Linux Fundamentals

This book is meant to be used in an instructor-led training. For self-study, the intent is to readthis book next to a working Linux computer so you can immediately do every subject, practicingeach command.This book is aimed at novice Linux system administrators (and might be interesting and usefulfor home users that want to know a bit more about their Linux system). However, this bookis not meant as an introduction to Linux desktop applications like text editors, browsers, mailclients, multimedia or office applications.
About Jack Berlin

About Linux Training - About Paul Cobbaut - About Prizm Share
Paul file root command echo txt user
About Jack Berlin

About Linux Training - About Paul Cobbaut - About Prizm Share
Paul file root command echo txt user
Linux Fundamentals
Paul Cobbaut
Linux Fundamentals
Paul Cobbaut
lt-2.0
Publication date Fri 17 Apr 2015 01:00:26 AM CEST
Abstract
This book is meant to be used in an instructor-led training. For self-study, the intent is to read
this book next to a working Linux computer so you can immediately do every subject, practicing
each command.
This book is aimed at novice Linux system administrators (and might be interesting and useful
for home users that want to know a bit more about their Linux system). However, this book
is not meant as an introduction to Linux desktop applications like text editors, browsers, mail
clients, multimedia or office applications.
More information and free .pdf available at http://linux-training.be .
Feel free to contact the author:
â€¢ Paul Cobbaut: paul.cobbaut@gmail.com, http://www.linkedin.com/in/cobbaut
Contributors to the Linux Training project are:
â€¢ Serge van Ginderachter: serge@ginsys.eu, build scripts and infrastructure setup
â€¢ Ywein Van den Brande: ywein@crealaw.eu, license and legal sections
â€¢ Hendrik De Vloed: hendrik.devloed@ugent.be, buildheader.pl script
We'd also like to thank our reviewers:
â€¢ Wouter Verhelst: wo@uter.be, http://grep.be
â€¢ Geert 
Goossens: 
mail.goossens.geert@gmail.com, 
http://www.linkedin.com/in/
geertgoossens
â€¢ Elie De Brauwer: elie@de-brauwer.be, http://www.de-brauwer.be
â€¢ Christophe Vandeplas: christophe@vandeplas.com, http://christophe.vandeplas.com
â€¢ Bert Desmet: bert@devnox.be, http://blog.bdesmet.be
â€¢ Rich Yonts: richyonts@gmail.com,
Copyright 2007-2015 Netsec BVBA, Paul Cobbaut
Permission is granted to copy, distribute and/or modify this document under the terms of the
GNU Free Documentation License, Version 1.3 or any later version published by the Free
Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled 'GNU Free Documentation
License'.
iv
Table of Contents
I. introduction to Linux .............................................................................................................  1
1. Linux history .............................................................................................................  3
1.1. 1969 ................................................................................................................  4
1.2. 1980s ...............................................................................................................  4
1.3. 1990s ...............................................................................................................  4
1.4. 2015 ................................................................................................................  5
2. distributions ...............................................................................................................  6
2.1. Red Hat ...........................................................................................................  7
2.2. Ubuntu .............................................................................................................  7
2.3. Debian .............................................................................................................  7
2.4. Other ...............................................................................................................  7
2.5. Which to choose ? .............................................................................................  8
3. licensing .....................................................................................................................  9
3.1. about software licenses ...................................................................................... 10
3.2. public domain software and freeware ...................................................................  10
3.3. Free Software or Open Source Software ...............................................................  10
3.4. GNU General Public License .............................................................................  11
3.5. using GPLv3 software .......................................................................................  11
3.6. BSD license ....................................................................................................  12
3.7. other licenses ................................................................................................... 12
3.8. combination of software licenses .........................................................................  12
II. installing Linux .................................................................................................................  13
4. installing Debian 8 ....................................................................................................  15
4.1. Debian ...........................................................................................................  16
4.2. Downloading ...................................................................................................  16
4.3. virtualbox networking .......................................................................................  32
4.4. setting the hostname .........................................................................................  34
4.5. adding a static ip address ...................................................................................  34
4.6. Debian package management .............................................................................. 35
5. installing CentOS 7 ...................................................................................................  36
5.1. download a CentOS 7 image ..............................................................................  37
5.2. Virtualbox .......................................................................................................  39
5.3. CentOS 7 installing ..........................................................................................  44
5.4. CentOS 7 first logon .........................................................................................  52
5.5. Virtualbox network interface ..............................................................................  53
5.6. configuring the network ....................................................................................  54
5.7. adding one static ip address ...............................................................................  54
5.8. package management ........................................................................................  55
5.9. logon from Linux and MacOSX .......................................................................... 56
5.10. logon from MS Windows .................................................................................  56
6. getting Linux at home ...............................................................................................  58
6.1. download a Linux CD image .............................................................................. 59
6.2. download Virtualbox ........................................................................................  59
6.3. create a virtual machine ....................................................................................  60
6.4. attach the CD image .........................................................................................  65
6.5. install Linux ....................................................................................................  68
III. first steps on the command line ...........................................................................................  69
7. man pages ................................................................................................................  71
7.1. man $command ...............................................................................................  72
7.2. man $configfile ................................................................................................  72
7.3. man $daemon ..................................................................................................  72
7.4. man -k (apropos) .............................................................................................. 72
7.5. whatis ............................................................................................................  72
7.6. whereis ...........................................................................................................  72
7.7. man sections ...................................................................................................  73
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7.8. man $section $file ............................................................................................  73
7.9. man man ........................................................................................................  73
7.10. mandb ..........................................................................................................  73
8. working with directories ............................................................................................  74
8.1. pwd ...............................................................................................................  75
8.2. cd ..................................................................................................................  75
8.3. absolute and relative paths .................................................................................  76
8.4. path completion ...............................................................................................  77
8.5. ls ...................................................................................................................  77
8.6. mkdir .............................................................................................................  79
8.7. rmdir .............................................................................................................. 79
8.8. practice: working with directories ........................................................................ 81
8.9. solution: working with directories .......................................................................  82
9. working with files .....................................................................................................  84
9.1. all files are case sensitive ..................................................................................  85
9.2. everything is a file ...........................................................................................  85
9.3. file ................................................................................................................  85
9.4. touch .............................................................................................................. 86
9.5. rm .................................................................................................................  87
9.6. cp ..................................................................................................................  88
9.7. mv .................................................................................................................  89
9.8. rename ...........................................................................................................  90
9.9. practice: working with files ................................................................................  91
9.10. solution: working with files ..............................................................................  92
10. working with file contents ........................................................................................  94
10.1. head .............................................................................................................  95
10.2. tail ...............................................................................................................  95
10.3. cat ...............................................................................................................  96
10.4. tac ...............................................................................................................  97
10.5. more and less ................................................................................................  98
10.6. strings ..........................................................................................................  98
10.7. practice: file contents ......................................................................................  99
10.8. solution: file contents ....................................................................................  100
11. the Linux file tree ..................................................................................................  101
11.1. filesystem hierarchy standard ..........................................................................  102
11.2. man hier ......................................................................................................  102
11.3. the root directory / ........................................................................................  102
11.4. binary directories ..........................................................................................  103
11.5. configuration directories .................................................................................  105
11.6. data directories .............................................................................................  107
11.7. in memory directories ....................................................................................  109
11.8. /usr Unix System Resources ............................................................................  114
11.9. /var variable data ..........................................................................................  116
11.10. practice: file system tree ...............................................................................  118
11.11. solution: file system tree ...............................................................................  120
IV. shell expansion ...............................................................................................................  122
12. commands and arguments ......................................................................................  125
12.1. arguments ....................................................................................................  126
12.2. white space removal ......................................................................................  126
12.3. single quotes ................................................................................................  127
12.4. double quotes ...............................................................................................  127
12.5. echo and quotes ............................................................................................  127
12.6. commands ...................................................................................................  128
12.7. aliases .........................................................................................................  129
12.8. displaying shell expansion ..............................................................................  130
12.9. practice: commands and arguments ..................................................................  131
12.10. solution: commands and arguments ................................................................  133
13. control operators ...................................................................................................  135
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13.1. ; semicolon ..................................................................................................  136
13.2. & ampersand ................................................................................................  136
13.3. $? dollar question mark .................................................................................. 136
13.4. && double ampersand ...................................................................................  137
13.5. || double vertical bar ......................................................................................  137
13.6. combining && and || .....................................................................................  137
13.7. # pound sign ................................................................................................  138
13.8. \ escaping special characters ...........................................................................  138
13.9. practice: control operators ..............................................................................  139
13.10. solution: control operators .............................................................................  140
14. shell variables ........................................................................................................  141
14.1. $ dollar sign ................................................................................................  142
14.2. case sensitive ...............................................................................................  142
14.3. creating variables ..........................................................................................  142
14.4. quotes .........................................................................................................  143
14.5. set ..............................................................................................................  143
14.6. unset ...........................................................................................................  143
14.7. $PS1 ...........................................................................................................  144
14.8. $PATH .......................................................................................................  145
14.9. env .............................................................................................................  146
14.10. export ........................................................................................................ 146
14.11. delineate variables .......................................................................................  147
14.12. unbound variables .......................................................................................  147
14.13. practice: shell variables ................................................................................  148
14.14. solution: shell variables ................................................................................  149
15. shell embedding and options ...................................................................................  150
15.1. shell embedding ............................................................................................ 151
15.2. shell options ................................................................................................  152
15.3. practice: shell embedding ...............................................................................  153
15.4. solution: shell embedding ...............................................................................  154
16. shell history ........................................................................................................... 155
16.1. repeating the last command ............................................................................  156
16.2. repeating other commands ..............................................................................  156
16.3. history ........................................................................................................  156
16.4. !n ...............................................................................................................  156
16.5. Ctrl-r ..........................................................................................................  157
16.6. $HISTSIZE ..................................................................................................  157
16.7. $HISTFILE ..................................................................................................  157
16.8. $HISTFILESIZE ...........................................................................................  157
16.9. prevent recording a command .........................................................................  158
16.10. (optional)regular expressions .........................................................................  158
16.11. (optional) Korn shell history .......................................................................... 158
16.12. practice: shell history ...................................................................................  159
16.13. solution: shell history ...................................................................................  160
17. file globbing ..........................................................................................................  161
17.1. * asterisk .....................................................................................................  162
17.2. ? question mark ............................................................................................  162
17.3. [] square brackets .......................................................................................... 163
17.4. a-z and 0-9 ranges ........................................................................................  164
17.5. $LANG and square brackets ...........................................................................  164
17.6. preventing file globbing .................................................................................  165
17.7. practice: shell globbing ..................................................................................  166
17.8. solution: shell globbing ..................................................................................  167
V. pipes and commands .........................................................................................................  169
18. I/O redirection ....................................................................................................... 171
18.1. stdin, stdout, and stderr ..................................................................................  172
18.2. output redirection ..........................................................................................  173
18.3. error redirection ............................................................................................  175
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18.4. output redirection and pipes ............................................................................  176
18.5. joining stdout and stderr .................................................................................  176
18.6. input redirection ...........................................................................................  177
18.7. confusing redirection .....................................................................................  178
18.8. quick file clear .............................................................................................  178
18.9. practice: input/output redirection ......................................................................  179
18.10. solution: input/output redirection ....................................................................  180
19. filters ....................................................................................................................  181
19.1. cat ..............................................................................................................  182
19.2. tee ..............................................................................................................  182
19.3. grep ............................................................................................................  182
19.4. cut .............................................................................................................. 184
19.5. tr ................................................................................................................ 184
19.6. wc ..............................................................................................................  185
19.7. sort .............................................................................................................  186
19.8. uniq ............................................................................................................  187
19.9. comm .........................................................................................................  188
19.10. od .............................................................................................................  189
19.11. sed ............................................................................................................ 190
19.12. pipe examples .............................................................................................  191
19.13. practice: filters ............................................................................................  192
19.14. solution: filters ............................................................................................ 193
20. basic Unix tools .....................................................................................................  195
20.1. find ............................................................................................................  196
20.2. locate ..........................................................................................................  197
20.3. date ............................................................................................................  197
20.4. cal ..............................................................................................................  198
20.5. sleep ...........................................................................................................  198
20.6. time ............................................................................................................  199
20.7. gzip - gunzip ................................................................................................ 200
20.8. zcat - zmore .................................................................................................  200
20.9. bzip2 - bunzip2 ............................................................................................  201
20.10. bzcat - bzmore ............................................................................................  201
20.11. practice: basic Unix tools .............................................................................. 202
20.12. solution: basic Unix tools .............................................................................  203
21. regular expressions ................................................................................................  205
21.1. regex versions ..............................................................................................  206
21.2. grep ............................................................................................................  207
21.3. rename ........................................................................................................  212
21.4. sed .............................................................................................................  215
21.5. bash history .................................................................................................  219
VI. vi .................................................................................................................................  220
22. Introduction to vi ..................................................................................................  222
22.1. command mode and insert mode ......................................................................  223
22.2. start typing (a A i I o O) ................................................................................  223
22.3. replace and delete a character (r x X) ...............................................................  224
22.4. undo and repeat (u .) .....................................................................................  224
22.5. cut, copy and paste a line (dd yy p P) ...............................................................  224
22.6. cut, copy and paste lines (3dd 2yy) ..................................................................  225
22.7. start and end of a line (0 or ^ and $) ................................................................  225
22.8. join two lines (J) and more .............................................................................  225
22.9. words (w b) .................................................................................................  226
22.10. save (or not) and exit (:w :q :q! ) ....................................................................  226
22.11. Searching (/ ?) ............................................................................................  226
22.12. replace all ( :1,$ s/foo/bar/g ) .........................................................................  227
22.13. reading files (:r :r !cmd) ................................................................................ 227
22.14. text buffers ................................................................................................  227
22.15. multiple files ..............................................................................................  227
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22.16. abbreviations ..............................................................................................  228
22.17. key mappings .............................................................................................  229
22.18. setting options ............................................................................................  229
22.19. practice: vi(m) ............................................................................................  230
22.20. solution: vi(m) ............................................................................................  231
VII. scripting .......................................................................................................................  232
23. scripting introduction .............................................................................................  234
23.1. prerequisites .................................................................................................  235
23.2. hello world ..................................................................................................  235
23.3. she-bang ......................................................................................................  235
23.4. comment .....................................................................................................  236
23.5. variables ......................................................................................................  236
23.6. sourcing a script ...........................................................................................  236
23.7. troubleshooting a script ..................................................................................  237
23.8. prevent setuid root spoofing ............................................................................  237
23.9. practice: introduction to scripting .....................................................................  238
23.10. solution: introduction to scripting ...................................................................  239
24. scripting loops .......................................................................................................  240
24.1. test [ ] .........................................................................................................  241
24.2. if then else ..................................................................................................  242
24.3. if then elif ...................................................................................................  242
24.4. for loop .......................................................................................................  242
24.5. while loop ...................................................................................................  243
24.6. until loop ..................................................................................................... 243
24.7. practice: scripting tests and loops .....................................................................  244
24.8. solution: scripting tests and loops ..................................................................... 245
25. scripting parameters ..............................................................................................  247
25.1. script parameters ...........................................................................................  248
25.2. shift through parameters .................................................................................  249
25.3. runtime input ...............................................................................................  249
25.4. sourcing a config file .....................................................................................  250
25.5. get script options with getopts .........................................................................  251
25.6. get shell options with shopt ............................................................................  252
25.7. practice: parameters and options ......................................................................  253
25.8. solution: parameters and options ......................................................................  254
26. more scripting .......................................................................................................  255
26.1. eval ............................................................................................................  256
26.2. (( )) ............................................................................................................  256
26.3. let ..............................................................................................................  257
26.4. case ............................................................................................................  258
26.5. shell functions ..............................................................................................  259
26.6. practice : more scripting .................................................................................  260
26.7. solution : more scripting .................................................................................  261
VIII. local user management ...................................................................................................  263
27. introduction to users ..............................................................................................  266
27.1. whoami .......................................................................................................  267
27.2. who ............................................................................................................  267
27.3. who am i .....................................................................................................  267
27.4. w ...............................................................................................................  267
27.5. id ...............................................................................................................  267
27.6. su to another user .........................................................................................  268
27.7. su to root ....................................................................................................  268
27.8. su as root ....................................................................................................  268
27.9. su - $username .............................................................................................  268
27.10. su - ...........................................................................................................  268
27.11. run a program as another user .......................................................................  269
27.12. visudo .......................................................................................................  269
27.13. sudo su - ...................................................................................................  270
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27.14. sudo logging ..............................................................................................  270
27.15. practice: introduction to users ........................................................................  271
27.16. solution: introduction to users ........................................................................  272
28. user management ...................................................................................................  274
28.1. user management ..........................................................................................  275
28.2. /etc/passwd ................................................................................................... 275
28.3. root ............................................................................................................  275
28.4. useradd .......................................................................................................  276
28.5. /etc/default/useradd ........................................................................................  276
28.6. userdel ........................................................................................................  276
28.7. usermod ......................................................................................................  276
28.8. creating home directories ................................................................................ 277
28.9. /etc/skel/ ......................................................................................................  277
28.10. deleting home directories ..............................................................................  277
28.11. login shell ..................................................................................................  278
28.12. chsh ..........................................................................................................  278
28.13. practice: user management ............................................................................  279
28.14. solution: user management ............................................................................  280
29. user passwords ......................................................................................................  282
29.1. passwd ........................................................................................................  283
29.2. shadow file ..................................................................................................  283
29.3. encryption with passwd ..................................................................................  284
29.4. encryption with openssl .................................................................................. 284
29.5. encryption with crypt .....................................................................................  285
29.6. /etc/login.defs ...............................................................................................  286
29.7. chage ..........................................................................................................  286
29.8. disabling a password .....................................................................................  287
29.9. editing local files ..........................................................................................  287
29.10. practice: user passwords ...............................................................................  288
29.11. solution: user passwords ...............................................................................  289
30. user profiles ..........................................................................................................  291
30.1. system profile ..............................................................................................  292
30.2. ~/.bash_profile .............................................................................................. 292
30.3. ~/.bash_login ................................................................................................  293
30.4. ~/.profile .....................................................................................................  293
30.5. ~/.bashrc .....................................................................................................  293
30.6. ~/.bash_logout ..............................................................................................  294
30.7. Debian overview ...........................................................................................  295
30.8. RHEL5 overview ..........................................................................................  295
30.9. practice: user profiles ....................................................................................  296
30.10. solution: user profiles ...................................................................................  297
31. groups ..................................................................................................................  298
31.1. groupadd .....................................................................................................  299
31.2. group file ....................................................................................................  299
31.3. groups .........................................................................................................  299
31.4. usermod ......................................................................................................  300
31.5. groupmod ....................................................................................................  300
31.6. groupdel ......................................................................................................  300
31.7. gpasswd ......................................................................................................  301
31.8. newgrp ........................................................................................................  302
31.9. vigr ............................................................................................................  302
31.10. practice: groups ..........................................................................................  303
31.11. solution: groups ..........................................................................................  304
IX. file security ....................................................................................................................  305
32. standard file permissions ........................................................................................  307
32.1. file ownership ..............................................................................................  308
32.2. list of special files .........................................................................................  310
32.3. permissions ..................................................................................................  311
Linux Fundamentals
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32.4. practice: standard file permissions ....................................................................  316
32.5. solution: standard file permissions .................................................................... 317
33. advanced file permissions .......................................................................................  319
33.1. sticky bit on directory ....................................................................................  320
33.2. setgid bit on directory ....................................................................................  320
33.3. setgid and setuid on regular files .....................................................................  321
33.4. setuid on sudo ..............................................................................................  321
33.5. practice: sticky, setuid and setgid bits ...............................................................  322
33.6. solution: sticky, setuid and setgid bits ...............................................................  323
34. access control lists ..................................................................................................  325
34.1. acl in /etc/fstab .............................................................................................  326
34.2. getfacl .........................................................................................................  326
34.3. setfacl .........................................................................................................  326
34.4. remove an acl entry .......................................................................................  327
34.5. remove the complete acl ................................................................................  327
34.6. the acl mask ................................................................................................  327
34.7. eiciel ..........................................................................................................  328
35. file links ................................................................................................................  329
35.1. inodes .........................................................................................................  330
35.2. about directories ...........................................................................................  331
35.3. hard links ....................................................................................................  332
35.4. symbolic links ..............................................................................................  333
35.5. removing links .............................................................................................  333
35.6. practice : links ..............................................................................................  334
35.7. solution : links ..............................................................................................  335
X. Appendices .....................................................................................................................  336
A. keyboard settings ....................................................................................................  338
A.1. about keyboard layout ....................................................................................  338
A.2. X Keyboard Layout ........................................................................................ 338
A.3. shell keyboard layout ...................................................................................... 338
B. hardware ...............................................................................................................  340
B.1. buses ...........................................................................................................  340
B.2. interrupts ......................................................................................................  341
B.3. io ports ........................................................................................................  342
B.4. dma .............................................................................................................  342
C. License ..................................................................................................................  344
Index ..................................................................................................................................  351
xi
List of Tables
2.1. choosing a Linux distro ......................................................................................................  8
4.1. Debian releases ................................................................................................................  16
22.1. getting to command mode ..............................................................................................  223
22.2. switch to insert mode ....................................................................................................  223
22.3. replace and delete .........................................................................................................  224
22.4. undo and repeat ............................................................................................................  224
22.5. cut, copy and paste a line ...............................................................................................  224
22.6. cut, copy and paste lines ................................................................................................  225
22.7. start and end of line ......................................................................................................  225
22.8. join two lines ...............................................................................................................  225
22.9. words .......................................................................................................................... 226
22.10. save and exit vi ..........................................................................................................  226
22.11. searching ...................................................................................................................  226
22.12. replace ......................................................................................................................  227
22.13. read files and input ......................................................................................................  227
22.14. text buffers ................................................................................................................  227
22.15. multiple files ..............................................................................................................  228
22.16. abbreviations ..............................................................................................................  228
30.1. Debian User Environment ..............................................................................................  295
30.2. Red Hat User Environment .............................................................................................  295
32.1. Unix special files ..........................................................................................................  310
32.2. standard Unix file permissions ........................................................................................  311
32.3. Unix file permissions position .........................................................................................  311
32.4. Octal permissions .......................................................................................................... 314
Part I. introduction to Linux
2
Table of Contents
1. Linux history .....................................................................................................................  3
1.1. 1969 ........................................................................................................................  4
1.2. 1980s .......................................................................................................................  4
1.3. 1990s .......................................................................................................................  4
1.4. 2015 ........................................................................................................................  5
2. distributions .......................................................................................................................  6
2.1. Red Hat ...................................................................................................................  7
2.2. Ubuntu .....................................................................................................................  7
2.3. Debian .....................................................................................................................  7
2.4. Other .......................................................................................................................  7
2.5. Which to choose ? .....................................................................................................  8
3. licensing .............................................................................................................................  9
3.1. about software licenses .............................................................................................  10
3.2. public domain software and freeware ...........................................................................  10
3.3. Free Software or Open Source Software .......................................................................  10
3.4. GNU General Public License .....................................................................................  11
3.5. using GPLv3 software ............................................................................................... 11
3.6. BSD license ............................................................................................................  12
3.7. other licenses ..........................................................................................................  12
3.8. combination of software licenses ................................................................................. 12
3
Chapter 1. Linux history
This chapter briefly tells the history of Unix and where Linux fits in.
If you are eager to start working with Linux without this blah, blah, blah over history,
distributions, and licensing then jump straight to Part II - Chapter 8. Working with
Directories page 73.
Linux history
4
1.1. 1969
All modern operating systems have their roots in 1969 when Dennis Ritchie and Ken
Thompson developed the C language and the Unix operating system at AT&T Bell Labs.
They shared their source code (yes, there was open source back in the Seventies) with the
rest of the world, including the hippies in Berkeley California. By 1975, when AT&T started
selling Unix commercially, about half of the source code was written by others. The hippies
were not happy that a commercial company sold software that they had written; the resulting
(legal) battle ended in there being two versions of Unix: the official AT&T Unix, and the
free BSD Unix.
Development of BSD descendants like FreeBSD, OpenBSD, NetBSD, DragonFly BSD and
PC-BSD is still active today.
https://en.wikipedia.org/wiki/Dennis_Ritchie
https://en.wikipedia.org/wiki/Ken_Thompson
https://en.wikipedia.org/wiki/BSD
https://en.wikipedia.org/wiki/Comparison_of_BSD_operating_systems
1.2. 1980s
In the Eighties many companies started developing their own Unix: IBM created AIX, Sun
SunOS (later Solaris), HP HP-UX and about a dozen other companies did the same. The
result was a mess of Unix dialects and a dozen different ways to do the same thing. And
here is the first real root of Linux, when Richard Stallman aimed to end this era of Unix
separation and everybody re-inventing the wheel by starting the GNU project (GNU is Not
Unix). His goal was to make an operating system that was freely available to everyone, and
where everyone could work together (like in the Seventies). Many of the command line tools
that you use today on Linux are GNU tools.
https://en.wikipedia.org/wiki/Richard_Stallman
https://en.wikipedia.org/wiki/IBM_AIX
https://en.wikipedia.org/wiki/HP-UX
1.3. 1990s
The Nineties started with Linus Torvalds, a Swedish speaking Finnish student, buying a
386 computer and writing a brand new POSIX compliant kernel. He put the source code
online, thinking it would never support anything but 386 hardware. Many people embraced
the combination of this kernel with the GNU tools, and the rest, as they say, is history.
http://en.wikipedia.org/wiki/Linus_Torvalds
https://en.wikipedia.org/wiki/History_of_Linux
https://en.wikipedia.org/wiki/Linux
https://lwn.net
http://www.levenez.com/unix/   (a huge Unix history poster)
Linux history
5
1.4. 2015
Today more than 97 percent of the world's supercomputers (including the complete top 10),
more than 80 percent of all smartphones, many millions of desktop computers, around 70
percent of all web servers, a large chunk of tablet computers, and several appliances (dvd-
players, washing machines, dsl modems, routers, self-driving cars, space station laptops...)
run Linux. Linux is by far the most commonly used operating system in the world.
Linux kernel version 4.0 was released in April 2015. Its source code grew by several hundred
thousand lines (compared to version 3.19 from February 2015) thanks to contributions of
thousands of developers paid by hundreds of commercial companies including Red Hat,
Intel, Samsung, Broadcom, Texas Instruments, IBM, Novell, Qualcomm, Nokia, Oracle,
Google, AMD and even Microsoft (and many more).
http://kernelnewbies.org/DevelopmentStatistics
http://kernel.org
http://www.top500.org
6
Chapter 2. distributions
This chapter gives a short overview of current Linux distributions.
A Linux distribution is a collection of (usually open source) software on top of a Linux
kernel. A distribution (or short, distro) can bundle server software, system management
tools, documentation and many desktop applications in a central secure software
repository. A distro aims to provide a common look and feel, secure and easy software
management and often a specific operational purpose.
Let's take a look at some popular distributions.
distributions
7
2.1. Red Hat
Red Hat is a billion dollar commercial Linux company that puts a lot of effort in developing
Linux. They have hundreds of Linux specialists and are known for their excellent support.
They give their products (Red Hat Enterprise Linux and Fedora) away for free. While Red
Hat Enterprise Linux (RHEL) is well tested before release and supported for up to seven
years after release, Fedora is a distro with faster updates but without support.
2.2. Ubuntu
Canonical started sending out free compact discs with Ubuntu Linux in 2004 and quickly
became popular for home users (many switching from Microsoft Windows). Canonical
wants Ubuntu to be an easy to use graphical Linux desktop without need to ever see a
command line. Of course they also want to make a profit by selling support for Ubuntu.
2.3. Debian
There is no company behind Debian. Instead there are thousands of well organised
developers that elect a Debian Project Leader every two years. Debian is seen as one of
the most stable Linux distributions. It is also the basis of every release of Ubuntu. Debian
comes in three versions: stable, testing and unstable. Every Debian release is named after
a character in the movie Toy Story.
2.4. Other
Distributions like CentOS, Oracle Enterprise Linux and Scientific Linux are based on
Red Hat Enterprise Linux and share many of the same principles, directories and
system administration techniques. Linux Mint, Edubuntu and many other *buntu named
distributions are based on Ubuntu and thus share a lot with Debian. There are hundreds of
other Linux distributions.
distributions
8
2.5. Which to choose ?
Below are some very personal opinions on some of the most popular Linux Distributions.
Keep in mind that any of the below Linux distributions can be a stable server and a nice
graphical desktop client.
Table 2.1. choosing a Linux distro
distribution name
reason(s) for using
Red Hat Enterprise (RHEL) You are a manager and you want a good support contract.
CentOS
You want Red Hat without the support contract from Red Hat.
Fedora
You want Red Hat on your laptop/desktop.
Linux Mint
You want a personal graphical desktop to play movies, music and games.
Debian
My personal favorite for servers, laptops, and any other device.
Ubuntu
Very popular, based on Debian, not my favorite.
Kali
You want a pointy-clicky hacking interface.
others
Advanced users may prefer Arch, Gentoo, OpenSUSE, Scientific, ...
When you are new to Linux in 2015, go for the latest Mint or Fedora. If you only want to
practice the Linux command line then install one Debian server and/or one CentOS server
(without graphical interface).
Here are some links to help you choose:
distrowatch.com
redhat.com
centos.org
debian.org
www.linuxmint.com
ubuntu.com
9
Chapter 3. licensing
This chapter briefly explains the different licenses used for distributing operating systems
software.
Many thanks go to Ywein Van den Brande for writing most of this chapter.
Ywein is an attorney at law, co-author of The International FOSS Law Book and author
of Praktijkboek Informaticarecht (in Dutch).
http://ifosslawbook.org
http://www.crealaw.eu
licensing
10
3.1. about software licenses
There are two predominant software paradigms: Free and Open Source Software (FOSS)
and proprietary software. The criteria for differentiation between these two approaches is
based on control over the software. With proprietary software, control tends to lie more
with the vendor, while with Free and Open Source Software it tends to be more weighted
towards the end user. But even though the paradigms differ, they use the same copyright
laws to reach and enforce their goals. From a legal perspective, Free and Open Source
Software can be considered as software to which users generally receive more rights via
their license agreement than they would have with a proprietary software license, yet the
underlying license mechanisms are the same.
Legal theory states that the author of FOSS, contrary to the author of public domain
software, has in no way whatsoever given up his rights on his work. FOSS supports on the
rights of the author (the copyright) to impose FOSS license conditions. The FOSS license
conditions need to be respected by the user in the same way as proprietary license conditions.
Always check your license carefully before you use third party software.
Examples of proprietary software are AIX from IBM, HP-UX from HP and Oracle
Database 11g. You are not authorised to install or use this software without paying a
licensing fee. You are not authorised to distribute copies and you are not authorised to modify
the closed source code.
3.2. public domain software and freeware
Software that is original in the sense that it is an intellectual creation of the author benefits
copyright protection. Non-original software does not come into consideration for copyright
protection and can, in principle, be used freely.
Public domain software is considered as software to which the author has given up all rights
and on which nobody is able to enforce any rights. This software can be used, reproduced or
executed freely, without permission or the payment of a fee. Public domain software can in
certain cases even be presented by third parties as own work, and by modifying the original
work, third parties can take certain versions of the public domain software out of the public
domain again.
Freeware is not public domain software or FOSS. It is proprietary software that you can use
without paying a license cost. However, the often strict license terms need to be respected.
Examples of freeware are Adobe Reader, Skype and Command and Conquer: Tiberian
Sun (this game was sold as proprietary in 1999 and is since 2011 available as freeware).
3.3. Free Software or Open Source Software
Both the Free Software (translates to vrije software in Dutch and to Logiciel Libre in
French) and the Open Source Software movement largely pursue similar goals and endorse
similar software licenses. But historically, there has been some perception of differentiation
due to different emphases. Where the Free Software movement focuses on the rights (the
licensing
11
four freedoms) which Free Software provides to its users, the Open Source Software
movement points to its Open Source Definition and the advantages of peer-to-peer software
development.
Recently, the term free and open source software or FOSS has arisen as a neutral alternative.
A lesser-used variant is free/libre/open source software (FLOSS), which uses libre to clarify
the meaning of free as in freedom rather than as in at no charge.
Examples of free software are gcc, MySQL and gimp.
Detailed information about the four freedoms can be found here:
http://www.gnu.org/philosophy/free-sw.html
The open source definition can be found at:
http://www.opensource.org/docs/osd
The above definition is based on the Debian Free Software Guidelines available here:
http://www.debian.org/social_contract#guidelines
3.4. GNU General Public License
More and more software is being released under the GNU GPL (in 2006 Java was released
under the GPL). This license (v2 and v3) is the main license endorsed by the Free Software
Foundation. Itâ€™s main characteristic is the copyleft principle. This means that everyone in the
chain of consecutive users, in return for the right of use that is assigned, needs to distribute
the improvements he makes to the software and his derivative works under the same
conditions to other users, if he chooses to distribute such improvements or derivative works.
In other words, software which incorporates GNU GPL software, needs to be distributed
in turn as GNU GPL software (or compatible, see below). It is not possible to incorporate
copyright protected parts of GNU GPL software in a proprietary licensed work. The GPL
has been upheld in court.
3.5. using GPLv3 software
You can use GPLv3 software almost without any conditions. If you solely run the software
you even donâ€™t have to accept the terms of the GPLv3. However, any other use - such as
modifying or distributing the software - implies acceptance.
In case you use the software internally (including over a network), you may modify the
software without being obliged to distribute your modification. You may hire third parties
to work on the software exclusively for you and under your direction and control. But if you
modify the software and use it otherwise than merely internally, this will be considered as
distribution. You must distribute your modifications under GPLv3 (the copyleft principle).
Several more obligations apply if you distribute GPLv3 software. Check the GPLv3 license
carefully.
You create output with GPLv3 software: The GPLv3 does not automatically apply to the
output.
licensing
12
3.6. BSD license
There are several versions of the original Berkeley Distribution License. The most common
one is the 3-clause license ("New BSD License" or "Modified BSD License").
This is a permissive free software license. The license places minimal restrictions on how
the software can be redistributed. This is in contrast to copyleft licenses such as the GPLv.
3 discussed above, which have a copyleft mechanism.
This difference is of less importance when you merely use the software, but kicks in when
you start redistributing verbatim copies of the software or your own modified versions.
3.7. other licenses
FOSS or not, there are many kind of licenses on software. You should read and understand
them before using any software.
3.8. combination of software licenses
When you use several sources or wishes to redistribute your software under a different
license, you need to verify whether all licenses are compatible. Some FOSS licenses (such
as BSD) are compatible with proprietary licenses, but most are not. If you detect a license
incompatibility, you must contact the author to negotiate different license conditions or
refrain from using the incompatible software.
Part II. installing Linux
14
Table of Contents
4. installing Debian 8 ............................................................................................................  15
4.1. Debian ...................................................................................................................  16
4.2. Downloading ...........................................................................................................  16
4.3. virtualbox networking ...............................................................................................  32
4.4. setting the hostname .................................................................................................  34
4.5. adding a static ip address ........................................................................................... 34
4.6. Debian package management .....................................................................................  35
5. installing CentOS 7 ...........................................................................................................  36
5.1. download a CentOS 7 image ......................................................................................  37
5.2. Virtualbox ............................................................................................................... 39
5.3. CentOS 7 installing ..................................................................................................  44
