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. [1] 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
# Invoke with command line parameter to exercise last section of this 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."'
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.
# Alias handles 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.
let count+=1
done
echo; echo
alias xyz="cat $1" # Try a positional parameter in an alias.
xyz # Assumes you invoke the script
#+ with a filename as a parameter.
# This seems to work,
#+ although the Bash documentation suggests that it shouldn't.
#
# However, as Steve Jacobson points out,
#+ the "$1" parameter expands immediately upon declaration of the alias,
#+ so, in the strictest sense, this is not an example
#+ of parameterizing an alias.
exit 0 |
The unalias command removes a previously set alias.
Example 24-2. unalias: Setting and unsetting an alias
#!/bin/bash shopt -s expand_aliases # Enables alias expansion. alias llm='ls -al | more' llm echo unalias llm # Unset alias. llm # Error message results, since 'llm' no longer recognized. exit 0 |
bash$ ./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 |
| [1] | However, aliases do seem to expand positional parameters. |