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:

In RHEL/CentOS 7 /bin/sh is a soft link to /bin/bash :

[trainee@centos7 ~]$ ls -l /bin/sh
lrwxrwxrwx. 1 root root 4 30 sept. 06:01 /bin/sh -> bash

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@centos7 ~]$ 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@centos7 ~]$ 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@centos7 ~]$ type ls
ls is aliased to `ls --color=auto'

An alias is defined using the alias command:

[trainee@centos7 ~]$ alias dir='ls -l'
[trainee@centos7 ~]$ dir
total 4
-rw-rw-r--. 1 trainee trainee   0 29 sept. 18:20 aac
-rw-rw-r--. 1 trainee trainee   0 29 sept. 18:20 abc
-rw-rw-r--. 1 trainee trainee   0 29 sept. 18:20 bca
drwxr-xr-x. 2 trainee trainee   6 30 avril 11:54 Desktop
drwxr-xr-x. 2 trainee trainee   6 30 avril 11:54 Documents
drwxr-xr-x. 2 trainee trainee   6 30 avril 11:54 Downloads
drwxr-xr-x. 2 trainee trainee   6 30 avril 11:54 Music
drwxr-xr-x. 2 trainee trainee   6 30 avril 11:54 Pictures
drwxr-xr-x. 2 trainee trainee   6 30 avril 11:54 Public
drwxr-xr-x. 2 trainee trainee   6 30 avril 11:54 Templates
drwxr-xr-x. 2 trainee trainee   6 30 avril 11:54 Videos
-rw-rw-r--. 1 trainee trainee 442 29 sept. 00:53 vitext
-rw-rw-r--. 1 trainee trainee   0 29 sept. 18:20 xyz

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

[trainee@centos7 ~]$ alias
alias dir='ls -l'
alias egrep='egrep --color=auto'
alias fgrep='fgrep --color=auto'
alias grep='grep --color=auto'
alias l.='ls -d .* --color=auto'
alias ll='ls -l --color=auto'
alias ls='ls --color=auto'
alias vi='vim'
alias which='alias | /usr/bin/which --tty-only --read-alias --show-dot --show-tilde'

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

[trainee@centos7 ~]$ \dir
aac  bca      Documents  Music	   Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext

To delete an alias, simply use the unalias command:

[trainee@centos7 ~]$ unalias dir
[trainee@centos7 ~]$ 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@centos7 ~]$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
games:x:12:100:games:/usr/games:/sbin/nologin
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin
nobody:x:99:99:Nobody:/:/sbin/nologin
avahi-autoipd:x:170:170:Avahi IPv4LL Stack:/var/lib/avahi-autoipd:/sbin/nologin
systemd-bus-proxy:x:999:997:systemd Bus Proxy:/:/sbin/nologin
systemd-network:x:998:996:systemd Network Management:/:/sbin/nologin
dbus:x:81:81:System message bus:/:/sbin/nologin
polkitd:x:997:995:User for polkitd:/:/sbin/nologin
abrt:x:173:173::/etc/abrt:/sbin/nologin
usbmuxd:x:113:113:usbmuxd user:/:/sbin/nologin
colord:x:996:993:User for colord:/var/lib/colord:/sbin/nologin
libstoragemgmt:x:995:992:daemon account for libstoragemgmt:/var/run/lsm:/sbin/nologin
setroubleshoot:x:994:991::/var/lib/setroubleshoot:/sbin/nologin
rpc:x:32:32:Rpcbind Daemon:/var/lib/rpcbind:/sbin/nologin
rtkit:x:172:172:RealtimeKit:/proc:/sbin/nologin
chrony:x:993:990::/var/lib/chrony:/sbin/nologin
unbound:x:992:989:Unbound DNS resolver:/etc/unbound:/sbin/nologin
tss:x:59:59:Account used by the trousers package to sandbox the tcsd daemon:/dev/null:/sbin/nologin
geoclue:x:991:988:User for geoclue:/var/lib/geoclue:/sbin/nologin
ntp:x:38:38::/etc/ntp:/sbin/nologin
sssd:x:990:987:User for sssd:/:/sbin/nologin
rpcuser:x:29:29:RPC Service User:/var/lib/nfs:/sbin/nologin
nfsnobody:x:65534:65534:Anonymous NFS User:/var/lib/nfs:/sbin/nologin
pulse:x:171:171:PulseAudio System Daemon:/var/run/pulse:/sbin/nologin
gdm:x:42:42::/var/lib/gdm:/sbin/nologin
gnome-initial-setup:x:989:984::/run/gnome-initial-setup/:/sbin/nologin
avahi:x:70:70:Avahi mDNS/DNS-SD Stack:/var/run/avahi-daemon:/sbin/nologin
postfix:x:89:89::/var/spool/postfix:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
tcpdump:x:72:72::/:/sbin/nologin
trainee:x:1000:1000:trainee:/home/trainee:/bin/bash
vboxadd:x:988:1::/var/run/vboxadd:/bin/false
named:x:25:25:Named:/var/named:/sbin/nologin

