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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><HTML><HEAD><TITLE>Aliases</TITLE><METANAME="GENERATOR"CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+"><LINKREL="HOME"TITLE="Advanced Bash-Scripting Guide"HREF="index.html"><LINKREL="UP"TITLE="Advanced Topics"HREF="part5.html"><LINKREL="PREVIOUS"TITLE="Recursion Without Local Variables"HREF="recurnolocvar.html"><LINKREL="NEXT"TITLE="List Constructs"HREF="list-cons.html"><METAHTTP-EQUIV="Content-Style-Type"CONTENT="text/css"><LINKREL="stylesheet"HREF="common/kde-common.css"TYPE="text/css"><METAHTTP-EQUIV="Content-Type"CONTENT="text/html; charset=iso-8859-1"><METAHTTP-EQUIV="Content-Language"CONTENT="en"><LINKREL="stylesheet"HREF="common/kde-localised.css"TYPE="text/css"TITLE="KDE-English"><LINKREL="stylesheet"HREF="common/kde-default.css"TYPE="text/css"TITLE="KDE-Default"></HEAD><BODYCLASS="CHAPTER"BGCOLOR="#FFFFFF"TEXT="#000000"LINK="#AA0000"VLINK="#AA0055"ALINK="#AA0000"STYLE="font-family: sans-serif;"><DIVCLASS="NAVHEADER"><TABLESUMMARY="Header navigation table"WIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><THCOLSPAN="3"ALIGN="center">Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH></TR><TR><TDWIDTH="10%"ALIGN="left"VALIGN="bottom"><AHREF="recurnolocvar.html"ACCESSKEY="P">Prev</A></TD><TDWIDTH="80%"ALIGN="center"VALIGN="bottom"></TD><TDWIDTH="10%"ALIGN="right"VALIGN="bottom"><AHREF="list-cons.html"ACCESSKEY="N">Next</A></TD></TR></TABLE><HRALIGN="LEFT"WIDTH="100%"></DIV><DIVCLASS="CHAPTER"><H1><ANAME="ALIASES"></A>Chapter 24. Aliases</H1><P><ANAME="ALIASREF"></A></P><P>A Bash <ICLASS="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 <BCLASS="COMMAND">alias lm="ls -l | more"</B> in the <AHREF="files.html#FILESREF1"><TTCLASS="FILENAME">~/.bashrc</TT> file</A>, then each <TTCLASS="USERINPUT"><B>lm</B></TT> typed at the command line will automatically be replaced by a <BCLASS="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 <BCLASS="COMMAND">alias rm="rm -i"</B> (interactive mode delete) may save a good deal of grief, since it can prevent inadvertently losing important files.</P><P>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. <ANAME="AEN17194"HREF="#FTN.AEN17194">[1]</A> Moreover, a script fails to expand an alias itself within <SPANCLASS="QUOTE">"compound constructs"</SPAN>, such as <AHREF="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 <AHREF="functions.html#FUNCTIONREF">function</A>.</P><DIVCLASS="EXAMPLE"><HR><ANAME="AL"></A><P><B>Example 24-1. Aliases within a script</B></P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #!/bin/bash 2 # alias.sh 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 # An alias can handle 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 # alias.sh: line 57: rrr: command not found 59 let count+=1 60 done 61 62 echo; echo 63 64 alias xyz='cat $0' # Script lists itself. 65 # Note strong quotes. 66 xyz 67 # This seems to work, 68 #+ although the Bash documentation suggests that it shouldn't. 69 # 70 # However, as Steve Jacobson points out, 71 #+ the "$0" parameter expands immediately upon declaration of the alias. 72 73 exit 0</PRE></TD></TR></TABLE><HR></DIV><P><ANAME="UNALIASREF"></A></P><P>The <BCLASS="COMMAND">unalias</B> command removes a previously set <ICLASS="FIRSTTERM">alias</I>.</P><DIVCLASS="EXAMPLE"><HR><ANAME="UNAL"></A><P><B>Example 24-2. <ICLASS="FIRSTTERM">unalias</I>: Setting and unsetting an alias</B></P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #!/bin/bash 2 # unalias.sh 3 4 shopt -s expand_aliases # Enables alias expansion. 5 6 alias llm='ls -al | more' 7 llm 8 9 echo 10 11 unalias llm # Unset alias. 12 llm 13 # Error message results, since 'llm' no longer recognized. 14 15 exit 0</PRE></TD></TR></TABLE><HR></DIV><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="SCREEN"> <TTCLASS="PROMPT">bash$ </TT><TTCLASS="USERINPUT"><B>./unalias.sh</B></TT> <TTCLASS="COMPUTEROUTPUT">total 6drwxrwxr-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></TR></TABLE></DIV><H3CLASS="FOOTNOTES">Notes</H3><TABLEBORDER="0"CLASS="FOOTNOTES"WIDTH="100%"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="5%"><ANAME="FTN.AEN17194"HREF="aliases.html#AEN17194">[1]</A></TD><TDALIGN="LEFT"VALIGN="TOP"WIDTH="95%"><P>However, aliases do seem to expand positional parameters.</P></TD></TR></TABLE><DIVCLASS="NAVFOOTER"><HRALIGN="LEFT"WIDTH="100%"><TABLESUMMARY="Footer navigation table"WIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top"><AHREF="recurnolocvar.html"ACCESSKEY="P">Prev</A></TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><AHREF="index.html"ACCESSKEY="H">Home</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top"><AHREF="list-cons.html"ACCESSKEY="N">Next</A></TD></TR><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top">Recursion Without Local Variables</TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><AHREF="part5.html"ACCESSKEY="U">Up</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top">List Constructs</TD></TR></TABLE></DIV></BODY></HTML>⌨️ 快捷键说明
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