Command Line Interface

A shell is a Command Line Interpreter (C.L.I). It is used to give instructions or commands to the operating system (OS).

The word shell is generic. There are many shells under Unix and Linux such as:

When using Debian 8 /bin/sh is a soft link to /bin/dash :

trainee@debian8:~$ ls -l /bin/sh
lrwxrwxrwx 1 root root 4 Nov  8  2014 /bin/sh -> dash

This unit covers the /bin/bash shell. The /bin/bash shell allows you to:

• Recall previously typed commands
• Auto-generate the end of a file name
• Use Aliases
• Use tables
• Use C language numerical and math variables
• Manage strings
• Use Functions

A command always starts with a keyword. This keyword is interpreted by the shell, in the order shown, as one of the following:

• An Alias,
• A Function,
• A Built-in Command,
• An External Command.

The /bin/bash shell comes with a set of built-in or internal commands. External commands are executable binaries or scripts generally found in one of the following directories:

• /bin,
• /sbin,
• /usr/bin,
• /usr/sbin.

To check if a command is internal to the shell or external, use the type command:

trainee@debian8:~$ type cd
cd is a shell builtin

External commands are either binaries or scripts that can be found in /bin, /sbin, /usr/bin or /usr/sbin :

trainee@debian8:~$ type passwd
passwd is /usr/bin/passwd

Aliases are strings that are aliased to a command, a command and some options or even several commands. Aliases are specific to the shell in which they are created and unless specified in one of the start-up files, they disappear when the shell is closed:

trainee@debian8:~$ type ls
ls is aliased to `ls --color=auto

An alias is defined using the alias command:

trainee@debian8:~$ alias dir='ls -l'
trainee@debian8:~$ dir
total 36
-rw-r--r-- 1 trainee trainee    0 Aug 19 17:08 aac
-rw-r--r-- 1 trainee trainee    0 Aug 19 17:08 abc
-rw-r--r-- 1 trainee trainee    0 Aug 19 17:08 bca
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Desktop
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Documents
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Downloads
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Music
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Pictures
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Public
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Templates
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Videos
-rw-r--r-- 1 trainee trainee  391 Aug 18 23:34 vitext
-rw-r--r-- 1 trainee trainee    0 Aug 19 17:08 xyz

The list of currently defined aliases is obtained by using the alias command with no options:

trainee@debian8:~$ alias
alias dir='ls -l'
alias ls='ls --color=auto'

To force the shell to use the command and not the alias, you can precede the command with the \ character:

trainee@debian8:~$ \dir
aac  bca      Documents  Music	   Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext

To delete an alias, simply use the unalias command:

trainee@debian8:~$ unalias dir
trainee@debian8:~$ dir
aac  bca      Documents  Music	   Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext

Each user's shell is defined by root in the /etc/passwd file:

trainee@debian8:~$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
systemd-timesync:x:100:103:systemd Time Synchronization,,,:/run/systemd:/bin/false
systemd-network:x:101:104:systemd Network Management,,,:/run/systemd/netif:/bin/false
systemd-resolve:x:102:105:systemd Resolver,,,:/run/systemd/resolve:/bin/false
systemd-bus-proxy:x:103:106:systemd Bus Proxy,,,:/run/systemd:/bin/false
trainee:x:1000:1000:trainee,,,:/home/trainee:/bin/bash
sshd:x:104:65534::/var/run/sshd:/usr/sbin/nologin
Debian-exim:x:105:110::/var/spool/exim4:/bin/false
messagebus:x:106:111::/var/run/dbus:/bin/false
statd:x:107:65534::/var/lib/nfs:/bin/false
avahi-autoipd:x:108:113:Avahi autoip daemon,,,:/var/lib/avahi-autoipd:/bin/false
avahi:x:109:115:Avahi mDNS daemon,,,:/var/run/avahi-daemon:/bin/false
colord:x:110:117:colord colour management daemon,,,:/var/lib/colord:/bin/false
dnsmasq:x:111:65534:dnsmasq,,,:/var/lib/misc:/bin/false
speech-dispatcher:x:112:29:Speech Dispatcher,,,:/var/run/speech-dispatcher:/bin/sh
pulse:x:113:119:PulseAudio daemon,,,:/var/run/pulse:/bin/false
rtkit:x:114:121:RealtimeKit,,,:/proc:/bin/false
saned:x:115:122::/var/lib/saned:/bin/false
usbmux:x:116:46:usbmux daemon,,,:/var/lib/usbmux:/bin/false
lightdm:x:117:124:Light Display Manager:/var/lib/lightdm:/bin/false

However, each user can change his shell using the chsh command. The shells available to users are listed in the /etc/shells file:

trainee@debian8:~$ cat /etc/shells
# /etc/shells: valid login shells
/bin/sh
/bin/dash
/bin/bash
/bin/rbash
/usr/bin/screen

