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. [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
1 #!/bin/bash
2 # May need to be invoked with #!/bin/bash2 on older systems.
3
4 shopt -s expand_aliases
5 # Must set this option, else script will not expand aliases.
6
7
8 # First, some fun.
9 alias Jesse_James='echo "\"Alias Jesse James\" was a 1959 comedy starring Bob Hope."'
10 Jesse_James
11
12 echo; echo; echo;
13
14 alias ll="ls -l"
15 # May use either single (') or double (") quotes to define an alias.
16
17 echo "Trying aliased \"ll\":"
18 ll /usr/X11R6/bin/mk* #* Alias works.
19
20 echo
21
22 directory=/usr/X11R6/bin/
23 prefix=mk* # See if wild-card causes problems.
24 echo "Variables \"directory\" + \"prefix\" = $directory$prefix"
25 echo
26
27 alias lll="ls -l $directory$prefix"
28
29 echo "Trying aliased \"lll\":"
30 lll # Long listing of all files in /usr/X11R6/bin stating with mk.
31 # Alias handles concatenated variables, including wild-card o.k.
32
33
34
35
36 TRUE=1
37
38 echo
39
40 if [ TRUE ]
41 then
42 alias rr="ls -l"
43 echo "Trying aliased \"rr\" within if/then statement:"
44 rr /usr/X11R6/bin/mk* #* Error message results!
45 # Aliases not expanded within compound statements.
46 echo "However, previously expanded alias still recognized:"
47 ll /usr/X11R6/bin/mk*
48 fi
49
50 echo
51
52 count=0
53 while [ $count -lt 3 ]
54 do
55 alias rrr="ls -l"
56 echo "Trying aliased \"rrr\" within \"while\" loop:"
57 rrr /usr/X11R6/bin/mk* #* Alias will not expand here either.
58 let count+=1
59 done
60
61 echo; echo
62
63 alias xyz="cat $1" # Try a positional parameter in an alias.
64 xyz # If you invoke the script with a filename as a parameter.
65 # This seems to work,
66 #+ although the Bash documentation suggests that it shouldn't.
67
68 exit 0 |
 | The unalias command removes a previously set alias. |
Example 24-2. unalias: Setting and unsetting an alias
1 #!/bin/bash 2 3 shopt -s expand_aliases # Enables alias expansion. 4 5 alias llm='ls -al | more' 6 llm 7 8 echo 9 10 unalias llm # Unset alias. 11 llm 12 # Error message results, since 'llm' no longer recognized. 13 14 exit 0 |
bash$ ./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 |