5.4. CentOS 7 first logon ................................................................................................. 52
5.5. Virtualbox network interface ......................................................................................  53
5.6. configuring the network ............................................................................................  54
5.7. adding one static ip address .......................................................................................  54
5.8. package management ................................................................................................  55
5.9. logon from Linux and MacOSX .................................................................................. 56
5.10. logon from MS Windows ......................................................................................... 56
6. getting Linux at home .......................................................................................................  58
6.1. download a Linux CD image .....................................................................................  59
6.2. download Virtualbox ................................................................................................  59
6.3. create a virtual machine ............................................................................................  60
6.4. attach the CD image .................................................................................................  65
6.5. install Linux ............................................................................................................  68
15
Chapter 4. installing Debian 8
This module is a step by step demonstration of an actual installation of Debian 8 (also known
as Jessie).
We start by downloading an image from the internet and install Debian 8 as a virtual machine
in Virtualbox. We will also do some basic configuration of this new machine like setting
an ip address and fixing a hostname.
This procedure should be very similar for other versions of Debian, and also for distributions
like Linux Mint, xubuntu/ubuntu/kubuntu or Mepis. This procedure can also be helpful
if you are using another virtualization solution.
Go to the next chapter if you want to install CentOS, Fedora, Red Hat Enterprise
Linux, ....
installing Debian 8
16
4.1. Debian
Debian is one of the oldest Linux distributions. I use Debian myself on almost every
computer that I own (including raspbian on the Raspberry Pi).
Debian comes in releases named after characters in the movie Toy Story. The Jessie release
contains about 36000 packages.
Table 4.1. Debian releases
name
number
year
Woody
3.0
2002
Sarge
3.1
2005
Etch
4.0
2007
Lenny
5.0
2009
Squeeze
6.0
2011
Wheezy
7
2013
Jessie
8
2015
There is never a fixed date for the next Debian release. The next version is released when
it is ready.
4.2. Downloading
All these screenshots were made in November 2014, which means Debian 8 was still in
'testing' (but in 'freeze', so there will be no major changes when it is released).
Download Debian here:
installing Debian 8
17
After a couple of clicks on that website, I ended up downloading Debian 8 (testing) here. It
should be only one click once Debian 8 is released (somewhere in 2015).
You have many other options to download and install Debian. We will discuss them much
later.
This small screenshot shows the downloading of a netinst .iso file. Most of the software will
be downloaded during the installation. This also means that you will have the most recent
version of all packages when the install is finished.
I already have Debian 8 installed on my laptop (hence the paul@debian8 prompt). Anyway,
this is the downloaded file just before starting the installation.
paul@debian8:~$ ls -hl debian-testing-amd64-netinst.iso
-rw-r--r-- 1 paul paul 231M Nov 10 17:59 debian-testing-amd64-netinst.iso
installing Debian 8
18
Create a new virtualbox machine (I already have five, you might have zero for now). Click
the New button to start a wizard that will help you create a virtual machine.
The machine needs a name, this screenshot shows that I named it server42.
installing Debian 8
19
Most of the defaults in Virtualbox are ok.
512MB of RAM is enough to practice all the topics in this book.
We do not care about the virtual disk format.
installing Debian 8
20
Choosing dynamically allocated will save you some disk space (for a small performance
hit).
8GB should be plenty for learning about Linux servers.
This finishes the wizard. You virtual machine is almost ready to begin the installation.
installing Debian 8
21
First, make sure that you attach the downloaded .iso image to the virtual CD drive. (by
opening Settings, Storage followed by a mouse click on the round CD icon)
Personally I also disable sound and usb, because I never use these features. I also remove
the floppy disk and use a PS/2 mouse pointer. This is probably not very important, but I like
the idea that it saves some resources.
Now boot the virtual machine and begin the actual installation. After a couple of seconds
you should see a screen similar to this. Choose Install to begin the installation of Debian.
installing Debian 8
22
First select the language you want to use.
Choose your country. This information will be used to suggest a download mirror.
installing Debian 8
23
Choose the correct keyboard. On servers this is of no importance since most servers are
remotely managed via ssh.
Enter a hostname (with fqdn to set a dnsdomainname).
installing Debian 8
24
Give the root user a password. Remember this password (or use hunter2).
It is adviced to also create a normal user account. I don't give my full name, Debian 8 accepts
an identical username and full name paul.
installing Debian 8
25
The use entire disk refers to the virtual disk that you created before in Virtualbox..
Again the default is probably what you want. Only change partitioning if you really know
what you are doing.
installing Debian 8
26
Accept the partition layout (again only change if you really know what you are doing).
This is the point of no return, the magical moment where pressing yes will forever erase
data on the (virtual) computer.
installing Debian 8
27
Software is downloaded from a mirror repository, preferably choose one that is close by (as
in the same country).
This setup was done in Belgium.
installing Debian 8
28
Leave the proxy field empty (unless you are sure that you are behind a proxy server).
Choose whether you want to send anonymous statistics to the Debian project (it gathers data
about installed packages). You can view the statistics here http://popcon.debian.org/.
installing Debian 8
29
Choose what software to install, we do not need any graphical stuff for this training.
The latest versions are being downloaded.
installing Debian 8
30
Say yes to install the bootloader on the virtual machine.
Booting for the first time shows the grub screen
installing Debian 8
31
A couple seconds later you should see a lot of text scrolling of the screen (dmesg). After
which you are presented with this getty and are allowed your first logon.
You should now be able to log on to your virtual machine with the root account. Do you
remember the password ? Was it hunter2 ?
The screenshots in this book will look like this from now on. You can just type those
commands in the terminal (after you logged on).
root@server42:~# who am i
root     tty1         2014-11-10 18:21
root@server42:~# hostname
server42
root@server42:~# date
Mon Nov 10 18:21:56 CET 2014
installing Debian 8
32
4.3. virtualbox networking
You can also log on from remote (or from your Windows/Mac/Linux host computer) using
ssh or putty. Change the network settings in the virtual machine to bridge. This will enable
your virtual machine to receive an ip address from your local dhcp server.
The default virtualbox networking is to attach virtual network cards to nat. This screenshiot
shows the ip address 10.0.2.15 when on nat:
root@server42:~# ifconfig
eth0      Link encap:Ethernet  HWaddr 08:00:27:f5:74:cf
          inet addr:10.0.2.15  Bcast:10.0.2.255  Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:fef5:74cf/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:11 errors:0 dropped:0 overruns:0 frame:0
          TX packets:19 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:2352 (2.2 KiB)  TX bytes:1988 (1.9 KiB)
lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)
By shutting down the network interface and enabling it again, we force Debian to renew an
ip address from the bridged network.
root@server42:~# # do not run ifdown while connected over ssh!
root@server42:~# ifdown eth0
Killed old client process
Internet Systems Consortium DHCP Client 4.3.1
Copyright 2004-2014 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Listening on LPF/eth0/08:00:27:f5:74:cf
Sending on   LPF/eth0/08:00:27:f5:74:cf
installing Debian 8
33
Sending on   Socket/fallback
DHCPRELEASE on eth0 to 10.0.2.2 port 67
root@server42:~# # now enable bridge in virtualbox settings
root@server42:~# ifup eth0
Internet Systems Consortium DHCP Client 4.3.1
Copyright 2004-2014 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Listening on LPF/eth0/08:00:27:f5:74:cf
Sending on   LPF/eth0/08:00:27:f5:74:cf
Sending on   Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 8
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 8
DHCPREQUEST on eth0 to 255.255.255.255 port 67
DHCPOFFER from 192.168.1.42
DHCPACK from 192.168.1.42
bound to 192.168.1.111 -- renewal in 2938 seconds.
root@server42:~# ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 08:00:27:f5:74:cf
          inet addr:192.168.1.111  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:fef5:74cf/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:15 errors:0 dropped:0 overruns:0 frame:0
          TX packets:31 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:3156 (3.0 KiB)  TX bytes:3722 (3.6 KiB)
root@server42:~#
Here is an example of ssh to this freshly installed computer. Note that Debian 8 has disabled
remote root access, so i need to use the normal user account.
paul@debian8:~$ ssh paul@192.168.1.111
paul@192.168.1.111's password:
The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.
Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
paul@server42:~$
paul@server42:~$ su -
Password:
root@server42:~#
TODO: putty screenshot here...
installing Debian 8
34
4.4. setting the hostname
The hostname of the server is asked during installation, so there is no need to configure this
manually.
root@server42:~# hostname
server42
root@server42:~# cat /etc/hostname
server42
root@server42:~# dnsdomainname
paul.local
root@server42:~# grep server42 /etc/hosts
127.0.1.1       server42.paul.local     server42
root@server42:~#
4.5. adding a static ip address
This example shows how to add a static ip address to your server.
You can use ifconfig to set a static address that is active until the next reboot (or until the
next ifdown).
a
root@server42:~# ifconfig eth0:0 10.104.33.39
Adding a couple of lines to the /etc/network/interfaces file to enable an extra ip address
forever.
root@server42:~# vi /etc/network/interfaces
root@server42:~# tail -4 /etc/network/interfaces
auto eth0:0
iface eth0:0 inet static
address 10.104.33.39
netmask 255.255.0.0
root@server42:~# ifconfig
eth0      Link encap:Ethernet  HWaddr 08:00:27:f5:74:cf
          inet addr:192.168.1.111  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:fef5:74cf/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:528 errors:0 dropped:0 overruns:0 frame:0
          TX packets:333 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:45429 (44.3 KiB)  TX bytes:48763 (47.6 KiB)
eth0:0    Link encap:Ethernet  HWaddr 08:00:27:f5:74:cf
          inet addr:10.104.33.39  Bcast:10.255.255.255  Mask:255.0.0.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)
root@server42:~#
installing Debian 8
35
4.6. Debian package management
To get all information about the newest packages form the online repository:
root@server42:~# aptitude update
Get: 1 http://ftp.be.debian.org jessie InRelease [191 kB]
Get: 2 http://security.debian.org jessie/updates InRelease [84.1 kB]
Get: 3 http://ftp.be.debian.org jessie-updates InRelease [117 kB]
Get: 4 http://ftp.be.debian.org jessie-backports InRelease [118 kB]
Get: 5 http://security.debian.org jessie/updates/main Sources [14 B]
Get: 6 http://ftp.be.debian.org jessie/main Sources/DiffIndex [7,876 B]
... (output truncated)
To download and apply all updates for all installed packages:
root@server42:~# aptitude upgrade
Resolving dependencies...
The following NEW packages will be installed:
  firmware-linux-free{a} irqbalance{a} libnuma1{a} linux-image-3.16.0-4-amd64{a}
The following packages will be upgraded:
  busybox file libc-bin libc6 libexpat1 libmagic1 libpaper-utils libpaper1 libsqlite3-0
  linux-image-amd64 locales multiarch-support
12 packages upgraded, 4 newly installed, 0 to remove and 0 not upgraded.
Need to get 44.9 MB of archives. After unpacking 161 MB will be used.
Do you want to continue? [Y/n/?]
... (output truncated)
To install new software (vim and tmux in this example):
root@server42:~# aptitude install vim tmux
The following NEW packages will be installed:
  tmux vim vim-runtime{a}
0 packages upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
Need to get 6,243 kB of archives. After unpacking 29.0 MB will be used.
Do you want to continue? [Y/n/?]
Get: 1 http://ftp.be.debian.org/debian/ jessie/main tmux amd64 1.9-6 [245 kB]
Get: 2 http://ftp.be.debian.org/debian/ jessie/main vim-runtime all 2:7.4.488-1 [5,046 kB]
Get: 3 http://ftp.be.debian.org/debian/ jessie/main vim amd64 2:7.4.488-1 [952 kB]
Refer to the package management chapter in LinuxAdm.pdf for more information.
36
Chapter 5. installing CentOS 7
This module is a step by step demonstration of an actual installation of CentOS 7.
We start by downloading an image from the internet and install CentOS 7 as a virtual
machine in Virtualbox. We will also do some basic configuration of this new machine like
setting an ip address and fixing a hostname.
This procedure should be very similar for other versions of CentOS, and also for
distributions like RHEL (Red Hat Enterprise Linux) or Fedora. This procedure can also be
helpful if you are using another virtualization solution.
installing CentOS 7
37
5.1. download a CentOS 7 image
This demonstration uses a laptop computer with Virtualbox to install CentOS 7 as a virtual
machine. The first task is to download an .iso image of CentOS 7.
The CentOS 7 website looks like this today (November 2014). They change the look
regularly, so it may look different when you visit it.
You can download a full DVD, which allows for an off line installation of a graphical
CentOS 7 desktop. You can select this because it should be easy and complete, and should
get you started with a working CentOS 7 virtual machine.
installing CentOS 7
38
But I clicked instead on 'alternative downloads', selected CentOS 7 and x86_64 and ended
up on a mirror list. Each mirror is a server that contains copies of CentOS 7 media. I
selected a Belgian mirror because I currently am in Belgium.
There is again the option for full DVD's and more. This demonstration will use the minimal
.iso file, because it is much smaller in size. The download takes a couple of minutes.
Verify the size of the file after download to make sure it is complete. Probably a right click
on the file and selecting 'properties' (if you use Windows or Mac OSX).
I use Linux on the laptop already:
paul@debian8:~$ ls -lh CentOS-7.0-1406-x86_64-Minimal.iso
-rw-r--r-- 1 paul paul 566M Nov  1 14:45 CentOS-7.0-1406-x86_64-Minimal.iso
Do not worry if you do no understand the above command. Just try to make sure that the
size of this file is the same as the size that is mentioned on the CentOS 7 website.
installing CentOS 7
39
5.2. Virtualbox
This screenshot shows up when I start Virtualbox. I already have four virtual machines, you
might have none.
Below are the steps for creating a new virtual machine. Start by clicking New and give your
machine a name (I chose server33). Click Next.
installing CentOS 7
40
A Linux computer without graphical interface will run fine on half a gigabyte of RAM.
A Linux virtual machine will need a virtual hard drive.
installing CentOS 7
41
Any format will do for our purpose, so I left the default vdi.
The default dynamically allocated type will save disk space (until we fill the virtual disk
up to 100 percent). It makes the virtual machine a bit slower than fixed size, but the fixed
size speed improvement is not worth it for our purpose.
installing CentOS 7
42
The name of the virtual disk file on the host computer will be server33.vdi in my case (I left
it default and it uses the vm name). Also 16 GB should be enough to practice Linux. The
file will stay much smaller than 16GB, unless you copy a lot of files to the virtual machine.
You should now be back to the start screen of Virtualbox. If all went well, then you should
see the machine you just created in the list.
installing CentOS 7
43
After finishing the setup, we go into the Settings of our virtual machine and attach the .iso
file we downloaded before. Below is the default screenshot.
This is a screenshot with the .iso file properly attached.
installing CentOS 7
44
5.3. CentOS 7 installing
The screenshots below will show every step from starting the virtual machine for the first
time (with the .iso file attached) until the first logon.
You should see this when booting, otherwise verify the attachment of the .iso file form the
previous steps. Select Test this media and install CentOS 7.
installing CentOS 7
45
Carefully select the language in which you want your CentOS. I always install operating
systems in English, even though my native language is not English.
Also select the right keyboard, mine is a US qwerty, but yours may be different.
You should arrive at a summary page (with one or more warnings).
installing CentOS 7
46
Start by configuring the network. During this demonstration I had a DHCP server running
at 192.168.1.42, yours is probably different. Ask someone (a network administator ?) for
help if this step fails.
Select your time zone, and activate ntp.
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47
Choose a mirror that is close to you. If you can't find a local mirror, then you can copy the
one from this screenshot (it is a general CentOS mirror).
It can take a couple of seconds before the mirror is verified.
installing CentOS 7
48
I did not select any software here (because I want to show it all in this training).
After configuring network, location, software and all, you should be back on this page. Make
sure there are no warnings anymore (and that you made the correct choice everywhere).
installing CentOS 7
49
You can enter a root password and create a user account while the installation is
downloading from the internet. This is the longest step, it can take several minutes (or up to
an hour if you have a slow internet connection).
If you see this, then the installation was successful.
Time to reboot the computer and start CentOS 7 for the first time.
installing CentOS 7
50
This screen will appear briefly when the virtual machines starts. You don't have to do
anything.
After a couple of seconds, you should see a logon screen. This is called a tty or a getty. Here
you can type root as username. The login process will then ask your password (nothing will
appear on screen when you type your password).
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51
And this is what it looks like after logon. You are logged on to your own Linux machine,
very good.
All subsequent screenshots will be text only, no images anymore.
For example this screenshot shows three commands being typed on my new CentOS 7
install.
[root@localhost ~]# who am i
root     pts/0        2014-11-01 22:14
[root@localhost ~]# hostname
localhost.localdomain
[root@localhost ~]# date
Sat Nov  1 22:14:37 CET 2014
When using ssh the same commands will give this screenshot:
[root@localhost ~]# who am i
root     pts/0        2014-11-01 21:00 (192.168.1.35)
[root@localhost ~]# hostname
localhost.localdomain
[root@localhost ~]# date
Sat Nov  1 22:10:04 CET 2014
[root@localhost ~]#
If the last part is a bit too fast, take a look at the next topic CentOS 7 first logon.
installing CentOS 7
52
5.4. CentOS 7 first logon
All you have to log on, after finishing the installation, is this screen in Virtualbox.
This is workable to learn Linux, and you will be able to practice a lot. But there are more
ways to access your virtual machine, the next chapters discuss some of these and will also
introduce some basic system configuration.
5.4.1. setting the hostname
Setting the hostname is a simple as changing the /etc/hostname file. As you can see here,
it is set to localhost.localdomain by default.
[root@localhost ~]# cat /etc/hostname
localhost.localdomain
You could do echo server33.netsec.local > /etc/hostname followed by a reboot. But there
is also the new CentOS 7 way of setting a new hostname.
[root@localhost ~]# nmtui
The above command will give you a menu to choose from with a set system hostname
option. Using this nmtui option will edit the /etc/hostname file for you.
[root@localhost ~]# cat /etc/hostname
server33.netsec.local
[root@localhost ~]# hostname
server33.netsec.local
[root@localhost ~]# dnsdomainname
netsec.local
For some reason the documentation on the centos.org and docs.redhat.com websites tell
you to also execute this command:
[root@localhost ~]# systemctl restart systemd-hostnamed
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53
5.5. Virtualbox network interface
By default Virtualbox will connect your virtual machine over a nat interface. This will
show up as a 10.0.2.15 (or similar).
[root@server33 ~]# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast s\
tate UP qlen 1000
    link/ether 08:00:27:1c:f5:ab brd ff:ff:ff:ff:ff:ff
    inet 10.0.2.15/24 brd 10.0.2.255 scope global dynamic enp0s3
       valid_lft 86399sec preferred_lft 86399sec
    inet6 fe80::a00:27ff:fe1c:f5ab/64 scope link
       valid_lft forever preferred_lft forever
You can change this to bridge (over your wi-fi or over the ethernet cable) and thus make it
appear as if your virtual machine is directly on your local network (receiving an ip address
from your real dhcp server).
You can make this change while the vm is running, provided that you execute this command:
[root@server33 ~]# systemctl restart network
[root@server33 ~]# ip a s dev enp0s3
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast s\
tate UP qlen 1000
    link/ether 08:00:27:1c:f5:ab brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.110/24 brd 192.168.1.255 scope global dynamic enp0s3
       valid_lft 7199sec preferred_lft 7199sec
    inet6 fe80::a00:27ff:fe1c:f5ab/64 scope link
       valid_lft forever preferred_lft forever
[root@server33 ~]#
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54
5.6. configuring the network
The new way of changing network configuration is through the nmtui tool. If you want to
manually play with the files in /etc/sysconfig/network-scripts then you will first need to
verify (and disable) NetworkManager on that interface.
Verify whether an interface is controlled by NetworkManager using the nmcli command
(connected means managed bu NM).
[root@server33 ~]# nmcli dev status
DEVICE  TYPE      STATE      CONNECTION
enp0s3  ethernet  connected  enp0s3
lo      loopback  unmanaged  --
Disable NetworkManager on an interface (enp0s3 in this case):
echo 'NM_CONTROLLED=no' >> /etc/sysconfig/network-scripts/ifcfg-enp0s3
You can restart the network without a reboot like this:
[root@server33 ~]# systemctl restart network
Also, forget ifconfig and instead use ip a.
[root@server33 ~]# ip a s dev enp0s3 | grep inet
    inet 192.168.1.110/24 brd 192.168.1.255 scope global dynamic enp0s3
    inet6 fe80::a00:27ff:fe1c:f5ab/64 scope link
[root@server33 ~]#
5.7. adding one static ip address
This example shows how to add one static ip address to your computer.
[root@server33 ~]# nmtui edit enp0s3
In this interface leave the IPv4 configuration to automatic, and add an ip address just below.
          IPv4 CONFIGURATION <Automatic>                         <Hide>
          Addresses 10.104.33.32/16__________ <Remove>
Execute this command after exiting nmtui.
[root@server33 ~]# systemctl restart network
And verify with ip (not with ifconfig):
[root@server33 ~]# ip a s dev enp0s3 | grep inet
    inet 192.168.1.110/24 brd 192.168.1.255 scope global dynamic enp0s3
    inet 10.104.33.32/16 brd 10.104.255.255 scope global enp0s3
    inet6 fe80::a00:27ff:fe1c:f5ab/64 scope link
[root@server33 ~]#
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55
5.8. package management
Even with a network install, CentOS 7 did not install the latest version of some packages.
Luckily there is only one command to run (as root). This can take a while.
[root@server33 ~]# yum update
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: centos.weepeetelecom.be
 * extras: centos.weepeetelecom.be
 * updates: centos.weepeetelecom.be
Resolving Dependencies
--> Running transaction check
---> Package NetworkManager.x86_64 1:0.9.9.1-13.git20140326.4dba720.el7 \
will be updated
... (output truncated)
You can also use yum to install one or more packages. Do not forget to run yum update
from time to time.
[root@server33 ~]# yum update -y && yum install vim -y
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: centos.weepeetelecom.be
... (output truncated)
Refer to the package management chapter for more information on installing and removing
packages.
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56
5.9. logon from Linux and MacOSX
You can now open a terminal on Linux or MacOSX and use ssh to log on to your virtual
machine.
paul@debian8:~$ ssh root@192.168.1.110
root@192.168.1.110's password:
Last login: Sun Nov  2 11:53:57 2014
[root@server33 ~]# hostname
server33.netsec.local
[root@server33 ~]#
5.10. logon from MS Windows
There is no ssh installed on MS Windows, but you can download putty.exe from http://
www.chiark.greenend.org.uk/~sgtatham/putty/download.html (just Google it).
Use putty.exe as shown in this screenshot (I saved the ip address by giving it a name
'server33' and presing the 'save' button).
installing CentOS 7
57
The first time you will get a message about keys, accept this (this is explained in the ssh
chapter).
Enter your userid (or root) and the correct password (nothing will appear on the screen when
typing a password).
58
Chapter 6. getting Linux at home
This chapter shows a Ubuntu install in Virtualbox. Consider it legacy and use CentOS7
or Debian8 instead (each have their own chapter now).
This book assumes you have access to a working Linux computer. Most companies have
one or more Linux servers, if you have already logged on to it, then you 're all set (skip this
chapter and go to the next).
Another option is to insert a Ubuntu Linux CD in a computer with (or without) Microsoft
Windows and follow the installation. Ubuntu will resize (or create) partitions and setup a
menu at boot time to choose Windows or Linux.
If you do not have access to a Linux computer at the moment, and if you are unable or unsure
about installing Linux on your computer, then this chapter proposes a third option: installing
Linux in a virtual machine.
Installation in a virtual machine (provided by Virtualbox) is easy and safe. Even when you
make mistakes and crash everything on the virtual Linux machine, then nothing on the real
computer is touched.
This chapter gives easy steps and screenshots to get a working Ubuntu server in a Virtualbox
virtual machine. The steps are very similar to installing Fedora or CentOS or even Debian,
and if you like you can also use VMWare instead of Virtualbox.
getting Linux at home
59
6.1. download a Linux CD image
Start by downloading a Linux CD image (an .ISO file) from the distribution of your choice
from the Internet. Take care selecting the correct cpu architecture of your computer; choose
i386 if unsure. Choosing the wrong cpu type (like x86_64 when you have an old Pentium)
will almost immediately fail to boot the CD.
6.2. download Virtualbox
Step two (when the .ISO file has finished downloading) is to download Virtualbox. If you are
currently running Microsoft Windows, then download and install Virtualbox for Windows!
getting Linux at home
60
6.3. create a virtual machine
Now start Virtualbox. Contrary to the screenshot below, your left pane should be empty.
Click New to create a new virtual machine. We will walk together through the wizard. The
screenshots below are taken on Mac OSX; they will be slightly different if you are running
Microsoft Windows.
getting Linux at home
61
Name your virtual machine (and maybe select 32-bit or 64-bit).
Give the virtual machine some memory (512MB if you have 2GB or more, otherwise select
256MB).
getting Linux at home
62
Select to create a new disk (remember, this will be a virtual disk).
If you get the question below, choose vdi.
getting Linux at home
63
Choose dynamically allocated (fixed size is only useful in production or on really old, slow
hardware).
Choose between 10GB and 16GB as the disk size.
getting Linux at home
64
Click create to create the virtual disk.
Click create to create the virtual machine.
getting Linux at home
65
6.4. attach the CD image
Before we start the virtual computer, let us take a look at some settings (click Settings).
Do not worry if your screen looks different, just find the button named storage.
getting Linux at home
66
Remember the .ISO file you downloaded? Connect this .ISO file to this virtual machine by
clicking on the CD icon next to Empty.
Now click on the other CD icon and attach your ISO file to this virtual CD drive.
getting Linux at home
67
Verify that your download is accepted. If Virtualbox complains at this point, then you
probably did not finish the download of the CD (try downloading it again).
It could be useful to set the network adapter to bridge instead of NAT. Bridged usually will
connect your virtual computer to the Internet.
getting Linux at home
68
6.5. install Linux
The virtual machine is now ready to start. When given a choice at boot, select install and
follow the instructions on the screen. When the installation is finished, you can log on to
the machine and start practising Linux!
Part III. first steps on
the command line
70
Table of Contents
7. man pages ........................................................................................................................  71
7.1. man $command .......................................................................................................  72
7.2. man $configfile ........................................................................................................ 72
7.3. man $daemon ..........................................................................................................  72
7.4. man -k (apropos) ...................................................................................................... 72
7.5. whatis ....................................................................................................................  72
7.6. whereis ...................................................................................................................  72
7.7. man sections ...........................................................................................................  73
7.8. man $section $file ....................................................................................................  73
7.9. man man ................................................................................................................  73
7.10. mandb ..................................................................................................................  73
8. working with directories ....................................................................................................  74
8.1. pwd .......................................................................................................................  75
8.2. cd ..........................................................................................................................  75
8.3. absolute and relative paths .........................................................................................  76
8.4. path completion .......................................................................................................  77
8.5. ls ...........................................................................................................................  77
8.6. mkdir .....................................................................................................................  79
8.7. rmdir ...................................................................................................................... 79
8.8. practice: working with directories ...............................................................................  81
8.9. solution: working with directories ...............................................................................  82
9. working with files .............................................................................................................  84
9.1. all files are case sensitive ..........................................................................................  85
9.2. everything is a file ...................................................................................................  85
9.3. file ........................................................................................................................  85
9.4. touch ...................................................................................................................... 86
9.5. rm .........................................................................................................................  87
9.6. cp ..........................................................................................................................  88
9.7. mv .........................................................................................................................  89
9.8. rename ...................................................................................................................  90
9.9. practice: working with files ........................................................................................  91
9.10. solution: working with files ......................................................................................  92
10. working with file contents ................................................................................................  94
10.1. head .....................................................................................................................  95
10.2. tail .......................................................................................................................  95
10.3. cat .......................................................................................................................  96
10.4. tac .......................................................................................................................  97
10.5. more and less ........................................................................................................  98
10.6. strings ..................................................................................................................  98
10.7. practice: file contents ..............................................................................................  99
10.8. solution: file contents ............................................................................................  100
11. the Linux file tree ..........................................................................................................  101
11.1. filesystem hierarchy standard ..................................................................................  102
11.2. man hier ..............................................................................................................  102
11.3. the root directory / ................................................................................................  102
11.4. binary directories ..................................................................................................  103
11.5. configuration directories .........................................................................................  105
11.6. data directories .....................................................................................................  107
11.7. in memory directories ............................................................................................  109
11.8. /usr Unix System Resources ....................................................................................  114
11.9. /var variable data ..................................................................................................  116
11.10. practice: file system tree .......................................................................................  118
11.11. solution: file system tree ....................................................................................... 120
71
Chapter 7. man pages
This chapter will explain the use of man pages (also called manual pages) on your Unix
or Linux computer.
You will learn the man command together with related commands like whereis, whatis
and mandb.
Most Unix files and commands have pretty good man pages to explain their use. Man
pages also come in handy when you are using multiple flavours of Unix or several Linux
distributions since options and parameters sometimes vary.
man pages
72
7.1. man $command
Type man followed by a command (for which you want help) and start reading. Press q to
quit the manpage. Some man pages contain examples (near the end).
paul@laika:~$ man whois
Reformatting whois(1), please wait...
7.2. man $configfile
Most configuration files have their own manual.
paul@laika:~$ man syslog.conf
Reformatting syslog.conf(5), please wait...
7.3. man $daemon
This is also true for most daemons (background programs) on your system..
paul@laika:~$ man syslogd
Reformatting syslogd(8), please wait...
7.4. man -k (apropos)
man -k (or apropos) shows a list of man pages containing a string.
paul@laika:~$ man -k syslog
lm-syslog-setup (8)  - configure laptop mode to switch syslog.conf ...
logger (1)           - a shell command interface to the syslog(3) ...
syslog-facility (8)  - Setup and remove LOCALx facility for sysklogd
syslog.conf (5)      - syslogd(8) configuration file
syslogd (8)          - Linux system logging utilities.
syslogd-listfiles (8) - list system logfiles
7.5. whatis
To see just the description of a manual page, use whatis followed by a string.
paul@u810:~$ whatis route
route (8)            - show / manipulate the IP routing table
7.6. whereis
The location of a manpage can be revealed with whereis.
paul@laika:~$ whereis -m whois
whois: /usr/share/man/man1/whois.1.gz
This file is directly readable by man.
paul@laika:~$ man /usr/share/man/man1/whois.1.gz
man pages
73
7.7. man sections
By now you will have noticed the numbers between the round brackets. man man will
explain to you that these are section numbers. Executable programs and shell commands
reside in section one.
1 Executable programs or shell commands
2 System calls (functions provided by the kernel)
3 Library calls (functions within program libraries)
4 Special files (usually found in /dev)
5 File formats and conventions eg /etc/passwd
6 Games
7 Miscellaneous (including macro packages and conventions), e.g. man(7)
8 System administration commands (usually only for root)
9 Kernel routines [Non standard]
7.8. man $section $file
Therefor, when referring to the man page of the passwd command, you will see it written
as passwd(1); when referring to the passwd file, you will see it written as passwd(5). The
screenshot explains how to open the man page in the correct section.
[paul@RHEL52 ~]$ man passwd      # opens the first manual found
[paul@RHEL52 ~]$ man 5 passwd    # opens a page from section 5
7.9. man man
If you want to know more about man, then Read The Fantastic Manual (RTFM).
Unfortunately, manual pages do not have the answer to everything...
paul@laika:~$ man woman
No manual entry for woman
7.10. mandb
Should you be convinced that a man page exists, but you can't access it, then try running
mandb on Debian/Mint.
root@laika:~# mandb
0 man subdirectories contained newer manual pages.
0 manual pages were added.
0 stray cats were added.
0 old database entries were purged.
Or run makewhatis on CentOS/Redhat.
[root@centos65 ~]# apropos scsi
scsi: nothing appropriate
[root@centos65 ~]# makewhatis 
[root@centos65 ~]# apropos scsi
hpsa                 (4)  - HP Smart Array SCSI driver
lsscsi               (8)  - list SCSI devices (or hosts) and their attributes
sd                   (4)  - Driver for SCSI Disk Drives
st                   (4)  - SCSI tape device
74
Chapter 8. working with directories
This module is a brief overview of the most common commands to work with directories:
pwd, cd, ls, mkdir and rmdir. These commands are available on any Linux (or Unix)
system.
This module also discusses absolute and relative paths and path completion in the bash
shell.
working with directories
75
8.1. pwd
The you are here sign can be displayed with the pwd command (Print Working Directory).
Go ahead, try it: Open a command line interface (also called a terminal, console or xterm)
and type pwd. The tool displays your current directory.
paul@debian8:~$ pwd
/home/paul
8.2. cd
You can change your current directory with the cd command (Change Directory).
paul@debian8$ cd /etc
paul@debian8$ pwd
/etc
paul@debian8$ cd /bin
paul@debian8$ pwd
/bin
paul@debian8$ cd /home/paul/
paul@debian8$ pwd
/home/paul
8.2.1. cd ~
The cd is also a shortcut to get back into your home directory. Just typing cd without a target
directory, will put you in your home directory. Typing cd ~ has the same effect.
paul@debian8$ cd /etc
paul@debian8$ pwd
/etc
paul@debian8$ cd
paul@debian8$ pwd
/home/paul
paul@debian8$ cd ~
paul@debian8$ pwd
/home/paul
8.2.2. cd ..
To go to the parent directory (the one just above your current directory in the directory
tree), type cd .. .
paul@debian8$ pwd
/usr/share/games
paul@debian8$ cd ..
paul@debian8$ pwd
/usr/share
To stay in the current directory, type cd . ;-) We will see useful use of the . character
representing the current directory later.
working with directories
76
8.2.3. cd -
Another useful shortcut with cd is to just type cd - to go to the previous directory.
paul@debian8$ pwd
/home/paul
paul@debian8$ cd /etc
paul@debian8$ pwd
/etc
paul@debian8$ cd -
/home/paul
paul@debian8$ cd -
/etc
8.3. absolute and relative paths
You should be aware of absolute and relative paths in the file tree. When you type a path
starting with a slash (/), then the root of the file tree is assumed. If you don't start your path
with a slash, then the current directory is the assumed starting point.
The screenshot below first shows the current directory /home/paul. From within this
directory, you have to type cd /home instead of cd home to go to the /home directory.
paul@debian8$ pwd
/home/paul
paul@debian8$ cd home
bash: cd: home: No such file or directory
paul@debian8$ cd /home
paul@debian8$ pwd
/home
When inside /home, you have to type cd paul instead of cd /paul to enter the subdirectory
paul of the current directory /home.
paul@debian8$ pwd
/home
paul@debian8$ cd /paul
bash: cd: /paul: No such file or directory
paul@debian8$ cd paul
paul@debian8$ pwd
/home/paul
In case your current directory is the root directory /, then both cd /home and cd home will
get you in the /home directory.
paul@debian8$ pwd
/
paul@debian8$ cd home
paul@debian8$ pwd
/home
paul@debian8$ cd /
paul@debian8$ cd /home 
paul@debian8$ pwd
/home
This was the last screenshot with pwd statements. From now on, the current directory will
often be displayed in the prompt. Later in this book we will explain how the shell variable
$PS1 can be configured to show this.
working with directories
77
8.4. path completion
The tab key can help you in typing a path without errors. Typing cd /et followed by the tab
key will expand the command line to cd /etc/. When typing cd /Et followed by the tab key,
nothing will happen because you typed the wrong path (upper case E).
You will need fewer key strokes when using the tab key, and you will be sure your typed
path is correct!
8.5. ls
You can list the contents of a directory with ls.
paul@debian8:~$ ls
allfiles.txt  dmesg.txt  services   stuff  summer.txt
paul@debian8:~$
8.5.1. ls -a
A frequently used option with ls is -a to show all files. Showing all files means including
the hidden files. When a file name on a Linux file system starts with a dot, it is considered
a hidden file and it doesn't show up in regular file listings.
paul@debian8:~$ ls
allfiles.txt  dmesg.txt  services  stuff  summer.txt
paul@debian8:~$ ls -a
.   allfiles.txt   .bash_profile  dmesg.txt   .lesshst  stuff
..  .bash_history  .bashrc        services    .ssh      summer.txt 
paul@debian8:~$
8.5.2. ls -l
Many times you will be using options with ls to display the contents of the directory in
different formats or to display different parts of the directory. Typing just ls gives you a
list of files in the directory. Typing ls -l (that is a letter L, not the number 1) gives you a
long listing.
paul@debian8:~$ ls -l
total 17296
-rw-r--r-- 1 paul paul 17584442 Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 paul paul    96650 Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 paul paul    19558 Sep 17 00:04 services
drwxr-xr-x 2 paul paul     4096 Sep 17 00:04 stuff
-rw-r--r-- 1 paul paul        0 Sep 17 00:04 summer.txt
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8.5.3. ls -lh
Another frequently used ls option is -h. It shows the numbers (file sizes) in a more human
readable format. Also shown below is some variation in the way you can give the options
to ls. We will explain the details of the output later in this book.
Note that we use the letter L as an option in this screenshot, not the number 1.
paul@debian8:~$ ls -l -h
total 17M
-rw-r--r-- 1 paul paul  17M Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 paul paul  95K Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 paul paul  20K Sep 17 00:04 services
drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff
-rw-r--r-- 1 paul paul    0 Sep 17 00:04 summer.txt
paul@debian8:~$ ls -lh
total 17M
-rw-r--r-- 1 paul paul  17M Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 paul paul  95K Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 paul paul  20K Sep 17 00:04 services
drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff
-rw-r--r-- 1 paul paul    0 Sep 17 00:04 summer.txt
paul@debian8:~$ ls -hl
total 17M
-rw-r--r-- 1 paul paul  17M Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 paul paul  95K Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 paul paul  20K Sep 17 00:04 services
drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff
-rw-r--r-- 1 paul paul    0 Sep 17 00:04 summer.txt
paul@debian8:~$ ls -h -l
total 17M
-rw-r--r-- 1 paul paul  17M Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 paul paul  95K Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 paul paul  20K Sep 17 00:04 services
drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff
-rw-r--r-- 1 paul paul    0 Sep 17 00:04 summer.txt
paul@debian8:~$
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8.6. mkdir
Walking around the Unix file tree is fun, but it is even more fun to create your own directories
with mkdir. You have to give at least one parameter to mkdir, the name of the new directory
to be created. Think before you type a leading / .
paul@debian8:~$ mkdir mydir
paul@debian8:~$ cd mydir
paul@debian8:~/mydir$ ls -al
total 8
drwxr-xr-x  2 paul paul 4096 Sep 17 00:07 .
drwxr-xr-x 48 paul paul 4096 Sep 17 00:07 ..
paul@debian8:~/mydir$ mkdir stuff
paul@debian8:~/mydir$ mkdir otherstuff
paul@debian8:~/mydir$ ls -l
total 8
drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 otherstuff
drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 stuff
paul@debian8:~/mydir$
8.6.1. mkdir -p
The following command will fail, because the parent directory of threedirsdeep does not
exist.
paul@debian8:~$ mkdir mydir2/mysubdir2/threedirsdeep
mkdir: cannot create directory â€˜mydir2/mysubdir2/threedirsdeepâ€™: No such fi\
le or directory
When given the option -p, then mkdir will create parent directories as needed.
paul@debian8:~$ mkdir -p mydir2/mysubdir2/threedirsdeep
paul@debian8:~$ cd mydir2
paul@debian8:~/mydir2$ ls -l
total 4
drwxr-xr-x 3 paul paul 4096 Sep 17 00:11 mysubdir2
paul@debian8:~/mydir2$ cd mysubdir2
paul@debian8:~/mydir2/mysubdir2$ ls -l
total 4
drwxr-xr-x 2 paul paul 4096 Sep 17 00:11 threedirsdeep
paul@debian8:~/mydir2/mysubdir2$ cd threedirsdeep/
paul@debian8:~/mydir2/mysubdir2/threedirsdeep$ pwd
/home/paul/mydir2/mysubdir2/threedirsdeep
8.7. rmdir
When a directory is empty, you can use rmdir to remove the directory.
paul@debian8:~/mydir$ ls -l
total 8
drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 otherstuff
drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 stuff
paul@debian8:~/mydir$ rmdir otherstuff
paul@debian8:~/mydir$ cd ..
paul@debian8:~$ rmdir mydir
rmdir: failed to remove â€˜mydirâ€™: Directory not empty
paul@debian8:~$ rmdir mydir/stuff
paul@debian8:~$ rmdir mydir
paul@debian8:~$
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8.7.1. rmdir -p
And similar to the mkdir -p option, you can also use rmdir to recursively remove
directories.
paul@debian8:~$ mkdir -p test42/subdir
paul@debian8:~$ rmdir -p test42/subdir
paul@debian8:~$
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81
8.8. practice: working with directories
1. Display your current directory.
2. Change to the /etc directory.
3. Now change to your home directory using only three key presses.
4. Change to the /boot/grub directory using only eleven key presses.
5. Go to the parent directory of the current directory.
6. Go to the root directory.
7. List the contents of the root directory.
8. List a long listing of the root directory.
9. Stay where you are, and list the contents of /etc.
10. Stay where you are, and list the contents of /bin and /sbin.
11. Stay where you are, and list the contents of ~.
12. List all the files (including hidden files) in your home directory.
13. List the files in /boot in a human readable format.
14. Create a directory testdir in your home directory.
15. Change to the /etc directory, stay here and create a directory newdir in your home
directory.
16. Create in one command the directories ~/dir1/dir2/dir3 (dir3 is a subdirectory from dir2,
and dir2 is a subdirectory from dir1 ).
17. Remove the directory testdir.
18. If time permits (or if you are waiting for other students to finish this practice), use and
understand pushd and popd. Use the man page of bash to find information about these
commands.
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8.9. solution: working with directories
1. Display your current directory.
pwd
2. Change to the /etc directory.
cd /etc
3. Now change to your home directory using only three key presses.
cd (and the enter key)
4. Change to the /boot/grub directory using only eleven key presses.
cd /boot/grub (use the tab key)
5. Go to the parent directory of the current directory.
cd .. (with space between cd and ..)
6. Go to the root directory.
cd /
7. List the contents of the root directory.
ls
8. List a long listing of the root directory.
ls -l
9. Stay where you are, and list the contents of /etc.
ls /etc
10. Stay where you are, and list the contents of /bin and /sbin.
ls /bin /sbin
11. Stay where you are, and list the contents of ~.
ls ~
12. List all the files (including hidden files) in your home directory.
ls -al ~
13. List the files in /boot in a human readable format.
ls -lh /boot
14. Create a directory testdir in your home directory.
mkdir ~/testdir
15. Change to the /etc directory, stay here and create a directory newdir in your home
directory.
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83
cd /etc ; mkdir ~/newdir
16. Create in one command the directories ~/dir1/dir2/dir3 (dir3 is a subdirectory from dir2,
and dir2 is a subdirectory from dir1 ).
mkdir -p ~/dir1/dir2/dir3
17. Remove the directory testdir.
rmdir testdir
18. If time permits (or if you are waiting for other students to finish this practice), use and
understand pushd and popd. Use the man page of bash to find information about these
commands.
man bash           # opens the manual
/pushd             # searches for pushd
n                  # next (do this two/three times)
The Bash shell has two built-in commands called pushd and popd. Both commands work
with a common stack of previous directories. Pushd adds a directory to the stack and changes
to a new current directory, popd removes a directory from the stack and sets the current
directory.
paul@debian7:/etc$ cd /bin
paul@debian7:/bin$ pushd /lib
/lib /bin
paul@debian7:/lib$ pushd /proc
/proc /lib /bin
paul@debian7:/proc$ popd
/lib /bin
paul@debian7:/lib$ popd
/bin
84
Chapter 9. working with files
In this chapter we learn how to recognise, create, remove, copy and move files using
commands like file, touch, rm, cp, mv and rename.
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85
9.1. all files are case sensitive
Files on Linux (or any Unix) are case sensitive. This means that FILE1 is different from
file1, and /etc/hosts is different from /etc/Hosts (the latter one does not exist on a typical
Linux computer).
This screenshot shows the difference between two files, one with upper case W, the other
with lower case w.
paul@laika:~/Linux$ ls
winter.txt  Winter.txt
paul@laika:~/Linux$ cat winter.txt
It is cold.
paul@laika:~/Linux$ cat Winter.txt
It is very cold!
9.2. everything is a file
A directory is a special kind of file, but it is still a (case sensitive!) file. Each terminal
window (for example /dev/pts/4), any hard disk or partition (for example /dev/sdb1) and
any process are all represented somewhere in the file system as a file. It will become clear
throughout this course that everything on Linux is a file.
9.3. file
The file utility determines the file type. Linux does not use extensions to determine the
file type. The command line does not care whether a file ends in .txt or .pdf. As a system
administrator, you should use the file command to determine the file type. Here are some
examples on a typical Linux system.
paul@laika:~$ file pic33.png
pic33.png: PNG image data, 3840 x 1200, 8-bit/color RGBA, non-interlaced
paul@laika:~$ file /etc/passwd
/etc/passwd: ASCII text
paul@laika:~$ file HelloWorld.c
HelloWorld.c: ASCII C program text
The file command uses a magic file that contains patterns to recognise file types. The magic
file is located in /usr/share/file/magic. Type man 5 magic for more information.
It is interesting to point out file -s for special files like those in /dev and /proc.
root@debian6~# file /dev/sda
/dev/sda: block special
root@debian6~# file -s /dev/sda
/dev/sda: x86 boot sector; partition 1: ID=0x83, active, starthead...
root@debian6~# file /proc/cpuinfo 
/proc/cpuinfo: empty
root@debian6~# file -s /proc/cpuinfo
/proc/cpuinfo: ASCII C++ program text
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9.4. touch
9.4.1. create an empty file
One easy way to create an empty file is with touch. (We will see many other ways for
creating files later in this book.)
This screenshot starts with an empty directory, creates two files with touch and the lists
those files.
paul@debian7:~$ ls -l
total 0
paul@debian7:~$ touch file42
paul@debian7:~$ touch file33
paul@debian7:~$ ls -l
total 0
-rw-r--r-- 1 paul paul 0 Oct 15 08:57 file33
-rw-r--r-- 1 paul paul 0 Oct 15 08:56 file42
paul@debian7:~$
9.4.2. touch -t
The touch command can set some properties while creating empty files. Can you determine
what is set by looking at the next screenshot? If not, check the manual for touch.
paul@debian7:~$ touch -t 200505050000 SinkoDeMayo
paul@debian7:~$ touch -t 130207111630 BigBattle.txt
paul@debian7:~$ ls -l
total 0
-rw-r--r-- 1 paul paul 0 Jul 11  1302 BigBattle.txt
-rw-r--r-- 1 paul paul 0 Oct 15 08:57 file33
-rw-r--r-- 1 paul paul 0 Oct 15 08:56 file42
-rw-r--r-- 1 paul paul 0 May  5  2005 SinkoDeMayo
paul@debian7:~$
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9.5. rm
9.5.1. remove forever
When you no longer need a file, use rm to remove it. Unlike some graphical user interfaces,
the command line in general does not have a waste bin or trash can to recover files. When
you use rm to remove a file, the file is gone. Therefore, be careful when removing files!
paul@debian7:~$ ls
BigBattle.txt  file33  file42  SinkoDeMayo
paul@debian7:~$ rm BigBattle.txt
paul@debian7:~$ ls
file33  file42  SinkoDeMayo
paul@debian7:~$
9.5.2. rm -i
To prevent yourself from accidentally removing a file, you can type rm -i.
paul@debian7:~$ ls
file33  file42  SinkoDeMayo
paul@debian7:~$ rm -i file33
rm: remove regular empty file `file33'? yes
paul@debian7:~$ rm -i SinkoDeMayo
rm: remove regular empty file `SinkoDeMayo'? n
paul@debian7:~$ ls
file42  SinkoDeMayo
paul@debian7:~$
9.5.3. rm -rf
By default, rm -r will not remove non-empty directories. However rm accepts several
options that will allow you to remove any directory. The rm -rf statement is famous because
it will erase anything (providing that you have the permissions to do so). When you are
logged on as root, be very careful with rm -rf (the f means force and the r means recursive)
since being root implies that permissions don't apply to you. You can literally erase your
entire file system by accident.
paul@debian7:~$ mkdir test
paul@debian7:~$ rm test
rm: cannot remove `test': Is a directory
paul@debian7:~$ rm -rf test
paul@debian7:~$ ls test
ls: cannot access test: No such file or directory
paul@debian7:~$
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9.6. cp
9.6.1. copy one file
To copy a file, use cp with a source and a target argument.
paul@debian7:~$ ls
file42  SinkoDeMayo
paul@debian7:~$ cp file42 file42.copy
paul@debian7:~$ ls
file42  file42.copy  SinkoDeMayo
9.6.2. copy to another directory
If the target is a directory, then the source files are copied to that target directory.
paul@debian7:~$ mkdir dir42
paul@debian7:~$ cp SinkoDeMayo dir42
paul@debian7:~$ ls dir42/
SinkoDeMayo
9.6.3. cp -r
To copy complete directories, use cp -r (the -r option forces recursive copying of all files
in all subdirectories).
paul@debian7:~$ ls
dir42  file42  file42.copy  SinkoDeMayo
paul@debian7:~$ cp -r dir42/ dir33
paul@debian7:~$ ls
dir33  dir42  file42  file42.copy  SinkoDeMayo
paul@debian7:~$ ls dir33/
SinkoDeMayo
9.6.4. copy multiple files to directory
You can also use cp to copy multiple files into a directory. In this case, the last argument
(a.k.a. the target) must be a directory.
paul@debian7:~$ cp file42 file42.copy SinkoDeMayo dir42/
paul@debian7:~$ ls dir42/
file42  file42.copy  SinkoDeMayo
9.6.5. cp -i
To prevent cp from overwriting existing files, use the -i (for interactive) option.
paul@debian7:~$ cp SinkoDeMayo file42
paul@debian7:~$ cp SinkoDeMayo file42
paul@debian7:~$ cp -i SinkoDeMayo file42
cp: overwrite `file42'? n
paul@debian7:~$
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9.7. mv
9.7.1. rename files with mv
Use mv to rename a file or to move the file to another directory.
paul@debian7:~$ ls
dir33  dir42  file42  file42.copy  SinkoDeMayo
paul@debian7:~$ mv file42 file33
paul@debian7:~$ ls
dir33  dir42  file33  file42.copy  SinkoDeMayo
paul@debian7:~$
When you need to rename only one file then mv is the preferred command to use.
9.7.2. rename directories with mv
The same mv command can be used to rename directories.
paul@debian7:~$ ls -l
total 8
drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 dir33
drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 dir42
-rw-r--r-- 1 paul paul    0 Oct 15 09:38 file33
-rw-r--r-- 1 paul paul    0 Oct 15 09:16 file42.copy
-rw-r--r-- 1 paul paul    0 May  5  2005 SinkoDeMayo
paul@debian7:~$ mv dir33 backup
paul@debian7:~$ ls -l
total 8
drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 backup
drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 dir42
-rw-r--r-- 1 paul paul    0 Oct 15 09:38 file33
-rw-r--r-- 1 paul paul    0 Oct 15 09:16 file42.copy
-rw-r--r-- 1 paul paul    0 May  5  2005 SinkoDeMayo
paul@debian7:~$
9.7.3. mv -i
The mv also has a -i switch similar to cp and rm.
this screenshot shows that mv -i will ask permission to overwrite an existing file.
paul@debian7:~$ mv -i file33 SinkoDeMayo
mv: overwrite `SinkoDeMayo'? no
paul@debian7:~$
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9.8. rename
9.8.1. about rename
The rename command is one of the rare occasions where the Linux Fundamentals book
has to make a distinction between Linux distributions. Almost every command in the
Fundamentals part of this book works on almost every Linux computer. But rename is
different.
Try to use mv whenever you need to rename only a couple of files.
9.8.2. rename on Debian/Ubuntu
The rename command on Debian uses regular expressions (regular expression or shor regex
are explained in a later chapter) to rename many files at once.
Below a rename example that switches all occurrences of txt to png for all file names ending
in .txt.
paul@debian7:~/test42$ ls
abc.txt  file33.txt  file42.txt
paul@debian7:~/test42$ rename 's/\.txt/\.png/' *.txt
paul@debian7:~/test42$ ls
abc.png  file33.png  file42.png
This second example switches all (first) occurrences of file into document for all file names
ending in .png.
paul@debian7:~/test42$ ls
abc.png  file33.png  file42.png
paul@debian7:~/test42$ rename 's/file/document/' *.png
paul@debian7:~/test42$ ls
abc.png  document33.png  document42.png
paul@debian7:~/test42$
9.8.3. rename on CentOS/RHEL/Fedora
On Red Hat Enterprise Linux, the syntax of rename is a bit different. The first example
below renames all *.conf files replacing any occurrence of .conf with .backup.
[paul@centos7 ~]$ touch one.conf two.conf three.conf
[paul@centos7 ~]$ rename .conf .backup *.conf
[paul@centos7 ~]$ ls
one.backup  three.backup  two.backup
[paul@centos7 ~]$
The second example renames all (*) files replacing one with ONE.
[paul@centos7 ~]$ ls
one.backup  three.backup  two.backup
[paul@centos7 ~]$ rename one ONE *
[paul@centos7 ~]$ ls
ONE.backup  three.backup  two.backup
[paul@centos7 ~]$
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91
9.9. practice: working with files
1. List the files in the /bin directory
2. Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd.
3a. Download wolf.jpg and LinuxFun.pdf from http://linux-training.be (wget http://
linux-training.be/files/studentfiles/wolf.jpg and wget http://linux-training.be/files/books/
LinuxFun.pdf)
wget http://linux-training.be/files/studentfiles/wolf.jpg
wget http://linux-training.be/files/studentfiles/wolf.png
wget http://linux-training.be/files/books/LinuxFun.pdf
3b. Display the type of file of wolf.jpg and LinuxFun.pdf
3c. Rename wolf.jpg to wolf.pdf (use mv).
3d. Display the type of file of wolf.pdf and LinuxFun.pdf.
4. Create a directory ~/touched and enter it.
5. Create the files today.txt and yesterday.txt in touched.
6. Change the date on yesterday.txt to match yesterday's date.
7. Copy yesterday.txt to copy.yesterday.txt
8. Rename copy.yesterday.txt to kim
9. Create a directory called ~/testbackup and copy all files from ~/touched into it.
10. Use one command to remove the directory ~/testbackup and all files into it.
11. Create a directory ~/etcbackup and copy all *.conf files from /etc into it. Did you include
all subdirectories of /etc ?
12. Use rename to rename all *.conf files to *.backup . (if you have more than one distro
available, try it on all!)
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92
9.10. solution: working with files
1. List the files in the /bin directory
ls /bin
2. Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd.
file /bin/cat /etc/passwd /usr/bin/passwd
3a. Download wolf.jpg and LinuxFun.pdf from http://linux-training.be (wget http://
linux-training.be/files/studentfiles/wolf.jpg and wget http://linux-training.be/files/books/
LinuxFun.pdf)
wget http://linux-training.be/files/studentfiles/wolf.jpg
wget http://linux-training.be/files/studentfiles/wolf.png
wget http://linux-training.be/files/books/LinuxFun.pdf
3b. Display the type of file of wolf.jpg and LinuxFun.pdf
file wolf.jpg LinuxFun.pdf
3c. Rename wolf.jpg to wolf.pdf (use mv).
mv wolf.jpg wolf.pdf
3d. Display the type of file of wolf.pdf and LinuxFun.pdf.
file wolf.pdf LinuxFun.pdf
4. Create a directory ~/touched and enter it.
mkdir ~/touched ; cd ~/touched
5. Create the files today.txt and yesterday.txt in touched.
touch today.txt yesterday.txt
6. Change the date on yesterday.txt to match yesterday's date.
touch -t 200810251405 yesterday.txt (substitute 20081025 with yesterday)
7. Copy yesterday.txt to copy.yesterday.txt
cp yesterday.txt copy.yesterday.txt
8. Rename copy.yesterday.txt to kim
mv copy.yesterday.txt kim
9. Create a directory called ~/testbackup and copy all files from ~/touched into it.
mkdir ~/testbackup ; cp -r ~/touched ~/testbackup/ 
10. Use one command to remove the directory ~/testbackup and all files into it.
rm -rf ~/testbackup 
11. Create a directory ~/etcbackup and copy all *.conf files from /etc into it. Did you include
all subdirectories of /etc ?
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93
cp -r /etc/*.conf ~/etcbackup
Only *.conf files that are directly in /etc/ are copied.
12. Use rename to rename all *.conf files to *.backup . (if you have more than one distro
available, try it on all!)
On RHEL: touch 1.conf 2.conf ; rename conf backup *.conf
On Debian: touch 1.conf 2.conf ; rename 's/conf/backup/' *.conf
94
Chapter 10. working with file contents
In this chapter we will look at the contents of text files with head, tail, cat, tac, more, less
and strings.
We will also get a glimpse of the possibilities of tools like cat on the command line.
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95
10.1. head
You can use head to display the first ten lines of a file.
paul@debian7~$ head /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
lp:x:7:7:lp:/var/spool/lpd:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
root@debian7~#
The head command can also display the first n lines of a file.
paul@debian7~$ head -4 /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
paul@debian7~$
And head can also display the first n bytes.
paul@debian7~$ head -c14 /etc/passwd
root:x:0:0:roopaul@debian7~$
10.2. tail
Similar to head, the tail command will display the last ten lines of a file.
paul@debian7~$ tail /etc/services
vboxd           20012/udp
binkp           24554/tcp                       # binkp fidonet protocol
asp             27374/tcp                       # Address Search Protocol
asp             27374/udp
csync2          30865/tcp                       # cluster synchronization tool
dircproxy       57000/tcp                       # Detachable IRC Proxy
tfido           60177/tcp                       # fidonet EMSI over telnet
fido            60179/tcp                       # fidonet EMSI over TCP
# Local services
paul@debian7~$
You can give tail the number of lines you want to see.
paul@debian7~$ tail -3 /etc/services
fido            60179/tcp                       # fidonet EMSI over TCP
# Local services
paul@debian7~$
The tail command has other useful options, some of which we will use during this course.
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10.3. cat
The cat command is one of the most universal tools, yet all it does is copy standard input to
standard output. In combination with the shell this can be very powerful and diverse. Some
examples will give a glimpse into the possibilities. The first example is simple, you can use
cat to display a file on the screen. If the file is longer than the screen, it will scroll to the end.
paul@debian8:~$ cat /etc/resolv.conf
domain linux-training.be
search linux-training.be
nameserver 192.168.1.42
10.3.1. concatenate
cat is short for concatenate. One of the basic uses of cat is to concatenate files into a bigger
(or complete) file.
paul@debian8:~$ echo one >part1
paul@debian8:~$ echo two >part2
paul@debian8:~$ echo three >part3
paul@debian8:~$ cat part1
one
paul@debian8:~$ cat part2
two
paul@debian8:~$ cat part3
three
paul@debian8:~$ cat part1 part2 part3
one
two
three
paul@debian8:~$ cat part1 part2 part3 >all
paul@debian8:~$ cat all
one
two
three
paul@debian8:~$
10.3.2. create files
You can use cat to create flat text files. Type the cat > winter.txt command as shown in the
screenshot below. Then type one or more lines, finishing each line with the enter key. After
the last line, type and hold the Control (Ctrl) key and press d.
paul@debian8:~$ cat > winter.txt
It is very cold today!
paul@debian8:~$ cat winter.txt
It is very cold today!
paul@debian8:~$
The Ctrl d key combination will send an EOF (End of File) to the running process ending
the cat command.
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10.3.3. custom end marker
You can choose an end marker for cat with << as is shown in this screenshot. This
construction is called a here directive and will end the cat command.
paul@debian8:~$ cat > hot.txt <<stop
> It is hot today!
> Yes it is summer.
> stop
paul@debian8:~$ cat hot.txt
It is hot today!
Yes it is summer.
paul@debian8:~$
10.3.4. copy files
In the third example you will see that cat can be used to copy files. We will explain in detail
what happens here in the bash shell chapter.
paul@debian8:~$ cat winter.txt
It is very cold today!
paul@debian8:~$ cat winter.txt > cold.txt
paul@debian8:~$ cat cold.txt 
It is very cold today!
paul@debian8:~$
10.4. tac
Just one example will show you the purpose of tac (cat backwards).
paul@debian8:~$ cat count
one
two
three
four
paul@debian8:~$ tac count 
four
three
two
one
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10.5. more and less
The more command is useful for displaying files that take up more than one screen. More
will allow you to see the contents of the file page by page. Use the space bar to see the next
page, or q to quit. Some people prefer the less command to more.
10.6. strings
With the strings command you can display readable ascii strings found in (binary) files.
This example locates the ls binary then displays readable strings in the binary file (output
is truncated).
paul@laika:~$ which ls
/bin/ls
paul@laika:~$ strings /bin/ls
/lib/ld-linux.so.2
librt.so.1
__gmon_start__
_Jv_RegisterClasses
clock_gettime
libacl.so.1
...
 
