What is an alias
An alias is a shorthand shell notation that allows you to customize and abbreviate commands. Aliases are available in all shells. A common syntax to define an alias on command line is as follows:
$ alias name=command_string
If the first word on the command line is an alias, the shell replaces that word with the text of the alias. The shell maintains a list of aliases that it searches when a command is entered. The following rules apply while creating an alias:
- There can be no space on either side of the equal sign.
- The command string must be quoted if it includes any options, metacharacters, or spaces.
- Each command in a single alias must be separated with a semicolon.
You can group several commands under a single alias name. Individual commands are separated by semicolons. For example:
$ alias info='uname -s; id; date'
Linux uid=1001(user) gid=1001(user) Tue Dec 5 15:11:58 UTC 2017
In the following example, an alias is created using a pipe (|) to direct the output of the ls -l command to the more command. When the new alias is invoked, a directory list appears.
$ alias ll='ls -l | more'
$ cd /usr/bin $ ll total 121528 -rwxr-xr-x. 1 root root 41496 Nov 5 2016 [ -rwxr-xr-x. 1 root root 107856 Aug 2 17:46 a2p -rwxr-xr-x. 1 root root 52640 Oct 19 20:40 ab -rwxr-xr-x. 1 root root 29112 Sep 6 16:47 addr2line -rwxr-xr-x. 1 root root 29 Sep 6 16:25 alias -rwxr-xr-x. 1 root root 12930120 Oct 25 18:22 amazon-ssm-agent -rwxr-xr-x. 1 root root 7200 Jun 16 2016 animate lrwxrwxrwx. 1 root root 6 Sep 29 2014 apropos -> whatis -rwxr-xr-x. 1 root root 62680 Sep 6 16:47 ar ... --More--
The shell contains several predefined aliases. You can display these predefined aliases by using the alias command. For Example :
$ alias alias egrep='egrep --color=auto' alias fgrep='fgrep --color=auto' alias grep='grep --color=auto' alias info='uname -s; id; date' alias l.='ls -d .* --color=auto' alias ll='ls -l | more' alias ls='ls --color=auto' alias vi='vim' alias which='alias | /usr/bin/which --tty-only --read-alias --show-dot --show-tilde'
User-defined aliases are defined by a user, usually to abbreviate or customize frequently used commands. For example, the history command is aliased as h using the alias command in the following code:
$ alias h=history ... 46 cd /usr 47 ls -lrt 48 cd 49 cd /usr/bin 50 ll 51 clear 52 alias 53 alias h=history 54 h
Using the rm, cp, and mv commands can inadvertently result in loss of data. As a precaution, you can alias these commands with the interactive option. For example, the rm command is aliased with the –i option as coded as follows:
$ alias rm='rm -i' $ rm file1 rm: remove file1: (yes/no)? no
Similarly, creating a ‘cp -i‘ and ‘mv -i‘ alias ensures that the shell prompts you for confirmation before overwriting existing files.
Deactivating an Alias
You can deactivate an alias temporarily by placing a backslash (\) in front of the alias on the command line. For example, in the following code, the backslash prevents the shell from looking in the alias list. This allows the shell to run the original rm command to remove the file1 file.
$ rm file1 rm: remove file1 (yes/no)? no
$ \rm file1 $ ls file1 file1: No such file or directory
Or if the alias like h for history would give an error as below if you use the backslash.
$ \h -bash: h: command not found
Removing an Alias
The unalias command removes aliases from the alias list.
$ unalias alias_name
For example, the h alias that was created earlier is removed using the unalias command.
$ unalias h
$ h -bash: h: command not found