
a mouse unless you use a GUI. Since it is difficult to operate via the keyboard, but
easy (and not worth explaining) with a mouse in a GUI, we will not consider the
menu bar further.
     In the second-to-last line of the screen you find the name of the current buffer, Status line
which usually corresponds to the name of the loaded file. If you did not give a
file name when launching emacs , the editor calls the buffer *scratch* . That line also
gives information about the current line number (after L , thus line 1 here) and the
position of the visible part of the text in relation to the text as a whole (here All ,
i. e., all of the text is visible—otherwise a percentage). The beginning of the line
displays the state of the buffer (table 5.6).
   3 Honest!    Try Esc x tetris ↩ or Esc x doctor ↩ .
   4 Emacs     advocates claim that the only reason for the continued existence of vi is to edit the Emacs
Makefile .
58                                                                  5 Editors: vi and emacs

                             Figure 5.2: The emacs launch screen

        As with most editors (excluding vi ), you can enter text immediately after
     launching the program. The cursor can be moved freely around the text, you
     can delete single characters using the ⇐ key, the return key starts a new line—
     everything works as usual. Should characters extend past the right margin, Emacs
     puts a backslash at the right end of the line and continues it in a new line on screen,
     the actual line length can therefore by far exceed that of a screen line.
        At the end the possibly most important information on the launch screen: With
     the Ctrl + x u key sequence, unsaved modifications can be undone. The Ctrl +
      x Ctrl + c key sequence quits Emacs, asking whether modified and unsaved
     buffers should be saved first.

     Loading and Saving Files To edit a file using emacs , that file’s buffer must be “cur-
     rent”. The Ctrl + x Ctrl + f command will read the file into a buffer if it isn’t
     already available, or make that file’s buffer the current buffer if the file has al-
     ready been loaded. The command will prompt for a file name; Emacs supports
     file name completion using Tab , similar to that of the shell.
         The Ctrl + x Ctrl + v command will not create a new buffer for the desig-
     nated file. Instead, the contents of the current buffer is overwritten and the buffer
     name changed accordingly. This will of course cause the previous contents of the
     buffer to be lost.
         Furthermore, you can use Ctrl + x i to insert a file’s contents at the current
     cursor position in a buffer. The contents of the current buffer can be stored using
      Ctrl + x Ctrl + s (“save”). If the buffer does not have a name, the program will
     prompt for one, otherwise the message “Wrote ⟨file⟩” presently appears in the mini
     buffer line. On the first save operation, an existing file will be made into a “backup
     copy” by appending a tilde to the file name.
         To save the contents of a named buffer in a different file, use the command
5.3 The Challenger—Emacs                                                                           59

                                Table 5.7: Cursor movement commands in emacs

          Command                         Result
        Ctrl +f           or        →     One character to the right (engl. forward)
        Ctrl +b           or        ←     One character to the left (engl. back)
        Ctrl + n           or       ↓     One line down (engl. next)
        Ctrl + p           or       ↑     One line up (engl. previous)
                Esc         f             Jump to space character in front of next word
                Esc       b               Jump to first character of previous word
               Ctrl     +       a         Jump to beginning of line
               Ctrl     +       e         Jump to end of line
                Esc         a             Jump to beginning of sentence
                Esc         e             Jump to end of sentence
               Ctrl     +       v         Scroll up one screenful
                Esc         v             Scroll back one screenful
                Esc       <               Jump to beginning of buffer
                Esc       >               Jump to end of buffer

                                        Table 5.8: Deletion commands in emacs

     Command                        Result
         Ctrl   +     d             Deletes the character “under” the cursor
          Esc       d               Deletes from the character under the cursor to the end of
                                    the word containing that character
         Ctrl   +     k             Deletes from the cursor to the end of the line (engl. kill).
                                    A second Ctrl + k deletes the end of the line as well.
          Esc       k               Deletes the sentence containing the cursor
        Ctrl    +     w             Deletes from the “mark” (set using Ctrl + ) to the cur-
                                    rent cursor position

 Ctrl + x Ctrl + w . The program will prompt for a new file name and leaves the
original file undisturbed.
    You can move between buffers using the Ctrl + x b command. The Ctrl + x
 Ctrl + b command will display a list of buffers.

Moving about the Text As a rule, the cursor keys can be used to move about
the text character by character. Since older terminals in particular often do not
feature such keys, there are equivalent Ctrl commands. In a similar way, you can
also move through the text by words or sentences (table 5.7).

Deleting To remove an arbitrary character, you must first position the cursor on
that character. Then the commands in table 5.8 are available.

5.3.3      Extended Functions
Cutting and Pasting Text If more than a single character has been deleted at one
go, that sequence of characters is placed to the “kill buffer”. In fact, the text has
been “cut” instead of deleted.
   The contents of the kill buffer can be inserted at the current cursor position us-
ing Ctrl + y . Older deletions are also availabe: Esc y replaces the inserted text
by the previous contents of the kill buffer. This can be repeated to reach progres-
sively older material.
60                                                                                               5 Editors: vi and emacs

                                                      Table 5.9: Text-correcting commands in emacs

                                Command                        Result
                                 Ctrl     +       t            Swaps the character at the current cursor position
                                                               with the immediately preceding one (if the cursor
                                                               is at the end of the line, the preceding character
                                                               will be swapped with the one before that)
                                    Esc       t                Swaps the word beginning to the left of the cursor
                                                               position with the one beginning to the right of the
                                                               cursor. Punctuation is not moved
                         Ctrl   +   x     Ctrl         +   t   Swaps the current and preceding lines
                                    Esc   c                    Capitalizes the character at the current cursor po-
                                                               sition and forces all others in the word to lower-
                                    Esc   u                    Capitalizes all characters from the cursor position
                                                               to the end of the word
                                    Esc       l                Forces all characters from the cursor position to
                                                               the end of the word to lowercase

                   Correcting Typos Less well-practised typists will frequently enter two characters
                   in the wrong sequence, or type lowercase characters as caps—this list could go on
                   and on. The emacs editor offers a few commands to help with correcting these
                   mistakes, which are described in table 5.9.

                   Searching Owing to the large number of search functions in emacs , we shall re-
                   strict ourselves to introducing the two basic search mechanisms: The Ctrl + s
                   command searches from the current cursor position towards the end of the buffer,
                   while Ctrl + r searches in the other direction (towards the beginning). The char-
                   acter sequence in question is displayed in the mini buffer, and the cursor jumps to
                   the next matching place in the buffer while the search string is still being entered
                   (“incremental search”).

                   Additional Functions As mentioned earlier, Emacs offers an enormous number
                   of functions, far exceeding the usual requirements for a text editor. Some of these
                   special extensions are mentioned here:

      file types      • emacs is able to recognize various file types by their file name extensions.
                        For example, if a C source file (extension .c ) is opened, the editor automat-
                        ically enters “C mode”. This allows automatic indentation and matching
                        of parentheses and braces to make programming easier. Even the C com-
                        piler can be called directly from the editor. The same works for most current
                        programming languages.

                      • If you receive new electronic mail while working in emacs , a notification
                        will appear in the status line. The Esc x rmail command changes to “mail
                        mode”, which lets you read, compose and send messages.
                      • File management in Emacs is possible through “dired” (“directory editor”):
                         Ctrl + x d displays a list of all files in the current directory. You can use
                        the cursor keys to move around in the list, and pressing the return key will
                        open the file under the cursor. You can also delete, copy or rename files.
     Emacs Lisp       • The editor can be extended using a dialect of the Lisp programming lan-
                        guage. Lisp is a list-oriented language fairly unlike other extension lan-
                        guages, but Emacs Lisp is a very powerful tool. Emacs comes with a manual
                        explaining the language in great detail.
5.4 Other Editors                                                                       61

C 5.3 [5] Work through the interactive Emacs tutorial. (The English language
  version is available via Ctrl + h t ; versions in other languages can be in-
  voked using the Emacs “Help” menu.)

5.4       Other Editors
We have already alluded to the fact that your choice of editor is just as much down
to your personal preferences and probably says as much about you as a user as
your choice of car: Do you drive a polished BMW or are you happy with a dented
Astra? Or would you rather prefer a Land Rover? As far as choice is concerned,
the editor market offers no less than the vehicle market. We have presented two
fairly important contenders, but of course there are many others. kate on KDE
and gedit on GNOME, for example, are straightforward and easy-to-learn editors
with a graphical user interface that are perfectly adequate for the requirements
of a normal user. Do browse through the package lists of your distribution and
check whether you will find the editor of your dreams there.

Commands in this Chapter
ed         Primitive (but useful) line-oriented text editor                 ed (1) 51
elvis      Popular “clone” of the vi editor                              elvis (1) 50
emacs      Powerful extensible screen-oriented text editor emacs (1), Info: emacs 55
ex         Powerful line-oriented text editor (really vi )                  vi (1) 50
jove       Text editor inspired by emacs                                  jove (1) 56
sed        Stream-oriented editor, copies its input to its output making changes in
           the process                                                     sed (1) 51
vi         Screen-oriented text editor                                      vi (1) 50
vim        Popular “clone” of the vi editor                                vim (1) 50

      • Text editors are important for changing configuration files and program-
        ming. They often offer special features to make these tasks easier.
      • vi is a traditional, very widespread and powerful text editor with an id-
        iosyncratic user interface.
      • Emacs is a freely available, modern editor with many special features.
                                                                                           $ echo tux
                                                                                           $ ls
                                                                                           $ /bin/su -

Files: Care and Feeding

6.1  File and Path Names . . . . . . . . . . . . .            .   .   .   .   .   .   64
   6.1.1 File Names . . . . . . . . . . . . . .               .   .   .   .   .   .   64
   6.1.2 Directories . . . . . . . . . . . . . .              .   .   .   .   .   .   65
   6.1.3 Absolute and Relative Path Names . . . . .           .   .   .   .   .   .   66
6.2 Directory Commands . . . . . . . . . . . .                .   .   .   .   .   .   67
   6.2.1 The Current Directory: cd & Co. . . . . . .          .   .   .   .   .   .   67
   6.2.2 Listing Files and Directories—ls . . . . . .         .   .   .   .   .   .   68
   6.2.3 Creating and Deleting Directories: mkdir and rmdir   .   .   .   .   .   .   69
6.3 File Search Patterns . . . . . . . . . . . . .            .   .   .   .   .   .   70
   6.3.1 Simple Search Patterns . . . . . . . . . .           .   .   .   .   .   .   70
   6.3.2 Character Classes . . . . . . . . . . . .            .   .   .   .   .   .   72
   6.3.3 Braces . . . . . . . . . . . . . . . .               .   .   .   .   .   .   73
6.4 Handling Files . . . . . . . . . . . . . . .              .   .   .   .   .   .   74
   6.4.1 Copying, Moving and Deleting—cp and Friends.         .   .   .   .   .   .   74
   6.4.2 Linking Files—ln and ln -s . . . . . . . .           .   .   .   .   .   .   76
   6.4.3 Displaying File Content—more and less . . . .        .   .   .   .   .   .   80
   6.4.4 Searching Files—find . . . . . . . . . .             .   .   .   .   .   .   81
   6.4.5 Finding Files Quickly—locate and slocate . . .       .   .   .   .   .   .   84

      • Being familiar with Linux conventions concerning file and directory names
      • Knowing the most important commands to work with files and directories
      • Being able to use shell filename search patterns

      • Using a shell (qv. chapter 2)
      • Use of a text editor (qv. chapter 5)

grd1-dateien.tex   (be27bba8095b329b )
64                                                                                    6 Files: Care and Feeding

                           6.1      File and Path Names
                           6.1.1     File Names
                          One of the most important services of an operating system like Linux consists
                          of storing data on permanent storage media like hard disks or USB keys and re-
                          trieving them later. To make this bearable for humans, similar data are usually
                    files collected into “files” that are stored on the medium under a name.

                           B Even if this seems trivial to you, it is by no means a given. In former times,
                             some operating systems made it necessary to know abominations like track
                             numbers on a disk in order to retrieve one’s data.

                           Thus, before we can explain to you how to handle files, we need to explain to
                        you how Linux names files.
     Allowed characters    In Linux file names, you are essentially allowed to use any character that your
                        computer can display (and then some). However, since some of the characters
                        have a special meaning, we would recommend against their use in file names.
                        Only two characters are completely disallowed, the slash and the zero byte (the
                        character with ASCII value 0). Other characters like spaces, umlauts, or dollar
                        signs may be used freely, but must usually be escaped on the command line by
                        means of a backslash or quotes in order to avoid misinterpretations by the shell.

                           A An easy trap for beginners to fall into is the fact that Linux distinguishes
             Letter case     uppercase and lowercase letters in file names. Unlike Windows, where up-
                             percase and lowercase letters in file names are displayed but treated the
                             same, Linux considers x- files and X- Files two different file names.

                            Under Linux, file names may be “quite long”—there is no definite upper
                         bound, since the maximum depends on the “file system”, which is to say the
                         specific way bytes are arranged on the medium (there are several methods on
                         Linux). A typical upper limit is 255 characters—but since such a name would
                         take somewhat more than three lines on a standard text terminal this shouldn’t
                         really cramp your style.
                            A further difference from DOS and Windows computers is that Linux does not
                suffixes use suffixes to characterise a file’s “type”. Hence, the dot is a completely ordi-
                         nary character within a file name. You are free to store a text as mumble.txt , but
                         mumble would be just as acceptable in principle. This should of course not turn you
                         off using suffixes completely—you do after all make it easier to identify the file

                           B Some programs insist on their input files having specific suffixes. The C
                             compiler, gcc , for example, considers files with names ending in “.c ” C
                             source code, those ending in “.s ” assembly language source code, and
                             those ending in “.o ” precompiled object files.

      special characters      You may freely use umlauts and other special characters in file names. How-
                           ever, if files are to be used on other systems it is best to stay away from special
                           characters in file names, as it is not guaranteed that they will show up as the same
                           characters elsewhere.

         locale settings A What happens to special characters also depends on your locale settings,
                                   since there is no general standard for representing characters exceeding the
                                   ASCII character set (128 characters covering mostly the English language,
                                   digits and the most common special characters). Widely used encodings
                                   are, for example, ISO 8859-1 and ISO 8859-15 (popularly know as ISO-Latin-
                                   1 and ISO-Latin-9, respectively … don’t ask) as well as ISO 10646, casually
                                   and not quite correctly called “Unicod” and usually encoded as “UTF-8”.
                                   File names you created while encoding 𝑋 was active may look completely
                                   different when you look at the directory while encoding 𝑌 is in force. The
                                   whole topic is nothing you want to think about during meals.
6.1 File and Path Names                                                                                            65

A Should you ever find yourself facing a pile of files whose names are encoded
  according to the wrong character set, the convmv program, which can con-                   convmv
  vert file names between various character encodings, may be able to help
  you. (You will probably have to install it yourself since it is not part of
  the standard installation of most distributions.) However, you should re-
  ally get down to this only after working through the rest of this chapter, as
  we haven’t even explained the regular mv yet …

   All characters from the following set may be used freely in file names:                    Portable file names


However, you should pay attention to the following hints:
   • To allow moving files between Linux and older Unix systems, the length of
     a file name should be at most 14 characters. (Make that “ancient”, really.)
   • File names should always start with one of the letters or a digit; the other
     four characters can be used without problems only inside a file name.

These conventions are easiest to understand by looking at some examples. Allow-
able file names would be, for instance:


On the contrary, problems would be possible (if not likely or even assured) with:

-10°F                                       Starts with ‘‘- ’’, includes special character
.profile                                                                   Will be hidden
3/4-metre                                                       Contains illegal character
Smörrebröd                                                             Contains umlauts

   As another peculiarity, file names starting with a dot (“. ”) will be skipped in Hidden files
some places, for example when the files within a directory are listed—files with
such names are considered “hidden”. This feature is often used for files contain-
ing settings for programs and which should not distract users from more impor-
tant files in directory listings.

B For DOS and Windows experts: These systems allow “hiding” files by
  means of a “file attribute” which can be set independently of the file’s
  name. Linux and Unix do not support such a thing.

6.1.2    Directories
Since potentially many users may work on the same Linux system, it would be
problematic if each file name could occur just once. It would be difficult to make
clear to user Joe that he cannot create a file called letter.txt since user Sue already
has a file by that name. In addition, there must be a (convenient) way of ensuring
that Joe cannot read all of Sue’s files and the other way round.
   For this reason, Linux supports the idea of hierarchical “directories” which are
used to group files. File names do not need to be unique within the whole system,
but only within the same directory. This means in particular that the system can
assign different directories to Joe and Sue, and that within those they may call
their files whatever they please without having to worry about each other’s files.
66                                                                                   6 Files: Care and Feeding

                         In addition, we can forbid Joe from accessing Sue’s directory (and vice versa) and
                         no longer need to worry about the individual files within them.
                            On Linux, directories are simply files, even though you cannot access them
                         using the same methods you would use for “plain” files. However, this implies
                         that the rules we discussed for file names (see the previous section) also apply to
                   slash the names of directories. You merely need to learn that the slash (“/ ”) serves to
                         separate file names from directory names and directory names from one another.
                         joe/letter.txt would be the file letter.txt in the directory joe .
                            Directories may contain other directories (this is the term “hierarchical” we
          directory tree mentioned earlier), which results in a tree-like structure (inventively called a “di-
                         rectory tree”). A Linux system has a special directory which forms the root of the
                         tree and is therefore called the “root directory”. Its name is “/ ” (slash).

                          B In spite of its name, the root directory has nothing to do with the system
                            administrator, root . It’s just that their names are similar.

                          B The slash does double duty here—it serves both as the name of the root
                            directory and as the separator between other directory names. We’ll come
                            back to this presently.

                             The basic installation of common Linux distributions usually contains tens of
                          thousands of files in a directory hierarchy that is mostly structured according to
                          certain conventions. We shall tell you more about this directory hierarchy in chap-
                          ter 10.

                          6.1.3    Absolute and Relative Path Names
                         Every file in a Linux system is described by a name which is constructed by start-
                         ing at the root directory and mentioning every directory down along the path to
                         the one containing the file, followed by the name of the file itself. For example,
                         /home/joe/letter.txt names the file letter.txt , which is located within the joe direc-
                         tory, which in turn is located within the home directory, which in turn is a direct
                         descendant of the root directory. A name that starts with the root directory is
     absolute path name called an “absolute path name”—we talk about “path names” since the name de-
                         scribes a “path” through the directory tree, which may contain directory and file
                         names (i. e., it is a collective term).
                             Each process within a Linux system has a “current directory” (often also called
                         “working directory”). File names are searched within this directory; letter.txt
                         is thus a convenient abbreviation for “the file called letter.txt in the current di-
                         rectory”, and sue/letter.txt stands for “the file letter.txt within the sue directory
                         within the current directory”. Such names, which start from the current directory,
     relative path names are called “relative path names”.

                          B It is trivial to tell absolute from relative path names: A path name starting
                            with a “/ ” is absolute; all others are relative.

                          B The current directory is “inherited” between parent and child processes. So
                            if you start a new shell (or any program) from a shell, that new shell uses
                            the same current directory as the shell you used to start it. In your new
                            shell, you can change into another directory using the cd command, but the
                            current directory of the old shell does not change—if you leave the new
                            shell, you are back to the (unchanged) current directory of the old shell.

              shortcuts      There are two convenient shortcuts in relative path names (and even absolute
                          ones): The name “.. ” always refers to the directory above the directory in question
                          in the directory tree—for example, in the case of /home/joe , /home . This frequently
                          allows you to refer conveniently to files in a “side branch” of the directory tree
                          as viewed from the current directory, without having to resort to absolute path
                          names. Assume /home/joe has the subdirectories letters and novels . With letters
                          as the current directory, you could refer to the ivanhoe.txt file within the novels
6.2 Directory Commands                                                                             67

directory by means of the relative path name ../novels/ivanhoe.txt , without having
to use the unwieldy absolute path name /home/joe/novels/ivanhoe.txt .
   The second shortcut does not make quite as obvious sense: the “. ” name within
a directory always stands for the directory itself. It is not immediately clear why
one would need a method to refer to a directory which one has already reached,
but there are situations where this comes in quite handy. For example, you may
know (or could look up in Chapter 9) that the shell searches program files for
external commands in the directories listed in the environment variable PATH . If
you, as a software developer, want to invoke a program, let’s call it prog , which (a)
resides in a file within the current directory, and (b) this directory is not listed in
PATH (always a good idea for security reasons), you can still get the shell to start
your file as a program by saying

$ ./prog

without having to enter an absolute path name.

B As a Linux user you have a “home directory” which you enter immediately
  after logging in to the system. The system administrator determines that
  directory’s name when they create your user account, but it is usually called
  the same as your user name and located below /home —something like /home/
  joe for the user joe .

6.2     Directory Commands
6.2.1      The Current Directory: cd & Co.
You can use the cd shell command to change the current directory: Simply give Changing directory
the desired directory as a parameter:

$ cd letters                                            Change to the letters directory
$ cd ..                                                  Change to the directory above

   If you do not give a parameter you will end up in your home directory:

$ cd
$ pwd

You can output the absolute path name of the current directory using the pwd current directory
(“print working directory”) command.
   Possibly you can also see the current directory as part of your prompt: Depend- prompt
ing on your system settings there might be something like

joe@red:~/letters> _

where ~/letters is short for /home/joe/letters ; the tilde (“~ ”) stands for the current
user’s home directory.

B The “cd - ” command changes to the directory that used to be current before
  the most recent cd command. This makes it convenient to alternate between
  two directories.

C 6.1 [2] In the shell, is cd an internal or an external command? Why?

C 6.2 [3] Read about the pushd , popd , and dirs commands in the bash man page.
  Convince yourself that these commands work as described there.
68                                                                                              6 Files: Care and Feeding

                                     Table 6.1: Some file type designations in ls

                           File type          Colour    Suffix (ls -F )    Type letter (ls -l )
                           plain file           black         none                 -
                           executable file      green           *                  -
                           directory            blue           /                  d
                           link                cyan            @                  l

                                              Table 6.2: Some ls options

     Option                       Result
     -a   or --all                Displays hidden files as well
     -i   or --inode              Displays the unique file number (inode number)
     -l   or --format=long        Displays extra information
     -o   or --no-color           Omits colour-coding the output
     -p   or -F                   Marks file type by adding a special character
     -r   or --reverse            Reverses sort order
     -R   or --recursive          Recurses into subdirectories (DOS: DIR/S )
     -S   or --sort=size          Sorts files by size (longest first)
     -t   or --sort=time          Sorts file by modification time (newest first)
     -X   or --sort=extension     Sorts file by extension (“file type”)

                                  6.2.2    Listing Files and Directories—ls
                                To find one’s way around the directory tree, it is important to be able to find out
                                which files and directories are located within a directory. The ls (“list”) command
                                does this.
                 Tabular format    Without options, this information is output as a multi-column table sorted by
                                file name. With colour screens being the norm rather than the exception today, it
                                has become customary to display the names of files of different types in various
                                colours. (We have not talked about file types yet; this topic will be mentioned in
                                Chapter 10.)

                                  B Thankfully, by now most distributions have agreed about the colours to use.
                                    Table 6.1 shows the most common assignment.

                                  B On monochrome monitors—which can still be found—, the options -F or -p
                                    recommend themselves. These will cause special characters to be appended
                                    to the file names according to the file’s type. A subset of these characters is
                                    given in table 6.1.

                   Hidden files     You can display hidden files (whose names begin with a dot) by giving the -a
                                 (“all”) option. Another very useful option is -l (a lowercase “L”, for “long”, rather
          Additional information than the digit “1”). This displays not only the file names, but also some additional
                                 information about each file.

                                  B Some Linux distributions pre-set abbreviations for some combinations of
                                    helpful options; the SUSE distributions, for example, use a simple l as an
                                    abbreviation of “ls -alF ”. “ll ” and “la ” are also abbreviations for ls variants.

                                    Here is an example of ls without and with -l :

                                  $ ls
                                  $ ls -l
6.2 Directory Commands                                                                                  69

-rw-r--r--    1   joe users   4711   Oct 4 11:11 file.txt
-rw-r--r--    1   joe users    333   Oct 2 13:21 file2.dat

In the first case, all visible (non-hidden) files in the directory are listed; the second
case adds the extra information.
    The different parts of the long format have the following meanings: The first Long format
character gives the file type (see chapter 10); plain files have “- ”, directories “d ”
and so on (“type character” in table 6.1).
    The next nine characters show the access permissions. Next there are a refer-
ence counter, the owner of the file (joe here), and the file’s group (users ). After the
size of file in bytes, you can see the date and time of the last modification of the
file’s content. On the very right there is the file’s name.

A Depending on the language you are using, the date and time columns in par-
  ticular may look completely different than the ones in our example (which
  we generated using the minimal language environment “C ”). This is usu-
  ally not a problem in interactive use, but may prove a major nuisance if you
  try to take the output of “ls -l ” apart in a shell script. (Without wanting to
  anticipate the training manual Advanced Linux, we recommend setting the
  language environment to a defined value in shell scripts.)

B If you want to see the extra information for a directory (such as /tmp ), “ls -l
  /tmp ” doesn’t really help, because ls will list the data for all the files within
  /tmp . Use the -d option to suppress this and obtain the information about
  /tmp itself.

  ls supports many more options than the ones mentioned here; a few of the
more important ones are shown in table 6.2.

        In the LPI exams, Linux Essentials and LPI-101, nobody expects you to know
        all 57 varieties of ls options by heart. However, you may wish to commit the
        most import half dozen or so—the content of Table 6.2, approximately—to

C 6.3 [1] Which files does the /boot directory contain? Does the directory have
  subdirectories and, if so, which ones?

C 6.4 [2] Explain the difference between ls with a file name argument and ls
  with a directory name argument.

C 6.5 [2] How do you tell ls to display information about a directory rather
  than the files in that directory, if a directory name is passed to the program?
  (Hint: Documentation.)

6.2.3     Creating and Deleting Directories: mkdir and rmdir
To keep your own files in good order, it makes sense to create new directories. You
can keep files in these “folders” according to their subject matter (for example).
Of course, for further structuring, you can create further directories within such
directories—your ambition will not be curbed by arbitrary limits.
   To create new directories, the mkdir command is available. It requires one or Creating directories
more directory names as arguments, otherwise you will only obtain an error mes-
sage instead of a new directory. To create nested directories in a single step, you
can use the -p option, otherwise the command assumes that all directories in a
path name except the last one already exist. For example:
70                                                                                       6 Files: Care and Feeding

                            $ mkdir pictures/holiday
                            mkdir: cannot create directory `pictures/holiday': No such file 
                              or directory
                            $ mkdir -p pictures/holiday
                            $ cd pictures
                            $ ls -F

     Removing directories       Sometimes a directory is no longer required. To reduce clutter, you can remove
                            it using the rmdir (“remove directory”) command.
                                As with mkdir , at least one path name of a directory to be deleted must be given.
                            In addition, the directories in question must be empty, i. e., they may not contain
                            entries for files, subdirectories, etc. Again, only the last directory in every name
                            will be removed:
                            $ rmdir pictures/holiday
                            $ ls -F

                            With the -p option, all empty subdirectories mentioned in a name can be removed
                            in one step, beginning with the one on the very right.

                            $ mkdir -p pictures/holiday/summer
                            $ rmdir pictures/holiday/summer
                            $ ls -F pictures
                            $ rmdir -p pictures/holiday
                            $ ls -F pictures
                            ls: pictures: No such file or directory

                            C 6.6 [!2] In your home directory, create a directory grd1- test with subdirecto-
                              ries dir1 , dir2 , and dir3 . Change into directory grd1- test/dir1 and create (e. g.,
                              using a text editor) a file called hello containing “hello ”. In grd1- test/dir2 ,
                              create a file howdy containing “howdy ”. Check that these files do exist. Delete
                              the subdirectory dir3 using rmdir . Next, attempt to remove the subdirectory
                              dir2 using rmdir . What happens, and why?

                            6.3     File Search Patterns
                            6.3.1    Simple Search Patterns
                           You will often want to apply a command to several files at the same time. For
                           example, if you want to copy all files whose names start with “p ” and end with
                           “.c ” from the prog1 directory to the prog2 directory, it would be quite tedious to
                           have to name every single file explictly—at least if you need to deal with more
          search patterns than a couple of files! It is much more convenient to use the shell’s search patterns.
                  asterisk     If you specify a parameter containing an asterisk on the shell command line—
6.3 File Search Patterns                                                                          71

—the shell replaces this parameter in the actual program invocation by a sorted list
of all file names that “match” the parameter. “Match” means that in the actual file
name there may be an arbitrary-length sequence of arbitrary characters in place
of the asterisk. For example, names like


are eligible (note in particular the last name in the example—“arbitrary length”
does include “length zero”!). The only character the asterisk will not match is—
can you guess it?—the slash; it is usually better to restrict a search pattern like the
asterisk to the current directory.

B You can test these search patterns conveniently using echo . The
       $ echo prog1/p*.c

       command will output the matching file names without any obligation or
       consequence of any kind.

B If you really want to apply a command to all files in the directory tree starting
  with a particular directory, there are ways to do that, too. We will discuss
  this in section 6.4.4.

   The search pattern “* ” describes “all files in the current directory”—excepting All files
hidden files whose name starts with a dot. To avert possibly inconvenient sur-
prises, search patterns diligently ignore hidden files unless you explicitly ask for
them to be included by means of something like “.* ”.

A You may have encountered the asterisk at the command line of operating
  systems like DOS or Windows1 and may be used to specifying the “*.* ”
  pattern to refer to all files in a directory. On Linux, this is not correct—the
  “*.* ” pattern matches “all files whose name contains a dot”, but the dot isn’t
  mandatory. The Linux equivalent, as we said, is “* ”.

   A question mark as a search pattern stands for exactly one arbitrary character question mark
(again excluding the slash). A pattern like


thus matches the names

(among others). Note that there must be one character—the “nothing” option
does not exist here.
 You should take particular care to remember a very important fact: The expan-
 sion of search pattern is the responsibility of the shell! The commands that you ex-
 ecute usually know nothing about search patterns and don’t care about them,
 either. All they get to see are lists of path names, but not where they come
 from—i. e., whether they have been typed in directly or resulted from the ex-
 pansion of search patterns.

  1 You’re   probably too young for CP/M.
72                                                                         6 Files: Care and Feeding

              B Incidentally, nobody says that the results of search patterns always need to
                be interpreted as path names. For example, if a directory contains a file
                called “- l ”, a “ls * ” in that directory will yield an interesting and perhaps
                surprising result (see exercise 6.9).

