RH12405 - The Command Line Interface

• RH12405 - The Command Line Interface
  • Contents
  • The Shell
  • LAB #1 - The /bin/bash Shell
    • 1.1 - Shell Internal and External Commands
    • 1.2 - Aliases
    • 1.3 - Defining a user's shell
    • 1.4 - The Prompt
    • 1.5 - Recalling commands
    • 1.6 - Generating file name endings
    • 1.7 - The interactive shell
      • Character *
      • Character ?
      • Character [ ]
    • 1.8 - The extglob option
      • ?(expression)
      • *(expression)
      • +(expression)
      • @(expression)
      • !(expression)
      • Escape characters
  • 1.9 - Return Codes
  • 1.10 - Redirections
  • 1.11 - Pipes
  • 1.12 - Command substitution
  • 1.13 - Command chaining
  • 1.14 - Displaying shell variables
    • Main variables
    • Internationalization and Localization Variables
    • Special variables
  • 1.15 - The env command
  • 1.16 - Bash Shell options
    • Examples
      • noclobber
      • noglob
      • nounset

A shell is a command line interpreter (C.L.I). It is used as an interface to give instructions or commands to the operating system.

The word shell is generic. There are many shells in the Unix world, for example :

Under RHEL 9 the /bin/sh shell is a symbolic link to /bin/bash :

[trainee@redhat9 ~]$ ls -l /bin/sh
lrwxrwxrwx. 1 root root 4 Feb 15 2024 /bin/sh -> bash

This module is about using the bash shell under Linux. The bash shell allows you to:

• Recall commands
• Generate the end of file names
• Use aliases
• Use array variables
• Use numeric variables and C language arithmetic
• Managing character strings
• Using functions

A command always begins with a keyword. This keyword is interpreted by the shell according to the type of command and in the following order:

1. Aliases
2. Functions
3. Built-in shell commands
4. External commands

Built-in shell commands are commands such as cd. To check the type of command, use the type command:

[trainee@redhat9 ~]$ type cd
cd is a shell builtin

Commands external to the shell are executable binaries or scripts, generally located in /bin, /sbin, /usr/bin or /usr/sbin :

[trainee@redhat9 ~]$ type ifconfig
ifconfig is /usr/sbin/ifconfig

Aliases are names used to designate a command or a sequence of commands and are specific only to the shell that created them and to the user's environment :

[trainee@redhat9 ~]$ type ls
ls is aliased to `ls --color=auto'

An alias is defined using the alias command:

[trainee@redhat9 ~]$ alias dir=‘ls -l’
[trainee@redhat9 ~]$ dir
total 4
-rw-r--r--. 1 trainee trainee 0 Sep 25 15:11 aac
-rw-r--r--. 1 trainee trainee 0 Sep 25 15:11 abc
-rw-r--r--. 1 trainee trainee 0 Sep 25 15:11 bca
drwxr-xr-x. 2 trainee trainee 6 Oct 19 2023 Desktop
drwxr-xr-x. 2 trainee trainee 6 Oct 19 2023 Documents
drwxr-xr-x. 2 trainee trainee 6 Oct 19 2023 Downloads
drwxr-xr-x. 2 trainee trainee 6 Oct 19 2023 Music
drwxr-xr-x. 2 trainee trainee 6 Oct 19 2023 Pictures
drwxr-xr-x. 2 trainee trainee 6 Oct 19 2023 Public
drwxr-xr-x. 2 trainee trainee 6 Oct 19 2023 Templates
drwxr-xr-x. 2 trainee trainee 6 Oct 19 2023 Videos
-rw-r--r--. 1 trainee trainee 442 Sep 25 14:24 vitext
-rw-r--r--. 1 trainee trainee 0 Sep 25 15:11 xyz

The list of defined aliases can be viewed using the alias command:

[trainee@redhat9 ~]$ 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 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 execution of a command and not the alias you must precede the command with the \ character:

[trainee@redhat9 ~]$ \dir
aac abc bca Desktop Documents Downloads Music Pictures Public Templates Videos vitext xyz

To delete an alias, use the unalias command:

[trainee@redhat9 ~]$ unalias dir
[trainee@redhat9 ~]$ dir
aac abc bca Desktop Documents Downloads Music Pictures Public Templates Videos vitext xyz

The user shell is defined by root in the last field of the /etc/passwd file:

