Chapter 25. Aliases
A Bash alias is essentially nothing
more than a keyboard shortcut, an abbreviation, a means of
avoiding typing a long command sequence. If, for example,
we include alias lm="ls -l | more" in
file, then each lm
typed at the command-line will automatically be replaced by a
ls -l | more. This can save a great deal of
typing at the command-line and avoid having to remember complex
combinations of commands and options. Setting alias
rm="rm -i" (interactive mode delete) may save a
good deal of grief, since it can prevent inadvertently deleting
In a script, aliases have very limited usefulness. It would be
nice if aliases could assume some of the functionality of
the C preprocessor, such as macro expansion,
but unfortunately Bash does not expand arguments within the
Moreover, a script fails to expand an alias itself
within "compound constructs," such as if/then statements, loops, and
functions. An added limitation is that an alias will not expand
recursively. Almost invariably, whatever we would like an alias
to do could be accomplished much more effectively with a function.
Example 25-1. Aliases within a script
shopt -s expand_aliases
# Must set this option, else script will not expand aliases.
# First, some fun.
alias Jesse_James='echo "\"Alias Jesse James\" was a 1959 comedy starring Bob Hope."'
echo; echo; echo;
alias ll="ls -l"
# May use either single (') or double (") quotes to define an alias.
echo "Trying aliased \"ll\":"
ll /usr/X11R6/bin/mk* #* Alias works.
prefix=mk* # See if wild card causes problems.
echo "Variables \"directory\" + \"prefix\" = $directory$prefix"
alias lll="ls -l $directory$prefix"
echo "Trying aliased \"lll\":"
lll # Long listing of all files in /usr/X11R6/bin stating with mk.
# An alias can handle concatenated variables -- including wild card -- o.k.
if [ TRUE ]
alias rr="ls -l"
echo "Trying aliased \"rr\" within if/then statement:"
rr /usr/X11R6/bin/mk* #* Error message results!
# Aliases not expanded within compound statements.
echo "However, previously expanded alias still recognized:"
while [ $count -lt 3 ]
alias rrr="ls -l"
echo "Trying aliased \"rrr\" within \"while\" loop:"
rrr /usr/X11R6/bin/mk* #* Alias will not expand here either.
# alias.sh: line 57: rrr: command not found
alias xyz='cat $0' # Script lists itself.
# Note strong quotes.
# This seems to work,
#+ although the Bash documentation suggests that it shouldn't.
# However, as Steve Jacobson points out,
#+ the "$0" parameter expands immediately upon declaration of the alias.
The unalias command removes a previously
Example 25-2. unalias: Setting and unsetting
shopt -s expand_aliases # Enables alias expansion.
alias llm='ls -al | more'
unalias llm # Unset alias.
# Error message results, since 'llm' no longer recognized.
drwxrwxr-x 2 bozo bozo 3072 Feb 6 14:04 .
drwxr-xr-x 40 bozo bozo 2048 Feb 6 14:04 ..
-rwxr-xr-x 1 bozo bozo 199 Feb 6 14:04 unalias.sh
./unalias.sh: llm: command not found