Chapter 24. 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 the ~/.bashrc 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 losing important files.
In a script, aliases have very limited usefulness. It would be
quite 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 alias body.
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 24-1. Aliases within a script
#!/bin/bash
# alias.sh
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."'
Jesse_James
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.
echo
directory=/usr/X11R6/bin/
prefix=mk* # See if wild card causes problems.
echo "Variables \"directory\" + \"prefix\" = $directory$prefix"
echo
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.
TRUE=1
echo
if [ TRUE ]
then
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:"
ll /usr/X11R6/bin/mk*
fi
echo
count=0
while [ $count -lt 3 ]
do
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
let count+=1
done
echo; echo
alias xyz='cat $0' # Script lists itself.
# Note strong quotes.
xyz
# This seems to work,
#+ although the Bash documentation suggests that it shouldn't.
#
# However, as Steve Jacobson points out,
#+ the "$0" parameter expands immediately upon declaration of the alias.
exit 0 |
The unalias command removes a previously
set alias.
Example 24-2. unalias: Setting and unsetting an alias
#!/bin/bash
# unalias.sh
shopt -s expand_aliases # Enables alias expansion.
alias llm='ls -al | more'
llm
echo
unalias llm # Unset alias.
llm
# Error message results, since 'llm' no longer recognized.
exit 0 |
bash$ ./unalias.sh
total 6
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 |
