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 +Version : **2024.01**
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 +Last update : ~~LASTMOD~~
 +
 +====== RH12405 - The Command Line Interface======
 +
 +=====Contents=====
 +
 +  * **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)
 +        * Protecting Metacharacters
 +    * 1.9 - Exit 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
 +
 +=====The 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 :
 +
 +^ Shell ^ Name ^ Release Date ^ Inventer ^ Command ^ Comments ^
 +| tsh | Thompson Shell | 1971 | Ken Thompson | sh | The first shell |
 +| sh | Bourne Shell | 1977 | Stephen Bourne | sh | The shell common to all Unix and Linux OSs: /bin/sh |
 +| csh | C-Shell | 1978 | Bill Joy | csh | The BSD shell: /bin/csh |
 +| tcsh | Tenex C-Shell | 1979 | Ken Greer | tcsh | A fork of the csh shell: /bin/tcsh |
 +| ksh | Korn Shell | 1980 | David Korn | ksh | Open Source since 2005: /bin/ksh |
 +| bash| Bourne Again Shell | 1987 | Brian Fox | bash | The default shell for Linux, MacOS X, Solaris 11: /bin/bash |
 +| zsh | Z Shell | 1990 | Paul Falstad | zsh | Zsh is an extended Bourne shell with a large number of improvements, including some features of bash, ksh, and tcsh: /usr/bin/zsh |
 +
 +Under RHEL 9 the **/bin/sh** shell is a symbolic link to **/bin/bash** :
 +
 +<code>
 +[trainee@redhat9 ~]$ ls -l /bin/sh
 +lrwxrwxrwx. 1 root root 4 Feb 15 2024 /bin/sh -> bash
 +</code>
 +
 +=====LAB #1 - The /bin/bash Shell=====
 +
 +This module is about using the **bash** shell under Linux. The **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 begins with a keyword. This keyword is interpreted by the shell according to the type of command and in the following order:
 +
 +  * An Alias,
 +  * A Function,
 +  * A Built-in Command,
 +  * An External Command.
 +
 +====1.1 - Built-in and External Shell Commands====
 +
 +Built-in shell commands are commands such as **cd**. To check the type of command, use the **type** command:
 +
 +<code>
 +[trainee@redhat9 ~]$ type cd
 +cd is a shell builtin
 +</code>
 +
 +Commands external to the shell are executable binaries or scripts, generally located in /bin, /sbin, /usr/bin or /usr/sbin :
 +
 +<code>
 +[trainee@redhat9 ~]$ type ifconfig
 +ifconfig is /usr/sbin/ifconfig
 +</code>
 +
 +====1.2 - Aliases====
 +
 +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 :
 +
 +<code>
 +[trainee@redhat9 ~]$ type ls
 +ls is aliased to `ls --color=auto'
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note that in this case the **ls** alias is indeed an alias that uses the ls **command** itself.
 +</WRAP>
 +
 +An alias is defined using the **alias** command:
 +
 +<code>
 +[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
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note that the **dir** command actually exists. Creating an alias that is called **dir** implies that the alias will be executed instead of the **dir** command.
 +</WRAP>
 +
 +The list of defined aliases can be viewed using the **alias** command:
 +
 +<code>
 +[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’
 +</code>
 +
 +<WRAP centre round important 60%>
 +**Important**: Note that this list contains, indiscriminately, aliases defined in system startup files as well as the **dir** alias created by **trainee** that is only available to **trainee** in the current terminal.
 +</WRAP>
 +
 +To force execution of a command and not the alias you must precede the command with the **\** character:
 +
 +<code>
 +[trainee@redhat9 ~]$ \dir
 +aac abc bca Desktop Documents Downloads Music Pictures Public Templates Videos vitext xyz
 +</code>
 +
 +To delete an alias, use the **unalias** command:
 +
 +<code>
 +[trainee@redhat9 ~]$ unalias dir
 +[trainee@redhat9 ~]$ dir
 +aac abc bca Desktop Documents Downloads Music Pictures Public Templates Videos vitext xyz
 +</code>
 +
 +====1.3 - Defining a user's shell====
 +
 +The user shell is defined by **root** in the last field of the **/etc/passwd** file:
 +
 +<code>
 +[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
 +</code>
 +
 +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:
 +
 +<code>
 +[trainee@redhat9 ~]$ cat /etc/shells
 +/bin/sh
 +/bin/bash
 +/usr/bin/sh
 +/usr/bin/bash
 +</code>
 +
 +Then use the **echo** command to display the current **trainee** shell:
 +
 +<code>
 +[trainee@redhat9 ~]$ echo $SHELL
 +/bin/bash
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note that under RHEL 9, the system informs us that the current shell of the **trainee** user is **/bin/bash** and not **/usr/bin/bash**. This is because the /bin directory is a symbolic link pointing to the /usr/bin directory.
