alias (command)

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For the aliasing of variables in a programming language, see Aliasing (computing).

In computing, alias is a command in various command line interpreters (shells) such as Unix shells, 4DOS/4NT and Windows PowerShell, which enables a replacement of a word by another string. It is mainly used for abbreviating a system command, or for adding default arguments to a regularly used command. Aliasing functionality in the MS-DOS and Microsoft Windows operating systems is provided by the DOSKey command-line utility.

An alias will last for the life of the shell session. Regularly used aliases can be set from the shell's configuration file (~/.cshrc or the systemwide /etc/csh.cshrc for csh, or ~/.bashrc or the systemwide /etc/bashrc or /etc/bash.bashrc for bash) so that they will be available upon the start of the corresponding shell session. The alias commands may either be written in the config file directly or sourced from a separate file, typically named .alias (or .alias-bash, .alias-csh, etc., if multiple shells may be used).

Creating aliases[edit]

Aliases can be created by supplying name/value pairs as arguments for the alias command. In Unix shells the syntax is:

alias copy='cp'

The corresponding syntax in the C shell or tcsh shell is:

alias copy "cp"

This alias means that when the command copy is read in the shell, it will be replaced with cp and that command will be executed instead.

In the 4DOS/4NT shell the following syntax is used to define cp as an alias for the 4DOS copy command:

alias cp copy

To create a new alias in Windows PowerShell, the new verb can be used with the alias cmdlet:

new-alias ci copy-item

This creates a new alias called ci that will be replaced with the copy-item cmdlet when executed.

History[edit]

In Unix, aliases were introduced in the C shell and thus survive in descendent shells such as tcsh and bash. C shell aliases were strictly limited to one line in a shell language where all complex constructs required more, but still useful for creating simple shortcut commands. Aliases were absent from the Bourne shell, which had the more powerful facility of functions. The alias concept was imported into Bourne Again Shell (bash) and the Korn shell (ksh). Shells such as these, that support both functions and aliases, recommend using functions where possible. Cases where aliases are necessary include the use of chained aliases (bash and ksh).

Viewing currently defined aliases[edit]

To view defined aliases the following commands can be used:

 alias          # Used without arguments; displays a list of all current aliases
 alias -p       # Analogous to the above; not available in 4DOS/4NT and PowerShell
 alias myAlias  # Displays the command for a defined alias

Overriding aliases[edit]

In Unix shells, if an alias exists for a command, it is possible to override the alias by surrounding the command with quotes or prefixing it with a backslash. For example, consider the following alias definition:

alias ls='ls -la'

To override this alias and execute the ls command as it was originally defined, the following syntax can be used:

'ls'

or

\ls

In the 4DOS/4NT shell it is possible to override an alias by prefixing it with an asterisk. For example, consider the following alias definition:

alias dir = *dir /2/p

The asterisk in the 2nd instance of dir causes the unaliased dir to be invoked, preventing recursive alias expansion. Also the user can get the unaliased behaviour of dir at the command line by using the same syntax:

*dir

Changing aliases[edit]

In Windows PowerShell, the set verb can be used with the alias cmdlet to change an existing alias:

set-alias ci cls

The alias ci will now point to the cls command.

In the 4DOS/4NT shell, the eset command provides an interactive command line to edit an existing alias:

eset /a cp

The /a causes the alias cp to be edited, as opposed to an environment variable of the same name.

Removing aliases[edit]

In Unix shells and 4DOS/4NT, aliases can be removed by executing the unalias command:

 unalias copy          # Removes the copy alias
 unalias -a            # The -a switch will remove all aliases; not available in 4DOS/4NT
unalias *             # 4DOS/4NT equivalent of `unalias -a` - wildcards are supported

In Windows PowerShell, the alias can be removed from the alias:\ drive using remove-item:

 remove-item alias:ci  # Removes the ci alias

Features[edit]

Chaining[edit]

An alias usually replaces just the first word. But some shells, such as bash and ksh allow a sequence or words to be replaced; this particular feature is unavailable through the function mechanism.

The usual syntax is to define the first alias with a trailing space character. For instance, using the two aliases:

 alias list='ls '      # note the trailing space to trigger chaining
 alias long='-Flas'    # options to ls for a long listing

allows:

 list long myfile      # becomes "ls -Flas myfile" when run

for a long listing, where "long" is also checked for being an alias.

Quoting quotes[edit]

To define an alias with single quotes, which itself needs to contain single quotes, you need to use several concatenated quoted strings. For example, to define an alias which would do:

$ perl -pe 's/^(.*) foo/$1 bar/;'

You cannot do

$ alias foo2bar='perl -pe \'s/^(.*) foo/$1 bar/;\'' # WRONG: backslashes do not escape the next character inside single quotes

However, you can:

$ alias foo2bar='perl -pe '\''s/^(.*) foo/$1 bar/;'\''' # Put single quotes around your \' like '\''

But you can use single quotes quoted inside double quotes

$ alias foo2bar='perl -pe '"'"'s/^(.*) foo/$1 bar/;'"'"''

.[1]

You may also consider using a function instead of an alias.

Command arguments[edit]

In the C Shell, arguments can be embedded inside the command using the string \!*. For example, with this alias:

alias ls-more 'ls \!* | more'

ls-more /etc /usr expands to ls /etc /usr | more to list the contents of the directories /etc and /usr, pausing after every screenful. Without \!*,

alias ls-more 'ls | more'

would instead expand to ls | more /etc /usr which incorrectly attempts to open the directories in more.[2]

The Bash and Korn shells instead use shell functions — see Alternatives below.

Typical aliases[edit]

Some commonly used, but deprecated, aliases in the Bash shell:

alias ls='ls --color=auto' # use colors
alias la='ls -Fa'          # list all files
alias ll='ls -Fls'         # long listing format

alias rm='rm -i'           # prompt before overwrite (but dangerous, see rm for a better approach)
alias cp='cp -i'           # prompt before overwrite (same general problem as the rm)
alias mv='mv -i'           # prompt before overwrite (same general problem as the rm)

alias vi='vim'             # use improved vi editor

Standard aliases of Windows PowerShell include:

new-alias cd set-location

new-alias ls get-childitem
new-alias dir get-childitem

new-alias echo write-output
new-alias ps get-process
new-alias kill stop-process

Alternatives[edit]

Aliases should usually be kept simple. Where it would not be simple, the recommendation is usually to use one of the following:

  • Shell scripts, which essentially provide the full ability to create new system commands.
  • Symbolic links, either in /usr/local/bin if for all users, or in a user's $HOME/bin directory if for personal use. This method is useful for providing an additional way of calling the command, and in some cases may allow access to a buried command function for the small number of commands that use their invocation name to select the mode of operation.
  • Shell functions, especially if the command being created needs to modify the internal runtime environment of the shell itself (such as environment variables), needs to change the shell's current working directory, or must be implemented in a way which guarantees they it appear in the command search path for anything but an interactive shell (especially any "safer" version of rm, cp, mv and so forth).

The most common form of aliases, which just add a few options to a command and then include the rest of the command line, can be converted easily to shell functions following this pattern:

 alias ll='ls -Flas'              # long listing, alias
 ll () { ls -Flas "$@" ; }        # long listing, function

To make ls itself a function (note that "command ls" is Bash-specific, and that older Bourne shells would have used "/bin/ls" instead):

 ls () { command ls --color=auto "$@" ; }

References[edit]

  1. ^ See this explanation
  2. ^ Examples of passing arguments given to a command alias

External links[edit]