              B What happens if the shell cannot find a file whose name matches the search
                pattern? In this case the command in question is passed the search pattern
                as such; what it makes of that is its own affair. Typically such search patterns
                are interpreted as file names, but the “file” in question is not found and an
                error message is issued. However, there are commands that can do useful
                things with search patterns that you pass them—with them, the challenge
                is really to ensure that the shell invoking the command does not try to cut
                in with its own expansion. (Cue: quotes)

              6.3.2      Character Classes
              A somewhat more precise specification of the matching characters in a search pat-
              tern is offered by “character classes”: In a search pattern of the form


              the square brackets match exactly those characters that are enumerated within
              them (no others). The pattern in the example therefore matches


              but not
              prog.c                                           There needs to be exactly one character
              prog4.c                                                           4 was not enumerated
              proga.c                                                                        a neither
              prog12.c                                                  Exactly one character, please

     ranges     As a more convenient notation, you may specify ranges as in


              The square brackets in the first line match all digits, the ones in the second all
              uppercase letters.

              A Note that in the common character encodings the letters are not contiguous:
                A pattern like


                      not only matches progQ.c and progx.c , but also prog_.c . (Check an ASCII table,
                      e. g. using “man ascii ”.) If you want to match “uppercase and lowercase
                      letters only”, you need to use


              A A construct like

                      does not catch umlauts, even if they look suspiciously like letters.
6.3 File Search Patterns                                                                                     73

   As a further convenience, you can specify negated character classes, which are negated classes
interpreted as “all characters except these”: Something like


matches all names where the character between “g ” and “. ” is not a letter. As
usual, the slash is excepted.

6.3.3     Braces
The expansion of braces in expressions like


is often mentioned in conjunction with shell search patterns, even though it is
really just a distant relative. The shell replaces this by

red.txt yellow.txt blue.txt

In general, a word on the command line that contains several comma-separated
pieces of text within braces is replaced by as many words as there are pieces of
text between the braces, where in each of these words the whole brace expression
is replaced by one of the pieces. This replacement is purely based on the command
line text and is completely independent of the existence or non-existence of any files or
directories—unlike search patterns, which always produce only those names that
actually exist as path names on the system.
    You can have more than one brace expression in a word, which will result in
the cartesian product, in other words all possible combinations:                         cartesian product



a1.dat a2.dat a3.dat b1.dat b2.dat b3.dat c1.dat c2.dat c3.dat

   This is useful, for example, to create new directories systematically; the usual
search patterns cannot help there, since they can only find things that already
$ mkdir -p revenue/200{8,9}/q{1,2,3,4}

C 6.7 [!1] The current directory contains the files
        prog.c    prog1.c   prog2.c   progabc.c   prog
        p.txt     p1.txt    p21.txt   p22.txt     p22.dat

        Which of these names match the search patterns (a) prog*.c , (b) prog?.c , (c)
        p?*.txt , (d) p[12]* , (e) p* , (f) *.* ?

C 6.8 [!2] What is the difference between “ls ” and “ls * ”? (Hint: Try both in a
  directory containing subdirectories.)

C 6.9 [2] Explain why the following command leads to the output shown:
74                                                                                           6 Files: Care and Feeding

                                               Table 6.3: Options for cp

             Option               Result
     -b             (backup)      Makes backup copies of existing target files by appending a tilde to their
     -f                (force)    Overwrites existing target files without prompting
     -i   (engl. interactive)     Asks (once per file) whether existing target files should be overwritten
     -p      (engl. preserve)     Tries to preserve all attributes of the source file for the copy
     -R     (engl. recursive)     Copies directories with all their content
     -u        (engl. update)     Copies only if the source file is newer than the target file (or the target file
                                  doesn’t exist)
     -v      (engl. verbose)      Displays all activity on screen

                                         $ ls
                                         -l file1     file2   file3
                                         $ ls *
                                         -rw-r--r--   1 joe users 0 Dec 19 11:24 file1
                                         -rw-r--r--   1 joe users 0 Dec 19 11:24 file2
                                         -rw-r--r--   1 joe users 0 Dec 19 11:24 file3

                                 C 6.10 [2] Why does it make sense for “* ” not to match file names starting with
                                   a dot?

                                 6.4       Handling Files
                                 6.4.1     Copying, Moving and Deleting—cp and Friends
                Copying files You can copy arbitrary files using the cp (“copy”) command. There are two basic
                  1 ∶ 1 copy         If you tell cp the source and target file names (two arguments), then a 1 ∶ 1 copy
                                 of the content of the source file will be placed in the target file. Normally cp does
                                 not ask whether it should overwrite the target file if it already exists, but just does
                                 it—caution (or the -i option) is called for here.
                                     You can also give a target directory name instead of a target file name. The
                                 source file will then be copied to that directory, keeping its old name.

                                 $ cp list list2
                                 $ cp /etc/passwd .
                                 $ ls -l
                                 -rw-r--r-- 1 joe       users    2500   Oct 4 11:11 list
                                 -rw-r--r-- 1 joe       users    2500   Oct 4 11:25 list2
                                 -rw-r--r-- 1 joe       users    8765   Oct 4 11:26 passwd

                                 In this example, we first created an exact copy of file list under the name list2 .
                                 After that, we copied the /etc/passwd file to the current directory (represented by
                                 the dot as a target directory name). The most important cp options are listed in
                                 table 6.3.
            List of source files    Instead of a single source file, a longer list of source files (or a shell wildcard
                                 pattern) is allowed. However, this way it is not possible to copy a file to a different
                                 name, but only to a different directory. While in DOS it is possible to use “COPY
                                 *.TXT *.BAK ” to make a backup copy of every TXT file to a file with the same name
                                 and a BAK suffix, the Linux command “cp *.txt *.bak ” usually fails with an error
6.4 Handling Files                                                                                        75

B To understand this, you have to visualise how the shell executes this com-
  mand. It tries first to replace all wildcard patterns with the corresponding
  file names, for example *.txt by letter1.txt and letter2.txt . What happens
  to *.bak depends on the expansion of *.txt and on whether there are match-
  ing file names for *.bak in the current directory—but the outcome will al-
  most never be what a DOS user would expect! Usually the shell will pass
  the cp command the unexpanded *.bak wildcard pattern as the final argu-
  ment, which fails from the point of view of cp since this is (unlikely to be)
  an existing directory name.

    While the cp command makes an exact copy of a file, physically duplicating the
file on the storage medium or creating a new, identical copy on a different storage
medium, the mv (“move”) command serves to move a file to a different place or Move/rename files
change its name. This is strictly an operation on directory contents, unless the file
is moved to a different file system—for example from a hard disk partition to a
USB key. In this case it is necessary to move the file around physically, by copying
it to the new place and removing it from the old.
    The syntax and rules of mv are identical to those of cp —you can again specify
a list of source files instead of merely one, and in this case the command expects
a directory name as the final argument. The main difference is that mv lets you
rename directories as well as files.
    The -b , -f , -i , -u , and -v options of mv correspond to the eponymous ones de-
scribed with cp .

$ mv passwd list2
$ ls -l
-rw-r--r-- 1 joe     users   2500   Oct 4 11:11 list
-rw-r--r-- 1 joe     users   8765   Oct 4 11:26 list2

In this example, the original file list2 is replaced by the renamed file passwd . Like
cp , mv does not ask for confirmation if the target file name exists, but overwrites
the file mercilessly.
     The command to delete files is called rm (“remove”). To delete a file, you must Deleting files
have write permission in the corresponding directory. Therefore you are “lord of
the manor” in your own home directory, where you can remove even files that do
not properly belong to you.

A Write permission on a file, on the other hand, is completely irrelevant as far
  as deleting that file is concerned, as is the question to which user or group
  the file belongs.

   rm goes about its work just as ruthlessly as cp or mv —the files in question are Deleting is forever!
obliterated from the file system without confirmation. You should be especially
careful, in particular when shell wildcard patterns are used. Unlike in DOS, the
dot in a Linux file name is a character without special significance. For this rea-
son, the “rm * ” command deletes all non-hidden files from the current directory.
Subdirectories will remain unscathed; with “rm -r * ” they can also be removed.

A As the system administrator, you can trash the whole system with a com-
  mand such as “rm -rf / ”; utmost care is required! It is easy to type “rm -rf
  foo * ” instead of “rm -rf foo* ”.

  Where rm removes all files whose names are passed to it, “rm -i ” proceeds a little
more carefully:

$ rm -i lis*
rm: remove 'list'? n
rm: remove 'list2'? y
$ ls -l
-rw-r--r-- 1 joe users       2500   Oct 4 11:11 list
76                                                                                      6 Files: Care and Feeding

                             The example illustrates that, for each file, rm asks whether it should be removed
                             (“y ” for “yes”) or not (“n ” for “no”).

                              B Desktop environments such as KDE usually support the notion of a “dust-
                                bin” which receives files deleted from within the file manager, and which
                                makes it possible to retrieve files that have been removed inadvertently.
                                There are similar software packages for the command line.

                             In addition to the -i and -r options, rm allows cp ’s -v and -f options, with similar

                              C 6.11 [!2] Create, within your home directory, a copy of the file /etc/services
                                called myservices . Rename this file to srv.dat and copy it to the /tmp directory
                                (keeping the new name intact). Remove both copies of the file.

                              C 6.12 [1] Why doesn’t mv have an -R option (like cp has)?

                              C 6.13 [!2] Assume that one of your directories contains a file called “- file ”
                                (with a dash at the start of the name). How would you go about removing
                                this file?

                              C 6.14 [2] If you have a directory where you do not want to inadvertently fall
                                victim to a “rm * ”, you can create a file called “- i ” there, as in

                                     $ > -i

                                     (will be explained in more detail in chapter 8). What happens if you now
                                     execute the “rm * ” command, and why?

                             6.4.2     Linking Files—ln and ln -s
                             Linux allows you to create references to files, so-called “links”, and thus to assign
                             several names to the same file. But what purpose does this serve? The applica-
                             tions range from shortcuts for file and directory names to a “safety net” against
                             unwanted file deletions, to convenience for programmers, to space savings for
                             large directory trees that should be available in several versions with only small
                                The ln (“link”) command assigns a new name (second argument) to a file in
                             addition to its existing one (first argument):

                              $ ln list list2
                              $ ls -l
                              -rw-r--r-- 2 joe       users   2500   Oct 4 11:11 list
                              -rw-r--r-- 2 joe       users   2500   Oct 4 11:11 list2

     A file with multiple names The directory now appears to contain a new file called list2 . Actually, there are
            reference counter just two references to the same file. This is hinted at by the reference counter in
                            the second column of the “ls -l ” output. Its value is 2, denoting that the file really
                            has two names. Whether the two file names really refer to the same file can only be
                            decided using the “ls -i ” command. If this is the case, the file number in the first
              inode numbers column must be identical for both files. File numbers, also called inode numbers,
                            identify files uniquely within their file system:

                              $ ls -i
                              876543 list     876543 list2
6.4 Handling Files                                                                        77

B “Inode” is short for “indirection node”. Inodes store all the information that
  the system has about a file, except for the name. There is exactly one inode
  per file.

If you change the content of one of the files, the other’s content changes as well,
since in fact there is only one file (with the unique inode number 876543). We only
gave that file another name.

B Directories are simply tables mapping file names to inode numbers. Obvi-
  ously there can be several entries in a table that contain different names but
  the same inode number. A directory entry with a name and inode number
  is called a “link”.

   You should realise that, for a file with two links, it is quite impossible to find
out which name is “the original”, i. e., the first parameter within the ln command.
From the system’s point of view both names are completely equivalent and indis-

A Incidentally, links to directories are not allowed on Linux. The only excep-
  tions are “. ” and “.. ”, which the system maintains for each directory. Since
  directories are also files and have their own inode numbers, you can keep
  track of how the file system fits together internally. (See also Exercise 6.19).

   Deleting one of the two files decrements the number of names for file no.
876543 (the reference counter is adjusted accordingly). Not until the reference
counter reachers the value of 0 will the file’s content actually be removed.

$ rm list
$ ls -li
876543 -rw-r--r--    1   joe   users   2500   Oct 4 11:11 list2

B Since inode numbers are only unique within the same physical file system
  (disk partition, USB key, …), such links are only possible within the same
  file system where the file resides.

B The explanation about deleting a file’s content was not exactly correct: If the
  last file name is removed, a file can no longer be opened, but if a process is
  still using the file it can go on to do so until it explicitly closes the file or ter-
  minates. In Unix software this is a common idiom for handling temporary
  files that are supposed to disappear when the program exits: You create
  them for reading and writing and “delete” them immediately afterwards
  without closing them within your program. You can then write data to the
  file and later jump back to the beginning to reread them.

B You can invoke ln not just with two file name arguments but also with one
  or with many. In the first case, a link with the same name as the original
  will be created in the current directory (which should really be different
  from the one where the file is located), in the second case all named files
  will be “linked” under their original names into the diréctory given as the
  last argument (think mv ).

    You can use the “cp -l ” command to create a “link farm”. This means that link farm
instead of copying the files to the destination (as would otherwise be usual), links
to the originals will be created:

$ mkdir prog-1.0.1                                                     New directory
$ cp -l prog-1.0/* prog-1.0.1
78                                                                                                 6 Files: Care and Feeding

                        The advantage of this approach is that the files still exist only once on the disk, and
                        thus take up space only once. With today’s prices for disk storage this may not be
                        compellingly necessary—but a common application of this idea, for example, con-
                        sists of making periodic backup copies of large file hierarchies which should ap-
                        pear on the backup medium (disk or remote computer) as separate, date-stamped
                        file hierarchies. Experience teaches that most files only change very rarely, and
                        if these files then need to be stored just once instead of over and over again, this
                        tends to add up over time. In addition, the files do not need to be written to the
                        backup medium time and again, and that can save considerable time.

                         B Backup packages that adopt this idea include, for example, Rsnapshot (http:
                           // ) or Dirvish ( ).

                        A This approach should be taken with a certain amount of caution. Using
                          links may let you “deduplicate” identical files, but not identical directo-
                          ries. This means that for every date-stamped file hierarchy on the backup
                          medium, all directories must be created anew, even if the directories only
                          contain links to existing files. This can lead to very complicated directory
                          structures and, in the extreme case, to consistency checks on the backup
                          medium failing because the computer does not have enough virtual mem-
                          ory to check the directory hierarchy.

                        A You will also need to watch out if – as alluded to in the example – you make
                          a “copy” of a program’s source code as a link farm (which in the case of,
                          e. g., the Linux source code could really pay off): Before you can modify a
                          file in your newly-created version, you will need to ensure that it is really a
                          separate file and not just a link to the original (which you will very probably
                          not want to change). This means that you either need to manually replace
                          the link to the file by an actual copy of the file, or else use an editor which
                          writes modified versions as separate files automatically2 .

                             This is not all, however: There are two different kinds of link in Linux systems.
                          The type explained above is the default case for the ln command and is called a
                          “hard link”. It always uses a file’s inode number for identification. In addition,
          symbolic links there are symbolic links (also called “soft links” in contrast to “hard links”). Sym-
                          bolic links are really files containing the name of the link’s “target file”, together
                          with a flag signifying that the file is a symbolic link and that accesses should be
                          redirected to the target file. Unlike with hard links, the target file does not “know”
                          about the symbolic link. Creating or deleting a symbolic link does not impact the
                          target file in any way; when the target file is removed, however, the symbolic link
                          “dangles”, i.e., points nowhere (accesses elicit an error message).
     Links to directories    In contrast to hard links, symbolic links allow links to directories as well as files
                          on different physical file systems. In practice, symbolic links are often preferred,
                          since it is easier to keep track of the linkage by means of the path name.

                         B Symbolic links are popular if file or directory names change but a certain
                           backwards compatibility is desired. For example, it was agreed that user
                           mailboxes (that store unread e-mail) should be stored in the /var/mail di-
                           rectory. Traditionally, this directory was called /var/spool/mail , and many
                           programs hard-code this value internally. To ease a transition to /var/mail ,
                           a distribution can set up a symbolic link under the name of /var/spool/mail
                           which points to /var/mail . (This would be impossible using hard links, since
                           hard links to directories are not allowed.)

                            To create a symbolic link, you must pass the -s option to ln :

                         $ ln -s /var/log short
                         $ ls -l
                           2 If you use Vim (a. k. a vi , you can add the “set backupcopy=auto,breakhardlink ” command to the .vimrc

                        file in your home directory.
6.4 Handling Files                                                                                                 79

-rw-r--r--   1   joe   users   2500   Oct 4 11:11 liste2
lrwxrwxrwx   1   joe   users     14   Oct 4 11:40 short -> /var/log
$ cd short
$ pwd -P

Besides the -s option to create “soft links”, the ln command supports (among oth-
ers) the -b , -f , -i , and -v options discussed earlier on.
   To remove symbolic links that are no longer required, delete them using rm just
like plain files. This operation applies to the link rather than the link’s target.

$ cd
$ rm short
$ ls

   As you have seen above, “ls -l ” will, for symbolic links, also display the file
that the link is pointing to. With the -L and -H options, you can get ls to resolve
symbolic links directly:

$ mkdir dir
$ echo XXXXXXXXXX >dir/file
$ ln -s file dir/symlink
$ ls -l dir
total 4
-rw-r--r-- 1 hugo users 11 Jan 21 12:29 file
lrwxrwxrwx 1 hugo users 5 Jan 21 12:29 symlink -> file
$ ls -lL dir
-rw-r--r-- 1 hugo users 11 Jan 21 12:29 file
-rw-r--r-- 1 hugo users 11 Jan 21 12:29 symlink
$ ls -lH dir
-rw-r--r-- 1 hugo users 11 Jan 21 12:29 file
lrwxrwxrwx 1 hugo users 5 Jan 21 12:29 symlink -> file
$ ls -l dir/symlink
lrwxrwxrwx 1 hugo users 5 Jan 21 12:29 dir/symlink -> file
$ ls -lH dir/symlink
-rw-r--r-- 1 hugo users 11 Jan 21 12:29 dir/symlink

The difference between -L and -H is that the -L option always resolves symbolic links
and displays information about the actual file (the name shown is still always the
one of the link, though). The -H , as illustrated by the last three commands in the
example, does that only for links that have been directly given on the command
   By analogy to “cp -l ”, the “cp -s ” command creates link farms based on sym-         cp   and symbolic links
bolic links. These, however, are not quite as useful as the hard-link-based ones
shown above. “cp -a ” copies directory hierarchies as they are, keeping symbolic
links as they are; “cp -L ” arranges to replace symbolic links by their targets in the
copy, and “cp -P ” precludes that.

C 6.15 [!2] In your home directory, create a file with arbitrary content (e. g.,
  using “echo Hello >~/hello ” or a text editor). Create a hard link to that file
  called link . Make sure that the file now has two names. Try changing the
  file with a text editor. What happens?

C 6.16 [!2] Create a symbolic link called ~/symlink to the file in the previous ex-
  ercise. Check whether accessing the file via the symbolic link works. What
  happens if you delete the file (name) the symbolic link is pointing to?
80                                                                                                   6 Files: Care and Feeding

                                                          Table 6.4: Keyboard commands for more

                                         Key                  Result
                                          ↩                   Scrolls up a line
                                                              Scrolls up a screenful
                                              b               Scrolls back a screenful
                                              h               Displays help
                                              q               Quits more
                                   / ⟨word⟩ ↩                 Searches for ⟨word⟩
                               !   ⟨command⟩ ↩                Executes ⟨command⟩ in a subshell
                                              v               Invokes editor (vi )
                                       Ctrl       +   l       Redraws the screen

                       C 6.17 [!2] What directory does the .. link in the “/ ” directory point to?

                       C 6.18 [3] Consider the following command and its output:
                               $ ls -ai /
                                    2 .                   330211   etc         1   proc   4303 var
                                    2 ..                       2   home    65153   root
                                 4833 bin                 244322   lib    313777   sbin
                               228033 boot                460935   mnt    244321   tmp
                               330625 dev                 460940   opt    390938   usr

                               Obviously, the / and /home directories have the same inode number. Since
                               the two evidently cannot be the same directory—can you explain this phe-

                       C 6.19 [3] We mentioned that hard links to directories are not allowed. What
                         could be a reason for this?

                       C 6.20 [3] How can you tell from the output of “ls -l ~ ” that a subdirectory of
                         ~ contains no further subdirectories?

                       C 6.21 [2] How do “ls -lH ” and “ls -lL ” behave if a symbolic link points to a
                         different symbolic link?

                       C 6.22 [3] What is the maximum length of a “chain” of symbolic links? (In
                         other words, if you start with a symbolic link to a file, how often can you
                         create a symbolic link that points to the previous symbolic link?)

                       C 6.23 [4] (Brainteaser/research exercise:) What requires more space on disk,
                         a hard link or a symbolic link? Why?

                       6.4.3       Displaying File Content—more and less
     display of text files A convenient display of text files on screen is possible using the more command,
                      which lets you view long documents page by page. The output is stopped after
                      one screenful, and “--More-- ” appears in the final line (possibly followed by the
                      percentage of the file already displayed). The output is continued after a key press.
                      The meanings of various keys are explained in table 6.4.
              Options     more also understands some options. With -s (“squeeze”), runs of empty lines
                      are compressed to just one, the -l option ignores page ejects (usually represented
                      by “^L ”) which would otherwise stop the output. The -n ⟨number⟩ option sets the
                      number of screen lines to ⟨number⟩, otherwise more takes the number from the
                      terminal definition pointed to by TERM .
                          more ’s output is still subject to vexing limitations such as the general impossibil-
                      ity of moving back towards the beginning of the output. Therefore, the improved
6.4 Handling Files                                                                                           81

                                         Table 6.5: Keyboard commands for less

                             Key                     Result
        ↓   or          e    or      j   or      ↩   Scrolls up one line
                         f   or                      Scrolls up one screenful
                ↑       or    y    or        k       Scrolls back one line
                              b                      Scrolls back one screenful
                    Home      or g                   Jumps to the beginning of the text
        End             or Shift ⇑ +             g   Jumps to the end of the text
                p       ⟨percent⟩ ↩                  Jumps to position ⟨percent⟩ (in %) of the text
                              h                      Displays help
                              q                      Quits less
                    /   ⟨word⟩           ↩           Searches for ⟨word⟩ towards the end
                              n                      Continues search towards the end
                  ⟨word⟩ ↩
                    ?                                Searches for ⟨word⟩ towards the beginning
                 Shift ⇑ + n                         Continues search towards the beginning
            !   ⟨command⟩ ↩                          Executes ⟨command⟩ in subshell
                              v                      Invokes editor (vi )
                r       or    Ctrl       +   l       Redraws screen

version less (a weak pun—think “less is more”) is more [sic!] commonly seen to-                       less
day. less lets you use the cursor keys to move around the text as usual, the search
routines have been extended and allow searching both towards the end as well
as towards the beginning of the text. The most common keyboard commands are
summarised in table 6.5.
    As mentioned in chapter 4, less usually serves as the display program for man-
ual pages via man . All the commands are therefore available when perusing man-
ual pages.

6.4.4       Searching Files—find
Who does not know the following feeling: “There used to be a file foobar … but
where did I put it?” Of course you can tediously sift through all your directories
by hand. But Linux would not be Linux if it did not have something handy to help
    The find command searches the directory tree recursively for files matching a
set of criteria. “Recursively” means that it considers subdirectories, their subdirec-
tories and so on. find ’s result consists of the path names of matching files, which
can then be passed on to other programs. The following example introduces the
command structure:

$ find . -user joe -print

This searches the current directory including all subdirectories for files belonging
to the user joe . The -print command displays the result (a single file in our case)
on the terminal. For convenience, if you do not specify what to do with matching
files, -print will be assumed.
    Note that find needs some arguments to go about its task.

Starting Directory The starting directory should be selected with care. If you
pick the root directory, the required file(s)—if they exist—will surely be found,
but the search may take a long time. Of course you only get to search those files
where you have appropriate privileges.
82                                                                                              6 Files: Care and Feeding

Absolute or relative path names?    B An absolute path name for the start directory causes the file names in the
                                      output to be absolute, a relative path name for the start directory accord-
                                      ingly produces relative path names.

                   Directory list      Instead of a single start directory, you can specify a list of directories that will
                                    be searched in turn.

                                    Test Conditions These options describe the requirements on the files in detail.
                                    Table 6.6 shows the most important tests. The find documentation explains many

                                                             Table 6.6: Test conditions for find

                                         Test    Description
                                        -name    Specifies a file name pattern. All shell search pattern characters
                                                 are allowed. The -iname option ignores case differences.
                                        -type    Specifies a file type (see section 10.2). This includes:
                                                   b   block device file
                                                   c   character device file
                                                   d   directory
                                                   f   plain file
                                                   l   symbolic link
                                                   p   FIFO (named pipe)
                                                   s   Unix domain socket
                                        -user    Specifies a user that the file must belong to. User names as well
                                                 as numeric UIDs can be given.
                                        -group   Specifies a group that the file must belong to. As with -user , a
                                                 numeric GID can be specified as well as a group name.
                                        -size    Specifies a particular file size. Plain numbers signify 512-byte
                                                 blocks; bytes or kibibytes can be given by appending c or k , re-
                                                 spectively. A preceding plus or minus sign stands for a lower or
                                                 upper limit; -size +10k , for example, matches all files bigger than
                                                 10 KiB.
                                        -atime   (engl. access) allows searching for files based on the time of last
                                                 access (reading or writing). This and the next two tests take their
                                                 argument in days; …min instead of …time produces 1-minute ac-
                                        -mtime   (engl. modification) selects according to the time of modification.
                                        -ctime   (engl. change) selects according to the time of the last inode
                                                 change (including access to content, permission change, renam-
                                                 ing, etc.)
                                        -perm    Specifies a set of permissions that a file must match. The per-
                                                 missions are given as an octal number (see the chmod command.
                                                 To search for a permission in particular, the octal number must
                                                 be preceded by a minus sign, e.g., -perm -20 matches all files with
                                                 group write permission, regardless of their other permissions.
                                        -links   Specifies a reference count value that eligible files must match.
                                         -inum   Finds links to a file with a given inode number.

                  Multiple tests        If multiple tests are given at the same time, they are implicitly ANDed together—
                                    all of them must match. find does support additional logical operators (see ta-
                                    ble 6.7).
                                        In order to avoid mistakes when evaluating logical operators, the tests are best
                                    enclosed in parentheses. The parentheses must of course be escaped from the
                                    $ find . \( -type d -o -name "A*" \) -print
6.4 Handling Files                                                                                 83

                                       Table 6.7: Logical operators for find

       Option    Operator     Meaning
       !             Not      The following test must not match
       -a            And      Both tests to the left and right of -a must match
       -o             Or      At least one of the tests to the left and right of -o must match

$ _

This example lists all names that either refer to directories or that begin with “A ”
or both.

Actions As mentioned before, the search results can be displayed on the screen
using the -print option. In addition to this, there are two options, -exec and -
ok , which execute commands incorporating the file names. The single difference Executing commands
between -ok and -exec is that -ok asks the user for confirmation before actually exe-
cuting the command; with -exec , this is tacitly assumed. We will restrict ourselves
to discussing -exec .
     There are some general rules governing the -exec option:
   • The command following -exec must be terminated with a semicolon (“; ”).
     Since the semicolon is a special character in most shells, it must be escaped
     (e.g., as “\\; ” or using quotes) in order to make it visible to find .
   • Two braces (“{} ”) within the command are replaced by the file name that
     was found. It is best to enclose the braces in quotes to avoid problems with
     spaces in file names.
For example:

$ find . -user joe -exec ls -l '{}' \;
-rw-r--r-- 1 joe users      4711 Oct 4 11:11 file.txt
$ _

This example searches for all files within the current directory (and below) be-
longing to user test , and executes the “ls -l ” command for each of them. The
following makes more sense:

$ find . -atime +13 -exec rm -i '{}' \;

This interactively deletes all files within the current directory (and below) that
have not been accessed for two weeks.

B Sometimes—say, in the last example above—it is very inefficient to use -
  exec to start a new process for every single file name found. In this case,
  the xargs command, which collects as many file names as possible before
  actually executing a command, can come in useful:

       $ find . -atime +13 | xargs -r rm -i

      xargs  reads its standard input up to a (configurable) maximum of characters
      or lines and uses this material as arguments for the specified command (here
      rm ). On input, arguments are separated by space characters (which can be
      escaped using quotes or “\ ”) or newlines. The command is invoked as often
84                                                                6 Files: Care and Feeding

             as necessary to exhaust the input.—The -r option ensures that rm is executed
             only if find actually sends a file name; otherwise it would be executed at least

     B Weird filenames can get the find /xargs combination in trouble, for example
       ones that contain spaces or, indeed, newlines which may be mistaken as
       separators. The silver bullet consists of using the “-print0 ” option to find ,
       which outputs the file names just as “-print ” does, but uses null bytes to
       separate them instead of newlines. Since the null byte is not a valid character
       in path names, confusion is no longer possible. xargs must be invoked using
       the “-0 ” option to understand this kind of input:

             $ find . -atime +13 -print0 | xargs -0r rm -i

     C 6.24 [!2] Find all files on your system which are longer than 1 MiB, and
       output their names.

     C 6.25 [2] How could you use find to delete a file with an unusual name (e. g.,
       containing invisible control characters or umlauts that older shells cannot
       deal with)?

     C 6.26 [3] (Second time through the book.) How would you ensure that files
       in /tmp which belong to you are deleted once you log out?

     6.4.5     Finding Files Quickly—locate and slocate
     The find command searches files according to many different criteria but needs to
     walk the complete directory tree below the starting directory. Depending on the
     tree size, this may take considerable time. For the typical application—searching
     files with particular names—there is an accelerated method.
        The locate command lists all files whose names match a given shell wildcard
     pattern. In the most trivial case, this is a simple string of characters:

     $ locate letter.txt

     A Although locate is a fairly important service (as emphasised by the fact that
       it is part of the LPIC1 curriculum), not all Linux distributions include it as
       part of the default installation.

             For example, if you are using a SUSE distribution, you must explicitly install
             the findutils-locate package before being able to use locate .