[trainee@redhat9 ~]$ 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
systemd-coredump:x:999:997:systemd Core Dumper:/:/sbin/nologin
dbus:x:81:81:System message bus:/:/sbin/nologin
polkitd:x:998:996:User for polkitd:/:/sbin/nologin
avahi:x:70:70:Avahi mDNS/DNS-SD Stack:/var/run/avahi-daemon:/sbin/nologin
tss:x:59:59:Account used for TPM access:/dev/null:/sbin/nologin
colord:x:997:993:User for colord:/var/lib/colord:/sbin/nologin
clevis:x:996:992:Clevis Decryption Framework unprivileged user:/var/cache/clevis:/usr/sbin/nologin
rtkit:x:172:172:RealtimeKit:/proc:/sbin/nologin
sssd:x:995:991:User for sssd:/:/sbin/nologin
geoclue:x:994:990:User for geoclue:/var/lib/geoclue:/sbin/nologin
libstoragemgmt:x:988:988:daemon account for libstoragemgmt:/:/usr/sbin/nologin
systemd-oom:x:987:987:systemd Userspace OOM Killer:/:/usr/sbin/nologin
setroubleshoot:x:986:986:SELinux troubleshoot server:/var/lib/setroubleshoot:/sbin/nologin
pipewire:x:985:984:PipeWire System Daemon:/var/run/pipewire:/sbin/nologin
flatpak:x:984:983:User for flatpak system helper:/:/sbin/nologin
gdm:x:42:42::/var/lib/gdm:/sbin/nologin
cockpit-ws:x:983:982:User for cockpit web service:/nonexisting:/sbin/nologin
cockpit-wsinstance:x:982:981:User for cockpit-ws instances:/nonexisting:/sbin/nologin
gnome-initial-setup:x:981:980::/run/gnome-initial-setup/:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/usr/share/empty.sshd:/sbin/nologin
chrony:x:980:979:chrony system user:/var/lib/chrony:/sbin/nologin
dnsmasq:x:979:978:Dnsmasq DHCP and DNS server:/var/lib/dnsmasq:/sbin/nologin
tcpdump:x:72:72::/:/sbin/nologin
trainee:x:1000:1000:trainee:/home/trainee:/bin/bash

However the user can change their shell using the chsh command. The shells available to system users are listed in the /etc/shells file. Enter the cat /etc/shells command:

[trainee@redhat9 ~]$ cat /etc/shells
/bin/sh
/bin/bash
/usr/bin/sh
/usr/bin/bash

Then use the echo command to display the current trainee shell:

[trainee@redhat9 ~]$ echo $SHELL
/bin/bash

Then change the trainee shell using the chsh command, specifying the value of /bin/sh for the new shell:

[trainee@redhat9 ~]$ chsh
Changing shell for trainee.
New shell [/bin/bash]: /bin/sh
Password: trainee
Shell changed.

Next, check the active shell for trainee :

[trainee@redhat9 ~]$ echo $SHELL
/bin/bash

Lastly check the shell stipulated in the /etc/passwd file for trainee:

[trainee@redhat9 ~]$ cat /etc/passwd | grep trainee
trainee:x:1000:1000:trainee:/home/trainee:/bin/sh

Change your shell to /bin/bash again using the chsh command:

[trainee@redhat9 ~]$ chsh
Changing shell for trainee.
New shell [/bin/sh]: /bin/bash
Password: trainee
Shell changed.

A user's prompt depends on their status:

• $ for a normal user,
• # for root.

The /bin/bash shell can be used to recall the last commands entered. To see the list of memorised commands, use the history command:

[trainee@redhat9 ~]$ history | more
    1 su -
    2 exit
    3 su -
    4 clear
    5 cd /
    6 ls -l
    7 cd afs
    8 ls
    9 cd /
   10 su -
   11 cd ~
   12 vi vitext
   13 view vitext
   14 vi vitext
   15 vi .exrc
   16 vi vitext
   17 su -
   18 stty -a
   19 date
   20 who
   21 df
   22 df -h
   23 free
   24 free -h
   25 whoami
   26 su -
   27 pwd
   28 cd /tmp
   29 pwd
   30 ls
   31 su -
   32 touch test
   33 ls
   34 echo fenestros
   35 cp test ~
   36 ls -l ~
   37 file ~/test
--More--
[q]

