Introduction to Linux for Users and Administrators

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Introduction to Linux
for Users and Administrators

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Introduction to Linux for Users and Administrators
Revision: grd1:62f570f98f89998d:2015-08-04
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Contents

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
5

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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Preface
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
information.

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
info@tuxcademy.org

(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
much!

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
Introduction

Contents
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

Goals
• 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-
ware”

Prerequisites
• 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.

Exercises
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

55MiB
Linux 2.0
Linux 2.1
50MiB Linux 2.2
Linux 2.3
Linux 2.4
Linux 2.5
45MiB Linux 2.6

40MiB

35MiB

30MiB

25MiB

20MiB

15MiB

10MiB

5MiB
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
GPL.
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
“freeware”.
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
ways:
• 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
freedoms.
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.

Exercises
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-
sions.
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 2.6.38.1, 2.6.38.2, …, 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.
kernel.org . 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.

Exercises
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-
ported?

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-
mally.
• 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
protocols.
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
level:
• 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-
sition.
• 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.

Exercises
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

Users
elect Developers

Officers
Project leader
appoints appoints

Technical committee Project secretary
appoints/approves

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
eer
ol unt
V Software in the
Public Interest
(SPI)

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-
tioned.

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

Summary
• 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-
ware”.
• 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.

Bibliography
DFSG “Debian Free Software Guidelines”. http://www.debian.org/social_contract

GPL-Urteil06 Landgericht Frankfurt am Main. “Urteil 2-6 0 224/06”, July 2006.
http://www.jbb.de/urteil_lg_frankfurt_gpl.pdf

GPL91 Free Software Foundation, Inc. “GNU General Public License, Version 2”,
June 1991. http://www.gnu.org/licenses/gpl.html

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.
http://cm.bell- labs.com/cm/cs/who/dmr/hist.html

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
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

2
Using the Linux System

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

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

Prerequisites
• 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
buddy).

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.

Exercises
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
password.

Exercises
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
Ubuntu.

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
privileges.

Exercises
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

Summary
• 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
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

3
Who’s Afraid Of The Big Bad
Shell?

Contents
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

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

Prerequisites
• 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-
dows.
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.

Exercises
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

Exercises
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
include
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

Summary
• 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
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

4
Getting Help

Contents
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

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

Prerequisites
• 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
chapter.

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
MANPATH .
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-
played.

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 ”.

Exercises
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.

Exercises
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
originals.
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” (http://www.tldp.org ) 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
http://www.tldp.org/
man pages and HOWTOs (among other things).
http://www.linux.org/ A general “portal” for Linux enthusiasts.
http://www.linuxwiki.de/A “free-form text information database for everything
pertaining to Linux” (in German).
http://lwn.net/ 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.
http://freecode.com/ 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
packages.
A site collecting “headlines” from other in-
http://www.linux- knowledge- portal.de/
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-
self).

B A news archive is accessible on http://groups.google.com/ (formerly De-
jaNews)

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.
kernel.org . 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

Summary
• “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
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

5
Editors: vi and emacs

Contents
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

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

Prerequisites
• 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 OpenOﬃce.org 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.

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

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
position.
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).:

:5,$s/red/blue/

replaces the ﬁrst occurrence of red on each line by blue , where the ﬁrst four lines
are not considered.
:5,$s/red/blue/g

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:
:/^BEGIN/,/^END/s/red/blue/g

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
⟨replacement⟩
: q Quits vi
: q! Quits vi even if the buﬀer contents is un-
saved
: x oder :e wq Saves the buﬀer contents and quits vi

Exercises
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-
ingly.

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-
case
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

Exercises
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

Summary
• 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
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

6
Files: Care and Feeding

Contents
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

Goals
• 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

Prerequisites
• 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
content.

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

ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz
0123456789+-._

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:

X-files
foo.txt.bak
50.something
7_of_9

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
/home/joe

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.

Exercises
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
file.txt
file2.dat
$ 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
memory.