     The “* ”, “? ”, and “[ …] ” characters mean the same thing to locate as they do to
     the shell. But while a query without wildcard characters locates all file names that
     contain the pattern anywhere, a query with wildcard characters returns only those
     names which the pattern describes completely—from beginning to end. Therefore
     pattern queries to locate usually start with “* ”:

     $ locate "*/letter.t*"
6.4 Handling Files                                                                       85

B Be sure to put quotes around locate queries including shell wildcard char-
  acters, to keep the shell from trying to expand them.

The slash (“/ ”) is not handled specially:

$ locate Letters/granny

   locate is so fast because it does not walk the file system tree, but checks a
“database” of file names that must have been previously created using the updat-
edb program. This means that locate does not catch files that have been added to
the system since the last database update, and conversely may output the names
of files that have been deleted in the meantime.

B You can get locate to return existing files only by using the “-e ” option, but
  this negates locate ’s speed advantage.

   The updatedb program constructs the database for locate . Since this may take
considerable time, your system administrator usually sets this up to run when the
system does not have a lot to do, anyway, presumably late at night.

B The cron service which is necessary for this will be explained in detail in
  Advanced Linux. For now, remember that most Linux distributions come
  with a mechanism which causes updatedb to be run every so often.

As the system administrator, you can tell updatedb which files to consider when
setting up the database. How that happens in detail depends on your distribution:
updatedb itself does not read a configuration file, but takes its settings from the
command line and (partly) environment variables. Even so, most distributions
call updatedb from a shell script which usually reads a file like /etc/updatedb.conf or
/etc/sysconfig/locate , where appropriate environment variables can be set up.

B You may find such a file, e.g., in /etc/cron.daily (details may vary according
  to your distribution).

   You can, for instance, cause updatedb to search certain directories and omit oth-
ers; the program also lets you specify “network file systems” that are used by sev-
eral computers and that should have their own database in their root directories,
such that only one computer needs to construct the database.

B An important configuration setting is the identity of the user that runs up-
  datedb . There are essentially two possibilities:

         • updatedb runs as root and can thus enter every file in its database. This
           also means that users can ferret out file names in directories that they
           would not otherwise be able to look into, for example, other users’
           home directories.
         • updatedb runs with restricted privileges, such as those of user nobody . In
           this case, only names within directories readable by nobody end up in
           the database.

B The slocate program—an alternative to the usual locate —circumvents this
  problem by storing a file’s owner, group and permissions in the database in
  addition to the file’s name. It outputs a file name only if the user who runs
  slocate can, in fact, access the file in question. slocate comes with an updatedb
  program, too, but this is merely another name for slocate itself.

B In many cases, slocate is installed such that it can also be invoked using the
  locate command.
86                                                              6 Files: Care and Feeding

     C 6.27 [!1] README is a very popular file name. Give the absolute path names of
       all files on your system called README .

     C 6.28 [2] Create a new file in your home directory and convince yourself by
       calling locate that this file is not listed (use an appropriately outlandish file
       name to make sure). Call updatedb (possibly with administrator privileges).
       Does locate find your file afterwards? Delete the file and repeat these steps.

     C 6.29 [1] Convince yourself that the slocate program works, by searching for
       files like /etc/shadow as normal user.

     Commands in this Chapter
     cd       Changes a shell’s current working directory                    bash (1) 67
     convmv   Converts file names between character encodings               convmv (1) 64
     cp       Copies files                                                      cp (1) 74
     find     Searches files matching certain given criteria     find (1), Info: find 81
     less     Displays texts (such as manual pages) by page                  less (1) 80
     ln       Creates (“hard” or symbolic) links                               ln (1) 76
     locate   Finds files by name in a file name database                    locate (1) 84
     ls       Lists file information or directory contents                      ls (1) 67
     mkdir    Creates new directories                                       mkdir (1) 69
     more     Displays text data by page                                     more (1) 80
     mv       Moves files to different directories or renames them               mv (1) 75
     pwd      Displays the name of the current working directory pwd (1), bash (1) 67
     rm       Removes files or directories                                      rm (1) 75
     rmdir    Removes (empty) directories                                   rmdir (1) 70
     slocate  Searches file by name in a file name database, taking file permissions into
              account                                                     slocate (1) 85
     updatedb Creates the file name database for locate                  updatedb (1) 85
     xargs    Constructs command lines from its standard input
                                                               xargs (1), Info: find 83

        • Nearly all possible characters may occur in file names. For portability’s sake,
          however, you should restrict yourself to letters, digits, and some special
        • Linux distinguishes between uppercase and lowercase letters in file names.
        • Absolute path names always start with a slash and mention all directories
          from the root of the directory tree to the directory or file in question. Relative
          path names start from the “current directory”.
        • You can change the current directory of the shell using the cd command.
          You can display its name using pwd .
        • ls displays information about files and directories.
        • You can create or remove directories using mkdir and rmdir .
        • The cp , mv and rm commands copy, move, and delete files and directories.
        • The ln command allows you to create “hard” and “symbolic” links.
        • more and less display files (and command output) by pages on the terminal.
        • find searches for files or directories matching certain criteria.
                                                                                   $ echo tux
                                                                                   $ ls
                                                                                   $ /bin/su -

Regular Expressions

7.1  Regular Expressions: The Basics . . . . . . . . . . . . . . 88
   7.1.1 Regular Expressions: Extras . . . . . . . . . . . . . . 88
7.2 Searching Files for Text—grep . . . . . . . . . . . . . . . 89

      • Understanding and being able to formulate simple and extended regular
      • Knowing the grep program and its variants, fgrep and egrep

      • Basic knowledge of Linux, the shell, and Linux commands (e. g., from the
        preceding chapters)
      • Handling of files and directories (chapter 6)
      • Use of a text editor (chapter 5)

grd1-regex.tex   (be27bba8095b329b )
88                                                                                                  7 Regular Expressions

                      7.1       Regular Expressions: The Basics
                       Many Linux commands are used for text processing—patterns of the form “do 𝑥𝑦𝑧
                       for all lines that look like this” appear over and over again. A very powerful tool to
                       describe bits of text, most commonly lines of files, is called “regular expressions”1 .
                       At first glance, regular expressions resemble the shell’s file name search patterns
                       (section 6.3), but they work differently and offer more possibilities.
                           Regular expressions are often constructed “recursively” from primitives that
                       are themselves considered regular expressions. The simplest regular expressions
           characters are letters, digits and many other characters from the usual character set, which
                       stand for themselves. “a ”, for example, is a regular expression matching the “a ”
     Character classes character; the regular expression “abc ” matches the string “abc ”. Character classes
                       can be defined in a manner similar to shell search patterns; therefore, the regular
                       expression “[a-e] ” matches exactly one character out of “a ” to “e ”, and “a[xy]b ”
                       matches either “axb ” or “ayb ”. As in the shell, ranges can be concatenated—
        complement ”[A-Za-z] ” matches all uppercase and lowercase letters—but the complement of
                       a range is constructed slightly differently: “[^abc] ” matches all characters except
                       “a ”, “b ”, and “c ”. (In the shell, that was “[!abc] ”.) The dot, “. ”, corresponds to
                       the question mark in shell search patterns, in that it will match a single arbitrary
                       character—the only exception is the newline character, “\n ”. Thus, “a.c ” matches
                       “abc ”, “a/c ” and so on, but not the multi-line construction

                       This is due to the fact that most programs operate on a per-line basis, and multi-
                       line constructions would be more difficult to process. (Which is not to say that it
                       wouldn’t sometimes be nice to be able to do it.)
                           While shell search patterns must always match beginning at the start of a file
                       name, in programs selecting lines based on regular expressions it usually suffices
                       if the regular expression matches anywhere in a line. You can restrict this, how-
            Line start ever: A regular expression starting with a caret (“^ ”) matches only at the begin-
                       ning of a line, and a regular expression finishing with a dollar sign (“$ ”) matches
            Line end only at the end. The newline character at the end of each line is ignored, so you
                       can use “xyz$ ” to select all lines ending in “xyz ”, instead of having to write “xyz\n$ ”.

                       B Strictly speaking, “^ ” and “$ ” match conceptual “invisible” characters at the
                         beginning of a line and immediately to the left of the newline character at
                         the end of a line, respectively.

                           Finally, you can use the asterisk (“* ”) to denote that the preceding regular ex-
           Repetition pression may be repeated arbitrarily many times (including not at all). The aster-
                    isk itself does not stand for any characters in the input, but only modifies the pre-
                    ceding expression—consequently, the shell search pattern “a*.txt ” corresponds to
                    the regular expression “^a.*\\.txt ” (remember the “anchoring” of the expression
                    to the beginning and end of the input line and that an unescaped dot matches any
         precedence character). Repetition has precedence over concatenation; “ab* ” is a single “a ” fol-
                    lowed by arbitrarily many “b ” (including none at all), not an arbitrary number of
                    repetitions of “ab ”.

                      7.1.1      Regular Expressions: Extras
                      The previous section’s explanations apply to nearly all Linux programs that deal
           extensions with regular expressions. Various programs support different extensions provid-
                         1 This is originally a term from computer science and describes a method of characterization of

                      sets of strings that result from the concatenation of “letters”, choices from a set of letters, and their
                      potentially unbounded repetition. Routines to recognize regular expressions are elementary building
                      blocks of many programs such as programming language compilers. Regular expressions appeared
                      very early in the history of Unix; most of the early Unix developers had a computer science background,
                      so the idea was well-known to them.
7.2 Searching Files for Text—grep                                                                89

ing either notational convenience or additional functionality. The most advanced
implementations today are found in modern scripting languages like Tcl, Perl or
Python, whose implementations by now far exceed the power of regular expres-
sions in their original computer science sense.
   Some common extensions are:

Word brackets The “\< ” matches the beginning of a word (a place where a non-
     letter precedes a letter). Analogously, “\> ” matches the end of a word (where
     a letter is followed by a non-letter).
Grouping Parentheses (“( …) ”) allow for the repetition of concatenations of reg-
    ular expressions: “a(bc)* ” matches a “a ” followed by arbitrarily many repe-
    titions of “bc ”.
Alternative With the vertical bar (“| ”) you can select between several regular ex-
     pressions. The expression “motor (bike|cycle|boat)” matches “motor bike ”,
     “motor cycle ”, and “motor boat ” but nothing else.

Optional Expression The question mark (“? ”) makes the preceding regular ex-
     pression optional, i. e., it must occur either once or not at all. “ferry(man)? ”
     matches either “ferry ” or “ferryman ”.
At-Least-Once Repetition The plus sign (“+ ”) corresponds to the repetition op-
     erator “* ”, except that the preceding regular expression must occur at least
Given Number of Repetitions You can specify a minimum and maximum num-
     ber of repetitions in braces: “ab{2,4} ” matches “abb ”, “abbb ”, and “abbbb ”, but
     not “ab ” or “abbbbb ”. You may omit the minimum as well as the maximum
     number; if there is no minimum number, 0 is assumed, if there is no maxi-
     mum number, “infinity” is assumed.

Back-Reference With an expression like “\\ 𝑛” you may call for a repetition of
     that part of the input that matched the parenthetical expression no. 𝑛 in the
     regular expression. “(ab)\\1 ”, for example, matches “abab ”, and if, when
     processing “(ab*a)x\1 ”, the parentheses matched abba , then the whole ex-
     pression matches abbaxabba (and nothing else). More detail is available in
     the documentation of GNU grep .
Non-Greedy Matching The “* ”, “+ ”, and “? ” operators are usually “greedy”, i. e.,
    they try to match as much of the input as possible: “^a.*a ” applied to the in-
    put string “abacada ” matches “abacada ”, not “aba ” or “abaca ”. However, there
    are corresponding “non-greedy” versions “*? ”, “+? ”, and “?? ” which try
    to match as little of the input as possible. In our example, “^a.*?a ” would
    match “aba ”. The braces operator may also offer a non-greedy version.
   Not every program supports every extension. table 7.1 shows an overview of
the most important programs. Emacs, Perl and Tcl in particular support lots of
extensions that have not been discussed here.

7.2     Searching Files for Text—grep
Possibly one of the most important Linux programs using regular expressions is
grep . It searches one or more files for lines matching a given regular expression.
Matching lines are output, non-matching lines are discarded.
   There are two varieties of grep : Traditionally, the stripped-down fgrep (“fixed”) Varieties
does not allow regular expressions—it is restricted to character strings—but is
very fast. egrep (“extended”) offers additional regular expression operators, but is
a bit slower and needs more memory.
90                                                                                                  7 Regular Expressions

                                          Table 7.1: Regular expression support

                 Extension              GNU grep     GNU egrep    trad egrep   vim   emacs   Perl    Tcl
                Word brackets           •           •              •       •1     •1      •4    •4
                Grouping               •1           •              •       •1     •1       •    •
                Alternative            •1           •              •       •2     •1       •    •
                Option                 •1           •              •       •3      •       •    •
                At-least-once          •1           •              •       •1      •       •    •
                Limits                 •1           •              ∘       •1     •1       •    •
                Back-Reference          ∘           •              •        ∘      •       •    •
                Non-Greedy              ∘           ∘              ∘       •4      •       •    •
     •: supported; ∘: not supported
     Notes: 1. Requires a preceding backslash (“\ ”), e. g. “ab\+ ” instead of “ab+ ”. 2. Needs no parenthe-
     ses; alternatives always refer to the complete expression. 3. Uses “\= ” instead of “? ”. 4. Completely
     different syntax (see documentation).

                                           Table 7.2: Options for grep (selected)

           Option           Result
      -c       (count)      Outputs just the number of matching lines
      -i      (ignore)      Uppercase and lowercase letters are equivalent
      -l          (list)    Outputs just the names of matching files, no actual matches
      -n    (number)        Includes line numbers of matching lines in the output
      -r   (recursive)      Searches files in subdirectories as well
      -v      (invert)      Outputs only lines that do not match the regular expression

                                     B These observations used to be true to some extent. In particular, grep and
                                       egrep used completely different algorithms for regular expression evalua-
                                       tion, which could lead to wildly diverging performance results depending
                                       on the size and structure of the regular expressions as well as the size of the
                                       input. With the common Linux implementation of grep , all three variants
                                       are, in fact, the same program; they differ mostly in the allowable syntax for
                                       their search patterns.

                           syntax  grep ’s syntax requires at least a regular expression to search for. This is followed
                                by the name of a text file (or files) to be searched. If no file name is specified, grep
                                refers to standard input (see chapter 8).
             regular expression    The regular expression to search in the input may contain, besides the basic
                                regular expressions from section 7.1, most of the extensions from section 7.1.1.
                                With grep , however, the operators “\+ ”, “\? ”, and “\{ ” must be preceded by a back-
                                slash. (For egrep , this is not necessary.) There are unfortunately no “non-greedy”

                                     B You should put the regular expression in single quotes to prevent the shell
                                       from trying to expand it, especially if it is more complicated than a simple
                                       character string, and definitely if it resembles a shell search pattern.

                                  In addition to the regular expression and file names, various options can be passed
                                  on the command line (see table 7.2).
           Search pattern in file    With the -f (“file”) option, the search pattern can be read from a file. If that
                                  file contains several lines, the content of every line will be considered a search
                                  pattern in its own right, to be searched simultaneously. This can simplify things
                                  considerably especially for frequently used search patterns.
                                     As mentioned above, fgrep does not allow regular expressions as search pat-
                                  terns. egrep , on the other hand, makes most extensions for regular expressions
                                  more conveniently available (table 7.1).
7.2 Searching Files for Text—grep                                                               91

   Finally some examples for grep . The frog.txt file contains the Brothers Grimm
fairytale of the Frog King (see appendix B). All lines containing the character se-
quence frog can be easily found as follows:

$ grep frog frog.txt
frog stretching forth its big, ugly head from the water. »Ah, old
»Be quiet, and do not weep,« answered the frog, »I can help you, but
»Whatever you will have, dear frog,« said she, »My clothes, my pearls

To find all lines containing exactly the word “frog ” (and not combinations like
“bullfrog ” or “frogspawn ”), you need the word bracket extension:

$ grep \<frog\> frog.txt
frog stretching forth its big, ugly head from the water. »Ah, old

(it turns out that this does not in fact make a difference in the English translation).
It is as simple to find all lines beginning with “frog ”:

$ grep ^frog frog.txt
frog stretching forth its big, ugly head from the water. »Ah, old
frog, that he had caused three iron bands to be laid round his heart,

   A different example: The file /usr/share/dict/words contains a list of English
words (frequently called the “dictionary”)2 . We’re interested in all words con-
taining three or more “a ”:

$ grep -n 'a.*a.*a' /usr/share/dict/words
                                                                     … 7030 more words …

(in order: an African animal (Orycteropus afer), a banana plant used for fibre (Musa
textilis), the Brazilian name for the avocado (Persea sp.), a facial muscle and two
adjectives from the same—medical—area of interest.)

B With more complicated regular expressions, it can quickly become unclear
  why grep outputs one line but not another. This can be mitigated to a certain
  extent by using the --color option, which displays the matching part(s) in a
  file in a particular colour:

          $ grep --color root /etc/passwd

      A command like export GREP_OPTIONS='--color=auto' (for example, in ~/.profile )
      enables this option on a permanent basis; the auto argument suppresses
      colour output if the output is sent to a pipe or file.

C 7.1 [2] Are the ? and + regular expressions operators really necessary?
  2 The   size of the dictionary may vary wildly depending on the distribution.
92                                                                  7 Regular Expressions

     C 7.2 [!1] In frog.txt , find all lines containing the words “king” or “king’s

     C 7.3 [!2] In /etc/passwd there is a list of users on the system (most of the time,
       anyway). Every line of the file consists of a sequence of fields separated by
       colons. The last field in each line gives the login shell for that user. Give a
       grep command line to find all users that use bash as their login shell.

     C 7.4 [3] Search /usr/share/dict/words for all words containing exactly the five
       vowels “a ”, “e ”, “i ”, “o ”, and “u ”, in that order (possibly with consonants in
       front, in between, and at the end).

     C 7.5 [4] Give a command to locate and output all lines from the “Frog King”
       in which a word of at least four letters occurs twice.

     Commands in this Chapter
     egrep   Searches files for lines matching specific regular expressions; extended
             regular expressions are allowed                              grep (1) 89
     fgrep   Searches files for lines with specific content; no regular expressions al-
             lowed                                                       fgrep (1) 89
     grep    Searches files for lines matching a given regular expression grep (1) 89

        • Regular expressions are a powerful method for describing sets of character
        • grep and its relations search a file’s content for lines matching regular ex-
                                                                                         $ echo tux
                                                                                         $ ls
                                                                                         $ /bin/su -

Standard I/O and Filter

8.1  I/O Redirection and Command Pipelines . . . .     .   .   .   .   .   .   .    94
   8.1.1 Standard Channels . . . . . . . . . .         .   .   .   .   .   .   .    94
   8.1.2 Redirecting Standard Channels . . . . . .     .   .   .   .   .   .   .    95
   8.1.3 Command Pipelines . . . . . . . . . .         .   .   .   .   .   .   .    98
8.2 Filter Commands . . . . . . . . . . . . .          .   .   .   .   .   .   .    99
8.3 Reading and Writing Files . . . . . . . . . .      .   .   .   .   .   .   .   100
   8.3.1 Outputting and Concatenating Text Files—cat   .   .   .   .   .   .   .   100
   8.3.2 Beginning and End—head and tail . . . . .     .   .   .   .   .   .   .   100
8.4 Data Management . . . . . . . . . . . .            .   .   .   .   .   .   .   101
   8.4.1 Sorted Files—sort and uniq . . . . . . .      .   .   .   .   .   .   .   101
   8.4.2 Columns and Fields—cut , paste etc. . . . .   .   .   .   .   .   .   .   106

      • Mastering shell I/O redirection
      • Knowing the most important filter commands

      • Shell operation (see chapter 2)
      • Use of a text editor (see chapter 5)
      • File and directory handling (see chapter 6)

grd1-filter-opt.tex   (be27bba8095b329b )
94                                                                       8 Standard I/O and Filter Commands

                                                   stdin                      stdout
                                   Keyboard                   Process                     Screen

                                                   stdin                      stdout
                                   Keyboard                   Process                     Screen


                                               Figure 8.1: Standard channels on Linux

                           8.1     I/O Redirection and Command Pipelines
                           8.1.1       Standard Channels
                           Many Linux commands—like grep and friends from chapter 7—are designed to
                           read input data, manipulate it in some way, and output the result of these manip-
                           ulations. For example, if you enter

                           $ grep xyz

                           you can type lines of text on the keyboard, and grep will only let those pass that
                           contain the character sequence, “xyz”:

                           $ grep xyz
                           abc def
                           xyz 123
                           xyz 123
                           aaa bbb
                            Ctrl + d

                           (The key combination at the end lets grep know that the input is at an end.)
          standard input      We say that grep reads data from “standard input”—in this case, the keyboard—
          standard output and writes to “standard output”—in this case, the console screen or, more likely,
                           a terminal program in a graphical desktop environment. The third of these
     standard error output “standard channels” is “standard error output”; while the “payload data” grep
                           produces are written to standard output, standard error output takes any error
                           messages (e. g., about a non-existent input file or a syntax error in the regular
                              In this chapter you will learn how to redirect a program’s standard output to
                           a file or take a program’s standard input from a file. Even more importantly, you
                           will learn how to feed one program’s output directly (without the detour via a
                           file) into another program as that program’s input. This opens the door to using
                           the Linux commands, which taken on their own are all fairly simple, as building
                           blocks to construct very complex applications. (Think of a Lego set.)

                           B We will not be able to exhaust this topic in this chapter. Do look forward
                             to the manual, Advanced Linux, where constructing shell scripts with the
                             commands from the Unix “toolchest” plays a very important rôle! Here is
                             where you learn the very important fundamentals of cleverly combining
                             Linux commands even on the command line.
8.1 I/O Redirection and Command Pipelines                                                                     95

                                      Table 8.1: Standard channels on Linux

        Channel    Name                       Abbreviation     Device       Use
            0      standard input             stdin            keyboard     Input for programs
            1      standard output            stdout           screen       Output of programs
            2      standard error output      stderr           screen       Programs’ error messages

   The standard channels are summarised once more in table 8.1. In the pa- standard channels
tois, they are normally referred to using their abbreviated names—stdin , stdout
and stderr for standard input, standard output, and standard error output. These
channels are respectively assigned the numbers 0, 1, and 2, which we are going to
use later on.
   The shell can redirect these standard channels for individual commands, with- Redirection
out the programs in question noticing anything. These always use the standard
channels, even though the output might no longer be written to the screen or ter-
minal window but some arbitrary other file. That file could be a different device,
like a printer—but it is also possible to specify a text file which will receive the
output. That file does not even have to exist but will be created if required.
   The standard input channel can be redirected in the same way. A program no
longer receives its input from the keyboard, but takes it from the specified file,
which can refer to another device or a file in the proper sense.

B The keyboard and screen of the “terminal” you are working on (no matter
  whether this is a Linux text console, a “genuine” terminal on a serial port,
  a terminal window in a graphical environment, or a network session using,
  say, the secure shell) can be accessed by means of the /dev/tty file—if you
  want to read data this means the keyboard, for output the screen (the other
  way round would be quite silly). The

        $ grep xyz /dev/tty

        would be equivalent to our example earlier on in this section. You can find
        out more about such “special files” from chapter 10.)

8.1.2     Redirecting Standard Channels
You can redirect the standard output channel using the shell operator “> ” (the Redirecting standard output
“greater-than” sign). In the following example, the output of “ls -laF ” is redi-
rected to a file called filelist ; the screen output consists merely of

$ ls -laF >filelist
$ __

If the filelist file does not exist it is created. Should a file by that name exist,
however, its content will be overwritten. The shell arranges for this even before
the program in question is invoked—the output file will thus be created even if
the actual command invocation contained typos, or if the program did not indeed
write any output at all (in which case the filelist file will remain empty).

B If you want to avoid overwriting existing files using shell output redirection, Protecting existing files
  you can give the bash command “set -o noclobber ”. In this case, if output is
  redirected to an existing file, an error occurs.

   You can look at the filelist file in the usual way, e. g., using less :

$ less inhalt
total 7
96                                                                                       8 Standard I/O and Filter Commands

                                  drwxr-xr-x   12   joe    users    1024   Aug   26   18:55   ./
                                  drwxr-xr-x    5   root   root     1024   Aug   13   12:52   ../
                                  drwxr-xr-x    3   joe    users    1024   Aug   20   12:30   photos/
                                  -rw-r--r--    1   joe    users       0   Sep    6   13:50   filelist
                                  -rw-r--r--    1   joe    users   15811   Aug   13   12:33   pingu.gif
                                  -rw-r--r--    1   joe    users   14373   Aug   13   12:33   hobby.txt
                                  -rw-r--r--    2   joe    users    3316   Aug   20   15:14   chemistry.txt

                                  If you look closely at the content of filelist , you can see a directory entry for
                                  filelist with size 0. This is due to the shell’s way of doing things: When parsing
                                  the command line, it notices the output redirection first and creates a new filelist
                                  file (or removes its content). After that, the shell executes the command, in this
                                  case ls , while connecting ls ’s standard output to the filelist file instead of the

                                  B The file’s length in the ls output is 0 because the ls command looked at the
                                    file information for filelist before anything was written to that file – even
                                    though there are three other entries above that of filelist . This is because
                                    ls first reads all directory entries, then sorts them by file name, and only
                                    then starts writing to the file. Thus ls sees the newly created (or emptied)
                                    file filelist , with no content so far.
              Appending stan-        If you want to append a command’s output to an existing file without replacing
           dard output to a file its previous content, use the >> operator. If that file does not exist, it will be created
                                  in this case, too.
                                  $ date >> filelist
                                  $ less filelist
                                  total 7
                                  drwxr-xr-x 12 joe     users       1024   Aug   26   18:55   ./
                                  drwxr-xr-x   5 root   root        1024   Aug   13   12:52   ../
                                  drwxr-xr-x   3 joe    users       1024   Aug   20   12:30   photos/
                                  -rw-r--r--   1 joe    users          0   Sep    6   13:50   filelist
                                  -rw-r--r--   1 joe    users      15811   Aug   13   12:33   pingu.gif
                                  -rw-r--r--   1 joe    users      14373   Aug   13   12:33   hobby.txt
                                  -rw-r--r--   2 joe    users       3316   Aug   20   15:14   chemistry.txt
                                  Wed Oct 22 12:31:29 CEST 2003

                              In this example, the current date and time was appended to the filelist file.
                                 Another way to redirect the standard output of a command is by using “back-
         command substitution ticks” (` …` ). This is also called command substitution: The standard output of a
                              command in backticks will be inserted into the command line instead of the com-
                              mand (and backticks); whatever results from the replacement will be executed.
                              For example:

                                  $ cat dates                                                                  Our little diary
                                  22/12 Get presents
                                  23/12 Get Christmas tree
                                  24/12 Christmas Eve
                                  $ date +%d/%m                                                               What’s the date?
                                  $ grep `̂
                                          d ate +%d/%m.` dates                                                     What’s up?
                                  23/12 Get Christmas tree

                                  B A possibly more convenient syntax for “`date` ” is “$(date) ”. This makes it
                                    easier to nest such calls. However, this syntax is only supported by modern
                                    shells such as bash .
     Redirecting standard input      You can use < , the “less-than” sign, to redirect the standard input channel. This
                                  will read the content of the specified file instead of keyboard input:
8.1 I/O Redirection and Command Pipelines                                                                   97

$ wc -w <frog.txt

In this example, the wc filter command counts the words in file frog.txt .

B There is no << redirection operator to concatenate multiple input files; to
  pass the content of several files as a command’s input you need to use cat :

       $ cat file1 file2 file3 | wc -w

      (We shall find out more about the “| ” operator in the next section.) Most
      programs, however, do accept one or more file names as command line ar-

   Of course, standard input and standard output may be redirected at the same Simultaneous redirection
time. The output of the word-count example is written to a file called wordcount
$ wc -w <frog.txt >wordcount
$ cat wordcount

   Besides the standard input and standard output channels, there is also the stan- standard error output
dard error output channel. If errors occur during a program’s operation, the cor-
responding messages will be written to that channel. That way you will see them
even if standard output has been redirected to a file. If you want to redirect stan-
dard error output to a file as well, you must state the channel number for the
redirection operator—this is optional for stdin (0< ) and stdout (1> ) but mandatory
for stderr (2> ).
   You can use the >& operator to redirect a channel to a different one:

make >make.log 2>&1

redirects standard output and standard error output of the make command to make.
log .

B Watch out: Order is important here! The two commands
       make >make.log 2>&1
       make 2>&1 >make.log

      lead to completely different results. In the second case, standard error out-
      put will be redirected to wherever standard output goes (/dev/tty , where
      standard error output would go anyway), and then standard output will
      be sent to make.log , which, however, does not change the target for standard
      error output.

C 8.1 [2] You can use the -U option to get ls to output a directory’s entries with-
  out sorting them. Even so, after “ls -laU >filelist ”, the entry for filelist in
  the output file gives length zero. What could be the reason?

C 8.2 [!2] Compare the output of the commands “ls /tmp ” and “ls /tmp >ls-
  tmp.txt ” (where, in the second case, we consider the content of the ls- tmp.txt
  to be the output). Do you notice something? If so, how could you explain
  the phenomenon?
98                                                                                8 Standard I/O and Filter Commands

                                                    stdin                                stdout
                                    Command                           tee                              Command


                                                      Figure 8.2: The tee command

                            C 8.3 [!2] Why isn’t it possible to replace a file by a new version in one step,
                              for example using “grep xyz file >file ”?

                            C 8.4 [!1] And what is wrong with “cat foo >>foo ”, assuming a non-empty file
                              foo ?

                            C 8.5 [2] In the shell, how would you output an error message such that it goes
                              to standard error output?