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

[trainee@centos7 ~]$ cat /etc/shells
/bin/sh
/bin/bash
/sbin/nologin
/usr/bin/sh
/usr/bin/bash
/usr/sbin/nologin
/bin/tcsh
/bin/csh

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

[trainee@centos7 ~]$ echo $SHELL
/bin/bash

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

[trainee@centos7 ~]$ chsh
Changing shell for trainee.
New shell [/bin/bash]: /bin/sh
Password: trainee
Shell changed.

Now check your current shell:

[trainee@centos7 ~]$ 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@centos7 ~]$ 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@centos7 ~]$ chsh
Changing shell for trainee.
New shell [/bin/sh]: /bin/bash
Password: trainee
Shell changed.

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@centos7 ~]$ history | more
    1  su -
    2  df -h
    3  su -
    4  exit
    5  su -
    6  su -
    7  vi vitext
    8  view vitext
    9  vi vitext
   10  locale
   11  LANG=en_GB.UTF-8
   12  export LANG
   13  locale
   14  vi vitext
   15  vi .exrc
   16  vi vitext
   17  clear
   18  stty -a
   19  date
   20  locale
   21  who
   22  df
   23  df -h
--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@centos7 ~]$ ls
aac  bca      Documents  Music     Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext
[trainee@centos7 ~]$ !!
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@centos7 ~]$ !123
ls
aac  bca      Documents  Music     Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext

The environmental variables associated with the history are set system-wide in the /etc/profile file:

[trainee@centos7 ~]$ cat /etc/profile | grep HISTSIZE
HISTSIZE=1000
export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE HISTCONTROL

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@centos7 ~]$ nl .bash_history | more
     1	su -
     2	df -h
     3	su -
     4	exit
     5	su -
     6	su -
     7	vi vitext
     8	view vitext
     9	vi vitext
    10	locale
    11	LANG=en_GB.UTF-8
    12	export LANG
    13	locale
    14	vi vitext
    15	vi .exrc
    16	vi vitext
    17	clear
    18	stty -a
    19	date
    20	locale
    21	who
    22	df
    23	df -h
--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@centos7 ~]$ ls .bash
.bash_history  .bash_logout   .bash_profile  .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@centos7 ~]$ mo
mobj_dump        modutil          mount.cifs       mount.nfs4       mousetweaks
modifyrepo       mokutil          mount.fuse       mountpoint       
modinfo          more             mount.glusterfs  mountstats       
modprobe         mount            mount.nfs        mount.vboxsf 

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@centos7 ~]$ mkdir training
[trainee@centos7 ~]$ cd training
[trainee@centos7 training]$ touch f1 f2 f3 f4 f5

Now use the Metacharacter *:

[trainee@centos7 training]$ echo f*
f1 f2 f3 f4 f5

Create two more files:

[trainee@centos7 training]$ touch f52 f62

Now use the Metacharacter ?:

[trainee@centos7 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@centos7 training]$ touch a100

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

[trainee@centos7 training]$ echo [a-f]*
a100 f1 f2 f3 f4 f5 f52 f62
[trainee@centos7 training]$ echo [af]*
a100 f1 f2 f3 f4 f5 f52 f62

[trainee@centos7 training]$ echo [!a]*
f1 f2 f3 f4 f5 f52 f62

[trainee@centos7 training]$ echo [a-b]*
a100

[trainee@centos7 training]$ echo [a-f]
[a-f]

