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Command Line Interface

Dernière mise-à-jour : 2020/01/30 03:28

Command Line Interface

The Shell

A shell is a Command Line Interpreter (C.L.I). It is used to give instructions or commands to the operating system (OS).

The word shell is generic. There are many shells under Unix and Linux such as:

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

When using SLES 12 /bin/sh is a soft link to /bin/bash :

trainee@SLES12SP1:~> ls -l /bin/sh
lrwxrwxrwx 1 root root 4  1 mai    2016 /bin/sh -> bash

/bin/bash

This unit covers the /bin/bash shell. The /bin/bash shell allows you to:

Recall previously typed commands
Auto-generate the end of a file name
Use Aliases
Use tables
Use C language numerical and math variables
Manage strings
Use Functions

A command always starts with a keyword. This keyword is interpreted by the shell, in the order shown, as one of the following:

An Alias,
A Function,
A Built-in Command,
An External Command.

Internal And External Commands

The /bin/bash shell comes with a set of built-in or internal commands. External commands are executable binaries or scripts generally found in one of the following directories:

/bin,
/sbin,
/usr/bin,
/usr/sbin.

To check if a command is internal to the shell or external, use the type command:

trainee@SLES12SP1:~> type cd
cd is a shell builtin

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

trainee@SLES12SP1:~> type passwd
passwd is /usr/bin/passwd

Aliases

Aliases are strings that are aliased to a command, a command and some options or even several commands. Aliases are specific to the shell in which they are created and unless specified in one of the start-up files, they disappear when the shell is closed:

trainee@SLES12SP1:~> type ls
ls is aliased to `_ls'

[stextbox id='black' image='null'] Important: Note that the ls alias is an alias to the ls command itself. [/stextbox]

An alias is defined using the alias command:

trainee@SLES12SP1:~> alias dir='ls -l'
trainee@SLES12SP1:~> dir
total 4
-rw-r--r-- 1 trainee users   0  1 oct.  06:55 aac
-rw-r--r-- 1 trainee users   0  1 oct.  06:55 abc
-rw-r--r-- 1 trainee users   0  1 oct.  06:55 bca
drwxr-xr-x 1 trainee users   0  1 mai    2016 bin
drwxr-xr-x 1 trainee users   0  2 mai    2016 Desktop
drwxr-xr-x 1 trainee users   0  2 mai    2016 Documents
drwxr-xr-x 1 trainee users   0  2 mai    2016 Downloads
drwxr-xr-x 1 trainee users   0  2 mai    2016 Music
drwxr-xr-x 1 trainee users   0  2 mai    2016 Pictures
drwxr-xr-x 1 trainee users   0  2 mai    2016 Public
drwxr-xr-x 1 trainee users  20  1 mai    2016 public_html
drwxr-xr-x 1 trainee users   0  2 mai    2016 Templates
drwxr-xr-x 1 trainee users   0  2 mai    2016 Videos
-rw-r--r-- 1 trainee users 391 30 sept. 10:27 vitext
-rw-r--r-- 1 trainee users   0  1 oct.  06:55 xyz

[stextbox id='black' image='null'] Important: Note that dir exists as a command. By creating an alias of the same name, the alias will be executed in place of the command. [/stextbox]

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

trainee@SLES12SP1:~> alias
alias +='pushd .'
alias -='popd'
alias ..='cd ..'
alias ...='cd ../..'
alias aumix='padsp aumix'
alias beep='echo -en "\007"'
alias cd..='cd ..'
alias dir='ls -l'
alias egrep='egrep --color=auto'
alias fgrep='fgrep --color=auto'
alias grep='grep --color=auto'
alias l='ls -alF'
alias la='ls -la'
alias ll='ls -l'
alias ls='_ls'
alias ls-l='ls -l'
alias md='mkdir -p'
alias o='less'
alias rd='rmdir'
alias rehash='hash -r'
alias sox='padsp sox'
alias timidity='timidity -Oe'
alias unmount='echo "Error: Try the command: umount" 1>&2; false'
alias you='if test "$EUID" = 0 ; then /sbin/yast2 online_update ; else su - -c "/sbin/yast2 online_update" ; fi'