                            8.1.3    Command Pipelines
                            Output redirection is frequently used to store the result of a program in order to
                            continue processing it with a different command. However, this type of interme-
                            diate storage is not only quite tedious, but you must also remember to get rid of
                            the intermediate files once they are no longer required. Therefore, Linux offers a
                   pipes    way of linking commands directly via pipes: A program’s output automatically
                            becomes another program’s input.
     direct connection of      This direct connection of several commands into a pipeline is done using the
       several commands     | operator. Instead of first redirecting the output of “ls -laF ” to a file and then
                 pipeline   looking at that file using less , you can do the same thing in one step without an
                            intermediate file:
                            $ ls -laF | less
                            total 7
                            drwxr-xr-x 12 joe     users      1024   Aug   26   18:55   ./
                            drwxr-xr-x   5 root   root       1024   Aug   13   12:52   ../
                            drwxr-xr-x   3 joe    users      1024   Aug   20   12:30   photos/
                            -rw-r--r--   1 joe    users       449   Sep    6   13:50   filelist
                            -rw-r--r--   1 joe    users     15811   Aug   13   12:33   pingu.gif
                            -rw-r--r--   1 joe    users     14373   Aug   13   12:33   hobby.txt
                            -rw-r--r--   2 joe    users      3316   Aug   20   15:14   chemistry.txt

                            These command pipelines can be almost any length. Besides, the final result can
                            be redirected to a file:
                            $ cut -d: -f1 /etc/passwd | sort | pr -2 >userlst

                          This command pipeline takes all user names from the first comma-separated col-
                          umn of /etc/passwd file, sorts them alphabetically and writes them to the userlst
                          file in two columns. The commands used here will be described in the remainder
                          of this chapter.
                              Sometimes it is helpful to store the data stream inside a command pipeline at
      intermediate result a certain point, for example because the intermediate result at that stage is useful
                          for different tasks. The tee command copies the data stream and sends one copy
                          to standard output and another copy to a file. The command name should be
                          obvious if you know anything about plumbing (see figure 8.2).
                              The tee command with no options creates the specified file or overwrites it if it
                          exists; with -a (“append”), the output can be appended to an existing file.
8.2 Filter Commands                                                                                 99

$ ls -laF | tee list | less
total 7
drwxr-xr-x 12 joe     users     1024   Aug   26   18:55   ./
drwxr-xr-x   5 root   root      1024   Aug   13   12:52   ../
drwxr-xr-x   3 joe    users     1024   Aug   20   12:30   photos/
-rw-r--r--   1 joe    users      449   Sep    6   13:50   content
-rw-r--r--   1 joe    users    15811   Aug   13   12:33   pingu.gif
-rw-r--r--   1 joe    users    14373   Aug   13   12:33   hobby.txt
-rw-r--r--   2 joe    users     3316   Aug   20   15:14   chemistry.txt

In this example the content of the current directory is written both to the list file
and the screen. (The list file does not show up in the ls output because it is only
created afterwards by tee .)

C 8.6 [!2] How would you write the same intermediate result to several files
  at the same time?

8.2     Filter Commands
One of the basic ideas of Unix—and, consequently, Linux—is the “toolkit princi- toolkit principle
ple”. The system comes with a great number of system programs, each of which
performs a (conceptually) simple task. These programs can be used as “building
blocks” to construct other programs, to save the authors of those programs from
having to develop the requisite functions themselves. For example, not every pro-
gram contains its own sorting routines, but many programs avail themselves of
the sort command provided by Linux. This modular structure has several advan-
   • It makes life easier for programmers, who do not need to develop (or incor-
     porate) new sorting routines all the time.
   • If sort receives a bug fix or performance improvement, all programs using
     sort benefit from it, too—and in most cases do not even need to be changed.

Tools that take their input from standard input and write their output to standard
output are called “filter commands” or “filters” for short. Without input redirec-
tion, a filter will read its input from the keyboard. To finish off keyboard input for
such a program, you must enter the key sequence Ctrl + d , which is interpreted
as “end of file” by the terminal driver.

B Note that the last applies to keyboard input only. Files on the disk may of
  course contain the Ctrl + d character (ASCII 4), without the system believ-
  ing that the file ended at that point. This as opposed to a certain very pop-
  ular operating system, which traditionally has a somewhat quaint notion of
  the meaning of the Control-Z (ASCII 26) character even in text files …

   Many “normal” commands, such as the aforementioned grep , operate like fil-
ters if you do not specify input file names for them to work on.
   In the remainder of the chapter you will become familiar with a selection of the
most important such commands. Some commands have crept in that are not tech-
nically genuine filter commands, but all of them form important building blocks
for pipelines.
100                                                                         8 Standard I/O and Filter Commands

                                                    Table 8.2: Options for cat (selection)

                                 Option    Result
                                   -b      (engl. number non-blank lines) Numbers all non-blank lines in
                                           the output, starting at 1.
                                   -E      (engl. end-of-line) Displays a $ at the end of each line (useful
                                           to detect otherwise invisible space characters).
                                   -n      (engl. number) Numbers all lines in the output, starting at 1.
                                   -s      (engl. squeeze) Replaces sequences of empty lines by a single
                                           empty line.
                                   -T      (engl. tabs) Displays tab characters as “^I ”.
                                   -v      (engl. visible) Makes control characters 𝑐 visible as “^ 𝑐”, char-
                                           acters 𝛼 with character codes greater than 127 as “M- 𝛼”.
                                   -A      (engl. show all) Same as -vET .

                           8.3     Reading and Writing Files
                           8.3.1    Outputting and Concatenating Text Files—cat
      concatenating files The cat (“concatenate”) command is really intended to join several files named on
                           the command line into one. If you pass just a single file name, the content of that
                           file will be written to standard output. If you do not pass a file name at all, cat
                           reads its standard input—this may seem useless, but cat offers options to number
                           lines, make line ends and special characters visible or compress runs of blank lines
                           into one (table 8.2).

              text files   B It goes without saying that only text files lead to sensible screen output with
                             cat . If you apply the command to other types of files (such as the binary file
                             /bin/cat ), it is more than probable—on a text terminal at least—that the shell
                             prompt will consist of unreadable characters once the output is done. In this
                             case you can restore the normal character set by (blindly) typing reset . If you
                             redirect cat output to a file this is of course not a problem.

                           B The “Useless Use of cat Award” goes to people using cat where it is extra-
                             neous. In most cases, commands do accept filenames and don’t just read
                             their standard input, so cat is not required to pass a single file to them on
                             standard input. A command like “cat data.txt | grep foo ” is unnecessary if
                             you can just as well write “grep foo data.txt ”. Even if grep could only read its
                             standard input, “grep foo <data.txt ” would be shorter and would not involve
                             an additional cat process.

                           C 8.7 [2] How can you check whether a directory contains files with “weird”
                             names (e. g., ones with spaces at the end or invisible control characters in
                             the middle)?

                           8.3.2    Beginning and End—head and tail
                           Sometimes you are only interested in part of a file: The first few lines to check
                           whether it is the right file, or, in particular with log files, the last few entries. The
                           head and tail commands deliver exactly that—by default, the first ten and the last
                           ten lines of every file passed as an argument, respectively (or else as usual the first
                           or last ten lines of their standard input). The -n option lets you specify a different
                           number of lines: “head -n 20 ” returns the first 20 lines of its standard input, “tail
                           -n 5 data.txt ” the last 5 lines of file data.txt .
8.4 Data Management                                                                                   101

B Tradition dictates that you can specify the number 𝑛 of desired lines directly
  as “- 𝑛”. Officially this is no longer allowed, but the Linux versions of head
  and tail still support it.
   You can use the -c option to specify that the count should be in bytes, not lines:
“head -c 20 ” displays the first 20 bytes of standard input, no matter how many
lines they occupy. If you append a “b ”, “k ”, or “m ” (for “blocks”, “kibibytes”, and
“mebibytes”, respectively) to the count, the count will be multiplied by 512, 1024,
or 1048576, respectively.

B head also lets you use a minus sign: “head -c -20 ” displays all of its standard
  input but the last 20 bytes.

B By way of revenge, tail can do something that head does not support: If the
  number of lines starts with “+ ”, it displays everything starting with the given
        $ tail -n +3 file                                               Everything from line 3

   The tail command also supports the important -f option. This makes tail wait
after outputting the current end of file, to also output data that is appended later
on. This is very useful if you want to keep an eye on some log files. If you pass
several file names to tail -f , it puts a header line in front of each block of output
lines telling what file the new data was written to.

C 8.8 [!2] How would you output just the 13th line of the standard input?
C 8.9 [3] Check out “tail -f ”: Create a file and invoke “tail -f ” on it. Then,
  from another window or virtual console, append something to the file us-
  ing, e. g., “echo >>… ”, and observe the output of tail . What does it look like
  when tail is watching several files simultaneously?

C 8.10 [3] What happens to “tail -f ” if the file being observed shrinks?
C 8.11 [3] Explain the output of the following commands:
        $ echo Hello >/tmp/hello
        $ echo "Hiya World" >/tmp/hello

        when you have started the command

        $ tail -f /tmp/hello

        in a different window after the first echo above.

8.4      Data Management
8.4.1     Sorted Files—sort and uniq
The sort command lets you sort the lines of text files according to predetermined
criteria. The default setting is ascending (from A to Z) according to the ASCII default setting
values1 of the first few characters of each line. This is why special characters such
as German umlauts are frequently sorted incorrectly. For example, the character
code of “Ä” is 143, so that character ends up far beyond “Z” with its character code
of 91. Even the lowercase latter “a” is considered “greater than” the uppercase
letter “Z”.
   1 Of course ASCII only goes up to 127. What is really meant here is ASCII together with whatever

extension for the characters with codes from 128 up is currently used, for example ISO-8859-1, also
known as ISO-Latin-1.
102                                                                       8 Standard I/O and Filter Commands

                       B Of course, sort can adjust itself to different languages and cultures. To sort
                         according to German conventions, set one of the environment variables LANG ,
                         LC_ALL , or LC_COLLATE to a value such as “de ”, “de_DE ”, or “de_DE@UTF-8 ” (the
                         actual value depends on your distribution). If you want to set this up for
                         a single sort invocation only, do

                             $ … | LC_COLLATE=de_DE.UTF-8 sort

                            The value of LC_ALL has precedence over the value of LC_COLLATE and that,
                            again, has precedence over the value of LANG . As a side effect, German sort
                            order causes the case of letters to be ignored when sorting.

                        Unless you specify otherwise, the sort proceeds “lexicographically” considering
                        all of the input line. That is, if the initial characters of two lines compare equal,
                        the first differing character within the line governs their relative positioning. Of
                        course sort can sort not just according to the whole line, but more specifically ac-
      Sorting by fields cording to the values of certain “columns” or fields of a (conceptual) table. Fields
                        are numbered starting at 1; with the “-k 2 ” option, the first field would be ignored
                        and the second field of each line considered for sorting. If the values of two lines
                        are equal in the second field, the rest of the line will be looked at, unless you spec-
                        ify the last field to be considered using something like “-k 2,3 ”. Incidentally, it is
                        permissible to specify several -k options with the same sort command.

                       B In addition, sort supports an obsolete form of position specification: Here
                         fields are numbered starting at 0, the initial field is specified as “+ 𝑚” and
                         the final field as “- 𝑛”. To complete the differences to the modern form, the
                         final field is specified “exclusively”—you give the first field that should not
                         be taken into account for sorting. The examples above would, respectively,
                         be “+1 ”, “+1 -3 ”, and “+1 -2 ”.

            separator The space character serves as the separator between fields. If several spaces occur
                      in sequence, only the first is considered a separator; the others are considered
                      part of the value of the following field. Here is a little example, namely the list
                      of participants for the annual marathon run of the Lameborough Track & Field
                      Club. To start, we ensure that we use the system’s standard language environment
                      (“POSIX ”) by resetting the corresponding environment variables. (Incidentally, the
                      fourth column gives a runner’s bib number.)

                       $ unset LANG LC_ALL LC_COLLATE
                       $ cat participants.dat
                       Smith      Herbert Pantington AC            123   Men
                       Prowler    Desmond Lameborough TFC          13    Men
                       Fleetman   Fred     Rundale Sportsters      217   Men
                       Jumpabout Mike      Fairing Track Society   154   Men
                       de Leaping Gwen     Fairing Track Society   26    Ladies
                       Runnington Vivian   Lameborough TFC         117   Ladies
                       Sweat      Susan    Rundale Sportsters      93    Ladies
                       Runnington Kathleen Lameborough TFC         119   Ladies
                       Longshanks Loretta Pantington AC            55    Ladies
                       O'Finnan   Jack     Fairing Track Society   45    Men
                       Oblomovsky Katie    Rundale Sportsters      57    Ladies

                      Let’s try a list sorted by last name first. This is easy in principle, since the last
                      names are at the front of each line:
                       $ sort participants.dat
                       Fleetman   Fred     Rundale Sportsters    217 Men
                       Jumpabout Mike      Fairing Track Society 154 Men
                       Longshanks Loretta Pantington AC          55 Ladies
8.4 Data Management                                                                      103

O'Finnan     Jack       Fairing Track Society   45    Men
Oblomovsky   Katie      Rundale Sportsters      57    Ladies
Prowler      Desmond    Lameborough TFC         13    Men
Runnington   Kathleen   Lameborough TFC         119   Ladies
Runnington   Vivian     Lameborough TFC         117   Ladies
Smith        Herbert    Pantington AC           123   Men
Sweat        Susan      Rundale Sportsters      93    Ladies
de Leaping   Gwen       Fairing Track Society   26    Ladies

You will surely notice the two small problems with this list: “Oblomovsky” should
really be in front of “O’Finnan”, and “de Leaping” should end up at the front of
the list, not the end. These will disappear if we specify “English” sorting rules:

$ LC_COLLATE=en_GB sort participants.dat
de Leaping Gwen     Fairing Track Society       26    Ladies
Fleetman   Fred     Rundale Sportsters          217   Men
Jumpabout Mike      Fairing Track Society       154   Men
Longshanks Loretta Pantington AC                55    Ladies
Oblomovsky Katie    Rundale Sportsters          57    Ladies
O'Finnan   Jack     Fairing Track Society       45    Men
Prowler    Desmond Lameborough TFC              13    Men
Runnington Kathleen Lameborough TFC             119   Ladies
Runnington Vivian   Lameborough TFC             117   Ladies
Smith      Herbert Pantington AC                123   Men
Sweat      Susan    Rundale Sportsters          93    Ladies

(en_GB is short for “British English”; en_US , for “American English”, would also work
here.) Let’s sort according to the first name next:

$ sort -k 2,2 participants.dat
Smith      Herbert Pantington AC                123   Men
Sweat      Susan    Rundale Sportsters          93    Ladies
Prowler    Desmond Lameborough TFC              13    Men
Fleetman   Fred     Rundale Sportsters          217   Men
O'Finnan   Jack     Fairing Track Society       45    Men
Jumpabout Mike      Fairing Track Society       154   Men
Runnington Kathleen Lameborough TFC             119   Ladies
Oblomovsky Katie    Rundale Sportsters          57    Ladies
de Leaping Gwen     Fairing Track Society       26    Ladies
Longshanks Loretta Pantington AC                55    Ladies
Runnington Vivian   Lameborough TFC             117   Ladies

This illustrates the property of sort mentioned above: The first of a sequence of
spaces is considered the separator, the others are made part of the following field’s
value. As you can see, the first names are listed alphabetically but only within the
same length of last name. This can be fixed using the -b option, which treats runs
of space characters like a single space:

$ sort -b -k 2,2 participants.dat
Prowler    Desmond Lameborough TFC              13    Men
Fleetman   Fred     Rundale Sportsters          217   Men
Smith      Herbert Pantington AC                123   Men
O'Finnan   Jack     Fairing Track Society       45    Men
Runnington Kathleen Lameborough TFC             119   Ladies
Oblomovsky Katie    Rundale Sportsters          57    Ladies
de Leaping Gwen     Fairing Track Society       26    Ladies
Longshanks Loretta Pantington AC                55    Ladies
Jumpabout Mike      Fairing Track Society       154   Men
104                                                                                      8 Standard I/O and Filter Commands

                                           Table 8.3: Options for sort (selection)

       Option                           Result
       -b                    (blank)    Ignores leading blanks in field contents
       -d               (dictionary)    Sorts in “dictionary order”, i. e., only letters, digits and spaces are taken
                                        into account
       -f                      (fold)   Makes uppercase and lowercase letters equivalent
       -i                   (ignore)    Ignores non-printing characters
       -k   ⟨field⟩[, ⟨field’⟩] (key)     Sort according to ⟨field⟩ (up to and including ⟨field’⟩)
       -n                 (numeric)     Considers field value as a number and sorts according to its numeric
                                        value; leading blanks will be ignored
       -o datei            (output)     Writes results to a file, whose name may match the original input file
       -r                  (reverse)    Sorts in descending order, i. e., Z to A
       -t ⟨char⟩        (terminate)     The ⟨char⟩ character is used as the field separator
       -u                  (unique)     Writes only the first of a sequence of equal output lines

                                    Sweat      Susan      Rundale Sportsters      93 Ladies
                                    Runnington Vivian     Lameborough TFC         117 Ladies

                                    This sorted list still has a little blemish; see exercise 8.14.
More detailed field specification      The sort field can be specified in even more detail, as the following example
                                    $ sort -br   -k 2.2 participants.dat
                                    Sweat        Susan    Rundale Sportsters      93    Ladies
                                    Fleetman     Fred     Rundale Sportsters      217   Men
                                    Longshanks   Loretta Pantington AC            55    Ladies
                                    Runnington   Vivian   Lameborough TFC         117   Ladies
                                    Jumpabout    Mike     Fairing Track Society   154   Men
                                    Prowler      Desmond Lameborough TFC          13    Men
                                    Smith        Herbert Pantington AC            123   Men
                                    de Leaping   Gwen     Fairing Track Society   26    Ladies
                                    Oblomovsky   Katie    Rundale Sportsters      57    Ladies
                                    Runnington   Kathleen Lameborough TFC         119   Ladies
                                    O'Finnan     Jack     Fairing Track Society   45    Men

                                   Here, the participants.dat file is sorted in descending order (-r ) according to the
                                   second character of the second table field, i. e., the second character of the first
                                   name (very meaningful!). In this case as well it is necessary to ignore leading
                                   spaces using the -b option. (The blemish from exercise 8.14 still manifests itself
                                      With the -t (“terminate”) option you can select an arbitrary character in place
                   field separator of the field separator. This is a good idea in principle, since the fields then may
                                   contain spaces. Here is a more usable (if less readable) version of our example file:

                                    Smith:Herbert:Pantington AC:123:Men
                                    Prowler:Desmond:Lameborough TFC:13:Men
                                    Fleetman:Fred:Rundale Sportsters:217:Men
                                    Jumpabout:Mike:Fairing Track Society:154:Men
                                    de Leaping:Gwen:Fairing Track Society:26:Ladies
                                    Runnington:Vivian:Lameborough TFC:117:Ladies
                                    Sweat:Susan:Rundale Sportsters:93:Ladies
                                    Runnington:Kathleen:Lameborough TFC:119:Ladies
                                    Longshanks:Loretta: Pantington AC:55:Ladies
                                    O'Finnan:Jack:Fairing Track Society:45:Men
                                    Oblomovsky:Katie:Rundale Sportsters:57:Ladies
8.4 Data Management                                                                                           105

Sorting by first name now leads to correct results using “LC_COLLATE=en_GB sort -t:
-k2,2 ”. It is also a lot easier to sort, e. g., by a participant’s number (now field 4, no
matter how many spaces occur in their club’s name:

$ sort -t: -k4 participants0.dat
Runnington:Vivian:Lameborough TFC:117:Ladies
Runnington:Kathleen:Lameborough TFC:119:Ladies
Smith:Herbert:Pantington AC:123:Men
Prowler:Desmond:Lameborough TFC:13:Men
Jumpabout:Mike:Fairing Track Society:154:Men
Fleetman:Fred:Rundale Sportsters:217:Men
de Leaping:Gwen:Fairing Track Society:26:Ladies
O'Finnan:Jack:Fairing Track Society:45:Men
Longshanks:Loretta: Pantington AC:55:Ladies
Oblomovsky:Katie:Rundale Sportsters:57:Ladies
Sweat:Susan:Rundale Sportsters:93:Ladies

Of course the “number” sort is done lexicographically, unless otherwise specified—“117”
and “123” are put before “13”, and that in turn before “154”. This can be fixed by
giving the -n option to force a numeric comparison:                               numeric comparison

$ sort -t: -k4 -n participants0.dat
Prowler:Desmond:Lameborough TFC:13:Men
de Leaping:Gwen:Fairing Track Society:26:Ladies
O'Finnan:Jack:Fairing Track Society:45:Men
Longshanks:Loretta: Pantington AC:55:Ladies
Oblomovsky:Katie:Rundale Sportsters:57:Ladies
Sweat:Susan:Rundale Sportsters:93:Ladies
Runnington:Vivian:Lameborough TFC:117:Ladies
Runnington:Kathleen:Lameborough TFC:119:Ladies
Smith:Herbert:Pantington AC:123:Men
Jumpabout:Mike:Fairing Track Society:154:Men
Fleetman:Fred:Rundale Sportsters:217:Men

These and some more important options for sort are shown in table 8.3; studying
the program’s documentation is well worthwhile. sort is a versatile and powerful
command which will save you a lot of work.
    The uniq command does the important job of letting through only the first of a            uniq   command
sequence of equal lines in the input (or the last, just as you prefer). What is con-
sidered “equal” can, as usual, be specified using options. uniq differs from most
of the programs we have seen so far in that it does not accept an arbitrary number
of named input files but just one; a second file name, if it is given, is considered
the name of the desired output file (if not, standard output is assumed). If no file
is named in the uniq call, uniq reads standard input (as it ought).
    uniq works best if the input lines are sorted such that all equal lines occur one
after another. If that is not the case, it is not guaranteed that each line occurs only
once in the output:

$ cat uniq-test
$ uniq uniq-test
106                                                                         8 Standard I/O and Filter Commands


                         Compare this to the output of “sort -u ”:

                         $ sort -u uniq-test

                         C 8.12 [!2] Sort the list of participants in participants0.dat (the file with colon
                           separators) according to the club’s name and, within clubs, the last and first
                           names of the runners (in that order).

                         C 8.13 [3] How can you sort the list of participants by club name in ascending
                           order and, within clubs, by number in descending order? (Hint: Read the

                         C 8.14 [!2] What is the “blemish” alluded to in the examples and why does it

                         C 8.15 [2] A directory contains files with the following names:
                                 01-2002.txt   01-2003.txt   02-2002.txt   02-2003.txt
                                 03-2002.txt   03-2003.txt   04-2002.txt   04-2003.txt
                                 11-2002.txt   11-2003.txt   12-2002.txt   12-2003.txt

                                 Give a sort command to sort the output of ls into “chronologically correct”


                         8.4.2     Columns and Fields—cut , paste etc.
       Cutting columns While you can locate and “cut out” lines of a text file using grep , the cut command
                         works through a text file “by column”. This works in one of two ways:
      Absolute columns      One possibility is the absolute treatment of columns. These columns corre-
                         spond to single characters in a line. To cut out such columns, the column number
                         must be given after the -c option (“column”). To cut several columns in one step,
                         these can be specified as a comma-separated list. Even column ranges may be

                         $ cut -c 12,1-5 participants.dat
                         de LeG
8.4 Data Management                                                                                                  107

In this example, the first letter of the first name and the first five letters of the
last name are extracted. It also illustrates the notable fact that the output always
contains the columns in the same order as in input. Even if the selected column
ranges overlap, every input character is output at most once:

$ cut -c 1-5,2-6,3-7 participants.dat
de Leap

   The second method is to cut relative fields, which are delimited by separator Relative fields
characters. If you want to cut delimited fields, cut needs the -f (“field”) option
and the desired field number. The same rules as for columns apply. The -c and -f
options are mutually exclusive.
   The default separator is the tab character; other separators may be specified separators
with the -d option (“delimiter”):

$ cut -d: -f 1,4 participants0.dat
de Leaping:26

In this way, the participants’ last names (column 1) and numbers (column 4) are
taken from the list. For readability, only the first few lines are displayed.

B Incidentally, using the --output-delimiter option you can specify a different
  separator character for the output fields than is used for the input fields:

       $ cut -d: --output-delimiter=': ' -f 1,4 participants0.dat
       Smith: 123
       Prowler: 13
       Fleetman: 217
       Jumpabout: 154
       de Leaping: 26

B If you really want to change the order of columns and fields, you have to
  bring in the big guns, such as awk or perl ; you could do it using the paste
  command, which will be introduced presently, but that is rather tedious.

    When files are treated by fields (rather than columns), the -s option (“sepa-         Suppressing no-field lines
rator”) is helpful. If “cut -f ” encounters lines that do not contain the separator
character, these are normally output in their entirety; -s suppresses these lines.
    The paste command joins the lines of the specified files. It is thus frequently       Joining lines of files
used together with cut . As you will have noticed immediately, paste is not a filter
command. You may however give a minus sign in place of one of the input file-
names for paste to read its standard input at that point. Its output always goes to
standard output.
    As we said, paste works by lines. If two file names are specified, the first line      Join files “in parallel”
of the first file and the first of the second are joined (using a tab character as the
separator) to form the first line of the output. The same is done with all other lines
in the files. To specify a different separator, use the -d option.                        separator
    By way of an example, we can construct a version of the list of marathon run-
ners with the participants’ numbers in front:
108                                                                           8 Standard I/O and Filter Commands

                            $ cut -d: -f4 participants0.dat >number.dat
                            $ cut -d: -f1-3,5 participants0.dat \
                            >   | paste -d: number.dat - >p-number.dat
                            $ cat p-number.dat
                            123:Smith:Herbert:Pantington AC:Men
                            13:Prowler:Desmond:Lameborough TFC:Men
                            217:Fleetman:Fred:Rundale Sportsters:Men
                            154:Jumpabout:Mike:Fairing Track Society:Men
                            26:de Leaping:Gwen:Fairing Track Society:Ladies
                            117:Runnington:Vivian:Lameborough TFC:Ladies
                            93:Sweat:Susan:Rundale Sportsters:Ladies
                            119:Runnington:Kathleen:Lameborough TFC:Ladies
                            55:Longshanks:Loretta: Pantington AC:Ladies
                            45:O'Finnan:Jack:Fairing Track Society:Men
                            57:Oblomovsky:Katie:Rundale Sportsters:Ladies

                            This file may now conveniently be sorted by number using “sort -n p-number.dat ”.
      Join files serially       With -s (“serial”), the given files are processed in sequence. First, all the lines
                            of the first file are joined into one single line (using the separator character), then
                            all lines from the second file make up the second line of the output etc.

                            $ cat list1
                            $ cat list2
                            $ paste -s list*
                            Wood    Bell     Potter
                            Keeper Chaser Seeker

                            All files matching the list* wildcard pattern—in this case, list1 and list2 —are
                            joined using paste . The -s option causes every line of these files to make up one
                            column of the output.

                            C 8.16 [!2] Generate a new version of the participants.dat file (the one with
                              fixed-width columns) in which the participant numbers and club affiliations
                              do not occur.

                            C 8.17 [!2] Generate a new version of the participants0.dat file (the one with
                              fields separated using colons) in which the participant numbers and club
                              affiliations do not occur.

                            C 8.18 [3] Generate a version of participants0.dat in which the fields are not
                              separated by colons but by the string “,␣ ” (a comma followed by a space

                            C 8.19 [3] How many groups are used as primary groups by users on your
                              system? (The primary group of a user is the fourth field in /etc/passwd .)
8.4 Data Management                                                                  109

Commands in this Chapter
cat     Concatenates files (among other things)                         cat (1) 100
cut     Extracts fields or columns from its input                       cut (1) 106
head    Displays the beginning of a file                               head (1) 100
paste   Joins lines from different input files                         paste (1) 107
reset   Resets a terminal’s character set to a “reasonable” value     tset (1) 100
sort    Sorts its input by line                                       sort (1) 101
tail    Displays a file’s end                                          tail (1) 100
uniq    Replaces sequences of identical lines in its input by single specimens
                                                                      uniq (1) 105

   • Every Linux program supports the standard I/O channels stdin , stdout , and
     stderr .
   • Standard output and standard error output can be redirected using opera-
     tors > and >> , standard input using operator < .
   • Pipelines can be used to connect the standard output and input of programs
     directly (without intermediate files).
   • Using the tee command, intermediate results of a pipeline can be stored to
   • Filter commands (or “filters”) read their standard input, manipulate it, and
     write the results to standard output.
   • sort is a versatile program for sorting.
   • The cut command cuts specified ranges of columns or fields from every line
     of its input.
   • With paste , the lines of files can be joined.
                                                                                                       $ echo tux
                                                                                                       $ ls
                                                                                                       $ /bin/su -

More About The Shell

9.1     Simple Commands: sleep , echo , and date .   .   .   .   .   .   .   .   .   .   .   .   112
9.2     Shell Variables and The Environment. .       .   .   .   .   .   .   .   .   .   .   .   113
9.3     Command Types—Reloaded . . . . .             .   .   .   .   .   .   .   .   .   .   .   115
9.4     The Shell As A Convenient Tool. . . .        .   .   .   .   .   .   .   .   .   .   .   116
9.5     Commands From A File . . . . . .             .   .   .   .   .   .   .   .   .   .   .   119
9.6     The Shell As A Programming Language.         .   .   .   .   .   .   .   .   .   .   .   120

      • Knowing about shell variables and evironment variables

      • Basic shell knowledge (Chapter 3)
      • File management and simple filter commands (Chapter 6, Chapter 8)
      • Use of a text editor (Chapter 5)

grd1-shell2-opt.tex   (be27bba8095b329b )
112                                                                                    9 More About The Shell

                        9.1     Simple Commands: sleep , echo , and date
                        To give you some tools for experiments, we shall now explain some very simple

                        sleep This command does nothing for the number of seconds specified as the
                        argument. You can use it if you want your shell to take a little break:

                         $ sleep 10
                                                                Nothing happens for approximately 10 seconds
                         $ _

      Output arguments echo     The command echo outputs its arguments (and nothing else), separated by
                        spaces. It is still interesting and useful, since the shell replaces variable references
                        (see section 9.2) and similar things first:

                         $ p=Planet
                         $ echo Hello $p
                         Hello Planet
                         $ echo Hello ${p}oid
                         Hello Planetoid

                        (The second echo illustrates what to do if you want to append something directly
                        to the value of a variable.)

                         B If echo is called with the -n option, it does not write a line terminator at the
                           end of its output:

                               $ echo -n Hello

          date and time date   The date command displays the current date and time. You have consider-
                        able leeway in determining the format of the output—call “date --help ”, or read
                        the online documentation using “man date ”.

                         B (When reading through this manual for the second time:) In particular, date
                           serves as a world clock, if you first set the TZ environment variable to the
                           name of a time zone or important city (usually capital):

                               $ date
                               Thu Oct 5 14:26:07 CEST 2006
                               $ export TZ=Asia/Tokyo
                               $ date
                               Tue Oct 5 21:26:19 JST 2006
                               $ unset TZ

                               You can find out about valid time zone and city names by rooting around
                               in /usr/share/zoneinfo .