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10.7. practice: file contents
1. Display the first 12 lines of /etc/services.
2. Display the last line of /etc/passwd.
3. Use cat to create a file named count.txt that looks like this:
One
Two
Three
Four
Five
4. Use cp to make a backup of this file to cnt.txt.
5. Use cat to make a backup of this file to catcnt.txt.
6. Display catcnt.txt, but with all lines in reverse order (the last line first).
7. Use more to display /etc/services.
8. Display the readable character strings from the /usr/bin/passwd command.
9. Use ls to find the biggest file in /etc.
10. Open two terminal windows (or tabs) and make sure you are in the same directory in
both. Type echo this is the first line > tailing.txt in the first terminal, then issue tail -f
tailing.txt in the second terminal. Now go back to the first terminal and type echo This is
another line >> tailing.txt (note the double >>), verify that the tail -f in the second terminal
shows both lines. Stop the tail -f with Ctrl-C.
11. Use cat to create a file named tailing.txt that contains the contents of tailing.txt followed
by the contents of /etc/passwd.
12. Use cat to create a file named tailing.txt that contains the contents of tailing.txt preceded
by the contents of /etc/passwd.
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10.8. solution: file contents
1. Display the first 12 lines of /etc/services.
head -12 /etc/services
2. Display the last line of /etc/passwd.
tail -1 /etc/passwd
3. Use cat to create a file named count.txt that looks like this:
cat > count.txt
One
Two
Three
Four
Five (followed by Ctrl-d)
4. Use cp to make a backup of this file to cnt.txt.
cp count.txt cnt.txt
5. Use cat to make a backup of this file to catcnt.txt.
cat count.txt > catcnt.txt
6. Display catcnt.txt, but with all lines in reverse order (the last line first).
tac catcnt.txt
7. Use more to display /etc/services.
more /etc/services
8. Display the readable character strings from the /usr/bin/passwd command.
strings /usr/bin/passwd
9. Use ls to find the biggest file in /etc.
ls -lrS /etc
10. Open two terminal windows (or tabs) and make sure you are in the same directory in
both. Type echo this is the first line > tailing.txt in the first terminal, then issue tail -f
tailing.txt in the second terminal. Now go back to the first terminal and type echo This is
another line >> tailing.txt (note the double >>), verify that the tail -f in the second terminal
shows both lines. Stop the tail -f with Ctrl-C.
11. Use cat to create a file named tailing.txt that contains the contents of tailing.txt followed
by the contents of /etc/passwd.
cat /etc/passwd >> tailing.txt
12. Use cat to create a file named tailing.txt that contains the contents of tailing.txt preceded
by the contents of /etc/passwd.
mv tailing.txt tmp.txt ; cat /etc/passwd tmp.txt > tailing.txt
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Chapter 11. the Linux file tree
This chapter takes a look at the most common directories in the Linux file tree. It also shows
that on Unix everything is a file.
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11.1. filesystem hierarchy standard
Many Linux distributions partially follow the Filesystem Hierarchy Standard. The FHS
may help make more Unix/Linux file system trees conform better in the future. The FHS
is available online at http://www.pathname.com/fhs/ where we read: "The filesystem
hierarchy standard has been designed to be used by Unix distribution developers, package
developers, and system implementers. However, it is primarily intended to be a reference
and is not a tutorial on how to manage a Unix filesystem or directory hierarchy."
11.2. man hier
There are some differences in the filesystems between Linux distributions. For help about
your machine, enter man hier to find information about the file system hierarchy. This
manual will explain the directory structure on your computer.
11.3. the root directory /
All Linux systems have a directory structure that starts at the root directory. The root
directory is represented by a forward slash, like this: /. Everything that exists on your Linux
system can be found below this root directory. Let's take a brief look at the contents of the
root directory.
[paul@RHELv4u3 ~]$ ls /
bin   dev  home  media  mnt  proc  sbin     srv  tftpboot  usr
boot  etc  lib   misc   opt  root  selinux  sys  tmp       var
 
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11.4. binary directories
Binaries are files that contain compiled source code (or machine code). Binaries can be
executed on the computer. Sometimes binaries are called executables.
11.4.1. /bin
The /bin directory contains binaries for use by all users. According to the FHS the /bin
directory should contain /bin/cat and /bin/date (among others).
In the screenshot below you see common Unix/Linux commands like cat, cp, cpio, date, dd,
echo, grep, and so on. Many of these will be covered in this book.
paul@laika:~$ ls /bin
archdetect       egrep             mt               setupcon
autopartition    false             mt-gnu           sh
bash             fgconsole         mv               sh.distrib
bunzip2          fgrep             nano             sleep
bzcat            fuser             nc               stralign
bzcmp            fusermount        nc.traditional   stty
bzdiff           get_mountoptions  netcat           su
bzegrep          grep              netstat          sync
bzexe            gunzip            ntfs-3g          sysfs
bzfgrep          gzexe             ntfs-3g.probe    tailf
bzgrep           gzip              parted_devices   tar
bzip2            hostname          parted_server    tempfile
bzip2recover     hw-detect         partman          touch
bzless           ip                partman-commit   true
bzmore           kbd_mode          perform_recipe   ulockmgr
cat              kill              pidof            umount
...
11.4.2. other /bin directories
You can find a /bin subdirectory in many other directories. A user named serena could put
her own programs in /home/serena/bin.
Some applications, often when installed directly from source will put themselves in /opt. A
samba server installation can use /opt/samba/bin to store its binaries.
11.4.3. /sbin
/sbin contains binaries to configure the operating system. Many of the system binaries
require root privilege to perform certain tasks.
Below a screenshot containing system binaries to change the ip address, partition a disk
and create an ext4 file system.
paul@ubu1010:~$ ls -l /sbin/ifconfig /sbin/fdisk /sbin/mkfs.ext4
-rwxr-xr-x 1 root root 97172 2011-02-02 09:56 /sbin/fdisk
-rwxr-xr-x 1 root root 65708 2010-07-02 09:27 /sbin/ifconfig
-rwxr-xr-x 5 root root 55140 2010-08-18 18:01 /sbin/mkfs.ext4
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11.4.4. /lib
Binaries found in /bin and /sbin often use shared libraries located in /lib. Below is a
screenshot of the partial contents of /lib.
paul@laika:~$ ls /lib/libc*
/lib/libc-2.5.so     /lib/libcfont.so.0.0.0  /lib/libcom_err.so.2.1    
/lib/libcap.so.1     /lib/libcidn-2.5.so     /lib/libconsole.so.0      
/lib/libcap.so.1.10  /lib/libcidn.so.1       /lib/libconsole.so.0.0.0  
/lib/libcfont.so.0   /lib/libcom_err.so.2    /lib/libcrypt-2.5.so
/lib/modules
Typically, the Linux kernel loads kernel modules from /lib/modules/$kernel-version/.
This directory is discussed in detail in the Linux kernel chapter.
/lib32 and /lib64
We currently are in a transition between 32-bit and 64-bit systems. Therefore, you may
encounter directories named /lib32 and /lib64 which clarify the register size used during
compilation time of the libraries. A 64-bit computer may have some 32-bit binaries and
libraries for compatibility with legacy applications. This screenshot uses the file utility to
demonstrate the difference.
paul@laika:~$ file /lib32/libc-2.5.so 
/lib32/libc-2.5.so: ELF 32-bit LSB shared object, Intel 80386, \
version 1 (SYSV), for GNU/Linux 2.6.0, stripped
paul@laika:~$ file /lib64/libcap.so.1.10 
/lib64/libcap.so.1.10: ELF 64-bit LSB shared object, AMD x86-64, \
version 1 (SYSV), stripped
   
The ELF (Executable and Linkable Format) is used in almost every Unix-like operating
system since System V.
11.4.5. /opt
The purpose of /opt is to store optional software. In many cases this is software from outside
the distribution repository. You may find an empty /opt directory on many systems.
A large package can install all its files in /bin, /lib, /etc subdirectories within /opt/
$packagename/. If for example the package is called wp, then it installs in /opt/wp, putting
binaries in /opt/wp/bin and manpages in /opt/wp/man.
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11.5. configuration directories
11.5.1. /boot
The /boot directory contains all files needed to boot the computer. These files don't change
very often. On Linux systems you typically find the /boot/grub directory here. /boot/grub
contains /boot/grub/grub.cfg (older systems may still have /boot/grub/grub.conf) which
defines the boot menu that is displayed before the kernel starts.
11.5.2. /etc
All of the machine-specific configuration files should be located in /etc. Historically /etc
stood for etcetera, today people often use the Editable Text Configuration backronym.
Many times the name of a configuration files is the same as the application, daemon, or
protocol with .conf added as the extension.
paul@laika:~$ ls /etc/*.conf
/etc/adduser.conf        /etc/ld.so.conf       /etc/scrollkeeper.conf
/etc/brltty.conf         /etc/lftp.conf        /etc/sysctl.conf
/etc/ccertificates.conf  /etc/libao.conf       /etc/syslog.conf
/etc/cvs-cron.conf       /etc/logrotate.conf   /etc/ucf.conf
/etc/ddclient.conf       /etc/ltrace.conf      /etc/uniconf.conf
/etc/debconf.conf        /etc/mke2fs.conf      /etc/updatedb.conf
/etc/deluser.conf        /etc/netscsid.conf    /etc/usplash.conf
/etc/fdmount.conf        /etc/nsswitch.conf    /etc/uswsusp.conf
/etc/hdparm.conf         /etc/pam.conf         /etc/vnc.conf
/etc/host.conf           /etc/pnm2ppa.conf     /etc/wodim.conf
/etc/inetd.conf          /etc/povray.conf      /etc/wvdial.conf
/etc/kernel-img.conf     /etc/resolv.conf
paul@laika:~$ 
There is much more to be found in /etc.
/etc/init.d/
A lot of Unix/Linux distributions have an /etc/init.d directory that contains scripts to start
and stop daemons. This directory could disappear as Linux migrates to systems that replace
the old init way of starting all daemons.
/etc/X11/
The graphical display (aka X Window System or just X) is driven by software from the
X.org foundation. The configuration file for your graphical display is /etc/X11/xorg.conf.
/etc/skel/
The skeleton directory /etc/skel is copied to the home directory of a newly created user. It
usually contains hidden files like a .bashrc script.
/etc/sysconfig/
This directory, which is not mentioned in the FHS, contains a lot of Red Hat Enterprise
Linux configuration files. We will discuss some of them in greater detail. The screenshot
below is the /etc/sysconfig directory from RHELv4u4 with everything installed.
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paul@RHELv4u4:~$ ls /etc/sysconfig/
apmd         firstboot     irda              network      saslauthd
apm-scripts  grub          irqbalance        networking   selinux
authconfig   hidd          keyboard          ntpd         spamassassin
autofs       httpd         kudzu             openib.conf  squid
bluetooth    hwconf        lm_sensors        pand         syslog
clock        i18n          mouse             pcmcia       sys-config-sec
console      init          mouse.B           pgsql        sys-config-users
crond        installinfo   named             prelink      sys-logviewer
desktop      ipmi          netdump           rawdevices   tux
diskdump     iptables      netdump_id_dsa    rhn          vncservers
dund         iptables-cfg  netdump_id_dsa.p  samba        xinetd
paul@RHELv4u4:~$
The file /etc/sysconfig/firstboot tells the Red Hat Setup Agent not to run at boot time. If
you want to run the Red Hat Setup Agent at the next reboot, then simply remove this file,
and run chkconfig --level 5 firstboot on. The Red Hat Setup Agent allows you to install
the latest updates, create a user account, join the Red Hat Network and more. It will then
create the /etc/sysconfig/firstboot file again.
paul@RHELv4u4:~$ cat /etc/sysconfig/firstboot 
RUN_FIRSTBOOT=NO
The /etc/sysconfig/harddisks file contains some parameters to tune the hard disks. The file
explains itself.
You can see hardware detected by kudzu in /etc/sysconfig/hwconf. Kudzu is software from
Red Hat for automatic discovery and configuration of hardware.
The keyboard type and keymap table are set in the /etc/sysconfig/keyboard file. For more
console keyboard information, check the manual pages of keymaps(5), dumpkeys(1),
loadkeys(1) and the directory /lib/kbd/keymaps/.
root@RHELv4u4:/etc/sysconfig# cat keyboard 
KEYBOARDTYPE="pc"
KEYTABLE="us"
We will discuss networking files in this directory in the networking chapter.
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11.6. data directories
11.6.1. /home
Users can store personal or project data under /home. It is common (but not mandatory by
the fhs) practice to name the users home directory after the user name in the format /home/
$USERNAME. For example:
paul@ubu606:~$ ls /home 
geert  annik  sandra  paul  tom
Besides giving every user (or every project or group) a location to store personal files, the
home directory of a user also serves as a location to store the user profile. A typical Unix
user profile contains many hidden files (files whose file name starts with a dot). The hidden
files of the Unix user profiles contain settings specific for that user.
paul@ubu606:~$ ls -d /home/paul/.*
/home/paul/.              /home/paul/.bash_profile  /home/paul/.ssh
/home/paul/..             /home/paul/.bashrc        /home/paul/.viminfo
/home/paul/.bash_history  /home/paul/.lesshst
  
11.6.2. /root
On many systems /root is the default location for personal data and profile of the root user.
If it does not exist by default, then some administrators create it.
11.6.3. /srv
You may use /srv for data that is served by your system. The FHS allows locating cvs,
rsync, ftp and www data in this location. The FHS also approves administrative naming in /
srv, like /srv/project55/ftp and /srv/sales/www.
On Sun Solaris (or Oracle Solaris) /export is used for this purpose.
11.6.4. /media
The /media directory serves as a mount point for removable media devices such as CD-
ROM's, digital cameras, and various usb-attached devices. Since /media is rather new in the
Unix world, you could very well encounter systems running without this directory. Solaris
9 does not have it, Solaris 10 does. Most Linux distributions today mount all removable
media in /media.
paul@debian5:~$ ls /media/
cdrom  cdrom0  usbdisk
11.6.5. /mnt
The /mnt directory should be empty and should only be used for temporary mount points
(according to the FHS).
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Unix and Linux administrators used to create many directories here, like /mnt/something/.
You likely will encounter many systems with more than one directory created and/or
mounted inside /mnt to be used for various local and remote filesystems.
11.6.6. /tmp
Applications and users should use /tmp to store temporary data when needed. Data stored
in /tmp may use either disk space or RAM. Both of which are managed by the operating
system. Never use /tmp to store data that is important or which you wish to archive.
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11.7. in memory directories
11.7.1. /dev
Device files in /dev appear to be ordinary files, but are not actually located on the hard disk.
The /dev directory is populated with files as the kernel is recognising hardware.
common physical devices
Common hardware such as hard disk devices are represented by device files in /dev. Below
a screenshot of SATA device files on a laptop and then IDE attached drives on a desktop.
(The detailed meaning of these devices will be discussed later.)
#
# SATA or SCSI or USB
#
paul@laika:~$ ls /dev/sd*
/dev/sda  /dev/sda1  /dev/sda2  /dev/sda3  /dev/sdb  /dev/sdb1  /dev/sdb2
#
# IDE or ATAPI
#
paul@barry:~$ ls /dev/hd*
/dev/hda  /dev/hda1  /dev/hda2  /dev/hdb  /dev/hdb1  /dev/hdb2  /dev/hdc
   
Besides representing physical hardware, some device files are special. These special devices
can be very useful.
/dev/tty and /dev/pts
For example, /dev/tty1 represents a terminal or console attached to the system. (Don't
break your head on the exact terminology of 'terminal' or 'console', what we mean here is
a command line interface.) When typing commands in a terminal that is part of a graphical
interface like Gnome or KDE, then your terminal will be represented as /dev/pts/1 (1 can
be another number).
/dev/null
On Linux you will find other special devices such as /dev/null which can be considered
a black hole; it has unlimited storage, but nothing can be retrieved from it. Technically
speaking, anything written to /dev/null will be discarded. /dev/null can be useful to discard
unwanted output from commands. /dev/null is not a good location to store your backups ;-).
11.7.2. /proc conversation with the kernel
/proc is another special directory, appearing to be ordinary files, but not taking up disk
space. It is actually a view of the kernel, or better, what the kernel manages, and is a means
to interact with it directly. /proc is a proc filesystem.
paul@RHELv4u4:~$ mount -t proc
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none on /proc type proc (rw)
  
When listing the /proc directory you will see many numbers (on any Unix) and some
interesting files (on Linux)
mul@laika:~$ ls /proc
1      2339   4724  5418  6587  7201       cmdline      mounts
10175  2523   4729  5421  6596  7204       cpuinfo      mtrr
10211  2783   4741  5658  6599  7206       crypto       net
10239  2975   4873  5661  6638  7214       devices      pagetypeinfo
141    29775  4874  5665  6652  7216       diskstats    partitions
15045  29792  4878  5927  6719  7218       dma          sched_debug
1519   2997   4879  6     6736  7223       driver       scsi
1548   3      4881  6032  6737  7224       execdomains  self
1551   30228  4882  6033  6755  7227       fb           slabinfo
1554   3069   5     6145  6762  7260       filesystems  stat
1557   31422  5073  6298  6774  7267       fs           swaps
1606   3149   5147  6414  6816  7275       ide          sys
180    31507  5203  6418  6991  7282       interrupts   sysrq-trigger
181    3189   5206  6419  6993  7298       iomem        sysvipc
182    3193   5228  6420  6996  7319       ioports      timer_list
18898  3246   5272  6421  7157  7330       irq          timer_stats
19799  3248   5291  6422  7163  7345       kallsyms     tty
19803  3253   5294  6423  7164  7513       kcore        uptime
19804  3372   5356  6424  7171  7525       key-users    version
1987   4      5370  6425  7175  7529       kmsg         version_signature
1989   42     5379  6426  7188  9964       loadavg      vmcore
2      45     5380  6430  7189  acpi       locks        vmnet
20845  4542   5412  6450  7191  asound     meminfo      vmstat
221    46     5414  6551  7192  buddyinfo  misc         zoneinfo
2338   4704   5416  6568  7199  bus        modules
  
Let's investigate the file properties inside /proc. Looking at the date and time will display
the current date and time showing the files are constantly updated (a view on the kernel).
paul@RHELv4u4:~$ date
Mon Jan 29 18:06:32 EST 2007
paul@RHELv4u4:~$ ls -al /proc/cpuinfo 
-r--r--r--  1 root root 0 Jan 29 18:06 /proc/cpuinfo
paul@RHELv4u4:~$ 
paul@RHELv4u4:~$  ...time passes...
paul@RHELv4u4:~$ 
paul@RHELv4u4:~$ date
Mon Jan 29 18:10:00 EST 2007
paul@RHELv4u4:~$ ls -al /proc/cpuinfo 
-r--r--r--  1 root root 0 Jan 29 18:10 /proc/cpuinfo
  
Most files in /proc are 0 bytes, yet they contain data--sometimes a lot of data. You can see
this by executing cat on files like /proc/cpuinfo, which contains information about the CPU.
paul@RHELv4u4:~$ file /proc/cpuinfo 
/proc/cpuinfo: empty
paul@RHELv4u4:~$ cat /proc/cpuinfo 
processor       : 0
vendor_id       : AuthenticAMD
cpu family      : 15
model           : 43
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model name      : AMD Athlon(tm) 64 X2 Dual Core Processor 4600+
stepping        : 1
cpu MHz         : 2398.628
cache size      : 512 KB
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 1
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge...
bogomips        : 4803.54
  
Just for fun, here is /proc/cpuinfo on a Sun Sunblade 1000...
paul@pasha:~$ cat /proc/cpuinfo
cpu : TI UltraSparc III (Cheetah)
fpu : UltraSparc III integrated FPU
promlib : Version 3 Revision 2
prom : 4.2.2
type : sun4u
ncpus probed : 2
ncpus active : 2
Cpu0Bogo : 498.68
Cpu0ClkTck : 000000002cb41780
Cpu1Bogo : 498.68
Cpu1ClkTck : 000000002cb41780
MMU Type : Cheetah
State:
CPU0: online
CPU1: online 
  
Most of the files in /proc are read only, some require root privileges, some files are writable,
and many files in /proc/sys are writable. Let's discuss some of the files in /proc.
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/proc/interrupts
On the x86 architecture, /proc/interrupts displays the interrupts.
paul@RHELv4u4:~$ cat /proc/interrupts 
           CPU0       
  0:   13876877    IO-APIC-edge  timer
  1:         15    IO-APIC-edge  i8042
  8:          1    IO-APIC-edge  rtc
  9:          0   IO-APIC-level  acpi
 12:         67    IO-APIC-edge  i8042
 14:        128    IO-APIC-edge  ide0
 15:     124320    IO-APIC-edge  ide1
169:     111993   IO-APIC-level  ioc0
177:       2428   IO-APIC-level  eth0
NMI:          0 
LOC:   13878037 
ERR:          0
MIS:          0
On a machine with two CPU's, the file looks like this.
paul@laika:~$ cat /proc/interrupts 
          CPU0      CPU1       
  0:    860013        0  IO-APIC-edge     timer
  1:      4533        0  IO-APIC-edge     i8042
  7:         0        0  IO-APIC-edge     parport0
  8:   6588227        0  IO-APIC-edge     rtc
 10:      2314        0  IO-APIC-fasteoi  acpi
 12:       133        0  IO-APIC-edge     i8042
 14:         0        0  IO-APIC-edge     libata
 15:     72269        0  IO-APIC-edge     libata
 18:         1        0  IO-APIC-fasteoi  yenta
 19:    115036        0  IO-APIC-fasteoi  eth0
 20:    126871        0  IO-APIC-fasteoi  libata, ohci1394
 21:     30204        0  IO-APIC-fasteoi  ehci_hcd:usb1, uhci_hcd:usb2
 22:      1334        0  IO-APIC-fasteoi  saa7133[0], saa7133[0]
 24:    234739        0  IO-APIC-fasteoi  nvidia
NMI:        72       42 
LOC:    860000   859994 
ERR:         0
/proc/kcore
The physical memory is represented in /proc/kcore. Do not try to cat this file, instead use a
debugger. The size of /proc/kcore is the same as your physical memory, plus four bytes.
paul@laika:~$ ls -lh /proc/kcore 
-r-------- 1 root root 2.0G 2007-01-30 08:57 /proc/kcore
paul@laika:~$
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11.7.3. /sys Linux 2.6 hot plugging
The /sys directory was created for the Linux 2.6 kernel. Since 2.6, Linux uses sysfs
to support usb and IEEE 1394 (FireWire) hot plug devices. See the manual pages
of udev(8) (the successor of devfs) and hotplug(8) for more info (or visit http://linux-
hotplug.sourceforge.net/ ).
Basically the /sys directory contains kernel information about hardware.
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11.8. /usr Unix System Resources
Although /usr is pronounced like user, remember that it stands for Unix System Resources.
The /usr hierarchy should contain shareable, read only data. Some people choose to mount
/usr as read only. This can be done from its own partition or from a read only NFS share
(NFS is discussed later).
11.8.1. /usr/bin
The /usr/bin directory contains a lot of commands.
paul@deb508:~$ ls /usr/bin | wc -l
1395
(On Solaris the /bin directory is a symbolic link to /usr/bin.)
11.8.2. /usr/include
The /usr/include directory contains general use include files for C.
paul@ubu1010:~$ ls /usr/include/
aalib.h        expat_config.h      math.h           search.h
af_vfs.h       expat_external.h    mcheck.h         semaphore.h
aio.h          expat.h             memory.h         setjmp.h
AL             fcntl.h             menu.h           sgtty.h
aliases.h      features.h          mntent.h         shadow.h
...
11.8.3. /usr/lib
The /usr/lib directory contains libraries that are not directly executed by users or scripts.
paul@deb508:~$ ls /usr/lib | head -7
4Suite
ao
apt
arj
aspell
avahi
bonobo
11.8.4. /usr/local
The /usr/local directory can be used by an administrator to install software locally.
paul@deb508:~$ ls /usr/local/
bin  etc  games  include  lib  man  sbin  share  src
paul@deb508:~$ du -sh /usr/local/
128K /usr/local/
11.8.5. /usr/share
The /usr/share directory contains architecture independent data. As you can see, this is a
fairly large directory.
paul@deb508:~$ ls /usr/share/ | wc -l
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263
paul@deb508:~$ du -sh /usr/share/
1.3G /usr/share/
This directory typically contains /usr/share/man for manual pages.
paul@deb508:~$ ls /usr/share/man
cs  fr    hu  it.UTF-8  man2 man6  pl.ISO8859-2  sv
de  fr.ISO8859-1  id  ja   man3 man7  pl.UTF-8     tr
es  fr.UTF-8   it  ko   man4 man8  pt_BR     zh_CN
fi  gl    it.ISO8859-1 man1   man5 pl    ru     zh_TW
And it contains /usr/share/games for all static game data (so no high-scores or play logs).
paul@ubu1010:~$ ls /usr/share/games/
openttd  wesnoth
11.8.6. /usr/src
The /usr/src directory is the recommended location for kernel source files.
paul@deb508:~$ ls -l /usr/src/
total 12
drwxr-xr-x  4 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-686
drwxr-xr-x 18 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-common
drwxr-xr-x  3 root root 4096 2009-10-28 16:01 linux-kbuild-2.6.26
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11.9. /var variable data
Files that are unpredictable in size, such as log, cache and spool files, should be located in
/var.
11.9.1. /var/log
The /var/log directory serves as a central point to contain all log files.
[paul@RHEL4b ~]$ ls /var/log
acpid           cron.2    maillog.2   quagga           secure.4
amanda          cron.3    maillog.3   radius           spooler
anaconda.log    cron.4    maillog.4   rpmpkgs          spooler.1
anaconda.syslog cups      mailman     rpmpkgs.1        spooler.2
anaconda.xlog   dmesg     messages    rpmpkgs.2        spooler.3
audit           exim      messages.1  rpmpkgs.3        spooler.4
boot.log        gdm       messages.2  rpmpkgs.4        squid
boot.log.1      httpd     messages.3  sa               uucp
boot.log.2      iiim      messages.4  samba            vbox
boot.log.3      iptraf    mysqld.log  scrollkeeper.log vmware-tools-guestd
boot.log.4      lastlog   news        secure           wtmp
canna           mail      pgsql       secure.1         wtmp.1
cron            maillog   ppp         secure.2         Xorg.0.log
cron.1          maillog.1 prelink.log secure.3         Xorg.0.log.old
11.9.2. /var/log/messages
A typical first file to check when troubleshooting on Red Hat (and derivatives) is the /var/
log/messages file. By default this file will contain information on what just happened to the
system. The file is called /var/log/syslog on Debian and Ubuntu.
[root@RHEL4b ~]# tail /var/log/messages
Jul 30 05:13:56 anacron: anacron startup succeeded
Jul 30 05:13:56 atd: atd startup succeeded
Jul 30 05:13:57 messagebus: messagebus startup succeeded
Jul 30 05:13:57 cups-config-daemon: cups-config-daemon startup succeeded
Jul 30 05:13:58 haldaemon: haldaemon startup succeeded
Jul 30 05:14:00 fstab-sync[3560]: removed all generated mount points
Jul 30 05:14:01 fstab-sync[3628]: added mount point /media/cdrom for...
Jul 30 05:14:01 fstab-sync[3646]: added mount point /media/floppy for...
Jul 30 05:16:46 sshd(pam_unix)[3662]: session opened for user paul by... 
Jul 30 06:06:37 su(pam_unix)[3904]: session opened for user root by paul
11.9.3. /var/cache
The /var/cache directory can contain cache data for several applications.
paul@ubu1010:~$ ls /var/cache/
apt      dictionaries-common    gdm       man        software-center
binfmts  flashplugin-installer  hald      pm-utils
cups     fontconfig             jockey    pppconfig
debconf  fonts                  ldconfig  samba
11.9.4. /var/spool
The /var/spool directory typically contains spool directories for mail and cron, but also
serves as a parent directory for other spool files (for example print spool files).
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11.9.5. /var/lib
The /var/lib directory contains application state information.
Red Hat Enterprise Linux for example keeps files pertaining to rpm in /var/lib/rpm/.
11.9.6. /var/...
/var also contains Process ID files in /var/run (soon to be replaced with /run) and temporary
files that survive a reboot in /var/tmp and information about file locks in /var/lock. There
will be more examples of /var usage further in this book.
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11.10. practice: file system tree
1. Does the file /bin/cat exist ? What about /bin/dd and /bin/echo. What is the type of these
files ?
2. What is the size of the Linux kernel file(s) (vmlinu*) in /boot ?
3. Create a directory ~/test. Then issue the following commands:
cd ~/test
dd if=/dev/zero of=zeroes.txt count=1 bs=100
od zeroes.txt
dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/zero
to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ?
4. Now issue the following command:
dd if=/dev/random of=random.txt count=1 bs=100 ; od random.txt
dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/
random to ~/test/random.txt. Can you describe the functionality of /dev/random ?
5. Issue the following two commands, and look at the first character of each output line.
ls -l /dev/sd* /dev/hd*
ls -l /dev/tty* /dev/input/mou*
The first ls will show block(b) devices, the second ls shows character(c) devices. Can you
tell the difference between block and character devices ?
6. Use cat to display /etc/hosts and /etc/resolv.conf. What is your idea about the purpose
of these files ?
7. Are there any files in /etc/skel/ ? Check also for hidden files.
8. Display /proc/cpuinfo. On what architecture is your Linux running ?
9. Display /proc/interrupts. What is the size of this file ? Where is this file stored ?
10. Can you enter the /root directory ? Are there (hidden) files ?
11. Are ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are these
binaries in /sbin and not in /bin ?
12. Is /var/log a file or a directory ? What about /var/spool ?
13. Open two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (Ctrl-Alt-F1,
Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from one terminal
to the other.
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119
14. Read the man page of random and explain the difference between /dev/random and /
dev/urandom.
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11.11. solution: file system tree
1. Does the file /bin/cat exist ? What about /bin/dd and /bin/echo. What is the type of these
files ?
ls /bin/cat ; file /bin/cat
ls /bin/dd ; file /bin/dd
ls /bin/echo ; file /bin/echo
2. What is the size of the Linux kernel file(s) (vmlinu*) in /boot ?
ls -lh /boot/vm*
3. Create a directory ~/test. Then issue the following commands:
cd ~/test
dd if=/dev/zero of=zeroes.txt count=1 bs=100
od zeroes.txt
dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/zero
to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ?
/dev/zero is a Linux special device. It can be considered a source of zeroes. You cannot send
something to /dev/zero, but you can read zeroes from it.
4. Now issue the following command:
dd if=/dev/random of=random.txt count=1 bs=100 ; od random.txt
dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/
random to ~/test/random.txt. Can you describe the functionality of /dev/random ?
/dev/random acts as a random number generator on your Linux machine.
5. Issue the following two commands, and look at the first character of each output line.
ls -l /dev/sd* /dev/hd*
ls -l /dev/tty* /dev/input/mou*
The first ls will show block(b) devices, the second ls shows character(c) devices. Can you
tell the difference between block and character devices ?
Block devices are always written to (or read from) in blocks. For hard disks, blocks of 512
bytes are common. Character devices act as a stream of characters (or bytes). Mouse and
keyboard are typical character devices.
6. Use cat to display /etc/hosts and /etc/resolv.conf. What is your idea about the purpose
of these files ?
/etc/hosts contains hostnames with their ip address
/etc/resolv.conf should contain the ip address of a DNS name server.
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121
7. Are there any files in /etc/skel/ ? Check also for hidden files.
Issue "ls -al /etc/skel/". Yes, there should be hidden files there.
8. Display /proc/cpuinfo. On what architecture is your Linux running ?
The file should contain at least one line with Intel or other cpu.
9. Display /proc/interrupts. What is the size of this file ? Where is this file stored ?
The size is zero, yet the file contains data. It is not stored anywhere because /proc is a
virtual file system that allows you to talk with the kernel. (If you answered "stored in RAM-
memory, that is also correct...).
10. Can you enter the /root directory ? Are there (hidden) files ?
Try "cd /root". The /root directory is not accessible for normal users on most modern Linux systems.
11. Are ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are these
binaries in /sbin and not in /bin ?
Because those files are only meant for system administrators.
12. Is /var/log a file or a directory ? What about /var/spool ?
Both are directories.
13. Open two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (Ctrl-Alt-F1,
Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from one terminal
to the other.
tty-terminal: echo Hello > /dev/tty1
pts-terminal: echo Hello > /dev/pts/1
14. Read the man page of random and explain the difference between /dev/random and /
dev/urandom.
man 4 random
Part IV. shell expansion
123
Table of Contents
12. commands and arguments ..............................................................................................  125
12.1. arguments ............................................................................................................  126
12.2. white space removal ..............................................................................................  126
12.3. single quotes ........................................................................................................  127
12.4. double quotes .......................................................................................................  127
12.5. echo and quotes ....................................................................................................  127
12.6. commands ...........................................................................................................  128
12.7. aliases .................................................................................................................  129
12.8. displaying shell expansion ......................................................................................  130
12.9. practice: commands and arguments ..........................................................................  131
12.10. solution: commands and arguments ........................................................................  133
13. control operators ...........................................................................................................  135
13.1. ; semicolon ..........................................................................................................  136
13.2. & ampersand ........................................................................................................ 136
13.3. $? dollar question mark .........................................................................................  136
13.4. && double ampersand ...........................................................................................  137
13.5. || double vertical bar ..............................................................................................  137
13.6. combining && and || .............................................................................................  137
13.7. # pound sign ........................................................................................................  138
13.8. \ escaping special characters ...................................................................................  138
13.9. practice: control operators ......................................................................................  139
13.10. solution: control operators ..................................................................................... 140
14. shell variables ................................................................................................................  141
14.1. $ dollar sign ........................................................................................................  142
14.2. case sensitive .......................................................................................................  142
14.3. creating variables ..................................................................................................  142
14.4. quotes .................................................................................................................  143
14.5. set ......................................................................................................................  143
14.6. unset ...................................................................................................................  143
14.7. $PS1 ...................................................................................................................  144
14.8. $PATH ...............................................................................................................  145
14.9. env .....................................................................................................................  146
14.10. export ...............................................................................................................  146
14.11. delineate variables ...............................................................................................  147
14.12. unbound variables ...............................................................................................  147
14.13. practice: shell variables ........................................................................................  148
14.14. solution: shell variables ........................................................................................  149
15. shell embedding and options ...........................................................................................  150
15.1. shell embedding .................................................................................................... 151
15.2. shell options ........................................................................................................  152
15.3. practice: shell embedding .......................................................................................  153
15.4. solution: shell embedding .......................................................................................  154
16. shell history ................................................................................................................... 155
16.1. repeating the last command ....................................................................................  156
16.2. repeating other commands ......................................................................................  156
16.3. history ................................................................................................................  156
16.4. !n .......................................................................................................................  156
16.5. Ctrl-r ..................................................................................................................  157
16.6. $HISTSIZE ..........................................................................................................  157
16.7. $HISTFILE ..........................................................................................................  157
16.8. $HISTFILESIZE ...................................................................................................  157
16.9. prevent recording a command .................................................................................  158
16.10. (optional)regular expressions .................................................................................  158
16.11. (optional) Korn shell history .................................................................................  158
16.12. practice: shell history ...........................................................................................  159
shell expansion
124
16.13. solution: shell history ...........................................................................................  160
17. file globbing ..................................................................................................................  161
17.1. * asterisk .............................................................................................................  162
17.2. ? question mark ....................................................................................................  162
17.3. [] square brackets .................................................................................................. 163
17.4. a-z and 0-9 ranges ................................................................................................  164
17.5. $LANG and square brackets ...................................................................................  164
17.6. preventing file globbing .........................................................................................  165
17.7. practice: shell globbing ..........................................................................................  166
17.8. solution: shell globbing ..........................................................................................  167
125
Chapter 12. commands and
arguments
This chapter introduces you to shell expansion by taking a close look at commands and
arguments. Knowing shell expansion is important because many commands on your
Linux system are processed and most likely changed by the shell before they are executed.
The command line interface or shell used on most Linux systems is called bash, which
stands for Bourne again shell. The bash shell incorporates features from sh (the original
Bourne shell), csh (the C shell), and ksh (the Korn shell).
This chapter frequently uses the echo command to demonstrate shell features. The echo
command is very simple: it echoes the input that it receives.
paul@laika:~$ echo Burtonville
Burtonville
paul@laika:~$ echo Smurfs are blue
Smurfs are blue
commands and arguments
126
12.1. arguments
One of the primary features of a shell is to perform a command line scan. When you enter
a command at the shell's command prompt and press the enter key, then the shell will start
scanning that line, cutting it up in arguments. While scanning the line, the shell may make
many changes to the arguments you typed.
This process is called shell expansion. When the shell has finished scanning and modifying
that line, then it will be executed.
12.2. white space removal
Parts that are separated by one or more consecutive white spaces (or tabs) are considered
separate arguments, any white space is removed. The first argument is the command to be
executed, the other arguments are given to the command. The shell effectively cuts your
command into one or more arguments.
This explains why the following four different command lines are the same after shell
expansion.
[paul@RHELv4u3 ~]$ echo Hello World
Hello World
[paul@RHELv4u3 ~]$ echo Hello   World
Hello World
[paul@RHELv4u3 ~]$ echo   Hello   World
Hello World
[paul@RHELv4u3 ~]$    echo      Hello      World
Hello World
The echo command will display each argument it receives from the shell. The echo
command will also add a new white space between the arguments it received.
commands and arguments
127
12.3. single quotes
You can prevent the removal of white spaces by quoting the spaces. The contents of the
quoted string are considered as one argument. In the screenshot below the echo receives
only one argument.
[paul@RHEL4b ~]$ echo 'A line with      single    quotes'
A line with      single    quotes
[paul@RHEL4b ~]$
12.4. double quotes
You can also prevent the removal of white spaces by double quoting  the spaces. Same as
above, echo only receives one argument.
[paul@RHEL4b ~]$ echo "A line with      double    quotes"
A line with      double    quotes
[paul@RHEL4b ~]$
Later in this book, when discussing variables we will see important differences between
single and double quotes.
12.5. echo and quotes
Quoted lines can include special escaped characters recognised by the echo command (when
using echo -e). The screenshot below shows how to use \n for a newline and \t for a tab
(usually eight white spaces).
[paul@RHEL4b ~]$ echo -e "A line with \na newline"
A line with 
a newline
[paul@RHEL4b ~]$ echo -e 'A line with \na newline'
A line with 
a newline
[paul@RHEL4b ~]$ echo -e "A line with \ta tab"
A line with     a tab
[paul@RHEL4b ~]$ echo -e 'A line with \ta tab'
A line with     a tab
[paul@RHEL4b ~]$
The echo command can generate more than white spaces, tabs and newlines. Look in the
man page for a list of options.
commands and arguments
128
12.6. commands
12.6.1. external or builtin commands ?
Not all commands are external to the shell, some are builtin. External commands are
programs that have their own binary and reside somewhere in the file system. Many external
commands are located in /bin or /sbin. Builtin commands are an integral part of the shell
program itself.
12.6.2. type
To find out whether a command given to the shell will be executed as an external command
or as a builtin command, use the type command.
paul@laika:~$ type cd
cd is a shell builtin
paul@laika:~$ type cat
cat is /bin/cat
As you can see, the cd command is builtin and the cat command is external.
You can also use this command to show you whether the command is aliased or not.
paul@laika:~$ type ls
ls is aliased to `ls --color=auto'
12.6.3. running external commands
Some commands have both builtin and external versions. When one of these commands is
executed, the builtin version takes priority. To run the external version, you must enter the
full path to the command.
paul@laika:~$ type -a echo
echo is a shell builtin
echo is /bin/echo
paul@laika:~$ /bin/echo Running the external echo command... 
Running the external echo command...
12.6.4. which
The which command will search for binaries in the $PATH environment variable (variables
will be explained later). In the screenshot below, it is determined that cd is builtin, and ls,
cp, rm, mv, mkdir, pwd, and which are external commands.
[root@RHEL4b ~]# which cp ls cd mkdir pwd 
/bin/cp
/bin/ls
/usr/bin/which: no cd in (/usr/kerberos/sbin:/usr/kerberos/bin:...
/bin/mkdir
/bin/pwd
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129
12.7. aliases
12.7.1. create an alias
The shell allows you to create aliases. Aliases are often used to create an easier to remember
name for an existing command or to easily supply parameters.
[paul@RHELv4u3 ~]$ cat count.txt 
one
two
three
[paul@RHELv4u3 ~]$ alias dog=tac
[paul@RHELv4u3 ~]$ dog count.txt 
three
two
one
12.7.2. abbreviate commands
An alias can also be useful to abbreviate an existing command.
paul@laika:~$ alias ll='ls -lh --color=auto'
paul@laika:~$ alias c='clear'
paul@laika:~$
12.7.3. default options
Aliases can be used to supply commands with default options. The example below shows
how to set the -i option default when typing rm.
[paul@RHELv4u3 ~]$ rm -i winter.txt 
rm: remove regular file `winter.txt'? no
[paul@RHELv4u3 ~]$ rm winter.txt 
[paul@RHELv4u3 ~]$ ls winter.txt
ls: winter.txt: No such file or directory
[paul@RHELv4u3 ~]$ touch winter.txt
[paul@RHELv4u3 ~]$ alias rm='rm -i'
[paul@RHELv4u3 ~]$ rm winter.txt 
rm: remove regular empty file `winter.txt'? no
[paul@RHELv4u3 ~]$
Some distributions enable default aliases to protect users from accidentally erasing files ('rm
-i', 'mv -i', 'cp -i')
12.7.4. viewing aliases
You can provide one or more aliases as arguments to the alias command to get their
definitions. Providing no arguments gives a complete list of current aliases.
paul@laika:~$ alias c ll
alias c='clear'
alias ll='ls -lh --color=auto'
commands and arguments
130
12.7.5. unalias
You can undo an alias with the unalias command.
[paul@RHEL4b ~]$ which rm
/bin/rm
[paul@RHEL4b ~]$ alias rm='rm -i'
[paul@RHEL4b ~]$ which rm
alias rm='rm -i'
        /bin/rm
[paul@RHEL4b ~]$ unalias rm
[paul@RHEL4b ~]$ which rm
/bin/rm
[paul@RHEL4b ~]$
12.8. displaying shell expansion
You can display shell expansion with set -x, and stop displaying it with set +x. You might
want to use this further on in this course, or when in doubt about exactly what the shell is
doing with your command.
[paul@RHELv4u3 ~]$ set -x
++ echo -ne '\033]0;paul@RHELv4u3:~\007'
[paul@RHELv4u3 ~]$ echo $USER
+ echo paul
paul
++ echo -ne '\033]0;paul@RHELv4u3:~\007'
[paul@RHELv4u3 ~]$ echo \$USER
+ echo '$USER'
$USER
++ echo -ne '\033]0;paul@RHELv4u3:~\007'
[paul@RHELv4u3 ~]$ set +x
+ set +x
[paul@RHELv4u3 ~]$ echo $USER
paul
commands and arguments
131
12.9. practice: commands and arguments
1. How many arguments are in this line (not counting the command itself).
touch '/etc/cron/cron.allow' 'file 42.txt' "file 33.txt"
2. Is tac a shell builtin command ?
3. Is there an existing alias for rm ?
4. Read the man page of rm, make sure you understand the -i option of rm. Create and
remove a file to test the -i option.
5. Execute: alias rm='rm -i' . Test your alias with a test file. Does this work as expected ?
6. List all current aliases.
7a. Create an alias called 'city' that echoes your hometown.
7b. Use your alias to test that it works.
8. Execute set -x to display shell expansion for every command.
9. Test the functionality of set -x by executing your city and rm aliases.
10 Execute set +x to stop displaying shell expansion.
11. Remove your city alias.
12. What is the location of the cat and the passwd commands ?
13. Explain the difference between the following commands:
echo
/bin/echo
14. Explain the difference between the following commands:
echo Hello
echo -n Hello
15. Display A B C with two spaces between B and C.
(optional)16. Complete the following command (do not use spaces) to display exactly the
following output:
4+4     =8
10+14   =24
17. Use echo to display the following exactly:
??\\
commands and arguments
132
Find two solutions with single quotes, two with double quotes and one without quotes (and
say thank you to RenÃ© and Darioush from Google for this extra).
18. Use one echo command to display three words on three lines.
commands and arguments
133
12.10. solution: commands and arguments
1. How many arguments are in this line (not counting the command itself).
touch '/etc/cron/cron.allow' 'file 42.txt' "file 33.txt"
answer: three
2. Is tac a shell builtin command ?
type tac
3. Is there an existing alias for rm ?
alias rm
4. Read the man page of rm, make sure you understand the -i option of rm. Create and
remove a file to test the -i option.
man rm
touch testfile
rm -i testfile
5. Execute: alias rm='rm -i' . Test your alias with a test file. Does this work as expected ?
touch testfile
rm testfile (should ask for confirmation)
6. List all current aliases.
alias
7a. Create an alias called 'city' that echoes your hometown.
alias city='echo Antwerp'
7b. Use your alias to test that it works.
city (it should display Antwerp)
8. Execute set -x to display shell expansion for every command.
set -x
9. Test the functionality of set -x by executing your city and rm aliases.
shell should display the resolved aliases and then execute the command:
paul@deb503:~$ set -x
paul@deb503:~$ city
+ echo antwerp
antwerp
10 Execute set +x to stop displaying shell expansion.
set +x
11. Remove your city alias.
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134
unalias city
12. What is the location of the cat and the passwd commands ?
which cat (probably /bin/cat)
which passwd (probably /usr/bin/passwd)
13. Explain the difference between the following commands:
echo
/bin/echo
The echo command will be interpreted by the shell as the built-in echo command. The /bin/
echo command will make the shell execute the echo binary located in the /bin directory.
14. Explain the difference between the following commands:
echo Hello
echo -n Hello
The -n option of the echo command will prevent echo from echoing a trailing newline. echo
Hello will echo six characters in total, echo -n hello only echoes five characters.
(The -n option might not work in the Korn shell.)
15. Display A B C with two spaces between B and C.
echo "A B  C"
16. Complete the following command (do not use spaces) to display exactly the following
output:
4+4     =8
10+14   =24
The solution is to use tabs with \t.
echo -e "4+4\t=8" ; echo -e "10+14\t=24"
17. Use echo to display the following exactly:
??\\
echo '??\\'
echo -e '??\\\\'
echo "??\\\\"
echo -e "??\\\\\\"
echo ??\\\\
Find two solutions with single quotes, two with double quotes and one without quotes (and
say thank you to RenÃ© and Darioush from Google for this extra).
18. Use one echo command to display three words on three lines.
echo -e "one \ntwo \nthree"
135
Chapter 13. control operators
In this chapter we put more than one command on the command line using control
operators. We also briefly discuss related parameters ($?) and similar special characters(&).
control operators
136
13.1. ; semicolon
You can put two or more commands on the same line separated by a semicolon ; . The shell
will scan the line until it reaches the semicolon. All the arguments before this semicolon
will be considered a separate command from all the arguments after the semicolon. Both
series will be executed sequentially with the shell waiting for each command to finish before
starting the next one.
[paul@RHELv4u3 ~]$ echo Hello
Hello
[paul@RHELv4u3 ~]$ echo World
World
[paul@RHELv4u3 ~]$ echo Hello ; echo World
Hello
World
[paul@RHELv4u3 ~]$
13.2. & ampersand
When a line ends with an ampersand &, the shell will not wait for the command to finish.
You will get your shell prompt back, and the command is executed in background. You will
get a message when this command has finished executing in background.
[paul@RHELv4u3 ~]$ sleep 20 &
[1] 7925
[paul@RHELv4u3 ~]$ 
...wait 20 seconds...
[paul@RHELv4u3 ~]$ 
[1]+  Done                     sleep 20
The technical explanation of what happens in this case is explained in the chapter about
processes.
13.3. $? dollar question mark
The exit code of the previous command is stored in the shell variable $?. Actually $? is a
shell parameter and not a variable, since you cannot assign a value to $?.
paul@debian5:~/test$ touch file1
paul@debian5:~/test$ echo $?
0
paul@debian5:~/test$ rm file1
paul@debian5:~/test$ echo $?
0
paul@debian5:~/test$ rm file1
rm: cannot remove `file1': No such file or directory
paul@debian5:~/test$ echo $?
1
paul@debian5:~/test$
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137
13.4. && double ampersand
The shell will interpret && as a logical AND. When using && the second command is
executed only if the first one succeeds (returns a zero exit status).
paul@barry:~$ echo first && echo second
first
second
paul@barry:~$ zecho first && echo second
-bash: zecho: command not found
Another example of the same logical AND principle. This example starts with a working cd
followed by ls, then a non-working cd which is not followed by ls.
[paul@RHELv4u3 ~]$ cd gen && ls
file1  file3  File55  fileab  FileAB   fileabc
file2  File4  FileA   Fileab  fileab2
[paul@RHELv4u3 gen]$ cd gen && ls
-bash: cd: gen: No such file or directory
13.5. || double vertical bar
The || represents a logical OR. The second command is executed only when the first
command fails (returns a non-zero exit status).
paul@barry:~$ echo first || echo second ; echo third
first
third
paul@barry:~$ zecho first || echo second ; echo third
-bash: zecho: command not found
second
third
paul@barry:~$
Another example of the same logical OR principle.
[paul@RHELv4u3 ~]$ cd gen || ls
[paul@RHELv4u3 gen]$ cd gen || ls
-bash: cd: gen: No such file or directory
file1  file3  File55  fileab  FileAB   fileabc
file2  File4  FileA   Fileab  fileab2
13.6. combining && and ||
You can use this logical AND and logical OR to write an if-then-else structure on the
command line. This example uses echo to display whether the rm command was successful.
paul@laika:~/test$ rm file1 && echo It worked! || echo It failed!
It worked!
paul@laika:~/test$ rm file1 && echo It worked! || echo It failed!
rm: cannot remove `file1': No such file or directory
It failed!
paul@laika:~/test$
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138
13.7. # pound sign
Everything written after a pound sign (#) is ignored by the shell. This is useful to write a
shell comment, but has no influence on the command execution or shell expansion.
paul@debian4:~$ mkdir test    # we create a directory
paul@debian4:~$ cd test       #### we enter the directory
paul@debian4:~/test$ ls       # is it empty ?
paul@debian4:~/test$
13.8. \ escaping special characters
The backslash \ character enables the use of control characters, but without the shell
interpreting it, this is called escaping characters.
[paul@RHELv4u3 ~]$ echo hello \; world
hello ; world
[paul@RHELv4u3 ~]$ echo hello\ \ \ world
hello   world
[paul@RHELv4u3 ~]$ echo escaping \\\ \#\ \&\ \"\ \'
escaping \ # & " '
[paul@RHELv4u3 ~]$ echo escaping \\\?\*\"\'
escaping \?*"'
13.8.1. end of line backslash
Lines ending in a backslash are continued on the next line. The shell does not interpret the
newline character and will wait on shell expansion and execution of the command line until
a newline without backslash is encountered.
[paul@RHEL4b ~]$ echo This command line \
> is split in three \
> parts
This command line is split in three parts
[paul@RHEL4b ~]$
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139
13.9. practice: control operators
0. Each question can be answered by one command line!
1. When you type passwd, which file is executed ?
2. What kind of file is that ?
3. Execute the pwd command twice. (remember 0.)
4. Execute ls after cd /etc, but only if cd /etc did not error.
5. Execute cd /etc after cd etc, but only if cd etc fails.
6. Echo it worked when touch test42 works, and echo it failed when the touch failed. All
on one command line as a normal user (not root). Test this line in your home directory and
in /bin/ .
7. Execute sleep 6, what is this command doing ?
8. Execute sleep 200 in background (do not wait for it to finish).
9. Write a command line that executes rm file55. Your command line should print 'success'
if file55 is removed, and print 'failed' if there was a problem.
(optional)10. Use echo to display "Hello World with strange' characters \ * [ } ~ \
\ ." (including all quotes)
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140
13.10. solution: control operators
0. Each question can be answered by one command line!
1. When you type passwd, which file is executed ?
which passwd
2. What kind of file is that ?
file /usr/bin/passwd
3. Execute the pwd command twice. (remember 0.)
pwd ; pwd
4. Execute ls after cd /etc, but only if cd /etc did not error.
cd /etc && ls
5. Execute cd /etc after cd etc, but only if cd etc fails.
cd etc || cd /etc
6. Echo it worked when touch test42 works, and echo it failed when the touch failed. All
on one command line as a normal user (not root). Test this line in your home directory and
in /bin/ .
paul@deb503:~$ cd ; touch test42 && echo it worked || echo it failed
it worked
paul@deb503:~$ cd /bin; touch test42 && echo it worked || echo it failed
touch: cannot touch `test42': Permission denied
it failed
7. Execute sleep 6, what is this command doing ?
pausing for six seconds
8. Execute sleep 200 in background (do not wait for it to finish).
sleep 200 &
9. Write a command line that executes rm file55. Your command line should print 'success'
if file55 is removed, and print 'failed' if there was a problem.
rm file55 && echo success || echo failed
(optional)10. Use echo to display "Hello World with strange' characters \ * [ } ~ \
\ ." (including all quotes)
echo \"Hello World with strange\' characters \\ \* \[ \} \~ \\\\ \. \"
or
echo \""Hello World with strange' characters \ * [ } ~ \\ . "\"
141
Chapter 14. shell variables
In this chapter we learn to manage environment variables in the shell. These variables are
often needed by applications.
shell variables
142
14.1. $ dollar sign
Another important character interpreted by the shell is the dollar sign $. The shell will look
for an environment variable named like the string following the dollar sign and replace it
with the value of the variable (or with nothing if the variable does not exist).
These are some examples using $HOSTNAME, $USER, $UID, $SHELL, and $HOME.
[paul@RHELv4u3 ~]$ echo This is the $SHELL shell
This is the /bin/bash shell
[paul@RHELv4u3 ~]$ echo This is $SHELL on computer $HOSTNAME
This is /bin/bash on computer RHELv4u3.localdomain
[paul@RHELv4u3 ~]$ echo The userid of $USER is $UID
The userid of paul is 500
[paul@RHELv4u3 ~]$ echo My homedir is $HOME
My homedir is /home/paul
14.2. case sensitive
This example shows that shell variables are case sensitive!
[paul@RHELv4u3 ~]$ echo Hello $USER
Hello paul
[paul@RHELv4u3 ~]$ echo Hello $user
Hello
14.3. creating variables
This example creates the variable $MyVar and sets its value. It then uses echo to verify
the value.
[paul@RHELv4u3 gen]$ MyVar=555
[paul@RHELv4u3 gen]$ echo $MyVar
555
[paul@RHELv4u3 gen]$
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143
14.4. quotes
Notice that double quotes still allow the parsing of variables, whereas single quotes prevent
this.
[paul@RHELv4u3 ~]$ MyVar=555
[paul@RHELv4u3 ~]$ echo $MyVar
555
[paul@RHELv4u3 ~]$ echo "$MyVar"
555
[paul@RHELv4u3 ~]$ echo '$MyVar'
$MyVar
The bash shell will replace variables with their value in double quoted lines, but not in single
quoted lines.
paul@laika:~$ city=Burtonville
paul@laika:~$ echo "We are in $city today."
We are in Burtonville today.
paul@laika:~$ echo 'We are in $city today.'
We are in $city today. 
14.5. set
You can use the set command to display a list of environment variables. On Ubuntu and
Debian systems, the set command will also list shell functions after the shell variables. Use
set | more to see the variables then.
14.6. unset
Use the unset command to remove a variable from your shell environment.
[paul@RHEL4b ~]$ MyVar=8472
[paul@RHEL4b ~]$ echo $MyVar
8472
[paul@RHEL4b ~]$ unset MyVar
[paul@RHEL4b ~]$ echo $MyVar
[paul@RHEL4b ~]$
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144
14.7. $PS1
The $PS1 variable determines your shell prompt. You can use backslash escaped special
characters like \u for the username or \w for the working directory. The bash manual has
a complete reference.
In this example we change the value of $PS1 a couple of times.
paul@deb503:~$ PS1=prompt
prompt
promptPS1='prompt '
prompt 
prompt PS1='> '
> 
> PS1='\u@\h$ '
paul@deb503$ 
paul@deb503$ PS1='\u@\h:\W$'
paul@deb503:~$
To avoid unrecoverable mistakes, you can set normal user prompts to green and the root
prompt to red. Add the following to your .bashrc for a green user prompt:
# color prompt by paul
RED='\[\033[01;31m\]'
WHITE='\[\033[01;00m\]'
GREEN='\[\033[01;32m\]'
BLUE='\[\033[01;34m\]'
export PS1="${debian_chroot:+($debian_chroot)}$GREEN\u$WHITE@$BLUE\h$WHITE\w\$ "
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145
14.8. $PATH
The $PATH variable is determines where the shell is looking for commands to execute
(unless the command is builtin or aliased). This variable contains a list of directories,
separated by colons.
[[paul@RHEL4b ~]$ echo $PATH
/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:
The shell will not look in the current directory for commands to execute! (Looking for
executables in the current directory provided an easy way to hack PC-DOS computers). If
you want the shell to look in the current directory, then add a . at the end of your $PATH.
[paul@RHEL4b ~]$ PATH=$PATH:.
[paul@RHEL4b ~]$ echo $PATH
/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:.
[paul@RHEL4b ~]$
Your path might be different when using su instead of su - because the latter will take on
the environment of the target user. The root user typically has /sbin directories added to the
$PATH variable.
[paul@RHEL3 ~]$ su
Password: 
[root@RHEL3 paul]# echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin
[root@RHEL3 paul]# exit
[paul@RHEL3 ~]$ su -
Password: 
[root@RHEL3 ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:
[root@RHEL3 ~]#
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146
14.9. env
The env command without options will display a list of exported variables. The difference
with set with options is that set lists all variables, including those not exported to child shells.
But env can also be used to start a clean shell (a shell without any inherited environment).
The env -i command clears the environment for the subshell.
Notice in this screenshot that bash will set the $SHELL variable on startup.
[paul@RHEL4b ~]$ bash -c 'echo $SHELL $HOME $USER'
/bin/bash /home/paul paul
[paul@RHEL4b ~]$ env -i bash -c 'echo $SHELL $HOME $USER'
/bin/bash
[paul@RHEL4b ~]$
You can use the env command to set the $LANG, or any other, variable for just one instance
of bash with one command. The example below uses this to show the influence of the
$LANG variable on file globbing (see the chapter on file globbing).
[paul@RHEL4b test]$ env LANG=C bash -c 'ls File[a-z]'
Filea  Fileb
[paul@RHEL4b test]$ env LANG=en_US.UTF-8 bash -c 'ls File[a-z]'
Filea  FileA  Fileb  FileB
[paul@RHEL4b test]$
14.10. export
You can export shell variables to other shells with the export command. This will export
the variable to child shells.
[paul@RHEL4b ~]$ var3=three
[paul@RHEL4b ~]$ var4=four
[paul@RHEL4b ~]$ export var4
[paul@RHEL4b ~]$ echo $var3 $var4
three four
[paul@RHEL4b ~]$ bash
[paul@RHEL4b ~]$ echo $var3 $var4
four
But it will not export to the parent shell (previous screenshot continued).
[paul@RHEL4b ~]$ export var5=five
[paul@RHEL4b ~]$ echo $var3 $var4 $var5
four five
[paul@RHEL4b ~]$ exit
exit
[paul@RHEL4b ~]$ echo $var3 $var4 $var5
three four
[paul@RHEL4b ~]$
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147
14.11. delineate variables
Until now, we have seen that bash interprets a variable starting from a dollar sign, continuing
until the first occurrence of a non-alphanumeric character that is not an underscore. In some
situations, this can be a problem. This issue can be resolved with curly braces like in this
example.
[paul@RHEL4b ~]$ prefix=Super
[paul@RHEL4b ~]$ echo Hello $prefixman and $prefixgirl
Hello  and
[paul@RHEL4b ~]$ echo Hello ${prefix}man and ${prefix}girl
Hello Superman and Supergirl
[paul@RHEL4b ~]$
14.12. unbound variables
The example below tries to display the value of the $MyVar variable, but it fails because the
variable does not exist. By default the shell will display nothing when a variable is unbound
(does not exist).
[paul@RHELv4u3 gen]$ echo $MyVar
    
