LCE405 - Command Line Interface

• LCE405 - Command Line Interface
  • Contents
  • The Shell
  • /bin/bash
    • Internal And External Commands
    • Aliases
    • The Prompt
    • The history Command
    • The TAB key
    • Metacharacters
    • Protecting Metacharacters
    • Exit Status
    • Redirections
    • Pipes
    • Command Substitution
    • Conditional Command Execution
  • Environment Variables
    • Principal Variables
    • Internationalisation and Localisation
    • Special Variables
    • The env Command
  • Bash Shell Options
    • noclobber
    • noglob
    • nounset
  • Basic Shell Scripting
    • Execution
    • The read command
    • The test Command
    • The [[ expression ]] Command
    • Shell Operators
    • The expr Command
    • The let Command
    • Control Structures
    • Loops
    • Start-up Scripts
    • LAB #1 - Start-up Scripts

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 8 /bin/sh is a soft link to /bin/bash :

[trainee@centos8 ~]$ ls -l /bin/sh
lrwxrwxrwx. 1 root root 4 Jul 21  2020 /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:

[trainee@centos7 ~]$ type cd
cd is a shell builtin

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

[trainee@centos8 ~]$ type cd
cd is a shell builtin

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@centos8 ~]$ type ls
ls is aliased to `ls --color=auto'

An alias is defined using the alias command:

[trainee@centos8 ~]$ alias dir='ls -l'
[trainee@centos8 ~]$ dir
total 0
-rw-rw-r--. 1 trainee trainee 0 Apr 20 03:46 aac
-rw-rw-r--. 1 trainee trainee 0 Apr 20 03:46 abc
-rw-rw-r--. 1 trainee trainee 0 Apr 20 03:46 bca
-rw-rw-r--. 1 trainee trainee 0 Apr 20 03:46 xyz

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

[trainee@centos8 ~]$ 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; declare -f) | /usr/bin/which --tty-only --read-alias --read-functions --show-tilde --show-dot'
alias xzegrep='xzegrep --color=auto'
alias xzfgrep='xzfgrep --color=auto'
alias xzgrep='xzgrep --color=auto'
alias zegrep='zegrep --color=auto'
alias zfgrep='zfgrep --color=auto'
alias zgrep='zgrep --color=auto'

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

[trainee@centos8 ~]$ \dir
aac  abc  bca  xyz

To delete an alias, simply use the unalias command:

[trainee@centos8 ~]$ unalias dir
[trainee@centos8 ~]$ dir
aac  abc  bca  xyz

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

[trainee@centos8 ~]$ 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:65534:65534:Kernel Overflow User:/:/sbin/nologin
dbus:x:81:81:System message bus:/:/sbin/nologin
systemd-coredump:x:999:997:systemd Core Dumper:/:/sbin/nologin
systemd-resolve:x:193:193:systemd Resolver:/:/sbin/nologin
tss:x:59:59:Account used by the trousers package to sandbox the tcsd daemon:/dev/null:/sbin/nologin
polkitd:x:998:996:User for polkitd:/:/sbin/nologin
unbound:x:997:994:Unbound DNS resolver:/etc/unbound:/sbin/nologin
libstoragemgmt:x:996:993:daemon account for libstoragemgmt:/var/run/lsm:/sbin/nologin
cockpit-ws:x:995:991:User for cockpit-ws:/nonexisting:/sbin/nologin
sssd:x:994:990:User for sssd:/:/sbin/nologin
setroubleshoot:x:993:989::/var/lib/setroubleshoot:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
chrony:x:992:988::/var/lib/chrony:/sbin/nologin
tcpdump:x:72:72::/:/sbin/nologin
trainee:x:1000:1000:trainee:/home/trainee:/bin/bash
cockpit-wsinstance:x:991:987:User for cockpit-ws instances:/nonexisting:/sbin/nologin
rngd:x:990:986:Random Number Generator Daemon:/var/lib/rngd:/sbin/nologin
gluster:x:989:985:GlusterFS daemons:/run/gluster:/sbin/nologin
qemu:x:107:107:qemu user:/:/sbin/nologin
rpc:x:32:32:Rpcbind Daemon:/var/lib/rpcbind:/sbin/nologin
rpcuser:x:29:29:RPC Service User:/var/lib/nfs:/sbin/nologin
saslauth:x:988:76:Saslauthd user:/run/saslauthd:/sbin/nologin
radvd:x:75:75:radvd user:/:/sbin/nologin
dnsmasq:x:983:983:Dnsmasq DHCP and DNS server:/var/lib/dnsmasq:/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@centos8 ~]$ cat /etc/shells
/bin/sh
/bin/bash
/usr/bin/sh
/usr/bin/bash