[stextbox id='black' image='null'] Important: In the above list you can see, without distinction, the system wide aliases created by system start up scripts and the user created alias dir. The latter is only available for trainee and will disappear when the current session is terminated. [/stextbox]

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

trainee@SLES12SP1:~> \dir
aac  bca  Desktop    Downloads	Pictures  public_html  Videos  xyz
abc  bin  Documents  Music	Public	  Templates    vitext

To delete an alias, simply use the unalias command:

trainee@SLES12SP1:~> unalias dir
trainee@SLES12SP1:~> dir
aac  bca  Desktop    Downloads	Pictures  public_html  Videos  xyz
abc  bin  Documents  Music	Public	  Templates    vitext

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

trainee@SLES12SP1:~> cat /etc/passwd
at:x:25:25:Batch jobs daemon:/var/spool/atjobs:/bin/bash
bin:x:1:1:bin:/bin:/bin/bash
daemon:x:2:2:Daemon:/sbin:/bin/bash
ftp:x:40:49:FTP account:/srv/ftp:/bin/bash
ftpsecure:x:488:65534:Secure FTP User:/var/lib/empty:/bin/false
games:x:12:100:Games account:/var/games:/bin/bash
gdm:x:486:485:Gnome Display Manager daemon:/var/lib/gdm:/bin/false
lp:x:4:7:Printing daemon:/var/spool/lpd:/bin/bash
mail:x:8:12:Mailer daemon:/var/spool/clientmqueue:/bin/false
man:x:13:62:Manual pages viewer:/var/cache/man:/bin/bash
messagebus:x:499:499:User for D-Bus:/var/run/dbus:/bin/false
news:x:9:13:News system:/etc/news:/bin/bash
nobody:x:65534:65533:nobody:/var/lib/nobody:/bin/bash
nscd:x:496:495:User for nscd:/run/nscd:/sbin/nologin
ntp:x:74:492:NTP daemon:/var/lib/ntp:/bin/false
openslp:x:494:2:openslp daemon:/var/lib/empty:/sbin/nologin
polkitd:x:497:496:User for polkitd:/var/lib/polkit:/sbin/nologin
postfix:x:51:51:Postfix Daemon:/var/spool/postfix:/bin/false
pulse:x:490:489:PulseAudio daemon:/var/lib/pulseaudio:/sbin/nologin
root:x:0:0:root:/root:/bin/bash
rpc:x:495:65534:user for rpcbind:/var/lib/empty:/sbin/nologin
rtkit:x:491:490:RealtimeKit:/proc:/bin/false
scard:x:487:487:Smart Card Reader:/var/run/pcscd:/usr/sbin/nologin
sshd:x:498:498:SSH daemon:/var/lib/sshd:/bin/false
statd:x:489:65534:NFS statd daemon:/var/lib/nfs:/sbin/nologin
usbmux:x:493:65534:usbmuxd daemon:/var/lib/usbmuxd:/sbin/nologin
uucp:x:10:14:Unix-to-Unix CoPy system:/etc/uucp:/bin/bash
vnc:x:492:491:user for VNC:/var/lib/empty:/sbin/nologin
wwwrun:x:30:8:WWW daemon apache:/var/lib/wwwrun:/bin/false
trainee:x:1000:100:trainee:/home/trainee:/bin/bas

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

trainee@SLES12SP1:~> cat /etc/shells
/bin/ash
/bin/bash
/bin/csh
/bin/dash
/bin/false
/bin/ksh
/bin/ksh93
/bin/mksh
/bin/pdksh
/bin/sh
/bin/tcsh
/bin/true
/bin/zsh
/usr/bin/csh
/usr/bin/dash
/usr/bin/ksh
/usr/bin/ksh93
/usr/bin/mksh
/usr/bin/passwd
/usr/bin/pdksh
/usr/bin/bash
/usr/bin/tcsh
/usr/bin/zsh

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

trainee@SLES12SP1:~> echo $SHELL
/bin/bash

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

trainee@SLES12SP1:~> chsh
Password: trainee
Changing the login shell for trainee
Enter the new value, or press ENTER for the default
	Login Shell [/bin/bash]: /bin/sh

