465 lines
7.3 KiB
HTML
465 lines
7.3 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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>Aliases</TITLE
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><DIV
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CLASS="CHAPTER"
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><H1
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><A
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NAME="ALIASES"
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></A
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>Chapter 25. Aliases</H1
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><P
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><A
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NAME="ALIASREF"
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></A
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></P
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><P
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>A Bash <I
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CLASS="FIRSTTERM"
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>alias</I
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> is essentially nothing
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more than a keyboard shortcut, an abbreviation, a means of
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avoiding typing a long command sequence. If, for example,
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we include <B
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CLASS="COMMAND"
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>alias lm="ls -l | more"</B
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> in
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the <A
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HREF="sample-bashrc.html"
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><TT
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CLASS="FILENAME"
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>~/.bashrc</TT
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>
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file</A
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>, then each <TT
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CLASS="USERINPUT"
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><B
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>lm</B
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></TT
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>
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<A
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NAME="AEN18669"
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HREF="#FTN.AEN18669"
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><SPAN
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CLASS="footnote"
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>[1]</SPAN
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></A
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>
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typed at the command-line will automatically be replaced by a
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<B
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CLASS="COMMAND"
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>ls -l | more</B
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>. This can save a great deal of
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typing at the command-line and avoid having to remember complex
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combinations of commands and options. Setting <B
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CLASS="COMMAND"
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>alias
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rm="rm -i"</B
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> (interactive mode delete) may save a
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good deal of grief, since it can prevent inadvertently deleting
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important files.</P
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><P
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>In a script, aliases have very limited usefulness. It would be
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nice if aliases could assume some of the functionality of
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the <B
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CLASS="COMMAND"
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>C</B
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> preprocessor, such as macro expansion,
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but unfortunately Bash does not expand arguments within the
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alias body.
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<A
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NAME="AEN18676"
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HREF="#FTN.AEN18676"
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><SPAN
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CLASS="footnote"
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>[2]</SPAN
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></A
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>
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Moreover, a script fails to expand an alias itself
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within <SPAN
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CLASS="QUOTE"
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>"compound constructs,"</SPAN
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> such as <A
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HREF="tests.html#IFTHEN"
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>if/then</A
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> statements, loops, and
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functions. An added limitation is that an alias will not expand
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recursively. Almost invariably, whatever we would like an alias
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to do could be accomplished much more effectively with a <A
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HREF="functions.html#FUNCTIONREF"
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>function</A
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>.</P
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><DIV
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CLASS="EXAMPLE"
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><A
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NAME="AL"
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></A
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><P
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><B
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>Example 25-1. Aliases within a script</B
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></P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="PROGRAMLISTING"
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>#!/bin/bash
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# alias.sh
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shopt -s expand_aliases
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# Must set this option, else script will not expand aliases.
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# First, some fun.
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alias Jesse_James='echo "\"Alias Jesse James\" was a 1959 comedy starring Bob Hope."'
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Jesse_James
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echo; echo; echo;
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alias ll="ls -l"
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# May use either single (') or double (") quotes to define an alias.
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echo "Trying aliased \"ll\":"
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ll /usr/X11R6/bin/mk* #* Alias works.
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echo
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directory=/usr/X11R6/bin/
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prefix=mk* # See if wild card causes problems.
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echo "Variables \"directory\" + \"prefix\" = $directory$prefix"
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echo
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alias lll="ls -l $directory$prefix"
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echo "Trying aliased \"lll\":"
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lll # Long listing of all files in /usr/X11R6/bin stating with mk.
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# An alias can handle concatenated variables -- including wild card -- o.k.
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TRUE=1
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echo
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if [ TRUE ]
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then
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alias rr="ls -l"
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echo "Trying aliased \"rr\" within if/then statement:"
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rr /usr/X11R6/bin/mk* #* Error message results!
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# Aliases not expanded within compound statements.
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echo "However, previously expanded alias still recognized:"
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ll /usr/X11R6/bin/mk*
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fi
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echo
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count=0
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while [ $count -lt 3 ]
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do
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alias rrr="ls -l"
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echo "Trying aliased \"rrr\" within \"while\" loop:"
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rrr /usr/X11R6/bin/mk* #* Alias will not expand here either.
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# alias.sh: line 57: rrr: command not found
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let count+=1
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done
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echo; echo
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alias xyz='cat $0' # Script lists itself.
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# Note strong quotes.
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xyz
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# This seems to work,
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#+ although the Bash documentation suggests that it shouldn't.
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#
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# However, as Steve Jacobson points out,
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#+ the "$0" parameter expands immediately upon declaration of the alias.
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exit 0</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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><P
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><A
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NAME="UNALIASREF"
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></A
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></P
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><P
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>The <B
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CLASS="COMMAND"
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>unalias</B
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> command removes a previously
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set <I
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CLASS="FIRSTTERM"
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>alias</I
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>.</P
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><DIV
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CLASS="EXAMPLE"
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><A
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NAME="UNAL"
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></A
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><P
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><B
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>Example 25-2. <I
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CLASS="FIRSTTERM"
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>unalias</I
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>: Setting and unsetting
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an alias</B
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></P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="PROGRAMLISTING"
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>#!/bin/bash
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# unalias.sh
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shopt -s expand_aliases # Enables alias expansion.
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alias llm='ls -al | more'
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llm
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echo
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unalias llm # Unset alias.
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llm
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# Error message results, since 'llm' no longer recognized.
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exit 0</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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><TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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><TT
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CLASS="PROMPT"
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>bash$ </TT
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><TT
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CLASS="USERINPUT"
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><B
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>./unalias.sh</B
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></TT
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>
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<TT
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CLASS="COMPUTEROUTPUT"
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>total 6
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drwxrwxr-x 2 bozo bozo 3072 Feb 6 14:04 .
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drwxr-xr-x 40 bozo bozo 2048 Feb 6 14:04 ..
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-rwxr-xr-x 1 bozo bozo 199 Feb 6 14:04 unalias.sh
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./unalias.sh: llm: command not found</TT
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></PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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><H3
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CLASS="FOOTNOTES"
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>Notes</H3
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><TABLE
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CLASS="FOOTNOTES"
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WIDTH="100%"
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="5%"
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><A
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NAME="FTN.AEN18669"
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HREF="aliases.html#AEN18669"
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><SPAN
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CLASS="footnote"
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>[1]</SPAN
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></A
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="95%"
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><P
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> ... as the first word of a command string.
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Obviously, an alias is only meaningful at the
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<EM
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>beginning</EM
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> of a command.</P
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></TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="5%"
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><A
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NAME="FTN.AEN18676"
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HREF="aliases.html#AEN18676"
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><SPAN
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CLASS="footnote"
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>[2]</SPAN
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></A
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="95%"
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><P
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>However, aliases do seem to expand positional
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parameters.</P
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></TD
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></TR
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></TABLE
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VALIGN="top"
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ACCESSKEY="P"
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>Prev</A
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WIDTH="33%"
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VALIGN="top"
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>Recursion Without Local Variables</TD
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VALIGN="top"
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