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

[trainee@centos8 ~]$ echo $SHELL
/bin/bash

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

[trainee@centos8 ~]$ chsh
Changing shell for trainee.
New shell [/bin/bash]
/bin/sh
Password: trainee
Shell changed.

Now check your current shell:

[trainee@centos8 ~]$ 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@centos8 ~]$ 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@centos8 ~]$ 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@centos8 ~]$ history | more
    1  su -
    2  exit
    3  su -
    4  nmcli c show
    5  stty -a
    6  date
    7  who
    8  df
    9  df -h
   10  free free -h
   11  free
   12  free -h
   13  whoami
   14  su -
   15  pwd
   16  cd /tmp
   17  pwd
   18  ls
   19  su -
   20  touch test
   21  ls
   22  echo fenestros
   23  cp test ~
--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@centos8 ~]$ ls
aac  abc  bca  xyz
[trainee@centos8 ~]$ !!
ls
aac  abc  bca  xyz

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

[trainee@centos8 ~]$ history
    1  su -
...
   80  history | more
   81  ls
   82  history
[trainee@centos8 ~]$ !81
ls
aac  abc  bca  xyz

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

[trainee@centos8 ~]$ 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@centos8 ~]$ nl .bash_history | tail
    54	ls
    55	ls | sort
    56	ls | sort -r
    57	more /etc/services
    58	less /etc/services
    59	find acc
    60	find aac
    61	su -
    62	sleep 10
    63	su -

/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@centos8 ~]$ 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@centos8 ~]$ mo
modinfo                more                   mount.nfs4
modprobe               mount                  mountpoint
modulemd-validator     mount.fuse             mountstats
modulemd-validator-v1  mount.nfs 

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@centos8 ~]$ mkdir training
[trainee@centos8 ~]$ cd training
[trainee@centos8 training]$ touch f1 f2 f3 f4 f5
[trainee@centos8 training]$ ls
f1  f2  f3  f4  f5

Now use the Metacharacter *:

[trainee@centos8 training]$ echo f*
f1 f2 f3 f4 f5

Create two more files:

[trainee@centos8 training]$ touch f52 f62

Now use the Metacharacter ?:

[trainee@centos8 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@centos8 training]$ touch a100

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

[trainee@centos8 training]$ echo [a-f]*
a100 f1 f2 f3 f4 f5 f52 f62
[trainee@centos8 training]$ echo [af]*
a100 f1 f2 f3 f4 f5 f52 f62

[trainee@centos8 training]$ echo [!a]*
f1 f2 f3 f4 f5 f52 f62

[trainee@centos8 training]$ echo [a-b]*
a100

[trainee@centos8 training]$ echo [a-f]
[a-f]

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

[trainee@centos8 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@centos8 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@centos8 training]$ touch f f.txt f123.txt f123123.txt f123123123.txt

Execute the following command:

[trainee@centos8 training]$ ls f?(123).txt
f123.txt  f.txt

Execute the following command:

[trainee@centos8 training]$ ls f*(123).txt
f123123123.txt  f123123.txt  f123.txt  f.txt