[paul@RHELv4u3 gen]$
There is, however, the nounset shell option that you can use to generate an error when a
variable does not exist.
paul@laika:~$ set -u
paul@laika:~$ echo $Myvar
bash: Myvar: unbound variable
paul@laika:~$ set +u
paul@laika:~$ echo $Myvar
paul@laika:~$
In the bash shell set -u is identical to set -o nounset and likewise set +u is identical to set
+o nounset.
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148
14.13. practice: shell variables
1. Use echo to display Hello followed by your username. (use a bash variable!)
2. Create a variable answer with a value of 42.
3. Copy the value of $LANG to $MyLANG.
4. List all current shell variables.
5. List all exported shell variables.
6. Do the env and set commands display your variable ?
6. Destroy your answer variable.
7. Create two variables, and export one of them.
8. Display the exported variable in an interactive child shell.
9. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use
echo and the two variables to echo Dumbledore.
10. Find the list of backslash escaped characters in the manual of bash. Add the time to your
PS1 prompt.
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149
14.14. solution: shell variables
1. Use echo to display Hello followed by your username. (use a bash variable!)
echo Hello $USER
2. Create a variable answer with a value of 42.
answer=42
3. Copy the value of $LANG to $MyLANG.
MyLANG=$LANG
4. List all current shell variables.
set
set|more on Ubuntu/Debian
5. List all exported shell variables.
env
export
declare -x
6. Do the env and set commands display your variable ?
env | more
set | more
6. Destroy your answer variable.
unset answer
7. Create two variables, and export one of them.
var1=1; export var2=2
8. Display the exported variable in an interactive child shell.
bash
echo $var2
9. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use
echo and the two variables to echo Dumbledore.
varx=Dumb; vary=do
echo ${varx}le${vary}re
solution by Yves from Dexia : echo $varx'le'$vary're'
solution by Erwin from Telenet : echo "$varx"le"$vary"re
10. Find the list of backslash escaped characters in the manual of bash. Add the time to your
PS1 prompt.
PS1='\t \u@\h \W$ '
150
Chapter 15. shell embedding and
options
This chapter takes a brief look at child shells, embedded shells and shell options.
shell embedding and options
151
15.1. shell embedding
Shells can be embedded on the command line, or in other words, the command line scan
can spawn new processes containing a fork of the current shell. You can use variables to
prove that new shells are created. In the screenshot below, the variable $var1 only exists in
the (temporary) sub shell.
[paul@RHELv4u3 gen]$ echo $var1
[paul@RHELv4u3 gen]$ echo $(var1=5;echo $var1)
5
[paul@RHELv4u3 gen]$ echo $var1
[paul@RHELv4u3 gen]$
You can embed a shell in an embedded shell, this is called nested embedding of shells.
This screenshot shows an embedded shell inside an embedded shell.
paul@deb503:~$ A=shell
paul@deb503:~$ echo $C$B$A $(B=sub;echo $C$B$A; echo $(C=sub;echo $C$B$A))
shell subshell subsubshell
15.1.1. backticks
Single embedding can be useful to avoid changing your current directory. The screenshot
below uses backticks instead of dollar-bracket to embed.
[paul@RHELv4u3 ~]$ echo `cd /etc; ls -d * | grep pass`
passwd passwd- passwd.OLD
[paul@RHELv4u3 ~]$
You can only use the $() notation to nest embedded shells, backticks cannot do this.
15.1.2. backticks or single quotes
Placing the embedding between backticks uses one character less than the dollar and
parenthesis combo. Be careful however, backticks are often confused with single quotes.
The technical difference between ' and ` is significant!
[paul@RHELv4u3 gen]$ echo `var1=5;echo $var1`
5
[paul@RHELv4u3 gen]$ echo 'var1=5;echo $var1'
var1=5;echo $var1
[paul@RHELv4u3 gen]$
shell embedding and options
152
15.2. shell options
Both set and unset are builtin shell commands. They can be used to set options of the bash
shell itself. The next example will clarify this. By default, the shell will treat unset variables
as a variable having no value. By setting the -u option, the shell will treat any reference to
unset variables as an error. See the man page of bash for more information.
[paul@RHEL4b ~]$ echo $var123
[paul@RHEL4b ~]$ set -u
[paul@RHEL4b ~]$ echo $var123
-bash: var123: unbound variable
[paul@RHEL4b ~]$ set +u
[paul@RHEL4b ~]$ echo $var123
[paul@RHEL4b ~]$
To list all the set options for your shell, use echo $-. The noclobber (or -C) option will be
explained later in this book (in the I/O redirection chapter).
[paul@RHEL4b ~]$ echo $-
himBH
[paul@RHEL4b ~]$ set -C ; set -u
[paul@RHEL4b ~]$ echo $-
himuBCH
[paul@RHEL4b ~]$ set +C ; set +u
[paul@RHEL4b ~]$ echo $-
himBH
[paul@RHEL4b ~]$
When typing set without options, you get a list of all variables without function when the
shell is on posix mode. You can set bash in posix mode typing set -o posix.
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153
15.3. practice: shell embedding
1. Find the list of shell options in the man page of bash. What is the difference between set
-u and set -o nounset?
2. Activate nounset in your shell. Test that it shows an error message when using non-
existing variables.
3. Deactivate nounset.
4. Execute cd /var and ls in an embedded shell.
The echo command is only needed to show the result of the ls command. Omitting will result
in the shell trying to execute the first file as a command.
5. Create the variable embvar in an embedded shell and echo it. Does the variable exist in
your current shell now ?
6. Explain what "set -x" does. Can this be useful ?
(optional)7. Given the following screenshot, add exactly four characters to that command
line so that the total output is FirstMiddleLast.
[paul@RHEL4b ~]$ echo  First; echo  Middle; echo  Last
8. Display a long listing (ls -l) of the passwd command using the which command inside
an embedded shell.
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154
15.4. solution: shell embedding
1. Find the list of shell options in the man page of bash. What is the difference between set
-u and set -o nounset?
read the manual of bash (man bash), search for nounset -- both mean the same thing.
2. Activate nounset in your shell. Test that it shows an error message when using non-
existing variables.
set -u
OR
set -o nounset
Both these lines have the same effect.
3. Deactivate nounset.
set +u
OR
set +o nounset
4. Execute cd /var and ls in an embedded shell.
echo $(cd /var ; ls)
The echo command is only needed to show the result of the ls command. Omitting will result
in the shell trying to execute the first file as a command.
5. Create the variable embvar in an embedded shell and echo it. Does the variable exist in
your current shell now ?
echo $(embvar=emb;echo $embvar) ; echo $embvar #the last echo fails
$embvar does not exist in your current shell
6. Explain what "set -x" does. Can this be useful ?
It displays shell expansion for troubleshooting your command.
(optional)7. Given the following screenshot, add exactly four characters to that command
line so that the total output is FirstMiddleLast.
[paul@RHEL4b ~]$ echo  First; echo  Middle; echo  Last
echo -n First; echo -n Middle; echo Last
8. Display a long listing (ls -l) of the passwd command using the which command inside
an embedded shell.
ls -l $(which passwd)
155
Chapter 16. shell history
The shell makes it easy for us to repeat commands, this chapter explains how.
shell history
156
16.1. repeating the last command
To repeat the last command in bash, type !!. This is pronounced as bang bang.
paul@debian5:~/test42$ echo this will be repeated > file42.txt
paul@debian5:~/test42$ !!
echo this will be repeated > file42.txt
paul@debian5:~/test42$ 
16.2. repeating other commands
You can repeat other commands using one bang followed by one or more characters. The
shell will repeat the last command that started with those characters.
paul@debian5:~/test42$ touch file42
paul@debian5:~/test42$ cat file42
paul@debian5:~/test42$ !to
touch file42
paul@debian5:~/test42$
16.3. history
To see older commands, use history to display the shell command history (or use history
n to see the last n commands).
paul@debian5:~/test$ history 10
38  mkdir test
39  cd test
40  touch file1
41  echo hello > file2
42  echo It is very cold today > winter.txt
43  ls
44  ls -l
45  cp winter.txt summer.txt
46  ls -l
47  history 10
16.4. !n
When typing ! followed by the number preceding the command you want repeated, then the
shell will echo the command and execute it.
paul@debian5:~/test$ !43
ls
file1  file2  summer.txt  winter.txt
shell history
157
16.5. Ctrl-r
Another option is to use ctrl-r to search in the history. In the screenshot below i only typed
ctrl-r followed by four characters apti and it finds the last command containing these four
consecutive characters.
paul@debian5:~$ 
(reverse-i-search)`apti': sudo aptitude install screen
16.6. $HISTSIZE
The $HISTSIZE variable determines the number of commands that will be remembered in
your current environment. Most distributions default this variable to 500 or 1000.
paul@debian5:~$ echo $HISTSIZE
500
You can change it to any value you like.
paul@debian5:~$ HISTSIZE=15000
paul@debian5:~$ echo $HISTSIZE
15000
16.7. $HISTFILE
The $HISTFILE variable points to the file that contains your history. The bash shell defaults
this value to ~/.bash_history.
paul@debian5:~$ echo $HISTFILE
/home/paul/.bash_history
A session history is saved to this file when you exit the session!
Closing a gnome-terminal with the mouse, or typing reboot as root will NOT save your
terminal's history.
16.8. $HISTFILESIZE
The number of commands kept in your history file can be set using $HISTFILESIZE.
paul@debian5:~$ echo $HISTFILESIZE
15000
shell history
158
16.9. prevent recording a command
You can prevent a command from being recorded in history using a space prefix.
paul@debian8:~/github$ echo abc
abc
paul@debian8:~/github$  echo def
def
paul@debian8:~/github$ echo ghi
ghi
paul@debian8:~/github$ history 3
 9501  echo abc
 9502  echo ghi
 9503  history 3
16.10. (optional)regular expressions
It is possible to use regular expressions when using the bang to repeat commands. The
screenshot below switches 1 into 2.
paul@debian5:~/test$ cat file1
paul@debian5:~/test$ !c:s/1/2
cat file2
hello
paul@debian5:~/test$
16.11. (optional) Korn shell history
Repeating a command in the Korn shell is very similar. The Korn shell  also has the history
command, but uses the letter r to recall lines from history.
This screenshot shows the history command. Note the different meaning of the parameter.
$ history 17
17  clear
18  echo hoi
19  history 12
20  echo world
21  history 17
Repeating with r can be combined with the line numbers given by the history command, or
with the first few letters of the command.
$ r e
echo world
world
$ cd /etc
$ r
cd /etc
$
shell history
159
16.12. practice: shell history
1. Issue the command echo The answer to the meaning of life, the universe and
everything is 42.
2. Repeat the previous command using only two characters (there are two solutions!)
3. Display the last 5 commands you typed.
4. Issue the long echo from question 1 again, using the line numbers you received from the
command in question 3.
5. How many commands can be kept in memory for your current shell session ?
6. Where are these commands stored when exiting the shell ?
7. How many commands can be written to the history file when exiting your current shell
session ?
8. Make sure your current bash shell remembers the next 5000 commands you type.
9. Open more than one console (by press Ctrl-shift-t in gnome-terminal, or by opening an
extra putty.exe in MS Windows) with the same user account. When is command history
written to the history file ?
shell history
160
16.13. solution: shell history
1. Issue the command echo The answer to the meaning of life, the universe and
everything is 42.
echo The answer to the meaning of life, the universe and everything is 42
2. Repeat the previous command using only two characters (there are two solutions!)
!!
OR
!e
3. Display the last 5 commands you typed.
paul@ubu1010:~$ history 5
 52  ls -l
 53  ls
 54  df -h | grep sda
 55  echo The answer to the meaning of life, the universe and everything is 42
 56  history 5
You will receive different line numbers.
4. Issue the long echo from question 1 again, using the line numbers you received from the
command in question 3.
paul@ubu1010:~$ !55
echo The answer to the meaning of life, the universe and everything is 42
The answer to the meaning of life, the universe and everything is 42
5. How many commands can be kept in memory for your current shell session ?
echo $HISTSIZE
6. Where are these commands stored when exiting the shell ?
echo $HISTFILE
7. How many commands can be written to the history file when exiting your current shell
session ?
echo $HISTFILESIZE
8. Make sure your current bash shell remembers the next 5000 commands you type.
HISTSIZE=5000
9. Open more than one console (by press Ctrl-shift-t in gnome-terminal, or by opening an
extra putty.exe in MS Windows) with the same user account. When is command history
written to the history file ?
when you type exit
161
Chapter 17. file globbing
The shell is also responsible for file globbing (or dynamic filename generation). This chapter
will explain file globbing.
file globbing
162
17.1. * asterisk
The asterisk * is interpreted by the shell as a sign to generate filenames, matching the asterisk
to any combination of characters (even none). When no path is given, the shell will use
filenames in the current directory. See the man page of glob(7) for more information. (This
is part of LPI topic 1.103.3.)
[paul@RHELv4u3 gen]$ ls
file1  file2  file3  File4  File55  FileA  fileab  Fileab  FileAB  fileabc
[paul@RHELv4u3 gen]$ ls File*
File4  File55  FileA  Fileab  FileAB
[paul@RHELv4u3 gen]$ ls file*
file1  file2  file3  fileab  fileabc
[paul@RHELv4u3 gen]$ ls *ile55
File55
[paul@RHELv4u3 gen]$ ls F*ile55
File55
[paul@RHELv4u3 gen]$ ls F*55
File55
[paul@RHELv4u3 gen]$
17.2. ? question mark
Similar to the asterisk, the question mark ? is interpreted by the shell as a sign to generate
filenames, matching the question mark with exactly one character.
[paul@RHELv4u3 gen]$ ls
file1  file2  file3  File4  File55  FileA  fileab  Fileab  FileAB  fileabc
[paul@RHELv4u3 gen]$ ls File?
File4  FileA
[paul@RHELv4u3 gen]$ ls Fil?4
File4
[paul@RHELv4u3 gen]$ ls Fil??
File4  FileA
[paul@RHELv4u3 gen]$ ls File??
File55  Fileab  FileAB
[paul@RHELv4u3 gen]$
file globbing
163
17.3. [] square brackets
The square bracket [ is interpreted by the shell as a sign to generate filenames, matching
any of the characters between [ and the first subsequent ]. The order in this list between the
brackets is not important. Each pair of brackets is replaced by exactly one character.
[paul@RHELv4u3 gen]$ ls 
file1  file2  file3  File4  File55  FileA  fileab  Fileab  FileAB  fileabc
[paul@RHELv4u3 gen]$ ls File[5A]
FileA
[paul@RHELv4u3 gen]$ ls File[A5]
FileA
[paul@RHELv4u3 gen]$ ls File[A5][5b]
File55
[paul@RHELv4u3 gen]$ ls File[a5][5b]
File55  Fileab
[paul@RHELv4u3 gen]$ ls File[a5][5b][abcdefghijklm]
ls: File[a5][5b][abcdefghijklm]: No such file or directory
[paul@RHELv4u3 gen]$ ls file[a5][5b][abcdefghijklm]
fileabc
[paul@RHELv4u3 gen]$
You can also exclude characters from a list between square brackets with the exclamation
mark !. And you are allowed to make combinations of these wild cards.
[paul@RHELv4u3 gen]$ ls 
file1  file2  file3  File4  File55  FileA  fileab  Fileab  FileAB  fileabc
[paul@RHELv4u3 gen]$ ls file[a5][!Z]
fileab
[paul@RHELv4u3 gen]$ ls file[!5]*
file1  file2  file3  fileab  fileabc
[paul@RHELv4u3 gen]$ ls file[!5]?
fileab
[paul@RHELv4u3 gen]$
file globbing
164
17.4. a-z and 0-9 ranges
The bash shell will also understand ranges of characters between brackets.
[paul@RHELv4u3 gen]$ ls
file1  file3  File55  fileab  FileAB   fileabc
file2  File4  FileA   Fileab  fileab2
[paul@RHELv4u3 gen]$ ls file[a-z]*
fileab  fileab2  fileabc
[paul@RHELv4u3 gen]$ ls file[0-9]
file1  file2  file3
[paul@RHELv4u3 gen]$ ls file[a-z][a-z][0-9]*
fileab2
[paul@RHELv4u3 gen]$
17.5. $LANG and square brackets
But, don't forget the influence of the LANG variable. Some languages include lower case
letters in an upper case range (and vice versa).
paul@RHELv4u4:~/test$ ls [A-Z]ile?
file1  file2  file3  File4
paul@RHELv4u4:~/test$ ls [a-z]ile?
file1  file2  file3  File4
paul@RHELv4u4:~/test$ echo $LANG
en_US.UTF-8
paul@RHELv4u4:~/test$ LANG=C
paul@RHELv4u4:~/test$ echo $LANG
C
paul@RHELv4u4:~/test$ ls [a-z]ile?
file1  file2  file3
paul@RHELv4u4:~/test$ ls [A-Z]ile?
File4
paul@RHELv4u4:~/test$
If $LC_ALL is set, then this will also need to be reset to prevent file globbing.
file globbing
165
17.6. preventing file globbing
The screenshot below should be no surprise. The echo * will echo a * when in an empty
directory. And it will echo the names of all files when the directory is not empty.
paul@ubu1010:~$ mkdir test42
paul@ubu1010:~$ cd test42
paul@ubu1010:~/test42$ echo *
*
paul@ubu1010:~/test42$ touch file42 file33
paul@ubu1010:~/test42$ echo *
file33 file42
Globbing can be prevented using quotes or by escaping the special characters, as shown in
this screenshot.
paul@ubu1010:~/test42$ echo *
file33 file42
paul@ubu1010:~/test42$ echo \*
*
paul@ubu1010:~/test42$ echo '*'
*
paul@ubu1010:~/test42$ echo "*"
*
file globbing
166
17.7. practice: shell globbing
1. Create a test directory and enter it.
2. Create the following files :
file1
file10
file11
file2
File2
File3
file33
fileAB
filea
fileA
fileAAA
file(
file 2
(the last one has 6 characters including a space)
3. List (with ls) all files starting with file
4. List (with ls) all files starting with File
5. List (with ls) all files starting with file and ending in a number.
6. List (with ls) all files starting with file and ending with a letter
7. List (with ls) all files starting with File and having a digit as fifth character.
8. List (with ls) all files starting with File and having a digit as fifth character and nothing
else.
9. List (with ls) all files starting with a letter and ending in a number.
10. List (with ls) all files that have exactly five characters.
11. List (with ls) all files that start with f or F and end with 3 or A.
12. List (with ls) all files that start with f have i or R as second character and end in a number.
13. List all files that do not start with the letter F.
14. Copy the value of $LANG to $MyLANG.
15. Show the influence of $LANG in listing A-Z or a-z ranges.
16. You receive information that one of your servers was cracked, the cracker probably
replaced the ls command. You know that the echo command is safe to use. Can echo replace
ls ? How can you list the files in the current directory with echo ?
17. Is there another command besides cd to change directories ?
file globbing
167
17.8. solution: shell globbing
1. Create a test directory and enter it.
mkdir testdir; cd testdir
2. Create the following files :
file1
file10
file11
file2
File2
File3
file33
fileAB
filea
fileA
fileAAA
file(
file 2
(the last one has 6 characters including a space)
touch file1 file10 file11 file2 File2 File3
touch file33 fileAB filea fileA fileAAA
touch "file("
touch "file 2"
3. List (with ls) all files starting with file
ls file*
4. List (with ls) all files starting with File
ls File*
5. List (with ls) all files starting with file and ending in a number.
ls file*[0-9]
6. List (with ls) all files starting with file and ending with a letter
ls file*[a-z]
7. List (with ls) all files starting with File and having a digit as fifth character.
ls File[0-9]*
8. List (with ls) all files starting with File and having a digit as fifth character and nothing
else.
ls File[0-9]
9. List (with ls) all files starting with a letter and ending in a number.
ls [a-z]*[0-9]
10. List (with ls) all files that have exactly five characters.
file globbing
168
ls ?????
11. List (with ls) all files that start with f or F and end with 3 or A.
ls [fF]*[3A]
12. List (with ls) all files that start with f have i or R as second character and end in a number.
ls f[iR]*[0-9]
13. List all files that do not start with the letter F.
ls [!F]*
14. Copy the value of $LANG to $MyLANG.
MyLANG=$LANG
15. Show the influence of $LANG in listing A-Z or a-z ranges.
see example in book
16. You receive information that one of your servers was cracked, the cracker probably
replaced the ls command. You know that the echo command is safe to use. Can echo replace
ls ? How can you list the files in the current directory with echo ?
echo *
17. Is there another command besides cd to change directories ?
pushd popd
Part V. pipes and commands
170
Table of Contents
18. I/O redirection ............................................................................................................... 171
18.1. stdin, stdout, and stderr ..........................................................................................  172
18.2. output redirection ..................................................................................................  173
18.3. error redirection ....................................................................................................  175
18.4. output redirection and pipes ....................................................................................  176
18.5. joining stdout and stderr ......................................................................................... 176
18.6. input redirection ...................................................................................................  177
18.7. confusing redirection .............................................................................................  178
18.8. quick file clear .....................................................................................................  178
18.9. practice: input/output redirection ..............................................................................  179
18.10. solution: input/output redirection ............................................................................  180
19. filters ............................................................................................................................  181
19.1. cat ......................................................................................................................  182
19.2. tee ......................................................................................................................  182
19.3. grep ....................................................................................................................  182
19.4. cut ...................................................................................................................... 184
19.5. tr ........................................................................................................................ 184
19.6. wc ......................................................................................................................  185
19.7. sort .....................................................................................................................  186
19.8. uniq ....................................................................................................................  187
19.9. comm .................................................................................................................  188
19.10. od .....................................................................................................................  189
19.11. sed .................................................................................................................... 190
19.12. pipe examples .....................................................................................................  191
19.13. practice: filters ....................................................................................................  192
19.14. solution: filters .................................................................................................... 193
20. basic Unix tools .............................................................................................................  195
20.1. find ....................................................................................................................  196
20.2. locate ..................................................................................................................  197
20.3. date ....................................................................................................................  197
20.4. cal ......................................................................................................................  198
20.5. sleep ...................................................................................................................  198
20.6. time ....................................................................................................................  199
20.7. gzip - gunzip ........................................................................................................ 200
20.8. zcat - zmore .........................................................................................................  200
20.9. bzip2 - bunzip2 ....................................................................................................  201
20.10. bzcat - bzmore ....................................................................................................  201
20.11. practice: basic Unix tools .....................................................................................  202
20.12. solution: basic Unix tools .....................................................................................  203
21. regular expressions ........................................................................................................  205
21.1. regex versions ......................................................................................................  206
21.2. grep ....................................................................................................................  207
21.3. rename ................................................................................................................  212
21.4. sed .....................................................................................................................  215
21.5. bash history .........................................................................................................  219
171
Chapter 18. I/O redirection
One of the powers of the Unix command line is the use of input/output redirection and
pipes.
This chapter explains redirection of input, output and error streams.
I/O redirection
172
18.1. stdin, stdout, and stderr
The bash shell has three basic streams; it takes input from stdin (stream 0), it sends output
to stdout (stream 1)  and it sends error messages to stderr (stream 2) .
The drawing below has a graphical interpretation of these three streams.
The keyboard often serves as stdin, whereas stdout and stderr both go to the display. This
can be confusing to new Linux users because there is no obvious way to recognize stdout
from stderr. Experienced users know that separating output from errors can be very useful.
The next sections will explain how to redirect these streams.
I/O redirection
173
18.2. output redirection
18.2.1. > stdout
stdout can be redirected with a greater than sign. While scanning the line, the shell will
see the > sign and will clear the file.
The > notation is in fact the abbreviation of 1> (stdout being referred to as stream 1).
[paul@RHELv4u3 ~]$ echo It is cold today!
It is cold today!
[paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt
[paul@RHELv4u3 ~]$ cat winter.txt 
It is cold today!
[paul@RHELv4u3 ~]$
Note that the bash shell effectively removes the redirection from the command line before
argument 0 is executed. This means that in the case of this command:
echo hello > greetings.txt
the shell only counts two arguments (echo = argument 0, hello = argument 1). The redirection
is removed before the argument counting takes place.
18.2.2. output file is erased
While scanning the line, the shell will see the > sign and will clear the file! Since this
happens before resolving argument 0, this means that even when the command fails, the
file will have been cleared!
[paul@RHELv4u3 ~]$ cat winter.txt 
It is cold today!
[paul@RHELv4u3 ~]$ zcho It is cold today! > winter.txt
-bash: zcho: command not found
[paul@RHELv4u3 ~]$ cat winter.txt 
[paul@RHELv4u3 ~]$
I/O redirection
174
18.2.3. noclobber
Erasing a file while using > can be prevented by setting the noclobber option.
[paul@RHELv4u3 ~]$ cat winter.txt 
It is cold today!
[paul@RHELv4u3 ~]$ set -o noclobber
[paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt
-bash: winter.txt: cannot overwrite existing file
[paul@RHELv4u3 ~]$ set +o noclobber
[paul@RHELv4u3 ~]$
18.2.4. overruling noclobber
The noclobber can be overruled with >|.
[paul@RHELv4u3 ~]$ set -o noclobber
[paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt
-bash: winter.txt: cannot overwrite existing file
[paul@RHELv4u3 ~]$ echo It is very cold today! >| winter.txt
[paul@RHELv4u3 ~]$ cat winter.txt 
It is very cold today!
[paul@RHELv4u3 ~]$
18.2.5. >> append
Use >> to append output to a file.
[paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt
[paul@RHELv4u3 ~]$ cat winter.txt 
It is cold today!
[paul@RHELv4u3 ~]$ echo Where is the summer ? >> winter.txt
[paul@RHELv4u3 ~]$ cat winter.txt 
It is cold today!
Where is the summer ?
[paul@RHELv4u3 ~]$
I/O redirection
175
18.3. error redirection
18.3.1. 2> stderr
Redirecting stderr is done with 2>. This can be very useful to prevent error messages from
cluttering your screen.
The screenshot below shows redirection of stdout to a file, and stderr to /dev/null. Writing
1> is the same as >.
[paul@RHELv4u3 ~]$ find / > allfiles.txt 2> /dev/null
[paul@RHELv4u3 ~]$
18.3.2. 2>&1
To redirect both stdout and stderr to the same file, use 2>&1.
[paul@RHELv4u3 ~]$ find / > allfiles_and_errors.txt 2>&1
[paul@RHELv4u3 ~]$
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1
directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the
file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error made a copy of the
standard output before the standard output was redirected to dirlist.
I/O redirection
176
18.4. output redirection and pipes
By default you cannot grep inside stderr when using pipes on the command line, because
only stdout is passed.
paul@debian7:~$ rm file42 file33 file1201 | grep file42
rm: cannot remove â€˜file42â€™: No such file or directory
rm: cannot remove â€˜file33â€™: No such file or directory
rm: cannot remove â€˜file1201â€™: No such file or directory
With 2>&1 you can force stderr to go to stdout. This enables the next command in the
pipe to act on both streams.
paul@debian7:~$ rm file42 file33 file1201 2>&1 | grep file42
rm: cannot remove â€˜file42â€™: No such file or directory
You cannot use both 1>&2 and 2>&1 to switch stdout and stderr.
paul@debian7:~$ rm file42 file33 file1201 2>&1 1>&2 | grep file42
rm: cannot remove â€˜file42â€™: No such file or directory
paul@debian7:~$ echo file42 2>&1 1>&2 | sed 's/file42/FILE42/' 
FILE42
You need a third stream to switch stdout and stderr after a pipe symbol.
paul@debian7:~$ echo file42 3>&1 1>&2 2>&3 | sed 's/file42/FILE42/' 
file42
paul@debian7:~$ rm file42 3>&1 1>&2 2>&3 | sed 's/file42/FILE42/' 
rm: cannot remove â€˜FILE42â€™: No such file or directory
18.5. joining stdout and stderr
The &> construction will put both stdout and stderr in one stream (to a file).
paul@debian7:~$ rm file42 &> out_and_err
paul@debian7:~$ cat out_and_err 
rm: cannot remove â€˜file42â€™: No such file or directory
paul@debian7:~$ echo file42 &> out_and_err
paul@debian7:~$ cat out_and_err 
file42
paul@debian7:~$ 
I/O redirection
177
18.6. input redirection
18.6.1. < stdin
Redirecting stdin is done with < (short for 0<).
[paul@RHEL4b ~]$ cat < text.txt
one
two
[paul@RHEL4b ~]$ tr 'onetw' 'ONEZZ' < text.txt
ONE
ZZO
[paul@RHEL4b ~]$
18.6.2. << here document
The here document (sometimes called here-is-document) is a way to append input until a
certain sequence (usually EOF) is encountered. The EOF marker can be typed literally or
can be called with Ctrl-D.
[paul@RHEL4b ~]$ cat <<EOF > text.txt
> one
> two
> EOF
[paul@RHEL4b ~]$ cat text.txt 
one
two
[paul@RHEL4b ~]$ cat <<brol > text.txt
> brel
> brol
[paul@RHEL4b ~]$ cat text.txt 
brel
[paul@RHEL4b ~]$
  