The command history is in emacs mode by default. As a result, the last command is recalled using the [Up Arrow] key or the [CTRL]-[P] keys and the next command is recalled using the [Down Arrow] key or the [CTRL]-[N] keys :

To move through the history line :

To search the history, use the keys :

It is also possible to recall the last command in the history using the !! characters:

[trainee@redhat9 ~]$ ls
aac abc bca Desktop Documents Downloads Music Pictures Public Templates Videos vitext xyz
[trainee@redhat9 ~]$ !!
ls
aac abc bca Desktop Documents Downloads Music Pictures Public Templates Videos vitext xyz

You can recall a specific command from the history by using the ! character followed by the number of the command to be recalled:

[trainee@redhat9 ~]$ history
    1 su -
    2 exit
    3 su -
    4 clear
    5 cd /
    6 ls -l
    7 cd afs
    8 ls
    9 cd /
   10 su -
...
   85 echo $SHELL
   86 cat /etc/passwd | grep trainee
   87 chsh
   88 history | more
   89 clear
   90 ls
   91 history
[trainee@redhat9 ~]$ !90
ls
aac abc bca Desktop Documents Downloads Music Pictures Public Templates Videos vitext xyz

The command callback function is set up for all users in the /etc/profile file. In this file, variables concerning the order callback can be defined. The most important is HISTSIZE :

[trainee@redhat9 ~]$ cat /etc/profile | grep HISTSIZE
HISTSIZE=1000
export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE HISTCONTROL

You will notice that in the previous case, the value of HISTSIZE is 1000. This implies that the last thousand commands are memorised.

The memorised commands are stored in the ~/.bash_history file. The commands for the current session are not saved in this file until the session is closed:

[trainee@redhat9 ~]$ nl .bash_history | tail
    59 ls
    60 ls | sort
    61 ls | sort -r
    62 more /etc/services
    63 less /etc/services
    64 find acc
    65 find aac
    66 su -
    67 sleep 10
    68 su -

The /bin/bash shell is used to generate file name endings. This is accomplished by using the [Tab] key. In the following example, the command entered is :

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

[trainee@redhat9 ~]$ ls .bash
.bash_history .bash_logout .bash_profile .bashrc 

The same possibility exists for generating command name endings. In this case enter the following command:

$ mo [Tab][Tab]

Press the Tab key twice. You will get a window similar to this one:

[trainee@redhat9 ~]$ mo
modinfo modulemd-validator more mount.composefs mount.fuse3         
modprobe monitor-sensor mount mount.fuse mountpoint 

When using the shell, we often need to execute a command on several files instead of processing them individually. For this purpose we can use special characters.

In your home directory, create a training directory. Then create 5 files in this directory named f1, f2, f3, f4 and f5 respectively:

[trainee@redhat9 ~]$ mkdir training
[trainee@redhat9 ~]$ cd training
[trainee@redhat9 training]$ touch f1 f2 f3 f4 f5
[trainee@redhat9 training]$ ls
f1 f2 f3 f4 f5

To demonstrate the use of the special character *, enter the following command:

[trainee@redhat9 training]$ echo f*
f1 f2 f3 f4 f5

Now create the f52 and f62 files:

[trainee@redhat9 training]$ touch f52 f62

Then enter the following command:

[trainee@redhat9 training]$ echo f?2
f52 f62

Usage can take several different forms:

To demonstrate the use of the characters [ and ], create the file a100 :

[trainee@redhat9 training]$ touch a100

Then enter the following commands and note the result:

[trainee@redhat9 training]$ echo [a-f]*
a100 f1 f2 f3 f4 f5 f52 f62

[trainee@redhat9 training]$ echo [af]*
a100 f1 f2 f3 f4 f5 f52 f62

[trainee@redhat9 training]$ echo [!a]*
f1 f2 f3 f4 f5 f52 f62

[trainee@redhat9 training]$ echo [a-b]*
a100

[trainee@redhat9 training]$ echo [a-f]
[a-f]

Enable the extglob option in the bash shell so that you can use ?(expression), *(expression), +(expression), @(expression) and !(expression) :

[trainee@redhat9 training]$ shopt -s extglob

The shopt command is used to enable or disable options for optional shell behaviour. The list of options can be viewed by executing the shopt command without options:

[trainee@redhat9 training]$ shopt
autocd off
assoc_expand_once off
cdable_vars off
cdspell off
checkhash off
checkjobs off
checkwinsize on
cmdhist on
compat31 off
compat32 off
compat40 off
compat41 off
compat42 off
compat43 off
compat44 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
globasciiranges on
globstar off
gnu_errfmt off
histappend on
histreedit off
histverify off
hostcomplete off
huponexit off
inherit_errexit off
interactive_comments on
lastpipe off
lithist off
localvar_inherit off
localvar_unset off
login_shell on
mailwarn off
no_empty_cmd_completion off
nocaseglob off
nocasematch off
nullglob off
progcomp on
progcomp_alias off
promptvars on
restricted_shell off
shift_verbose off
sourcepath on
syslog_history off
xpg_echo off

Create the files f, f.txt, f123.txt, f123123.txt, f123123123.txt :

[trainee@redhat9 training]$ touch f f.txt f123.txt f123123.txt f123123123.txt

Enter the following command:

[trainee@redhat9 training]$ ls f?(123).txt
f123.txt f.txt

Enter the following command:

[trainee@redhat9 training]$ ls f*(123).txt
f123123123.txt f123123.txt f123123.txt f.txt

Enter the following command:

[trainee@redhat9 training]$ ls f+(123).txt
f123123123.txt f123123.txt f123.txt

Enter the following command:

[trainee@redhat9 training]$ ls f@(123).txt
f123.txt

Enter the following command:

[trainee@redhat9 training]$ ls f!(123).txt
f123123123.txt f123123.txt f.txt

In order to use a special character in a literal context, an escape character must be used. There are three escape characters:

To illustrate the use of escape characters, consider the following command:

echo * is a special character [Enter].

When you enter this command in your training directory, you will get a window similar to this one:

[trainee@redhat9 training]$ echo * is a special character
a100 f f1 f123123123.txt f123123.txt f123123.txt f2 f3 f4 f5 f52 f62 f.txt is a special character

[trainee@redhat9 training]$ echo \* is a special character
* is a special character

[trainee@redhat9 training]$ echo ‘* is a special character
* is a special character

[trainee@redhat9 training]$ echo ‘* is a special character’ * is a special character
* is a special character

Each command returns a code at the end of its execution. The special variable $? is used to store the return code of the last command executed.

For example:

[trainee@redhat9 training]$ cd ..
[trainee@redhat9 ~]$ mkdir codes
[trainee@redhat9 ~]$ echo $?
0
[trainee@redhat9 ~]$ touch codes/exit.txt
[trainee@redhat9 ~]$ rmdir codes
rmdir: failed to remove ‘codes’: Directory not empty
[trainee@redhat9 ~]$ echo $?
1

In this example the codes directory was created successfully. The return code stored in the $? variable is a zero.

Deleting the directory encountered an error because codes contained the file return. The return code stored in the $? variable is one.

If the return code is zero, the last command was executed without error.

If the return code is other than zero, the last command was completed with an error.

Your dialogue with the Linux system uses input and output channels. The keyboard is called the standard input channel and the screen is called the standard output channel:

In other words, when you type a command on the keyboard, you see the result of that command on the screen.

Sometimes, however, it is useful to redirect the standard output channel to a file. In this way, the result of a command such as free can be stored in a file for future reference:

This effect is achieved by using a redirection:

[trainee@redhat9 ~]$ pwd
/home/trainee
[trainee@redhat9 ~]$ cd training
[trainee@redhat9 training]$ free > file
[trainee@redhat9 training]$ cat file
               total used free shared buff/cache available
Mem: 7869560 996400 4964048 15324 2229600 6873160
Swap: 5242876 0 5242876

If the target file does not exist, it is created and its contents will be the result of the free command.

On the other hand if the file already exists, it will be overwritten :

[trainee@redhat9 training]$ date > file
[trainee@redhat9 training]$ cat file
Thu Sep 26 12:49:11 PM CEST 2024

To add additional data to the same target file, a double redirect must be used:

[trainee@redhat9 training]$ free >> file
[trainee@redhat9 training]$ cat file
Thu Sep 26 12:49:11 PM CEST 2024
               total used free shared buff/cache available
Mem: 7869560 996392 4964048 15324 2229608 6873168
Swap: 5242876 0 5242876

This way, the output of the free command will be added to the end of your file after the information in the free command.