Now use the echo command to view the contents of the system variable SHELL for your current session:

trainee@debian8:~$ echo $SHELL
/bin/bash

Now change your shell to /bin/sh using the chsh command:

trainee@debian8:~$ chsh
Password: trainee
Changing the login shell for trainee
Enter the new value, or press ENTER for the default
	Login Shell [/bin/bash]: /bin/sh

Now check your current shell:

trainee@debian8:~$ echo $SHELL
/bin/bash

At first glance nothing has happened. However if you view your entry in the /etc/passwd file you will notice that your login shell has changed:

trainee@debian8:~$ cat /etc/passwd | grep trainee
trainee:x:1000:1000:trainee,,,:/home/trainee:/bin/sh

Now change your shell back to /bin/bash using the chsh command:

trainee@debian8:~$ chsh
Password: trainee
Changing the login shell for trainee
Enter the new value, or press ENTER for the default
	Login Shell [/bin/sh]: /bin/bash

As you have already noticed, the prompt under Linux is different for a normal user and root:

• $ for a user,
• # for root.

/bin/bash keeps track of commands that have been previously executed. To access the command history, use the following command:

trainee@debian8:~$ history | more
    1  su -
    2  su -
    3  exit
    4  su -
    5  exit
    6  ls -l /var
    7  su -
    8  su -
    9  vi vitext
   10  view vitext
   11  clear
   12  stty -a
   13  date
   14  who
   15  df
   16  df -h
   17  free
   18  free -h
   19  whoami
   20  su -
   21  clear
   22  su -
   23  exit
--More--

The history command uses emacs style control characters. As a result you can navigate through the list as follows:

To move around in the history:

Pour rechercher dans l'historique il convient d'utiliser les touches :

It is also possible to recall the last command executed by using the !! characters:

trainee@debian8:~$ ls
aac  bca      Documents  Music     Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext
trainee@debian8:~$ !!
ls
aac  bca      Documents  Music     Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext

Alternatively, to execute a command in the list, you can use the list number preceded by the ! character:

trainee@debian8:~$ !52
ls
aac  bca      Documents  Music     Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext

The environmental variables associated with the history are set in the ~/.bashrc file where ~/ indicates the home directory of the user concerned:

trainee@debian8:~$ cat .bashrc | grep HISTSIZE
# for setting history length see HISTSIZE and HISTFILESIZE in bash(1)
HISTSIZE=1000

As you can see, in the previous case the HISTSIZE value is set to 1000. This means that the last 1,000 commands are held in the history.

The history command stores data in the ~/.bash_history file for each user. The commands for the current bash session are stored in the file when the session is closed:

trainee@debian8:~$ nl .bash_history | more
     1	su -
     2	su -
     3	exit
     4	su -
     5	exit
     6	ls -l /var
     7	su -
     8	su -
     9	vi vitext
    10	view vitext
    11	clear
    12	stty -a
    13	date
    14	who
    15	df
    16	df -h
    17	free
    18	free -h
    19	whoami
    20	su -
    21	clear
    22	su -
    23	exit
--More--

/bin/bash can auto-generate the end of a file name. Consider the following example:

$ ls .b [Tab][Tab][Tab]

By hitting the Tab key three times, the system shows you the files that match:

trainee@debian8:~$ ls .bash
.bash_history  .bash_logout   .bashrc

This same technique can also be used to auto-generate command names. Consider the following example:

$ mo [Tab][Tab]

By hitting the Tab twice the system lists all known commands available to the user and starting with mo:

trainee@debian8:~$ mo
moc         moggsplit   more        mount       mountpoint  mousepad  

It is often necessary and desirable to be able to work with several files at one time as opposed to repeating the operation on each file individually. For this reason, bash accepts the use of Metacharacters:

To illustrate the use of Metacharacters, you need to create a directory in your home directory and the create some files within it:

trainee@debian8:~$ mkdir training
trainee@debian8:~$ cd training
trainee@debian8:~/training$ touch f1 f2 f3 f4 f5

Now use the Metacharacter *:

trainee@debian8:~/training$ echo f*
f1 f2 f3 f4 f5

Create two more files:

trainee@debian8:~/training$ touch f52 f62

Now use the Metacharacter ?:

trainee@debian8:~/training$ echo f?2
f52 f62

The [] Metacharacter can take several forms:

To demonstrate the use of the metacharacter [], create a file called a100:

trainee@debian8:~/training$ touch a100

The use of this Metacharacter can be demonstrated with the following examples:

trainee@debian8:~/training$ echo [a-f]*
a100 f1 f2 f3 f4 f5 f52 f62
trainee@debian8:~/training$ echo [af]*
a100 f1 f2 f3 f4 f5 f52 f62

trainee@debian8:~/training$ echo [!a]*
f1 f2 f3 f4 f5 f52 f62

trainee@debian8:~/training$ echo [a-b]*
a100

trainee@debian8:~/training$ echo [a-f]
[a-f]

In order to use ?(expression), *(expression), +(expression), @(expression) and !(expression), you need to activate the extglob option:

trainee@debian8:~/training$ shopt -s extglob

The shopt command is used to activate and deactivate the shopt option of the shell.