[stextbox id='black' image='null'] Important: Note that the password will not be printed to standard output. [/stextbox]

Now check your current shell:

trainee@SLES12SP1:~> echo $SHELL
/bin/bash

At first glance nothing has happened. However if you view your entry in the /etc/passwd file you will notice that your login shell has changed:

trainee@SLES12SP1:~> cat /etc/passwd | grep trainee
trainee:x:1000:100:trainee:/home/trainee:/bin/sh

[stextbox id='black' image='null'] Important : The /bin/sh shell will be your active shell the next time you login. [/stextbox]

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

trainee@SLES12SP1:~> chsh
Password: trainee
Changing the login shell for trainee
Enter the new value, or press ENTER for the default
	Login Shell [/bin/sh]: /bin/bash

[stextbox id='black' image='null'] Important: Note that the password will not be printed to standard output. [/stextbox]

The Prompt

As you have already noticed, the prompt under Linux is different for a normal user and root:

> for a user,
# for root.

The history Command

/bin/bash keeps track of commands that have been previously executed. To access the command history, use the following command:

trainee@SLES12SP1:~> history | more
    1  su -
    2  su -
    3  clear
    4  cd /
    5  ls -l
    6  ls -l /var/run
    7  cd /mnt
    8  ls
    9  cd 
   10  mount
   11  mount --help
   12  cat /etc/fstab
   13  umount --help
   14  dumpe2fs /dev/sda1 | grep -i superbloc
   15  ls -ld /dev/console /dev/initctl /dev/loop0 /etc /etc/passwd
   16  ls -ld /dev/console /dev/initctl /etc /etc/passwd
   17  ls -ldi /dev/console /dev/initctl /etc /etc/passwd
   18  cd /tmp; mkdir inode; cd inode; touch file1; ls -ali
   19  ln file1 file2
   20  ls -ali
   21  ln -s file1 file3
   22  ls -ali
   23  su -
--More--

[stextbox id='black' image='null'] Important: The history is specific to each user. [/stextbox]

The history command uses emacs style control characters. As a result you can navigate through the list as follows:

Control Character	Action
[CTRL]-[P] (= Up Arrow)	Navigates backwards through the list
[CTRL]-[N] (= Down Arrow)	Navigates forwards through the list

To move around in the history:

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

Pour rechercher dans l'historique il convient d'utiliser les touches :

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 executed by using the !! characters:

trainee@SLES12SP1:~> ls
aac  bca  Desktop    Downloads  Pictures  public_html  Videos  xyz
abc  bin  Documents  Music      Public    Templates    vitext
trainee@SLES12SP1:~> !!
ls
aac  bca  Desktop    Downloads  Pictures  public_html  Videos  xyz
abc  bin  Documents  Music      Public    Templates    vitext

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

trainee@SLES12SP1:~> !131
ls
aac  bca  Desktop    Downloads  Pictures  public_html  Videos  xyz
abc  bin  Documents  Music      Public    Templates    vitext

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

trainee@SLES12SP1:~> cat /etc/profile | grep HISTSIZE
    HISTSIZE=1000
    export HISTSIZE

As you can see, in the previous case the HISTSIZE value is set to 1000. This means that the last 1,000 commands are held in the history.

The history command stores data in the ~/.bash_history file for each user. The commands for the current bash session are stored in the file when the session is closed:

trainee@SLES12SP1:~> nl .bash_history | more
     1	su -
     2	su -
     3	clear
     4	cd /
     5	ls -l
     6	ls -l /var/run
     7	cd /mnt
     8	ls
     9	cd -
    10	mount
    11	mount --help
    12	cat /etc/fstab
    13	umount --help
    14	dumpe2fs /dev/sda1 | grep -i superbloc
    15	ls -ld /dev/console /dev/initctl /dev/loop0 /etc /etc/passwd
    16	ls -ld /dev/console /dev/initctl /etc /etc/passwd
    17	ls -ldi /dev/console /dev/initctl /etc /etc/passwd
    18	cd /tmp; mkdir inode; cd inode; touch file1; ls -ali
    19	ln file1 file2
    20	ls -ali
    21	ln -s file1 file3
    22	ls -ali
    23	su -
--More--