Input and output channels are numbered:

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

The following command will create a file named errorlog that contains error messages from the execution of the rmdir command:

[trainee@redhat9 training]$ cd ..
[trainee@redhat9 ~]$ rmdir training/ 2>errorlog
[trainee@redhat9 ~]$ cat errorlog 
rmdir: failed to remove ‘training/’: Directory not empty

In fact the error is generated because the training directory is not empty.

We can also gather channels. To apply this, we need to understand that the shell processes commands from left to right.

In the following example, we join the output channel and the error channel:

[trainee@redhat9 ~]$ free > file 2>&1

The syntax 2>&1 sends the output of channel 2 to the same place as channel 1, namely the file named file.

It is possible to modify the standard input channel in order to read information from a file. In this case the redirection is obtained by using the < character:

[trainee@redhat9 ~]$ wc -w < errorlog
8

In this example the wc command counts the number of words ( -w ) in the errorlog file and displays it on the screen :

Other redirections exist:

It is also possible to link commands using a | pipe.

In this case, the output channel of the command to the left of the pipe is sent to the input channel of the command to the right of the pipe :

[trainee@redhat9 ~]$ ls | wc -w
17

This command, run in your home directory, takes the output of the ls command and asks the wc command to count the number of words included in the output of ls:

Remember that the standard output can only be redirected in one direction. In order to be able to redirect the standard output to a file and view it on screen, we need to use the tee command with a pipe:

[trainee@redhat9 ~]$ date | tee file1
Thu Sep 26 12:54:36 PM CEST 2024
[trainee@redhat9 ~]$ cat file1
Thu Sep 26 12:54:36 PM CEST 2024

This same technique allows us to create two files:

[trainee@redhat9 ~]$ date | tee file1 > file2
[trainee@redhat9 ~]$ cat file1
Thu Sep 26 12:55:11 PM CEST 2024
[trainee@redhat9 ~]$ cat file2
Thu Sep 26 12:55:11 PM CEST 2024

It is sometimes interesting, particularly in scripts, to replace a command with its output value. To illustrate this point, let's consider the following commands:

[trainee@redhat9 ~]$ echo date
date

[trainee@redhat9 ~]$ echo $(date)
Thu Sep 26 12:56:02 PM CEST 2024

[trainee@redhat9 ~]$ echo `date
Thu Sep 26 12:56:17 PM CEST 2024

It is possible to group commands together using a sub-shell :

$ (ls -l; ps; who) > list [Enter]

This example sends the results of the three commands to the list file, processing them in the background.

The commands can also be chained according to the return code of the previous command.

&& is used to ensure that the second command is executed if the output status value is 0, i.e. there were no errors.

|| is used to ensure the reverse.

The syntax of this command is :

Command1 && Command2

In this case, Command2 is only executed if Command1 has run without error.

Or :

Command1 || Command2

In this case, Command2 is executed if Command1 has encountered an error.

A shell variable can be displayed using the command :

$ echo $VARIABLE [Input]

Internationalization, also known as i18n because there are 18 letters between the letter I and the letter n in the word Internationalization, involves adapting software to parameters that vary from one region to another:

• word length
• accents
• writing from left to right or right to left,
• currency units
• typographic styles and editorial models,
• units of measurement,
• date and time display,
• print formats,
• keyboard format,
• keyboard format, etc.

The Localization, also called l10n because there are 10 letters between the letter L and the letter n in the word Localization, consists of modifying the internalisation according to a specific region.

The complete country code takes the following form: language-PAYS.character_set. For example, for the English language the language-PAYS values are :

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

The most important system variables containing regionalisation information are :

For example:

[trainee@redhat9 ~]$ echo $LC_ALL

[trainee@redhat9 ~]$ echo $LC_CTYPE

[trainee@redhat9 ~]$ echo $LANG
en_US.UTF-8
[trainee@redhat9 ~]$ locale
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8’
LC_NUMERIC="en_US.UTF-8’
LC_TIME="en_US.UTF-8’
LC_COLLATE="en_US.UTF-8’
LC_MONETARY="en_US.UTF-8
LC_MESSAGES="en_US.UTF-8’
LC_PAPER="en_US.UTF-8’
LC_NAME="en_US.UTF-8’
LC_ADDRESS="en_US.UTF-8’
LC_TELEPHONE="en_US.UTF-8’
LC_MEASUREMENT="en_US.UTF-8’
LC_IDENTIFICATION="en_US.UTF-8
LC_ALL=