18.6.3. <<< here string
The here string can be used to directly pass strings to a command. The result is the same
as using echo string | command (but you have one less process running).
paul@ubu1110~$ base64 <<< linux-training.be
bGludXgtdHJhaW5pbmcuYmUK
paul@ubu1110~$ base64 -d <<< bGludXgtdHJhaW5pbmcuYmUK
linux-training.be
See rfc 3548 for more information about base64.
I/O redirection
178
18.7. confusing redirection
The shell will scan the whole line before applying redirection. The following command line
is very readable and is correct.
cat winter.txt > snow.txt 2> errors.txt
But this one is also correct, but less readable.
2> errors.txt cat winter.txt > snow.txt
Even this will be understood perfectly by the shell.
< winter.txt > snow.txt 2> errors.txt cat
18.8. quick file clear
So what is the quickest way to clear a file ?
>foo
And what is the quickest way to clear a file when the noclobber option is set ?
>|bar
I/O redirection
179
18.9. practice: input/output redirection
1. Activate the noclobber shell option.
2. Verify that noclobber is active by repeating an ls on /etc/ with redirected output to a file.
3. When listing all shell options, which character represents the noclobber option ?
4. Deactivate the noclobber option.
5. Make sure you have two shells open on the same computer. Create an empty tailing.txt
file. Then type tail -f tailing.txt. Use the second shell to append a line of text to that file.
Verify that the first shell displays this line.
6. Create a file that contains the names of five people. Use cat and output redirection to
create the file and use a here document to end the input.
I/O redirection
180
18.10. solution: input/output redirection
1. Activate the noclobber shell option.
set -o noclobber
set -C
2. Verify that noclobber is active by repeating an ls on /etc/ with redirected output to a file.
ls /etc > etc.txt 
ls /etc > etc.txt (should not work)
4. When listing all shell options, which character represents the noclobber option ?
echo $- (noclobber is visible as C)
5. Deactivate the noclobber option.
set +o noclobber
6. Make sure you have two shells open on the same computer. Create an empty tailing.txt
file. Then type tail -f tailing.txt. Use the second shell to append a line of text to that file.
Verify that the first shell displays this line.
paul@deb503:~$ > tailing.txt
paul@deb503:~$ tail -f tailing.txt 
hello
world
in the other shell:
paul@deb503:~$ echo hello >> tailing.txt 
paul@deb503:~$ echo world >> tailing.txt
7. Create a file that contains the names of five people. Use cat and output redirection to
create the file and use a here document to end the input.
paul@deb503:~$ cat > tennis.txt << ace
> Justine Henin
> Venus Williams
> Serena Williams
> Martina Hingis
> Kim Clijsters
> ace
paul@deb503:~$ cat tennis.txt 
Justine Henin
Venus Williams
Serena Williams
Martina Hingis
Kim Clijsters
paul@deb503:~$
181
Chapter 19. filters
Commands that are created to be used with a pipe are often called filters. These filters
are very small programs that do one specific thing very efficiently. They can be used as
building blocks.
This chapter will introduce you to the most common filters. The combination of simple
commands and filters in a long pipe allows you to design elegant solutions.
filters
182
19.1. cat
When between two pipes, the cat command does nothing (except putting stdin on stdout).
[paul@RHEL4b pipes]$ tac count.txt | cat | cat | cat | cat | cat
five
four
three
two
one
[paul@RHEL4b pipes]$
19.2. tee
Writing long pipes in Unix is fun, but sometimes you may want intermediate results. This
is were tee comes in handy. The tee filter puts stdin on stdout and also into a file. So tee is
almost the same as cat, except that it has two identical outputs.
[paul@RHEL4b pipes]$ tac count.txt | tee temp.txt | tac
one
two
three
four
five
[paul@RHEL4b pipes]$ cat temp.txt 
five
four
three
two
one
[paul@RHEL4b pipes]$
19.3. grep
The grep filter is famous among Unix users. The most common use of grep is to filter lines
of text containing (or not containing) a certain string.
[paul@RHEL4b pipes]$ cat tennis.txt 
Amelie Mauresmo, Fra
Kim Clijsters, BEL
Justine Henin, Bel
Serena Williams, usa
Venus Williams, USA
[paul@RHEL4b pipes]$ cat tennis.txt | grep Williams
Serena Williams, usa
Venus Williams, USA
You can write this without the cat.
[paul@RHEL4b pipes]$ grep Williams tennis.txt 
Serena Williams, usa
Venus Williams, USA
One of the most useful options of grep is grep -i which filters in a case insensitive way.
[paul@RHEL4b pipes]$ grep Bel tennis.txt 
Justine Henin, Bel
[paul@RHEL4b pipes]$ grep -i Bel tennis.txt 
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183
Kim Clijsters, BEL
Justine Henin, Bel
[paul@RHEL4b pipes]$
Another very useful option is grep -v which outputs lines not matching the string.
[paul@RHEL4b pipes]$ grep -v Fra tennis.txt 
Kim Clijsters, BEL
Justine Henin, Bel
Serena Williams, usa
Venus Williams, USA
[paul@RHEL4b pipes]$
And of course, both options can be combined to filter all lines not containing a case
insensitive string.
[paul@RHEL4b pipes]$ grep -vi usa tennis.txt 
Amelie Mauresmo, Fra
Kim Clijsters, BEL
Justine Henin, Bel
[paul@RHEL4b pipes]$
With grep -A1 one line after the result is also displayed.
paul@debian5:~/pipes$ grep -A1 Henin tennis.txt 
Justine Henin, Bel
Serena Williams, usa
With grep -B1 one line before the result is also displayed.
paul@debian5:~/pipes$ grep -B1 Henin tennis.txt 
Kim Clijsters, BEL
Justine Henin, Bel
With grep -C1 (context) one line before and one after are also displayed. All three options
(A,B, and C) can display any number of lines (using e.g. A2, B4 or C20).
paul@debian5:~/pipes$ grep -C1 Henin tennis.txt 
Kim Clijsters, BEL
Justine Henin, Bel
Serena Williams, usa
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184
19.4. cut
The cut filter can select columns from files, depending on a delimiter or a count of bytes.
The screenshot below uses cut to filter for the username and userid in the /etc/passwd file.
It uses the colon as a delimiter, and selects fields 1 and 3.
[[paul@RHEL4b pipes]$ cut -d: -f1,3 /etc/passwd | tail -4 
Figo:510
Pfaff:511
Harry:516
Hermione:517
[paul@RHEL4b pipes]$
When using a space as the delimiter for cut, you have to quote the space.
[paul@RHEL4b pipes]$ cut -d" " -f1 tennis.txt 
Amelie
Kim
Justine
Serena
Venus
[paul@RHEL4b pipes]$
This example uses cut to display the second to the seventh character of /etc/passwd.
[paul@RHEL4b pipes]$ cut -c2-7 /etc/passwd | tail -4
igo:x:
faff:x
arry:x
ermion
[paul@RHEL4b pipes]$
19.5. tr
You can translate characters with tr. The screenshot shows the translation of all occurrences
of e to E.
[paul@RHEL4b pipes]$ cat tennis.txt | tr 'e' 'E'
AmEliE MaurEsmo, Fra
Kim ClijstErs, BEL
JustinE HEnin, BEl
SErEna Williams, usa
VEnus Williams, USA
Here we set all letters to uppercase by defining two ranges.
[paul@RHEL4b pipes]$ cat tennis.txt | tr 'a-z' 'A-Z'
AMELIE MAURESMO, FRA
KIM CLIJSTERS, BEL
JUSTINE HENIN, BEL
SERENA WILLIAMS, USA
VENUS WILLIAMS, USA
[paul@RHEL4b pipes]$
Here we translate all newlines to spaces.
[paul@RHEL4b pipes]$ cat count.txt 
one
two
filters
185
three
four
five
[paul@RHEL4b pipes]$ cat count.txt | tr '\n' ' '
one two three four five [paul@RHEL4b pipes]$
The tr -s filter can also be used to squeeze multiple occurrences of a character to one.
[paul@RHEL4b pipes]$ cat spaces.txt 
one    two        three
     four   five  six
[paul@RHEL4b pipes]$ cat spaces.txt | tr -s ' '
one two three
 four five six
[paul@RHEL4b pipes]$
You can also use tr to 'encrypt' texts with rot13.
[paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'nopqrstuvwxyzabcdefghijklm'
bar
gjb
guerr
sbhe
svir
[paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'n-za-m'
bar
gjb
guerr
sbhe
svir
[paul@RHEL4b pipes]$
This last example uses tr -d to delete characters.
paul@debian5:~/pipes$ cat tennis.txt | tr -d e
Amli Maursmo, Fra
Kim Clijstrs, BEL
Justin Hnin, Bl
Srna Williams, usa
Vnus Williams, USA
19.6. wc
Counting words, lines and characters is easy with wc.
[paul@RHEL4b pipes]$ wc tennis.txt 
  5  15 100 tennis.txt
[paul@RHEL4b pipes]$ wc -l tennis.txt 
5 tennis.txt
[paul@RHEL4b pipes]$ wc -w tennis.txt 
15 tennis.txt
[paul@RHEL4b pipes]$ wc -c tennis.txt 
100 tennis.txt
[paul@RHEL4b pipes]$
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186
19.7. sort
The sort filter will default to an alphabetical sort.
paul@debian5:~/pipes$ cat music.txt 
Queen
Brel
Led Zeppelin
Abba
paul@debian5:~/pipes$ sort music.txt 
Abba
Brel
Led Zeppelin
Queen
But the sort filter has many options to tweak its usage. This example shows sorting different
columns (column 1 or column 2).
[paul@RHEL4b pipes]$ sort -k1 country.txt 
Belgium, Brussels, 10
France, Paris, 60
Germany, Berlin, 100
Iran, Teheran, 70
Italy, Rome, 50
[paul@RHEL4b pipes]$ sort -k2 country.txt 
Germany, Berlin, 100
Belgium, Brussels, 10
France, Paris, 60
Italy, Rome, 50
Iran, Teheran, 70
The screenshot below shows the difference between an alphabetical sort and a numerical
sort (both on the third column).
[paul@RHEL4b pipes]$ sort -k3 country.txt 
Belgium, Brussels, 10
Germany, Berlin, 100
Italy, Rome, 50
France, Paris, 60
Iran, Teheran, 70
[paul@RHEL4b pipes]$ sort -n -k3 country.txt 
Belgium, Brussels, 10
Italy, Rome, 50
France, Paris, 60
Iran, Teheran, 70
Germany, Berlin, 100
filters
187
19.8. uniq
With uniq you can remove duplicates from a sorted list.
paul@debian5:~/pipes$ cat music.txt 
Queen
Brel
Queen
Abba
paul@debian5:~/pipes$ sort music.txt 
Abba
Brel
Queen
Queen
paul@debian5:~/pipes$ sort music.txt |uniq
Abba
Brel
Queen
uniq can also count occurrences with the -c option.
paul@debian5:~/pipes$ sort music.txt |uniq -c
      1 Abba
      1 Brel
      2 Queen
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188
19.9. comm
Comparing streams (or files) can be done with the comm. By default comm will output
three columns. In this example, Abba, Cure and Queen are in both lists, Bowie and Sweet
are only in the first file, Turner is only in the second.
paul@debian5:~/pipes$ cat > list1.txt
Abba
Bowie
Cure
Queen
Sweet
paul@debian5:~/pipes$ cat > list2.txt
Abba
Cure
Queen
Turner
paul@debian5:~/pipes$ comm list1.txt list2.txt 
                Abba
Bowie
                Cure
                Queen
Sweet
        Turner
The output of comm can be easier to read when outputting only a single column. The digits
point out which output columns should not be displayed.
paul@debian5:~/pipes$ comm -12 list1.txt list2.txt 
Abba
Cure
Queen
paul@debian5:~/pipes$ comm -13 list1.txt list2.txt 
Turner
paul@debian5:~/pipes$ comm -23 list1.txt list2.txt 
Bowie
Sweet
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189
19.10. od
European humans like to work with ascii characters, but computers store files in bytes. The
example below creates a simple file, and then uses od to show the contents of the file in
hexadecimal bytes
paul@laika:~/test$ cat > text.txt
abcdefg
1234567
paul@laika:~/test$ od -t x1 text.txt 
0000000 61 62 63 64 65 66 67 0a 31 32 33 34 35 36 37 0a
0000020
The same file can also be displayed in octal bytes.
paul@laika:~/test$ od -b text.txt 
0000000 141 142 143 144 145 146 147 012 061 062 063 064 065 066 067 012
0000020
And here is the file in ascii (or backslashed) characters.
paul@laika:~/test$ od -c text.txt 
0000000   a   b   c   d   e   f   g  \n   1   2   3   4   5   6   7  \n
0000020
filters
190
19.11. sed
The stream editor sed can perform editing functions in the stream, using regular
expressions.
paul@debian5:~/pipes$ echo level5 | sed 's/5/42/'
level42
paul@debian5:~/pipes$ echo level5 | sed 's/level/jump/'
jump5
  
Add g for global replacements (all occurrences of the string per line).
paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/'
jump5 level7
paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/g'
jump5 jump7
  
With d you can remove lines from a stream containing a character.
paul@debian5:~/test42$ cat tennis.txt 
Venus Williams, USA
Martina Hingis, SUI
Justine Henin, BE
Serena williams, USA
Kim Clijsters, BE
Yanina Wickmayer, BE
paul@debian5:~/test42$ cat tennis.txt | sed '/BE/d'
Venus Williams, USA
Martina Hingis, SUI
Serena williams, USA
filters
191
19.12. pipe examples
19.12.1. who | wc
How many users are logged  on to this system ?
[paul@RHEL4b pipes]$ who
root     tty1         Jul 25 10:50
paul     pts/0        Jul 25 09:29 (laika)
Harry    pts/1        Jul 25 12:26 (barry)
paul     pts/2        Jul 25 12:26 (pasha)
[paul@RHEL4b pipes]$ who | wc -l
4
19.12.2. who | cut | sort
Display a sorted  list of logged on users.
[paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort
Harry
paul
paul
root
Display a sorted list of logged on users, but every user only once .
[paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort | uniq
Harry
paul
root
19.12.3. grep | cut
Display a list of all bash user accounts on this computer. Users accounts are explained in
detail later.
paul@debian5:~$ grep bash /etc/passwd
root:x:0:0:root:/root:/bin/bash
paul:x:1000:1000:paul,,,:/home/paul:/bin/bash
serena:x:1001:1001::/home/serena:/bin/bash
paul@debian5:~$ grep bash /etc/passwd | cut -d: -f1
root
paul
serena
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192
19.13. practice: filters
1. Put a sorted list of all bash users in bashusers.txt.
2. Put a sorted list of all logged on users in onlineusers.txt.
3. Make a list of all filenames in /etc that contain the string conf in their filename.
4. Make a sorted list of all files in /etc that contain the case insensitive string conf in their
filename.
5. Look at the output of /sbin/ifconfig. Write a line that displays only ip address and the
subnet mask.
6. Write a line that removes all non-letters from a stream.
7. Write a line that receives a text file, and outputs all words on a separate line.
8. Write a spell checker on the command line. (There may be a dictionary in /usr/share/
dict/ .)
filters
193
19.14. solution: filters
1. Put a sorted list of all bash users in bashusers.txt.
grep bash /etc/passwd | cut -d: -f1 | sort > bashusers.txt
2. Put a sorted list of all logged on users in onlineusers.txt.
who | cut -d' ' -f1 | sort > onlineusers.txt
3. Make a list of all filenames in /etc that contain the string conf in their filename.
ls /etc | grep conf
4. Make a sorted list of all files in /etc that contain the case insensitive string conf in their
filename.
ls /etc | grep -i conf | sort
5. Look at the output of /sbin/ifconfig. Write a line that displays only ip address and the
subnet mask.
/sbin/ifconfig | head -2 | grep 'inet ' | tr -s ' ' | cut -d' ' -f3,5
6. Write a line that removes all non-letters from a stream.
paul@deb503:~$ cat text
This is, yes really! , a text with ?&* too many str$ange# characters ;-)
paul@deb503:~$ cat text | tr -d ',!$?.*&^%#@;()-'
This is yes really  a text with  too many strange characters
 
7. Write a line that receives a text file, and outputs all words on a separate line.
paul@deb503:~$ cat text2 
it is very cold today without the sun
paul@deb503:~$ cat text2 | tr ' ' '\n'
it
is
very
cold
today
without
the
sun
 
8. Write a spell checker on the command line. (There may be a dictionary in /usr/share/
dict/ .)
paul@rhel ~$ echo "The zun is shining today" > text
paul@rhel ~$ cat > DICT
is
shining
sun
the
filters
194
today
paul@rhel ~$ cat text | tr 'A-Z ' 'a-z\n' | sort | uniq | comm -23 - DICT
zun
 
You could also add the solution from question number 6 to remove non-letters, and tr -s '
' to remove redundant spaces.
195
Chapter 20. basic Unix tools
This chapter introduces commands to find or locate files and to compress files, together
with other common tools that were not discussed before. While the tools discussed here are
technically not considered filters, they can be used in pipes.
basic Unix tools
196
20.1. find
The find command can be very useful at the start of a pipe to search for files. Here are some
examples. You might want to add 2>/dev/null to the command lines to avoid cluttering your
screen with error messages.
Find all files in /etc and put the list in etcfiles.txt
find /etc > etcfiles.txt
Find all files of the entire system and put the list in allfiles.txt
find / > allfiles.txt
Find files that end in .conf in the current directory (and all subdirs).
find . -name "*.conf"
Find files of type file (not directory, pipe or etc.) that end in .conf.
find . -type f -name "*.conf"
Find files of type directory that end in .bak .
find /data -type d -name "*.bak"
Find files that are newer than file42.txt
find . -newer file42.txt
Find can also execute another command on every file found. This example will look for
*.odf files and copy them to /backup/.
find /data -name "*.odf" -exec cp {} /backup/ \;
Find can also execute, after your confirmation, another command on every file found. This
example will remove *.odf files if you approve of it for every file found.
find /data -name "*.odf" -ok rm {} \;
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197
20.2. locate
The locate tool is very different from find in that it uses an index to locate files. This is a
lot faster than traversing all the directories, but it also means that it is always outdated. If
the index does not exist yet, then you have to create it (as root on Red Hat Enterprise Linux)
with the updatedb command.
[paul@RHEL4b ~]$ locate Samba
warning: locate: could not open database: /var/lib/slocate/slocate.db:...
warning: You need to run the 'updatedb' command (as root) to create th...
Please have a look at /etc/updatedb.conf to enable the daily cron job.
[paul@RHEL4b ~]$ updatedb 
fatal error: updatedb: You are not authorized to create a default sloc...
[paul@RHEL4b ~]$ su -
Password: 
[root@RHEL4b ~]# updatedb
[root@RHEL4b ~]#
Most Linux distributions will schedule the updatedb to run once every day.
20.3. date
The date command can display the date, time, time zone and more.
paul@rhel55 ~$ date
Sat Apr 17 12:44:30 CEST 2010
A date string can be customised to display the format of your choice. Check the man page
for more options.
paul@rhel55 ~$ date +'%A %d-%m-%Y'
Saturday 17-04-2010
Time on any Unix is calculated in number of seconds since 1969 (the first second being the
first second of the first of January 1970). Use date +%s to display Unix time in seconds.
paul@rhel55 ~$ date +%s
1271501080
When will this seconds counter reach two thousand million ?
paul@rhel55 ~$ date -d '1970-01-01 + 2000000000 seconds'
Wed May 18 04:33:20 CEST 2033
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198
20.4. cal
The cal command displays the current month, with the current day highlighted.
paul@rhel55 ~$ cal
     April 2010     
Su Mo Tu We Th Fr Sa
             1  2  3
 4  5  6  7  8  9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30
You can select any month in the past or the future.
paul@rhel55 ~$ cal 2 1970
   February 1970    
Su Mo Tu We Th Fr Sa
 1  2  3  4  5  6  7
 8  9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
20.5. sleep
The sleep command is sometimes used in scripts to wait a number of seconds. This example
shows a five second sleep.
paul@rhel55 ~$ sleep 5
paul@rhel55 ~$
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199
20.6. time
The time command can display how long it takes to execute a command. The date command
takes only a little time.
paul@rhel55 ~$ time date
Sat Apr 17 13:08:27 CEST 2010
real    0m0.014s
user    0m0.008s
sys     0m0.006s
The sleep 5 command takes five real seconds to execute, but consumes little cpu time.
paul@rhel55 ~$ time sleep 5
real    0m5.018s
user    0m0.005s
sys     0m0.011s
This bzip2 command compresses a file and uses a lot of cpu time.
paul@rhel55 ~$ time bzip2 text.txt 
real    0m2.368s
user    0m0.847s
sys     0m0.539s
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200
20.7. gzip - gunzip
Users never have enough disk space, so compression comes in handy. The gzip command
can make files take up less space.
paul@rhel55 ~$ ls -lh text.txt 
-rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt
paul@rhel55 ~$ gzip text.txt 
paul@rhel55 ~$ ls -lh text.txt.gz 
-rw-rw-r-- 1 paul paul 760K Apr 17 13:11 text.txt.gz
You can get the original back with gunzip.
paul@rhel55 ~$ gunzip text.txt.gz 
paul@rhel55 ~$ ls -lh text.txt
-rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt
20.8. zcat - zmore
Text files that are compressed with gzip can be viewed with zcat and zmore.
paul@rhel55 ~$ head -4 text.txt 
/
/opt
/opt/VBoxGuestAdditions-3.1.6
/opt/VBoxGuestAdditions-3.1.6/routines.sh
paul@rhel55 ~$ gzip text.txt 
paul@rhel55 ~$ zcat text.txt.gz | head -4
/
/opt
/opt/VBoxGuestAdditions-3.1.6
/opt/VBoxGuestAdditions-3.1.6/routines.sh
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201
20.9. bzip2 - bunzip2
Files can also be compressed with bzip2 which takes a little more time than gzip, but
compresses better.
paul@rhel55 ~$ bzip2 text.txt 
paul@rhel55 ~$ ls -lh text.txt.bz2 
-rw-rw-r-- 1 paul paul 569K Apr 17 13:11 text.txt.bz2
Files can be uncompressed again with bunzip2.
paul@rhel55 ~$ bunzip2 text.txt.bz2 
paul@rhel55 ~$ ls -lh text.txt 
-rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt
20.10. bzcat - bzmore
And in the same way bzcat and bzmore can display files compressed with bzip2.
paul@rhel55 ~$ bzip2 text.txt 
paul@rhel55 ~$ bzcat text.txt.bz2 | head -4
/
/opt
/opt/VBoxGuestAdditions-3.1.6
/opt/VBoxGuestAdditions-3.1.6/routines.sh
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202
20.11. practice: basic Unix tools
1. Explain the difference between these two commands. This question is very important. If
you don't know the answer, then look back at the shell chapter.
find /data -name "*.txt"
find /data -name *.txt
2. Explain the difference between these two statements. Will they both work when there are
200 .odf files in /data ? How about when there are 2 million .odf files ?
find /data -name "*.odf" > data_odf.txt
find /data/*.odf > data_odf.txt
3. Write a find command that finds all files created after January 30th 2010.
4. Write a find command that finds all *.odf files created in September 2009.
5. Count the number of *.conf files in /etc and all its subdirs.
6. Here are two commands that do the same thing: copy *.odf files to /backup/ . What would
be a reason to replace the first command with the second ? Again, this is an important
question.
cp -r /data/*.odf /backup/
find /data -name "*.odf" -exec cp {} /backup/ \;
7. Create a file called loctest.txt. Can you find this file with locate ? Why not ? How do
you make locate find this file ?
8. Use find and -exec to rename all .htm files to .html.
9. Issue the date command. Now display the date in YYYY/MM/DD format.
10. Issue the cal command. Display a calendar of 1582 and 1752. Notice anything special ?
basic Unix tools
203
20.12. solution: basic Unix tools
1. Explain the difference between these two commands. This question is very important. If
you don't know the answer, then look back at the shell chapter.
find /data -name "*.txt"
find /data -name *.txt
When *.txt is quoted then the shell will not touch it. The find tool will look in the /data
for all files ending in .txt.
When *.txt is not quoted then the shell might expand this (when one or more files that ends
in .txt exist in the current directory). The find might show a different result, or can result
in a syntax error.
2. Explain the difference between these two statements. Will they both work when there are
200 .odf files in /data ? How about when there are 2 million .odf files ?
find /data -name "*.odf" > data_odf.txt
find /data/*.odf > data_odf.txt
The first find will output all .odf filenames in /data and all subdirectories. The shell will
redirect this to a file.
The second find will output all files named .odf in /data and will also output all files that
exist in directories named *.odf (in /data).
With two million files the command line would be expanded beyond the maximum that the
shell can accept. The last part of the command line would be lost.
3. Write a find command that finds all files created after January 30th 2010.
touch -t 201001302359 marker_date
find . -type f -newer marker_date 
There is another solution :
find . -type f -newerat "20100130 23:59:59"
4. Write a find command that finds all *.odf files created in September 2009.
touch -t 200908312359 marker_start
touch -t 200910010000 marker_end
find . -type f -name "*.odf" -newer marker_start ! -newer marker_end
The exclamation mark ! -newer can be read as not newer.
5. Count the number of *.conf files in /etc and all its subdirs.
find /etc -type f -name '*.conf' | wc -l
6. Here are two commands that do the same thing: copy *.odf files to /backup/ . What would
be a reason to replace the first command with the second ? Again, this is an important
question.
cp -r /data/*.odf /backup/
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204
find /data -name "*.odf" -exec cp {} /backup/ \;
The first might fail when there are too many files to fit on one command line.
7. Create a file called loctest.txt. Can you find this file with locate ? Why not ? How do
you make locate find this file ?
You cannot locate this with locate because it is not yet in the index.
updatedb
8. Use find and -exec to rename all .htm files to .html.
paul@rhel55 ~$ find . -name '*.htm'
./one.htm
./two.htm
paul@rhel55 ~$ find . -name '*.htm' -exec mv {} {}l \;
paul@rhel55 ~$ find . -name '*.htm*'
./one.html
./two.html
9. Issue the date command. Now display the date in YYYY/MM/DD format.
date +%Y/%m/%d
10. Issue the cal command. Display a calendar of 1582 and 1752. Notice anything special ?
cal 1582
The calendars are different depending on the country. Check http://linux-training.be/files/
studentfiles/dates.txt
205
Chapter 21. regular expressions
Regular expressions are a very powerful tool in Linux. They can be used with a variety of
programs like bash, vi, rename, grep, sed, and more.
This chapter introduces you to the basics of regular expressions.
regular expressions
206
21.1. regex versions
There are three different versions of regular expression syntax:
BRE: Basic Regular Expressions
ERE: Extended Regular Expressions
PRCE: Perl Regular Expressions
Depending on the tool being used, one or more of these syntaxes can be used.
For example the grep tool has the -E option to force a string to be read as ERE while -G
forces BRE and -P forces PRCE.
Note that grep also has -F to force the string to be read literally.
The sed tool also has options to choose a regex syntax.
Read the manual of the tools you use!
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21.2. grep
21.2.1. print lines matching a pattern
grep is a popular Linux tool to search for lines that match a certain pattern. Below are some
examples of the simplest regular expressions.
This is the contents of the test file. This file contains three lines (or three newline characters).
paul@rhel65:~$ cat names
Tania
Laura
Valentina
When grepping for a single character, only the lines containing that character are returned.
paul@rhel65:~$ grep u names
Laura
paul@rhel65:~$ grep e names
Valentina
paul@rhel65:~$ grep i names
Tania
Valentina
The pattern matching in this example should be very straightforward; if the given character
occurs on a line, then grep will return that line.
21.2.2. concatenating characters
Two concatenated characters will have to be concatenated in the same way to have a match.
This example demonstrates that ia will match Tania but not Valentina and in will match
Valentina but not Tania.
paul@rhel65:~$ grep a names
Tania
Laura
Valentina
paul@rhel65:~$ grep ia names
Tania
paul@rhel65:~$ grep in names
Valentina
paul@rhel65:~$
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21.2.3. one or the other
PRCE and ERE both use the pipe symbol to signify OR. In this example we grep for lines
containing the letter i or the letter a.
paul@debian7:~$ cat list 
Tania
Laura
paul@debian7:~$ grep -E 'i|a' list 
Tania
Laura
Note that we use the -E switch of grep to force interpretion of our string as an ERE.
We need to escape the pipe symbol in a BRE to get the same logical OR.
paul@debian7:~$ grep -G 'i|a' list 
paul@debian7:~$ grep -G 'i\|a' list 
Tania
Laura
21.2.4. one or more
The * signifies zero, one or more occurences of the previous and the + signifies one or more
of the previous.
paul@debian7:~$ cat list2
ll
lol
lool
loool
paul@debian7:~$ grep -E 'o*' list2
ll
lol
lool
loool
paul@debian7:~$ grep -E 'o+' list2
lol
lool
loool
paul@debian7:~$
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21.2.5. match the end of a string
For the following examples, we will use this file.
paul@debian7:~$ cat names 
Tania
Laura
Valentina
Fleur
Floor
The two examples below show how to use the dollar character to match the end of a string.
paul@debian7:~$ grep a$ names 
Tania
Laura
Valentina
paul@debian7:~$ grep r$ names 
Fleur
Floor
21.2.6. match the start of a string
The caret character (^) will match a string at the start (or the beginning) of a line.
Given the same file as above, here are two examples.
paul@debian7:~$ grep ^Val names 
Valentina
paul@debian7:~$ grep ^F names 
Fleur
Floor
Both the dollar sign and the little hat are called anchors in a regex.
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21.2.7. separating words
Regular expressions use a \b sequence to reference a word separator. Take for example this
file:
paul@debian7:~$ cat text
The governer is governing.
The winter is over.
Can you get over there?
Simply grepping for over will give too many results.
paul@debian7:~$ grep over text
The governer is governing.
The winter is over.
Can you get over there?
Surrounding the searched word with spaces is not a good solution (because other characters
can be word separators). This screenshot below show how to use \b to find only the searched
word:
paul@debian7:~$ grep '\bover\b' text
The winter is over.
Can you get over there?
paul@debian7:~$
Note that grep also has a -w option to grep for words.
paul@debian7:~$ cat text 
The governer is governing.
The winter is over.
Can you get over there?
paul@debian7:~$ grep -w over text
The winter is over.
Can you get over there?
paul@debian7:~$ 
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21.2.8. grep features
Sometimes it is easier to combine a simple regex with grep options, than it is to write a more
complex regex. These options where discussed before:
grep -i
grep -v
grep -w
grep -A5
grep -B5
grep -C5
21.2.9. preventing shell expansion of a regex
The dollar sign is a special character, both for the regex and also for the shell (remember
variables and embedded shells). Therefore it is advised to always quote the regex, this
prevents shell expansion.
paul@debian7:~$ grep 'r$' names 
Fleur
Floor
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21.3. rename
21.3.1. the rename command
On Debian Linux the /usr/bin/rename command is a link to /usr/bin/prename installed by
the perl package.
paul@pi ~ $ dpkg -S $(readlink -f $(which rename))
perl: /usr/bin/prename
Red Hat derived systems do not install the same rename command, so this section does not
describe rename on Red Hat (unless you copy the perl script manually).
There is often confusion on the internet about the rename command because solutions
that work fine in Debian (and Ubuntu, xubuntu, Mint, ...) cannot be used in Red Hat
(and CentOS, Fedora, ...).
21.3.2. perl
The rename command is actually a perl script that uses perl regular expressions. The
complete manual for these can be found by typing perldoc perlrequick (after installing
perldoc).
root@pi:~# aptitude install perl-doc
The following NEW packages will be installed:
  perl-doc
0 packages upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 8,170 kB of archives. After unpacking 13.2 MB will be used.
Get: 1 http://mirrordirector.raspbian.org/raspbian/ wheezy/main perl-do...
Fetched 8,170 kB in 19s (412 kB/s)
Selecting previously unselected package perl-doc.
(Reading database ... 67121 files and directories currently installed.)
Unpacking perl-doc (from .../perl-doc_5.14.2-21+rpi2_all.deb) ...
Adding 'diversion of /usr/bin/perldoc to /usr/bin/perldoc.stub by perl-doc'
Processing triggers for man-db ...
Setting up perl-doc (5.14.2-21+rpi2) ...
root@pi:~# perldoc perlrequick
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21.3.3. well known syntax
The most common use of the rename is to search for filenames matching a certain string
and replacing this string with an other string.
This is often presented as s/string/other string/ as seen in this example:
paul@pi ~ $ ls
abc       allfiles.TXT  bllfiles.TXT  Scratch   tennis2.TXT
abc.conf  backup        cllfiles.TXT  temp.TXT  tennis.TXT
paul@pi ~ $ rename 's/TXT/text/' *
paul@pi ~ $ ls
abc       allfiles.text  bllfiles.text  Scratch    tennis2.text
abc.conf  backup         cllfiles.text  temp.text  tennis.text
And here is another example that uses rename with the well know syntax to change the
extensions of the same files once more:
paul@pi ~ $ ls
abc       allfiles.text  bllfiles.text  Scratch    tennis2.text
abc.conf  backup         cllfiles.text  temp.text  tennis.text
paul@pi ~ $ rename 's/text/txt/' *.text
paul@pi ~ $ ls
abc       allfiles.txt  bllfiles.txt  Scratch   tennis2.txt
abc.conf  backup        cllfiles.txt  temp.txt  tennis.txt
paul@pi ~ $
These two examples appear to work because the strings we used only exist at the end of the
filename. Remember that file extensions have no meaning in the bash shell.
The next example shows what can go wrong with this syntax.
paul@pi ~ $ touch atxt.txt
paul@pi ~ $ rename 's/txt/problem/' atxt.txt
paul@pi ~ $ ls
abc       allfiles.txt  backup        cllfiles.txt  temp.txt     tennis.txt
abc.conf  aproblem.txt  bllfiles.txt  Scratch       tennis2.txt
paul@pi ~ $
Only the first occurrence of the searched string is replaced.
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21.3.4. a global replace
The syntax used in the previous example can be described as s/regex/replacement/. This
is simple and straightforward, you enter a regex between the first two slashes and a
replacement string between the last two.
This example expands this syntax only a little, by adding a modifier.
paul@pi ~ $ rename -n 's/TXT/txt/g' aTXT.TXT
aTXT.TXT renamed as atxt.txt
paul@pi ~ $
The syntax we use now can be described as s/regex/replacement/g where s signifies switch
and g stands for global.
Note that this example used the -n switch to show what is being done (instead of actually
renaming the file).
21.3.5. case insensitive replace
Another modifier that can be useful is i. this example shows how to replace a case insensitive
string with another string.
paul@debian7:~/files$ ls
file1.text  file2.TEXT  file3.txt
paul@debian7:~/files$ rename 's/.text/.txt/i' *
paul@debian7:~/files$ ls
file1.txt  file2.txt  file3.txt
paul@debian7:~/files$ 
21.3.6. renaming extensions
Command line Linux has no knowledge of MS-DOS like extensions, but many end users
and graphical application do use them.
Here is an example on how to use rename to only rename the file extension. It uses the
dollar sign to mark the ending of the filename.
paul@pi ~ $ ls *.txt
allfiles.txt  bllfiles.txt  cllfiles.txt  really.txt.txt  temp.txt  tennis.txt
paul@pi ~ $ rename 's/.txt$/.TXT/' *.txt
paul@pi ~ $ ls *.TXT
allfiles.TXT  bllfiles.TXT    cllfiles.TXT    really.txt.TXT
temp.TXT      tennis.TXT
paul@pi ~ $
Note that the dollar sign in the regex means at the end. Without the dollar sign this
command would fail on the really.txt.txt file.
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21.4. sed
21.4.1. stream editor
The stream editor or short sed uses regex for stream editing.
In this example sed is used to replace a string.
echo Sunday | sed 's/Sun/Mon/'
Monday
The slashes can be replaced by a couple of other characters, which can be handy in some
cases to improve readability.
echo Sunday | sed 's:Sun:Mon:'
Monday
echo Sunday | sed 's_Sun_Mon_'
Monday
echo Sunday | sed 's|Sun|Mon|'
Monday
21.4.2. interactive editor
While sed is meant to be used in a stream, it can also be used interactively on a file.
paul@debian7:~/files$ echo Sunday > today
paul@debian7:~/files$ cat today 
Sunday
paul@debian7:~/files$ sed -i 's/Sun/Mon/' today
paul@debian7:~/files$ cat today 
Monday
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21.4.3. simple back referencing
The ampersand character can be used to reference the searched (and found) string.
In this example the ampersand is used to double the occurence of the found string.
echo Sunday | sed 's/Sun/&&/'
SunSunday
echo Sunday | sed 's/day/&&/'
Sundayday
21.4.4. back referencing
Parentheses (often called round brackets) are used to group sections of the regex so they
can leter be referenced.
Consider this simple example:
paul@debian7:~$ echo Sunday | sed 's_\(Sun\)_\1ny_'
Sunnyday
paul@debian7:~$ echo Sunday | sed 's_\(Sun\)_\1ny \1_'
Sunny Sunday
21.4.5. a dot for any character
In a regex a simple dot can signify any character.
paul@debian7:~$ echo 2014-04-01 | sed 's/....-..-../YYYY-MM-DD/'
YYYY-MM-DD
paul@debian7:~$ echo abcd-ef-gh | sed 's/....-..-../YYYY-MM-DD/'
YYYY-MM-DD
21.4.6. multiple back referencing
When more than one pair of parentheses is used, each of them can be referenced separately
by consecutive numbers.
paul@debian7:~$ echo 2014-04-01 | sed 's/\(....\)-\(..\)-\(..\)/\1+\2+\3/'
2014+04+01
paul@debian7:~$ echo 2014-04-01 | sed 's/\(....\)-\(..\)-\(..\)/\3:\2:\1/'
01:04:2014
This feature is called grouping.
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21.4.7. white space
The \s can refer to white space such as a space or a tab.
This example looks for white spaces (\s) globally and replaces them with 1 space.
paul@debian7:~$ echo -e 'today\tis\twarm'
today   is      warm
paul@debian7:~$ echo -e 'today\tis\twarm' | sed 's_\s_ _g'
today is warm
21.4.8. optional occurrence
A question mark signifies that the previous is optional.
The example below searches for three consecutive letter o, but the third o is optional.
paul@debian7:~$ cat list2
ll
lol
lool
loool
paul@debian7:~$ grep -E 'ooo?' list2
lool
loool
paul@debian7:~$ cat list2 | sed 's/ooo\?/A/'
ll
lol
lAl
lAl
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21.4.9. exactly n times
You can demand an exact number of times the oprevious has to occur.
This example wants exactly three o's.
paul@debian7:~$ cat list2
ll
lol
lool
loool
paul@debian7:~$ grep -E 'o{3}' list2
loool
paul@debian7:~$ cat list2 | sed 's/o\{3\}/A/'
ll
lol
lool
lAl
paul@debian7:~$
21.4.10. between n and m times
And here we demand exactly from minimum 2 to maximum 3 times.
paul@debian7:~$ cat list2
ll
lol
lool
loool
paul@debian7:~$ grep -E 'o{2,3}' list2
lool
loool
paul@debian7:~$ grep 'o\{2,3\}' list2
lool
loool
paul@debian7:~$ cat list2 | sed 's/o\{2,3\}/A/'
ll
lol
lAl
lAl
paul@debian7:~$
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219
21.5. bash history
The bash shell can also interprete some regular expressions.
This example shows how to manipulate the exclamation mask history feature of the bash
shell.
paul@debian7:~$ mkdir hist
paul@debian7:~$ cd hist/
paul@debian7:~/hist$ touch file1 file2 file3
paul@debian7:~/hist$ ls -l file1
-rw-r--r-- 1 paul paul 0 Apr 15 22:07 file1
paul@debian7:~/hist$ !l
ls -l file1
-rw-r--r-- 1 paul paul 0 Apr 15 22:07 file1
paul@debian7:~/hist$ !l:s/1/3
ls -l file3
-rw-r--r-- 1 paul paul 0 Apr 15 22:07 file3
paul@debian7:~/hist$
This also works with the history numbers in bash.
paul@debian7:~/hist$ history 6
 2089  mkdir hist
 2090  cd hist/
 2091  touch file1 file2 file3
 2092  ls -l file1
 2093  ls -l file3
 2094  history 6
paul@debian7:~/hist$ !2092
ls -l file1
-rw-r--r-- 1 paul paul 0 Apr 15 22:07 file1
paul@debian7:~/hist$ !2092:s/1/2
ls -l file2
-rw-r--r-- 1 paul paul 0 Apr 15 22:07 file2
paul@debian7:~/hist$
Part VI. vi
221
Table of Contents
22. Introduction to vi ..........................................................................................................  222
22.1. command mode and insert mode .............................................................................. 223
22.2. start typing (a A i I o O) ........................................................................................  223
22.3. replace and delete a character (r x X) .......................................................................  224
22.4. undo and repeat (u .) .............................................................................................  224
22.5. cut, copy and paste a line (dd yy p P) .......................................................................  224
22.6. cut, copy and paste lines (3dd 2yy) ..........................................................................  225
22.7. start and end of a line (0 or ^ and $) ........................................................................  225
22.8. join two lines (J) and more .....................................................................................  225
22.9. words (w b) .........................................................................................................  226
22.10. save (or not) and exit (:w :q :q! ) ............................................................................  226
22.11. Searching (/ ?) ....................................................................................................  226
22.12. replace all ( :1,$ s/foo/bar/g ) .................................................................................  227
22.13. reading files (:r :r !cmd) .......................................................................................  227
22.14. text buffers ........................................................................................................  227
22.15. multiple files ......................................................................................................  227
22.16. abbreviations ......................................................................................................  228
22.17. key mappings .....................................................................................................  229
22.18. setting options ....................................................................................................  229
22.19. practice: vi(m) ....................................................................................................  230
22.20. solution: vi(m) ....................................................................................................  231
222
Chapter 22. Introduction to vi
The vi editor is installed on almost every Unix. Linux will very often install vim (vi
improved) which is similar. Every system administrator should know vi(m), because it is
an easy tool to solve problems.
The vi editor is not intuitive, but once you get to know it, vi becomes a very powerful
application. Most Linux distributions will include the vimtutor which is a 45 minute lesson
in vi(m).
Introduction to vi
223
22.1. command mode and insert mode
The vi editor starts in command mode. In command mode, you can type commands. Some
commands will bring you into insert mode. In insert mode, you can type text. The escape
key will return you to command mode.
Table 22.1. getting to command mode
key
action
Esc
set vi(m) in command mode.
22.2. start typing (a A i I o O)
The difference between a A i I o and O is the location where you can start typing. a will
append after the current character and A will append at the end of the line. i will insert before
the current character and I will insert at the beginning of the line. o will put you in a new
line after the current line and O will put you in a new line before the current line.
Table 22.2. switch to insert mode
command action
a
start typing after the current character
A
start typing at the end of the current line
i
start typing before the current character
I
start typing at the start of the current line
o
start typing on a new line after the current line
O
start typing on a new line before the current line
Introduction to vi
224
22.3. replace and delete a character (r x X)
When in command mode (it doesn't hurt to hit the escape key more than once) you can use
the x key to delete the current character. The big X key (or shift x) will delete the character
left of the cursor. Also when in command mode, you can use the r key to replace one single
character. The r key will bring you in insert mode for just one key press, and will return you
immediately to command mode.
Table 22.3. replace and delete
command
action
x
delete the character below the cursor
X
delete the character before the cursor
r
replace the character below the cursor
p
paste after the cursor (here the last deleted character)
xp
switch two characters
22.4. undo and repeat (u .)
When in command mode, you can undo your mistakes with u. You can do your mistakes
twice with . (in other words, the . will repeat your last command).
Table 22.4. undo and repeat
command action
u
undo the last action
.
repeat the last action
22.5. cut, copy and paste a line (dd yy p P)
When in command mode, dd will cut the current line. yy will copy the current line. You can
paste the last copied or cut line after (p) or before (P) the current line.
Table 22.5. cut, copy and paste a line
command action
dd
cut the current line
yy
(yank yank) copy the current line
p
paste after the current line
P
paste before the current line
Introduction to vi
225
22.6. cut, copy and paste lines (3dd 2yy)
When in command mode, before typing dd or yy, you can type a number to repeat the
command a number of times. Thus, 5dd will cut 5 lines and 4yy will copy (yank) 4 lines.
That last one will be noted by vi in the bottom left corner as "4 line yanked".
Table 22.6. cut, copy and paste lines
command action
3dd
cut three lines
4yy
copy four lines
22.7. start and end of a line (0 or ^ and $)
When in command mode, the 0 and the caret ^ will bring you to the start of the current line,
whereas the $ will put the cursor at the end of the current line. You can add 0 and $ to the d
command, d0 will delete every character between the current character and the start of the
line. Likewise d$ will delete everything from the current character till the end of the line.
Similarly y0 and y$ will yank till start and end of the current line.
Table 22.7. start and end of line
command action
0
jump to start of current line
^
jump to start of current line
$
jump to end of current line
d0
delete until start of line
d$
delete until end of line
22.8. join two lines (J) and more
When in command mode, pressing J will append the next line to the current line. With yyp
you duplicate a line and with ddp you switch two lines.
Table 22.8. join two lines
command action
J
join two lines
yyp
duplicate a line
ddp
switch two lines
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226
22.9. words (w b)
When in command mode, w will jump to the next word and b will move to the previous
word. w and b can also be combined with d and y to copy and cut words (dw db yw yb).
Table 22.9. words
command action
w
forward one word
b
back one word
3w
forward three words
dw
delete one word
yw
yank (copy) one word
5yb
yank five words back
7dw
delete seven words
22.10. save (or not) and exit (:w :q :q! )
Pressing the colon : will allow you to give instructions to vi (technically speaking, typing
the colon will open the ex editor). :w will write (save) the file, :q will quit an unchanged
file without saving, and :q! will quit vi discarding any changes. :wq will save and quit and
is the same as typing ZZ in command mode.
Table 22.10. save and exit vi
command
action
:w
save (write)
:w fname
save as fname
:q
quit
:wq
save and quit
ZZ
save and quit
:q!
quit (discarding your changes)
:w!
save (and write to non-writable file!)
The last one is a bit special. With :w! vi will try to chmod the file to get write permission
(this works when you are the owner) and will chmod it back when the write succeeds. This
should always work when you are root (and the file system is writable).
22.11. Searching (/ ?)
When in command mode typing / will allow you to search in vi for strings (can be a regular
expression). Typing /foo will do a forward search for the string foo and typing ?bar will do
a backward search for bar.
Table 22.11. searching
command
action
/string
forward search for string
Introduction to vi
227
command
action
?string
backward search for string
n
go to next occurrence of search string
/^string
forward search string at beginning of line
/string$
forward search string at end of line
/br[aeio]l
search for bral brel bril and brol
/\<he\>
search for the word he (and not for here or the)
22.12. replace all ( :1,$ s/foo/bar/g )
To replace all occurrences of the string foo with bar, first switch to ex mode with : . Then
tell vi which lines to use, for example 1,$ will do the replace all from the first to the last
line. You can write 1,5 to only process the first five lines. The s/foo/bar/g will replace all
occurrences of foo with bar.
Table 22.12. replace
command
action
:4,8 s/foo/bar/g
replace foo with bar on lines 4 to 8
:1,$ s/foo/bar/g
replace foo with bar on all lines
22.13. reading files (:r :r !cmd)
When in command mode, :r foo will read the file named foo, :r !foo will execute the
command foo. The result will be put at the current location. Thus :r !ls will put a listing of
the current directory in your text file.
Table 22.13. read files and input
command action
:r fname
(read) file fname and paste contents
:r !cmd
execute cmd and paste its output
22.14. text buffers
There are 36 buffers in vi to store text. You can use them with the " character.
Table 22.14. text buffers
command action
"add
delete current line and put text in buffer a
"g7yy
copy seven lines into buffer g
"ap
paste from buffer a
22.15. multiple files
You can edit multiple files with vi. Here are some tips.
Introduction to vi
228
Table 22.15. multiple files
command
action
vi file1 file2 file3
start editing three files
:args
lists files and marks active file
:n
start editing the next file
:e
toggle with last edited file
:rew
rewind file pointer to first file
22.16. abbreviations
With :ab you can put abbreviations in vi. Use :una to undo the abbreviation.
Table 22.16. abbreviations
command
action
:ab str long string
abbreviate str to be 'long string'
:una str
un-abbreviate str
Introduction to vi
229
22.17. key mappings
Similarly to their abbreviations, you can use mappings with :map for command mode and
:map! for insert mode.
This example shows how to set the F6 function key to toggle between set number and set
nonumber. The <bar> separates the two commands, set number! toggles the state and set
number? reports the current state.
:map <F6> :set number!<bar>set number?<CR>
22.18. setting options
Some options that you can set in vim.
:set number  ( also try :se nu )
:set nonumber
:syntax on
:syntax off
:set all  (list all options)
:set tabstop=8
:set tx   (CR/LF style endings)
:set notx
 