The list of all the options can be displayed by simply using the shopt command:

trainee@debian8:~/training$ shopt
autocd         	off
cdable_vars    	off
cdspell        	off
checkhash      	off
checkjobs      	off
checkwinsize   	on
cmdhist        	on
compat31       	off
compat32       	off
compat40       	off
compat41       	off
compat42       	off
complete_fullquote	on
direxpand      	off
dirspell       	off
dotglob        	off
execfail       	off
expand_aliases 	on
extdebug       	off
extglob        	on
extquote       	on
failglob       	off
force_fignore  	on
globstar       	off
globasciiranges	off
gnu_errfmt     	off
histappend     	on
histreedit     	off
histverify     	off
hostcomplete   	off
huponexit      	off
interactive_comments	on
lastpipe       	off
lithist        	off
login_shell    	on
mailwarn       	off
no_empty_cmd_completion	off
nocaseglob     	off
nocasematch    	off
nullglob       	off
progcomp       	on
promptvars     	on
restricted_shell	off
shift_verbose  	off
sourcepath     	on
xpg_echo       	off

Create the following files:

trainee@debian8:~/training$ touch f f.txt f123.txt f123123.txt f123123123.txt

Execute the following command:

trainee@debian8:~/training$ ls f?(123).txt
f123.txt  f.txt

Execute the following command:

trainee@debian8:~/training$ ls f*(123).txt
f123123123.txt  f123123.txt  f123.txt  f.txt

Execute the following command:

trainee@debian8:~/training$ ls f+(123).txt
f123123123.txt  f123123.txt  f123.txt

Execute the following command:

trainee@debian8:~/training$ ls f@(123).txt
f123.txt

Execute the following command:

trainee@debian8:~/training$ ls f!(123).txt
f123123123.txt  f123123.txt  f.txt

To cancel the wild card effect of a special character, the character needs to be escaped or “protected”:

For example:

trainee@debian8:~/training$ echo * est un caractère spécial
a100 f f1 f123123123.txt f123123.txt f123.txt f2 f3 f4 f5 f52 f62 f.txt est un caractère spécial

trainee@debian8:~/training$ echo \* est un caractère spécial
* est un caractère spécial

trainee@debian8:~/training$ echo "* est un caractère spécial"
* est un caractère spécial

trainee@debian8:~/training$ echo '* est un caractère spécial'
* est un caractère spécial

Each command returns an exit status when it is executed. This exit status is stored in a special variable: $?.

For example:

trainee@debian8:~/training$ cd ..
trainee@debian8:~$ mkdir codes
trainee@debian8:~$ echo $?
0
trainee@debian8:~$ touch codes/exit.txt
trainee@debian8:~$ rmdir codes
rmdir: failed to remove ‘codes’: Directory not empty
trainee@debian8:~$ echo $?
1

As you can see when the exit status is 0, the command has executed correctly. If the exit status is anything else, the command has executed with errors.

Your dialogue with the system uses three file descriptors:

• Standard Input - the keyboard,
• Standard output - the screen,
• Standard error - contains any eventual errors.

The standard output can be redirected using the > character:

trainee@debian8:~$ pwd
/home/trainee
trainee@debian8:~$ cd training
trainee@debian8:~/training$ free > file
trainee@debian8:~/training$ cat file
             total       used       free     shared    buffers     cached
Mem:        506268     429624      76644       4804      59540     130524
-/+ buffers/cache:     239560     266708
Swap:      2046972          0    2046972

Repeating a single redirection will replace the file:

trainee@debian8:~/training$ date > file
trainee@debian8:~/training$ cat file
Mon 28 Nov 14:48:03 GMT 2016

To add additional data to the file, you need to use a double redirection:

trainee@debian8:~/training$ free >> file
trainee@debian8:~/training$ cat file
Mon 28 Nov 14:48:03 GMT 2016
             total       used       free     shared    buffers     cached
Mem:        506268     431668      74600       4804      60264     130912
-/+ buffers/cache:     240492     265776
Swap:      2046972          0    20469722

File descriptors are numbered for ease of use :

• 0 = Standard Input
• 1 = Standard Output
• 2 = Standard Error

For example:

trainee@debian8:~/training$ cd ..
trainee@debian8:~$ rmdir training/ 2>errorlog
trainee@debian8:~$ cat errorlog
rmdir: failed to remove ‘training/’: Directory not empty

You can join file descriptors using the & character:

trainee@debian8:~$ free > file 2>&1

Any errors are sent to the same destination as the standard output, in the case, file.