Exercises
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
holiday/

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

pictures/

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

Exercises
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—
like
prog1/p*.c
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

prog1/p1.c
prog1/polly.c
prog1/pop-rock.c
prog1/p.c

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

p?.c

thus matches the names
p1.c
pa.c
p-.c
p..c

(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

prog[123].c

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

prog1.c
prog2.c
prog3.c

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

prog[1-9].c
[A-Z]bracadabra.txt

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

prog[A-z].c

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

prog[A-Za-z].c

A A construct like
prog[A-Za-z].c

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

prog[!A-Za-z].c

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

{red,yellow,blue}.txt

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

{a,b,c}{1,2,3}.dat

produces

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
exist:
$ mkdir -p revenue/200{8,9}/q{1,2,3,4}

Exercises
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
names
-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
approaches:
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
message.
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
results.

Exercises
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
diﬀerences.
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-
tinguishable.

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:
//www.rsnapshot.org/ ) or Dirvish (http://www.dirvish.org/ ).

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
/var/log

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
liste2

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
line.
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.

Exercises
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-
nomenon?

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
you.
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
./list

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
more.

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-
curacy.
-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
shell:
$ 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

./..
./bilder
./Attic
$ _

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
once.

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

Exercises
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
/home/joe/Letters/letter.txt
/home/joe/Letters/grannyletter.txt
/home/joe/Letters/grannyletter.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*"
/home/joe/Letters/letter.txt
/home/joe/Letters/letter.tab

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
/home/joe/Letters/grannyletter.txt
/home/joe/Letters/grannyletter.txt~

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

Exercises
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

Summary
• Nearly all possible characters may occur in ﬁle names. For portability’s sake,
however, you should restrict yourself to letters, digits, and some special
characters.
• 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
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

7
Regular Expressions

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

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

Prerequisites
• 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
a
c

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
once.
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”
operators.

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
8:aardvark
21:abaca
22:abacate
… 7030 more words …
234831:zygomaticoauricularis
234832:zygomaticofacial
234834:zygomaticomaxillary

(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
root:x:0:0:root:/root:/bin/bash

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.

Exercises
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
daughter”.

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

Summary
• Regular expressions are a powerful method for describing sets of character
strings.
• grep and its relations search a ﬁle’s content for lines matching regular ex-
pressions.
$ echo tux
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

8
Standard I/O and Filter
Commands

Contents
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

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

Prerequisites
• 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

File

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
YYYxyzZZZ
YYYxyzZZZ
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
expression).
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
terminal.

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?
23/12
$ 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
1397

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-
guments.

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
here:
$ wc -w <frog.txt >wordcount
$ cat wordcount
1397

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.

Exercises
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

File

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 .)

Exercises
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-
tages:
• 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.

Exercises
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
line:
$ 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.

Exercises
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
shows:
$ 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
here.)
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
Hipp
Hopp
Hopp
Hipp
Hipp
Hopp
$ uniq uniq-test
Hipp
Hopp
106 8 Standard I/O and Filter Commands

Hipp
Hopp

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

$ sort -u uniq-test
Hipp
Hopp

Exercises
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
documentation!)

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

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”
order:

01-2002.txt
02-2002.txt

12-2002.txt
01-2003.txt

12-2003.txt

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
speciﬁed.

$ cut -c 12,1-5 participants.dat
SmithH
ProwlD
FleetF
JumpaM
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
Smith
Prowler
Fleetma
Jumpabo
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
Smith:123
Prowler:13
Fleetman:217
Jumpabout:154
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
Wood
Bell
Potter
$ cat list2
Keeper
Chaser
Seeker
$ 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.

Exercises
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
character).

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

Summary
• 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
ﬁles.
• 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
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

9
More About The Shell

Contents
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

Goals
• Knowing about shell variables and evironment variables

Prerequisites
• 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
commands:

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
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.

Exercises
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
week)
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
foo
$ echo $foo
bar

(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
bar
$ 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
/home/joe/bin:/usr/local/bin:/usr/bin:/bin:/usr/games

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
/bin/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
command.
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/passwd.org /usr/share/passwd
/usr/share/man/man1/passwd.1.gz /usr/share/man/man1/passwd.1ssl.gz
/usr/share/man/man5/passwd.5.gz

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

Exercises
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
time.

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
question.)

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-
colon:
$ 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,
respectively
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
exit
$ 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
it?):

$ bash -c "exit 0" && echo "Successful"
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"
Unsuccessful

Exercises
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
incantation
#!/bin/bash

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
foo=bar

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-
amples.