You can set these options (and much more) in ~/.vimrc for vim or in ~/.exrc for standard vi.
paul@barry:~$ cat ~/.vimrc
set number
set tabstop=8
set textwidth=78
map <F6> :set number!<bar>set number?<CR>
paul@barry:~$
 
Introduction to vi
230
22.19. practice: vi(m)
1. Start the vimtutor and do some or all of the exercises. You might need to run aptitude
install vim on xubuntu.
2. What 3 key sequence in command mode will duplicate the current line.
3. What 3 key sequence in command mode will switch two lines' place (line five becomes
line six and line six becomes line five).
4. What 2 key sequence in command mode will switch a character's place with the next one.
5. vi can understand macro's. A macro can be recorded with q followed by the name of
the macro. So qa will record the macro named a. Pressing q again will end the recording.
You can recall the macro with @ followed by the name of the macro. Try this example: i 1
'Escape Key' qa yyp 'Ctrl a' q 5@a (Ctrl a will increase the number with one).
6. Copy /etc/passwd to your ~/passwd. Open the last one in vi and press Ctrl v. Use the arrow
keys to select a Visual Block, you can copy this with y or delete it with d. Try pasting it.
7. What does dwwP do when you are at the beginning of a word in a sentence ?
Introduction to vi
231
22.20. solution: vi(m)
1. Start the vimtutor and do some or all of the exercises. You might need to run aptitude
install vim on xubuntu.
vimtutor
2. What 3 key sequence in command mode will duplicate the current line.
yyp
3. What 3 key sequence in command mode will switch two lines' place (line five becomes
line six and line six becomes line five).
ddp
4. What 2 key sequence in command mode will switch a character's place with the next one.
xp
5. vi can understand macro's. A macro can be recorded with q followed by the name of
the macro. So qa will record the macro named a. Pressing q again will end the recording.
You can recall the macro with @ followed by the name of the macro. Try this example: i 1
'Escape Key' qa yyp 'Ctrl a' q 5@a (Ctrl a will increase the number with one).
6. Copy /etc/passwd to your ~/passwd. Open the last one in vi and press Ctrl v. Use the arrow
keys to select a Visual Block, you can copy this with y or delete it with d. Try pasting it.
cp /etc/passwd ~
vi passwd
(press Ctrl-V)
7. What does dwwP do when you are at the beginning of a word in a sentence ?
dwwP can switch the current word with the next word.
Part VII. scripting
233
Table of Contents
23. scripting introduction .....................................................................................................  234
23.1. prerequisites .........................................................................................................  235
23.2. hello world ..........................................................................................................  235
23.3. she-bang ..............................................................................................................  235
23.4. comment .............................................................................................................  236
23.5. variables ..............................................................................................................  236
23.6. sourcing a script ...................................................................................................  236
23.7. troubleshooting a script ..........................................................................................  237
23.8. prevent setuid root spoofing ....................................................................................  237
23.9. practice: introduction to scripting .............................................................................  238
23.10. solution: introduction to scripting ...........................................................................  239
24. scripting loops ...............................................................................................................  240
24.1. test [ ] .................................................................................................................  241
24.2. if then else ..........................................................................................................  242
24.3. if then elif ...........................................................................................................  242
24.4. for loop ...............................................................................................................  242
24.5. while loop ...........................................................................................................  243
24.6. until loop ............................................................................................................  243
24.7. practice: scripting tests and loops .............................................................................  244
24.8. solution: scripting tests and loops ............................................................................  245
25. scripting parameters ......................................................................................................  247
25.1. script parameters ...................................................................................................  248
25.2. shift through parameters .........................................................................................  249
25.3. runtime input .......................................................................................................  249
25.4. sourcing a config file ............................................................................................. 250
25.5. get script options with getopts .................................................................................  251
25.6. get shell options with shopt ....................................................................................  252
25.7. practice: parameters and options ..............................................................................  253
25.8. solution: parameters and options ..............................................................................  254
26. more scripting ...............................................................................................................  255
26.1. eval ....................................................................................................................  256
26.2. (( )) ....................................................................................................................  256
26.3. let ......................................................................................................................  257
26.4. case ....................................................................................................................  258
26.5. shell functions ......................................................................................................  259
26.6. practice : more scripting .........................................................................................  260
26.7. solution : more scripting .........................................................................................  261
234
Chapter 23. scripting introduction
Shells like bash and Korn have support for programming constructs that can be saved as
scripts. These scripts in turn then become more shell commands. Many Linux commands
are scripts. User profile scripts are run when a user logs on and init scripts are run when
a daemon is stopped or started.
This means that system administrators also need basic knowledge of scripting to understand
how their servers and their applications are started, updated, upgraded, patched, maintained,
configured and removed, and also to understand how a user environment is built.
The goal of this chapter is to give you enough information to be able to read and understand
scripts. Not to become a writer of complex scripts.
scripting introduction
235
23.1. prerequisites
You should have read and understood part III shell expansion and part IV pipes and
commands before starting this chapter.
23.2. hello world
Just like in every programming course, we start with a simple hello_world script. The
following script will output Hello World.
echo Hello World
After creating this simple script in vi or with echo, you'll have to chmod +x hello_world
to make it executable. And unless you add the scripts directory to your path, you'll have to
type the path to the script for the shell to be able to find it.
[paul@RHEL4a ~]$ echo echo Hello World > hello_world
[paul@RHEL4a ~]$ chmod +x hello_world 
[paul@RHEL4a ~]$ ./hello_world 
Hello World
[paul@RHEL4a ~]$
23.3. she-bang
Let's expand our example a little further by putting #!/bin/bash on the first line of the script.
The #! is called a she-bang (sometimes called sha-bang), where the she-bang is the first
two characters of the script.
#!/bin/bash
echo Hello World
You can never be sure which shell a user is running. A script that works flawlessly in bash
might not work in ksh, csh, or dash. To instruct a shell to run your script in a certain shell,
you can start your script with a she-bang followed by the shell it is supposed to run in. This
script will run in a bash shell.
#!/bin/bash
echo -n hello
echo A bash subshell `echo -n hello`
  
This script will run in a Korn shell (unless /bin/ksh is a hard link to /bin/bash). The /etc/
shells file contains a list of shells on your system.
#!/bin/ksh
echo -n hello
echo a Korn subshell `echo -n hello`
  
scripting introduction
236
23.4. comment
Let's expand our example a little further by adding comment lines.
#!/bin/bash
#
# Hello World Script
#
echo Hello World
23.5. variables
Here is a simple example of a variable inside a script.
#!/bin/bash
#
# simple variable in script
#
var1=4
echo var1 = $var1
Scripts can contain variables, but since scripts are run in their own shell, the variables do
not survive the end of the script.
[paul@RHEL4a ~]$ echo $var1
[paul@RHEL4a ~]$ ./vars
var1 = 4
[paul@RHEL4a ~]$ echo $var1
[paul@RHEL4a ~]$
23.6. sourcing a script
Luckily, you can force a script to run in the same shell; this is called sourcing a script.
[paul@RHEL4a ~]$ source ./vars
var1 = 4
[paul@RHEL4a ~]$ echo $var1
4
[paul@RHEL4a ~]$ 
  
The above is identical to the below.
[paul@RHEL4a ~]$ . ./vars
var1 = 4
[paul@RHEL4a ~]$ echo $var1
4
[paul@RHEL4a ~]$ 
  
scripting introduction
237
23.7. troubleshooting a script
Another way to run a script in a separate shell is by typing bash with the name of the script
as a parameter.
paul@debian6~/test$ bash runme
42
Expanding this to bash -x allows you to see the commands that the shell is executing (after
shell expansion).
paul@debian6~/test$ bash -x runme
+ var4=42
+ echo 42
42
paul@debian6~/test$ cat runme
# the runme script
var4=42
echo $var4
paul@debian6~/test$
Notice the absence of the commented (#) line, and the replacement of the variable before
execution of echo.
23.8. prevent setuid root spoofing
Some user may try to perform setuid based script root spoofing. This is a rare but possible
attack. To improve script security and to avoid interpreter spoofing, you need to add -- after
the #!/bin/bash, which disables further option processing so the shell will not accept any
options.
#!/bin/bash -
or
#!/bin/bash --
Any arguments after the -- are treated as filenames and arguments. An argument of - is
equivalent to --.
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238
23.9. practice: introduction to scripting
0. Give each script a different name, keep them for later!
1. Write a script that outputs the name of a city.
2. Make sure the script runs in the bash shell.
3. Make sure the script runs in the Korn shell.
4. Create a script that defines two variables, and outputs their value.
5. The previous script does not influence your current shell (the variables do not exist outside
of the script). Now run the script so that it influences your current shell.
6. Is there a shorter way to source the script ?
7. Comment your scripts so that you know what they are doing.
scripting introduction
239
23.10. solution: introduction to scripting
0. Give each script a different name, keep them for later!
1. Write a script that outputs the name of a city.
$ echo 'echo Antwerp' > first.bash
$ chmod +x first.bash 
$ ./first.bash 
Antwerp
2. Make sure the script runs in the bash shell.
$ cat first.bash
#!/bin/bash
echo Antwerp
3. Make sure the script runs in the Korn shell.
$ cat first.bash
#!/bin/ksh
echo Antwerp
Note that while first.bash will technically work as a Korn shell script, the name ending
in .bash is confusing.
4. Create a script that defines two variables, and outputs their value.
$ cat second.bash
#!/bin/bash
var33=300
var42=400
echo $var33 $var42
5. The previous script does not influence your current shell (the variables do not exist outside
of the script). Now run the script so that it influences your current shell.
source second.bash
6. Is there a shorter way to source the script ?
. ./second.bash
7. Comment your scripts so that you know what they are doing.
$ cat second.bash
#!/bin/bash
# script to test variables and sourcing
# define two variables
var33=300
var42=400
# output the value of these variables
echo $var33 $var42
240
Chapter 24. scripting loops
scripting loops
241
24.1. test [ ]
The test command can test whether something is true or false. Let's start by testing whether
10 is greater than 55.
[paul@RHEL4b ~]$ test 10 -gt 55 ; echo $?
1
[paul@RHEL4b ~]$
The test command returns 1 if the test fails. And as you see in the next screenshot, test returns
0 when a test succeeds.
[paul@RHEL4b ~]$ test 56 -gt 55 ; echo $?
0
[paul@RHEL4b ~]$
If you prefer true and false, then write the test like this.
[paul@RHEL4b ~]$ test 56 -gt 55 && echo true || echo false
true
[paul@RHEL4b ~]$ test 6 -gt 55 && echo true || echo false
false
The test command can also be written as square brackets, the screenshot below is identical
to the one above.
[paul@RHEL4b ~]$ [ 56 -gt 55 ] && echo true || echo false
true
[paul@RHEL4b ~]$ [ 6 -gt 55 ] && echo true || echo false
false
Below are some example tests. Take a look at man test to see more options for tests.
[ -d foo ]             Does the directory foo exist ?
[ -e bar ]             Does the file bar exist ?
[ '/etc' = $PWD ]      Is the string /etc equal to the variable $PWD ?
[ $1 != 'secret' ]     Is the first parameter different from secret ?
[ 55 -lt $bar ]        Is 55 less than the value of $bar ?
[ $foo -ge 1000 ]      Is the value of $foo greater or equal to 1000 ?
[ "abc" < $bar ]       Does abc sort before the value of $bar ?
[ -f foo ]             Is foo a regular file ?
[ -r bar ]             Is bar a readable file ?
[ foo -nt bar ]        Is file foo newer than file bar ?
[ -o nounset ]         Is the shell option nounset set ?
Tests can be combined with logical AND and OR.
paul@RHEL4b:~$ [ 66 -gt 55 -a 66 -lt 500 ] && echo true || echo false
true
paul@RHEL4b:~$ [ 66 -gt 55 -a 660 -lt 500 ] && echo true || echo false
false
paul@RHEL4b:~$ [ 66 -gt 55 -o 660 -lt 500 ] && echo true || echo false
true
scripting loops
242
24.2. if then else
The if then else construction is about choice. If a certain condition is met, then execute
something, else execute something else. The example below tests whether a file exists, and
if the file exists then a proper message is echoed.
#!/bin/bash
if [ -f isit.txt ]
then echo isit.txt exists!
else echo isit.txt not found!
fi
If we name the above script 'choice', then it executes like this.
[paul@RHEL4a scripts]$ ./choice 
isit.txt not found!
[paul@RHEL4a scripts]$ touch isit.txt
[paul@RHEL4a scripts]$ ./choice 
isit.txt exists!
[paul@RHEL4a scripts]$
24.3. if then elif
You can nest a new if inside an else with elif. This is a simple example.
#!/bin/bash
count=42
if [ $count -eq 42 ]
then
  echo "42 is correct."
elif [ $count -gt 42 ]
then
  echo "Too much."
else
  echo "Not enough."
fi
24.4. for loop
The example below shows the syntax of a classical for loop in bash.
for i in 1 2 4
do
   echo $i
done
An example of a for loop combined with an embedded shell.
#!/bin/ksh
for counter in `seq 1 20`
do
   echo counting from 1 to 20, now at $counter
   sleep 1
done
The same example as above can be written without the embedded shell using the bash
{from..to} shorthand.
scripting loops
243
#!/bin/bash
for counter in {1..20}
do
   echo counting from 1 to 20, now at $counter
   sleep 1
done
This for loop uses file globbing (from the shell expansion). Putting the instruction on the
command line has identical functionality.
kahlan@solexp11$ ls
count.ksh  go.ksh
kahlan@solexp11$ for file in *.ksh ; do cp $file $file.backup ; done
kahlan@solexp11$ ls                                                 
count.ksh  count.ksh.backup  go.ksh  go.ksh.backup 
24.5. while loop
Below a simple example of a while loop.
i=100;
while [ $i -ge 0 ] ;
do
   echo Counting down, from 100 to 0, now at $i;
   let i--;
done
Endless loops can be made with while true or while : , where the colon is the equivalent
of no operation in the Korn and bash shells.
#!/bin/ksh
# endless loop
while :
do
 echo hello
 sleep 1
done
24.6. until loop
Below a simple example of an until loop.
let i=100;
until [ $i -le 0 ] ;
do
   echo Counting down, from 100 to 1, now at $i;
   let i--;
done
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244
24.7. practice: scripting tests and loops
1. Write a script that uses a for loop to count from 3 to 7.
2. Write a script that uses a for loop to count from 1 to 17000.
3. Write a script that uses a while loop to count from 3 to 7.
4. Write a script that uses an until loop to count down from 8 to 4.
5. Write a script that counts the number of files ending in .txt in the current directory.
6. Wrap an if statement around the script so it is also correct when there are zero files ending
in .txt.
scripting loops
245
24.8. solution: scripting tests and loops
1. Write a script that uses a for loop to count from 3 to 7.
#!/bin/bash
for i in 3 4 5 6 7
do
 echo Counting from 3 to 7, now at $i
done
2. Write a script that uses a for loop to count from 1 to 17000.
#!/bin/bash
for i in `seq 1 17000`
do
 echo Counting from 1 to 17000, now at $i
done
3. Write a script that uses a while loop to count from 3 to 7.
#!/bin/bash
i=3
while [ $i -le 7 ]
do
 echo Counting from 3 to 7, now at $i
 let i=i+1
done
4. Write a script that uses an until loop to count down from 8 to 4.
#!/bin/bash
i=8
until [ $i -lt 4 ]
do
 echo Counting down from 8 to 4, now at $i
 let i=i-1
done
5. Write a script that counts the number of files ending in .txt in the current directory.
#!/bin/bash
let i=0
for file in *.txt
do
 let i++
done
echo "There are $i files ending in .txt"
 
6. Wrap an if statement around the script so it is also correct when there are zero files ending
in .txt.
#!/bin/bash
ls *.txt > /dev/null 2>&1
if [ $? -ne 0 ] 
scripting loops
246
then echo "There are 0 files ending in .txt"
else
 let i=0
 for file in *.txt
 do
  let i++
 done
 echo "There are $i files ending in .txt"
fi
247
Chapter 25. scripting parameters
scripting parameters
248
25.1. script parameters
A bash shell script can have parameters. The numbering you see in the script below
continues if you have more parameters. You also have special parameters containing the
number of parameters, a string of all of them, and also the process id, and the last return
code. The man page of bash has a full list.
#!/bin/bash
echo The first argument is $1
echo The second argument is $2
echo The third argument is $3
echo \$ $$  PID of the script
echo \# $#  count arguments
echo \? $?  last return code
echo \* $*  all the arguments
Below is the output of the script above in action.
[paul@RHEL4a scripts]$ ./pars one two three
The first argument is one
The second argument is two
The third argument is three
$ 5610 PID of the script
# 3 count arguments
? 0 last return code
* one two three all the arguments
Once more the same script, but with only two parameters.
[paul@RHEL4a scripts]$ ./pars 1 2
The first argument is 1
The second argument is 2
The third argument is
$ 5612 PID of the script
# 2 count arguments
? 0 last return code
* 1 2 all the arguments
[paul@RHEL4a scripts]$
Here is another example, where we use $0. The $0 parameter contains the name of the script.
paul@debian6~$ cat myname 
echo this script is called $0
paul@debian6~$ ./myname 
this script is called ./myname
paul@debian6~$ mv myname test42
paul@debian6~$ ./test42 
this script is called ./test42
scripting parameters
249
25.2. shift through parameters
The shift statement can parse all parameters one by one. This is a sample script.
kahlan@solexp11$ cat shift.ksh 
#!/bin/ksh                                
                                          
if [ "$#" == "0" ] 
 then
  echo You have to give at least one parameter.
  exit 1
fi
while (( $# ))
 do
  echo You gave me $1
  shift
 done
Below is some sample output of the script above.
kahlan@solexp11$ ./shift.ksh one  
You gave me one
kahlan@solexp11$ ./shift.ksh one two three 1201 "33 42"
You gave me one                           
You gave me two
You gave me three
You gave me 1201
You gave me 33 42
kahlan@solexp11$ ./shift.ksh                           
You have to give at least one parameter.
25.3. runtime input
You can ask the user for input with the read command in a script.
#!/bin/bash
echo -n Enter a number:
read number
  
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250
25.4. sourcing a config file
The source (as seen in the shell chapters) can be used to source a configuration file.
Below a sample configuration file for an application.
[paul@RHEL4a scripts]$ cat myApp.conf 
# The config file of myApp
# Enter the path here
myAppPath=/var/myApp
# Enter the number of quines here
quines=5
And here an application that uses this file.
[paul@RHEL4a scripts]$ cat myApp.bash 
#!/bin/bash
#
# Welcome to the myApp application
# 
. ./myApp.conf
echo There are $quines quines
The running application can use the values inside the sourced configuration file.
[paul@RHEL4a scripts]$ ./myApp.bash 
There are 5 quines
[paul@RHEL4a scripts]$
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251
25.5. get script options with getopts
The getopts function allows you to parse options given to a command. The following script
allows for any combination of the options a, f and z.
kahlan@solexp11$ cat options.ksh 
#!/bin/ksh
while getopts ":afz" option;
do
 case $option in
  a)
   echo received -a
   ;;
  f)
   echo received -f
   ;;
  z)
   echo received -z
   ;;
  *)
   echo "invalid option -$OPTARG" 
   ;;
 esac
done
This is sample output from the script above. First we use correct options, then we enter twice
an invalid option.
kahlan@solexp11$ ./options.ksh        
kahlan@solexp11$ ./options.ksh -af
received -a
received -f
kahlan@solexp11$ ./options.ksh -zfg
received -z
received -f
invalid option -g
kahlan@solexp11$ ./options.ksh -a -b -z
received -a
invalid option -b
received -z
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252
You can also check for options that need an argument, as this example shows.
kahlan@solexp11$ cat argoptions.ksh 
#!/bin/ksh
while getopts ":af:z" option;
do
 case $option in
  a)
   echo received -a
   ;;
  f)
   echo received -f with $OPTARG
   ;;
  z)
   echo received -z
   ;;
  :)
   echo "option -$OPTARG needs an argument"
   ;;
  *)
   echo "invalid option -$OPTARG" 
   ;;
 esac
done
This is sample output from the script above.
kahlan@solexp11$ ./argoptions.ksh -a -f hello -z
received -a
received -f with hello
received -z
kahlan@solexp11$ ./argoptions.ksh -zaf 42       
received -z
received -a
received -f with 42
kahlan@solexp11$ ./argoptions.ksh -zf   
received -z
option -f needs an argument
25.6. get shell options with shopt
You can toggle the values of variables controlling optional shell behaviour with the shopt
built-in shell command. The example below first verifies whether the cdspell option is set;
it is not. The next shopt command sets the value, and the third shopt command verifies that
the option really is set. You can now use minor spelling mistakes in the cd command. The
man page of bash has a complete list of options.
paul@laika:~$ shopt -q cdspell ; echo $?
1
paul@laika:~$ shopt -s cdspell
paul@laika:~$ shopt -q cdspell ; echo $?
0
paul@laika:~$ cd /Etc
/etc
scripting parameters
253
25.7. practice: parameters and options
1. Write a script that receives four parameters, and outputs them in reverse order.
2. Write a script that receives two parameters (two filenames) and outputs whether those
files exist.
3. Write a script that asks for a filename. Verify existence of the file, then verify that you
own the file, and whether it is writable. If not, then make it writable.
4. Make a configuration file for the previous script. Put a logging switch in the config file,
logging means writing detailed output of everything the script does to a log file in /tmp.
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254
25.8. solution: parameters and options
1. Write a script that receives four parameters, and outputs them in reverse order.
echo $4 $3 $2 $1
2. Write a script that receives two parameters (two filenames) and outputs whether those
files exist.
#!/bin/bash
if [ -f $1 ]
then echo $1 exists!
else echo $1 not found!
fi
if [ -f $2 ]
then echo $2 exists!
else echo $2 not found!
fi
 
3. Write a script that asks for a filename. Verify existence of the file, then verify that you
own the file, and whether it is writable. If not, then make it writable.
4. Make a configuration file for the previous script. Put a logging switch in the config file,
logging means writing detailed output of everything the script does to a log file in /tmp.
255
Chapter 26. more scripting
more scripting
256
26.1. eval
eval reads arguments as input to the shell (the resulting commands are executed). This allows
using the value of a variable as a variable.
paul@deb503:~/test42$ answer=42
paul@deb503:~/test42$ word=answer
paul@deb503:~/test42$ eval x=\$$word ; echo $x
42
Both in bash and Korn the arguments can be quoted.
kahlan@solexp11$ answer=42
kahlan@solexp11$ word=answer
kahlan@solexp11$ eval "y=\$$word" ; echo $y
42
Sometimes the eval is needed to have correct parsing of arguments. Consider this example
where the date command receives one parameter 1 week ago.
paul@debian6~$ date --date="1 week ago"
Thu Mar  8 21:36:25 CET 2012
When we set this command in a variable, then executing that variable fails unless we use
eval.
paul@debian6~$ lastweek='date --date="1 week ago"'
paul@debian6~$ $lastweek
date: extra operand `ago"'
Try `date --help' for more information.
paul@debian6~$ eval $lastweek
Thu Mar  8 21:36:39 CET 2012
26.2. (( ))
The (( )) allows for evaluation of numerical expressions.
paul@deb503:~/test42$ (( 42 > 33 )) && echo true || echo false
true
paul@deb503:~/test42$ (( 42 > 1201 )) && echo true || echo false
false
paul@deb503:~/test42$ var42=42
paul@deb503:~/test42$ (( 42 == var42 )) && echo true || echo false
true
paul@deb503:~/test42$ (( 42 == $var42 )) && echo true || echo false
true
paul@deb503:~/test42$ var42=33
paul@deb503:~/test42$ (( 42 == var42 )) && echo true || echo false
false
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257
26.3. let
The let built-in shell function instructs the shell to perform an evaluation of arithmetic
expressions. It will return 0 unless the last arithmetic expression evaluates to 0.
[paul@RHEL4b ~]$ let x="3 + 4" ; echo $x
7
[paul@RHEL4b ~]$ let x="10 + 100/10" ; echo $x
20
[paul@RHEL4b ~]$ let x="10-2+100/10" ; echo $x
18
[paul@RHEL4b ~]$ let x="10*2+100/10" ; echo $x
30
  
The shell can also convert between different bases.
[paul@RHEL4b ~]$ let x="0xFF" ; echo $x
255
[paul@RHEL4b ~]$ let x="0xC0" ; echo $x
192
[paul@RHEL4b ~]$ let x="0xA8" ; echo $x
168
[paul@RHEL4b ~]$ let x="8#70" ; echo $x
56
[paul@RHEL4b ~]$ let x="8#77" ; echo $x
63
[paul@RHEL4b ~]$ let x="16#c0" ; echo $x
192
  
There is a difference between assigning a variable directly, or using let to evaluate the
arithmetic expressions (even if it is just assigning a value).
kahlan@solexp11$ dec=15 ; oct=017 ; hex=0x0f 
kahlan@solexp11$ echo $dec $oct $hex 
15 017 0x0f 
kahlan@solexp11$ let dec=15 ; let oct=017 ; let hex=0x0f
kahlan@solexp11$ echo $dec $oct $hex
15 15 15
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258
26.4. case
You can sometimes simplify nested if statements with a case construct.
[paul@RHEL4b ~]$ ./help
What animal did you see ? lion
You better start running fast!
[paul@RHEL4b ~]$ ./help
What animal did you see ? dog
Don't worry, give it a cookie.
[paul@RHEL4b ~]$ cat help
#!/bin/bash
#
# Wild Animals Helpdesk Advice
#
echo -n "What animal did you see ? "
read animal
case $animal in
        "lion" | "tiger")
                echo "You better start running fast!"
        ;;
        "cat")
                echo "Let that mouse go..."
        ;;
        "dog")
                echo "Don't worry, give it a cookie."
        ;;
        "chicken" | "goose" | "duck" )
                echo "Eggs for breakfast!"
        ;;
        "liger")
                echo "Approach and say 'Ah you big fluffy kitty...'."
        ;;
        "babelfish")
                echo "Did it fall out your ear ?"
        ;;
        *)
                echo "You discovered an unknown animal, name it!"
        ;;
esac
[paul@RHEL4b ~]$    
  
more scripting
259
26.5. shell functions
Shell functions can be used to group commands in a logical way.
kahlan@solexp11$ cat funcs.ksh 
#!/bin/ksh
                                          
function greetings {
echo Hello World!
echo and hello to $USER to!
}
echo We will now call a function
greetings
echo The end
This is sample output from this script with a function.
kahlan@solexp11$ ./funcs.ksh              
We will now call a function
Hello World!
and hello to kahlan to!
The end
A shell function can also receive parameters.
kahlan@solexp11$ cat addfunc.ksh 
#!/bin/ksh
function plus {
let result="$1 + $2"
echo  $1 + $2 = $result
}
plus 3 10
plus 20 13
plus 20 22
This script produces the following output.
kahlan@solexp11$ ./addfunc.ksh 
3 + 10 = 13
20 + 13 = 33
20 + 22 = 42
more scripting
260
26.6. practice : more scripting
1. Write a script that asks for two numbers, and outputs the sum and product (as shown here).
Enter a number: 5
Enter another number: 2
Sum:       5 + 2 = 7
Product:   5 x 2 = 10
 
2. Improve the previous script to test that the numbers are between 1 and 100, exit with an
error if necessary.
3. Improve the previous script to congratulate the user if the sum equals the product.
4. Write a script with a case insensitive case statement, using the shopt nocasematch option.
The nocasematch option is reset to the value it had before the scripts started.
5. If time permits (or if you are waiting for other students to finish this practice), take a look
at Linux system scripts in /etc/init.d and /etc/rc.d and try to understand them. Where does
execution of a script start in /etc/init.d/samba ? There are also some hidden scripts in ~, we
will discuss them later.
more scripting
261
26.7. solution : more scripting
1. Write a script that asks for two numbers, and outputs the sum and product (as shown here).
Enter a number: 5
Enter another number: 2
Sum:       5 + 2 = 7
Product:   5 x 2 = 10
 