It is also possible to have a reverse redirection:

trainee@debian8:~$ wc -w < errorlog
8

In this case wc -w counts the number of words in the file.

Other redirections exist :

A pipe is used to present the standard output on the first command to the standard input of the second command:

trainee@debian8:~$ ls | wc -w
17

Standard output can generally only be redirected to a single destination. To redirect to two destinations at once, you need to use the tee command:

trainee@debian8:~$ date | tee file1
Mon 28 Nov 15:14:18 GMT 2016
trainee@debian8:~$ cat file1
Mon 28 Nov 15:14:18 GMT 2016

Alternatively, tee can be used to redirect to two files at the same time:

trainee@debian8:~$ date | tee file1 > file2
trainee@debian8:~$ cat file1
Mon 28 Nov 15:15:52 GMT 2016
trainee@debian8:~$ cat file2
Mon 28 Nov 15:15:52 GMT 2016

Command substitution permits in-line execution of a command:

trainee@debian8:~$ echo date
date
trainee@debian8:~$ echo $(date)
Mon 28 Nov 15:19:29 GMT 2016
trainee@debian8:~$ echo `date`
Mon 28 Nov 15:19:30 GMT 2016

Commands can be grouped using brackets:

$ (ls -l; ps; who) > list

Conditional command execution can be obtained by using the exit status value and either && or ||.

For example,

• Command1 && Command2,
  • Command2 will execute if the exit status of Command1 is 0,
• Command1 || Command2,
  • Command2 will execute if the exit status of Command1 anything other than 0.

The contents of a shell variable can be displayed on standard output using the echo command:

$ echo $VARIABLE [Enter]

Internationalisation, also called i18n since there are 18 letters between the I and n, consists of modifying software so that it conforms to regional parameters:

• Text processing differences,
• Writing direction,
• Different systems of numerals,
• Telephone numbers, addresses and international postal codes,
• Weights and measures,
• Date/time format,
• Paper sizes,
• Keyboard layout,
• etc …

Localisation, also called L10n since there are 10 letters between the L and n, consists of modifying the Internationalisation so that it conforms to a specific locale:

• en_GB = Great Britain,
• en_US = USA,
• en_AU = Australia,
• en_NZ = New Zealand,
• en_ZA = South Africa,
• en_CA = Canada.

The most important variables are:

trainee@debian8:~$ echo $LC_ALL
en_GB.UTF-8
trainee@debian8:~$ echo $LC_CTYPE

trainee@debian8:~$ echo $LANG
en_GB.UTF-8

trainee@debian8:~$ locale
LANG=en_GB.UTF-8
LANGUAGE=en_GB:en
LC_CTYPE="en_GB.UTF-8"
LC_NUMERIC="en_GB.UTF-8"
LC_TIME="en_GB.UTF-8"
LC_COLLATE="en_GB.UTF-8"
LC_MONETARY="en_GB.UTF-8"
LC_MESSAGES="en_GB.UTF-8"
LC_PAPER="en_GB.UTF-8"
LC_NAME="en_GB.UTF-8"
LC_ADDRESS="en_GB.UTF-8"
LC_TELEPHONE="en_GB.UTF-8"
LC_MEASUREMENT="en_GB.UTF-8"
LC_IDENTIFICATION="en_GB.UTF-8"
LC_ALL=en_GB.UTF-8

The env command can be used to run a program in a modified environment or just list the values of all environmental variables associated with the user calling the program env:

trainee@debian8:~$ env
XDG_SESSION_ID=1
TERM=xterm-256color
SHELL=/bin/bash
SSH_CLIENT=10.0.2.2 44524 22
OLDPWD=/home/trainee/training
SSH_TTY=/dev/pts/0
LC_ALL=en_GB.UTF-8
USER=trainee
LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arc=01;31:*.arj=01;31:*.taz=01;31:*.lha=01;31:*.lz4=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.tzo=01;31:*.t7z=01;31:*.zip=01;31:*.z=01;31:*.Z=01;31:*.dz=01;31:*.gz=01;31:*.lrz=01;31:*.lz=01;31:*.lzo=01;31:*.xz=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.alz=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.cab=01;31:*.jpg=01;35:*.jpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.axv=01;35:*.anx=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=00;36:*.au=00;36:*.flac=00;36:*.m4a=00;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=00;36:*.wav=00;36:*.axa=00;36:*.oga=00;36:*.spx=00;36:*.xspf=00;36:
MAIL=/var/mail/trainee
PATH=/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games
PWD=/home/trainee
LANG=en_GB.UTF-8
SHLVL=1
HOME=/home/trainee
LANGUAGE=en_GB:en
LOGNAME=trainee
SSH_CONNECTION=10.0.2.2 44524 10.0.2.15 22
XDG_RUNTIME_DIR=/run/user/1000
_=/usr/bin/env