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

[trainee@centos7 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@centos7 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
direxpand      	off
dirspell       	off
dotglob        	off
execfail       	off
expand_aliases 	on
extdebug       	off
extglob        	on
extquote       	on
failglob       	off
force_fignore  	on
globstar       	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       	of

Create the following files:

[trainee@centos7 training]$ touch f f.txt f123.txt f123123.txt f123123123.txt

Execute the following command:

[trainee@centos7 training]$ ls f?(123).txt
f123.txt  f.txt

Execute the following command:

[trainee@centos7 training]$ ls f*(123).txt
f123123123.txt  f123123.txt  f123.txt  f.txt

Execute the following command:

[trainee@centos7 training]$ ls f+(123).txt
f123123123.txt  f123123.txt  f123.txt

Execute the following command:

[trainee@centos7 training]$ ls f@(123).txt
f123.txt

Execute the following command:

[trainee@centos7 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@centos7 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@centos7 training]$ echo \* est un caractère spécial
* est un caractère spécial

[trainee@centos7 training]$ echo "* est un caractère spécial"
* est un caractère spécial

[trainee@centos7 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@centos7 training]$ cd ..
[trainee@centos7 ~]$ mkdir codes
[trainee@centos7 ~]$ echo $?
0
[trainee@centos7 ~]$ touch codes/exit.txt
[trainee@centos7 ~]$ rmdir codes
rmdir: failed to remove ‘codes’: Directory not empty
[trainee@centos7 ~]$ 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@centos7 ~]$ pwd
/home/trainee
[trainee@centos7 ~]$ cd training
[trainee@centos7 training]$ free > file
[trainee@centos7 training]$ cat file
              total        used        free      shared  buff/cache   available
Mem:         500780      192692       38916        4824      269172      260472
Swap:       2096124           0     2096124

Repeating a single redirection will replace the file:

[trainee@centos7 training]$ date > file
[trainee@centos7 training]$ cat file
Mon 28 Nov 15:48:09 CET 2016

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

[trainee@centos7 training]$ free >> file
[trainee@centos7 training]$ cat file
Mon 28 Nov 15:48:09 CET 2016
              total        used        free      shared  buff/cache   available
Mem:         500780      192792       38516        4824      269472      260376
Swap:       2096124           0     2096124

File descriptors are numbered for ease of use :

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

For example:

[trainee@centos7 training]$ cd ..
[trainee@centos7 ~]$ rmdir training/ 2>errorlog
[trainee@centos7 ~]$ cat errorlog
rmdir: failed to remove ‘training/’: Directory not empty

You can join file descriptors using the & character:

[trainee@centos7 ~]$ 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@centos7 ~]$ 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@centos7 ~]$ 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@centos7 ~]$ date | tee file1
Mon 28 Nov 16:14:24 CET 2016
[trainee@centos7 ~]$ cat file1
Mon 28 Nov 16:14:24 CET 2016

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

[trainee@centos7 ~]$ date | tee file1 > file2
[trainee@centos7 ~]$ cat file1
Mon 28 Nov 16:15:57 CET 2016
[trainee@centos7 ~]$ cat file2
Mon 28 Nov 16:15:57 CET 2016

Command substitution permits in-line execution of a command:

[trainee@centos7 ~]$ echo date
date
[trainee@centos7 ~]$ echo $(date)
Mon 28 Nov 16:19:35 CET 2016
[trainee@centos7 ~]$ echo `date`
Mon 28 Nov 16:19:35 CET 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@centos7 ~]$ echo $LC_ALL
en_GB.UTF-8
[trainee@centos7 ~]$ echo $LC_CTYPE