#!/bin/bash
echo -n "Enter a number : "
read n1
echo -n "Enter another number : "
read n2
let sum="$n1+$n2"
let pro="$n1*$n2"
echo -e "Sum\t: $n1 + $n2 = $sum" 
echo -e "Product\t: $n1 * $n2 = $pro"
2. Improve the previous script to test that the numbers are between 1 and 100, exit with an
error if necessary.
echo -n "Enter a number between 1 and 100 : "
read n1
if [ $n1 -lt 1 -o $n1 -gt 100 ]
then
       echo Wrong number... 
       exit 1
fi
3. Improve the previous script to congratulate the user if the sum equals the product.
if [ $sum -eq $pro ] 
then echo Congratulations $sum == $pro
fi
4. Write a script with a case insensitive case statement, using the shopt nocasematch option.
The nocasematch option is reset to the value it had before the scripts started.
#!/bin/bash
#
# Wild Animals Case Insensitive Helpdesk Advice
#
if shopt -q nocasematch; then
  nocase=yes;
else
  nocase=no;
  shopt -s nocasematch;
fi
echo -n "What animal did you see ? "
read animal
case $animal in
more scripting
262
  "lion" | "tiger")
    echo "You better start running fast!"
  ;;
  "cat")
    echo "Let that mouse go..."
  ;;
  "dog")
    echo "Don't worry, give it a cookie."
  ;;
  "chicken" | "goose" | "duck" )
    echo "Eggs for breakfast!"
  ;;
  "liger")
    echo "Approach and say 'Ah you big fluffy kitty.'"
  ;;
  "babelfish")
    echo "Did it fall out your ear ?"
  ;;
  *)
    echo "You discovered an unknown animal, name it!"
  ;;
esac
if [ nocase = yes ] ; then
        shopt -s nocasematch;
else
        shopt -u nocasematch;
fi
5. If time permits (or if you are waiting for other students to finish this practice), take a look
at Linux system scripts in /etc/init.d and /etc/rc.d and try to understand them. Where does
execution of a script start in /etc/init.d/samba ? There are also some hidden scripts in ~, we
will discuss them later.
Part VIII. local user management
264
Table of Contents
27. introduction to users ......................................................................................................  266
27.1. whoami ...............................................................................................................  267
27.2. who ....................................................................................................................  267
27.3. who am i .............................................................................................................  267
27.4. w .......................................................................................................................  267
27.5. id .......................................................................................................................  267
27.6. su to another user .................................................................................................  268
27.7. su to root ............................................................................................................  268
27.8. su as root ............................................................................................................  268
27.9. su - $username .....................................................................................................  268
27.10. su - ...................................................................................................................  268
27.11. run a program as another user ...............................................................................  269
27.12. visudo ...............................................................................................................  269
27.13. sudo su - ...........................................................................................................  270
27.14. sudo logging ......................................................................................................  270
27.15. practice: introduction to users ................................................................................  271
27.16. solution: introduction to users ................................................................................  272
28. user management ...........................................................................................................  274
28.1. user management ..................................................................................................  275
28.2. /etc/passwd ........................................................................................................... 275
28.3. root ....................................................................................................................  275
28.4. useradd ...............................................................................................................  276
28.5. /etc/default/useradd ................................................................................................  276
28.6. userdel ................................................................................................................  276
28.7. usermod ..............................................................................................................  276
28.8. creating home directories .......................................................................................  277
28.9. /etc/skel/ ..............................................................................................................  277
28.10. deleting home directories ......................................................................................  277
28.11. login shell ..........................................................................................................  278
28.12. chsh ..................................................................................................................  278
28.13. practice: user management ....................................................................................  279
28.14. solution: user management ....................................................................................  280
29. user passwords ..............................................................................................................  282
29.1. passwd ................................................................................................................  283
29.2. shadow file ..........................................................................................................  283
29.3. encryption with passwd ..........................................................................................  284
29.4. encryption with openssl .........................................................................................  284
29.5. encryption with crypt ............................................................................................. 285
29.6. /etc/login.defs .......................................................................................................  286
29.7. chage ..................................................................................................................  286
29.8. disabling a password .............................................................................................  287
29.9. editing local files ..................................................................................................  287
29.10. practice: user passwords .......................................................................................  288
29.11. solution: user passwords .......................................................................................  289
30. user profiles ..................................................................................................................  291
30.1. system profile ......................................................................................................  292
30.2. ~/.bash_profile ...................................................................................................... 292
30.3. ~/.bash_login ........................................................................................................ 293
30.4. ~/.profile .............................................................................................................  293
30.5. ~/.bashrc .............................................................................................................  293
30.6. ~/.bash_logout ......................................................................................................  294
30.7. Debian overview ...................................................................................................  295
30.8. RHEL5 overview ..................................................................................................  295
30.9. practice: user profiles ............................................................................................  296
30.10. solution: user profiles ........................................................................................... 297
local user management
265
31. groups ..........................................................................................................................  298
31.1. groupadd .............................................................................................................  299
31.2. group file ............................................................................................................  299
31.3. groups .................................................................................................................  299
31.4. usermod ..............................................................................................................  300
31.5. groupmod ............................................................................................................  300
31.6. groupdel ..............................................................................................................  300
31.7. gpasswd ..............................................................................................................  301
31.8. newgrp ................................................................................................................  302
31.9. vigr ....................................................................................................................  302
31.10. practice: groups ..................................................................................................  303
31.11. solution: groups ..................................................................................................  304
266
Chapter 27. introduction to users
This little chapter will teach you how to identify your user account on a Unix computer using
commands like who am i, id, and more.
In a second part you will learn how to become another user with the su command.
And you will learn how to run a program as another user with sudo.
introduction to users
267
27.1. whoami
The whoami command tells you your username.
[paul@centos7 ~]$ whoami
paul
[paul@centos7 ~]$
27.2. who
The who command will give you information about who is logged on the system.
[paul@centos7 ~]$ who
root     pts/0        2014-10-10 23:07 (10.104.33.101)
paul     pts/1        2014-10-10 23:30 (10.104.33.101)
laura    pts/2        2014-10-10 23:34 (10.104.33.96)
tania    pts/3        2014-10-10 23:39 (10.104.33.91)
[paul@centos7 ~]$
27.3. who am i
With who am i the who command will display only the line pointing to your current session.
[paul@centos7 ~]$ who am i
paul     pts/1        2014-10-10 23:30 (10.104.33.101)
[paul@centos7 ~]$
27.4. w
The w command shows you who is logged on and what they are doing.
[paul@centos7 ~]$ w
 23:34:07 up 31 min,  2 users,  load average: 0.00, 0.01, 0.02
USER     TTY        LOGIN@   IDLE   JCPU   PCPU WHAT
root     pts/0     23:07   15.00s  0.01s  0.01s top
paul     pts/1     23:30    7.00s  0.00s  0.00s w
[paul@centos7 ~]$
27.5. id
The id command will give you your user id, primary group id, and a list of the groups that
you belong to.
paul@debian7:~$ id
uid=1000(paul) gid=1000(paul) groups=1000(paul)
On RHEL/CentOS you will also get SELinux context information with this command.
[root@centos7 ~]# id
uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r\
:unconfined_t:s0-s0:c0.c1023
introduction to users
268
27.6. su to another user
The su command allows a user to run a shell as another user.
laura@debian7:~$ su tania
Password:
tania@debian7:/home/laura$
27.7. su to root
Yes you can also su to become root, when you know the root password.
laura@debian7:~$ su root
Password:
root@debian7:/home/laura#
27.8. su as root
You need to know the password of the user you want to substitute to, unless your are logged
in as root. The root user can become any existing user without knowing that user's password.
root@debian7:~# id
uid=0(root) gid=0(root) groups=0(root)
root@debian7:~# su - valentina
valentina@debian7:~$
27.9. su - $username
By default, the su command maintains the same shell environment. To become another user
and also get the target user's environment, issue the su - command followed by the target
username.
root@debian7:~# su laura
laura@debian7:/root$ exit
exit
root@debian7:~# su - laura
laura@debian7:~$ pwd
/home/laura
27.10. su -
When no username is provided to su or su -, the command will assume root is the target.
tania@debian7:~$ su -
Password:
root@debian7:~#
introduction to users
269
27.11. run a program as another user
The sudo program allows a user to start a program with the credentials of another user.
Before this works, the system administrator has to set up the /etc/sudoers file. This can be
useful to delegate administrative tasks to another user (without giving the root password).
The screenshot below shows the usage of sudo. User paul received the right to run useradd
with the credentials of root. This allows paul to create new users on the system without
becoming root and without knowing the root password.
First the command fails for paul.
paul@debian7:~$ /usr/sbin/useradd -m valentina
useradd: Permission denied.
useradd: cannot lock /etc/passwd; try again later.
But with sudo it works.
paul@debian7:~$ sudo /usr/sbin/useradd -m valentina
[sudo] password for paul:
paul@debian7:~$
27.12. visudo
Check the man page of visudo before playing with the /etc/sudoers file. Editing the sudoers
is out of scope for this fundamentals book.
paul@rhel65:~$ apropos visudo
visudo               (8)  - edit the sudoers file
paul@rhel65:~$
introduction to users
270
27.13. sudo su -
On some Linux systems like Ubuntu and Xubuntu, the root user does not have a password
set. This means that it is not possible to login as root (extra security). To perform tasks as
root, the first user is given all sudo rights via the /etc/sudoers. In fact all users that are
members of the admin group can use sudo to run all commands as root.
root@laika:~# grep admin /etc/sudoers 
# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL
The end result of this is that the user can type sudo su - and become root without having to
enter the root password. The sudo command does require you to enter your own password.
Thus the password prompt in the screenshot below is for sudo, not for su.
paul@laika:~$ sudo su -
Password:
root@laika:~#
27.14. sudo logging
Using sudo without authorization will result in a severe warning:
paul@rhel65:~$ sudo su -
We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:
    #1) Respect the privacy of others.
    #2) Think before you type.
    #3) With great power comes great responsibility.
[sudo] password for paul:
paul is not in the sudoers file.  This incident will be reported.
paul@rhel65:~$
The root user can see this in the /var/log/secure on Red Hat and in /var/log/auth.log on
Debian).
root@rhel65:~# tail /var/log/secure | grep sudo | tr -s ' '
Apr 13 16:03:42 rhel65 sudo: paul : user NOT in sudoers ; TTY=pts/0 ; PWD=\
/home/paul ; USER=root ; COMMAND=/bin/su -
root@rhel65:~#
introduction to users
271
27.15. practice: introduction to users
1. Run a command that displays only your currently logged on user name.
2. Display a list of all logged on users.
3. Display a list of all logged on users including the command they are running at this very
moment.
4. Display your user name and your unique user identification (userid).
5. Use su to switch to another user account (unless you are root, you will need the password
of the other account). And get back to the previous account.
6. Now use su - to switch to another user and notice the difference.
Note that su - gets you into the home directory of Tania.
7. Try to create a new user account (when using your normal user account). this should fail.
(Details on adding user accounts are explained in the next chapter.)
8. Now try the same, but with sudo before your command.
introduction to users
272
27.16. solution: introduction to users
1. Run a command that displays only your currently logged on user name.
laura@debian7:~$ whoami
laura
laura@debian7:~$ echo $USER
laura
2. Display a list of all logged on users.
laura@debian7:~$ who
laura     pts/0        2014-10-13 07:22 (10.104.33.101)
laura@debian7:~$
3. Display a list of all logged on users including the command they are running at this very
moment.
laura@debian7:~$ w
 07:47:02 up 16 min,  2 users,  load average: 0.00, 0.00, 0.00
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
root     pts/0    10.104.33.101    07:30    6.00s  0.04s  0.00s w
root     pts/1    10.104.33.101    07:46    6.00s  0.01s  0.00s sleep 42
laura@debian7:~$
4. Display your user name and your unique user identification (userid).
laura@debian7:~$ id
uid=1005(laura) gid=1007(laura) groups=1007(laura)
laura@debian7:~$
5. Use su to switch to another user account (unless you are root, you will need the password
of the other account). And get back to the previous account.
laura@debian7:~$ su tania
Password:
tania@debian7:/home/laura$ id
uid=1006(tania) gid=1008(tania) groups=1008(tania)
tania@debian7:/home/laura$ exit
laura@debian7:~$
6. Now use su - to switch to another user and notice the difference.
laura@debian7:~$ su - tania
Password:
tania@debian7:~$ pwd
/home/tania
tania@debian7:~$ logout
laura@debian7:~$
Note that su - gets you into the home directory of Tania.
introduction to users
273
7. Try to create a new user account (when using your normal user account). this should fail.
(Details on adding user accounts are explained in the next chapter.)
laura@debian7:~$ useradd valentina
-su: useradd: command not found
laura@debian7:~$ /usr/sbin/useradd valentina
useradd: Permission denied.
useradd: cannot lock /etc/passwd; try again later.
It is possible that useradd is located in /sbin/useradd on your computer.
8. Now try the same, but with sudo before your command.
laura@debian7:~$ sudo /usr/sbin/useradd valentina
[sudo] password for laura:
laura is not in the sudoers file.  This incident will be reported.
laura@debian7:~$
Notice that laura has no permission to use the sudo on this system.
274
Chapter 28. user management
This chapter will teach you how to use useradd, usermod and userdel to create, modify
and remove user accounts.
You will need root access on a Linux computer to complete this chapter.
user management
275
28.1. user management
User management on Linux can be done in three complementary ways. You can use the
graphical tools provided by your distribution. These tools have a look and feel that depends
on the distribution. If you are a novice Linux user on your home system, then use the
graphical tool that is provided by your distribution. This will make sure that you do not run
into problems.
Another option is to use command line tools like useradd, usermod, gpasswd, passwd and
others. Server administrators are likely to use these tools, since they are familiar and very
similar across many different distributions. This chapter will focus on these command line
tools.
A third and rather extremist way is to edit the local configuration files directly using vi (or
vipw/vigr). Do not attempt this as a novice on production systems!
28.2. /etc/passwd
The local user database on Linux (and on most Unixes) is /etc/passwd.
[root@RHEL5 ~]# tail /etc/passwd
inge:x:518:524:art dealer:/home/inge:/bin/ksh
ann:x:519:525:flute player:/home/ann:/bin/bash
frederik:x:520:526:rubius poet:/home/frederik:/bin/bash
steven:x:521:527:roman emperor:/home/steven:/bin/bash
pascale:x:522:528:artist:/home/pascale:/bin/ksh
geert:x:524:530:kernel developer:/home/geert:/bin/bash
wim:x:525:531:master damuti:/home/wim:/bin/bash
sandra:x:526:532:radish stresser:/home/sandra:/bin/bash
annelies:x:527:533:sword fighter:/home/annelies:/bin/bash
laura:x:528:534:art dealer:/home/laura:/bin/ksh
As you can see, this file contains seven columns separated by a colon. The columns contain
the username, an x, the user id, the primary group id, a description, the name of the home
directory, and the login shell.
More information can be found by typing man 5 passwd.
[root@RHEL5 ~]# man 5 passwd
28.3. root
The root user also called the superuser is the most powerful account on your Linux system.
This user can do almost anything, including the creation of other users. The root user always
has userid 0 (regardless of the name of the account).
[root@RHEL5 ~]# head -1 /etc/passwd
root:x:0:0:root:/root:/bin/bash
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276
28.4. useradd
You can add users with the useradd command. The example below shows how to add a
user named yanina (last parameter) and at the same time forcing the creation of the home
directory (-m), setting the name of the home directory (-d), and setting a description (-c).
[root@RHEL5 ~]# useradd -m -d /home/yanina -c "yanina wickmayer" yanina
[root@RHEL5 ~]# tail -1 /etc/passwd
yanina:x:529:529:yanina wickmayer:/home/yanina:/bin/bash
The user named yanina received userid 529 and primary group id 529.
28.5. /etc/default/useradd
Both Red Hat Enterprise Linux and Debian/Ubuntu have a file called /etc/default/useradd
that contains some default user options. Besides using cat to display this file, you can also
use useradd -D.
[root@RHEL4 ~]# useradd -D
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/bash
SKEL=/etc/skel
28.6. userdel
You can delete the user yanina with userdel. The -r option of userdel will also remove the
home directory.
[root@RHEL5 ~]# userdel -r yanina
28.7. usermod
You can modify the properties of a user with the usermod command. This example uses
usermod to change the description of the user harry.
[root@RHEL4 ~]# tail -1 /etc/passwd
harry:x:516:520:harry potter:/home/harry:/bin/bash
[root@RHEL4 ~]# usermod -c 'wizard' harry
[root@RHEL4 ~]# tail -1 /etc/passwd
harry:x:516:520:wizard:/home/harry:/bin/bash
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277
28.8. creating home directories
The easiest way to create a home directory is to supply the -m option with useradd (it is
likely set as a default option on Linux).
A less easy way is to create a home directory manually with mkdir which also requires
setting the owner and the permissions on the directory with chmod and chown (both
commands are discussed in detail in another chapter).
[root@RHEL5 ~]# mkdir /home/laura
[root@RHEL5 ~]# chown laura:laura /home/laura
[root@RHEL5 ~]# chmod 700 /home/laura
[root@RHEL5 ~]# ls -ld /home/laura/
drwx------ 2 laura laura 4096 Jun 24 15:17 /home/laura/
28.9. /etc/skel/
When using useradd the -m option, the /etc/skel/ directory is copied to the newly created
home directory. The /etc/skel/ directory contains some (usually hidden) files that contain
profile settings and default values for applications. In this way /etc/skel/ serves as a default
home directory and as a default user profile.
[root@RHEL5 ~]# ls -la /etc/skel/
total 48
drwxr-xr-x  2 root root  4096 Apr  1 00:11 .
drwxr-xr-x 97 root root 12288 Jun 24 15:36 ..
-rw-r--r--  1 root root    24 Jul 12  2006 .bash_logout
-rw-r--r--  1 root root   176 Jul 12  2006 .bash_profile
-rw-r--r--  1 root root   124 Jul 12  2006 .bashrc
28.10. deleting home directories
The -r option of userdel will make sure that the home directory is deleted together with the
user account.
[root@RHEL5 ~]# ls -ld /home/wim/
drwx------ 2 wim wim 4096 Jun 24 15:19 /home/wim/
[root@RHEL5 ~]# userdel -r wim
[root@RHEL5 ~]# ls -ld /home/wim/
ls: /home/wim/: No such file or directory
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278
28.11. login shell
The /etc/passwd file specifies the login shell for the user. In the screenshot below you can
see that user annelies will log in with the /bin/bash shell, and user laura with the /bin/ksh
shell.
[root@RHEL5 ~]# tail -2 /etc/passwd
annelies:x:527:533:sword fighter:/home/annelies:/bin/bash
laura:x:528:534:art dealer:/home/laura:/bin/ksh
You can use the usermod command to change the shell for a user.
[root@RHEL5 ~]# usermod -s /bin/bash laura
[root@RHEL5 ~]# tail -1 /etc/passwd
laura:x:528:534:art dealer:/home/laura:/bin/bash
28.12. chsh
Users can change their login shell with the chsh command. First, user harry obtains a list of
available shells (he could also have done a cat /etc/shells) and then changes his login shell
to the Korn shell (/bin/ksh). At the next login, harry will default into ksh instead of bash.
[laura@centos7 ~]$ chsh -l
/bin/sh
/bin/bash
/sbin/nologin
/usr/bin/sh
/usr/bin/bash
/usr/sbin/nologin
/bin/ksh
/bin/tcsh
/bin/csh
[laura@centos7 ~]$
Note that the -l option does not exist on Debian and that the above screenshot assumes that
ksh and csh shells are installed.
The screenshot below shows how laura can change her default shell (active on next login).
[laura@centos7 ~]$ chsh -s /bin/ksh
Changing shell for laura.
Password: 
Shell changed.
user management
279
28.13. practice: user management
1. Create a user account named serena, including a home directory and a description (or
comment) that reads Serena Williams. Do all this in one single command.
2. Create a user named venus, including home directory, bash shell, a description that reads
Venus Williams all in one single command.
3. Verify that both users have correct entries in /etc/passwd, /etc/shadow and /etc/group.
4. Verify that their home directory was created.
5. Create a user named einstime with /bin/date as his default logon shell.
7. What happens when you log on with the einstime user ? Can you think of a useful real
world example for changing a user's login shell to an application ?
8. Create a file named welcome.txt and make sure every new user will see this file in their
home directory.
9. Verify this setup by creating (and deleting) a test user account.
10. Change the default login shell for the serena user to /bin/bash. Verify before and after
you make this change.
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28.14. solution: user management
1. Create a user account named serena, including a home directory and a description (or
comment) that reads Serena Williams. Do all this in one single command.
root@debian7:~# useradd -m -c 'Serena Williams' serena
2. Create a user named venus, including home directory, bash shell, a description that reads
Venus Williams all in one single command.
root@debian7:~# useradd -m -c "Venus Williams" -s /bin/bash venus
3. Verify that both users have correct entries in /etc/passwd, /etc/shadow and /etc/group.
root@debian7:~# tail -2 /etc/passwd
serena:x:1008:1010:Serena Williams:/home/serena:/bin/sh
venus:x:1009:1011:Venus Williams:/home/venus:/bin/bash
root@debian7:~# tail -2 /etc/shadow
serena:!:16358:0:99999:7:::
venus:!:16358:0:99999:7:::
root@debian7:~# tail -2 /etc/group
serena:x:1010:
venus:x:1011:
4. Verify that their home directory was created.
root@debian7:~# ls -lrt /home | tail -2
drwxr-xr-x 2 serena    serena    4096 Oct 15 10:50 serena
drwxr-xr-x 2 venus     venus     4096 Oct 15 10:59 venus
root@debian7:~#
5. Create a user named einstime with /bin/date as his default logon shell.
root@debian7:~# useradd -s /bin/date einstime
Or even better:
root@debian7:~# useradd -s $(which date) einstime
7. What happens when you log on with the einstime user ? Can you think of a useful real
world example for changing a user's login shell to an application ?
root@debian7:~# su - einstime
Wed Oct 15 11:05:56 UTC 2014 # You get the output of the date command
root@debian7:~#
It can be useful when users need to access only one application on the server. Just logging
in opens the application for them, and closing the application automatically logs them out.
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281
8. Create a file named welcome.txt and make sure every new user will see this file in their
home directory.
root@debian7:~# echo Hello > /etc/skel/welcome.txt
9. Verify this setup by creating (and deleting) a test user account.
root@debian7:~# useradd -m test
root@debian7:~# ls -l /home/test
total 4
-rw-r--r-- 1 test test 6 Oct 15 11:16 welcome.txt
root@debian7:~# userdel -r test
root@debian7:~#
10. Change the default login shell for the serena user to /bin/bash. Verify before and after
you make this change.
root@debian7:~# grep serena /etc/passwd
serena:x:1008:1010:Serena Williams:/home/serena:/bin/sh
root@debian7:~# usermod -s /bin/bash serena
root@debian7:~# grep serena /etc/passwd
serena:x:1008:1010:Serena Williams:/home/serena:/bin/bash
root@debian7:~#
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Chapter 29. user passwords
This chapter will tell you more about passwords for local users.
Three methods for setting passwords are explained; using the passwd command, using
openssel passwd, and using the crypt function in a C program.
The chapter will also discuss password settings and disabling, suspending or locking
accounts.
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29.1. passwd
Passwords of users can be set with the passwd command. Users will have to provide their
old password before twice entering the new one.
[tania@centos7 ~]$ passwd
Changing password for user tania.
Changing password for tania.
(current) UNIX password:
New password:
BAD PASSWORD: The password is shorter than 8 characters
New password:
BAD PASSWORD: The password is a palindrome
New password:
BAD PASSWORD: The password is too similar to the old one
passwd: Have exhausted maximum number of retries for service
As you can see, the passwd tool will do some basic verification to prevent users from using
too simple passwords. The root user does not have to follow these rules (there will be
a warning though). The root user also does not have to provide the old password before
entering the new password twice.
root@debian7:~# passwd tania
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
29.2. shadow file
User passwords are encrypted and kept in /etc/shadow. The /etc/shadow file is read only
and can only be read by root. We will see in the file permissions section how it is possible
for users to change their password. For now, you will have to know that users can change
their password with the /usr/bin/passwd command.
[root@centos7 ~]# tail -4 /etc/shadow
paul:$6$ikp2Xta5BT.Tml.p$2TZjNnOYNNQKpwLJqoGJbVsZG5/Fti8ovBRd.VzRbiDSl7TEq\
IaSMH.TeBKnTS/SjlMruW8qffC0JNORW.BTW1:16338:0:99999:7:::
tania:$6$8Z/zovxj$9qvoqT8i9KIrmN.k4EQwAF5ryz5yzNwEvYjAa9L5XVXQu.z4DlpvMREH\
eQpQzvRnqFdKkVj17H5ST.c79HDZw0:16356:0:99999:7:::
laura:$6$glDuTY5e$/NYYWLxfHgZFWeoujaXSMcR.Mz.lGOxtcxFocFVJNb98nbTPhWFXfKWG\
SyYh1WCv6763Wq54.w24Yr3uAZBOm/:16356:0:99999:7:::
valentina:$6$jrZa6PVI$1uQgqR6En9mZB6mKJ3LXRB4CnFko6LRhbh.v4iqUk9MVreui1lv7\
GxHOUDSKA0N55ZRNhGHa6T2ouFnVno/0o1:16356:0:99999:7:::
[root@centos7 ~]#
The /etc/shadow file contains nine colon separated columns. The nine fields contain (from
left to right) the user name, the encrypted password (note that only inge and laura have an
encrypted password), the day the password was last changed (day 1 is January 1, 1970),
number of days the password must be left unchanged, password expiry day, warning number
of days before password expiry, number of days after expiry before disabling the account,
and the day the account was disabled (again, since 1970). The last field has no meaning yet.
All the passwords in the screenshot above are hashes of hunter2.
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29.3. encryption with passwd
Passwords are stored in an encrypted format. This encryption is done by the crypt function.
The easiest (and recommended) way to add a user with a password to the system is to add
the user with the useradd -m user command, and then set the user's password with passwd.
[root@RHEL4 ~]# useradd -m xavier
[root@RHEL4 ~]# passwd xavier
Changing password for user xavier.
New UNIX password: 
Retype new UNIX password: 
passwd: all authentication tokens updated successfully.
[root@RHEL4 ~]#
29.4. encryption with openssl
Another way to create users with a password is to use the -p option of useradd, but that
option requires an encrypted password. You can generate this encrypted password with the
openssl passwd command.
The openssl passwd command will generate several distinct hashes for the same password,
for this it uses a salt.
paul@rhel65:~$ openssl passwd hunter2
86jcUNlnGDFpY
paul@rhel65:~$ openssl passwd hunter2
Yj7mDO9OAnvq6
paul@rhel65:~$ openssl passwd hunter2
YqDcJeGoDbzKA
paul@rhel65:~$
This salt can be chosen and is visible as the first two characters of the hash.
paul@rhel65:~$ openssl passwd -salt 42 hunter2
42ZrbtP1Ze8G.
paul@rhel65:~$ openssl passwd -salt 42 hunter2
42ZrbtP1Ze8G.
paul@rhel65:~$ openssl passwd -salt 42 hunter2
42ZrbtP1Ze8G.
paul@rhel65:~$
This example shows how to create a user with password.
root@rhel65:~# useradd -m -p $(openssl passwd hunter2) mohamed
Note that this command puts the password in your command history!
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29.5. encryption with crypt
A third option is to create your own C program using the crypt function, and compile this
into a command.
paul@rhel65:~$ cat MyCrypt.c
#include <stdio.h>
#define __USE_XOPEN
#include <unistd.h>
int main(int argc, char** argv)
{
 if(argc==3)
   {
       printf("%s\n", crypt(argv[1],argv[2]));
   }
   else
   {
       printf("Usage: MyCrypt $password $salt\n" );
   }
  return 0;
}
This little program can be compiled with gcc like this.
paul@rhel65:~$ gcc MyCrypt.c -o MyCrypt -lcrypt
To use it, we need to give two parameters to MyCrypt. The first is the unencrypted password,
the second is the salt. The salt is used to perturb the encryption algorithm in one of 4096
different ways. This variation prevents two users with the same password from having the
same entry in /etc/shadow.
paul@rhel65:~$ ./MyCrypt hunter2 42
42ZrbtP1Ze8G.
paul@rhel65:~$ ./MyCrypt hunter2 33
33d6taYSiEUXI
Did you notice that the first two characters of the password are the salt?
The standard output of the crypt function is using the DES algorithm which is old and can
be cracked in minutes. A better method is to use md5 passwords which can be recognized
by a salt starting with $1$.
paul@rhel65:~$ ./MyCrypt hunter2 '$1$42'
$1$42$7l6Y3xT5282XmZrtDOF9f0
paul@rhel65:~$ ./MyCrypt hunter2 '$6$42'
$6$42$OqFFAVnI3gTSYG0yI9TZWX9cpyQzwIop7HwpG1LLEsNBiMr4w6OvLX1KDa./UpwXfrFk1i...
The md5 salt can be up to eight characters long. The salt is displayed in /etc/shadow between
the second and third $, so never use the password as the salt!
paul@rhel65:~$ ./MyCrypt hunter2 '$1$hunter2'
$1$hunter2$YVxrxDmidq7Xf8Gdt6qM2.
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29.6. /etc/login.defs
The /etc/login.defs file contains some default settings for user passwords like password
aging and length settings. (You will also find the numerical limits of user ids and group ids
and whether or not a home directory should be created by default).
root@rhel65:~# grep ^PASS /etc/login.defs
PASS_MAX_DAYS   99999
PASS_MIN_DAYS   0
PASS_MIN_LEN    5
PASS_WARN_AGE   7
Debian also has this file.
root@debian7:~# grep PASS /etc/login.defs
#  PASS_MAX_DAYS   Maximum number of days a password may be used.
#  PASS_MIN_DAYS   Minimum number of days allowed between password changes.
#  PASS_WARN_AGE   Number of days warning given before a password expires.
PASS_MAX_DAYS   99999
PASS_MIN_DAYS   0
PASS_WARN_AGE   7
#PASS_CHANGE_TRIES
#PASS_ALWAYS_WARN
#PASS_MIN_LEN
#PASS_MAX_LEN
# NO_PASSWORD_CONSOLE
root@debian7:~#
29.7. chage
The chage command can be used to set an expiration date for a user account (-E), set a
minimum (-m) and maximum (-M) password age, a password expiration date, and set the
number of warning days before the password expiration date. Much of this functionality is
also available from the passwd command. The -l option of chage will list these settings for
a user.
root@rhel65:~# chage -l paul
Last password change                                    : Mar 27, 2014
Password expires                                        : never
Password inactive                                       : never
Account expires                                         : never
Minimum number of days between password change          : 0
Maximum number of days between password change          : 99999
Number of days of warning before password expires       : 7
root@rhel65:~#
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287
29.8. disabling a password
Passwords in /etc/shadow cannot begin with an exclamation mark. When the second field
in /etc/passwd starts with an exclamation mark, then the password can not be used.
Using this feature is often called locking, disabling, or suspending a user account. Besides
vi (or vipw) you can also accomplish this with usermod.
The first command in the next screenshot will show the hashed password of laura in /etc/
shadow. The next command disables the password of laura, making it impossible for Laura
to authenticate using this password.
root@debian7:~# grep laura /etc/shadow | cut -c1-70
laura:$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJV
root@debian7:~# usermod -L laura
As you can see below, the password hash is simply preceded with an exclamation mark.
root@debian7:~# grep laura /etc/shadow | cut -c1-70
laura:!$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJ
root@debian7:~#
The root user (and users with sudo rights on su) still will be able to su into the laura account
(because the password is not needed here). Also note that laura will still be able to login
if she has set up passwordless ssh!
root@debian7:~# su - laura
laura@debian7:~$ 
You can unlock the account again with usermod -U.
root@debian7:~# usermod -U laura
root@debian7:~# grep laura /etc/shadow | cut -c1-70
laura:$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJV
Watch out for tiny differences in the command line options of passwd, usermod, and
useradd on different Linux distributions. Verify the local files when using features like
"disabling, suspending, or locking" on user accounts and their passwords.
29.9. editing local files
If you still want to manually edit the /etc/passwd or /etc/shadow, after knowing these
commands for password management, then use vipw instead of vi(m) directly. The vipw
tool will do proper locking of the file.
[root@RHEL5 ~]# vipw /etc/passwd
vipw: the password file is busy (/etc/ptmp present)
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29.10. practice: user passwords
1. Set the password for serena to hunter2.
2. Also set a password for venus and then lock the venus user account with usermod. Verify
the locking in /etc/shadow before and after you lock it.
3. Use passwd -d to disable the serena password. Verify the serena line in /etc/shadow
before and after disabling.
4. What is the difference between locking a user account and disabling a user account's
password like we just did with usermod -L and passwd -d?
5. Try changing the password of serena to serena as serena.
6. Make sure serena has to change her password in 10 days.
7. Make sure every new user needs to change their password every 10 days.
8. Take a backup as root of /etc/shadow. Use vi to copy an encrypted hunter2 hash from
venus to serena. Can serena now log on with hunter2 as a password ?
9. Why use vipw instead of vi ? What could be the problem when using vi or vim ?
10. Use chsh to list all shells (only works on RHEL/CentOS/Fedora), and compare to cat /
etc/shells.
11. Which useradd option allows you to name a home directory ?
12. How can you see whether the password of user serena is locked or unlocked ? Give a
solution with grep and a solution with passwd.
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29.11. solution: user passwords
1. Set the password for serena to hunter2.
root@debian7:~# passwd serena
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
2. Also set a password for venus and then lock the venus user account with usermod. Verify
the locking in /etc/shadow before and after you lock it.
root@debian7:~# passwd venus
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
root@debian7:~# grep venus /etc/shadow | cut -c1-70
venus:$6$gswzXICW$uSnKFV1kFKZmTPaMVS4AvNA/KO27OxN0v5LHdV9ed0gTyXrjUeM/
root@debian7:~# usermod -L venus
root@debian7:~# grep venus /etc/shadow | cut -c1-70
venus:!$6$gswzXICW$uSnKFV1kFKZmTPaMVS4AvNA/KO27OxN0v5LHdV9ed0gTyXrjUeM
Note that usermod -L precedes the password hash with an exclamation mark (!).
3. Use passwd -d to disable the serena password. Verify the serena line in /etc/shadow
before and after disabling.
root@debian7:~# grep serena /etc/shadow | cut -c1-70
serena:$6$Es/omrPE$F2Ypu8kpLrfKdW0v/UIwA5jrYyBD2nwZ/dt.i/IypRgiPZSdB/B
root@debian7:~# passwd -d serena
passwd: password expiry information changed.
root@debian7:~# grep serena /etc/shadow
serena::16358:0:99999:7:::
root@debian7:~#
4. What is the difference between locking a user account and disabling a user account's
password like we just did with usermod -L and passwd -d?
Locking will prevent the user from logging on to the system with his password by putting
a ! in front of the password in /etc/shadow.
Disabling with passwd will erase the password from /etc/shadow.
5. Try changing the password of serena to serena as serena.
log on as serena, then execute: passwd serena... it should fail!
6. Make sure serena has to change her password in 10 days.
chage -M 10 serena
7. Make sure every new user needs to change their password every 10 days.
vi /etc/login.defs (and change PASS_MAX_DAYS to 10)
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290
8. Take a backup as root of /etc/shadow. Use vi to copy an encrypted hunter2 hash from
venus to serena. Can serena now log on with hunter2 as a password ?
Yes.
9. Why use vipw instead of vi ? What could be the problem when using vi or vim ?
vipw will give a warning when someone else is already using that file (with vipw).
10. Use chsh to list all shells (only works on RHEL/CentOS/Fedora), and compare to cat /
etc/shells.
chsh -l
cat /etc/shells
11. Which useradd option allows you to name a home directory ?
-d
12. How can you see whether the password of user serena is locked or unlocked ? Give a
solution with grep and a solution with passwd.
grep serena /etc/shadow
passwd -S serena
291
Chapter 30. user profiles
Logged on users have a number of preset (and customized) aliases, variables, and functions,
but where do they come from ? The shell uses a number of startup files that are executed
(or rather sourced) whenever the shell is invoked. What follows is an overview of startup
scripts.
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292
30.1. system profile
Both the bash and the ksh shell will verify the existence of /etc/profile and source it if it
exists.
When reading this script, you will notice (both on Debian and on Red Hat Enterprise Linux)
that it builds the PATH environment variable (among others). The script might also change
the PS1 variable, set the HOSTNAME and execute even more scripts like /etc/inputrc
This screenshot uses grep to show PATH manipulation in /etc/profile on Debian.
root@debian7:~# grep PATH /etc/profile
  PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
  PATH="/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games"
export PATH
root@debian7:~#
This screenshot uses grep to show PATH manipulation in /etc/profile on RHEL7/CentOS7.
[root@centos7 ~]# grep PATH /etc/profile
    case ":${PATH}:" in
                PATH=$PATH:$1
                PATH=$1:$PATH
export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE HISTCONTROL
[root@centos7 ~]#
The root user can use this script to set aliases, functions, and variables for every user on
the system.
30.2. ~/.bash_profile
When this file exists in the home directory, then bash will source it. On Debian Linux 5/6/7
this file does not exist by default.
RHEL7/CentOS7 uses a small ~/.bash_profile where it checks for the existence of
~/.bashrc and then sources it. It also adds $HOME/bin to the $PATH variable.
[root@rhel7 ~]# cat /home/paul/.bash_profile
# .bash_profile
# Get the aliases and functions
if [ -f ~/.bashrc ]; then
        . ~/.bashrc
fi
# User specific environment and startup programs
PATH=$PATH:$HOME/.local/bin:$HOME/bin
export PATH
[root@rhel7 ~]#
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30.3. ~/.bash_login
When .bash_profile does not exist, then bash will check for ~/.bash_login and source it.
Neither Debian nor Red Hat have this file by default.
30.4. ~/.profile
When neither ~/.bash_profile and ~/.bash_login exist, then bash will verify the existence
of ~/.profile and execute it. This file does not exist by default on Red Hat.
On Debian this script can execute ~/.bashrc and will add $HOME/bin to the $PATH
variable.
root@debian7:~# tail -11 /home/paul/.profile
if [ -n "$BASH_VERSION" ]; then
    # include .bashrc if it exists
    if [ -f "$HOME/.bashrc" ]; then
        . "$HOME/.bashrc"
    fi
fi
# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
    PATH="$HOME/bin:$PATH"
fi
RHEL/CentOS does not have this file by default.
30.5. ~/.bashrc
The ~/.bashrc script is often sourced by other scripts. Let us take a look at what it does
by default.
Red Hat uses a very simple ~/.bashrc, checking for /etc/bashrc and sourcing it. It also leaves
room for custom aliases and functions.
[root@rhel7 ~]# cat /home/paul/.bashrc
# .bashrc
# Source global definitions
if [ -f /etc/bashrc ]; then
        . /etc/bashrc
fi
# Uncomment the following line if you don't like systemctl's auto-paging feature:
# export SYSTEMD_PAGER=
# User specific aliases and functions
On Debian this script is quite a bit longer and configures $PS1, some history variables and
a number af active and inactive aliases.
root@debian7:~# wc -l /home/paul/.bashrc
110 /home/paul/.bashrc
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30.6. ~/.bash_logout
When exiting bash, it can execute ~/.bash_logout.
Debian use this opportunity to clear the console screen.
serena@deb503:~$ cat .bash_logout
# ~/.bash_logout: executed by bash(1) when login shell exits.
# when leaving the console clear the screen to increase privacy
if [ "$SHLVL" = 1 ]; then
    [ -x /usr/bin/clear_console ] && /usr/bin/clear_console -q
fi
Red Hat Enterprise Linux 5 will simple call the /usr/bin/clear command in this script.
[serena@rhel53 ~]$ cat .bash_logout 
# ~/.bash_logout
/usr/bin/clear
Red Hat Enterprise Linux 6 and 7 create this file, but leave it empty (except for a comment).
paul@rhel65:~$ cat .bash_logout
# ~/.bash_logout
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30.7. Debian overview
Below is a table overview of when Debian is running any of these bash startup scripts.
Table 30.1. Debian User Environment
script
su
su -
ssh
gdm
~./bashrc
no
yes
yes
yes
~/.profile
no
yes
yes
yes
/etc/profile
no
yes
yes
yes
/etc/bash.bashrc
yes
no
no
yes
30.8. RHEL5 overview
Below is a table overview of when Red Hat Enterprise Linux 5 is running any of these bash
startup scripts.
Table 30.2. Red Hat User Environment
script
su
su -
ssh
gdm
~./bashrc
yes
yes
yes
yes
~/.bash_profile
no
yes
yes
yes
/etc/profile
no
yes
yes
yes
/etc/bashrc
yes
yes
yes
yes
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30.9. practice: user profiles
1. Make a list of all the profile files on your system.
2. Read the contents of each of these, often they source extra scripts.
3. Put a unique variable, alias and function in each of those files.
4. Try several different ways to obtain a shell (su, su -, ssh, tmux, gnome-terminal, Ctrl-
alt-F1, ...) and verify which of your custom variables, aliases and function are present in
your environment.
5. Do you also know the order in which they are executed?
6. When an application depends on a setting in $HOME/.profile, does it matter whether
$HOME/.bash_profile exists or not ?
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297
30.10. solution: user profiles
1. Make a list of all the profile files on your system.
ls -a ~ ; ls -l /etc/pro* /etc/bash*
2. Read the contents of each of these, often they source extra scripts.
3. Put a unique variable, alias and function in each of those files.
4. Try several different ways to obtain a shell (su, su -, ssh, tmux, gnome-terminal, Ctrl-
alt-F1, ...) and verify which of your custom variables, aliases and function are present in
your environment.
5. Do you also know the order in which they are executed?
same name aliases, functions and variables will overwrite each other
6. When an application depends on a setting in $HOME/.profile, does it matter whether
$HOME/.bash_profile exists or not ?
Yes it does matter. (man bash /INVOCATION)
298
Chapter 31. groups
Users can be listed in groups. Groups allow you to set permissions on the group level instead
of having to set permissions for every individual user.
Every Unix or Linux distribution will have a graphical tool to manage groups. Novice users
are advised to use this graphical tool. More experienced users can use command line tools to
manage users, but be careful: Some distributions do not allow the mixed use of GUI and CLI
tools to manage groups (YaST in Novell Suse). Senior administrators can edit the relevant
files directly with vi or vigr.
groups
299
31.1. groupadd
Groups can be created with the groupadd command. The example below shows the creation
of five (empty) groups.
root@laika:~# groupadd tennis
root@laika:~# groupadd football
root@laika:~# groupadd snooker
root@laika:~# groupadd formula1
root@laika:~# groupadd salsa
31.2. group file
Users can be a member of several groups. Group membership is defined by the /etc/group
file.
root@laika:~# tail -5 /etc/group
tennis:x:1006:
football:x:1007:
snooker:x:1008:
formula1:x:1009:
salsa:x:1010:
root@laika:~#
The first field is the group's name. The second field is the group's (encrypted) password (can
be empty). The third field is the group identification or GID. The fourth field is the list of
members, these groups have no members.
31.3. groups
A user can type the groups command to see a list of groups where the user belongs to.
[harry@RHEL4b ~]$ groups
harry sports
[harry@RHEL4b ~]$
groups
300
31.4. usermod
Group membership can be modified with the useradd or usermod command.
root@laika:~# usermod -a -G tennis inge
root@laika:~# usermod -a -G tennis katrien
root@laika:~# usermod -a -G salsa katrien
root@laika:~# usermod -a -G snooker sandra
root@laika:~# usermod -a -G formula1 annelies
root@laika:~# tail -5 /etc/group
tennis:x:1006:inge,katrien
football:x:1007:
snooker:x:1008:sandra
formula1:x:1009:annelies
salsa:x:1010:katrien
root@laika:~#
Be careful when using usermod to add users to groups. By default, the usermod command
will remove the user from every group of which he is a member if the group is not listed in
the command! Using the -a (append) switch prevents this behaviour.
31.5. groupmod
You can change the group name with the groupmod command.
root@laika:~# groupmod -n darts snooker 
root@laika:~# tail -5 /etc/group
tennis:x:1006:inge,katrien
football:x:1007:
formula1:x:1009:annelies
salsa:x:1010:katrien
darts:x:1008:sandra
31.6. groupdel
You can permanently remove a group with the groupdel command.
root@laika:~# groupdel tennis
root@laika:~#
groups
301
31.7. gpasswd
You can delegate control of group membership to another user with the gpasswd command.
In the example below we delegate permissions to add and remove group members to serena
for the sports group. Then we su to serena and add harry to the sports group.
[root@RHEL4b ~]# gpasswd -A serena sports
[root@RHEL4b ~]# su - serena
[serena@RHEL4b ~]$ id harry
uid=516(harry) gid=520(harry) groups=520(harry)
[serena@RHEL4b ~]$ gpasswd -a harry sports
Adding user harry to group sports
[serena@RHEL4b ~]$ id harry
uid=516(harry) gid=520(harry) groups=520(harry),522(sports)
[serena@RHEL4b ~]$ tail -1 /etc/group
sports:x:522:serena,venus,harry
[serena@RHEL4b ~]$ 
Group administrators do not have to be a member of the group. They can remove themselves
from a group, but this does not influence their ability to add or remove members.
[serena@RHEL4b ~]$ gpasswd -d serena sports
Removing user serena from group sports
[serena@RHEL4b ~]$ exit
Information about group administrators is kept in the /etc/gshadow file.
[root@RHEL4b ~]# tail -1 /etc/gshadow
sports:!:serena:venus,harry
[root@RHEL4b ~]#
To remove all group administrators from a group, use the gpasswd command to set an empty
administrators list.
[root@RHEL4b ~]# gpasswd -A "" sports
groups
302
31.8. newgrp
You can start a child shell with a new temporary primary group using the newgrp
command.
root@rhel65:~# mkdir prigroup
root@rhel65:~# cd prigroup/
root@rhel65:~/prigroup# touch standard.txt
root@rhel65:~/prigroup# ls -l
total 0
-rw-r--r--. 1 root root 0 Apr 13 17:49 standard.txt
root@rhel65:~/prigroup# echo $SHLVL
1
root@rhel65:~/prigroup# newgrp tennis
root@rhel65:~/prigroup# echo $SHLVL
2
root@rhel65:~/prigroup# touch newgrp.txt
root@rhel65:~/prigroup# ls -l
total 0
-rw-r--r--. 1 root tennis 0 Apr 13 17:49 newgrp.txt
-rw-r--r--. 1 root root   0 Apr 13 17:49 standard.txt
root@rhel65:~/prigroup# exit
exit
root@rhel65:~/prigroup#
31.9. vigr
Similar to vipw, the vigr command can be used to manually edit the /etc/group file, since
it will do proper locking of the file. Only experienced senior administrators should use vi
or vigr to manage groups.
groups
303
31.10. practice: groups
1. Create the groups tennis, football and sports.
2. In one command, make venus a member of tennis and sports.
3. Rename the football group to foot.
4. Use vi to add serena to the tennis group.
5. Use the id command to verify that serena is a member of tennis.
6. Make someone responsible for managing group membership of foot and sports. Test that
it works.
groups
304
31.11. solution: groups
1. Create the groups tennis, football and sports.
groupadd tennis ; groupadd football ; groupadd sports
2. In one command, make venus a member of tennis and sports.
usermod -a -G tennis,sports venus
3. Rename the football group to foot.
groupmod -n foot football
4. Use vi to add serena to the tennis group.
vi /etc/group
5. Use the id command to verify that serena is a member of tennis.
id (and after logoff logon serena should be member)
6. Make someone responsible for managing group membership of foot and sports. Test that
it works.
gpasswd -A (to make manager)
gpasswd -a (to add member)
Part IX. file security
306
Table of Contents
32. standard file permissions ................................................................................................  307
32.1. file ownership ......................................................................................................  308
32.2. list of special files ................................................................................................. 310
32.3. permissions ..........................................................................................................  311
32.4. practice: standard file permissions ............................................................................  316
32.5. solution: standard file permissions ...........................................................................  317
33. advanced file permissions ...............................................................................................  319
33.1. sticky bit on directory ............................................................................................  320
33.2. setgid bit on directory ............................................................................................  320
33.3. setgid and setuid on regular files .............................................................................  321
33.4. setuid on sudo ......................................................................................................  321
33.5. practice: sticky, setuid and setgid bits .......................................................................  322
33.6. solution: sticky, setuid and setgid bits .......................................................................  323
34. access control lists .......................................................................................................... 325
34.1. acl in /etc/fstab .....................................................................................................  326
34.2. getfacl .................................................................................................................  326
34.3. setfacl .................................................................................................................  326
34.4. remove an acl entry ............................................................................................... 327
34.5. remove the complete acl ........................................................................................  327
34.6. the acl mask ........................................................................................................  327
34.7. eiciel ..................................................................................................................  328
35. file links ........................................................................................................................  329
35.1. inodes .................................................................................................................  330
35.2. about directories ...................................................................................................  331
35.3. hard links ............................................................................................................  332
35.4. symbolic links ......................................................................................................  333
35.5. removing links .....................................................................................................  333
35.6. practice : links ......................................................................................................  334
35.7. solution : links ......................................................................................................  335
307
Chapter 32. standard file permissions
This chapter contains details about basic file security through file ownership and file
permissions.
standard file permissions
308
32.1. file ownership
32.1.1. user owner and group owner
The users and groups of a system can be locally managed in /etc/passwd and /etc/group,
or they can be in a NIS, LDAP, or Samba domain. These users and groups can own files.
Actually, every file has a user owner and a group owner, as can be seen in the following
screenshot.
paul@rhel65:~/owners$ ls -lh
total 636K
-rw-r--r--. 1 paul snooker 1.1K Apr  8 18:47 data.odt
-rw-r--r--. 1 paul paul    626K Apr  8 18:46 file1
-rw-r--r--. 1 root tennis   185 Apr  8 18:46 file2
-rw-rw-r--. 1 root root       0 Apr  8 18:47 stuff.txt
paul@rhel65:~/owners$
User paul owns three files; file1 has paul as user owner and has the group paul as group
owner, data.odt is group owned by the group snooker, file2 by the group tennis.
The last file is called stuff.txt and is owned by the root user and the root group.
32.1.2. listing user accounts
You can use the following command to list all local user accounts.
paul@debian7~$ cut -d: -f1 /etc/passwd | column 
root            ntp             sam             bert            naomi
daemon          mysql           tom             rino            matthias2
bin             paul            wouter          antonio         bram
sys             maarten         robrecht        simon           fabrice
sync            kevin           bilal           sven            chimene
games           yuri            dimitri         wouter2         messagebus
man             william         ahmed           tarik           roger
lp              yves            dylan           jan             frank
mail            kris            robin           ian             toon
news            hamid           matthias        ivan            rinus
uucp            vladimir        ben             azeddine        eddy
proxy           abiy            mike            eric            bram2
www-data        david           kevin2          kamel           keith
backup          chahid          kenzo           ischa           jesse
list            stef            aaron           bart            frederick
irc             joeri           lorenzo         omer            hans
gnats           glenn           jens            kurt            dries
nobody          yannick         ruben           steve           steve2
libuuid         christof        jelle           constantin      tomas
Debian-exim     george          stefaan         sam2            johan
statd           joost           marc            bjorn           tom2
sshd            arno            thomas          ronald
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309
32.1.3. chgrp
You can change the group owner of a file using the chgrp command.
root@rhel65:/home/paul/owners# ls -l file2
-rw-r--r--. 1 root tennis 185 Apr  8 18:46 file2
root@rhel65:/home/paul/owners# chgrp snooker file2
root@rhel65:/home/paul/owners# ls -l file2
-rw-r--r--. 1 root snooker 185 Apr  8 18:46 file2
root@rhel65:/home/paul/owners#
32.1.4. chown
The user owner of a file can be changed with chown command.
root@laika:/home/paul# ls -l FileForPaul 
-rw-r--r-- 1 root paul 0 2008-08-06 14:11 FileForPaul
root@laika:/home/paul# chown paul FileForPaul 
root@laika:/home/paul# ls -l FileForPaul 
-rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul
You can also use chown to change both the user owner and the group owner.
root@laika:/home/paul# ls -l FileForPaul 
-rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul
root@laika:/home/paul# chown root:project42 FileForPaul 
root@laika:/home/paul# ls -l FileForPaul 
-rw-r--r-- 1 root project42 0 2008-08-06 14:11 FileForPaul
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310
32.2. list of special files
When you use ls -l, for each file you can see ten characters before the user and group owner.
The first character tells us the type of file. Regular files get a -, directories get a d, symbolic
links are shown with an l, pipes get a p, character devices a c, block devices a b, and sockets
an s.
Table 32.1. Unix special files
first character
file type
-
normal file
d
directory
l
symbolic link
p
named pipe
b
block device
c
character device
s
socket
Below a screenshot of a character device (the console) and a block device (the hard disk).
paul@debian6lt~$ ls -ld /dev/console /dev/sda
crw-------   1 root root  5, 1 Mar 15 12:45 /dev/console
brw-rw----   1 root disk  8, 0 Mar 15 12:45 /dev/sda
And here you can see a directory, a regular file and a symbolic link.
paul@debian6lt~$ ls -ld /etc /etc/hosts /etc/motd
drwxr-xr-x 128 root root 12288 Mar 15 18:34 /etc
-rw-r--r--   1 root root   372 Dec 10 17:36 /etc/hosts
lrwxrwxrwx   1 root root    13 Dec  5 10:36 /etc/motd -> /var/run/motd
standard file permissions
311
32.3. permissions
32.3.1. rwx
The nine characters following the file type denote the permissions in three triplets. A
permission can be r for read access, w for write access, and x for execute. You need the r
permission to list (ls) the contents of a directory. You need the x permission to enter (cd) a
directory. You need the w permission to create files in or remove files from a directory.
Table 32.2. standard Unix file permissions
permission
on a file
on a directory
r (read)
read file contents (cat)
read directory contents (ls)
w (write)
change file contents (vi)
create files in (touch)
x (execute)
execute the file
enter the directory (cd)
32.3.2. three sets of rwx
We already know that the output of ls -l  starts with ten characters for each file. This
screenshot shows a regular file (because the first character is a - ).
paul@RHELv4u4:~/test$ ls -l proc42.bash
-rwxr-xr--  1 paul proj 984 Feb  6 12:01 proc42.bash
Below is a table describing the function of all ten characters.
Table 32.3. Unix file permissions position
position
characters
function
1
-
this is a regular file
2-4
rwx
permissions for the user owner
5-7
r-x
permissions for the group owner
8-10
r--
permissions for others
When you are the user owner of a file, then the user owner permissions apply to you. The
rest of the permissions have no influence on your access to the file.
When you belong to the group that is the group owner of a file, then the group owner
permissions apply to you. The rest of the permissions have no influence on your access to
the file.
When you are not the user owner of a file and you do not belong to the group owner, then
the others permissions apply to you. The rest of the permissions have no influence on your
access to the file.
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312
32.3.3. permission examples
Some example combinations on files and directories are seen in this screenshot. The name
of the file explains the permissions.
paul@laika:~/perms$ ls -lh
total 12K
drwxr-xr-x 2 paul paul 4.0K 2007-02-07 22:26 AllEnter_UserCreateDelete
-rwxrwxrwx 1 paul paul    0 2007-02-07 22:21 EveryoneFullControl.txt
-r--r----- 1 paul paul    0 2007-02-07 22:21 OnlyOwnersRead.txt
-rwxrwx--- 1 paul paul    0 2007-02-07 22:21 OwnersAll_RestNothing.txt
dr-xr-x--- 2 paul paul 4.0K 2007-02-07 22:25 UserAndGroupEnter
dr-x------ 2 paul paul 4.0K 2007-02-07 22:25 OnlyUserEnter
paul@laika:~/perms$
To summarise, the first rwx triplet represents the permissions for the user owner. The
second triplet corresponds to the group owner; it specifies permissions for all members
of that group. The third triplet defines permissions for all other users that are not the user
owner and are not a member of the group owner.
standard file permissions
313
32.3.4. setting permissions (chmod)
Permissions can be changed with chmod. The first example gives the user owner execute
permissions.
paul@laika:~/perms$ ls -l permissions.txt 
-rw-r--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod u+x permissions.txt 
paul@laika:~/perms$ ls -l permissions.txt 
-rwxr--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
This example removes the group owners read permission.
paul@laika:~/perms$ chmod g-r permissions.txt 
paul@laika:~/perms$ ls -l permissions.txt 
-rwx---r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
This example removes the others read permission.
paul@laika:~/perms$ chmod o-r permissions.txt 
paul@laika:~/perms$ ls -l permissions.txt 
-rwx------ 1 paul paul 0 2007-02-07 22:34 permissions.txt
This example gives all of them the write permission.
paul@laika:~/perms$ chmod a+w permissions.txt 
paul@laika:~/perms$ ls -l permissions.txt 
-rwx-w--w- 1 paul paul 0 2007-02-07 22:34 permissions.txt
You don't even have to type the a.
paul@laika:~/perms$ chmod +x permissions.txt 
paul@laika:~/perms$ ls -l permissions.txt 
-rwx-wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt
You can also set explicit permissions.
paul@laika:~/perms$ chmod u=rw permissions.txt 
paul@laika:~/perms$ ls -l permissions.txt 
-rw--wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt
Feel free to make any kind of combination.
paul@laika:~/perms$ chmod u=rw,g=rw,o=r permissions.txt 
paul@laika:~/perms$ ls -l permissions.txt 
-rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
Even fishy combinations are accepted by chmod.
paul@laika:~/perms$ chmod u=rwx,ug+rw,o=r permissions.txt 
paul@laika:~/perms$ ls -l permissions.txt 
-rwxrw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
standard file permissions
314
32.3.5. setting octal permissions
Most Unix administrators will use the old school octal system to talk about and set
permissions. Look at the triplet bitwise, equating r to 4, w to 2, and x to 1.
Table 32.4. Octal permissions
binary
octal
permission
000
0
---
001
1
--x
010
2
-w-
011
3
-wx
100
4
r--
101
5
r-x
110
6
rw-
111
7
rwx
This makes 777 equal to rwxrwxrwx and by the same logic, 654 mean rw-r-xr-- . The chmod
command will accept these numbers.
paul@laika:~/perms$ chmod 777 permissions.txt 
paul@laika:~/perms$ ls -l permissions.txt 
-rwxrwxrwx 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod 664 permissions.txt 
paul@laika:~/perms$ ls -l permissions.txt 
-rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod 750 permissions.txt 
paul@laika:~/perms$ ls -l permissions.txt 
-rwxr-x--- 1 paul paul 0 2007-02-07 22:34 permissions.txt
standard file permissions
315
32.3.6. umask
When creating a file or directory, a set of default permissions are applied. These default
permissions are determined by the umask. The umask specifies permissions that you do
not want set on by default. You can display the umask with the umask command.
[Harry@RHEL4b ~]$ umask
0002
[Harry@RHEL4b ~]$ touch test
[Harry@RHEL4b ~]$ ls -l test
-rw-rw-r--  1 Harry Harry 0 Jul 24 06:03 test
[Harry@RHEL4b ~]$
As you can also see, the file is also not executable by default. This is a general security
feature among Unixes; newly created files are never executable by default. You have to
explicitly do a chmod +x to make a file executable. This also means that the 1 bit in the
umask has no meaning--a umask of 0022 is the same as 0033.
32.3.7. mkdir -m
When creating directories with mkdir you can use the -m option to set the mode. This
screenshot explains.
paul@debian5~$ mkdir -m 700 MyDir
paul@debian5~$ mkdir -m 777 Public
paul@debian5~$ ls -dl MyDir/ Public/
drwx------ 2 paul paul 4096 2011-10-16 19:16 MyDir/
drwxrwxrwx 2 paul paul 4096 2011-10-16 19:16 Public/
32.3.8. cp -p
To preserve permissions and time stamps from source files, use cp -p.
paul@laika:~/perms$ cp file* cp
paul@laika:~/perms$ cp -p file* cpp
paul@laika:~/perms$ ll *
-rwx------ 1 paul paul    0 2008-08-25 13:26 file33
-rwxr-x--- 1 paul paul    0 2008-08-25 13:26 file42
cp:
total 0
-rwx------ 1 paul paul 0 2008-08-25 13:34 file33
-rwxr-x--- 1 paul paul 0 2008-08-25 13:34 file42
cpp:
total 0
-rwx------ 1 paul paul 0 2008-08-25 13:26 file33
-rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42
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316
32.4. practice: standard file permissions
1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there.
2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ?
3. As root, create a file in the users ~/permissions directory.
4. As normal user, look at who owns this file created by root.
5. Change the ownership of all files in ~/permissions to yourself.
6. Make sure you have all rights to these files, and others can only read.
7. With chmod, is 770 the same as rwxrwx--- ?
8. With chmod, is 664 the same as r-xr-xr-- ?
9. With chmod, is 400 the same as r-------- ?
10. With chmod, is 734 the same as rwxr-xr-- ?
11a. Display the umask in octal and in symbolic form.
11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works.
12. Create a file as root, give only read to others. Can a normal user read this file ? Test
writing to this file with vi.
13a. Create a file as normal user, give only read to others. Can another normal user read this
file ? Test writing to this file with vi.
13b. Can root read this file ? Can root write to this file with vi ?
14. Create a directory that belongs to a group, where every member of that group can read
and write to files, and create files. Make sure that people can only delete their own files.
standard file permissions
317
32.5. solution: standard file permissions
1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there.
mkdir ~/permissions ; touch ~/permissions/myfile.txt
2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ?
cp /etc/hosts ~/permissions/
The copy is owned by you.
3. As root, create a file in the users ~/permissions directory.
(become root)# touch /home/username/permissions/rootfile
4. As normal user, look at who owns this file created by root.
ls -l ~/permissions
The file created by root is owned by root.
5. Change the ownership of all files in ~/permissions to yourself.
chown user ~/permissions/*
You cannot become owner of the file that belongs to root.
6. Make sure you have all rights to these files, and others can only read.
chmod 644 (on files)
chmod 755 (on directories)
7. With chmod, is 770 the same as rwxrwx--- ?
yes
8. With chmod, is 664 the same as r-xr-xr-- ?
No
9. With chmod, is 400 the same as r-------- ?
yes
10. With chmod, is 734 the same as rwxr-xr-- ?
no
11a. Display the umask in octal and in symbolic form.
umask ; umask -S
11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works.
umask -S u=rwx,go=
standard file permissions
318
12. Create a file as root, give only read to others. Can a normal user read this file ? Test
writing to this file with vi.
(become root)
# echo hello > /home/username/root.txt 
# chmod 744 /home/username/root.txt
(become user)
vi ~/root.txt
13a. Create a file as normal user, give only read to others. Can another normal user read this
file ? Test writing to this file with vi.
echo hello > file ; chmod 744 file
Yes, others can read this file
13b. Can root read this file ? Can root write to this file with vi ?
Yes, root can read and write to this file. Permissions do not apply to root.
14. Create a directory that belongs to a group, where every member of that group can read
and write to files, and create files. Make sure that people can only delete their own files.
mkdir /home/project42 ; groupadd project42
chgrp project42 /home/project42 ; chmod 775 /home/project42
You can not yet do the last part of this exercise...
319
Chapter 33. advanced file
permissions
advanced file permissions
320
33.1. sticky bit on directory
You can set the sticky bit on a directory to prevent users from removing files that they do
not own as a user owner. The sticky bit is displayed at the same location as the x permission
for others. The sticky bit is represented by a t (meaning x is also there) or a T (when there
is no x for others).
root@RHELv4u4:~# mkdir /project55
root@RHELv4u4:~# ls -ld /project55
drwxr-xr-x  2 root root 4096 Feb  7 17:38 /project55
root@RHELv4u4:~# chmod +t /project55/
root@RHELv4u4:~# ls -ld /project55
drwxr-xr-t  2 root root 4096 Feb  7 17:38 /project55
root@RHELv4u4:~#
The sticky bit can also be set with octal permissions, it is binary 1 in the first of four triplets.
root@RHELv4u4:~# chmod 1775 /project55/
root@RHELv4u4:~# ls -ld /project55
drwxrwxr-t  2 root root 4096 Feb  7 17:38 /project55
root@RHELv4u4:~#
You will typically find the sticky bit on the /tmp directory.
root@barry:~# ls -ld /tmp
drwxrwxrwt 6 root root 4096 2009-06-04 19:02 /tmp
33.2. setgid bit on directory
setgid can be used on directories to make sure that all files inside the directory are owned
by the group owner of the directory. The setgid bit is displayed at the same location as the x
permission for group owner. The setgid bit is represented by an s (meaning x is also there)
or a S (when there is no x for the group owner). As this example shows, even though root
does not belong to the group proj55, the files created by root in /project55 will belong to
proj55 since the setgid is set.
root@RHELv4u4:~# groupadd proj55
root@RHELv4u4:~# chown root:proj55 /project55/
root@RHELv4u4:~# chmod 2775 /project55/
root@RHELv4u4:~# touch /project55/fromroot.txt
root@RHELv4u4:~# ls -ld /project55/
drwxrwsr-x  2 root proj55 4096 Feb  7 17:45 /project55/
root@RHELv4u4:~# ls -l /project55/
total 4
-rw-r--r--  1 root proj55 0 Feb  7 17:45 fromroot.txt
root@RHELv4u4:~#
You can use the find command to find all setgid directories.
paul@laika:~$ find / -type d -perm -2000 2> /dev/null
/var/log/mysql
/var/log/news
/var/local
...
advanced file permissions
321
33.3. setgid and setuid on regular files
These two permissions cause an executable file to be executed with the permissions of the
file owner instead of the executing owner. This means that if any user executes a program
that belongs to the root user, and the setuid bit is set on that program, then the program
runs as root. This can be dangerous, but sometimes this is good for security.
Take the example of passwords; they are stored in /etc/shadow which is only readable by
root. (The root user never needs permissions anyway.)
root@RHELv4u4:~# ls -l /etc/shadow
-r--------  1 root root 1260 Jan 21 07:49 /etc/shadow
  
Changing your password requires an update of this file, so how can normal non-root users
do this? Let's take a look at the permissions on the /usr/bin/passwd.
root@RHELv4u4:~# ls -l /usr/bin/passwd 
-r-s--x--x  1 root root 21200 Jun 17  2005 /usr/bin/passwd
  
When running the passwd program, you are executing it with root credentials.
You can use the find command to find all setuid programs.
paul@laika:~$ find /usr/bin -type f -perm -04000
/usr/bin/arping
/usr/bin/kgrantpty
/usr/bin/newgrp
/usr/bin/chfn
/usr/bin/sudo
/usr/bin/fping6
/usr/bin/passwd
/usr/bin/gpasswd
...
  