      Set the system time While every user is allowed to read the system time, only the system administra-
                        tor root may change the system time using the date command and an argument of
                        the form MMDDhhmm , where MM is the calendar month, DD the calendar day, hh the hour,
                        and mm the minute. You can optionally add two digits the year (plus possibly an-
                        other two for the century) and the seconds (separated with a dot), which should,
                        however, prove necessary only in very rare cases.
9.2 Shell Variables and The Environment                                                                113

$ date
Thu Oct 5 14:28:13 CEST 2006
$ date 08181715
date: cannot set date: Operation not permitted
Fri Aug 18 17:15:00 CEST 2006

B The date command only changes the internal time of the Linux system. This
  time will not necessarily be transferred to the CMOS clock on the computer’s
  mainboard, so a special command may be required to do so. Many distri-
  butions will do this automatically when the system is shut down.

C 9.1 [!3] Assume now is 22 October 2003, 12:34 hours and 56 seconds. Study
  the date documentation and state formatting instructions to achieve the fol-
  lowing output:
        1. 22-10-2003
        2. 03-294 (WK43) (Two-digit year, number of day within year, calendar
        3. 12h34m56s

C 9.2 [!2] What time is it now in Los Angeles?

9.2     Shell Variables and The Environment
Like most common shells, bash has features otherwise found in programming lan-
guages. For example, it is possible to store pieces of text or numbers in variables
and retrieve them later. Variables also control various aspects of the operation of
the shell itself.
   Within the shell, a variable is set by means of a command like “foo=bar ” (this Setting variables
command sets the foo variable to the textual value bar ). Take care not to insert
spaces in front of or behind the equals sign! You can retrieve the value of the
variable by using the variable name with a dollar sign in front:

$ foo=bar
$ echo foo
$ echo $foo

(note the difference).
    We distinguish environment variables from shell variables. Shell variables environment variables
are only visible in the shell in which they have been defined. On the other hand, shell variables
environment variables are passed to the child process when an external command
is started and can be used there. (The child process does not have to be a shell;
every Linux process has environment variables). All the environment variables of
a shell are also shell variables but not vice versa.
    Using the export command, you can declare an existing shell variable an envi- export
ronment variable:
$ foo=bar                                                    foo is now a shell variable
$ export foo                                     foo   is now an environment variable

Or you define a new variable as a shell and environment variable at the same time:
114                                                                                   9 More About The Shell

                                                  Table 9.1: Important Shell Variables

                               Variable      Meaning
                                  PWD        Name of the current directory
                                EDITOR       Name of the user’s favourite editor
                                  PS1        Shell command prompt template
                                  UID        Current user’s user name
                                 HOME        Current user’s home directory
                                 PATH        List of directories containing executable programs that are
                                             eligible as external commands
                                LOGNAME      Current user’s user name (again)

                          $ export foo=bar

                          The same works for several variables simultaneously:

                          $ export foo baz
                          $ export foo=bar baz=quux

                             You can display all environment variables using the export command (with no
                          parameters). The env command (also with no parameters) also displays the cur-
                          rent environment. All shell variables (including those which are also environment
                          variables) can be displayed using the set command. The most common variables
                          and their meanings are shown in table 9.1.

                          B The set command also does many other strange and wonderful things. You
                            will encounter it again in the Linup Front training manual Advanced Linux,
                            which covers shell programming.

                          B env , too, is actually intended to manipulate the process environment rather
                            than just display it. Consider the following example:

                                $ env foo=bar bash                                 Launch child shell with foo
                                $ echo $foo
                                $ exit                                                   Back to the parent shell
                                $ echo $foo
                                                                                                     Not defined
                                $ _

                          B At least with bash (and relations) you don’t really need env to execute com-
                            mands with an extended environment – a simple

                                $ foo=bar bash

                                does the same thing. However, env also allows you to remove variables from
                                the environment temporarily (how?).

      Delete a variable      If you have had enough of a shell variable, you can delete it using the unset
                          command. This also removes it from the environment. If you want to remove a
                          variable from the environment but keep it on as a shell variable, use “export -n ”:

                          $ export foo=bar                                      foo is an environment variable
                          $ export -n foo                                          foo is a shell variable (only)
                          $ unset foo                                               foo is gone and lost forever
9.3 Command Types—Reloaded                                                                                     115

9.3      Command Types—Reloaded
One application of shell variables is controlling the shell itself. Here’s another ex- Controlling the shell
ample: As we discussed in Chapter 3, the shell distinguishes internal and external
commands. External commands correspond to executable programs, which the
shell looks for in the directories that make up the value of the PATH environment
variable. Here is a typical value for PATH :

$ echo $PATH

Individual directories are separated in the list by colons, therefore the list in the
example consists of five directories. If you enter a command like

$ ls

the shell knows that this isn’t an internal command (it knows its internal com-
mands) and thus begins to search the directories in PATH , starting with the leftmost
directory. In particular, it checks whether the following files exist:

/home/joe/bin/ls                                                               Nope …
/usr/local/bin/ls                                                      Still no luck …
/usr/bin/ls                                                          Again no luck …
/bin/ls                                                                        Gotcha!
                                               The directory /usr/games is not checked.

This implies that the /bin/ls file will be used to execute the ls command.

B Of course this search is a fairly involved process, which is why the shell
  prepares for the future: If it has once identified the /bin/ls file as the im-
  plementation of the ls command, it remembers this correspondence for the
  time being. This process is called “hashing”, and you can see that it did take
  place by applying type to the ls command.

        $ type ls
        ls is hashed (/bin/ls)

B The hash command tells you which commands your bash has “hashed” and
  how often they have been invoked in the meantime. With “hash -r ” you can
  delete the shell’s complete hashing memory. There are a few other options
  which you can look up in the bash manual or find out about using “help hash ”.

B Strictly speaking, the PATH variable does not even need to be an environment
  variable—for the current shell a shell variable would do just fine (see Exer-
  cise 9.5). However it is convenient to define it as an environment variable so
  the shell’s child processes (often also shells) use the desired value.

  If you want to find out exactly which program the shell uses for a given external
command, you can use the which command:

$ which grep

whichuses the same method as the shell—it starts at the first directory in PATH and
checks whether the directory in question contains an executable file with the same
name as the desired command.
116                                                              9 More About The Shell

      B which knows nothing about the shell’s internal commands; even though
        something like “which test ” returns “/usr/bin/test ”, this does not imply
        that this program will, in fact, be executed, since internal commands have
        precedence. If you want to know for sure, you need to use the “type ” shell
         The whereis command not only returns the names of executable programs, but
      also documentation (man pages), source code and other interesting files pertain-
      ing to the command(s) in question. For example:

      $ whereis passwd
      passwd: /usr/bin/passwd /etc/passwd /etc/ /usr/share/passwd 
        /usr/share/man/man1/passwd.1.gz /usr/share/man/man1/passwd.1ssl.gz 

      This uses a hard-coded method which is explained (sketchily) in whereis (1).

      C 9.3 [!2] Convince yourself that passing (or not passing) environment and
        shell variables to child processes works as advertised, by working through
        the following command sequence:

            $ foo=bar                                              foo is a shell variable
            $ bash                                              New shell (child process)
            $ echo $foo
                                                                         foo is not defined
            $ exit                                                 Back to the parent shell
            $ export foo                                   foo is an environment variable
            $ bash                                               New shell (child process)
            $ echo $foo
            bar                                   Environment variable was passed along
            $ exit                                              Back to the parent shell

      C 9.4 [!2] What happens if you change an environment variable in the child
        process? Consider the following command sequence:

            $ foo=bar                                              foo is a shell variable
            $ bash                                              New shell (child process)
            $ echo $foo
            bar                                   Environment variable was passed along
            $ foo=baz                                                        New value
            $ exit                                              Back to the parent shell
            $ echo $foo                                               What do we get??

      C 9.5 [2] Make sure that the shell’s command line search works even if PATH is
        a “only” simple shell variable rather than an environment variable. What
        happens if you remove PATH completely?

      C 9.6 [!1] Which executable programs are used to handle the following com-
        mands: fgrep , sort , mount , xterm

      C 9.7 [!1] Which files on your system contain the documentation for the
        “crontab ” command?

      9.4    The Shell As A Convenient Tool
      Since the shell is the tool many Linux users use most often, its developers have
      spared no trouble to make its use convenient. Here are some more useful trifles:
9.4 The Shell As A Convenient Tool                                                                     117

Command Editor You can edit command lines like in a simple text editor. Hence,
you can move the cursor around in the input line and delete or add characters
arbitrarily before finishing the input using the return key. The behaviour of this
editor can be adapted to that of the most popular editors on Linux (chapter 5)
using the “set -o vi ” and “set -o emacs ” commands.

Aborting Commands With so many Linux commands around, it is easy to con-
fuse a name or pass a wrong parameter. Therefore you can abort a command
while it is being executed. You simply need to press the Ctrl + c keys at the same

The History The shell remembers ever so many of your most recent commands as
part of the “history”, and you can move through this list using the ↑ and ↓ cur-
sor keys. If you find a previous command that you like you can either re-execute
it unchanged using ↩ , or else edit it as described above. You can search the list
“incrementally” using Ctrl + r —simply type a sequence of characters, and the
shell shows you the most recently executed command containing this sequence.
The longer your sequence, the more precise the search.

B When you log out of the system, the shell stores the history in the hidden
  file ~/.bash_history and makes it available again after your next login. (You
  may use a different file name by setting the HISTFILE variable to the name in

B A consequence of the fact that the history is stored in a “plain” file is that
  you can edit it using a text editor (chapter 5 tells you how). So in case you ac-
  cidentally enter your password on the command line, you can (and should!)
  remove it from the history manually—in particular, if your system is one of
  the more freewheeling ones where home directories are visible to anybody.

Autocompletion A massive convenience is bash ’s ability to automatically com- Completing command and file
plete command and file names. If you hit the Tab key, the shell completes an names
incomplete input if the continuation can be identified uniquely. For the first word
of a command, bash considers all executable programs, within the rest of the com-
mand line all the files in the current or specified directory. If several commands
or files exist whose names start out equal, the shell completes the name as far as
possible and then signals acoustically that the command or file name may still be
incomplete. Another Tab press then lists the remaining possibilities.

B It is possible to adapt the shell’s completion mechanism to specific pro-
  grams. For example, on the command line of a FTP client it might offer
  the names of recently visited FTP servers in place of file names. Check the
  bash documentation for details.

   table 9.2 gives an overview of the most important key strokes within bash .

Multiple Commands On One Line You are perfectly free to enter several com-
mands on the same input line. You merely need to separate them using a semi-
$ echo Today is; date
Today is
Fri 5 Dec 12:12:47 CET 2008

In this instance the second command will be executed once the first is done.
118                                                                           9 More About The Shell

                                         Table 9.2: Key Strokes within bash

                       Key Stroke               Function
                        ↑  or ↓                 Scroll through most recent commands
                        Ctrl+r                  Search command history
                        ← bzw. →                Move cursor within current command line
                        Home oder Ctrl +    a   Jump to the beginning of the command line
                        End oder Ctrl + e       Jump to the end of the command line
                        ⇐ bzw. Del              Delete character in front of/under the cursor,
                        Ctrl   +   t            Swap the two characters in front of and under
                                                the cursor
                        Ctrl   +   l            Clear the screen
                        Ctrl   +   c            Interrupt a command
                        Ctrl   +   d            End the input (for login shells: log off)

                   Conditional Execution Sometimes it is useful to make the execution of the second
                   command depend on whether the first was executed correctly or not. Every Unix
      return value process yields a return value which states whether it was executed correctly or
                   whether errors of whatever kind have occurred. In the former case, the return
                   value is 0; in the latter, it is different from 0.

                  B You can find the return value of a child process of your shell by looking at
                    the $? variable:

                        $ bash                                                 Start a child shell …
                        $ exit 33                                      … and exit again immediately
                        $ echo $?
                        33                                             The value from our exit above
                        $ _

                       But this really has no bearing on the following.

                     With && as the “separator” between two commands (where there would other-
                 wise be the semicolon), the second command is only executed when the first has
                 exited successfully. To demonstrate this, we use the shell’s -c option, with which
                 you can pass a command to the child shell on the command line (impressive, isn’t

                  $ bash -c "exit 0" && echo "Successful"
                  $ bash -c "exit 33" && echo "Successful"
                                                                          Nothing -- 33 isn’t success!

                     Conversely, with || as the “separator”, the second command is only executed
                 if the first did not finish successfully:

                  $ bash -c "exit 0" || echo "Unsuccessful"
                  $ bash -c "exit 33" || echo "Unsuccessful"

                  C 9.8 [3] What is wrong about the command “echo "Hello!" ”? (Hint: Experi-
                    ment with commands of the form “!-2 ” or “!ls ”.)
9.5 Commands From A File                                                                         119

9.5       Commands From A File
You can store shell commands in a file and execute them en bloc. (Chapter 5 ex-
plains how to conveniently create files.) You just need to invoke the shell and pass
the file name as a parameter:

$ bash my-commands

Such a file is also called a shell script, and the shell has extensive programming shell script
features that we can only outline very briefly here. (The Linup Front training
manual Advanced Linux explains shell programming in great detail.)

B You can avoid having to prepend the bash command by inserting the magical

      as the first line of your file and making the file “executable”:

       $ chmod +x my-commands

      After this, the

       $ ./my-commands

      command will suffice.

   If you invoke a shell script as above, whether with a prepended bash or as an
executable file, it is executed in a subshell, a shell that is a child process of the subshell
current shell. This means that changes to, e. g., shell or environment variables
do not influence the current shell. For example, assume that the file assignment
contains the line

Consider the following command sequence:

$ foo=quux
$ bash assignment                                                  Contains foo=bar
$ echo $foo
quux                                     No change; assignment was only in subshell

This is generally considered a feature, but every now and then it would be quite
desirable to have commands from a file affect the current shell. That works, too:
The source command reads the lines in a file exactly as if you would type them
directly into the current shell—all changes to variables (among other things) hence
take effect in your current shell:

$ foo=quux
$ source assignment                                                Contains foo=bar
$ echo $foo
bar                                                           Variable was changed!

   A different name for the source command, by the way, is “. ”. (You read correctly
– dot!) Hence

$ source assignment

is equivalent to
120                                                                               9 More About The Shell

                       $ . assignment

                       B Like program files for external commands, the files to be read using source
                         or . are searched in the directories given by the PATH variable.

                       9.6    The Shell As A Programming Language
                       Being able to execute shell commands from a file is a good thing, to be sure.
                       However, it is even better to be able to structure these shell commands such that
                       they do not have to do the same thing every time, but—for example—can ob-
                       tain command-line parameters. The advantages are obvious: In often-used pro-
                       cedures you save a lot of tedious typing, and in seldom-used procedures you can
                       avoid mistakes that might creep in because you accidentally leave out some im-
                       portant step. We do not have space here for a full explanation of the shell als a
                       programming language, but fortunately there is enough room for a few brief ex-

                       Command-line parameters      When you pass command-line parameters to a shell
      Single parameters script, the shell makes them available in the variables $1 , $2 , …. Consider the
                       following example:

                       $ cat hello
                       echo Hello $1, are you free $2?
                       $ ./hello Joe today
                       Hello Joe, are you free today?
                       $ ./hello Sue tomorrow
                       Hello Sue, are you free tomorrow?

        All parameters The $* contains all parameters at once, and the number of parameters is in $# :

                       $ cat parameter
                       echo $# parameters: $*
                       $ ./parameter
                       0 parameters:
                       $ ./parameter dog
                       1 parameters: dog
                       $ ./parameter dog cat mouse tree
                       4 parameters: dog cat mouse tree

                       Loops The for command lets you construct loops that iterate over a list of words
                       (separated by white space):

                       $ for i in 1 2 3
                       > do
                       >    echo And $i!
                       > done
                       And 1!
                       And 2!
                       And 3!

                       Here, the i variable assumes each of the listed values in turn as the commands
                       between do and done are executed.
                          This is even more fun if the words are taken from a variable:
9.6 The Shell As A Programming Language                                                                         121

$ list='4 5 6'
$ for i in $list
> do
>    echo And $i!
> done
And 4!
And 5!
And 6!

   If you omit the “in …”, the loop iterates over the command line parameters:           Loop over parameters

$ cat sort-wc
# Sort files according to their line count
for f
    echo `wc -l <"$f» lines in $f
done | sort -n
$ ./sort-wc /etc/passwd /etc/fstab /etc/motd

(The “wc -l ” command counts the lines of its standard input or the file(s) passed
on the command line.) Do note that you can redirect the standard output of a loop
to sort using a pipe line!

Alternatives You can use the aforementioned && and || operators to execute cer-
tain commands only under specific circumstances. The

# grepcp REGEX
rm -rf backup; mkdir backup
for f in *.txt
     grep $1 "$f" && cp "$f" backup

script, for example, copies a file to the backup directory only if its name ends with
.txt (the for loop ensures this) and which contain at least one line matching the
regular expression that is passed as a parameter.
   A useful tool for alternatives is the test command, which can check a large           test
variety of conditions. It returns an exit code of 0 (success), if the condition holds,
else a non-zero exit code (failure). For example, consider

# filetest NAME1 NAME2 ...
for name
   test -d "$name" && echo $name: directory
   test -f "$name" && echo $name: file
   test -L "$name" && echo $name: symbolic link

This script looks at a number of file names passed as parameters and outputs for
each one whether it refers to a directory, a (plain) file, or a symbolic link.

A The test command exists both as a free-standing program in /bin/test and
  as a built-in command in bash and other shells. These variants can differ
  subtly especially as far as more outlandish tests are concerned. If in doubt,
  read the documentation.
122                                                                          9 More About The Shell

         if      You can use the if command to make more than one command depend on a
              condition (in a convenient and readable fashion). You may write “[ …] ” instead
              of “test …”:
              # filetest2 NAME1 NAME2 ...
              for name
                   if [ -L "$name" ]
                        echo $name: symbolic link
                   elif [ -d "$name" ]
                        echo $name: directory
                   elif [ -f "$name" ]
                        echo $name: file
                        echo $name: no idea

              If the command after the if signals “success” (exit code 0), the commands after
              then will be executed, up to the next elif , else , or fi . If on the other hand it sig-
              nals “failure”, the command after the next elif will be evaluated next and its exit
              code will be considered. The shell continues the pattern until the matching fi is
              reached. Commands after the else are executed if none of the if or elif commands
              resulted in “success”. The elif and else branches may be omitted if they are not

              More loops With the for loop, the number of trips through the loop is fixed at
              the beginning (the number of words in the list). However, we often need to deal
              with situations where it is not clear at the beginning how often a loop should be
      while   executed. To handle this, the shell offers the while loop, which (like if ) executes
              a command whose success or failure determines what to do about the loop: On
              success, the “dependent” commands will be executed, on failure execution will
              continue after the loop.
                 The following script reads a file like

              Aunt delightful tea cosy
              Uncle great football

              (whose name is passed on the command line) and constructs a thank-you e-mail
              message from each line (Linux is very useful in daily life):

              # birthday FILE
              while read name email present
                  (echo $name
                    echo ""
                    echo "Thank you very much for $present!"
                    echo "I enjoyed it very much."
                    echo ""
                    echo "Best wishes"
                    echo "Tim") | mail -s "Many thanks!" $email
              done <$1

       read   The read command reads the input file line by line and splits each line at the colons
9.6 The Shell As A Programming Language                                                      123

(variable IFS ) into the three fields name , email , and present which are then made avail-
able as variables inside the loop. Somewhat counterintuitively, the input redirec-
tion for the loop can be found at the very end.

A Please test this script with innocuous e-mail addresses only, lest your rela-
  tions become confused!

C 9.9 [1] What is the difference (as far as loop execution is concerned) between
       for f; do …; done

       for f in $*; do …; done

      ? (Try it, if necessary)

C 9.10 [2] In the sort- wc script, why do we use the
       wc -l <$f

      instead of
       wc -l $f

C 9.11 [2] Alter the grepcp such that the list of files to be considered is also
  taken from the command line. (Hint: The shift shell command removes the
  first command line parameter from $ and pulls all others up to close the gap.
  After a shift , the previous $2 is now $1 , $3 is $2 and so on.)

C 9.12 [2] Why does the filetest script output
       $ ./filetest foo
       foo: file
       foo: symbolic link

      for symbolic links (instead of just »foo: symbolic link «)?
124                                                            9 More About The Shell

      Commands in this Chapter
      .       Reads a file containing shell commands as if they had been entered on
              the command line                                            bash (1) 119
      date    Displays the date and time                                  date (1) 112
      env     Outputs the process environment, or starts programs with an adjusted
              environment                                                  env (1) 114
      export  Defines and manages environment variables                    bash (1) 113
      hash    Shows and manages ”‘seen”’ commands in bash                 bash (1) 115
      set     Manages shell variables and options                         bash (1) 114
      source  Reads a file containing shell commands as if they had been entered on
              the command line                                            bash (1) 119
      test    Evaluates logical expressions on the command line
                                                                test (1), bash (1) 121
      unset   Deletes shell or environment variables                      bash (1) 114
      whereis Searches executable programs, manual pages, and source code for given
              programs                                                whereis (1) 115
      which   Searches programs along PATH                              which (1) 115

          • The sleep command waits for the number of seconds specified as the argu-
          • The echo command outputs its arguments.
          • The date and time may be determined using date
          • Various bash features support interactive use, such as command and file
            name autocompletion, command line editing, alias names and variables.
                                                                                                           $ echo tux
                                                                                                           $ ls
                                                                                                           $ /bin/su -

The File System

10.1   Terms . . . . . . . . . .             .   .   .   .   .   .   .   .   .   .   .   .   .   .   126
10.2   File Types. . . . . . . . .           .   .   .   .   .   .   .   .   .   .   .   .   .   .   126
10.3   The Linux Directory Tree . . .        .   .   .   .   .   .   .   .   .   .   .   .   .   .   127
10.4   Directory Tree and File Systems.      .   .   .   .   .   .   .   .   .   .   .   .   .   .   135

   •   Understanding the terms “file” and “file system”
   •   Recognising the different file types
   •   Knowing your way around the directory tree of a Linux system
   •   Knowing how external file systems are integrated into the directory tree

   • Basic Linux knowledge (from the previous chapters)
   • Handling files and directories (Chapter 6)

grd1-dateisystem-opt.tex   (be27bba8095b329b )
126                                                                                   10 The File System

                                              Table 10.1: Linux file types

                       Type                     ls -l      ls -F   Create using …
                       plain file                    -      name    diverse programs
                       directory                    d      name/
                       symbolic link                l      name@   ln -s
                       device file               b   or c   name    mknod
                       FIFO (named pipe)            p      name|   mkfifo
                       Unix-domain socket           s      name=   no command

                  10.1      Terms
            file Generally speaking, a file is a self-contained collection of data. There is no re-
                  striction on the type of the data within the file; a file can be a text of a few letters
                  or a multi-megabyte archive containing a user’s complete life works. Files do not
                  need to contain plain text. Images, sounds, executable programs and lots of other
                  things can be placed on a storage medium as files. To guess at the type of data
           file   contained in a file you can use the file command:

                  $ file /bin/ls /usr/bin/groups /etc/passwd
                  /bin/ls:         ELF 32-bit LSB executable, Intel 80386, 
                    version 1 (SYSV), for GNU/Linux 2.4.1, 
                    dynamically linked (uses shared libs), for GNU/Linux 2.4.1, stripped
                  /usr/bin/groups: Bourne shell script text executable
                  /etc/passwd:     ASCII text

                  B file guesses the type of a file based on rules in the /usr/share/file directory.
                    /usr/share/file/magic contains a clear-text version of the rules. You can define
                    your own rules by putting them into the /etc/magic file. Check magic (5) for
                  To function properly, a Linux system normally requires several thousand different
                  files. Added to that are the many files created and owned by the system’s various
      file system    A file system determines the method of arranging and managing data on a
                  storage medium. A hard disk basically stores bytes that the system must be able
                  to find again somehow—and as efficiently and flexibly as possible at that, even
                  for very huge files. The details of file system operation may differ (Linux knows
                  lots of different file systems, such as ext2 , ext3 , ext4 , ReiserFS, XFS, JFS, btrfs, …)
                  but what is presented to the user is largely the same: a tree-structured hierarchy
                  of file and directory names with files of different types. (See also Chapter 6.)

                  B In the Linux community, the term “file system” carries several meanings. In
                    addition to the meaning presented here—“method of arranging bytes on a
                    medium”—, a file system is often considered what we have been calling a
                    “directory tree”. In addition, a specific medium (hard disk partition, USB
                    key, …) together with the data on it is often called a “file system”—in the
                    sense that we say, for example, that hard links (section 6.4.2) do not work
                    “across file system boundaries”, that is, between two different partitions on
                    hard disk or between the hard disk and a USB key.

                  10.2      File Types
                  Linux systems subscribe to the basic premise “Everything is a file”. This may seem
                  confusing at first, but is a very useful concept. Six file types may be distinguished
                  in principle:
10.3 The Linux Directory Tree                                                                        127

Plain files This group includes texts, graphics, sound files, etc., but also exe-
      cutable programs. Plain files can be generated using the usual tools like
      editors, cat , shell output redirection, and so on.
Directories Also called “folders”; their function, as we have mentioned, is to help
     structure storage. A directory is basically a table giving file names and as-
     sociated inode numbers. Directories are created using the mkdir command.
Symbolic links Contain a path specification redirecting accesses to the link to
    a different file (similar to “shortcuts” in Windows). See also section 6.4.2.
    Symbolic links are created using ln -s .

Device files These files serve as interfaces to arbitrary devices such as disk drives.
     For example, the file /dev/fd0 represents the first floppy drive. Every write
     or read access to such a file is redirected to the corresponding device. De-
     vice files are created using the mknod command; this is usually the system
     administrator’s prerogative and is thus not explained in more detail in this

FIFOs Often called “named pipes”. Like the shell’s pipes, they allow the direct
     communication between processes without using intermediate files. A pro-
     cess opens the FIFO for writing and another one for reading. Unlike the
     pipes that the shell uses for its pipelines, which behave like files from a pro-
     gram’s point of view but are “anonymous”—they do not exist within the file
     system but only between related processes—, FIFOs have file names and can
     thus be opened like files by arbitrary programs. Besides, FIFOs may have
     access rights (pipes may not). FIFOs are created using the mkfifo command.
Unix-domain sockets Like FIFOs, Unix-domain sockets are a method of inter-
     process communication. They use essentially the same programming in-
     terface as “real” network communications across TCP/IP, but only work
     for communication peers on the same computer. On the other hand, Unix-
     domain sockets are considerably more efficient than TCP/IP. Unlike FIFOs,
     Unix-domain sockets allow bi-directional communications—both partici-
     pating processes can send as well as receive data. Unix-domain sockets are
     used, e. g., by the X11 graphic system, if the X server and clients run on the
     same computer. There is no special program to create Unix-domain sockets.

C 10.1 [3] Check your system for examples of the various file types. (Table 10.1
  shows you how to recognise the files in question.)

10.3     The Linux Directory Tree
A Linux system consists of hundreds of thousands of files. In order to keep track,
there are certain conventions for the directory structure and the files comprising a
Linux system, the Filesystem Hierarchy Standard (FHS). Most distributions adhere FHS
to this standard (possibly with small deviations). The FHS describes all directories
immediately below the file system’s root as well as a second level below /usr .
    The file system tree starts at the root directory, “/ ” (not to be confused with root directory
/root , the home directory of user root ). The root directory contains either just sub-
directories or else additionally, if no /boot directory exists, the operating system
    You can use the “ls -la / ” command to list the root directory’s subdirectories.
The result should look similar to figure 10.1. The individual subdirectories follow
FHS and therefore contain approximately the same files on every distribution. We
shall now take a closer look at some of the directories:
128                                                                                              10 The File System

$ cd /
$ ls -l
insgesamt 125
drwxr-xr-x    2 root     root    4096 Dez 20 12:37 bin
drwxr-xr-x    2 root     root    4096 Jan 27 13:19 boot
lrwxrwxrwx    1 root     root      17 Dez 20 12:51 cdrecorder 
                                                                                             -> /media/cdrecorder
lrwxrwxrwx    1   root   root      12   Dez   20   12:51   cdrom -> /media/cdrom
drwxr-xr-x   27   root   root   49152   Mär    4   07:49   dev
drwxr-xr-x   40   root   root    4096   Mär    4   09:16   etc
lrwxrwxrwx    1   root   root      13   Dez   20   12:51   floppy -> /media/floppy
drwxr-xr-x    6   root   root    4096   Dez   20   16:28   home
drwxr-xr-x    6   root   root    4096   Dez   20   12:36   lib
drwxr-xr-x    6   root   root    4096   Feb    2   12:43   media
drwxr-xr-x    2   root   root    4096   Mär   21    2002   mnt
drwxr-xr-x   14   root   root    4096   Mär    3   12:54   opt
dr-xr-xr-x   95   root   root       0   Mär    4   08:49   proc
drwx------   11   root   root    4096   Mär    3   16:09   root
drwxr-xr-x    4   root   root    4096   Dez   20   13:09   sbin
drwxr-xr-x    6   root   root    4096   Dez   20   12:36   srv
drwxrwxrwt   23   root   root    4096   Mär    4   10:45   tmp
drwxr-xr-x   13   root   root    4096   Dez   20   12:55   usr
drwxr-xr-x   17   root   root    4096   Dez   20   13:02   var

                                  Figure 10.1: Content of the root directory (SUSE)

                                  B There is considerable consensus about the FHS, but it is just as “binding”
                                    as anything on Linux, i. e., not that much. On the one hand, there certainly
                                    are Linux systems (for example the one on your broadband router or PVR)
                                    that are mostly touched only by the manufacturer and where conforming
                                    to every nook and cranny of the FHS does not gain anything. On the other
                                    hand, you may do whatever you like on your own system, but must be pre-
                                    pared to bear the consequences—your distributor assures you to keep to his
                                    side of the FHS bargain, but also expects you not to complain if you are not
                                    playing completely by the rules and problems do occur. For example, if you
                                    install a program in /usr/bin and the file in question gets overwritten during
                                    the next system upgrade, this is your own fault since, according to the FHS,
                                    you are not supposed to put your own programs into /usr/bin (/usr/local/bin
                                    would have been correct).