The env command sends the values of the system variables in the environment of the user invoking it to the standard output:

[trainee@redhat9 ~]$ env
SHELL=/bin/bash
HISTCONTROL=ignoredups
HISTSIZE=1000
HOSTNAME=redhat9.ittraining.loc
PWD=/home/trainee
LOGNAME=trainee
XDG_SESSION_TYPE=tty
MOTD_SHOWN=pam
HOME=/home/trainee
LANG=en_US.UTF-8
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:mi=01;37;41: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:*.dz=01;31:*.gz=01;31:*. lrz=01;31:*.lz=01;31:*.lzo=01;31:*.xz=01;31:*.zst=01;31:*.tzst=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:*.wim=01;31:*.swm=01;31:*.dwm=01;31:*.esd=01;31:*.jpg=01;35:*.jpeg=01;35:*.mjpg=01;35:*. mjpeg=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:*.webp=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:*.ogv=01;35:*.ogx=01;35:*. aac=01;36:*.au=01;36:*.flac=01;36:*.m4a=01;36:*.mid=01;36:*.midi=01;36:*.mka=01;36:*.mp3=01;36:*.mpc=01;36:*.ogg=01;36:*.ra=01;36:*.wav=01;36:*.oga=01;36:*.opus=01;36:*.spx=01;36:*.xspf=01;36:
SSH_CONNECTION=10.0.2.1 37578 10.0.2.101 22
XDG_SESSION_CLASS=user
SELINUX_ROLE_REQUESTED=
TERM=xterm-256color
LESSOPEN=||/usr/bin/lesspipe.sh %s
USER=trainee
SELINUX_USE_CURRENT_RANGE=
SHLVL=1
XDG_SESSION_ID=4
XDG_RUNTIME_DIR=/run/user/1000
SSH_CLIENT=10.0.2.1 37578 22
which_declare=declare -f
XDG_DATA_DIRS=/home/trainee/.local/share/flatpak/exports/share:/var/lib/flatpak/exports/share:/usr/local/share:/usr/share
PATH=/home/trainee/.local/bin:/home/trainee/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin
SELINUX_LEVEL_REQUESTED=
DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
MAIL=/var/spool/mail/trainee
SSH_TTY=/dev/pts/0
BASH_FUNC_which%%=() { ( alias;
 eval ${which_declare} ) | /usr/bin/which --tty-only --read-alias --read-functions --show-tilde --show-dot $@
}
_=/usr/bin/env
OLDPWD=/home/trainee/training

The command can also be used to set a variable when executing a command. For example, to run xterm with the EDITOR variable set to vi:

$ env EDITOR=vim xterm

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

$ set -o [Enter]

For example:

[trainee@redhat9 ~]$ 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 activate an option it is again convenient to use the set command:

[trainee@redhat9 ~]$ set -o allexport
[trainee@redhat9 ~]$ set -o
allexport on
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

Note that the allexport option has been enabled.

To disable an option, we use the set command with the +o option:

$ set +o allexport [Enter]

[trainee@redhat9 ~]$ set +o allexport
[trainee@redhat9 ~]$ 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

Among the options, here is a description of the most interesting ones:

[trainee@redhat9 ~]$ set -o noclobber
[trainee@redhat9 ~]$ pwd > file
-bash: file: cannot overwrite existing file
[trainee@redhat9 ~]$ pwd >| file
[trainee@redhat9 ~]$ cat file
/home/trainee
[trainee@redhat9 ~]$ set +o noclobber

[trainee@redhat9 ~]$ set -o noglob
[trainee@redhat9 ~]$ echo *
*
[trainee@redhat9 ~]$ set +o noglob
[trainee@redhat9 ~]$ echo *
aac abc bca codes Desktop Documents Downloads errorlog file file1 file2 list Music Pictures Public Templates training Videos vitext xyz

[trainee@redhat9 ~]$ set -o nounset
[trainee@redhat9 ~]$ echo $FENESTROS
-bash: FENESTROS: unbound variable
[trainee@redhat9 ~]$ set +o nounset
[trainee@redhat9 ~]$ echo $FENESTROS

[trainee@redhat9 ~]$

Copyright © 2024 Hugh Norris.