[stextbox id='black' image='null'] Important : Note the use of the nl command to number the lines in the output of the contents of .bash_history file. [/stextbox]

The TAB key

/bin/bash can auto-generate the end of a file name. Consider the following example:

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

By hitting the Tab key three times, the system shows you the files that match:

trainee@SLES12SP1:~> ls .bash
.bash_history  .bashrc

This same technique can also be used to auto-generate command names. Consider the following example:

$ mo [Tab][Tab]

By hitting the Tab twice the system lists all known commands available to the user and starting with mo:

trainee@SLES12SP1:~> mo
modeprint       modsign-verify  mount           mouse-test      
modetest        more            mountpoint

Metacharacters

It is often necessary and desirable to be able to work with several files at one time as opposed to repeating the operation on each file individually. For this reason, bash accepts the use of Metacharacters:

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 …

To illustrate the use of Metacharacters, you need to create a directory in your home directory and the create some files within it:

trainee@SLES12SP1:~> mkdir training
trainee@SLES12SP1:~> cd training
trainee@SLES12SP1:~/training> touch f1 f2 f3 f4 f5

The * Metacharacter

Now use the Metacharacter *:

trainee@SLES12SP1:~/training> echo f*
f1 f2 f3 f4 f5

[stextbox id='black' image='null'] Important: Note that the * is used as a wild card which replaces 0 or more characters. [/stextbox]

The ? Metacharacter

Create two more files:

trainee@SLES12SP1:~/training> touch f52 f62

Now use the Metacharacter ?:

trainee@SLES12SP1:~/training> echo f?2
f52 f62

[stextbox id='black' image='null'] Important: Note that the ? is used as a wild card which replaces a single character. [/stextbox]

The [] Metacharacter

The [] Metacharacter can take several forms:

Metacharacter	Description
[xyz]	Represents either x or y or z
[m-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 metacharacter [], create a file called a100:

trainee@SLES12SP1:~/training> touch a100

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

trainee@SLES12SP1:~/training> echo [a-f]*
a100 f1 f2 f3 f4 f5 f52 f62
trainee@SLES12SP1:~/training> echo [af]*
a100 f1 f2 f3 f4 f5 f52 f62

[stextbox id='black' image='null'] Important: Note that all the files starting with either a, b, c, d, e or f are displayed. [/stextbox]

trainee@SLES12SP1:~/training> echo [!a]*
f1 f2 f3 f4 f5 f52 f62

[stextbox id='black' image='null'] Important: Note that all the files in the directory are displayed except the file starting with a . [/stextbox]

trainee@SLES12SP1:~/training> echo [a-b]*
a100

[stextbox id='black' image='null'] Important: Note that only the file starting with a is displayed since no file starting with b is present. [/stextbox]

trainee@SLES12SP1:~/training> echo [a-f]
[a-f]

[stextbox id='black' image='null'] Important: Note that in the above example, since no file called a, b, c, d, e or f exists in the directory, the echo command simply returns the filter used. [/stextbox]

The extglob Option

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

trainee@SLES12SP1:~/training> shopt -s extglob

The shopt command is used to activate and deactivate the shopt option of the shell.

The list of all the options can be displayed by simply using the shopt command:

trainee@SLES12SP1:~/training> shopt
autocd         	off
cdable_vars    	off
cdspell        	off
checkhash      	off
checkjobs      	off
checkwinsize   	on
cmdhist        	on
compat31       	off
compat32       	off
compat40       	off
compat41       	off
direxpand      	off
dirspell       	off
dotglob        	off
execfail       	off
expand_aliases 	on
extdebug       	off
extglob        	on
extquote       	on
failglob       	off
force_fignore  	on
globstar       	off
gnu_errfmt     	off
histappend     	on
histreedit     	off
histverify     	off
hostcomplete   	off
huponexit      	off
interactive_comments	on
lastpipe       	off
lithist        	off
login_shell    	on
mailwarn       	off
no_empty_cmd_completion	off
nocaseglob     	off
nocasematch    	off
nullglob       	off
progcomp       	on
promptvars     	on
restricted_shell	off
shift_verbose  	off
sourcepath     	on
xpg_echo       	off