                                 The Operating System Kernel—/boot The /boot directory contains the actual op-
                                 erating system: vmlinuz is the Linux kernel. In the /boot directory there are also
                                 other files required for the boot loader (LILO or GRUB).

                                 General Utilities—/bin In /bin there are the most important executable programs
                                 (mostly system programs) which are necessary for the system to boot. This in-
                                 cludes, for example, mount and mkdir . Many of these programs are so essential
                                 that they are needed not just during system startup, but also when the system
                                 is running—like ls and grep . /bin also contains programs that are necessary to get
                                 a damaged system running again if only the file system containing the root direc-
                                 tory is available. Additional programs that are not required on boot or for system
                                 repair can be found in /usr/bin .

                                 Special System Programs—/sbin Like /bin , /sbin contains programs that are nec-
                                 essary to boot or repair the system. However, for the most part these are system
10.3 The Linux Directory Tree                                                                         129

configuration tools that can really be used only by root . “Normal” users can use
some of these programs to query the system, but can’t change anything. As with
/bin , there is a directory called /usr/sbin containing more system programs.

System Libraries—/lib This is where the “shared libraries” used by programs
in /bin and /sbin reside, as files and (symbolic) links. Shared libraries are pieces
of code that are used by various programs. Such libraries save a lot of resources,
since many processes use the same basic parts, and these basic parts must then be
loaded into memory only once; in addition, it is easier to fix bugs in such libraries
when they are in the system just once and all programs fetch the code in question
from one central file. Incidentally, below /lib/modules there are kernel modules, kernel modules
i. e., kernel code which is not necessarily in use—device drivers, file systems, or
network protocols. These modules can be loaded by the kernel when they are
needed, and in many cases also be removed after use.

Device Files—/dev This directory and its subdirectories contain a plethora of en-
tries for device files. Device files form the interface between the shell (or, gener- Device files
ally, the part of the system that is accessible to command-line users or program-
mers) to the device drivers inside the kernel. They have no “content” like other
files, but refer to a driver within the kernel via “device numbers”.

B In former times it was common for Linux distributors to include an entry in
  /dev for every conceivable device. So even a laptop Linux system included
  the device files required for ten hard disks with 63 partitions each, eight
  ISDN adapters, sixteen serial and four parallel interfaces, and so on. Today
  the trend is away from overfull /dev directories with one entry for every
  imaginable device and towards systems more closely tied to the running
  kernel, which only contain entries for devices that actually exist. The magic
  word in this context is udev (short for userspace /dev ) and will be discussed in
  more detail in Linux Administration I.

    Linux distinguishes between character devices and block devices. A character character devices
device is, for instance, a terminal, a mouse or a modem—a device that provides block devices
or processes single characters. A block device treats data in blocks—this includes
hard disks or floppy disks, where bytes cannot be read singly but only in groups
of 512 (or some such). Depending on their flavour, device files are labelled in “ls
-l ” output with a “c ” or “b ”:

crw-rw-rw-   1   root   root   10,   4   Oct   16   11:11   amigamouse
brw-rw----   1   root   disk    8,   1   Oct   16   11:11   sda1
brw-rw----   1   root   disk    8,   2   Oct   16   11:11   sda2
crw-rw-rw-   1   root   root    1,   3   Oct   16   11:11   null

Instead of the file length, the list contains two numbers. The first is the “major
device number” specifying the device’s type and governing which kernel driver
is in charge of this device. For example, all SCSI hard disks have major device
number 8. The second number is the “minor device number”. This is used by the
driver to distinguish between different similar or related devices or to denote the
various partitions of a disk.
    There are several notable pseudo devices. The null device, /dev/null , is like a pseudo devices
“dust bin” for program output that is not actually required, but must be directed
somewhere. With a command like
$ program >/dev/null

the program’s standard output, which would otherwise be displayed on the ter-
minal, is discarded. If /dev/null is read, it pretends to be an empty file and returns
end-of-file at once. /dev/null must be accessible to all users for reading and writ-
130                                                                                10 The File System

         The “devices” /dev/random and /dev/urandom return random bytes of “crypto-
      graphic quality” that are created from “noise” in the system—such as the in-
      tervals between unpredictable events like key presses. Data from /dev/random is
      suitable for creating keys for common cryptographic algorithms. The /dev/zero
      file returns an unlimited supply of null bytes; you can use these, for example, to
      create or overwrite files with the dd command.

      Configuration Files—/etc The /etc directory is very important; it contains the
      configuration files for most programs. Files /etc/inittab and /etc/init.d/* , for ex-
      ample, contain most of the system-specific data required to start system services.
      Here is a more detailed descriptionof the most important files—except for a few
      of them, only user root has write permission but everyone may read them.
      /etc/fstab This describes all mountable file systems and their properties (type,
             access method, “mount point”).
      /etc/hosts  This file is one of the configuration files of the TCP/IP network. It maps
             the names of network hosts to their IP addresses. In small networks and on
             freestanding hosts this can replace a name server.
      /etc/inittab  The /etc/inittab file is the configuration file for the init program and
             thus for the system start.
      /etc/init.d/* This directory contains the “init scripts” for various system services.
             These are used to start up or shut down system services when the system is
             booted or switched off.

                    On Red Hat distributions, this directory is called /etc/rc.d/init.d .

      /etc/issue  This file contains the greeting that is output before a user is asked to
             log in. After the installation of a new system this frequently contains the
             name of the vendor.
      /etc/motd This file contains the “message of the day” that appears after a user has
             successfully logged in. The system administrator can use this file to notify
             users of important facts and events1 .
      /etc/mtab This is a list of all mounted file systems including their mount points.
            /etc/mtab differs from /etc/fstab in that it contains all currently mounted file
            systems, while /etc/fstab contains only settings and options for file systems
             that might be mounted—typically on system boot but also later. Even that
             list is not exhaustive, since you can mount file systems via the command
             line where and how you like.

             B We’re really not supposed to put that kind of information in a file
               within /etc , where files ought to be static. Apparently, tradition has
               carried the day here.

      /etc/passwd  In /etc/passwd there is a list of all users that are known to the system, to-
             gether with various items of user-specific information. In spite of the name
             of the file, on modern systems the passwords are not stored in this file but
             in another one called /etc/shadow . Unlike /etc/passwd , that file is not readable
             by normal users.

      Accessories—/opt This directory is really intended for third-party software—
      complete packages prepared by vendors that are supposed to be installable with-
      out conflicting with distribution files or locally-installed files. Such software pack-
      ages occupy a subdirectory /opt/ ⟨package⟩. By rights, the /opt directory should be
      completely empty after a distribution has been installed on an empty disk.
         1 There is a well-known claim that the only thing all Unix systems in the world have in common is

      the “message of the day” asking users to remove unwanted files since all the disks are 98% full.
10.3 The Linux Directory Tree                                                                             131

“Unchanging Files”—/usr In /usr there are various subdirectories containing
programs and data files that are not essential for booting or repairing the system
or otherwise indispensable. The most important directories include:
/usr/bin System programs that are not essential for booting or otherwise impor-
/usr/sbin    More system programs for root
/usr/lib    Further libraries (not used for programs in /bin or /sbin
/usr/localDirectory for files installed by the local system administrator. Corre-
      sponds to the /opt directory—the distribution may not put anything here
/usr/share Architecture-independent data. In principle, a Linux network consist-
      ing, e. g., of Intel, SPARC and PowerPC hosts could share a single copy of
      /usr/share on a central server. However, today disk space is so cheap that no
      distribution takes the trouble of actually implementing this.

/usr/share/doc    Documentation, e. g., HOWTOs
/usr/share/info    Info pages
/usr/share/man    Manual pages (in subdirectories)
/usr/src    Source code for the kernel and other programs (if available)

B The name /usr is often erroneously considered an acronym of “Unix system
  resources”. Originally this directory derives from the time when computers
  often had a small, fast hard disk and another one that was bigger but slower.
  All the frequently-used programs and files went to the small disk, while the
  big disk (mounted as /usr ) served as a repository for files and programs
  that were either less frequently used or too big. Today this separation can
  be exploited in another way: With care, you can put /usr on its own partition
  and mount that partition “read-only”. It is even possible to import /usr from Read-only /usr
  a remote server, even though the falling prices for disk storage no longer
  make this necessary (the common Linux distributions do not support this,

A Window into the Kernel—/proc This is one of the most interesting and impor-
tant directories. /proc is really a “pseudo file system”: It does not occupy space on pseudo file system
disk, but its subdirectories and files are created by the kernel if and when someone
is interested in their content. You will find lots of data about running processes
as well as other information the kernel possesses about the computer’s hardware.
For instance, in some files you will find a complete hardware analysis. The most
important files include:
/proc/cpuinfo    This contains information about the CPU’s type and clock frequency.

/proc/devicesThis is a complete list of devices supported by the kernel including
      their major device numbers. This list is consulted when device files are cre-
/proc/dma A list of DMA channels in use. On today’s PCI-based systems this is
      neither very interesting nor important.

/proc/interrupts A list of all hardware interrupts in use. This contains the inter-
      rupt number, number of interrupts triggered and the drivers handling that
      particular interrupt. (An interrupt occurs in this list only if there is a driver
      in the kernel claiming it.)

/proc/ioports    Like /proc/interrupts , but for I/O ports.
132                                                                              10 The File System

              /proc/kcore This file is conspicuous for its size. It makes available the computer’s
                    complete RAM and is required for debugging the kernel. This file requires
                    root privileges for reading. You do well to stay away from it!

              /proc/loadavg This file contains three numbers measuring the CPU load during
                    the last 1, 5 and 15 minutes. These values are usually output by the uptime
                          Displays the memory and swap usage. This file is used by the free
              /proc/mounts Another list of all currently mounted file systems, mostly identical to

              /proc/scsi In this directory there is a file called scsi listing the available SCSI de-
                    vices. There is another subdirectory for every type of SCSI host adapter in
                    the system containing a file 0 (1 , 2 , …, for multiple adapters of the same type)
                    giving information about the SCSI adapter.

              /proc/version   Contains the version number and compilation date of the current

               B Back when /proc had not been invented, programs like the process status
                 display tool, ps , which had to access kernel information, needed to include
                 considerable knowledge about internal kernel data structures as well as the
                 appropriate access rights to read the data in question from the running ker-
                 nel. Since these data structures used to change fairly rapidly, it was often
                 necessary to install a new version of these programs along with a new ver-
                 sion of the kernel. The /proc file system serves as an abstraction layer be-
                 tween these internal data structures and the utilities: Today you just need
                 to ensure that after an internal change the data formats in /proc remain the
                 same—and ps and friends continue working as usual.

              Hardware Control—/sys The Linux kernel has featured this directory since ver-
              sion 2.6. Like /proc , it is made available on demand by the kernel itself and al-
              lows, in an extensive hierarchy of subdirectories, a consistent view on the available
              hardware. It also supports management operations on the hardware via various
              special files.

               B Theoretically, all entries in /proc that have nothing to do with individual
                 processes should slowly migrate to /sys . When this strategic goal is going
                 to be achieved, however, is anybody’s guess.

                Dynamically Changing Files—/var This directory contains dynamically changing
                files, distributed across different directories. When executing various programs,
                the user often creates data (frequently without being aware of the fact). For ex-
                ample, the man command causes compressed manual page sources to be uncom-
                pressed, while formatted man pages may be kept around for a while in case they
                are required again soon. Similarly, when a document is printed, the print data
                must be stored before being sent to the printer, e. g., in /var/spool/cups . Files in
      log files /var/log record login and logout times and other system events (the “log files”),
                /var/spool/cron contains information about regular automatic command invoca-
                tions, and users’ unread electronic mail is kept in /var/mail .

               B Just so you heard about it once (it might be on the exam): On Linux, the
                 system log files are generally handled by the “syslog” service. A program
                 called syslogd accepts messages from other programs and sorts these ac-
                 cording to their origin and priority (from “debugging help” to “error” and
                 “emergency, system is crashing right now”) into files below /var/log , where
                 you can find them later on. Other than to files, the syslog service can also
10.3 The Linux Directory Tree                                                           133

      write its messages elsewhere, such as to the console or via the network to
      another computer serving as a central “management station” that consoli-
      dates all log messages from your data center.

B Besides the syslogd , some Linux distributions also contain a klogd service.
  Its job is to accept messages from the operating system kernel and to pass
  them on to syslogd . Other distributions do not need a separate klogd since
  their syslogd can do that job itself.

B The Linux kernel emits all sorts of messages even before the system is booted
  far enough to run syslogd (and possibly klogd ) to accept them. Since the mes-
  sages might still be important, the Linux kernel stores them internally, and
  you can access them using the dmesg command.

Transient Files—/tmp Many utilities require temporary file space, for example
some editors or sort . In /tmp , all programs can deposit temporary data. Many
distributions can be set up to clean out /tmp when the system is booted; thus you
should not put anything of lasting importance there.

B According to tradition, /tmp is emptied during system startup but /var/tmp
  isn’t. You should check what your distribution does.

Server Files—/srv       Here you will find files offered by various server programs,
such as
drwxr-xr-x     2 root       root        4096 Sep 13 01:14 ftp
drwxr-xr-x     5 root       root        4096 Sep 9 23:00 www

This directory is a relatively new invention, and it is quite possible that it does
not yet exist on your system. Unfortunately there is no other obvious place for
web pages, an FTP server’s documents, etc., that the FHS authors could agree on
(the actual reason for the introduction of /srv ), so that on a system without /srv ,
these files could end up somewhere completely different, e. g., in subdirectories
of /usr/local or /var .

Access to CD-ROM or Floppies—/media This directory is often generated auto-
matically; it contains additional empty directories, like /media/cdrom and /media/
floppy , that can serve as mount points for CD-ROMs and floppies. Depending
on your hardware setup you should feel free to add further directories such as
/media/dvd , if these make sense as mount points and have not been preinstalled by
your distribution vendor.

Access to Other Storage Media—/mnt This directory (also empty) serves as a
mount point for short-term mounting of additional storage media. With some
distributions, such as those by Red Hat, media mountpoints for CD-ROM, floppy,
… might show up here instead of below /media .

User Home Directories—/home This directory contains the home directories of
all users except root (whose home directory is located elsewhere).

B If you have more than a few hundred users, it is sensible, for privacy protec-
  tion and efficiency, not to keep all home directories as immediate children
  of /home . You could, for example, use the users’ primary group as a criterion
  for further subdivision:

134                                                                        10 The File System

                     Table 10.2: Directory division according to the FHS

                     static                            dynamic
             local   /etc , /bin , /sbin , /lib        /dev , /var/log
           remote    /usr , /opt                       /home , /var/mail

      Administrator’s Home Directory—/root The system administrator’s home direc-
      tory is located in /root . This is a completely normal home directory similar to that
      of the other users, with the marked difference that it is not located below /home but
      immediately below the root directory (/ ).
          The reason for this is that /home is often located on a file system on a separate
      partition or hard disk. However, root must be able to access their own user envi-
      ronment even if the separate /home file system is not accessible for some reason.

      Lost property—lost+found (ext file systems only; not mandated by FHS.) This di-
      rectory is used for files that look reasonable but do not seem to belong to any
      directory. The file system consistency checker creates liks to such files in the
      lost+found directory on the same file system, so the system administrator can fig-
      ure out where the file really belongs; lost+found is created “on the off-chance” for
      the file system consistency checker to find in a fixed place (by convention, on the
      ext file systems, it always uses inode number 11).

      B Another motivation for the directory arrangement is as follows: The FHS di-
        vides files and directories roughly according to two criteria—do they need
        to be available locally or can they reside on another computer and be ac-
        cessed via the network, and are their contents static (do files only change
        by explicit administrator action) or do they change while the system is run-
        ning? (Table 10.2)
            The idea behind this division is to simplify system administration: Direc-
            tories can be moved to file servers and maintained centrally. Directories
            that do not contain dynamic data can be mounted read-only and are more
            resilient to crashes.

      C 10.2 [1] How many programs does your system contain in the “usual”

      C 10.3 [I]f grep is called with more than one file name on the command line,
        it outputs the name of the file in question in front of every matching line.
        This is possibly a problem if you invoke grep with a shell wildcard pattern
        (such as “*.txt ”), since the exact format of the grep output cannot be fore-
        seen, which may mess up programs further down the pipeline. How can
        you enforce output of the file name, even if the search pattern expands to a
        single file name only? (Hint: There is a very useful “file” in /dev .)

      C 10.4 [T]he “cp foo.txt /dev/null ” command does basically nothing, but the
        “mv foo.txt /dev/null ”—assuming suitable access permissions—replaces
        /dev/null by foo.txt . Why?

      C 10.5 [2] On your system, which (if any) software packages are installed be-
        low /opt ? Which ones are supplied by the distribution and which ones are
        third-party products? Should a distribution install a “teaser version” of a
        third-party product below /opt or elsewhere? What do you think?

      C 10.6 [1] Why is it inadvisable to make backup copies of the directory tree
        rooted at /proc ?
10.4 Directory Tree and File Systems                                                                   135

10.4      Directory Tree and File Systems
A Linux system’s directory tree usually extends over more than one partition on
disk, and removable media like CD-ROM disks, USB keys as well as portable MP3
players, digital cameras and so on must be taken into account. If you know your
way around Microsoft Windows, you are probably aware that this problem is
solved there by means of identifying different “drives” by means of letters—on
Linux, all available disk partitions and media are integrated in the directory tree
starting at “/ ”.
   In general, nothing prevents you from installing a complete Linux system
on a single hard disk partition. However, it is common to put at least the /home partitioning
directory—where users’ home directories reside—on its own partition. The ad-
vantage of this approach is that you can re-install the actual operating system,
your Linux distribution, completely from scratch without having to worry about
the safety of your own data (you simply need to pay attention at the correct mo-
ment, namely when you pick the target partition(s) for the installation in your
distribution’s installer.) This also simplifies the creation of backup copies.
   On larger server systems it is also quite usual to assign other directories, typi- server systems
cally /tmp , /var/tmp , or /var/spool , their own partitions. The goal is to prevent users
from disturbing system operations by filling important partitions completely. For
example, if /var is full, no protocol messages can be written to disk, so we want to
keep users from filling up the file system with large amounts of unread mail, un-
printed print jobs, or giant files in /var/tmp . On the other hand, all these partitions
tend to clutter up the system.

B More information and strategies for partitioning are presented in the Linup
  Front training manual, Linux Administration I.

    The /etc/fstab file describes how the system is assembled from various disk         /etc/fstab
partitions. During startup, the system arranges for the various file systems to be
made available—the Linux insider says “mounted”—in the correct places, which
you as a normal user do not need to worry about. What you may in fact be inter-
ested in, though, is how to access your CD-ROM disks and USB keys, and these
need to be mounted, too. Hence we do well to cover this topic briefly even though
it is really administrator country.
    To mount a medium, you require both the name of the device file for the
medium (usually a block device such as /dev/sda1 ) and a directory somewhere in
the directory tree where the content of the medium should appear—the so-called
mount point. This can be any directory.

B The directory doesn’t even have to be empty, although you cannot access the
  original content once you have mounted another medium “over” it. (The
  content reappears after you unmount the medium.)

A In principile, somebody could mount a removable medium over an impor-
  tant system directory such as /etc (ideally with a file called passwd containing
  a root entry without a password). This is why mounting of file systems in
  arbitrary places within the directory tree is restricted to the system adminis-
  trator, who will have no need for shenanigans like these, as they are already
  root .

B Earlier on, we called the “device file for the medium” /dev/sda1 . This is really
  the first partition on the first SCSI disk drive in the system—the real name
  may be completely different depending on the type of medium you are us-
  ing. Still it is an obvious name for USB keys, which for technical reasons are
  treated by the system as if they were SCSI devices.

   With this information—device name and mount point—a system administra-
tor can mount the medium as follows:
136                                                                   10 The File System

      # mount /dev/sda1 /media/usb

      This means that a file called file on the medium would appear as /media/usb/file
      in the directory tree. With a command such as

      # umount /media/usb                                                  Note: no ‘‘n’’

      the administrator can also unmount the medium again.

      Commands in this Chapter
      dmesg     Outputs the content of the kernel message buffer           dmesg (8)    133
      file      Guesses the type of a file’s content, according to rules    file (1)    126
      free      Displays main memory and swap space usage                  free (1)    132
      klogd     Accepts kernel log messages                               klogd (8)    133
      mkfifo    Creates FIFOs (named pipes)                              mkfifo (1)    127
      mknod     Creates device files                                       mknod (1)    127
      syslogd   Handles system log messages                             syslogd (8)    133
      uptime    Outputs the time since the last system boot as well as the system     load
                averages                                                 uptime (1)    131

         • Files are self-contained collections of data stored under a name. Linux uses
           the “file” abstraction also for devices and other objects.
         • The method of arranging data and administrative information on a disk is
           called a file system. The same term covers the complete tree-structured hi-
           erarchy of directories and files in the system or a specific storage medium
           together with the data on it.
         • Linux file systems contain plain files, directories, symbolic links, device files
           (two kinds), FIFOs, and Unix-domain sockets.
         • The Filesystem Hierarchy Standard (FHS) describes the meaning of the most
           important directories in a Linux system and is adhered to by most Linux
                                                                                               $ echo tux
                                                                                               $ ls
                                                                                               $ /bin/su -

Archiving and Compressing Files

11.1   Archival and Compression . . . . . .         . . . .      .   .   .   .   .   .   138
11.2   Archiving Files Using tar . . . . . . .      . . . .      .   .   .   .   .   .   139
11.3   Compressing Files with gzip . . . . . .      . . . .      .   .   .   .   .   .   142
11.4   Compressing Files with bzip2 . . . . . .     . . . .      .   .   .   .   .   .   143
11.5   Archiving and Compressing Files Using zip   and unzip .   .   .   .   .   .   .   144

   •   Understanding the terms “archival” and “compression”
   •   Being able to use tar
   •   Being able to compress and uncompress files with gzip and bzip2
   •   Being able to process files with zip and unzip

   • Use of the shell (Chapter 3)
   • Handling files and directories (Chapter 6)
   • Use of filters (Chapter 8)

grd1-targz-opt.tex   (be27bba8095b329b )
138                                                                       11 Archiving and Compressing Files

                         11.1      Archival and Compression
                         “Archival” is the process of collecting many files into a single on. The typical ap-
                         plication is storing a directory tree on magnetic tape—the magnetic tape drive
                         appears within Linux as a device file onto which the output of the archival pro-
                         gram can be written. Conversely, you can read the tape drive’s device file using a
                         de-archiver and reconstruct the directory tree from the archived data. Since most
                         of the relevant programs can both create and unravel archives, we discuss both
                         operations under the heading of “archival”.
                            “Compression” is the rewriting of data into a representation that saves space
                         compared to the original. Here we are only interested in “lossless” compression,
                         where it is possible to reconstruct the original in identical form from the com-
                         pressed data.

                          B The alternative is to achieve a higher degree of compression by abandoning
                            the requirement of being able to recreate the original perfectly. This “lossy”
                            approach is taken by compression schemes like JPEG for photographs and
                            “MPEG-1 Audio Layer 3” (better known as “MP3”) for audio data. The se-
                            cret here is to get rid of extraneous data; with MP3, for example, we throw
                            out those parts of the signal that, based on a “psycho-acoustic model” of hu-
                            man hearing, the listener will not be able to make out, anyway, and encode
                            the rest as efficiently as possible. JPEG works along similar lines.
      run-length encoding As a simple illustration, you might represent a character string like


                         more compactly as


                         Here, “*4B ” stands for a sequence of four “B ” characters. This simple approach
                         is called “run-length encoding” and is found even today, for example, in fax ma-
                         chines (with refinements). “Real” compression programs like gzip or bzip2 use
                         more sophisticated methods.
                             While programs that combine archival and compression are used widely in the
                         Windows world (PKZIP, WinZIP and so on), both steps are commonly handled
                         separately on Linux and Unix. A popular approach is to archive a set of files first
                         using tar before compressing the output of tar using, say, gzip —PKZIP and friends
                         compress each file on its own and then collect the compressed files into a single
                         big one.
                             The advantage of this approach compared to that of PKZIP and its relatives is
                         that compression can take place across several original files, which yields higher
                         compression rates. However, this also counts as a disadvantage: If the compressed
                         archive is damaged (e. g., due to a faulty medium or flipped bits during transmis-
                         sion), the whole archive can become unusable starting at that point.

                          B Naturally even on Linux nobody keeps you from first compressing your files
                            and then archiving them. Unfortunately this is not as convenient as the
                            other approach.

                          B Of course there are Linux implementations of compression and archival
                            programs popular in the Windows world, like zip and rar .

                          C 11.1 [1] Why does the run-length encoding example use AA instead of *2A ?
                          C 11.2 [2] How would your represent the string “A*2B****A ” using the run-
                            length encoding method shown above?
11.2 Archiving Files Using tar                                                                            139

11.2        Archiving Files Using tar
The name tar derives from “tape archive”. The program writes individual files to
the archival file one after the other and annotates them with additional informa-
tion (like the date, access permissions, owner, …). Even though tar was originally
meant to be used with magnetic tape drives, tar archives can be written directly
on various media. Among other uses, tar files are the standard format for dissem-
inating the source code for Linux and other free software packages.
   The GNU implementation of tar commonly used on Linux includes various ex-
tensions not found in the tar implementations of other Unix variants. For example,
GNU tar supports creating multi-volume archives spanning several media. This multi-volume archives
even allows backup copies to floppy disk, which of course is only worthwhile for
small archives.

B A small remark on the side: The split command lets you cut large files like
  archives into convenient pieces that can be copied to floppy disks or sent via
  e-mail, and can be re-joined at their destination using cat .

    The advantages of tar include: It is straightforward to use, it is reliable and
works well, it can be used universally on all Unix and Linux systems. Its disad-
vantages are that faults on the medium may lead to problems, and not all versions
of tar can store device files (which is only an issue if you want to perform a full
backup of your system).
    tar archives can contain files and whole directory hierarchies. If Windows me-
dia have been mounted into the directory tree across the network, even their con-
tent can be archived using tar . Archives created using tar are normally uncom-
pressed, but can be compressed using external compression software (nowadays
usually gzip or bzip2 ). This is not a good idea as far as backup copies are concerned,
since bit errors in the compressed data usually lead to the loss of the remainder
of the archive.
    Typical suffixes for tar archives include .tar , .tar.bz2 , or .tar.gz , depending on
whether they have been compressed not at all, using bzip2 , or using gzip . The .tgz
suffix is also common when zipped tar -formatted data need to be stored on a DOS
file system. tar ’s syntax is

tar    ⟨options⟩ ⟨file⟩|| ⟨directory⟩ …

and the most important include:                                                           tar   options

-c   (“create”) creates a new archive
-f   ⟨file⟩ creates the new archive on (or reads an existing archive from) ⟨file⟩, where
         ⟨file⟩ can be a plain file or a device file (among others)
-M   handles multi-volume archives
-r   appends files to the archive (not for magnetic tape)
-t   displays the “table of contents” of the archive
-u   replaces files which are newer than their version inside the archive. If a file is
        not archived at all, it is inserted (not for magnetic tape)
-v   Verbose mode—displays what tar is doing at the moment
-x   extracts files and directories from an archive
-z   compresses or decompresses the archive using gzip
-Z   compresses or decompresses the archive using compress (not normally available
       on Linux)
-j   compresses or decompresses the archive using bzip2
140                                                                     11 Archiving and Compressing Files

      option syntax tar ’s option syntax is somewhat unusual, in that it is possible (as is elsewhere)
                   to “bundle” several options after a single dash, including (extraordinarily) ones
                   such as -f that take a parameter. Option parameters need to be specified after the
                   “bundle” and are matched to the corresponding parameter-taking options within
                   the bundle in turn.

                    B You may leave out the dash in front of the first “option bundle”—you will
                      often see commands like

                         B tar cvf …
                         However, we don’t recommend this.

                     The following example archives all files within the current directory whose
                   names begin with data to the file data.tar in the user’s home directory:

                    # tar -cvf ~/data.tar data*

                    The -c option arranges for the archive to be newly created, “-f ~/data.tar ” gives the
                    name for the archive. The -v option does not change anything about the result; it
                    only causes the names of files to appear on the screen as they are being archived.
                    (If one of the files to be archived is really a directory, the complete content of the
                    directory will also be added to the archive.)
        directories     tar also lets you archive complete directories. It is better to do this from the
                    enclosing directory, which will create a subdirectory in the archive which is also
                    recreated when the archive is unpacked. The following example shows this in
                    more detail.
                    # cd /
                    # tar -cvf /tmp/home.tar /home

                   The system administrator root stores an archive of the /home directory (i. e., all user
                   data) under the name of home.tar . This is stored in the /tmp directory.

                    B If files or directories are given using absolute path names, tar automatically
                      stores them as relative path names (in other words, the “/ ” at the start of
                      each name is removed). This avoids problems when unpacking the archive
                      on other computers (see Exercise 11.6).

                      You can display the “table of contents” of an archive using the -t option:

                    $ tar -tf data.tar

                   The -v option makes tar somewhat more talkative:

                    $ tar -tvf   data.tar
                    -rw-r--r--   joe/joe    7 2009-01-27 12:04 data1
                    -rw-r--r--   joe/joe    8 2009-01-27 12:04 data10
                    -rw-r--r--   joe/joe    7 2009-01-27 12:04 data2
11.2 Archiving Files Using tar                                                          141

You can unpack the data using the -x option:

$ tar -xf data.tar

In this case tar produces no output on the terminal at all—you have to give the -v
option again:

$ tar -xvf data.tar

B If the archive contains a directory hierarchy, this is faithfully reconstructed
  in the current direcotry. (You will remember that tar makes relative path
  names from all absolute ones.) You can unpack the archive relative to any
  directory—it always keeps its structure.

   You can also give file or directory names on unpacking. In this case only the
files or directories in question will be unpacked. However, you need to take care
to match the names in the archive exactly:

$ tar -cf data.tar ./data
$ tar -tvf data.tar
drwxr-xr-x joe/joe     0 2009-01-27 12:04 ./data/
-rw-r--r-- joe/joe     7 2009-01-27 12:04 ./data/data2

$ mkdir data-new
$ cd data-new
$ tar -xvf ../data.tar data/data2                                        ./   missing
tar: data/data2: Not found in archive
tar: Error exit delayed from previous errors

C 11.3 [!2] Store a list of the files in your home directory in a file called content .
  Create a tar archive from that file. Compare the original file and the archive.
  What do you notice?

