3.20. 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 (see Section 3.23), 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. Almost invariably, whatever we would like an alias to do could be accomplished much more effectively with a function.
Example 3-85. Aliases within a script
1 #!/bin/bash2
2
3 shopt -s expand_aliases
4 # Must set this option, else script will not expand aliases.
5
6
7 # First, some fun.
8 alias Jesse_James='echo "\"Alias Jesse James\" was a 1959 comedy starring Bob Hope."'
9 Jesse_James
10
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 [ $count -lt 3 ]
54 do
55 alias rrr="ls -l"
56 echo "Trying aliased \"rrr\" within \"while\" loop:"
57 rrr /usr/X11R6/bin/mk* # Alias will not expand here either.
58 let count+=1
59 done
60
61
62 exit 0 |
 | The unalias command removes a previously set alias. |
Example 3-86. unalias: Setting and unsetting an alias
1 #!/bin/bash 2 3 shopt -s expand_aliases # Enables alias expansion. 4 5 alias llm='ls -al | more' 6 llm 7 8 echo 9 10 unalias llm # Unset alias. 11 llm 12 # Error message results, since 'llm' no longer recognized. 13 14 exit 0 |
bash$ ./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 |