Execute the following command:

[trainee@centos8 training]$ ls f+(123).txt
f123123123.txt  f123123.txt  f123.txt

Execute the following command:

[trainee@centos8 training]$ ls f@(123).txt
f123.txt

Execute the following command:

[trainee@centos8 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@centos8 training]$ echo * is a metacharacter
a100 f f1 f123123123.txt f123123.txt f123.txt f2 f3 f4 f5 f52 f62 f.txt est un caractère spécial

[trainee@centos8 training]$ echo \* is a metacharacter
* is a metacharacter

[trainee@centos8 training]$ echo "* is a metacharacter"
* is a metacharacter

[trainee@centos8 training]$ echo '* is a metacharacter'
* is a metacharacter

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

For example:

[trainee@centos8 training]$ cd ..
[trainee@centos8 ~]$ mkdir codes
[trainee@centos8 ~]$ echo $?
0
[trainee@centos8 ~]$ touch codes/exit.txt
[trainee@centos8 ~]$ rmdir codes
rmdir: failed to remove ‘codes’: Directory not empty
[trainee@centos8 ~]$ 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@centos8 ~]$ pwd
/home/trainee
[trainee@centos8 ~]$ cd training
[trainee@centos8 training]$ free > file
[trainee@centos8 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@centos8 training]$ date > file
[trainee@centos8 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@centos8 training]$ free >> file
[trainee@centos8 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@centos8 training]$ cd ..
[trainee@centos8 ~]$ rmdir training/ 2>errorlog
[trainee@centos8 ~]$ cat errorlog
rmdir: failed to remove ‘training/’: Directory not empty

You can join file descriptors using the & character:

[trainee@centos8 ~]$ 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@centos8 ~]$ 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@centos8 ~]$ ls | wc -w
7

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

[trainee@centos8 ~]$ date | tee file1
Tue 20 Apr 10:39:47 EDT 2021
[trainee@centos8 ~]$ cat file1
Tue 20 Apr 10:39:47 EDT 2021

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

[trainee@centos8 ~]$ date | tee file1 > file2
[trainee@centos8 ~]$ cat file1
Tue 20 Apr 10:40:36 EDT 2021
[trainee@centos8 ~]$ cat file2
Tue 20 Apr 10:40:36 EDT 2021

Command substitution permits in-line execution of a command:

[trainee@centos8 ~]$ echo date
date
[trainee@centos8 ~]$ echo $(date)
Tue 20 Apr 10:41:33 EDT 2021
[trainee@centos8 ~]$ echo `date`
Tue 20 Apr 10:41:45 EDT 2021

Commands can be grouped using brackets:

$ (ls -l; ps; who) > list [Entrée]

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@centos8 ~]$ echo $LC_ALL

[trainee@centos8 ~]$ echo $LC_CTYPE

[trainee@centos8 ~]$ echo $LANG
en_GB.UTF-8
[trainee@centos8 ~]$ 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=