[trainee@centos7 ~]$ echo $LANG
en_GB.UTF-8 

[trainee@centos7 ~]$ locale
LANG=en_GB.UTF-8
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@centos7 ~]$ env
XDG_SESSION_ID=1
HOSTNAME=centos7.fenestros.loc
SELINUX_ROLE_REQUESTED=
TERM=xterm-256color
SHELL=/bin/bash
HISTSIZE=1000
SSH_CLIENT=10.0.2.2 33896 22
SELINUX_USE_CURRENT_RANGE=
SSH_TTY=/dev/pts/0
LC_ALL=en_GB.UTF-8
USER=trainee
LS_COLORS=rs=0:di=38;5;27:ln=38;5;51:mh=44;38;5;15:pi=40;38;5;11:so=38;5;13:do=38;5;5:bd=48;5;232;38;5;11:cd=48;5;232;38;5;3:or=48;5;232;38;5;9:mi=05;48;5;232;38;5;15:su=48;5;196;38;5;15:sg=48;5;11;38;5;16:ca=48;5;196;38;5;226:tw=48;5;10;38;5;16:ow=48;5;10;38;5;21:st=48;5;21;38;5;15:ex=38;5;34:*.tar=38;5;9:*.tgz=38;5;9:*.arc=38;5;9:*.arj=38;5;9:*.taz=38;5;9:*.lha=38;5;9:*.lz4=38;5;9:*.lzh=38;5;9:*.lzma=38;5;9:*.tlz=38;5;9:*.txz=38;5;9:*.tzo=38;5;9:*.t7z=38;5;9:*.zip=38;5;9:*.z=38;5;9:*.Z=38;5;9:*.dz=38;5;9:*.gz=38;5;9:*.lrz=38;5;9:*.lz=38;5;9:*.lzo=38;5;9:*.xz=38;5;9:*.bz2=38;5;9:*.bz=38;5;9:*.tbz=38;5;9:*.tbz2=38;5;9:*.tz=38;5;9:*.deb=38;5;9:*.rpm=38;5;9:*.jar=38;5;9:*.war=38;5;9:*.ear=38;5;9:*.sar=38;5;9:*.rar=38;5;9:*.alz=38;5;9:*.ace=38;5;9:*.zoo=38;5;9:*.cpio=38;5;9:*.7z=38;5;9:*.rz=38;5;9:*.cab=38;5;9:*.jpg=38;5;13:*.jpeg=38;5;13:*.gif=38;5;13:*.bmp=38;5;13:*.pbm=38;5;13:*.pgm=38;5;13:*.ppm=38;5;13:*.tga=38;5;13:*.xbm=38;5;13:*.xpm=38;5;13:*.tif=38;5;13:*.tiff=38;5;13:*.png=38;5;13:*.svg=38;5;13:*.svgz=38;5;13:*.mng=38;5;13:*.pcx=38;5;13:*.mov=38;5;13:*.mpg=38;5;13:*.mpeg=38;5;13:*.m2v=38;5;13:*.mkv=38;5;13:*.webm=38;5;13:*.ogm=38;5;13:*.mp4=38;5;13:*.m4v=38;5;13:*.mp4v=38;5;13:*.vob=38;5;13:*.qt=38;5;13:*.nuv=38;5;13:*.wmv=38;5;13:*.asf=38;5;13:*.rm=38;5;13:*.rmvb=38;5;13:*.flc=38;5;13:*.avi=38;5;13:*.fli=38;5;13:*.flv=38;5;13:*.gl=38;5;13:*.dl=38;5;13:*.xcf=38;5;13:*.xwd=38;5;13:*.yuv=38;5;13:*.cgm=38;5;13:*.emf=38;5;13:*.axv=38;5;13:*.anx=38;5;13:*.ogv=38;5;13:*.ogx=38;5;13:*.aac=38;5;45:*.au=38;5;45:*.flac=38;5;45:*.mid=38;5;45:*.midi=38;5;45:*.mka=38;5;45:*.mp3=38;5;45:*.mpc=38;5;45:*.ogg=38;5;45:*.ra=38;5;45:*.wav=38;5;45:*.axa=38;5;45:*.oga=38;5;45:*.spx=38;5;45:*.xspf=38;5;45:
MAIL=/var/spool/mail/trainee
PATH=/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/home/trainee/.local/bin:/home/trainee/bin
PWD=/home/trainee
LANG=fr_FR.UTF-8
SELINUX_LEVEL_REQUESTED=
HISTCONTROL=ignoredups
SHLVL=1
HOME=/home/trainee
LOGNAME=trainee
SSH_CONNECTION=10.0.2.2 33896 192.168.1.99 22
LESSOPEN=||/usr/bin/lesspipe.sh %s
XDG_RUNTIME_DIR=/run/user/1000
_=/usr/bin/env
OLDPWD=/home/trainee/training

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@centos7 ~]$ 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@centos7 ~]$ set -o allexport
[trainee@centos7 ~]$ set -o
allexport      	on
braceexpand    	on
...

