This article is reprinted from the book A Practical Guide to Linux Commands, Editors, and Shell Programming 3rd edition, with permission of the author and publisher Prentice Hall, all rights reserved.
An alias is a (usually short) name that the shell translates into another (usually longer) name or command. Aliases allow you to define new commands by substituting a string for the first token of a simple command. They are typically placed in the ~/.bashrc (bash) or ~/.tcshrc (tcsh) startup files so that they are available to interactive subshells.
Under bash the syntax of the alias builtin is
Under tcsh the syntax is
alias [name[ value]]
In the bash syntax no spaces are permitted around the equal sign. If value contains spaces or tabs, you must enclose value within quotation marks. Unlike aliases under tcsh, a bash alias does not accept an argument from the command line in value. Use a bash function when you need to use an argument.
An alias does not replace itself, which avoids the possibility of infinite recursion in handling an alias such as the following:
alias ls='ls -F'
You can nest aliases. Aliases are disabled for noninteractive shells (that is, shell scripts). Use the
unalias builtin to remove an alias. When you give an
alias builtin command without any arguments, the shell displays a list of all defined aliases:
alias ll='ls -l'
alias l='ls -ltr'
alias ls='ls -F'
alias zap='rm -i'
To view the alias for a particular name, enter the command alias followed by the name of the alias. Most Linux distributions define at least some aliases. Enter an alias command to see which aliases are in effect. You can delete the aliases you do not want from the appropriate startup file.
Single versus double quotation marks in aliases
The choice of single or double quotation marks is significant in the alias syntax when the alias includes variables. If you enclose value within double quotation marks, any variables that appear in value are expanded when the alias is created. If you enclose value within single quotation marks, variables are not expanded until the alias is used. The following example illustrates the difference.
The PWD keyword variable holds the pathname of the working directory. Max creates two aliases while he is working in his home directory. Because he uses double quotation marks when he creates the dirA alias, the shell substitutes the value of the working directory when he creates this alias. The alias dirA command displays the dirA alias and shows that the substitution has already taken place:
$ echo $PWD
$ alias dirA="echo Working directory is $PWD"
$ alias dirA
alias dirA='echo Working directory is /home/max'
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