RH12405 - The Command Line

• RH12405 - The Command Line * Contents * The Shell * LAB #1 - The /bin/bash Shell * 1.1 - 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 * The * Metacharacter * The ? Metacharacter * The [] Metacharacter * 1.8 - The extglob option * ?(expression) * *(expression) * +(expression) * @(expression) * !(expression) * Escape characters * 1.9 - Exit Codes * 1.10 - Redirections * 1.11 - Pipes * 1.12 - Command substitutions * 1.13 - Command chaining * 1.14 - Displaying shell variables * Main variables * Regionalisation and internationalisation variables * Special variables * 1.15 - The env command * 1.16 - Bash Shell options * Examples * noclobber * noglob * nounset =====Le Shell===== 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. Internal shell commands
4. External commands

Internal 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. ====1.5 - Recalling commands ==== The /bin/bash shell can be used to recall the last commands entered. To see the list of memorised commands, use the history command: <code> [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] </code>

  Important: The history is specific to each user.

  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 : ^ Control Character ^ Action ^ | [CTRL]-[P] (= Up Arrow) | Navigates backwards through the list | | [CTRL]-[N] (= Down Arrow) | Navigates forwards through the list | To move through the history line : ^ Control Character ^ Action ^ | [CTRL]-[A] | Move to the beginning of the line | | [CTRL]-[E] | Move to the end of the line | | [CTRL]-[B] | Move one character to the left | | [CTRL]-[F] | Move one character to the right | | [CTRL]-[D] | Delete the character under the cursor | To search the history, use the keys : ^ Control Character ^ Action ^ | [CTRL]-[R] string | Search backwards for string in the history. Using [CTRL]-[R] again will search for the previous occurence of string | | [CTRL]-[S] string | Search forwards for string in the history. Using [CTRL]-[S] again will search for the next occurence of string | | [CTRL]-[G] | Quit the search mode | It is also possible to recall the last command in the history using the !! characters: <code> [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 </code> You can recall a specific command from the history by using the ! character followed by the number of the command to be recalled: <code> [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 </code> 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 : <code> [trainee@redhat9 ~]$ cat /etc/profile | grep HISTSIZE HISTSIZE=1000 export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE HISTCONTROL </code> 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: <code> [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 - </code>

  Important: Note the use of the nl command to number the lines in the display of the contents of the .bash_history file.

  ====1.6 - Generating file name endings==== 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] <code> [trainee@redhat9 ~]$ ls .bash .bash_history .bash_logout .bash_profile .bashrc </code>

  Important: Note that by pressing the Tab key three times the shell offers 4 filename completion possibilities. This is because, without more information, the shell does not know which file is involved.

  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: <code> [trainee@redhat9 ~]$ mo modinfo modulemd-validator more mount.composefs mount.fuse3 modprobe monitor-sensor mount mount.fuse mountpoint </code> ====1.7 - The interactive shell ==== 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. ^ Metacharacter ^ Description ^ | * | Matches one or more characters | | ? | Matches a single character | | [abc] | Matches any one of the characters between square brackets | | [!abc] | Matches any character except those between square brackets | | [m-t] | Matches any character from m through to t | | [!m-t] | Matches any character other than m through to t | | ?(expression1|expression2| …) | Matches 0 or 1 occurence of expression1 OR 0 or 1 occurence of expression2 OR … | | *(expression1|expression2| …) | Matches 0 to x occurences of expression1 OR 0 to x occurences of expression2 OR … | | +(expression1|expression2| …) | Matches 1 to x occurences of expression1 OR 1 to x occurences of expression2 OR … | | @(expression1|expression2| …) | Matches 1 occurrence of expression1 OR 1 occurence of expression2 OR … | | !(expression1|expression2| …) | Matches 0 occurrences of expression1 OR 0 occurrences of expression2 OR … | ===The * Metacharacter=== In your home directory, create a training directory. Then create 5 files in this directory named f1, f2, f3, f4 and f5 respectively: <code> [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 </code> To demonstrate the use of the special character *, enter the following command: <code> [trainee@redhat9 training]$ echo f* f1 f2 f3 f4 f5 </code>

  Important: Note that the * character replaces a character or a string of characters.