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
#!/bin/bash
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
#!/bin/bash
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
#!/bin/bash
# Sort files according to their line count
for f
do
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

#!/bin/bash
# grepcp REGEX
rm -rf backup; mkdir backup
for f in *.txt
do
grep $1 "$f" && cp "$f" backup
done

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

#!/bin/bash
# filetest NAME1 NAME2 ...
for name
do
test -d "$name" && echo $name: directory
test -f "$name" && echo $name: file
test -L "$name" && echo $name: symbolic link
done

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 …”:
#!/bin/bash
# filetest2 NAME1 NAME2 ...
for name
do
if [ -L "$name" ]
then
echo $name: symbolic link
elif [ -d "$name" ]
echo $name: directory
elif [ -f "$name" ]
echo $name: file
else
echo $name: no idea
fi
done

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
required.

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 Maggie:maggie@example.net:the delightful tea cosy
Uncle Bob:bob@example.com:the 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):

#!/bin/bash
# birthday FILE
IFS=:
while read name email present
do
(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!

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

and
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

Summary
• The sleep command waits for the number of seconds speciﬁed as the argu-
ment.
• 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
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

10
The File System

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

Goals
• 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

Prerequisites
• 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/
mkdir
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
details.
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
users.
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
manual.

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.

Exercises
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
kernel.
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-
ing.
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-
tant
/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,
anyway).

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-
ated.
/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
program
Displays the memory and swap usage. This ﬁle is used by the free
/proc/meminfo
program
/proc/mounts Another list of all currently mounted ﬁle systems, mostly identical to
/etc/mtab

/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
kernel.

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:

/home/support/jim
/home/develop/bob

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.

Exercises
C 10.2 [1] How many programs does your system contain in the “usual”
places?

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

Summary
• 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
distributions.
$ echo tux
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

11
Archiving and Compressing Files

Contents
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

Goals
• 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

Prerequisites
• 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

ABBBBAACCCCCAAAABAAAAAC

more compactly as

A*4BAA*5C*4AB*5AC

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 .

Exercises
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*
data1
data10
data2
data3

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
data1
data10
data2

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
data1
data10
data2

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

Exercises
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
archive.

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
there.)
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

or
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

Exercises
C 11.7 [2] Compress the tar archive from Exercise 11.3 using maximum com-
pression.

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
detail.

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-
zip

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 test.zip file1 file2
adding: file1 (deflated 66%)
adding: file2 (deflated 62%)
$ _

(Here test.zip is the name of the resulting archive.)
You can use the -r option to tell zip to descend into subdirectories recursively:

$ zip -r test.zip 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
input:

$ 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
Archive: test.zip
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
Archive: test.zip
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
Archive: test.zip
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
unpacked:

$ rm -rf ziptest
$ unzip test ziptest/file1
Archive: test.zip
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
Archive: test.zip
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*"
Archive: test.zip
inflating: ziptest/file2
inflating: ziptest/file1
$ rm -rf ziptest
$ unzip test -x "*/t*"
Archive: test.zip
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).

Exercises
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 files.zip 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

Summary
• “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.

Bibliography
RFC1951 P. Deutsch. “DEFLATE Compressed Data Format Speciﬁcation version
1.3”, May 1996. http://www.ietf.org/rfc/rfc1951.txt
$ echo tux
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

A
Sample Solutions
This appendix contains sample solutions for selected exercises.

1.2 There is a copy of linux- 0.01.tar.gz on ftp.kernel.org .

1.3
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 ftp.kernel.org or in the weekly edition of http://lwn.net/ .

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
eﬃciency.

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
grd1-test/dir1/hello
A Sample Solutions 151

grd1-test/dir2/howdy
$ 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
simpliﬁcation:

$ k='[âeiou]*'
$ grep -i $̂
{ k}a${k}e${k}i${k}o${k}u${k}$ /usr/share/dict/words
abstemious
abstemiously
abstentious
acheilous
acheirous
acleistous
affectious
annelidous
arsenious
arterious
bacterious
caesious
facetious
facetiously
fracedinous
majestious

(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
Hello
orld

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
count.
A Sample Solutions 157

9.11 Try something like

#!/bin/bash
pattern=$1
shift

for f
do
grep $pattern "$f" && cp "$f" backup
done

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
manner.

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 files.zip "a/*" -x "*/*.c"
$ echo tux
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

B
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
plaything.

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
kingdom.

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
tux
$ ls
hallo.c
hallo.o
$ /bin/su -
Password:

C
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-
ployers.
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 http://www.lpi.org/ , 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