To run a program, such as xterm in a modified environment the command is:

$ env EDITOR=vim xterm

To view all the options of the bash shell, use the command set:

trainee@debian8:~$ set -o
allexport      	off
braceexpand    	on
emacs          	on
errexit        	off
errtrace       	off
functrace      	off
hashall        	on
histexpand     	on
history        	on
ignoreeof      	off
interactive-comments	on
keyword        	off
monitor        	on
noclobber      	off
noexec         	off
noglob         	off
nolog          	off
notify         	off
nounset        	off
onecmd         	off
physical       	off
pipefail       	off
posix          	off
privileged     	off
verbose        	off
vi             	off
xtrace         	off

To turn on an option you need to specify which option as an argument to the previous command:

trainee@debian8:~$ set -o allexport
trainee@debian8:~$ set -o
allexport      	on
braceexpand    	on
...

To turn off an option, use set with the +o option:

trainee@debian8:~$ set +o allexport
trainee@debian8:~$ set -o
allexport      	off
braceexpand    	on
...

These are the most interesting options:

trainee@debian8:~$ set -o noclobber
trainee@debian8:~$ pwd > file
-bash: file: cannot overwrite existing file
trainee@debian8:~$ pwd > file
-bash: file: cannot overwrite existing file
trainee@debian8:~$ pwd >| file
trainee@debian8:~$ set +o noclobber

trainee@debian8:~$ set -o noglob
trainee@debian8:~$ echo *
*
trainee@debian8:~$ set +o noglob
trainee@debian8:~$ echo *
aac abc bca codes Desktop Documents Downloads errorlog file file1 Music Pictures Public Templates training Videos vitext xyz

trainee@debian8:~$ set -o nounset
trainee@debian8:~$ echo $FENESTROS
-bash: FENESTROS: unbound variable
trainee@debian8:~$ set +o nounset
trainee@debian8:~$ echo $FENESTROS

trainee@debian8:~$ 

A script is a text file that is read by the system and it's contents executed. There are five ways to execute a script:

By stipulating the shell that will execute the script:

/bin/bash myscript

by a reverse redirection:

/bin/bash < myscript

By calling the script by it's name, provided that the script is executable and that it resides in a directory specified by your path :

myscript

By placing yourself in the directory where the script resides and using one of the two following possibilities :

. myscript et ./myscript

Comments in a script are lines starting with #. However, each script starts with a pseudo-comment that informs the system which shell should be used to execute the script:

#!/bin/sh

Since a script in it's simplest form is a list of commands that are sequentially executed, it is often useful to test those command prior to writing the script> Linux has a command that can help you debug a future script. The script command can be used to generate a log file, called typescript, that contains a record of everything occurred on standard output. To exit the recording mode, use exit:

trainee@debian8:~$ script
Script started, file is typescript
trainee@debian8:~$ pwd
/home/trainee
trainee@debian8:~$ ls
aac  bca    Desktop    Downloads  fichier1  file   Music     Public     training    Videos  xyz
abc  codes  Documents  errorlog   fichier2  file1  Pictures  Templates  typescript  vitext
trainee@debian8:~$ exit
exit
Script done, file is typescript
trainee@debian8:~$ cat typescript 
Script started on Tue 29 Nov 2016 02:56:33 GMT
trainee@debian8:~$ pwd
/home/trainee
trainee@debian8:~$ ls
aac  bca    Desktop    Downloads  fichier1  file   Music     Public     training    Videos  xyz
abc  codes  Documents  errorlog   fichier2  file1  Pictures  Templates  typescript  vitext
trainee@debian8:~$ exit
exit

Script done on Tue 29 Nov 2016 02:56:44 GMT

Lets start by creating a simple script called myscript:

$ vi myscript [Enter]

Edit the file as follows:

pwd
ls

Save the file and use the five ways to execute it.