 +</WRAP>
 +
 +Then change the **trainee** shell using the **chsh** command, specifying the value of **/bin/sh** for the new shell:
 +
 +<code>
 +[trainee@redhat9 ~]$ chsh
 +Changing shell for trainee.
 +New shell [/bin/bash]: /bin/sh
 +Password: trainee
 +Shell changed.
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note that the password entered will **not** be visible.
 +</WRAP>
 +
 +Next, check the active shell for **trainee** :
 +
 +<code>
 +[trainee@redhat9 ~]$ echo $SHELL
 +/bin/bash
 +</code>
 +
 +Lastly check the shell stipulated in the **/etc/passwd** file for **trainee**:
 +
 +<code>
 +[trainee@redhat9 ~]$ cat /etc/passwd | grep trainee
 +trainee:x:1000:1000:trainee:/home/trainee:/bin/sh
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: You will notice that the active shell is always **/bin/bash** whereas the shell stipulated in the /etc/passwd file is the **/bin/sh**. The **/bin/sh** shell will only become **trainee**'s active shell the next time it connects to the system.
 +</WRAP>
 +
 +Change your shell to **/bin/bash** again using the chsh command:
 +
 +<code>
 +[trainee@redhat9 ~]$ chsh
 +Changing shell for trainee.
 +New shell [/bin/sh]: /bin/bash
 +Password: trainee
 +Shell changed.
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note that the password entered will **not** be visible.
 +</WRAP>
 +
 +====1.4 - The Prompt====
 +
 +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>
 +
 +<WRAP center round important 60%>
 +**Important**: The history is specific to each user.
 +</WRAP>
 +
 +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>
 +
 +<WRAP center round important 60%>
 +**Important**: Note the use of the **nl** command to number the lines in the display of the contents of the **.bash_history** file.
 +</WRAP>
 +
 +====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>
 +
 +<WRAP center round important 60%>
 +**Important**: Note that by pressing the <key>tab</key> 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.
 +</WRAP>
 +
 +The same possibility exists for generating command name endings. In this case enter the following command:
 +
 +  $ mo [Tab][Tab]
 +
 +Press the <key>tab</key> 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 metacharacters. 
 +
 +^ 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 metacharacter *, enter the following command:
 +
 +<code>
 +[trainee@redhat9 training]$ echo f*
 +f1 f2 f3 f4 f5
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note that the ***** character replaces a character or a string of characters.
 +</WRAP>
 +
 +===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>
 +
 +<WRAP center round important 60%>
 +**Important**: Note that the **?** character replaces **a single** character.
 +</WRAP>
 +
 +===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>
 +
 +<WRAP center round important 60%>
 +**Important**: Note here that all files starting with the letters **a**, **b**, **c**, **d**, **e** or **f** are displayed on the screen.
 +</WRAP>
 +
 +<code>
 +[trainee@redhat9 training]$ echo [af]*
 +a100 f1 f2 f3 f4 f5 f52 f62
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note here that all files starting with the letters **a** or **f** are displayed on the screen.
 +</WRAP>
 +
 +<code>
 +[trainee@redhat9 training]$ echo [!a]*
 +f1 f2 f3 f4 f5 f52 f62
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note here that all files are displayed on the screen, with the exception of one file beginning with the letter **a** .
 +</WRAP>
 +
 +<code>
 +[trainee@redhat9 training]$ echo [a-b]*
 +a100
 +</code>
 +
 +<WRAP center round important 60%>
 +**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**.