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

[trainee@centos7 ~]$ set +o allexport
[trainee@centos7 ~]$ set -o
allexport      	off
braceexpand    	on
...

These are the most interesting options:

[trainee@centos7 ~]$ set -o noclobber
[trainee@centos7 ~]$ pwd > file
-bash: file: cannot overwrite existing file
[trainee@centos7 ~]$ pwd > file
-bash: file: cannot overwrite existing file
[trainee@centos7 ~]$ pwd >| file
[trainee@centos7 ~]$ set +o noclobber

[trainee@centos7 ~]$ set -o noglob
[trainee@centos7 ~]$ echo *
*
[trainee@centos7 ~]$ set +o noglob
[trainee@centos7 ~]$ echo *
aac abc bca codes Desktop Documents Downloads errorlog file file1 Music Pictures Public Templates training Videos vitext xyz

[trainee@centos7 ~]$ set -o nounset
[trainee@centos7 ~]$ echo $FENESTROS
-bash: FENESTROS: unbound variable
[trainee@centos7 ~]$ set +o nounset
[trainee@centos7 ~]$ echo $FENESTROS

[trainee@centos7 ~]$ 

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@centos7 ~]$ script
Script started, file is typescript
[trainee@centos7 ~]$ pwd
/home/trainee
[trainee@centos7 ~]$ ls
aac  bca    Desktop    Downloads  fichier1  file   Music     Public     training    Videos  xyz
abc  codes  Documents  errorlog   fichier2  file1  Pictures  Templates  typescript  vitext
[trainee@centos7 ~]$ exit
exit
Script done, file is typescript
[trainee@centos7 ~]$ cat typescript 
Script started on Tue 29 Nov 2016 03:58:33 CET
[trainee@centos7 ~]$ pwd
/home/trainee
[trainee@centos7 ~]$ ls
aac  bca    Desktop    Downloads  fichier1  file   Music     Public     training    Videos  xyz
abc  codes  Documents  errorlog   fichier2  file1  Pictures  Templates  typescript  vitext
[trainee@centos7 ~]$ exit
exit

Script done on Tue 29 Nov 2016 03:58:40 CET

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@centos7 ~]$ vi myscript
[trainee@centos7 ~]$ /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@centos7 ~]$ /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@centos7 ~]$ echo $PATH
/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/home/trainee/.local/bin:/home/trainee/bin

In the case of RHEL/CentOS, even though PATH contains $HOME/bin, the directory is not present:

[trainee@centos7 ~]$ ls
aac  codes      Downloads  fichier2  myscript  Public     typescript  xyz
abc  Desktop    errorlog   file      Music      Templates  Videos
bca  Documents  fichier1   file1     Pictures   training   vitext

So you need to create the directory:

[trainee@centos7 ~]$ mkdir bin

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

[trainee@centos7 ~]$ mv myscript ~/bin
[trainee@centos7 ~]$ chmod u+x ~/bin/myscript

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

[trainee@centos7 tmp]$ myscript
/tmp
hsperfdata_root  systemd-private-e526abcf335b40949dfc725f28456502-cups.service-u0xGiL

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

• ./myscript
• . myscript

[trainee@centos7 tmp]$ cd ~/bin
[trainee@centos7 bin]$ ./myscript 
/home/trainee/bin
myscript
[trainee@centos7 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@centos7 bin]$ read var1 var2 var3 var4
fenestros edu is great!
[trainee@centos7 bin]$ echo $var1
fenestros
[trainee@centos7 bin]$ echo $var2
edu
[trainee@centos7 bin]$ echo $var3
is
[trainee@centos7 bin]$ echo $var4
great!

[trainee@centos7 bin]$ read var1 var2
fenestros edu is great!
[trainee@centos7 bin]$ echo $var1
fenestros
[trainee@centos7 bin]$ echo $var2
edu is great!