C 11.4 [2] Create three or four empty files and add them to the archive you
  just created.

C 11.5 [2] Remove the original files and then unpack the content of the tar

C 11.6 [2] Why does GNU tar prophylactically remove the / at the beginning
  of the path name, if the name of a file or directory to be archived is given as
  an absolute path name? (Hint: Consider the

       # tar -cvf /tmp/etc-backup.tar /etc

      command and imagine what will happen if etc- backup.tar (a) contains abso-
      lute path names, and (b) is transferred to another computer and unpacked
142                                                      11 Archiving and Compressing Files

      11.3       Compressing Files with gzip
      The most common compression program for Linux is gzip by Jean-loup Gailly and
      Mark Adler. It is used to compress single files (which, as mentioned earlier, may
      be archives containing many files).

      B The gzip program (short for “GNU zip ”) was published in 1992 to avoid
        problems with the compress program, which was the standard compression
        tool on proprietary Unix versions. compress is based on the Lempel-Ziv-
        Welch algorithm (LZW), which used to be covered by US patent 4,558,302.
        This patent belonged to the Sperry (later Unisys) corporation and expired
        on 20 June 2003. On the other hand, gzip uses the DEFLATE method by Phil
        Katz [RFC1951], which is based on a non-patented precursor of LZW called
        LZ77 as well as the Huffman encoding scheme and is free of patent claims.
        Besides, it works better than LZW.

      B gzip can decompress files compressed using compress , because the Unisys
        patent only covered compression. You can recognise such files by the “.Z ”
        suffix of their names.

      B gzip is not to be confused with PKZIP and similar Windows programs with
        “ZIP” in their names. These programs can compress files and then archive
        them immediately; gzip only takes care of the compression and leaves the
        archiving to programs like tar or cpio .—gzip can unpack ZIP archives as long
        as the archive contains exactly one file which has been packed using the
        DEFLATE method.

         gzip processes and replaces single files, appending the File*.gz suffix to their
      names. This substitution happens independently of whether the resulting file is
      actually smaller than the original. If several files are to be compressed into a single
      archive, tar and gzip must be combined.
         The most important options of gzip include:
      -c   writes the compressed file to standard output, instead of replacing the original;
             the original remains unmodified
      -d   uncompresses the file (alternatively: gunzip works like gzip -d )
      -l   (“list”) displays important information about the compressed file, such as the
               file name, original and packed size

      -r   (“recursive”) compresses files in subdirectories
      -S   ⟨suffix⟩ uses the specified suffix in place of .gz
      -v   outputs the name and compression factor of every file
      -1 … -9   specifies a compression factor: -1 (or --fast ) works most quickly but does
              not compress as thoroughly, while -9 (or --best ) results in the best compres-
              sion at a slower speed; the default setting is -6 .
         The following command compresses the letter.tex file, stores the compressed
      file as letter.tex.gz and deletes the original:

      $ gzip letter.tex

      The file can be unpacked using

      $ gzip -d letter.tex

11.4 Compressing Files with bzip2                                                     143

$ gunzip letter.tex

Here the compressed file is saved as letter.tex.t instead of letter.tex.gz (-S .t ),
and the compression rate achieved for the file is output (-v ):

$ gzip -vS .t letter.tex

The -S option must also be specified on decompression, since “gzip -d ” expects a
file with a .gz suffix:

$ gzip -dS .t letter.tex

If all .tex files are to be compressed in a file tex- all.tar.gz , the command is

$ tar -cvzf tex-all.tar.gz *.tex

Remember that tar does not delete the original files! This can be unpacked using

$ tar -xvzf tex-all.tar.gz

C 11.7 [2] Compress the tar archive from Exercise 11.3 using maximum com-

C 11.8 [!3] Inspect the content of the compressed archive. Restore the original
  tar archive.

C 11.9 [!2] How would you go about packing all of the contents of your home
  directory into a gzip -compressed file?

11.4      Compressing Files with bzip2
bzip2 by Julian Seward is a compression program which is largely compatible to
gzip . However, it uses a different method which leads to higher compression ratios
but requires more time and memory to compress (to decompress, the difference
is not as significant).

B If you are desperate to know: bzip2 uses a “Burrows-Wheeler transforma-
  tion” to encode frequently-occurring substrings in the input to sequences of
  single characters. This intermediate result is sorted according to the “local
  frequency” of individual characters and the sorted result, after being run-
  length encoded, is encoded using the Huffman scheme. The Huffman code
  is then written to a file in a very compact manner.

B What about bzip ? bzip was a predecessor of bzip2 which used arithmetic en-
  coding rather than Huffman encoding after the block transformation. How-
  ever, the author decided to give arithmetic coding a wide berth due to the
  various software patent issues that surround it.

   Like gzip , bzip2 accepts one or more file names as parameters for compression.
The files are replaced by compressed versions, whose names end in .bz2 .
   The -c and -d options correspond to the eponymous options to gzip . However,
the “quality options” -1 to -9 work differently: They determine the block size used
during compression. The default value is -9 , while -1 does not offer a significant
speed gain.
144                                                         11 Archiving and Compressing Files

            B -9 uses a 900 KiB block size. This corresponds to a memory usage of approx-
              imately 3.7 MiB to decompress (7.6 MiB to compress), which on contempo-
              rary hardware should not present a problem. A further increase of the block
              size does not appear to yield an appreciable advantage.—It is worth empha-
              sising that the choice of block size on compression determines the amount of
              memory necessary during decompression, which you should keep in mind
              if you use your multi-Gibibyte PC to prepare .bz2 files for computers with
              very little memory (toasters, set-top boxes, …). bzip2 (1) explains this in more

               By analogy to gzip and gunzip , bunzip2 is used to decompress files compressed
            using bzip2 . (This is really just another name for the bzip2 program: You can also
            use “bzip2 -d ” to decompress files.)

            11.5     Archiving and Compressing Files Using zip and un-

            To exchange data with Windows computers or on the Internet, it often makes
            sense to use the widespread ZIP file format (although many file archive programs
            on Windows can also deal with .tar.gz today). On Linux, there are two separate
            programs zip (to create archives) and unzip (to unpack archives).

            B Depending on your distribution you may have to install these programs sep-
              arately. On Debian GNU/Linux, for example, there are two distinct pack-
              ages, zip and unzip .

      zip      The zip program combines archiving and compressing in a way that may be
            familiar to you from programs like PKZIP. In the simplest case, it collects the files
            passed on the command line:

            $ zip file1 file2
              adding: file1 (deflated 66%)
              adding: file2 (deflated 62%)
            $ _

            (Here is the name of the resulting archive.)
              You can use the -r option to tell zip to descend into subdirectories recursively:

            $ zip -r ziptest
              adding: ziptest/ (stored 0%)
              adding: ziptest/testfile (deflated 62%)
              adding: ziptest/file2 (deflated 62%)
              adding: ziptest/file1 (deflated 66%)

            With the -@ option, zip reads the names of the files to be archived from its standard

            $ find ziptest | zip -@ test
              adding: ziptest/ (stored 0%)
              adding: ziptest/testfile (deflated 62%)
              adding: ziptest/file2 (deflated 62%)
              adding: ziptest/file1 (deflated 66%)

            (You may omit the .zip suffix from the name of the archive file.)

            B zip knows about two methods of adding files to an archive. stored means that
              the file was stored without compression, while deflated denotes compres-
              sion (and the percentage states how much the file was compressed—“deflated
11.5 Archiving and Compressing Files Using zip and unzip                                           145

      62% ”,
           for example, means that, inside the archive, the file is only 38% of its
      original size). zip automatically chooses the more sensible approach, unless
      you disable compression completely using the -0 option.

B If you invoke zip with an existing ZIP archive as its first parameter and do
  not specify anything else, the files to be archived are added to the archive
  on top of its existing content (existing files with the same names are over-
  written). In this case zip behaves differently from tar and cpio (just so you
  know). If you want a “clean” archive, you must remove the file first.

B Besides stupidly adding of files, zip supports several other modes of opera-
  tion: The -u option “updates” the archive by adding files to the archive only
  if the file mentioned on the command line is newer than a pre-existing file of
  the same name in the archive (named files that are not in the archive yet are
  added in any case). The -f option “freshens” the archive—files inside the
  archive are overwritten with newer versions from the command line, but
  only if they actually exist in the archive already (no completely new files
  are added to the archive). The -d option considers the file names on the
  command line as the names of files within the archive and deletes those.

B Newer versions of zip also support the -FS (“filesystem sync”) mode: This
  mode “synchronises” an archive with the file system by doing essentially
  what -u does, but also deleting files from the archive that have not been
  named on the command line (or, in case of -r , are part of a directory being
  searched). The advantage of this method compared to a full reconstruction
  of the archive is that any preexisting unchanged files in the archive do not
  need to be compressed again.

   zip supports all sorts of options, and you can use “zip -h ” to look at a list (or
“ -h2 to look at a more verbose list). The man page, zip (1), is also very informative.
   You can unpack a ZIP archive again using unzip (this can also be a ZIP archive          unzip
from a Windows machine). It is best to take a peek inside the archive first, us-
ing the -v option, to see what is in there—this may save you some hassle with
subdirectories (or their absence).

$ unzip -v test                                             The .zip suffix may be omitted
 Length   Method    Size Cmpr       Date      Time     CRC-32      Name
-------- ------ ------- ----     ----------   -----   --------     ----
        0 Stored        0   0%   2012-02-29   09:29   00000000     ziptest/
   16163 Defl:N      6191 62%    2012-02-29   09:46   0d9df6ad     ziptest/testfile
   18092 Defl:N      6811 62%    2012-02-29   09:01   4e46f4a1     ziptest/file2
   35147 Defl:N     12119 66%    2012-02-29   09:01   6677f57c     ziptest/file1
--------          ------- ---                                      -------
 69402            25121 64%                                      4 files

  Calling unzip with the name of the archive as its single parameter suffices to
unpack the archive:

$ mv ziptest ziptest.orig
$ unzip test
   creating: ziptest/
  inflating: ziptest/testfile
  inflating: ziptest/file2
  inflating: ziptest/file1

Use the -d option to unpack the archive in a different directory than the current
one. This directory is created first if necessary:
146                                                     11 Archiving and Compressing Files

      $ unzip -d dir test
         creating: dir/ziptest/
        inflating: dir/ziptest/testfile
        inflating: dir/ziptest/file2
        inflating: dir/ziptest/file1

      If you name particular files on the command line, then only these files will be

      $ rm -rf ziptest
      $ unzip test ziptest/file1
        inflating: ziptest/file1

      (In this case, the ziptest directory will also be created.)

      B Alternatively, you can use the -x option to selectively exclude certain files
        from being unpacked:

             $ rm -rf ziptest
             $ unzip test -x ziptest/file1
                creating: ziptest/
               inflating: ziptest/testfile
               inflating: ziptest/file2

         You can also use shell search patterns to unpack certain files (or prevent them
      from being unpacked):

      $ rm -rf ziptest
      $ unzip test "ziptest/f*"
        inflating: ziptest/file2
        inflating: ziptest/file1
      $ rm -rf ziptest
      $ unzip test -x "*/t*"
         creating: ziptest/
        inflating: ziptest/file2
        inflating: ziptest/file1

      (Note the quotes, which are used to hide the search patterns from the actual shell
      so unzip gets to see them.) Unlike in the shell, the search patterns refer to the
      complete file names (including any “/ ”).
         As is to be expected, unzip also supports various other options. Look at the
      program’s help information using “unzip -h ” or “unzip -hh ”, or read unzip (1).

      C 11.10 [!2] Create some files inside your home directory and store them in a
        zip archive. Look at the content of the archive using “unzip -v ”. Unpack the
        archive inside the /tmp directory.

      C 11.11 [!1] What happens if a file that you are about to unpack using unzip
        already exists in the file system?
11.5 Bibliography                                                                    147

C 11.12 [2] A ZIP archive contains two subdirectories a and b , which
  in turn contain a mixture of files with various suffixes (e. g., .c , .txt , and
  .dat ). Give a unzip command to extract the complete content of a except for
  the .txt files (in one step).

Commands in this Chapter
bunzip2  File decompression program for .bz2 files                bzip2 (1)    144
bzip2    File compression program                                bzip2 (1)    139
gzip     File compression utility                                 gzip (1)    139
split    Splits a file into pieces up to a given maximum size     split (1)    139
tar      File archive manager                                      tar (1)    138
unzip    Decompression software for (Windows-style) ZIP archives
                                                                 unzip (1)    145
vimtutor Interactive introduction to vim                      vimtutor (1)    150
zip      Archival and compression software like PKZIP              zip (1)    144

   • “Archival” collects many files into one large file. “Compression” reversibly
     determines a more compact representation for a file.
   • tar is the most common archival program on Linux.
   • gzip is a program for compressing and decompressing arbitrary files. It can
     be used together with tar .
   • bzip2 is another compression program. It can achieve higher compression
     ratios than gzip , but also needs more time and memory.
   • The zip and unzip programs are available to process ZIP archives as used (for
     example) by the PKZIP program on Windows.

RFC1951 P. Deutsch. “DEFLATE Compressed Data Format Specification version
     1.3”, May 1996.          
                                                                                           $ echo tux
                                                                                           $ ls
                                                                                           $ /bin/su -

Sample Solutions
This appendix contains sample solutions for selected exercises.

1.2     There is a copy of linux- 0.01.tar.gz on .

      1. False. GPL software may be sold for arbitrary amounts of money, as long as
         the buyer receives the source code (etc.) and the GPL rights.

      2. False. Companies are encouraged to develop products based on GPL code,
         but these products must also be distributed under the GPL. Of course a
         company is not required to give away their product to the world at large—it
         only needs to make the source code available to its direct customers who
         bought the executables, but these may make full use of their rights to the
         software under the GPL.
      3. True.
      4. False. You may use a program freely without having accepted the GPL (it
         is not a contract). The GPL governs just the redistribution of the software,
         and you can peruse the GPL before doing that. (Interactive programs are
         supposed to call your attention to the GPL.) The observation is true that
         only those conditions can be valid that the software recipient could know
         before the purchase of the product; since the GPL gives to the recipient rights
         that he would otherwise not have had at all—such as the right to distribute
         original or modified code—this is not a problem: One may ignore the GPL
         completely and still do all with the software that copyright allows for. This
         is a marked difference to the EULAs of proprietary programs; these try to es-
         tablish a contract relationship in which the buyer explicitly gives away rights
         that he would otherwise have been entitled to by copyright law (such as the
         right to inspect the program to find out its structure). This of course only
         works before the purchase (if at all).

1.5 This exercise can of course not be answered correctly in printed courseware.
Look around—on or in the weekly edition of .

2.2 In both cases, the message “Login incorrect ” appears, but only after the pass-
word has been prompted for and entered. This is supposed to make it difficult
to guess valid user names (those that do not elicit an error message right away).
The way the system is set up, a “cracker” cannot tell whether the user name was
150                                                                       A Sample Solutions

      invalid already, or whether the password was wrong, which makes breaking into
      the system a lot more difficult.

      2.5 Decency forbids us from printing a sample program here. It is reasonably
      simple using the (deprecated) C function getpass (3).

      3.1 In the login shell, the output is “-bash ”, whereas in the “subshell” it is “bash ”.
      The minus sign at the beginning tells the shell to behave as a login shell rather
      than a “normal” shell, which pertains to the initialisation.

      3.2 alias is an internal command (does not work otherwise). rm is an external
      command. Within bash, echo and test are internal commands but are also avail-
      able as external commands (executable program files), since other shells do not
      implement them internally. In bash’s case, they are internal mostly for reasons of

      4.2   Try “apropos process ” or “man -k process ”.

      4.5 The format and tools for info files were written in the mid-1980s. HTML
      wasn’t even invented then.

      5.1 In theory, you could start every program on the system and check whether
      it behaves like a text editor … but that might take more time than this exercise
      is worth. You could, for example, begin with a command like “apropos edit ” and
      see which of the man pages in the output correspond with an actual text editor
      (rather than an editor for graphics, icons, X resources or some such). Text editors
      from graphical desktop environments such as KDE or GNOME frequently do not
      actually have man pages, but are documented within the desktop environment, so
      it can be useful to check the desktop’s menus for a submenu such as “Editors” or
      “Office”. The third possibility is to use a package management command—such
      as “rpm -qa ” or “dpkg -l ”—to obtain a list of installed software packages and check
      for text editors there.

      5.2   The program is called vimtutor .

      6.1 In Linux, the current directory is a process attribute, i. e., every process has
      its own current directory (with DOS, the current directory is a feature of the drive,
      which of course is inappropriate in a multi-user system). Therefore cd must be
      an internal command. If it was an external command, it would be executed in a
      new process, change that process’s current directory and quit, while the invoking
      shell’s current directory would remain unchanged throughout the process.

      6.4 If a file name is passed to ls , it outputs information about that file only. With
      a directory name, it outputs information about all the files in that directory.

      6.5   The -d option to ls does exactly that.

      6.6   This could look approximately like so:

      $ mkdir -p grd1-test/dir1 grd1-test/dir2 grd1-test/dir3
      $ cd grd1-test/dir1
      $ vi hello
      $ cd
      $ vi grd1-test/dir2/howdy
      $ ls grd1-test/dir1/hallo grd1-test/dir2/howdy
A Sample Solutions                                                                        151

$ rmdir grd1-test/dir3
$ rmdir grd1-test/dir2
rmdir: grd1-test/dir2: Directory not empty

To remove a directory using rmdir , it must be empty (except for the entries “. ” and
“.. ”, which cannot be removed).

6.7      The matching names are, respectively
    (a) prog.c , prog1.c , prog2.c , progabc.c
    (b) prog1.c , prog2.c

    (c) p1.txt , p2.txt , p21.txt , p22.txt
    (d) p1.txt , p21.txt , p22.txt , p22.dat
    (e) all names
    (f) all names except prog (does not contain a period)

6.8 “ls ” without arguments lists the content of the current directory. Directories
in the current directory are only mentioned by name. “ls ” with arguments, on the
other hand (and in particular “ls * ”—ls does not get to see the search pattern, after
all) lists information about the given arguments. For directories this means that
the content of the directories is listed as well.

6.9 The “- l ” file (visible in the output of the first command) is interpreted as an
option by the ls command. Thus it does not show up in the output of the second
command, since ls with path name arguments only outputs information about
the files specified as arguments.

6.10 If the asterisk matched file names starting with a dot, the recursive dele-
tion command “rm -r * ” would also apply to the “.. ” entry of a directory. This
would delete not just subdirectories of the current directory, but also the enclosing
directory and so on.

6.11      Here are the commands:
$   cd
$   cp   /etc/services myservices
$   mv   myservices src.dat
$   cp   src.dat /tmp
$   rm   src.dat /tmp/src.dat

6.12 When you rename a directory, all its files and subdirectories will automat-
ically be “moved” so as to be within the directory with its new name. An -R to mv
is therefore completely unnecessary.

6.13 The simple-minded approach—something like “rm -file ”—fails because rm
misinterprets the file name as a sequence of options. The same goes for commands
like “rm "-file" ” or “rm '-file' ”. The following methods work better:
      1. With “rm ./-file ”, the dash is no longer at the start of the parameter and
         thus no longer introduces an option.
      2. With “rm -- -file ”, you tell rm that there are definitely no options after the
         “-- ” but only path names. This also works with many other programs.
152                                                                       A Sample Solutions

      6.14 During the replacement of the “* ”, the “- i ” file is picked up as well. Since
      the file names are inserted into the command line in ASCII order, rm sees a param-
      eter list like
      -i a.txt b.jpg c.dat                                                       or whatever

      and considers the “- i ” the option -i , which makes it remove files only with confir-
      mation. We hope that this is sufficient to get you to think things over.

      6.15 If you edit the file via one link, the new content should also be visible via
      the other link. However, there are “clever” editors which do not overwrite your
      file when saving, but save a new file and rename it afterwards. In this case you
      will have two different files again.

      6.16 If the target of a symbolic link does not exist (any longer), accessing that
      “dangling” link will lead to an error message.

      6.17    To itself. You can recognise the file system root directory by this.

      6.18 On this system, the /home directory is on a separate partition and has inode
      number 2 on that partition, while the / directory is inode number 2 on its own
      file system. Since inode numbers are only unique within the same physical file
      system, the same number can show up for different files in “ls -i ” output; this is
      no cause for concern.

      6.19 Hard links are indistinguishable, equivalent names for the same file (or,
      hypothetically, directory). But every directory has a “link” called “.. ” referring to
      the directory “above”. There can be just one such link per directory, which does
      not agree with the idea of several equivalent names for that directory. Another
      argument against hard links on directories is that for every name in the file system
      tree there must be a unique path leading to the root directory (/ ) in a finite number
      of steps. If hard links to directories were allowed, a command sequence such as

      $ mkdir -p a/b
      $ cd a/b
      $ ln .. c

      could lead to a loop.

      6.20 The reference counter for the subdirectory has the value 2 (one link results
      from the name of the subdirectory in ~ , one from the “. ” link in the subdirectory
      itself). If there were additional subdirectories within the directory, their “.. ” links
      would increment the reference counter beyond its minimum value of 2.

      6.21 The chain of symbolic links will be followed until you reach something that
      is not a symbolic link. However, the maximum length of such chains is usually
      bounded (see Exercise 6.22).

      6.22 Examining this question becomes easier if you can use shell loops (see, e. g.,
      the Advanced Linux training manual). Something like

      $   touch d
      $   ln -s d L1
      $   i=1
      $   while ls -lH L$i >/dev/null
      >   do
      >      ln -s L$i L$((i+1))
A Sample Solutions                                                                     153

>   i=$((i+1))
> done

creates a “chain” of symbolic links where every link points to the previous one.
This is continued until the “ls -lH ” command fails. The error message will tell
you which length is still allowed. (On the author’s computer, the result is “40”,
which in real life should not unduly cramp anybody’s style.)

6.23 Hard links need hardly any space, since they are only additional directory
entries. Symbolic links are separate files and need one inode at least (every file
has its own inode). Also, some space is required to store the name of the target
file. In theory, disk space is assigned to files in units of the file system’s block
size (1 KiB or more, usually 4 KiB), but there is a special exception in the ext file
systems for “short” symbolic links (smaller than approximately 60 bytes), which
can be stored within the inode itself and do not require a full data block. Other
file systems such as the Reiser file system can handle short files of any type very
efficiently, thus the space required for symbolic links ought to be negligible.

6.24   One possible command could be “find / -size +1024k -print ”.

6.25   The basic approach is something like

find . -maxdepth 1   ⟨tests⟩ -ok rm '{}' \;

The ⟨tests⟩ should match the file as closely as possible. The “-maxdepth 1 ” option
restricts the search to the current directory (no subdirectories). In the simplest
case, use “ls -i ” to determine the file’s inode number (e.g., 4711) and then use

find . -maxdepth 1 -inum 4711 -exec rm -f '{}' \;

to delete the file.

6.26   Add a line like
find /tmp -user $LOGNAME -type f -exec rm '{}' \;

or—more efficiently—

find /tmp -user $LOGNAME -type f -print0 \
    | xargs -0 -r rm -f

to the file .bash_logout in your home directory. (The LOGNAME environment variable
contains the current user name.)

6.27   Use a command like “locate '*/README' ”. Of course, something like “find /
-name README ”
             would also do the trick, but it will take a lot longer.

6.28 Immediately after its creation the new file does not occur in the database
and thus cannot be found (you need to run updatedb first). The database also
doesn’t notice that you have deleted the file until you invoke updatedb again.—It
is best not to invoke updatedb directly but by means of the shell script that your
distribution uses (e. g., /etc/cron.daily/find on Debian GNU/Linux). This ensures
that updatedb uses the same parameters as always.

6.29 slocate should only return file names that the invoking user may access.
The /etc/shadow file, which contains the users’ encrypted passwords, is restricted
to the system administrator (see Linux Administration I).
154                                                                     A Sample Solutions

      7.1 The regular expression 𝑟+ is a mere abbreviation of 𝑟𝑟* , so we could do with-
      out + . Things are different with ? , for which there is no convenient substitute,
      at least if we must assume (as in grep vs. egrep ) that we cannot substitute 𝑟? by
      \(\| 𝑟\) (GNU grep supports the synonymous 𝑟{,1} —see table 7.1—but this is not
      supported by the grep implementations of the traditional Unix vendors.

      7.2   You want a sample solution for this? Don’t be ridiculous.—Well, if you insist
      egrep "\<king('s daughter)?\>" frog.txt

      7.3   One possibility might be

      grep :/bin/bash$ /etc/passwd

      7.4 We’re looking for words starting with a (possibly empty) sequence of conso-
      nants, then there is an “a ”, then possibly consonants again, then an “e ”, and so on.
      We must take care, in particular, not to let extra vowels “slip through”. The result-
      ing regular expression is fairly unsavoury, so we allow ourselves some notational

      $ k='[âeiou]*'
      $ grep -i $̂
                 { k}a${k}e${k}i${k}o${k}u${k}$ /usr/share/dict/words

      (You may look up the words on your own time.)

      7.5   Try

      egrep '(\<[A-Za-z]{4,}\>).*\<\1\>' frog.txt

      We need egrep for the back reference. The word brackets are also required (try it
      without them!).

      8.1   A (probable) explanation is that the ls program works roughly like this:

      Read directory information to list 𝑙;
      if ( option -U not specified) {
           Sort the entries of 𝑙;
      Write 𝑙 to standard output;
A Sample Solutions                                                                         155

That is, everything is being read, then sorted (or not), and then output.
    The other explanation is that, at the time the filelist entry is being read, there
has not in fact been anything written to the file to begin with. For efficiency, most
file-writing programs buffer their output internally and only call upon the oper-
ating system to write to the file if a substantial amount of data has been collected
(e. g. 8192 bytes). This can be observed with commands that produce very much
output relatively slowly; the output file will grow by 8192 bytes at a time.

8.2 When ls writes to the screen (or, generally, a “screen-like” device), it formats
the output differently from when it writes to a “real” file: It tries to display several
file names on the same line if the file names’ length permits, and can also colour
the file names according to their type. When output is redirected to a “real” file,
just the names will be output one per line, with no formatting.
   At first glance this seems to contradict the claim that programs do not know
whether their output goes to the screen or elsewhere. This claim is correct in the
normal case, but if a program is seriously interested in whether its output goes
to a screen-like device (a “terminal”) it can ask the system. In the case of ls , the
reasoning behind this is that terminal output is usually looked at by people who
deserve as much information as possible. Redirected output, on the other hand, is
processed by other programs and should therefore be simple; hence the limitation
to one file name per line and the omission of colors, which must be set up using
terminal control characters which would “pollute” the output.

8.3 The shell arranges for the output redirection before the command is invoked.
Therefore the command sees only an empty input file, which usually does not lead
to the desired result.

8.4 The file is read from the beginning, and all that is read is appended to the
file at the same time, so that it grows until it takes up all the free space on the disk.

8.5    You need to redirect standard output to standard error output:
echo Error >&2

8.6    There is nothing wrong in principle with
… | tee foo | tee bar | …

However, it is easier to write
… | tee foo bar | …

See also tee ’s documentation (man page or info page).

8.7    Pipe the list of file names through “cat -b ”.

8.8    One method would be “head -n 13 | tail -n 1 ”.

8.10    tail   notices it, emits a warning, and continues from the new end of file.

8.11    The tail window displays

The first line results from the first echo ; the second echo overwrites the complete
file, but “tail -f ” knows that it has already written the first six characters of the
file (“Hello ” and a newline character)—it just waits for the file to become longer,
and then outputs whatever is new, in particular, “orld ” (and a newline character).
156                                                                      A Sample Solutions

      8.14 The line containing the name “de Leaping” is sorted wrongly, since on that
      line the second field isn’t really the first name but the word “Leaping”. If you look
      closely at the examples you will note that the sorted output is always correct—
      regarding “Leaping”, not “Gwen”. This is a strong argument for the second type
      of input file, the one with the colon as the separator character.

      8.15 You can sort the lines by year using “sort -k 1.4,1.8 ”. If two lines are equal
      according to the sort key, sort makes an “emergency comparison” considering the
      whole line, which in this case leads to the months getting sorted correctly within
      every year. If you want to be sure and very explicit, you could also write “sorkt -k
      1.4,1.8 -k 1.1,1.2 ”.

      8.19     Use something like

      cut -d: -f 4 /etc/passwd | sort -u | wc -l

      The cut command isolates the group number in each line of the user database.
      “sort -u ” constructs a sorted list of all group numbers containing each group num-
      ber exactly once. Finally, “wc -l ” counts the number of lines in that list. The result
      is the number of different primary groups in use on the system.

      9.1     For example:
            1. %d-%m-%Y
            2. %y-%j (WK%V)
            3. %Hh%Mm%Ss

      9.2     We don’t know either, but try something like “TZ=America/Los_Angeles date ”.

      9.4 If you change an environment variable in the child process, its value in the
      parent process remains unmodified. There are ways and means to pass informa-
      tion back to the parent process but the environment is not one.

      9.5 Start a new shell and remove the PATH variable from the environment (with-
      out deleting the variable itself). Try starting external programs.—If PATH does not
      exist at all, the shell will not start external programs.

      9.6 Unfortunately we cannot offer a system-independent sample solution; you
      need to see for yourself (using which ).

      9.7  Using whereis , you should be able to locate two files called /usr/share/man/man1/
      crontab.1.gzand /usr/share/man/man5/crontab.5.gz . The former contains the docu-
      mentation for the actual crontab command, the latter the documentation for the
      format of the files that crontab creates. (The details are irrelevant for this exercise;
      see Advanced Linux.)

      9.8 bash uses character sequences of the form “! ⟨character⟩” to access previous
      commands (an alternative to keyboard functions such as Ctrl + r which have mi-
      grated from the C shell to bash ). The “!" ” character sequence, however, counts as
      a syntax error.

      9.9     None.

      9.10 If the file name is passed as a parameter, wc insists on outputting it together
      with the number of lines. If wc reads its standard input, it only outputs the line
A Sample Solutions                                                                        157

9.11   Try something like


for f
     grep $pattern "$f" && cp "$f" backup

After the shift , the regular expression is no longer the first parameter, and that
must be taken into account for “for f ”.

