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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><H1
><A
NAME="ALIASES"
></A
>Chapter 25. Aliases</H1
><P
><A
NAME="ALIASREF"
></A
></P
><P
>A Bash <I
CLASS="FIRSTTERM"
>alias</I
> is essentially nothing
	more than a keyboard shortcut, an abbreviation, a means of
	avoiding typing a long command sequence. If, for example,
	we include <B
CLASS="COMMAND"
>alias lm="ls -l | more"</B
> in
	the <A
HREF="sample-bashrc.html"
><TT
CLASS="FILENAME"
>~/.bashrc</TT
>
        file</A
>, then each <TT
CLASS="USERINPUT"
><B
>lm</B
></TT
>
          <A
NAME="AEN18669"
HREF="#FTN.AEN18669"
>[1]</A
>
	typed at the command-line will automatically be replaced by a
	<B
CLASS="COMMAND"
>ls -l | more</B
>. This can save a great deal of
	typing at the command-line and avoid having to remember complex
	combinations of commands and options. Setting <B
CLASS="COMMAND"
>alias
	rm="rm -i"</B
> (interactive mode delete) may save a
	good deal of grief, since it can prevent inadvertently deleting
	important files.</P
><P
>In a script, aliases have very limited usefulness. It would be
	nice if aliases could assume some of the functionality of
	the <B
CLASS="COMMAND"
>C</B
> preprocessor, such as macro expansion,
	but unfortunately Bash does not expand arguments within the
	alias body.
	  <A
NAME="AEN18676"
HREF="#FTN.AEN18676"
>[2]</A
>
	Moreover, a script fails to expand an alias itself
	within <SPAN
CLASS="QUOTE"
>"compound constructs,"</SPAN
> such as <A
HREF="tests.html#IFTHEN"
>if/then</A
> 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 <A
HREF="functions.html#FUNCTIONREF"
>function</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="AL"
></A
><P
><B
>Example 25-1. Aliases within a script</B
></P
><TABLE
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# alias.sh
   3&nbsp;
   4&nbsp;shopt -s expand_aliases
   5&nbsp;# Must set this option, else script will not expand aliases.
   6&nbsp;
   7&nbsp;
   8&nbsp;# First, some fun.
   9&nbsp;alias Jesse_James='echo "\"Alias Jesse James\" was a 1959 comedy starring Bob Hope."'
  10&nbsp;Jesse_James
  11&nbsp;
  12&nbsp;echo; echo; echo;
  13&nbsp;
  14&nbsp;alias ll="ls -l"
  15&nbsp;# May use either single (') or double (") quotes to define an alias.
  16&nbsp;
  17&nbsp;echo "Trying aliased \"ll\":"
  18&nbsp;ll /usr/X11R6/bin/mk*   #* Alias works.
  19&nbsp;
  20&nbsp;echo
  21&nbsp;
  22&nbsp;directory=/usr/X11R6/bin/
  23&nbsp;prefix=mk*  # See if wild card causes problems.
  24&nbsp;echo "Variables \"directory\" + \"prefix\" = $directory$prefix"
  25&nbsp;echo
  26&nbsp;
  27&nbsp;alias lll="ls -l $directory$prefix"
  28&nbsp;
  29&nbsp;echo "Trying aliased \"lll\":"
  30&nbsp;lll         # Long listing of all files in /usr/X11R6/bin stating with mk.
  31&nbsp;# An alias can handle concatenated variables -- including wild card -- o.k.
  32&nbsp;
  33&nbsp;
  34&nbsp;
  35&nbsp;
  36&nbsp;TRUE=1
  37&nbsp;
  38&nbsp;echo
  39&nbsp;
  40&nbsp;if [ TRUE ]
  41&nbsp;then
  42&nbsp;  alias rr="ls -l"
  43&nbsp;  echo "Trying aliased \"rr\" within if/then statement:"
  44&nbsp;  rr /usr/X11R6/bin/mk*   #* Error message results!
  45&nbsp;  # Aliases not expanded within compound statements.
  46&nbsp;  echo "However, previously expanded alias still recognized:"
  47&nbsp;  ll /usr/X11R6/bin/mk*
  48&nbsp;fi  
  49&nbsp;
  50&nbsp;echo
  51&nbsp;
  52&nbsp;count=0
  53&nbsp;while [ $count -lt 3 ]
  54&nbsp;do
  55&nbsp;  alias rrr="ls -l"
  56&nbsp;  echo "Trying aliased \"rrr\" within \"while\" loop:"
  57&nbsp;  rrr /usr/X11R6/bin/mk*   #* Alias will not expand here either.
  58&nbsp;                           #  alias.sh: line 57: rrr: command not found
  59&nbsp;  let count+=1
  60&nbsp;done 
  61&nbsp;
  62&nbsp;echo; echo
  63&nbsp;
  64&nbsp;alias xyz='cat $0'   # Script lists itself.
  65&nbsp;                     # Note strong quotes.
  66&nbsp;xyz
  67&nbsp;#  This seems to work,
  68&nbsp;#+ although the Bash documentation suggests that it shouldn't.
  69&nbsp;#
  70&nbsp;#  However, as Steve Jacobson points out,
  71&nbsp;#+ the "$0" parameter expands immediately upon declaration of the alias.
  72&nbsp;
  73&nbsp;exit 0</PRE
></TD
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><P
><A
NAME="UNALIASREF"
></A
></P
><P
>The <B
CLASS="COMMAND"
>unalias</B
> command removes a previously
      set <I
CLASS="FIRSTTERM"
>alias</I
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="UNAL"
></A
><P
><B
>Example 25-2. <I
CLASS="FIRSTTERM"
>unalias</I
>: Setting and unsetting
      an alias</B
></P
><TABLE
BORDER="0"
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WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# unalias.sh
   3&nbsp;
   4&nbsp;shopt -s expand_aliases  # Enables alias expansion.
   5&nbsp;
   6&nbsp;alias llm='ls -al | more'
   7&nbsp;llm
   8&nbsp;
   9&nbsp;echo
  10&nbsp;
  11&nbsp;unalias llm              # Unset alias.
  12&nbsp;llm
  13&nbsp;# Error message results, since 'llm' no longer recognized.
  14&nbsp;
  15&nbsp;exit 0</PRE
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CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>./unalias.sh</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>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</TT
></PRE
></TD
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><H3
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>Notes</H3
><TABLE
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><TD
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><A
NAME="FTN.AEN18669"
HREF="aliases.html#AEN18669"
>[1]</A
></TD
><TD
ALIGN="LEFT"
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WIDTH="95%"
><P
> ... as the first word of a command string.
          Obviously, an alias is only meaningful at the
          <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>beginning</I
></SPAN
> of a command.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
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WIDTH="5%"
><A
NAME="FTN.AEN18676"
HREF="aliases.html#AEN18676"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>However, aliases do seem to expand positional
	  parameters.</P
></TD
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