The contents of a variable can also be empty:

[trainee@centos7 bin]$ read var

↵ Enter

[trainee@centos7 bin]$ echo $?
0
[trainee@centos7 bin]$ echo $var

[trainee@centos7 bin]$ 

But not null:

[trainee@centos7 bin]$ read var

Ctrl+D

[trainee@centos7 bin]$ echo $?
1
[trainee@centos7 bin]$ echo $var

[trainee@centos7 bin]$ 

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

[trainee@centos7 bin]$ echo "$IFS" | od -c
0000000      \t  \n  \n
0000004

It is possible to change the contents of this variable:

[trainee@centos7 bin]$ OLDIFS="$IFS"
[trainee@centos7 bin]$ IFS=":"
[trainee@centos7 bin]$ echo "$IFS" | od -c
0000000   :  \n
0000002

Now test the new configuration:

[trainee@centos7 bin]$ read var1 var2 var3
fenestros:edu is:great!
[trainee@centos7 bin]$ echo $var1
fenestros
[trainee@centos7 bin]$ echo $var2
edu is
[trainee@centos7 bin]$ echo $var3
great!

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

[trainee@centos7 bin]$ IFS="$OLDIFS"
[trainee@centos7 bin]$ 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@centos7 bin]$ cd ../training/
[trainee@centos7 training]$ test -f a100
[trainee@centos7 training]$ echo $? 
0
[trainee@centos7 training]$ [ -f a100 ]
[trainee@centos7 training]$ echo $? 
0

Test whether the a101 file is an ordinary file:

[trainee@centos7 training]$ [ -f a101 ]
[trainee@centos7 training]$ echo $?
1

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

[trainee@centos7 training]$ [ -d /home/trainee/training ]
[trainee@centos7 training]$ echo $?
0

Test whether two strings are indentical:

[trainee@centos7 training]$ string1="root"
[trainee@centos7 training]$ string2="fenestros"
[trainee@centos7 training]$ [ $string1 = $string2 ]
[trainee@centos7 training]$ echo $?
1

Test if string1 is not zero in length:

[trainee@centos7 training]$ [ -n $string1 ]
[trainee@centos7 training]$ echo $?
0

Test if string1 is is zero in length:

[trainee@centos7 training]$ [ -z $string1 ]
[trainee@centos7 training]$ echo $?
1

Compare the two numbers value1 and value2 :

[trainee@centos7 training]$ read value1
35
[trainee@centos7 training]$ read value2
23
[trainee@centos7 training]$ [ $value1 -lt $value2 ]
[trainee@centos7 training]$ echo $?
1
[trainee@centos7 training]$ [ $value2 -lt $value1 ]
[trainee@centos7 training]$ echo $?
0
[trainee@centos7 training]$ [ $value2 -eq $value1 ]
[trainee@centos7 training]$ echo $?
1

Test if $file is not a directory:

[trainee@centos7 training]$ file=a1OO
[trainee@centos7 training]$ [ ! -d $file ]
[trainee@centos7 training]$ echo $?
0

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

[trainee@centos7 training]$ directory=/usr
[trainee@centos7 training]$ [ -d $directory -a -x $directory ]
[trainee@centos7 training]$ 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@centos7 training]$ [ -w a100 -a \( -d /usr -o -d /tmp \) ]
[trainee@centos7 training]$ echo $?
0

[trainee@centos7 training]$ [ -o allexport ]
[trainee@centos7 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@centos7 training]$ [[ -w a100 && ( -d /usr || -d /tmp ) ]]
[trainee@centos7 training]$ echo $?
0

[trainee@centos7 training]$ [[ -d /root ]] && echo "The root directory exists"
The root directory exists
[trainee@centos7 training]$ [[ -d /root ]] || echo "The root directory exists"
[trainee@centos7 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@centos7 training]$ x=2
[trainee@centos7 training]$ expr $x + 2
4

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

[trainee@centos7 training]$ expr $x+2
2+2

Certain operators need to be protected:

[trainee@centos7 training]$ expr $x * 2
expr: syntax error
[trainee@centos7 training]$ expr $x \* 2
4

Now put the result of a calculation in a variable:

[trainee@centos7 training]$ resultat=`expr $x + 10`
[trainee@centos7 training]$ 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@centos7 training]$ x=2
[trainee@centos7 training]$ ((x=$x+10))
[trainee@centos7 training]$ echo $x
12
[trainee@centos7 training]$ ((x=$x+20))
[trainee@centos7 training]$ 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 case of RHEL/CentOS, Bash executes ~/.bash_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@centos7 training]$ cat ~/.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

[trainee@centos7 training]$ cat ~/.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

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