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@centos8 ~]$ env
LS_COLORS=rs=0:di=38;5;33:ln=38;5;51:mh=00: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=01;05;37;41: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;40:*.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:*.dz=38;5;9:*.gz=38;5;9:*.lrz=38;5;9:*.lz=38;5;9:*.lzo=38;5;9:*.xz=38;5;9:*.zst=38;5;9:*.tzst=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:*.wim=38;5;9:*.swm=38;5;9:*.dwm=38;5;9:*.esd=38;5;9:*.jpg=38;5;13:*.jpeg=38;5;13:*.mjpg=38;5;13:*.mjpeg=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:*.ogv=38;5;13:*.ogx=38;5;13:*.aac=38;5;45:*.au=38;5;45:*.flac=38;5;45:*.m4a=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:*.oga=38;5;45:*.opus=38;5;45:*.spx=38;5;45:*.xspf=38;5;45:
SSH_CONNECTION=10.0.2.2 42834 10.0.2.15 22
LANG=en_GB.UTF-8
HISTCONTROL=ignoredups
GUESTFISH_RESTORE=\e[0m
HOSTNAME=centos8.ittraining.loc
GUESTFISH_INIT=\e[1;34m
XDG_SESSION_ID=9
USER=trainee
GUESTFISH_PS1=\[\e[1;32m\]><fs>\[\e[0;31m\] 
SELINUX_ROLE_REQUESTED=
PWD=/home/trainee
HOME=/home/trainee
SSH_CLIENT=10.0.2.2 42834 22
SELINUX_LEVEL_REQUESTED=
SSH_TTY=/dev/pts/0
MAIL=/var/spool/mail/trainee
TERM=xterm-256color
SHELL=/bin/bash
SELINUX_USE_CURRENT_RANGE=
SHLVL=1
LOGNAME=trainee
DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
XDG_RUNTIME_DIR=/run/user/1000
PATH=/home/trainee/.local/bin:/home/trainee/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin
GUESTFISH_OUTPUT=\e[0m
HISTSIZE=1000
LESSOPEN=||/usr/bin/lesspipe.sh %s
_=/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@centos8 ~]$ 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@centos8 ~]$ set -o allexport
[trainee@centos8 ~]$ set -o
allexport      	on
braceexpand    	on
...

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

[trainee@centos8 ~]$ set +o allexport
[trainee@centos8 ~]$ set -o
allexport      	off
braceexpand    	on
...

These are the most interesting options:

[trainee@centos8 ~]$ set -o noclobber
[trainee@centos8 ~]$ pwd > file
-bash: file: cannot overwrite existing file
[trainee@centos8 ~]$ pwd > file
-bash: file: cannot overwrite existing file
[trainee@centos8 ~]$ pwd >| file
[trainee@centos8 ~]$ set +o noclobber

[trainee@centos8 ~]$ set -o noglob
[trainee@centos8 ~]$ echo *
*
[trainee@centos8 ~]$ set +o noglob
[trainee@centos8 ~]$ echo *
aac abc bca codes Desktop Documents Downloads errorlog file file1 Music Pictures Public Templates training Videos vitext xyz

[trainee@centos8 ~]$ set -o nounset
[trainee@centos8 ~]$ echo $FENESTROS
-bash: FENESTROS: unbound variable
[trainee@centos8 ~]$ set +o nounset
[trainee@centos8 ~]$ echo $FENESTROS

[trainee@centos8 ~]$ 

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@centos8 ~]$ script
Script started, file is typescript
[trainee@centos8 ~]$ pwd
/home/trainee
[trainee@centos8 ~]$ ls
aac  abc  bca  codes  errorlog  file  file1  file2  training  typescript  xyz
[trainee@centos8 ~]$ exit
exit
Script done, file is typescript

[trainee@centos8 ~]$ cat typescript 
Script started on 2021-04-20 10:59:58-04:00
[trainee@centos8 ~]$ pwd
/home/trainee
[trainee@centos8 ~]$ ls
aac  abc  bca  codes  errorlog  file  file1  file2  training  typescript  xyz
[trainee@centos8 ~]$ exit
exit

Script done on 2021-04-20 11:00:09-04:00

Lets start by creating a simple script called myscript:

[trainee@centos8 ~]$ vi myscript
[trainee@centos8 ~]$ cat myscript
pwd
ls

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

As an argument de /bin/bash:

[trainee@centos8 ~]$ /bin/bash myscript
/home/trainee
aac  bca    errorlog  file1  myscript  typescript
abc  codes  file      file2  training  xyz

Using a redirection:

[trainee@centos8 ~]$ /bin/bash < myscript
/home/trainee
aac  bca    errorlog  file1  myscript  typescript
abc  codes  file      file2  training  xyz

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@centos8 ~]$ echo $PATH
/home/trainee/.local/bin:/home/trainee/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin

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

[trainee@centos8 ~]$ ls
aac  bca    errorlog  file1  myscript  typescript
abc  codes  file      file2  training  xyz

So you need to create the directory:

[trainee@centos8 ~]$ mkdir bin

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

[trainee@centos8 ~]$ mv myscript ~/bin
[trainee@centos8 ~]$ chmod u+x ~/bin/myscript

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

[trainee@centos8 ~]$ cd /tmp
[trainee@centos8 tmp]$ myscript
/tmp
expand
expand1
filepartaa
filepartab
filepartac
filepartad
filepartae
greptest
greptest1
greptest.patch
newfile
sales.awk
sales.txt
scriptawk
sedtest
sedtest1
systemd-private-d9ff2376a8a44f0392f860d80c839be4-chronyd.service-6im4Ii

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

[trainee@centos8 tmp]$ cd ~/bin
[trainee@centos8 bin]$ ./myscript
/home/trainee/bin
myscript
[trainee@centos8 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@centos8 bin]$ read var1 var2 var3 var4
fenestros edu is great!
[trainee@centos8 bin]$ echo $var1
fenestros
[trainee@centos8 bin]$ echo $var2
edu
[trainee@centos8 bin]$ echo $var3
is
[trainee@centos8 bin]$ echo $var4
great!

[trainee@centos8 bin]$ read var1 var2
fenestros edu is great!
[trainee@centos8 bin]$ echo $var1
fenestros
[trainee@centos8 bin]$ echo $var2
edu is great!

The contents of a variable can also be empty:

[trainee@centos8 bin]$ read var

↵ Entrée

[trainee@centos8 bin]$ echo $?
0
[trainee@centos8 bin]$ echo $var

[trainee@centos8 bin]$ 

But not null:

[trainee@centos8 bin]$ read var

Ctrl+D

[trainee@centos8 bin]$ echo $?
1
[trainee@centos8 bin]$ echo $var

[trainee@centos8 bin]$ 

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

[trainee@centos8 bin]$ echo "$IFS" | od -c
0000000      \t  \n  \n
0000004

It is possible to change the contents of this variable:

[trainee@centos8 bin]$ OLDIFS="$IFS"
[trainee@centos8 bin]$ IFS=":"
[trainee@centos8 bin]$ echo "$IFS" | od -c
0000000   :  \n
0000002

Now test the new configuration:

[trainee@centos8 bin]$ read var1 var2 var3
fenestros:edu is:great!
[trainee@centos8 bin]$ echo $var1
fenestros
[trainee@centos8 bin]$ echo $var2
edu is
[trainee@centos8 bin]$ echo $var3
great!

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

[trainee@centos8 bin]$ IFS="$OLDIFS"
[trainee@centos8 bin]$ 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@centos8 bin]$ cd ../training/
[trainee@centos8 training]$ test -f a100
[trainee@centos8 training]$ echo $? 
0
[trainee@centos8 training]$ [ -f a100 ]
[trainee@centos8 training]$ echo $? 
0

Test whether the a101 file is an ordinary file:

[trainee@centos8 training]$ [ -f a101 ]
[trainee@centos8 training]$ echo $?
1

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

[trainee@centos8 training]$ [ -d /home/trainee/training ]
[trainee@centos8 training]$ echo $?
0

Test whether two strings are indentical:

[trainee@centos8 training]$ string1="root"
[trainee@centos8 training]$ string2="fenestros"
[trainee@centos8 training]$ [ $string1 = $string2 ]
[trainee@centos8 training]$ echo $?
1

Test if string1 is not zero in length:

[trainee@centos8 training]$ [ -n $string1 ]
[trainee@centos8 training]$ echo $?
0

Compare the two numbers value1 and value2:

[trainee@centos8 training]$ read value1
35
[trainee@centos8 training]$ read value2
23
[trainee@centos8 training]$ [ $value1 -lt $value2 ]
[trainee@centos8 training]$ echo $?
1
[trainee@centos8 training]$ [ $value2 -lt $value1 ]
[trainee@centos8 training]$ echo $?
0
[trainee@centos8 training]$ [ $value2 -eq $value1 ]
[trainee@centos8 training]$ echo $?
1

Test if $file is not a directory:

[trainee@centos8 training]$ file=a1OO
[trainee@centos8 training]$ [ ! -d $file ]
[trainee@centos8 training]$ echo $?
0

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

[trainee@centos8 training]$ directory=/usr
[trainee@centos8 training]$ [ -d $directory -a -x $directory ]
[trainee@centos8 training]$ 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@centos8 training]$ [ -w a100 -a \( -d /usr -o -d /tmp \) ]
[trainee@centos8 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@centos8 training]$ [[ -w a100 && ( -d /usr || -d /tmp ) ]]
[trainee@centos8 training]$ echo $?
0

[trainee@centos8 training]$ [[ -d /root ]] && echo "The root directory exists"
The root directory exists
[trainee@centos8 training]$ [[ -d /root ]] || echo "The root directory exists"
[trainee@centos8 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@centos8 training]$ x=2
[trainee@centos8 training]$ expr $x + 2
4

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

[trainee@centos8 training]$ expr $x+2
2+2

Certain operators need to be protected:

[trainee@centos8 training]$ expr $x * 2
expr: syntax error
[trainee@centos8 training]$ expr $x \* 2
4

Now put the result of a calculation in a variable:

[trainee@centos8 training]$ resultat=`expr $x + 10`
[trainee@centos8 training]$ 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.

For example:

[trainee@centos8 training]$ x=2
[trainee@centos8 training]$ ((x=$x+10))
[trainee@centos8 training]$ echo $x
12
[trainee@centos8 training]$ ((x=$x+20))
[trainee@centos8 training]$ 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

As an example, create the following script called user_check:

[trainee@centos8 training]$ vi user_check
[trainee@centos8 training]$ cat user_check
#!/bin/bash
if [ $# -ne 1 ] ; then
  echo "Incorrect number of arguments"
  echo "Usage : $0 user name"
  exit 1
fi
if grep "^$1:" /etc/passwd > /dev/null
then
  echo "User $1 has an account on this system"
else
  echo "User $1 does not have an account on this system"
fi
exit 0

Test this script:

[trainee@centos8 training]$ chmod 770 user_check
[trainee@centos8 training]$ ./user_check
Incorrect number of arguments
Usage : ./user_check user name
[trainee@centos8 training]$ ./user_check root
User root has an account on this system
[trainee@centos8 training]$ ./user_check mickey mouse
Incorrect number of arguments
Usage : ./user_check user name
[trainee@centos8 training]$ ./user_check "mickey mouse"
User mickey mouse does not have an account on this system

The syntax is as follows:

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

For example:

  case "$1" in
      start)
          start
          ;;
      stop)
          stop
          ;;
      restart|reload)
          stop
          start
          ;;
      status)
          status
          ;;
      *)
          echo $"Usage: $0 {start|stop|restart|status}"
          exit 1
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 "Account fenestros$U created"
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@centos8 training]$ cat ~/.bash_profile
# .bash_profile

# Get the aliases and functions
if [ -f ~/.bashrc ]; then
	. ~/.bashrc
fi

# User specific environment and startup programs

[trainee@centos8 training]$ cat ~/.bashrc
# .bashrc

# Source global definitions
if [ -f /etc/bashrc ]; then
	. /etc/bashrc
fi

# User specific environment
PATH="$HOME/.local/bin:$HOME/bin:$PATH"
export PATH

# Uncomment the following line if you don't like systemctl's auto-paging feature:
# export SYSTEMD_PAGER=

# User specific aliases and functions

Copyright © 2023 Hugh Norris.