As an argument de /bin/bash:

trainee@debian8:~$ vi myscript
trainee@debian8:~$ /bin/bash myscript
/home/trainee
aac  codes	Downloads  fichier2  myscript	Public	   typescript  xyz
abc  Desktop	errorlog   file      Music	Templates  Videos
bca  Documents	fichier1   file1     Pictures	training   vitext

Using a redirection:

trainee@debian8:~$ /bin/bash < myscript
/home/trainee
aac  codes	Downloads  fichier2  myscript	Public	   typescript  xyz
abc  Desktop	errorlog   file      Music	Templates  Videos
bca  Documents	fichier1   file1     Pictures	training   vitext

In order to be able to call the script by it's name from another directory, such as /tmp, you need to move the script into the /home/trainee/bin directory and make it executable. Note that in this case, the the value of the environmental variable $PATH should contain a reference to /home/trainee/bin:

trainee@debian8:~$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games

As you can see, in the case of Debian, this is not the case. The reason for this becomes apparent when viewing the contents of the .profile file in /home/trainee:

trainee@debian8:~$ cat .profile
# ~/.profile: executed by the command interpreter for login shells.
...
# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
    PATH="$HOME/bin:$PATH"
fi

As you can see PATH is set so it includes the user's private bin only if the directory exists:

# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
    PATH="$HOME/bin:$PATH"
fi

To fix the value of the PATH variable, create the $HOME/bin directory and re-load the .profile file:

trainee@debian8:~$ mkdir bin
trainee@debian8:~$ source .profile
trainee@debian8:~$ echo $PATH
/home/trainee/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games

Now you need to move the script to $HOME/bin and make it executable:

trainee@debian8:~$ mv myscript ~/bin
trainee@debian8:~$ chmod u+x ~/bin/myscript

Move to /tmp and can call the script by just using it's name:

trainee@debian8:/tmp$ myscript
/tmp
hsperfdata_root  pulse-PKdhtXMmr18n

Now move back to ~/bin and use the following two commands to execute myscript:

• ./myscript
• . myscript

trainee@debian8:/tmp$ cd ~/bin
trainee@debian8:~/bin$ ./myscript
/home/trainee/bin
myscript
trainee@debian8:~/bin$ . myscript
/home/trainee/bin
myscript

The read command reads the standard input and stores the information in the variables that are specified as arguments. The separator between fields is a space, a tabultaion or a carriage return:

trainee@debian8:~/bin$ read var1 var2 var3 var4
fenestros edu is great!
trainee@debian8:~/bin$ echo $var1
fenestros
trainee@debian8:~/bin$ echo $var2
edu
trainee@debian8:~/bin$ echo $var3
is
trainee@debian8:~/bin$ echo $var4
great!

trainee@debian8:~/bin$ read var1 var2
fenestros edu is great!
trainee@debian8:~/bin$ echo $var1
fenestros
trainee@debian8:~/bin$ echo $var2
edu is great!

The contents of a variable can also be empty:

trainee@debian8:~/bin$ read var

↵ Enter

trainee@debian8:~/bin$ echo $?
0
trainee@debian8:~/bin$ echo $var

trainee@debian8:~/bin$ 

But not null:

trainee@debian8:~/bin$ read var

Ctrl+D

trainee@debian8:~/bin$ echo $?
1
trainee@debian8:~/bin$ echo $var

trainee@debian8:~/bin$ 

The IFS variable contains the default separator characters: SpaceBar, Tab ⇆ and ↵ Enter:

trainee@debian8:~/bin$ echo "$IFS" | od -c
0000000      \t  \n  \n
0000004

It is possible to change the contents of this variable:

trainee@debian8:~/bin$ OLDIFS="$IFS"
trainee@debian8:~/bin$ IFS=":"
trainee@debian8:~/bin$ echo "$IFS" | od -c
0000000   :  \n
0000002

Now test the new configuration:

trainee@debian8:~/bin$ read var1 var2 var3
fenestros:edu is:great!
trainee@debian8:~/bin$ echo $var1
fenestros
trainee@debian8:~/bin$ echo $var2
edu is
trainee@debian8:~/bin$ echo $var3
great!

Restore the old value of IFS before proceeding further: IFS=“$OLDIFS”

trainee@debian8:~/bin$ IFS="$OLDIFS"
trainee@debian8:~/bin$ echo "$IFS" | od -c
0000000      \t  \n  \n
0000004

The test command uses two forms:

test expression

or

[SpaceBarexpressionSpaceBar]

Test whether the a100 file is an ordinary file:

trainee@debian8:~/bin$ cd ../training/
trainee@debian8:~/training$ test -f a100
trainee@debian8:~/training$ echo $? 
0
trainee@debian8:~/training$ [ -f a100 ]
trainee@debian8:~/training$ echo $? 
0

Test whether the a101 file is an ordinary file:

trainee@debian8:~/training$ [ -f a101 ]
trainee@debian8:~/training$ echo $?
1

Test whether /home/trainee/training is a directory:

trainee@debian8:~/training$ [ -d /home/trainee/training ]
trainee@debian8:~/training$ echo $?
0

Test whether two strings are indentical:

trainee@debian8:~/training$ string1="root"
trainee@debian8:~/training$ string2="fenestros"
trainee@debian8:~/training$ [ $string1 = $string2 ]
trainee@debian8:~/training$ echo $?
1