9.12 If the -f file test is applied to a symbolic link, it always applies to the file (or
directory, or whatever) that the link refers to. Hence it also succeeds if the name
in question is really just a symbolic link. (Why does this problem not apply to
filetest2 ?)

10.2   You can find out about this using something like

ls /bin /sbin /usr/bin /usr/sbin | wc -l

Alternatively, you can hit    Tab   twice at a shell prompt—the shell will answer
something like

Display all 2371 possibilities? (y or n)

and that is—depending on PATH —your answer. (If you are logged in as a normal—
non-privileged—user, the files in /sbin and /usr/sbin will not normally be included
in the total.)

10.3 Use “grep ⟨pattern⟩ *.txt /dev/null ” instead of “grep ⟨pattern⟩ *.txt ”. Thus
grep always has at least two file name parameters, but /dev/null does not otherwise
change the output.—The GNU implementation of grep , which is commonly found
on Linux, supports an -H option which does the same thing but in a non-portable

10.4 With cp to an existing file, the file is opened for writing and truncated to
length 0, before the source data is written to it. For /dev/null , this makes the data
disappear. With mv to an existing file, the target file is first removed—and that is
a directory operation which, disregarding the special nature of /dev/null , simply
removes the name null from the directory /dev and creates a new file called null
with the content of foo.txt in its place.

10.6 It is inadvisable because firstly it doesn’t work right, secondly the data in
question isn’t worth backing up anyway since it is changing constantly (you would
be wasting lots of space on backup media and time for copying), and thirdly be-
cause such a backup could never be restored. Uncontrolled write operations to,
say, /proc/kcore will with great certainty lead to a system crash.

11.1   Because AA is shorter than *2A .

11.2 The main problem is representing the asterisk. In the simplest case you
could write something like “A*12B*4*A ”. Of course compression suffers by repre-
senting the single asterisk by three characters; you could institute an exception to
let, for example, ** stand for a single asterisk, but this makes the decompression
step more complicated.
158                                                                       A Sample Solutions

      11.3 Use “ls -l >content ” and “tar -cvf content.tar content ”. You will notice that
      the archive is considerably bigger than the original. This is due to the metadata in
      the archive. tar does not compress; it archives. To create an archive (a file) from a
      single file is not really a workable idea.

      11.4    For example, enter “touch file{1,2,3} ” and “tar -rvf content.tar file* ”.

      11.5    Unpack the archive using “tar -xvf content.tar ”.

      11.6 If you want to unpack etc- backup.tar on the other computer (e. g., because
      you want to see what is in there) and the archive contains absolute path names, the
      data will not be written to a subdirectory etc of the current directory, but they end
      up in the /etc directory of the remote computer. This is very likely not what you
      had in mind. (Of course you should take care not to unpack an archive containing
      relative path names while / is your current directory.)

      11.7    If you want to use gzip , enter “gzip -9 contents.tar ”.

      11.8 Take care: To handle gzip -compressed tar archives, tar requires the -z op-
      tion: “tar -tzf contents.tar.gz ”. To restore the original archive, you need the “gunzip
      contents.tar.gz ” command.

      11.9    Try “tar -cvzf /tmp/homearchive.tar ~ ”.

      11.11 unzip offers to ignore the file in the archive, to rename it, or to overwrite
      the existing file.

      11.12    Try something like

      $ unzip "a/*" -x "*/*.c"
                                                                                       $ echo tux
                                                                                       $ ls
                                                                                       $ /bin/su -

Example Files
In various places, the fairy tale The Frog King, more exactly The Frog King, or Iron
Henry, from German Children’s and Domestic Fairy Tales by the brothers Grimm, is
used as an example. The fairy tale is presented here in its entirety to allow for
comparisons with the examples.

The Frog King, or Iron Henry

In olden times when wishing still helped one, there lived a king whose
daughters were all beautiful, but the youngest was so beautiful that
the sun itself, which has seen so much, was astonished whenever it
shone in her face.

Close by the king's castle lay a great dark forest, and under an old
lime-tree in the forest was a well, and when the day was very warm,
the king's child went out into the forest and sat down by the side of
the cool fountain, and when she was bored she took a golden ball, and
threw it up on high and caught it, and this ball was her favorite

Now it so happened that on one occasion the princess's golden ball did
not fall into the little hand which she was holding up for it, but on
to the ground beyond, and rolled straight into the water. The king's
daughter followed it with her eyes, but it vanished, and the well was
deep, so deep that the bottom could not be seen. At this she began to
cry, and cried louder and louder, and could not be comforted.

And as she thus lamented someone said to her, »What ails you, king's
daughter? You weep so that even a stone would show pity.«

She looked round to the side from whence the voice came, and saw a
frog stretching forth its big, ugly head from the water. »Ah, old
water-splasher, is it you,« she said, »I am weeping for my golden
ball, which has fallen into the well.«

»Be quiet, and do not weep,« answered the frog, »I can help you, but
what will you give me if I bring your plaything up again?«

»Whatever you will have, dear frog,« said she, »My clothes, my pearls
and jewels, and even the golden crown which I am wearing.«

grd1-beispiele.tex   (be27bba8095b329b )
160                                                                           B Example Files

      The frog answered, »I do not care for your clothes, your pearls and
      jewels, nor for your golden crown, but if you will love me and let me
      be your companion and play-fellow, and sit by you at your little
      table, and eat off your little golden plate, and drink out of your
      little cup, and sleep in your little bed - if you will promise me this
      I will go down below, and bring you your golden ball up again.«

      »Oh yes,« said she, »I promise you all you wish, if you will but bring
      me my ball back again.« But she thought, »How the silly frog does
      talk. All he does is to sit in the water with the other frogs, and
      croak. He can be no companion to any human being.«

      But the frog when he had received this promise, put his head into the
      water and sank down; and in a short while came swimming up again with
      the ball in his mouth, and threw it on the grass. The king's daughter
      was delighted to see her pretty plaything once more, and picked it up,
      and ran away with it.

      »Wait, wait,« said the frog. »Take me with you. I can't run as you
      can.« But what did it avail him to scream his croak, croak, after her,
      as loudly as he could. She did not listen to it, but ran home and soon
      forgot the poor frog, who was forced to go back into his well again.

      The next day when she had seated herself at table with the king and
      all the courtiers, and was eating from her little golden plate,
      something came creeping splish splash, splish splash, up the marble
      staircase, and when it had got to the top, it knocked at the door and
      cried, »Princess, youngest princess, open the door for me.«

      She ran to see who was outside, but when she opened the door, there
      sat the frog in front of it. Then she slammed the door to, in great
      haste, sat down to dinner again, and was quite frightened.

      The king saw plainly that her heart was beating violently, and said,
      »My child, what are you so afraid of? Is there perchance a giant
      outside who wants to carry you away?«

      »Ah, no,« replied she. »It is no giant but a disgusting frog.«

      »What does that frog want from you?«

      »Yesterday as I was in the forest sitting by the well, playing, my
      golden ball fell into the water. And because I cried so, the frog
      brought it out again for me, and because he so insisted, I promised
      him he should be my companion, but I never thought he would be able to
      come out of his water. And now he is outside there, and wants to come
      in to me.«

      In the meantime it   knocked a second time, and cried, »Princess,
      youngest princess,   open the door for me, do you not know what you said
      to me yesterday by   the cool waters of the well. Princess, youngest
      princess, open the   door for me.«

      Then said the king, »That which you have promised must you perform.
      Go and let him in.«

      She went and opened the door, and the frog hopped in and followed her,
      step by step, to her chair. There he sat and cried, »Lift me up beside
B Example Files                                                                 161

you.« She delayed, until at last the king commanded her to do it. Once
the frog was on the chair he wanted to be on the table, and when he
was on the table he said, »Now, push your little golden plate nearer
to me that we may eat together.« The frog enjoyed what he ate, but
almost every mouthful she took choked her.

At length he said, »I have eaten and am satisfied, now I am tired,
carry me into your little room and make your little silken bed ready,
and we will both lie down and go to sleep.« The king's daughter began
to cry, for she was afraid of the cold frog which she did not like to
touch, and which was now to sleep in her pretty, clean little bed.

But the king grew angry and said, »He who helped you when you were in
trouble ought not afterwards to be despised by you.«

So she took hold of the frog with two fingers, carried him upstairs,
and put him in a corner, but when she was in bed he crept to her and
said, »I am tired, I want to sleep as well as you, lift me up or I
will tell your father.«

At this she was terribly angry, and took him up and threw him with all
her might against the wall. »Now, will you be quiet, odious frog,«
said she. But when he fell down he was no frog but a king's son with
kind and beautiful eyes. He by her father's will was now her dear
companion and husband. Then he told her how he had been bewitched by a
wicked witch, and how no one could have delivered him from the well
but herself, and that to-morrow they would go together into his

And indeed, the next morning a carriage came driving up with eight
white horses, which had white ostrich feathers on their heads, and
were harnessed with golden chains, and behind stood the young king's
servant Faithful Henry.

Faithful Henry had been so unhappy when his master was changed into a
frog, that he had caused three iron bands to be laid round his heart,
lest it should burst with grief and sadness. The carriage was to
conduct the young king into his kingdom. Faithful Henry helped them
both in, and placed himself behind again, and was full of joy because
of this deliverance.

And when they had driven a part of the way the king's son heard a
cracking behind him as if something had broken. So he turned round and
cried, »Henry, the carriage is breaking.« »No, master, it is not the
carriage. It is a band from my heart, which was put there in my great
pain when you were a frog and imprisoned in the well.«

Again and once again while they were on their way something cracked,
and each time the king's son thought the carriage was breaking, but it
was only the bands which were springing from the heart of Faithful
Henry because his master was set free and was happy.

   (Linup Front GmbH would like to point out that the authors strongly disap-
prove of any cruelty to animals.)
                                                                                                   $ echo tux
                                                                                                   $ ls
                                                                                                   $ /bin/su -

LPIC-1 Certification

C.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.

C.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.

grd1-objs-101.tex   (be27bba8095b329b )
164                                                                                         C 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      –

                            C.3 LPI Objectives In This Manual
                            103.1     Work on the command line
                            Weight       4
                            Description Candidates should be able to interact with shells and commands
                            using the command line. The objective assumes the Bash shell.
                            Key Knowledge Areas

                               • Use single shell commands and one line command sequences to perform
                                 basic tasks on the command line
                               • Use and modify the shell environment including defining, referencing and
                                 exporting environment variables
                               • Use and edit command history
                               • Invoke commands inside and outside the defined path

                            The following is a partial list of the used files, terms and utilities:

                               •   bash
                               •   echo
                               •   env
                               •   export
                               •   pwd
                               •   set
                               •   unset
                               •   man
                               •   uname
                               •   history
                               •   .bash_history
C LPIC-1 Certification                                                             165

103.2     Process text streams using filters
Weight      3
Description Candidates should should be able to apply filters to text streams.
Key Knowledge Areas

   • Send text files and output streams through text utility filters to modify the
     output using standard UNIX commands found in the GNU textutils pack-

The following is a partial list of the used files, terms and utilities:

   •   cat
   •   cut
   •   expand
   •   fmt
   •   head
   •   join
   •   less
   •   nl
   •   od
   •   paste
   •   pr
   •   sed
   •   sort
   •   split
   •   tail
   •   tr
   •   unexpand
   •   uniq
   •   wc

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
166                                                                   C LPIC-1 Certification

         •   file
         •   gzip
         •   gunzip
         •   bzip2
         •   xz
         •   file globbing

      103.4      Use streams, pipes and redirects
      Weight        4
      Description Candidates should be able to redirect streams and connect them in
      order to efficiently process textual data. Tasks include redirecting standard input,
      standard output and standard error, piping the output of one command to the
      input of another command, using the output of one command as arguments to
      another command and sending output to both stdout and a file.
      Key Knowledge Areas

         •   Redirecting standard input, standard output and standard error
         •   Pipe the output of one command to the input of another command
         •   Use the output of one command as arguments to another command
         •   Send output to both stdout and a file

      The following is a partial list of the used files, terms and utilities:

         • tee
         • xargs

      103.7      Search text files using regular expressions
      Weight         2
      Description Candidates should be able to manipulate files and text data using
      regular expressions. This objective includes creating simple regular expressions
      containing several notational elements. It also includes using regular expression
      tools to perform searches through a filesystem or file content.
      Key Knowledge Areas

         • Create simple regular expressions containing several notational elements
         • Use regular expression tools to perform searches through a filesystem or file

      The following is a partial list of the used files, terms and utilities:

         •   grep
         •   egrep
         •   fgrep
         •   sed
         •   regex (7)

      103.8      Perform basic file editing operations using vi
      Weight        3
      Description Candidates should be able to edit text files using vi . This objective
      includes vi navigation, basic vi modes, inserting, editing, deleting, copying and
      finding text.
      Key Knowledge Areas

         • Navigate a document using vi
         • Use basic vi modes
C LPIC-1 Certification                                                             167

   • Insert, edit, delete, copy and find text

The following is a partial list of the used files, terms and utilities:

   •   vi
   •   /, ?
   •   h, j, k, l
   •   i, o, a
   •   c , d , p , y , dd , yy
   •   ZZ , :w! , :q! , :e!

104.6       Create and change hard and symbolic links
Weight          2
Description Candidates should be able to create and manage hard and symbolic
links to a file.
Key Knowledge Areas

   •   Create links
   •   Identify hard and/or soft links
   •   Copying versus linking files
   •   Use links to support system administration tasks

The following is a partial list of the used files, terms and utilities:

   • ln
   • ls

104.7       Find system files and place files in the correct location
Weight        2
Description Candidates should be thoroughly familiar with the Filesystem Hi-
erarchy Standard (FHS), including typical file locations and directory classifica-
Key Knowledge Areas

   • Understand the correct locations of files under the FHS
   • Find files and commands on a Linux system
   • Know the location and purpose of important file and directories as defined
     in the FHS

The following is a partial list of the used files, terms and utilities:

   •   find
   •   locate
   •   updatedb
   •   whereis
   •   which
   •   type
   •   /etc/updatedb.conf
                                                                                       $ 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.

.       Reads a file containing shell commands as if they had been entered on
        the command line                                                bash (1) 119
apropos Shows all manual pages whose NAME sections contain a given keyword
                                                                      apropos (1) 45
bash    The “Bourne-Again-Shell”, an interactive command interpreter
                                                                         bash (1) 36
bunzip2 File decompression program for .bz2 files                       bzip2 (1) 144
bzip2   File compression program                                       bzip2 (1) 139
cat     Concatenates files (among other things)                           cat (1) 100
cd      Changes a shell’s current working directory                      bash (1) 67
convmv  Converts file names between character encodings                 convmv (1) 64
cp      Copies files                                                        cp (1) 74
cut     Extracts fields or columns from its input                         cut (1) 106
date    Displays the date and time                                 date (1) 112, 39
dmesg   Outputs the content of the kernel message buffer                dmesg (8) 133
echo    Writes all its parameters to standard output, separated by spaces
                                                               bash (1), echo (1) 39
ed      Primitive (but useful) line-oriented text editor                   ed (1) 51
egrep   Searches files for lines matching specific regular expressions; extended
        regular expressions are allowed                                  grep (1) 89
elvis   Popular “clone” of the vi editor                                elvis (1) 50
emacs   Powerful extensible screen-oriented text editor emacs (1), Info: emacs 55
env     Outputs the process environment, or starts programs with an adjusted
        environment                                                      env (1) 114
ex      Powerful line-oriented text editor (really vi )                    vi (1) 50
exit    Quits a shell                                                    bash (1) 32
export  Defines and manages environment variables                        bash (1) 113
fgrep   Searches files for lines with specific content; no regular expressions al-
        lowed                                                           fgrep (1) 89
file    Guesses the type of a file’s content, according to rules         file (1) 126
find    Searches files matching certain given criteria       find (1), Info: find 81
free    Displays main memory and swap space usage                       free (1) 132
grep    Searches files for lines matching a given regular expression grep (1) 89
groff   Sophisticated typesetting program                          groff (1) 43, 45
gzip    File compression utility                                        gzip (1) 139
hash    Shows and manages ”‘seen”’ commands in bash                     bash (1) 115
170                                                                      D Command Index

      head       Displays the beginning of a file                                 head (1) 100
      help       Displays on-line help for bash commands                     bash (1) 39, 42
      info       Displays GNU Info pages on a character-based terminal            info (1) 45
      jove       Text editor inspired by emacs                                    jove (1) 56
      kdesu      Starts a program as a different user on KDE           KDE: help:/kdesu 33
      klogd      Accepts kernel log messages                                   klogd (8) 133
      less       Displays texts (such as manual pages) by page               less (1) 44, 80
      ln         Creates (“hard” or symbolic) links                                 ln (1) 76
      locate     Finds files by name in a file name database                      locate (1) 84
      logout     Terminates a shell session                                       bash (1) 31
      ls         Lists file information or directory contents                        ls (1) 67
      man        Displays system manual pages                                      man (1) 42
      manpath    Determines the search path for system manual pages           manpath (1) 43
      mkdir      Creates new directories                                         mkdir (1) 69
      mkfifo     Creates FIFOs (named pipes)                                  mkfifo (1) 127
      mknod      Creates device files                                           mknod (1) 127
      more       Displays text data by page                                       more (1) 80
      mv         Moves files to different directories or renames them                 mv (1) 75
      paste      Joins lines from different input files                          paste (1) 107
      pwd        Displays the name of the current working directory pwd (1), bash (1) 67
      reset      Resets a terminal’s character set to a “reasonable” value       tset (1) 100
      rm         Removes files or directories                                        rm (1) 75
      rmdir      Removes (empty) directories                                     rmdir (1) 70
      sed        Stream-oriented editor, copies its input to its output making changes in
                 the process                                                       sed (1) 51
      set        Manages shell variables and options                             bash (1) 114
      slocate    Searches file by name in a file name database, taking file permissions into
                 account                                                      slocate (1) 85
      sort       Sorts its input by line                                         sort (1) 101
      source     Reads a file containing shell commands as if they had been entered on
                 the command line                                                bash (1) 119
      split      Splits a file into pieces up to a given maximum size           split (1) 139
      su         Starts a shell using a different user’s identity                    su (1) 32
      sudo       Allows normal users to execute certain commands with administrator
                 privileges                                                       sudo (8) 33
      syslogd    Handles system log messages                                 syslogd (8) 133
      tail       Displays a file’s end                                            tail (1) 100
      tar        File archive manager                                             tar (1) 138
      test       Evaluates logical expressions on the command line
                                                                       test (1), bash (1) 121
      type       Determines the type of command (internal, external, alias) bash (1) 39
      uniq       Replaces sequences of identical lines in its input by single specimens
                                                                                 uniq (1) 105
      unset      Deletes shell or environment variables                          bash (1) 114
      unzip      Decompression software for (Windows-style) ZIP archives
                                                                               unzip (1) 145
      updatedb    Creates the file name database for locate                   updatedb (1) 85
      uptime     Outputs the time since the last system boot as well as the system load
                 averages                                                     uptime (1) 131
      vi         Screen-oriented text editor                                        vi (1) 50
      vim        Popular “clone” of the vi editor                                  vim (1) 50
      vimtutor    Interactive introduction to vim                           vimtutor (1) 150
      whatis     Locates manual pages with a given keyword in its description
                                                                                whatis (1) 45
      whereis    Searches executable programs, manual pages, and source code for given
                 programs                                                    whereis (1) 115
      which      Searches programs along PATH                                  which (1) 115
D Command Index                                                                     171

xargs   Constructs command lines from its standard input
                                                       xargs (1),   Info: find 83
zip     Archival and compression software like PKZIP                  zip (1) 144
                                                                                        $ 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

. , 66                                     convmv , 64
. , 119                                    cp , 74–77, 79, 157
.. , 66                                          -a (option), 79
./ , 141                                         -i (option), 74
/ , 134                                          -L (option), 79
                                                 -l (option), 77, 79
Adler, Mark, 141                                 -P (option), 79
alias , 39, 150                                  -s (option), 79
apropos , 45                               cpio , 142, 144
awk , 107                                  cron , 85
                                           crontab , 116, 156
bash , 36–37, 40, 42, 47, 67, 91, 95–96,   cut , 106–107, 156
            113–115, 117, 119, 121,              -c (option), 106–107
            123–124, 156, 169–170                -d (option), 107
      -c (option), 118                           -f (option), 107
~/.bash_history , 117                            --output-delimiter (option),   107
Bell Laboratories, 14                            -s (option), 107
Berkeley, 14
/bin , 39, 128–129, 131                    date , 39,    112–113
/bin/ls , 115                              dd , 129
block devices, 129                         Debian Free Software Guidelines, 19
/boot , 127–128                            Debian project, 25
Bourne, Stephen L., 36                     definitions, 11
BSD, 14                                    /dev , 129, 157
BSD license, 18                            /dev/fd0 , 127
buffers, 51                                 /dev/null , 129, 134, 157
bunzip2 , 144                              /dev/random , 129
bzip , 143                                 /dev/tty , 95
bzip2 , 138–139, 143–144, 147              /dev/urandom , 129
      -1 (option), 143                     /dev/zero , 129
      -9 (option), 143                     dirs , 67
      -c (option), 143                     dmesg , 133
      -d (option), 143–144
                                           echo , 39, 71, 101, 112, 150, 155
C, 14                                               (option), 112
Canonical Ltd., 26                         ed , 51
cat , 97, 100, 126, 139                    egrep , 89–90, 153–154
cd , 39, 66–67, 86, 150                    elvis , 50
character devices, 129                     emacs , 55, 57–60, 89
chmod , 82                                 env , 114
command substitution, 96                   environment variable
compress , 139, 141–142                          LANG , 101–102
174                                                                                     Index

           LC_ALL , 101–102                     gzip ,   138–139, 141–144, 147, 158
           LC_COLLATE , 101–102                          -1 (option), 142
           LOGNAME , 153                                 -6 (option), 142
           MANPATH , 43                                  -9 (option), 142
           PATH , 66, 114–116, 119,  123,                --best (option), 142
                  156–157, 170                           -c (option), 142
            TERM , 80                                    -d (option), 142–143
            TZ , 112                                     --fast (option), 142
      environment variables, 113                         -l (option), 142
      /etc , 130                                         -r (option), 142
      /etc/cron.daily , 85                               -S (option), 142–143
      /etc/fstab , 130, 135                              -v (option), 142
      /etc/hosts , 130
      /etc/init.d/* , 130                       hash , 115
      /etc/inittab , 130                                 (option), 115
      /etc/issue , 130                          head , 100–101
      /etc/magic , 126                                -c (option), 101
      /etc/motd , 130                                 -n (option), 100
      /etc/mtab , 130, 132                            - 𝑛 (option), 100
      /etc/passwd , 91, 98, 108, 130            help , 39, 42, 115
      /etc/rc.d/init.d , 130                    /home , 80, 133–134, 140
      /etc/shadow , 86, 130, 153
      /etc/sysconfig/locate , 85                - i , 152
      /etc/updatedb.conf , 85                   id , 34
      ex , 50, 53, 55                           info , 45
      exit , 32, 37, 39, 118                    init , 130
      export , 113–114                          inode numbers, 76
            -n (option), 114
                                                jove ,56
      fgrep , 89–90, 116                        Joy, Bill, 50
      FHS, 127
      file , 126                                kate ,61
      find , 81–84, 153                         Katz, Phil, 141
            -exec (option), 83                  KDE, 61
            -maxdepth (option), 153             kdesu , 33
            -name (option), 153                 kernel modules, 129
            -ok (option), 83                    klogd , 133
            -print (option), 81, 83             Knoppix, 26
            -print0 (option), 83                Korn, David, 36
      Fox, Brian, 36                            Krafft, Martin F., 25
      free , 132
      Free Software Foundation, 15              LANG (environment variable), 101–102
      freeware, 17                              LC_ALL (environment variable), 101–102
      frog.txt , 90                             LC_COLLATE (environment variable),
      FSF, 15                                                 101–102
                                                less , 44, 80–81, 95, 98
      Gailly, Jean-loup, 141                    /lib , 129
      gcc , 64                                  /lib/modules , 129
      gedit , 61                                linux- *.tar.gz , 15
      GNOME, 61                                 linux- 0.01.tar.gz , 149
      GNU, 15                                   ln , 76–79, 127
      GPL, 15                                         -b (option), 79
      grep , 43, 89–91, 94, 99–100, 106, 128,         -f (option), 79
                  134, 153, 157                       -i (option), 79
            --color (option), 91                      -s (option), 78–79, 127
            -f (option), 90                           -v (option), 79
            -H (option), 157                    locate , 84–86, 153, 170
      groff , 43, 45, 50                              -e (option), 85
      gunzip , 142, 144                         LOGNAME (environment variable),       153
Index                                                                                    175

logout , 31                                     PDP-11, 14
lost+found , 134                                Perl, 88
ls , 45, 67–69, 71,    73, 76, 79, 96–97, 99,   perl , 107
               106, 115, 128, 150–151,          pipeline, 98
               154–155                          pipes, 98
        -a   (option), 68                       popd , 67
        -d   (option), 69, 150                  /proc , 131–132, 134
        -F   (option), 68                       /proc/cpuinfo , 131
        -H   (option), 79                       /proc/devices , 131
        -i   (option), 76                       /proc/dma , 131
        -L   (option), 79                       /proc/interrupts , 131
        -l   (option), 68–69, 79                /proc/ioports , 131
        -p   (option), 68                       /proc/kcore , 131, 157
        -U   (option), 97                       /proc/loadavg , 131
                                                /proc/meminfo , 132
man , 42–45, 72, 81, 132                        /proc/mounts , 132
      -a (option), 44
                                                /proc/scsi , 132
      -f (option), 45
                                                ps , 132
      -k (option), 45
                                                pseudo devices, 129
MANPATH (environment variable), 43
                                                “public-domain” software, 17
manpath , 43
                                                pushd , 67
/media , 133
                                                pwd , 67, 86
/media/cdrom , 133
                                                Python, 88
/media/dvd , 133
/media/floppy , 133                             Qt, 19
Minix, 14
mkdir , 69–70, 126–128                          Ramey, Chet, 36
      -p (option), 69                           rar , 138
mkfifo , 127                                    Red Hat, 20
mknod , 127                                     reference counter, 76
/mnt , 133                                      reset , 100
more , 80                                       return value, 117
      -l (option), 80                           Ritchie, Dennis, 14
      -n ⟨number⟩ (option), 80                  rm , 39, 75–76, 79, 83, 150–151
      -s (option), 80                                 -f (option), 76
Morton, Andrew, 20                                    -i (option), 75–76, 152
mount , 116, 128                                      -r (option), 76
Multics, 14                                           -v (option), 76
Murdock, Ian, 25                                rmdir , 70, 151
mv , 75–77, 151, 157                                  -p (option), 70
      -b (option), 75                           /root , 127, 133
      -f (option), 75                           root directory, 127
      -i (option), 75
      -R (option), 76, 151                      /sbin , 128–129,   131
      -u (option), 75                           sed , 51
      -v (option), 75                           set , 114
                                                Seward, Julian, 143
nobody ,
      85                                        shell script, 119
Novell, 24                                      shell variables, 113
objectives, 163                                 Shuttleworth, Mark, 26
Open Source, 15                                 SkoleLinux, 26
/opt , 130–131, 134                             slocate , 85–86, 153
                                                sort , 99, 101–103, 105–106, 109, 116,
paste , 107–108                                             121, 133, 156
     -d (option), 107                                 -b (option), 103–104
     -s (option), 108                                 -k (option), 102
PATH (environment variable),       66,                -n (option), 105
               114–116, 119, 123, 156–157,            -r (option), 104
               170                                    -t (option), 104
176                                                                     Index

            -u (option),   175          /usr/lib , 131
      -u , 106                          /usr/local , 131, 133
      source , 119                      /usr/local/bin , 127
      split , 139                       /usr/sbin , 131
      /srv , 133                        /usr/share , 131
      Stallman, Richard M., 15, 55      /usr/share/dict/words , 91–92
      standard channels, 94             /usr/share/doc , 131
      su , 32, 34                       /usr/share/file , 126
      sudo , 33                         /usr/share/file/magic , 126
      SUSE, 20                          /usr/share/info , 131
      symbolic links, 78                /usr/share/man , 43, 131
      /sys , 132                        /usr/share/zoneinfo , 112
      syslogd , 132–133                 /usr/src , 131

      tail ,   100–101, 155             /var , 132–133, 135
               -c (option), 101         /var/log , 132
               -f (option), 101         /var/mail , 78, 132
               -n (option), 100         /var/spool , 135
               - 𝑛 (option), 100        /var/spool/cron , 132
      Tanenbaum, Andrew S., 14          /var/spool/cups , 132
      tar , 138–144, 147, 157–158       /var/tmp , 133, 135
            -c (option), 139–140        Verisign, 26
            -f (option), 139–140        vi , 50–55, 57, 61, 78
            -j (option), 139            vim , 50, 55, 89, 147, 170
            -M (option), 139            vimtutor , 150
            -r (option), 139            vmlinuz , 128
            -t (option), 139–140        Volkerding, Patrick, 24
            -u (option), 139
            -v (option), 139–140        wc ,
                                           97, 121, 156
            -x (option), 139–140             -l (option), 121
            -Z (option), 139            whatis , 45
            -z (option), 139, 158       whereis , 115, 156
      Tcl, 88                           which , 115, 156
      tee , 98–99, 155
            -a (option), 98
                                        Xandros, 26
                                        xargs , 83
      TERM (environment variable), 80
                                             -0 (option), 83
      test , 39, 121, 150
                                             -r (option), 83
            -f (option), 157
                                        xterm , 116
      Thawte, 26
      Thompson, Ken, 14
                                        .zip , 144–145
      /tmp , 133, 135
                                        zip , 138, 144–147
      Torvalds, Linus, 14, 17, 20
                                              -@ (option), 144
      type , 39, 115
                                              -0 (option), 144
      TZ (environment variable), 112
                                              -d (option), 145
                                              -f (option), 145
      Ubuntu, 26
                                              -FS (option), 145
      uniq , 105
                                              -h (option), 145
      Unix, 14
                                              -h2 (option), 145
      unset , 114
                                              -r (option), 144–145
      unzip , 144–147, 158
                                              -u (option), 145
            -d (option), 145
            -h (option), 146
            -hh (option), 146
            -v (option), 145–146
            -x (option), 146
      updatedb , 85, 153
      uptime , 131
      /usr , 127, 130–131
      /usr/bin , 39, 127, 131