  ===The ? Metacharacter=== Now create the f52 and f62 files: <code> [trainee@redhat9 training]$ touch f52 f62 </code> Then enter the following command: <code> [trainee@redhat9 training]$ echo f?2 f52 f62 </code>

  Important: Note that the ? character replaces a single character.

  ===The [] Metacharacter=== Usage can take several different forms: ^ Metacharacter ^ Description ^ | [xyz] | Represents either x or y or z | | [m-t] | Represents a character in the range m to t | [!xyz] | Represents any character other than x or y or z | | [!m-t] | Represents any character outside of the range m to t | To demonstrate the use of the characters [ and ], create the file a100 : <code> [trainee@redhat9 training]$ touch a100 </code> Then enter the following commands and note the result: <code> [trainee@redhat9 training]$ echo [a-f]* a100 f1 f2 f3 f4 f5 f52 f62 </code>

  Important: Note here that all files starting with the letters a, b, c, d, e or f are displayed on the screen.

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

  Important: Note here that all files starting with the letters a or f are displayed on the screen.

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

  Important: Note here that all files are displayed on the screen, with the exception of one file beginning with the letter a .

  <code> [trainee@redhat9 training]$ echo [a-b]* a100 </code>

  Important: Note here that only the file starting with the letter a is displayed on the screen as there are no files starting with the letter b.

  <code> [trainee@redhat9 training]$ echo [a-f] [a-f] </code>

  Important: Note that in this case, there are no files named a, b, c, d, e or f. For this reason, having found no correspondence between the filter used and the objects in the current directory, the echo command returns the filter passed as an argument, i.e. [a-f].

  ====1.8 - The extglob option==== Enable the extglob option in the bash shell so that you can use ?(expression), *(expression), +(expression), @(expression) and !(expression) : <code> [trainee@redhat9 training]$ shopt -s extglob </code> 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: <code> [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 </code> ===?(expression)=== Create the files f, f.txt, f123.txt, f123123.txt, f123123123.txt : <code> [trainee@redhat9 training]$ touch f f.txt f123.txt f123123.txt f123123123.txt </code> Enter the following command: <code> [trainee@redhat9 training]$ ls f?(123).txt f123.txt f.txt </code>

  Important: Note here that the command displays files with names containing 0 or 1 occurrences of the string 123.

  ===*(expression)=== Enter the following command: <code> [trainee@redhat9 training]$ ls f*(123).txt f123123123.txt f123123.txt f123123.txt f.txt </code>

  Important: Note here that the command displays files with names containing from 0 up to x occurrences of the string 123.

  ===+(expression)=== Enter the following command: <code> [trainee@redhat9 training]$ ls f+(123).txt f123123123.txt f123123.txt f123.txt </code>

  Important : Note here that the command displays files with names containing between 1 and x occurrences of the string 123.

  ===@(expression)=== Enter the following command: <code> [trainee@redhat9 training]$ ls f@(123).txt f123.txt </code>

  Important: Note here that the command displays files with names containing 1 single occurrence of the string 123.

  ===!(expression)=== Enter the following command: <code> [trainee@redhat9 training]$ ls f!(123).txt f123123123.txt f123123.txt f.txt </code>

  Important: Note here that the command only displays files with a name that does not contain the string 123.

