Advanced Bash-Scripting HOWTO: A guide to shell scripting, using Bash | ||
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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 |