Test if string1 is not zero in length:

trainee@debian8:~/training$ [ -n $string1 ]
trainee@debian8:~/training$ echo $?
0

Test if string1 is is zero in length:

trainee@debian8:~/training$ [ -z $string1 ]
trainee@debian8:~/training$ echo $?
1

Compare the two numbers value1 and value2 :

trainee@debian8:~/training$ read value1
35
trainee@debian8:~/training$ read value2
23
trainee@debian8:~/training$ [ $value1 -lt $value2 ]
trainee@debian8:~/training$ echo $?
1
trainee@debian8:~/training$ [ $value2 -lt $value1 ]
trainee@debian8:~/training$ echo $?
0
trainee@debian8:~/training$ [ $value2 -eq $value1 ]
trainee@debian8:~/training$ echo $?
1

Test if $file is not a directory:

trainee@debian8:~/training$ file=a1OO
trainee@debian8:~/training$ [ ! -d $file ]
trainee@debian8:~/training$ echo $?
0

Test if $directory is a directory and if trainee can cd into it:

trainee@debian8:~/training$ directory=/usr
trainee@debian8:~/training$ [ -d $directory -a -x $directory ]
trainee@debian8:~/training$ echo $?
0

Test if trainee has the write permission for the a100 file and test if /usr is a directory or test if /tmp is a directory:

trainee@debian8:~/training$ [ -w a100 -a \( -d /usr -o -d /tmp \) ]
trainee@debian8:~/training$ echo $?
0

trainee@debian8:~/training$ [ -o allexport ]
trainee@debian8:~/training$ echo $?
1

The [[SpaceBarexpressionSpaceBar]] command is an improved test command with some minor changes to syntax:

and some additional operators :

Test if trainee has the write permission for the a100 file and test if /usr is a directory or test if /tmp is a directory:

trainee@debian8:~/training$ [[ -w a100 && ( -d /usr || -d /tmp ) ]]
trainee@debian8:~/training$ echo $?
0

trainee@debian8:~/training$ [[ -d /root ]] && echo "The root directory exists"
The root directory exists
trainee@debian8:~/training$ [[ -d /root ]] || echo "The root directory exists"
trainee@debian8:~/training$  

Theexpr command's syntax is as follows :

expr SpaceBar number1 SpaceBar operator SpaceBar number2 SpaceBar

ou

expr Tab ⇆ number1 Tab operator Tab ⇆ number2 ↵ Enter

ou

expr SpaceBar string SpaceBar : SpaceBar regular_expression SpaceBar

or

expr Tab ⇆ string Tab ⇆ : Tab ⇆ regular_expression ↵ Enter

Add two to the value of $x:

trainee@debian8:~/training$ x=2
trainee@debian8:~/training$ expr $x + 2
4

If the surrounding spaces are removed, the result is completely different:

trainee@debian8:~/training$ expr $x+2
2+2

Certain operators need to be protected:

trainee@debian8:~/training$ expr $x * 2
expr: syntax error
trainee@debian8:~/training$ expr $x \* 2
4

Now put the result of a calculation in a variable:

trainee@debian8:~/training$ resultat=`expr $x + 10`
trainee@debian8:~/training$ echo $resultat
12

The let command is equivalent to ((expression)). The ((expression)) command provides the following additional features when compared with the expr command :

• greater number of operators,
• no need for spaces or tabulations between arguments,
• no need to prefix variables with the $ character,
• the shell's special characters do not need to be escaped,
• variables are defined directly in the command,
• faster execution time.

trainee@debian8:~/training$ x=2
trainee@debian8:~/training$ ((x=$x+10))
trainee@debian8:~/training$ echo $x
12
trainee@debian8:~/training$ ((x=$x+20))
trainee@debian8:~/training$ echo $x
32

The syntax is as follows:

if condition
then
    command(s)
else 
    command(s)
fi

or:

if condition
then
    command(s)
    command(s)
fi

or finally:

if condition
then
     command(s)
elif condition
then
     command(s)
elif condition
then
     command(s)
else 
    command(s)

fi

The syntax is as follows:

case $variable in
model1) function
   ...
   ;;
model2) function
   ...
   ;;
model3 | model4 | model5 ) function
   ...
   ;;
esac

The syntax is as follows:

for variable in variable_list
do
    command(s)
done

The syntax is as follows:

while condition
do
    command(s)
done

U=1
while [ $U -lt $MAX_ACCOUNTS ]
do
useradd fenestros"$U" -c fenestros"$U" -d /home/fenestros"$U" -g staff -G audio,fuse -s /bin/bash 2>/dev/null
useradd fenestros"$U"$ -g machines -s /dev/false -d /dev/null 2>/dev/null
echo "Compte fenestros$U créé"
let U=U+1
done

When Bash is called as a login shell it executes the start-up scripts in the following order:

• /etc/profile,
• ~/.bash_profile or ~/.bash_login or ~/.profile dependant upon the distribution,

In the cas of Debian Bash executes ~/.profile.