  ===Escaping characters=== In order to use a special character in a literal context, an escape character must be used. There are three escape characters: ^ Character ^ Description ^ | Protects the character that follows it | | Protects any character, except the ‘ character itself, between the two ’. | Protects any character, with the exception of the ‘ character itself, $, \ and ’, between the two “ |. To illustrate the use of escape characters, consider the following command: echo * is a special character [Enter]. When you enter this command in your training directory, you will get a window similar to this one: <code> [trainee@redhat9 training]$ 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 </code> ====1.9 - Exit codes==== Each command returns a code at the end of its execution. The special variable $? is used to store the return code of the last command executed. For example: <code> [trainee@redhat9 training]$ 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 </code> 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 $? variable is one. If the return code is zero, the last command was executed without error. If the return code is other than zero, the last command was completed with an error. ====1.10 - Redirections==== Your dialogue with the Linux system uses input and output channels. The keyboard is called the standard input channel and the screen is called the standard output channel: In other words, when you type a command on the keyboard, you see the result of that command on the screen. Sometimes, however, it is useful to redirect the standard output channel to a file. In this way, the result of a command such as free can be stored in a file for future reference: This effect is achieved by using a redirection: <code> [trainee@redhat9 ~]$ 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 </code> 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 : <code> [trainee@redhat9 training]$ date > file [trainee@redhat9 training]$ cat file Thu Sep 26 12:49:11 PM CEST 2024 </code> To add additional data to the same target file, a double redirect must be used: <code> [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 </code> This way, the output of the free command will be added to the end of your file after the information in the free command.

  Important: Note that the standard output can only be redirected in one direction.

  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: <code> [trainee@redhat9 training]$ cd .. [trainee@redhat9 ~]$ rmdir training/ 2>errorlog [trainee@redhat9 ~]$ cat errorlog rmdir: failed to remove ‘training/’: Directory not empty </code> 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: <code> [trainee@redhat9 ~]$ free > file 2>&1 </code> 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: <code> [trainee@redhat9 ~]$ wc -w < errorlog 8 </code> 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: ^ Redirection ^ Definition ^ | &> | Join file descriptors 1 and 2. | | << | Takes the text typed on the next lines as standard input until EOF is found at the beginning of a line. | | <> | Allows the use of the same file as STDIN and STDOUT. | ====1.11 - Pipes==== 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 : <code> [trainee@redhat9 ~]$ ls | wc -w 17 </code> 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:

  Important: Note that it is possible to link several tubes in the same command.

  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: <code> [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 </code> This same technique allows us to create two files: <code> [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 </code>

  Important: By default the tee command overwrites the destination file. To add additional data to the same target file, the -a option to the tee command should be used.

  ====1.12 - Command substitutions==== 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: <code> [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 </code>

  Important: Note the format of each $(command) or `commande` substitution. On a French keyboard, the anti-coast is accessed using the Alt Gr and 77 keys.

  ====1.13 - Command chaining==== 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. ====1.14 - Displaying shell variables==== A shell variable can be displayed using the command : $ echo $VARIABLE [Input] === Main variables === ^ Variable ^ Description ^ | BASH | Complete path to current shell. | | BASH_VERSION |Shell version. | | EUID | EUID of the current user. | | UID | UID of the current user. | | PPID | PID of the parent of the current process. | | PWD | The current directory. | | OLDPWD | The previous current directory ( like the cd -command ).| | RANDOM | A random number between 0 and 32767. | | SECONDS | The numbers of seconds since the shell was started.| | LINES | The number of lines in a screen. | | COLUMNS | The number of columns in a screen . | | HISTFILE | The history file. | | HISTFILESIZE | The history file size. | | HISTSIZE | The number of commands that can be saved to the history file. | | HISTCMD | The current command's number in the History. | | HISTCONTROL | ignorespace or ignoredups or ignoreboth | | HOME | The user's home directory. | | HOSTTYPE | Machine type. | | OSTYPE | The OS type. | | MAIL | The file containing the user's mail. | | MAILCHECK | Frequency in seconds that a user's mail is checked. | | PATH | The paths to executables. | | PROMPT_COMMAND | Command executed before each prompt is displayed. | | PS1 | User's default prompt. | | PS2 | User's 2nd level default prompt. | | PS3 | User's 3rd level prompt. | | PS4 | User's 4th level prompt. | | SHELL | User's current shell. | | SHLVL | The number of shell instances. | | TMOUT | The number of seconds less 60 before an unused terminal gets sent the exit command. | === Regionalisation and Internationalisation Variables === Internationalisation, 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 Regionalisation, also called l10n because there are 10 letters between the letter L and the letter n in the word Localisation, 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.