 +</WRAP>
 +
 +<code>
 +[trainee@redhat9 training]$ echo [a-f]
 +[a-f]
 +</code>
 +
 +<WRAP centre round important 60%>
 +**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]**.
 +</WRAP>
 +
 +====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>
 +
 +<WRAP centre round important 60%>
 +**Important**: Note here that the command displays files with names containing 0 or 1 occurrences of the string **123**.
 +</WRAP>
 +
 +===*(expression)===
 +
 +Enter the following command:
 +
 +<code>
 +[trainee@redhat9 training]$ ls f*(123).txt
 +f123123123.txt f123123.txt f123123.txt f.txt
 +</code>
 +
 +<WRAP centre round important 60%>
 +**Important**: Note here that the command displays files with names containing from 0 up to x occurrences of the string **123**.
 +</WRAP>
 +
 +===+(expression)===
 +
 +Enter the following command:
 +
 +<code>
 +[trainee@redhat9 training]$ ls f+(123).txt
 +f123123123.txt f123123.txt f123.txt
 +</code>
 +
 +<WRAP centre round important 60%>
 +**Important** : Note here that the command displays files with names containing between 1 and x occurrences of the string **123**.
 +</WRAP>
 +
 +===@(expression)===
 +
 +Enter the following command:
 +
 +<code>
 +[trainee@redhat9 training]$ ls f@(123).txt
 +f123.txt
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note here that the command displays files with names containing 1 single occurrence of the string **123**.
 +</WRAP>
 +
 +===!(expression)===
 +
 +Enter the following command:
 +
 +<code>
 +[trainee@redhat9 training]$ ls f!(123).txt
 +f123123123.txt f123123.txt f.txt
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note here that the command only displays files with a name that does **not** contain the string **123**.
 +</WRAP>
 +
 +====Protecting Metacharacters====
 +
 +In order to use a metacharacter in a literal context, an escape character must be used. There are three escape characters:
 +
 +^ Character ^ Description ^
 +| \ | Escapes the character which immediately follows |
 +| ' ' | Protects any character between the two **'** |
 +| " " | Protects any character between the two **"** except the following: **$**, **\** and **'** |
 +
 +To illustrate the use of escape characters, consider the following command:
 +
 +  echo * is a metacharacter [Enter].
 +
 +When you enter this command in your **training** directory, you will get a window similar to this one:
 +
 +<code>
 +[trainee@redhat9 training]$ echo * is a metacharacter
 +a100 f f1 f123123123.txt f123123.txt f123123.txt f2 f3 f4 f5 f52 f62 f.txt is a metacharacter
 +
 +[trainee@redhat9 training]$ echo \* is a metacharacter
 +* is a metacharacter
 +
 +[trainee@redhat9 training]$ echo "* is a metacharacter"
 +* is a metacharacter
 +
 +[trainee@redhat9 training]$ echo ‘* is a metacharacter’
 +* is a metacharacter
 +</code>
 +
 +====1.9 - Exit codes====
 +
 +Each command returns an **exit status** when it is executed. This exit status is stored in a special variable: **$?**.
 +
 +For example:
 +
 +<code>
 +[trainee@redhat9 training]$ 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 exit code stored in the $? variable is a zero.
 +
 +Deleting the directory encountered an error because **codes** contained the file **return**. The exit code stored in the $? variable is **one**.
 +
 +If the exit code is **zero**, the last command was executed without error. 
 +
 +If the exit code is **other than zero**, the last command was completed with an error. 
 +
 +====1.10 - Redirections====
 +
 +Your dialogue with the Linux system uses input and output channels. The keyboard is called the **standard input channel** and the screen is called the **standard output channel**: 
 +
 +{{:free:stdin.png|}}
 +
 +In other words, when you type a command on the keyboard, you see the result of that command on the screen.
 +
 +Sometimes, however, it is useful to redirect the standard output channel to a file. In this way, the result of a command such as **free** can be stored in a file for future reference:
 +
 +{{:free:redirection.png|}} 
 +
 +This effect is achieved by using a **redirection**:
 +
 +<code>
 +[trainee@redhat9 ~]$ 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]$ 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.