When a login shell is terminated, Bash executes the ~/.bash_logout file if it exists.

Whan Bash is called as an interactive shell as opposed to a login shell, it executes only the ~/.bashrc file.

trainee@debian8:~/training$ cat ~/.profile
# ~/.profile: executed by the command interpreter for login shells.
# This file is not read by bash(1), if ~/.bash_profile or ~/.bash_login
# exists.
# see /usr/share/doc/bash/examples/startup-files for examples.
# the files are located in the bash-doc package.

# the default umask is set in /etc/profile; for setting the umask
# for ssh logins, install and configure the libpam-umask package.
#umask 022

# if running bash
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

trainee@debian8:~/training$ cat ~/.bashrc
# ~/.bashrc: executed by bash(1) for non-login shells.
# see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
# for examples

# If not running interactively, don't do anything
case $- in
    *i*) ;;
      *) return;;
esac

# don't put duplicate lines or lines starting with space in the history.
# See bash(1) for more options
HISTCONTROL=ignoreboth

# append to the history file, don't overwrite it
shopt -s histappend

# for setting history length see HISTSIZE and HISTFILESIZE in bash(1)
HISTSIZE=1000
HISTFILESIZE=2000

# check the window size after each command and, if necessary,
# update the values of LINES and COLUMNS.
shopt -s checkwinsize

# If set, the pattern "**" used in a pathname expansion context will
# match all files and zero or more directories and subdirectories.
#shopt -s globstar

# make less more friendly for non-text input files, see lesspipe(1)
#[ -x /usr/bin/lesspipe ] && eval "$(SHELL=/bin/sh lesspipe)"

# set variable identifying the chroot you work in (used in the prompt below)
if [ -z "${debian_chroot:-}" ] && [ -r /etc/debian_chroot ]; then
    debian_chroot=$(cat /etc/debian_chroot)
fi

# set a fancy prompt (non-color, unless we know we "want" color)
case "$TERM" in
    xterm-color) color_prompt=yes;;
esac

# uncomment for a colored prompt, if the terminal has the capability; turned
# off by default to not distract the user: the focus in a terminal window
# should be on the output of commands, not on the prompt
#force_color_prompt=yes

if [ -n "$force_color_prompt" ]; then
    if [ -x /usr/bin/tput ] && tput setaf 1 >&/dev/null; then
	# We have color support; assume it's compliant with Ecma-48
	# (ISO/IEC-6429). (Lack of such support is extremely rare, and such
	# a case would tend to support setf rather than setaf.)
	color_prompt=yes
    else
	color_prompt=
    fi
fi

if [ "$color_prompt" = yes ]; then
    PS1='${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '
else
    PS1='${debian_chroot:+($debian_chroot)}\u@\h:\w\$ '
fi
unset color_prompt force_color_prompt

# If this is an xterm set the title to user@host:dir
case "$TERM" in
xterm*|rxvt*)
    PS1="\[\e]0;${debian_chroot:+($debian_chroot)}\u@\h: \w\a\]$PS1"
    ;;
*)
    ;;
esac

# enable color support of ls and also add handy aliases
if [ -x /usr/bin/dircolors ]; then
    test -r ~/.dircolors && eval "$(dircolors -b ~/.dircolors)" || eval "$(dircolors -b)"
    alias ls='ls --color=auto'
    #alias dir='dir --color=auto'
    #alias vdir='vdir --color=auto'

    #alias grep='grep --color=auto'
    #alias fgrep='fgrep --color=auto'
    #alias egrep='egrep --color=auto'
fi

# colored GCC warnings and errors
#export GCC_COLORS='error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01'

# some more ls aliases
#alias ll='ls -l'
#alias la='ls -A'
#alias l='ls -CF'

# Alias definitions.
# You may want to put all your additions into a separate file like
# ~/.bash_aliases, instead of adding them here directly.
# See /usr/share/doc/bash-doc/examples in the bash-doc package.

if [ -f ~/.bash_aliases ]; then
    . ~/.bash_aliases
fi

# enable programmable completion features (you don't need to enable
# this, if it's already enabled in /etc/bash.bashrc and /etc/profile
# sources /etc/bash.bashrc).
if ! shopt -oq posix; then
  if [ -f /usr/share/bash-completion/bash_completion ]; then
    . /usr/share/bash-completion/bash_completion
  elif [ -f /etc/bash_completion ]; then
    . /etc/bash_completion
  fi
fi

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