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Command Line Interface

Dernière mise-à-jour : 2020/01/30 03:27

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 Debian 8 /bin/sh is a soft link to /bin/dash :

trainee@debian8:~$ ls -l /bin/sh
lrwxrwxrwx 1 root root 4 Nov  8  2014 /bin/sh -> dash

/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@debian8:~$ 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@debian8:~$ 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@debian8:~$ type ls
ls is aliased to `ls --color=auto

Important: Note that the ls alias is an alias to the ls command itself.

An alias is defined using the alias command:

trainee@debian8:~$ alias dir='ls -l'
trainee@debian8:~$ dir
total 36
-rw-r--r-- 1 trainee trainee    0 Aug 19 17:08 aac
-rw-r--r-- 1 trainee trainee    0 Aug 19 17:08 abc
-rw-r--r-- 1 trainee trainee    0 Aug 19 17:08 bca
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Desktop
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Documents
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Downloads
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Music
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Pictures
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Public
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Templates
drwxr-xr-x 2 trainee trainee 4096 May  1  2016 Videos
-rw-r--r-- 1 trainee trainee  391 Aug 18 23:34 vitext
-rw-r--r-- 1 trainee trainee    0 Aug 19 17:08 xyz

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.

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

trainee@debian8:~$ alias
alias dir='ls -l'
alias ls='ls --color=auto'

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.

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

trainee@debian8:~$ \dir
aac  bca      Documents  Music	   Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext

To delete an alias, simply use the unalias command:

trainee@debian8:~$ unalias dir
trainee@debian8:~$ dir
aac  bca      Documents  Music	   Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext

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

trainee@debian8:~$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
systemd-timesync:x:100:103:systemd Time Synchronization,,,:/run/systemd:/bin/false
systemd-network:x:101:104:systemd Network Management,,,:/run/systemd/netif:/bin/false
systemd-resolve:x:102:105:systemd Resolver,,,:/run/systemd/resolve:/bin/false
systemd-bus-proxy:x:103:106:systemd Bus Proxy,,,:/run/systemd:/bin/false
trainee:x:1000:1000:trainee,,,:/home/trainee:/bin/bash
sshd:x:104:65534::/var/run/sshd:/usr/sbin/nologin
Debian-exim:x:105:110::/var/spool/exim4:/bin/false
messagebus:x:106:111::/var/run/dbus:/bin/false
statd:x:107:65534::/var/lib/nfs:/bin/false
avahi-autoipd:x:108:113:Avahi autoip daemon,,,:/var/lib/avahi-autoipd:/bin/false
avahi:x:109:115:Avahi mDNS daemon,,,:/var/run/avahi-daemon:/bin/false
colord:x:110:117:colord colour management daemon,,,:/var/lib/colord:/bin/false
dnsmasq:x:111:65534:dnsmasq,,,:/var/lib/misc:/bin/false
speech-dispatcher:x:112:29:Speech Dispatcher,,,:/var/run/speech-dispatcher:/bin/sh
pulse:x:113:119:PulseAudio daemon,,,:/var/run/pulse:/bin/false
rtkit:x:114:121:RealtimeKit,,,:/proc:/bin/false
saned:x:115:122::/var/lib/saned:/bin/false
usbmux:x:116:46:usbmux daemon,,,:/var/lib/usbmux:/bin/false
lightdm:x:117:124:Light Display Manager:/var/lib/lightdm:/bin/false

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

trainee@debian8:~$ cat /etc/shells
# /etc/shells: valid login shells
/bin/sh
/bin/dash
/bin/bash
/bin/rbash
/usr/bin/screen

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

trainee@debian8:~$ echo $SHELL
/bin/bash

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

trainee@debian8:~$ 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

Important: Note that the password will not be printed to standard output.

Now check your current shell:

trainee@debian8:~$ 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@debian8:~$ cat /etc/passwd | grep trainee
trainee:x:1000:1000:trainee,,,:/home/trainee:/bin/sh

Important : The /bin/sh shell will be your active shell the next time you login.

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

trainee@debian8:~$ 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

Important: Note that the password will not be printed to standard output.

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@debian8:~$ history | more
    1  su -
    2  su -
    3  exit
    4  su -
    5  exit
    6  ls -l /var
    7  su -
    8  su -
    9  vi vitext
   10  view vitext
   11  clear
   12  stty -a
   13  date
   14  who
   15  df
   16  df -h
   17  free
   18  free -h
   19  whoami
   20  su -
   21  clear
   22  su -
   23  exit
--More--

Important: The history is specific to each user.

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@debian8:~$ ls
aac  bca      Documents  Music     Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext
trainee@debian8:~$ !!
ls
aac  bca      Documents  Music     Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext

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

trainee@debian8:~$ !52
ls
aac  bca      Documents  Music     Public     Videos  xyz
abc  Desktop  Downloads  Pictures  Templates  vitext

The environmental variables associated with the history are set in the ~/.bashrc file where ~/ indicates the home directory of the user concerned:

trainee@debian8:~$ cat .bashrc | grep HISTSIZE
# for setting history length see HISTSIZE and HISTFILESIZE in bash(1)
HISTSIZE=1000

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@debian8:~$ nl .bash_history | more
     1	su -
     2	su -
     3	exit
     4	su -
     5	exit
     6	ls -l /var
     7	su -
     8	su -
     9	vi vitext
    10	view vitext
    11	clear
    12	stty -a
    13	date
    14	who
    15	df
    16	df -h
    17	free
    18	free -h
    19	whoami
    20	su -
    21	clear
    22	su -
    23	exit
--More--

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

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@debian8:~$ ls .bash
.bash_history  .bash_logout   .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@debian8:~$ mo
moc         moggsplit   more        mount       mountpoint  mousepad

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@debian8:~$ mkdir training
trainee@debian8:~$ cd training
trainee@debian8:~/training$ touch f1 f2 f3 f4 f5

The * Metacharacter

Now use the Metacharacter *:

trainee@debian8:~/training$ echo f*
f1 f2 f3 f4 f5

Important: Note that the * is used as a wild card which replaces 0 or more characters.

The ? Metacharacter

Create two more files:

trainee@debian8:~/training$ touch f52 f62

Now use the Metacharacter ?:

trainee@debian8:~/training$ echo f?2
f52 f62

Important: Note that the ? is used as a wild card which replaces a single character.

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@debian8:~/training$ touch a100

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

trainee@debian8:~/training$ echo [a-f]*
a100 f1 f2 f3 f4 f5 f52 f62
trainee@debian8:~/training$ echo [af]*
a100 f1 f2 f3 f4 f5 f52 f62

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

trainee@debian8:~/training$ echo [!a]*
f1 f2 f3 f4 f5 f52 f62

Important: Note that all the files in the directory are displayed except the file starting with a .

trainee@debian8:~/training$ echo [a-b]*
a100

Important: Note that only the file starting with a is displayed since no file starting with b is present.

trainee@debian8:~/training$ echo [a-f]
[a-f]

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.

The extglob Option

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

trainee@debian8:~/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@debian8:~/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
compat42       	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
globstar       	off
globasciiranges	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