 +
 +<WRAP centre round important 60%>
 +**Important**: Note that the standard output can only be redirected in **one direction**. 
 +</WRAP>
 +
 +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.
 +
 +You can join file descriptors using the **&** character:
 +
 +<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:
 +
 +<WRAP center round important 60%>
 +**Important**: Note that it is possible to link several tubes in the same command.
 +</WRAP>
 +
 +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>
 +
 +<WRAP center round important 60%>
 +**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.
 +</WRAP>
 +
 +====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>
 +
 +<WRAP center round important 60%>
 +**Important**: Note the format of each **$(command)** or **`commande`** substitution. On a French keyboard, the anti-coast is accessed using the <key>Alt Gr</key> and <key>7</key><key>7</key> keys.
 +</WRAP>
 +
 +====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 exit 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. |
 +
 +=== Internationalization and Localization Variables ===
 +
 +**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: 
 +
 +  * 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 ...
 +
 +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 :
 +
 +^ Variable ^ Description ^
 +| LC_ALL | With a non-zero value, this takes precedence over the value of all other internationalisation variables. |
 +| LANG | Provides a default value for environment variables whose value is null or undefined. |
 +| LC_CTYPE | Determines the regional parameters for interpreting the sequence of bytes of text data in characters. |
 +
 +For example:
 +
 +<code>
 +[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=
 +</code>
 +
 +===Special variables===
 +
 +^ Variable ^ Description ^
 +| $LINENO | Contains the current line number of the script or function being executed |
 +| $$ | Contains the PID of the current process |
 +| $PPID | Contains the PID of the parent of the current process |
 +| $0 | Contains the name of the current script |
 +| $1, $2 ... | Contains respectively the 1st, 2nd etc arguments passed to the script |
 +| $# | Contains the total number of arguments passed to the script |
 +| $* | Contains all of the arguments passed to the script |
 +| $@ | Contains all of the arguments passed to the script |
 +
 +====1.15 - The env command ====
 +
 +The **env** command sends the values of the system variables in the environment of the user invoking it to the standard output:
 +
 +<code>
 +[trainee@redhat9 ~]$ 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
 +</code>
 +
 +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
 +
 +====1.16 - Bash Shell options====
 +
 +To view the bash shell options, use the **set** command:
 +
 +  $ set -o [Enter]
 +
 +For example:
 +
 +<code>
 +[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
 +</code>
 +
 +To activate an option it is again convenient to use the **set** command:
 +
 +<code>
 +[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
 +</code>
 +
 +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]
 +
 +<code>
 +[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
 +</code>
 +
 +These are the most interesting options:
 +
 +^ Option ^ Default value ^ Description ^
 +| allexport | off | The shell automatically exports all variables |
 +| emacs | on | emacs editing mode |
 +| noclobber  | off | Simple re-directions do not squash the target file if it exists |
 +| noglob | off | Turns off metacharacters |
 +| nounset | off | The shell will return an error if the variable is not set |
 +| verbose | off | Echos back the typed command |
 +| vi | off | vi editing mode |
 +
 +===Examples===
 +
 +==noclobber==
 +
 +<code>
 +[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
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note that the **noclobber** option can be bypassed by using the redirect followed by the **|** character.
 +</WRAP>
 +
 +==noglob==
 +
 +<code>
 +[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
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note that the effect of the metacharacter is cancelled under the influence of the **noglob** option.
 +</WRAP>
 +
 +==nounset==
 +
 +<code>
 +[trainee@redhat9 ~]$ set -o nounset
 +[trainee@redhat9 ~]$ echo $FENESTROS
 +-bash: FENESTROS: unbound variable
 +[trainee@redhat9 ~]$ set +o nounset
 +[trainee@redhat9 ~]$ echo $FENESTROS
 +
 +[trainee@redhat9 ~]$
 +</code>
 +
 +<WRAP center round important 60%>
 +**Important**: Note that the non-existent variable **$FENESTROS** is identified as such under the influence of the **nounset** option. Now the usual behavior of Linux is to return an empty line which does not indicate whether the variable does not exist or whether it is simply empty.
 +</WRAP>
 +
 +-----
 +Copyright © 2024 Hugh Norris.

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