In most cases, setting the setuid bit on executables is sufficient. Setting the setgid bit will
result in these programs to run with the credentials of their group owner.
33.4. setuid on sudo
The sudo binary has the setuid bit set, so any user can run it with the effective userid of root.
paul@rhel65:~$ ls -l $(which sudo)
---s--x--x. 1 root root 123832 Oct  7  2013 /usr/bin/sudo
paul@rhel65:~$
advanced file permissions
322
33.5. practice: sticky, setuid and setgid bits
1a. Set up a directory, owned by the group sports.
1b. Members of the sports group should be able to create files in this directory.
1c. All files created in this directory should be group-owned by the sports group.
1d. Users should be able to delete only their own user-owned files.
1e. Test that this works!
2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your
password as a normal user. Reset the permissions back and try again.
3. If time permits (or if you are waiting for other students to finish this practice), read about
file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and
test that it works.
advanced file permissions
323
33.6. solution: sticky, setuid and setgid bits
1a. Set up a directory, owned by the group sports.
groupadd sports
mkdir /home/sports
chown root:sports /home/sports
1b. Members of the sports group should be able to create files in this directory.
chmod 770 /home/sports
1c. All files created in this directory should be group-owned by the sports group.
chmod 2770 /home/sports
1d. Users should be able to delete only their own user-owned files.
chmod +t /home/sports
1e. Test that this works!
Log in with different users (group members and others and root), create files and watch the
permissions. Try changing and deleting files...
2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your
password as a normal user. Reset the permissions back and try again.
root@deb503:~# ls -l /usr/bin/passwd 
-rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd
root@deb503:~# chmod 755 /usr/bin/passwd 
root@deb503:~# ls -l /usr/bin/passwd 
-rwxr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd
 
A normal user cannot change password now.
root@deb503:~# chmod 4755 /usr/bin/passwd 
root@deb503:~# ls -l /usr/bin/passwd 
-rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd
 
3. If time permits (or if you are waiting for other students to finish this practice), read about
file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and
test that it works.
paul@laika:~$ sudo su -
[sudo] password for paul: 
root@laika:~# mkdir attr
root@laika:~# cd attr/
root@laika:~/attr# touch file42
root@laika:~/attr# lsattr
------------------ ./file42
root@laika:~/attr# chattr +i file42 
advanced file permissions
324
root@laika:~/attr# lsattr
----i------------- ./file42
root@laika:~/attr# rm -rf file42 
rm: cannot remove `file42': Operation not permitted
root@laika:~/attr# chattr -i file42 
root@laika:~/attr# rm -rf file42 
root@laika:~/attr#
325
Chapter 34. access control lists
Standard Unix permissions might not be enough for some organisations. This chapter
introduces access control lists or acl's to further protect files and directories.
access control lists
326
34.1. acl in /etc/fstab
File systems that support access control lists, or acls, have to be mounted with the acl
option listed in /etc/fstab. In the example below, you can see that the root file system has
acl support, whereas /home/data does not.
root@laika:~# tail -4 /etc/fstab
/dev/sda1        /              ext3     acl,relatime    0  1
/dev/sdb2        /home/data     auto     noacl,defaults  0  0
pasha:/home/r    /home/pasha    nfs      defaults        0  0
wolf:/srv/data   /home/wolf     nfs      defaults        0  0
34.2. getfacl
Reading acls can be done with /usr/bin/getfacl. This screenshot shows how to read the acl
of file33 with getfacl.
paul@laika:~/test$ getfacl file33
# file: file33
# owner: paul
# group: paul
user::rw-
group::r--
mask::rwx
other::r--
34.3. setfacl
Writing or changing acls can be done with /usr/bin/setfacl. These screenshots show how
to change the acl of file33 with setfacl.
First we add user sandra with octal permission 7 to the acl.
paul@laika:~/test$ setfacl -m u:sandra:7 file33
Then we add the group tennis with octal permission 6 to the acl of the same file.
paul@laika:~/test$ setfacl -m g:tennis:6 file33
The result is visible with getfacl.
paul@laika:~/test$ getfacl file33 
# file: file33
# owner: paul
# group: paul
user::rw-
user:sandra:rwx
group::r--
group:tennis:rw-
mask::rwx
other::r--
access control lists
327
34.4. remove an acl entry
The -x option of the setfacl command will remove an acl entry from the targeted file.
paul@laika:~/test$ setfacl -m u:sandra:7 file33 
paul@laika:~/test$ getfacl file33 | grep sandra
user:sandra:rwx
paul@laika:~/test$ setfacl -x sandra file33
paul@laika:~/test$ getfacl file33 | grep sandra
Note that omitting the u or g when defining the acl for an account will default it to a user
account.
34.5. remove the complete acl
The -b option of the setfacl command will remove the acl from the targeted file.
paul@laika:~/test$ setfacl -b file33 
paul@laika:~/test$ getfacl file33 
# file: file33
# owner: paul
# group: paul
user::rw-
group::r--
other::r--
34.6. the acl mask
The acl mask defines the maximum effective permissions for any entry in the acl. This
mask is calculated every time you execute the setfacl or chmod commands.
You can prevent the calculation by using the --no-mask switch.
paul@laika:~/test$ setfacl --no-mask -m u:sandra:7 file33
paul@laika:~/test$ getfacl file33
# file: file33
# owner: paul
# group: paul
user::rw-
user:sandra:rwx   #effective:rw-
group::r--
mask::rw-
other::r--
  
access control lists
328
34.7. eiciel
Desktop users might want to use eiciel to manage acls with a graphical tool.
You will need to install eiciel and nautilus-actions to have an extra tab in nautilus to
manage acls.
paul@laika:~$ sudo aptitude install eiciel nautilus-actions
  
329
Chapter 35. file links
An average computer using Linux has a file system with many hard links and symbolic
links.
To understand links in a file system, you first have to understand what an inode is.
file links
330
35.1. inodes
35.1.1. inode contents
An inode is a data structure that contains metadata about a file. When the file system stores
a new file on the hard disk, it stores not only the contents (data) of the file, but also extra
properties like the name of the file, the creation date, its permissions, the owner of the file,
and more. All this information (except the name of the file and the contents of the file) is
stored in the inode of the file.
The ls -l command will display some of the inode contents, as seen in this screenshot.
root@rhel53 ~# ls -ld /home/project42/
drwxr-xr-x 4 root pro42 4.0K Mar 27 14:29 /home/project42/
35.1.2. inode table
The inode table contains all of the inodes and is created when you create the file system
(with mkfs). You can use the df -i command to see how many inodes are used and free on
mounted file systems.
root@rhel53 ~# df -i
Filesystem            Inodes   IUsed   IFree IUse% Mounted on
/dev/mapper/VolGroup00-LogVol00
                     4947968  115326 4832642    3% /
/dev/hda1              26104      45   26059    1% /boot
tmpfs                  64417       1   64416    1% /dev/shm
/dev/sda1             262144    2207  259937    1% /home/project42
/dev/sdb1              74400    5519   68881    8% /home/project33
/dev/sdb5                  0       0       0    -  /home/sales
/dev/sdb6             100744      11  100733    1% /home/research
In the df -i screenshot above you can see the inode usage for several mounted file systems.
You don't see numbers for /dev/sdb5 because it is a fat file system.
35.1.3. inode number
Each inode has a unique number (the inode number). You can see the inode numbers with
the ls -li command.
paul@RHELv4u4:~/test$ touch file1
paul@RHELv4u4:~/test$ touch file2
paul@RHELv4u4:~/test$ touch file3
paul@RHELv4u4:~/test$ ls -li
total 12
817266 -rw-rw-r--  1 paul paul 0 Feb  5 15:38 file1
817267 -rw-rw-r--  1 paul paul 0 Feb  5 15:38 file2
817268 -rw-rw-r--  1 paul paul 0 Feb  5 15:38 file3
paul@RHELv4u4:~/test$
These three files were created one after the other and got three different inodes (the first
column). All the information you see with this ls command resides in the inode, except for
the filename (which is contained in the directory).
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331
35.1.4. inode and file contents
Let's put some data in one of the files.
paul@RHELv4u4:~/test$ ls -li
total 16
817266 -rw-rw-r--  1 paul paul  0 Feb  5 15:38 file1
817270 -rw-rw-r--  1 paul paul 92 Feb  5 15:42 file2
817268 -rw-rw-r--  1 paul paul  0 Feb  5 15:38 file3
paul@RHELv4u4:~/test$ cat file2
It is winter now and it is very cold.
We do not like the cold, we prefer hot summer nights.
paul@RHELv4u4:~/test$
The data that is displayed by the cat command is not in the inode, but somewhere else on
the disk. The inode contains a pointer to that data.
35.2. about directories
35.2.1. a directory is a table
A directory is a special kind of file that contains a table which maps filenames to inodes.
Listing our current directory with ls -ali will display the contents of the directory file.
paul@RHELv4u4:~/test$ ls -ali
total 32
817262 drwxrwxr-x   2 paul paul 4096 Feb  5 15:42 .
800768 drwx------  16 paul paul 4096 Feb  5 15:42 ..
817266 -rw-rw-r--   1 paul paul    0 Feb  5 15:38 file1
817270 -rw-rw-r--   1 paul paul   92 Feb  5 15:42 file2
817268 -rw-rw-r--   1 paul paul    0 Feb  5 15:38 file3
paul@RHELv4u4:~/test$
35.2.2. . and ..
You can see five names, and the mapping to their five inodes. The dot . is a mapping to itself,
and the dotdot .. is a mapping to the parent directory. The three other names are mappings
to different inodes.
file links
332
35.3. hard links
35.3.1. creating hard links
When we create a hard link to a file with ln, an extra entry is added in the directory. A new
file name is mapped to an existing inode.
paul@RHELv4u4:~/test$ ln file2 hardlink_to_file2
paul@RHELv4u4:~/test$ ls -li
total 24
817266 -rw-rw-r--  1 paul paul  0 Feb  5 15:38 file1
817270 -rw-rw-r--  2 paul paul 92 Feb  5 15:42 file2
817268 -rw-rw-r--  1 paul paul  0 Feb  5 15:38 file3
817270 -rw-rw-r--  2 paul paul 92 Feb  5 15:42 hardlink_to_file2
paul@RHELv4u4:~/test$
Both files have the same inode, so they will always have the same permissions and the same
owner. Both files will have the same content. Actually, both files are equal now, meaning
you can safely remove the original file, the hardlinked file will remain. The inode contains
a counter, counting the number of hard links to itself. When the counter drops to zero, then
the inode is emptied.
35.3.2. finding hard links
You can use the find command to look for files with a certain inode. The screenshot below
shows how to search for all filenames that point to inode 817270. Remember that an inode
number is unique to its partition.
paul@RHELv4u4:~/test$ find / -inum 817270 2> /dev/null
/home/paul/test/file2
/home/paul/test/hardlink_to_file2
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333
35.4. symbolic links
Symbolic links (sometimes called soft links) do not link to inodes, but create a name to
name mapping. Symbolic links are created with ln -s. As you can see below, the symbolic
link gets an inode of its own.
paul@RHELv4u4:~/test$ ln -s file2 symlink_to_file2
paul@RHELv4u4:~/test$ ls -li
total 32
817273 -rw-rw-r--  1 paul paul  13 Feb  5 17:06 file1
817270 -rw-rw-r--  2 paul paul 106 Feb  5 17:04 file2
817268 -rw-rw-r--  1 paul paul   0 Feb  5 15:38 file3
817270 -rw-rw-r--  2 paul paul 106 Feb  5 17:04 hardlink_to_file2
817267 lrwxrwxrwx  1 paul paul   5 Feb  5 16:55 symlink_to_file2 -> file2
paul@RHELv4u4:~/test$
Permissions on a symbolic link have no meaning, since the permissions of the target apply.
Hard links are limited to their own partition (because they point to an inode), symbolic links
can link anywhere (other file systems, even networked).
35.5. removing links
Links can be removed with rm.
paul@laika:~$ touch data.txt
paul@laika:~$ ln -s data.txt sl_data.txt
paul@laika:~$ ln data.txt hl_data.txt
paul@laika:~$ rm sl_data.txt 
paul@laika:~$ rm hl_data.txt
file links
334
35.6. practice : links
1. Create two files named winter.txt and summer.txt, put some text in them.
2. Create a hard link to winter.txt named hlwinter.txt.
3. Display the inode numbers of these three files, the hard links should have the same inode.
4. Use the find command to list the two hardlinked files
5. Everything about a file is in the inode, except two things : name them!
6. Create a symbolic link to summer.txt called slsummer.txt.
7. Find all files with inode number 2. What does this information tell you ?
8. Look at the directories /etc/init.d/ /etc/rc2.d/ /etc/rc3.d/ ... do you see the links ?
9. Look in /lib with ls -l...
10. Use find to look in your home directory for regular files that do not(!) have one hard link.
file links
335
35.7. solution : links
1. Create two files named winter.txt and summer.txt, put some text in them.
echo cold > winter.txt ; echo hot > summer.txt
2. Create a hard link to winter.txt named hlwinter.txt.
ln winter.txt hlwinter.txt
3. Display the inode numbers of these three files, the hard links should have the same inode.
ls -li winter.txt summer.txt hlwinter.txt
4. Use the find command to list the two hardlinked files
find . -inum xyz #replace xyz with the inode number
5. Everything about a file is in the inode, except two things : name them!
The name of the file is in a directory, and the contents is somewhere on the disk.
6. Create a symbolic link to summer.txt called slsummer.txt.
ln -s summer.txt slsummer.txt
7. Find all files with inode number 2. What does this information tell you ?
It tells you there is more than one inode table (one for every formatted partition + virtual
file systems)
8. Look at the directories /etc/init.d/ /etc/rc.d/ /etc/rc3.d/ ... do you see the links ?
ls -l /etc/init.d
ls -l /etc/rc2.d
ls -l /etc/rc3.d
9. Look in /lib with ls -l...
ls -l /lib
10. Use find to look in your home directory for regular files that do not(!) have one hard link.
find ~ ! -links 1 -type f
Part X. Appendices
337
Table of Contents
A. keyboard settings ............................................................................................................  338
A.1. about keyboard layout ............................................................................................  338
A.2. X Keyboard Layout ...............................................................................................  338
A.3. shell keyboard layout .............................................................................................  338
B. hardware .......................................................................................................................  340
B.1. buses ...................................................................................................................  340
B.2. interrupts ..............................................................................................................  341
B.3. io ports ................................................................................................................  342
B.4. dma .....................................................................................................................  342
C. License ..........................................................................................................................  344
338
Appendix A. keyboard settings
A.1. about keyboard layout
Many people (like US-Americans) prefer the default US-qwerty keyboard layout. So when
you are not from the USA and want a local keyboard layout on your system, then the best
practice is to select this keyboard at installation time. Then the keyboard layout will always
be correct. Also, whenever you use ssh to remotely manage a Linux system, your local
keyboard layout will be used, independent of the server keyboard configuration. So you will
not find much information on changing keyboard layout on the fly on linux, because not
many people need it. Below are some tips to help you.
A.2. X Keyboard Layout
This is the relevant portion in /etc/X11/xorg.conf, first for Belgian azerty, then for US-
qwerty.
[paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf 
        Option      "XkbModel" "pc105"
        Option      "XkbLayout" "be"
[paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf
        Option      "XkbModel" "pc105"
        Option      "XkbLayout" "us"
When in Gnome or KDE or any other graphical environment, look in the graphical menu in
preferences, there will be a keyboard section to choose your layout. Use the graphical menu
instead of editing xorg.conf.
A.3. shell keyboard layout
When in bash, take a look in the /etc/sysconfig/keyboard file. Below a sample US-qwerty
configuration, followed by a Belgian azerty configuration.
[paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard 
KEYBOARDTYPE="pc"
KEYTABLE="us"
 
[paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard 
KEYBOARDTYPE="pc"
KEYTABLE="be-latin1"
 
The keymaps themselves can be found in /usr/share/keymaps or /lib/kbd/keymaps.
[paul@RHEL5 ~]$ ls -l /lib/kbd/keymaps/
total 52
drwxr-xr-x 2 root root 4096 Apr  1 00:14 amiga
keyboard settings
339
drwxr-xr-x 2 root root 4096 Apr  1 00:14 atari
drwxr-xr-x 8 root root 4096 Apr  1 00:14 i386
drwxr-xr-x 2 root root 4096 Apr  1 00:14 include
drwxr-xr-x 4 root root 4096 Apr  1 00:14 mac
lrwxrwxrwx 1 root root    3 Apr  1 00:14 ppc -> mac
drwxr-xr-x 2 root root 4096 Apr  1 00:14 sun
340
Appendix B. hardware
B.1. buses
B.1.1. about buses
Hardware components communicate with the Central Processing Unit or cpu over a bus.
The most common buses today are usb, pci, agp, pci-express and pcmcia aka pc-card.
These are all Plag and Play buses.
Older x86 computers often had isa buses, which can be configured using jumpers or dip
switches.
B.1.2. /proc/bus
To list the buses recognised by the Linux kernel on your computer, look at the contents of
the /proc/bus/ directory (screenshot from Ubuntu 7.04 and RHEL4u4 below).
root@laika:~# ls /proc/bus/
input  pccard  pci  usb
  
[root@RHEL4b ~]# ls /proc/bus/
input  pci  usb
  
Can you guess which of these two screenshots was taken on a laptop ?
B.1.3. /usr/sbin/lsusb
To list all the usb devices connected to your system, you could read the contents of /proc/
bus/usb/devices (if it exists) or you could use the more readable output of lsusb, which is
executed here on a SPARC system with Ubuntu.
root@shaka:~# lsusb
Bus 001 Device 002: ID 0430:0100 Sun Microsystems, Inc. 3-button Mouse
Bus 001 Device 003: ID 0430:0005 Sun Microsystems, Inc. Type 6 Keyboard
Bus 001 Device 001: ID 04b0:0136 Nikon Corp. Coolpix 7900 (storage)
root@shaka:~# 
  
B.1.4. /var/lib/usbutils/usb.ids
The /var/lib/usbutils/usb.ids file contains a gzipped list of all known usb devices.
paul@barry:~$ zmore /var/lib/usbutils/usb.ids | head
------> /var/lib/usbutils/usb.ids <------
#
# List of USB ID's
#
# Maintained by Vojtech Pavlik <vojtech@suse.cz>
hardware
341
# If you have any new entries, send them to the maintainer.
# The latest version can be obtained from
#  http://www.linux-usb.org/usb.ids
#
# $Id: usb.ids,v 1.225 2006/07/13 04:18:02 dbrownell Exp $
  
B.1.5. /usr/sbin/lspci
To get a list of all pci devices connected, you could take a look at /proc/bus/pci or run lspci
(partial output below).
paul@laika:~$ lspci
...
00:06.0 FireWire (IEEE 1394): Texas Instruments TSB43AB22/A IEEE-139...
00:08.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-816...
00:09.0 Multimedia controller: Philips Semiconductors SAA7133/SAA713...
00:0a.0 Network controller: RaLink RT2500 802.11g Cardbus/mini-PCI 
00:0f.0 RAID bus controller: VIA Technologies, Inc. VIA VT6420 SATA ...
00:0f.1 IDE interface: VIA Technologies, Inc. VT82C586A/B/VT82C686/A...
00:10.0 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1....
00:10.1 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1....
...
  
B.2. interrupts
B.2.1. about interrupts
An interrupt request or IRQ is a request from a device to the CPU. A device raises an
interrupt when it requires the attention of the CPU (could be because the device has data
ready to be read by the CPU).
Since the introduction of pci, irq's can be shared among devices.
Interrupt 0 is always reserved for the timer, interrupt 1 for the keyboard. IRQ 2 is used as a
channel for IRQ's 8 to 15, and thus is the same as IRQ 9.
B.2.2. /proc/interrupts
You can see a listing of interrupts on your system in /proc/interrupts.
paul@laika:~$ cat /proc/interrupts 
      CPU0     CPU1       
0:  1320048     555  IO-APIC-edge      timer
1:    10224       7  IO-APIC-edge      i8042
7:        0       0  IO-APIC-edge      parport0
8:        2       1  IO-APIC-edge      rtc
10:     3062     21  IO-APIC-fasteoi   acpi
12:      131      2  IO-APIC-edge      i8042
15:    47073      0  IO-APIC-edge      ide1
18:        0      1  IO-APIC-fasteoi   yenta
19:    31056      1  IO-APIC-fasteoi   libata, ohci1394
20:    19042      1  IO-APIC-fasteoi   eth0
21:    44052      1  IO-APIC-fasteoi   uhci_hcd:usb1, uhci_hcd:usb2,...
22:   188352      1  IO-APIC-fasteoi   ra0
hardware
342
23:   632444      1  IO-APIC-fasteoi   nvidia
24:     1585      1  IO-APIC-fasteoi   VIA82XX-MODEM, VIA8237
  
B.2.3. dmesg
You can also use dmesg to find irq's allocated at boot time.
paul@laika:~$ dmesg | grep "irq 1[45]"
[ 28.930069] ata3: PATA max UDMA/133 cmd 0x1f0 ctl 0x3f6 bmdma 0x2090 irq 14
[ 28.930071] ata4: PATA max UDMA/133 cmd 0x170 ctl 0x376 bmdma 0x2098 irq 15
  
B.3. io ports
B.3.1. about io ports
Communication in the other direction, from CPU to device, happens through IO ports. The
CPU writes data or control codes to the IO port of the device. But this is not only a one way
communication, the CPU can also use a device's IO port to read status information about the
device. Unlike interrupts, ports cannot be shared!
B.3.2. /proc/ioports
You can see a listing of your system's IO ports via /proc/ioports.
[root@RHEL4b ~]# cat /proc/ioports 
0000-001f : dma1
0020-0021 : pic1
0040-0043 : timer0
0050-0053 : timer1
0060-006f : keyboard
0070-0077 : rtc
0080-008f : dma page reg
00a0-00a1 : pic2
00c0-00df : dma2
00f0-00ff : fpu
0170-0177 : ide1
02f8-02ff : serial
...
  
B.4. dma
B.4.1. about dma
A device that needs a lot of data, interrupts and ports can pose a heavy load on the cpu. With
dma or Direct Memory Access a device can gain (temporary) access to a specific range
of the ram memory.
B.4.2. /proc/dma
Looking at /proc/dma might not give you the information that you want, since it only
contains currently assigned dma channels for isa devices.
hardware
343
root@laika:~# cat /proc/dma 
1: parport0
4: cascade
  
pci devices that are using dma are not listed in /proc/dma, in this case dmesg can be useful.
The screenshot below shows that during boot the parallel port received dma channel 1, and
the Infrared port received dma channel 3.
root@laika:~# dmesg | egrep -C 1 'dma 1|dma 3'
[   20.576000] parport: PnPBIOS parport detected.
[   20.580000] parport0: PC-style at 0x378 (0x778), irq 7, dma 1...
[   20.764000] irda_init()
--
[   21.204000] pnp: Device 00:0b activated.
[   21.204000] nsc_ircc_pnp_probe() : From PnP, found firbase 0x2F8...
[   21.204000] nsc-ircc, chip->init
  
344
Appendix C. License
GNU Free Documentation License
Version 1.3, 3 November 2008
Copyright Â© 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other
functional and useful document "free" in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way
to get credit for their work, while not being considered responsible
for modifications made by others.
This License is a kind of "copyleft", which means that derivative
works of the document must themselves be free in the same sense. It
complements the GNU General Public License, which is a copyleft
license designed for free software.
We have designed this License in order to use it for manuals for free
software, because free software needs free documentation: a free
program should come with manuals providing the same freedoms that the
software does. But this License is not limited to software manuals; it
can be used for any textual work, regardless of subject matter or
whether it is published as a printed book. We recommend this License
principally for works whose purpose is instruction or reference.
1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium, that
contains a notice placed by the copyright holder saying it can be
distributed under the terms of this License. Such a notice grants a
world-wide, royalty-free license, unlimited in duration, to use that
work under the conditions stated herein. The "Document", below, refers
to any such manual or work. Any member of the public is a licensee,
and is addressed as "you". You accept the license if you copy, modify
or distribute the work in a way requiring permission under copyright
law.
A "Modified Version" of the Document means any work containing the
Document or a portion of it, either copied verbatim, or with
modifications and/or translated into another language.
A "Secondary Section" is a named appendix or a front-matter section of
the Document that deals exclusively with the relationship of the
publishers or authors of the Document to the Document's overall
subject (or to related matters) and contains nothing that could fall
directly within that overall subject. (Thus, if the Document is in
part a textbook of mathematics, a Secondary Section may not explain
any mathematics.) The relationship could be a matter of historical
connection with the subject or with related matters, or of legal,
commercial, philosophical, ethical or political position regarding
them.
The "Invariant Sections" are certain Secondary Sections whose titles
License
345
are designated, as being those of Invariant Sections, in the notice
that says that the Document is released under this License. If a
section does not fit the above definition of Secondary then it is not
allowed to be designated as Invariant. The Document may contain zero
Invariant Sections. If the Document does not identify any Invariant
Sections then there are none.
The "Cover Texts" are certain short passages of text that are listed,
as Front-Cover Texts or Back-Cover Texts, in the notice that says that
the Document is released under this License. A Front-Cover Text may be
at most 5 words, and a Back-Cover Text may be at most 25 words.
A "Transparent" copy of the Document means a machine-readable copy,
represented in a format whose specification is available to the
general public, that is suitable for revising the document
straightforwardly with generic text editors or (for images composed of
pixels) generic paint programs or (for drawings) some widely available
drawing editor, and that is suitable for input to text formatters or
for automatic translation to a variety of formats suitable for input
to text formatters. A copy made in an otherwise Transparent file
format whose markup, or absence of markup, has been arranged to thwart
or discourage subsequent modification by readers is not Transparent.
An image format is not Transparent if used for any substantial amount
of text. A copy that is not "Transparent" is called "Opaque".
Examples of suitable formats for Transparent copies include plain
ASCII without markup, Texinfo input format, LaTeX input format, SGML
or XML using a publicly available DTD, and standard-conforming simple
HTML, PostScript or PDF designed for human modification. Examples of
transparent image formats include PNG, XCF and JPG. Opaque formats
include proprietary formats that can be read and edited only by
proprietary word processors, SGML or XML for which the DTD and/or
processing tools are not generally available, and the
machine-generated HTML, PostScript or PDF produced by some word
processors for output purposes only.
The "Title Page" means, for a printed book, the title page itself,
plus such following pages as are needed to hold, legibly, the material
this License requires to appear in the title page. For works in
formats which do not have any title page as such, "Title Page" means
the text near the most prominent appearance of the work's title,
preceding the beginning of the body of the text.
The "publisher" means any person or entity that distributes copies of
the Document to the public.
A section "Entitled XYZ" means a named subunit of the Document whose
title either is precisely XYZ or contains XYZ in parentheses following
text that translates XYZ in another language. (Here XYZ stands for a
specific section name mentioned below, such as "Acknowledgements",
"Dedications", "Endorsements", or "History".) To "Preserve the Title"
of such a section when you modify the Document means that it remains a
section "Entitled XYZ" according to this definition.
The Document may include Warranty Disclaimers next to the notice which
states that this License applies to the Document. These Warranty
Disclaimers are considered to be included by reference in this
License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and has
no effect on the meaning of this License.
2. VERBATIM COPYING
You may copy and distribute the Document in any medium, either
License
346
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no
other conditions whatsoever to those of this License. You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute. However, you may accept
compensation in exchange for copies. If you distribute a large enough
number of copies you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and
you may publicly display copies.
3. COPYING IN QUANTITY
If you publish printed copies (or copies in media that commonly have
printed covers) of the Document, numbering more than 100, and the
Document's license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover. Both covers must also clearly and legibly identify you
as the publisher of these copies. The front cover must present the
full title with all words of the title equally prominent and visible.
You may add other material on the covers in addition. Copying with
changes limited to the covers, as long as they preserve the title of
the Document and satisfy these conditions, can be treated as verbatim
copying in other respects.
If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.
If you publish or distribute Opaque copies of the Document numbering
more than 100, you must either include a machine-readable Transparent
copy along with each Opaque copy, or state in or with each Opaque copy
a computer-network location from which the general network-using
public has access to download using public-standard network protocols
a complete Transparent copy of the Document, free of added material.
If you use the latter option, you must take reasonably prudent steps,
when you begin distribution of Opaque copies in quantity, to ensure
that this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you distribute an
Opaque copy (directly or through your agents or retailers) of that
edition to the public.
It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to
give them a chance to provide you with an updated version of the
Document.
4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it. In addition, you must do these things in the Modified Version:
   * A. Use in the Title Page (and on the covers, if any) a title
distinct from that of the Document, and from those of previous
versions (which should, if there were any, be listed in the History
section of the Document). You may use the same title as a previous
version if the original publisher of that version gives permission.
License
347
   * B. List on the Title Page, as authors, one or more persons or
entities responsible for authorship of the modifications in the
Modified Version, together with at least five of the principal authors
of the Document (all of its principal authors, if it has fewer than
five), unless they release you from this requirement.
   * C. State on the Title page the name of the publisher of the
Modified Version, as the publisher.
   * D. Preserve all the copyright notices of the Document.
   * E. Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.
   * F. Include, immediately after the copyright notices, a license
notice giving the public permission to use the Modified Version under
the terms of this License, in the form shown in the Addendum below.
   * G. Preserve in that license notice the full lists of Invariant
Sections and required Cover Texts given in the Document's license
notice.
   * H. Include an unaltered copy of this License.
   * I. Preserve the section Entitled "History", Preserve its Title,
and add to it an item stating at least the title, year, new authors,
and publisher of the Modified Version as given on the Title Page. If
there is no section Entitled "History" in the Document, create one
stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.
   * J. Preserve the network location, if any, given in the Document
for public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions it
was based on. These may be placed in the "History" section. You may
omit a network location for a work that was published at least four
years before the Document itself, or if the original publisher of the
version it refers to gives permission.
   * K. For any section Entitled "Acknowledgements" or "Dedications",
Preserve the Title of the section, and preserve in the section all the
substance and tone of each of the contributor acknowledgements and/or
dedications given therein.
   * L. Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles. Section numbers or the
equivalent are not considered part of the section titles.
   * M. Delete any section Entitled "Endorsements". Such a section
may not be included in the Modified Version.
   * N. Do not retitle any existing section to be Entitled
"Endorsements" or to conflict in title with any Invariant Section.
   * O. Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant. To do this, add their titles to the
list of Invariant Sections in the Modified Version's license notice.
These titles must be distinct from any other section titles.
You may add a section Entitled "Endorsements", provided it contains
nothing but endorsements of your Modified Version by various
partiesâ€”for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.
You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version. Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity. If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,
License
348
you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.
5. COMBINING DOCUMENTS
You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled "History"
in the various original documents, forming one section Entitled
"History"; likewise combine any sections Entitled "Acknowledgements",
and any sections Entitled "Dedications". You must delete all sections
Entitled "Endorsements".
6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other
documents released under this License, and replace the individual
copies of this License in the various documents with a single copy
that is included in the collection, provided that you follow the rules
of this License for verbatim copying of each of the documents in all
other respects.
You may extract a single document from such a collection, and
distribute it individually under this License, provided you insert a
copy of this License into the extracted document, and follow this
License in all other respects regarding verbatim copying of that
document.
7. AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an "aggregate" if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation's users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document's Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.
License
349
8. TRANSLATION
Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers. In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.
If a section in the Document is Entitled "Acknowledgements",
"Dedications", or "History", the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.
9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License. Any attempt otherwise
to copy, modify, sublicense, or distribute it is void, and will
automatically terminate your rights under this License.
However, if you cease all violation of this License, then your license
from a particular copyright holder is reinstated (a) provisionally,
unless and until the copyright holder explicitly and finally
terminates your license, and (b) permanently, if the copyright holder
fails to notify you of the violation by some reasonable means prior to
60 days after the cessation.
Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after
your receipt of the notice.
Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License. If your rights have been terminated and not permanently
reinstated, receipt of a copy of some or all of the same material does
not give you any rights to use it.
10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the
GNU Free Documentation License from time to time. Such new versions
will be similar in spirit to the present version, but may differ in
detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License "or any later version" applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation. If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation. If the Document specifies
License
350
that a proxy can decide which future versions of this License can be
used, that proxy's public statement of acceptance of a version
permanently authorizes you to choose that version for the Document.
11. RELICENSING
"Massive Multiauthor Collaboration Site" (or "MMC Site") means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works. A
public wiki that anybody can edit is an example of such a server. A
"Massive Multiauthor Collaboration" (or "MMC") contained in the site
means any set of copyrightable works thus published on the MMC site.
"CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.
"Incorporate" means to publish or republish a Document, in whole or in
part, as part of another Document.
An MMC is "eligible for relicensing" if it is licensed under this
License, and if all works that were first published under this License
somewhere other than this MMC, and subsequently incorporated in whole
or in part into the MMC, (1) had no cover texts or invariant sections,
and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009,
provided the MMC is eligible for relicensing.
351
Index
Symbols
; (shell), 136
!! (shell), 156
! (bash history), 156
! (file globbing), 163
? (file globbing), 162
/, 76, 102
/bin, 103, 128
/bin/bash, 125, 292
/bin/cat, 103
/bin/csh, 125
/bin/date, 103
/bin/ksh, 125, 292
/bin/rm, 129
/bin/sh, 125
/boot, 105
/boot/grub, 105
/boot/grub/grub.cfg, 105
/boot/grub/grub.conf, 105
/dev, 85, 109
/dev/null, 109, 175
/dev/pts/1, 109
/dev/random, 120
/dev/tty1, 109
/dev/urandom, 119, 121
/dev/zero, 120
/etc, 105
/etc/bashrc, 293
/etc/default/useradd, 276
/etc/fstab, 326
/etc/group, 299, 308
/etc/gshadow, 301
/etc/hosts, 120
/etc/init.d/, 105
/etc/inputrc, 292
/etc/login.defs, 286
/etc/passwd, 191, 275, 278, 287, 287, 308
/etc/profile, 292
/etc/resolv.conf, 120
/etc/shadow, 283, 285, 321
/etc/shells, 235, 278
/etc/skel, 105, 277
/etc/sudoers, 269, 270
/etc/sysconfig, 105
/etc/sysconfig/firstboot, 106
/etc/sysconfig/harddisks, 106
/etc/sysconfig/hwconf, 106
/etc/sysconfig/keyboard, 106
/etc/X11/xorg.conf, 105
/export, 107
/home, 107
/lib, 104
/lib/kbd/keymaps/, 106
/lib/modules, 104
/lib32, 104
/lib64, 104
/media, 107
/opt, 104
/proc, 85, 109
/proc/bus, 340
/proc/bus/pci, 341
/proc/bus/usb/devices, 340
/proc/cpuinfo, 110
/proc/dma, 342
/proc/interrupts, 112, 341
/proc/ioports, 342
/proc/kcore, 112
/proc/sys, 111
/root, 107
/run, 117
/sbin, 103, 128
/srv, 107
/sys, 113
/tmp, 108, 320
/usr, 114
/usr/bin, 114
/usr/bin/getfacl, 326
/usr/bin/passwd, 321
/usr/bin/setfacl, 326
/usr/include, 114
/usr/lib, 114
/usr/local, 114
/usr/share, 114
/usr/share/games, 115
/usr/share/man, 115
/usr/src, 115
/var, 116
/var/cache, 116
/var/lib, 117
/var/lib/rpm, 117
/var/lib/usbutils/usb.ids, 340
/var/lock, 117
/var/log, 116
/var/log/messages, 116
/var/log/syslog, 116
/var/run, 117
/var/spool, 116
/var/tmp, 117
., 75
.., 75
.. (directory), 331
. (directory), 331
. (shell), 236
.bash_history, 157
.bash_login, 293
.bash_logout, 294
.bash_profile, 292
.bashrc, 292, 293
.exrc, 229
.vimrc, 229
`(backtick), 151
~, 75
Index
352
'(single quote), 151
" (double quotes), 127
(( (shell), 256
-- (shell), 237
[ (file globbing), 163
[ (shell), 241
$? (shell variables), 136
$() embedded shell, 151
$ (shell variables), 142
$HISTFILE, 157
$HISTFILESIZE, 157
$HISTSIZE, 157
$LANG, 164
$PATH, 128, 145
$PS1, 76
* (file globbing), 162
\ (backslash), 138
&, 136
&&, 137
#!/bin/bash, 235
#! (shell), 235
# (pound sign), 138
>, 173
>>, 174
>|, 174
||, 137
1>, 175
2>, 175
2>&1, 175
777, 314
A
access control list, 326
acl, 328
acls, 326
agp, 340
AIX, 4
alias(bash), 129
alias(shell), 129
apropos, 72
arguments(shell), 126
B
backticks, 151
base64, 177
bash, 219, 248
bash history, 156
bash -x, 237
binaries, 103
Bourne again shell, 125
BSD, 4
bunzip2, 201
bus, 340
bzcat, 201
bzip2, 199, 201, 201
bzmore, 201
C
cal, 198
case, 258
case sensitive, 85
cat, 96, 182
cd, 75
cd -, 76
CentOS, 7
chage, 286
chgrp(1), 309
chkconfig, 106
chmod, 277, 314
chmod(1), 226, 313
chmod +x, 235, 315
chown, 277
chown(1), 309
chsh(1), 278
comm(1), 188
command line scan, 126
command mode(vi), 223
copyleft, 11
copyright, 10, 10
cp, 88
cp(1), 88
cpu, 340
crypt, 284
csh, 235
Ctrl d, 96
ctrl-r, 157
current directory, 75
cut, 191
cut(1), 184
D
daemon, 72
date, 197
Debian, 7
Dennis Ritchie, 4
devfs, 113
df -i, 330
directory, 331
distribution, 6
distributions, 102
dma, 342
dmesg(1), 342, 343
dumpkeys(1), 106
E
echo, 126
echo(1), 125, 127
echo $-, 152
echo *, 165
Edubuntu, 7
eiciel, 328
ELF, 104
elif, 242
embedding(shell), 151
Index
353
env(1), 146, 146
environment variable, 142
EOF, 96, 177
escaping (shell), 165
eval, 256
executables, 103
exit (bash), 157
export, 146
F
Fedora, 7
FHS, 102
file, 85
file(1), 104
file globbing, 161
file ownership, 308
Filesystem Hierarchy Standard, 102
filters, 181
find(1), 196, 320, 321, 332
FireWire, 113
for (bash), 242
FOSS, 10
four freedoms, 11
Free Software, 10
free software, 10
freeware, 10
function (shell), 259
G
gcc(1), 285
getfacl, 326
getopts, 251
GID, 299
glob(7), 162
GNU, 4
gpasswd, 301
GPL, 11
GPLv3, 11
grep, 206, 207, 210
grep(1), 182
grep -i, 182
grep -v, 183
groupadd(1), 299
groupdel(1), 300
groupmod(1), 300
groups, 298
groups(1), 299
gunzip(1), 200
gzip, 200
gzip(1), 200
H
hard link, 332
head(1), 95
here directive, 97
here document, 177
here string, 177
hidden files, 77
HP, 4
HP-UX, 4
http://www.pathname.com/fhs/, 102
I
IBM, 4
id, 267
IEEE 1394, 113
if then else (bash), 242
inode, 329, 332
inode table, 330
insert mode(vi), 223
interrupt, 341
IO Ports, 342
IRQ, 341
isa, 340
K
Ken Thompson, 4
kernel, 104
keymaps(5), 106
Korn shell, 158
Korn Shell, 278
ksh, 158, 235
kudzu, 106
L
less(1), 98
let, 257
Linus Torvalds, 4
Linux Mint, 7
ln, 333
ln(1), 332
loadkeys(1), 106
locate(1), 197
logical AND, 137
logical OR, 137
Logiciel Libre, 10
ls, 77, 311, 330
ls(1), 77, 330, 331
ls -l, 310
lspci, 341
lsusb, 340
M
magic, 85
makewhatis, 73
man(1), 72, 72, 73
mandb(1), 73
man hier, 102
man -k, 72
md5, 285
mkdir, 277
mkdir(1), 79, 315
mkdir -p, 79
mkfs, 330
Index
354
more(1), 98
mv, 89
N
noclobber, 174
nounset(shell), 147
O
octal permissions, 314
od(1), 189
OEL, 7
open source, 10
open source definition, 11
open source software, 10
openssl, 284
Oracle Enterprise Linux, 7
owner, 311
P
parent directory, 75
passwd, 283, 283, 284, 286
passwd(1), 73, 321
passwd(5), 73
path, 76, 77
pc-card, 340
pci, 340
pci-express, 340
pcmcia, 340
perl, 212
perldoc, 212
popd, 83
prename, 212
primary group, 276
proprietary, 10
public domain, 10
pushd, 83
pwd, 75
pwd(1), 76
R
random number generator, 120
read, 249
reboot, 157
Red Hat, 7
regular expressions, 158
rename, 90, 212, 213, 214
repository, 6
Richard Stallman, 4
rm, 87
rm(1), 333
rmdir(1), 79
rmdir -p, 80
rm -rf, 87
root, 103, 268, 269, 270, 275
root directory, 102
rpm, 117
S
salt (encryption), 285
Scientific, 7
sed, 190, 215, 216
set, 152
set(shell), 143
set +x, 130
setfacl, 326
setgid, 320, 320
setuid, 237, 321, 321, 321
set -x, 130
she-bang (shell), 235
shell, 291
shell comment, 138
shell embedding, 151
shell escaping, 138
shell expansion, 126, 126
shell functions, 259
shift, 249
shopt, 252
skeleton, 105
sleep, 198
soft link, 333
Solaris, 4
sort, 191
sort(1), 186
source, 236, 250
standard input, 96
standard output, 96
stderr, 172
stdin, 172, 182
stdout, 172, 182
sticky bit, 320
strings(1), 98
su, 268, 268, 287, 301
su -, 145
sudo, 269, 270, 287
sudo su -, 270
Sun, 4
SunOS, 4
superuser, 275
symbolic link, 333
sysfs, 113
System V, 104
T
tab key(bash), 77
tac, 97
tail(1), 95
tee(1), 182
test, 241
time, 199
touch(1), 86
tr, 185
tr(1), 184
type(shell), 128
Index
355
U
Ubuntu, 7
umask(1), 315
unalias(bash), 130
uniq, 191
uniq(1), 187
Unix, 4
unset, 152
unset(shell), 143
until (bash), 243
updatedb(1), 197
usb, 113, 340
useradd, 276, 277, 284
useradd(1), 277
useradd -D, 276
userdel(1), 276
usermod, 287, 287, 300
usermod(1), 276
V
vi, 302
vi(1), 222
vigr(1), 302
vim(1), 222
vimtutor(1), 222
vipw, 287
visudo, 269
vrije software, 10
W
w, 267
wc(1), 185
whatis(1), 72
whereis(1), 72
which(1), 128
while (bash), 243
white space(shell), 126
who, 191, 267
whoami, 267
who am i, 267
wild cards, 163
X
X, 105
X Window System, 105
Z
zcat, 200
zmore, 200