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<HTML><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><!-- Created on September, 12 2004 by texi2html 1.64 --><!-- Written by: Lionel Cons <Lionel.Cons@cern.ch> (original author) Karl Berry <karl@freefriends.org> Olaf Bachmann <obachman@mathematik.uni-kl.de> and many others.Maintained by: Olaf Bachmann <obachman@mathematik.uni-kl.de>Send bugs and suggestions to <texi2html@mathematik.uni-kl.de> --><HEAD><TITLE>Autoconf, Automake, and Libtool: Pattern Matching</TITLE><META NAME="description" CONTENT="Autoconf, Automake, and Libtool: Pattern Matching"><META NAME="keywords" CONTENT="Autoconf, Automake, and Libtool: Pattern Matching"><META NAME="resource-type" CONTENT="document"><META NAME="distribution" CONTENT="global"><META NAME="Generator" CONTENT="texi2html 1.64"><script language="Javascript"><!-- // Check the browser version. function checkVersion() { if (navigator.appVersion.charAt(0)>=3) return true; if (navigator.appVersion.charAt(0)>=4) return true; else return false; 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There are many Unix commands which are regularly used fromshell scripts, and which provide some sort of pattern matchingmechanism: <CODE>expr</CODE>, <CODE>egrep</CODE> and <CODE>sed</CODE>, to name afew. Unfortunately they each have different quoting rules regardingwhether particular meta-characters must be backslash escaped to revertto their literal meaning and vice-versa. There is no real logicto the particular dialect of regular expressions accepted by thesecommands. To confirm the correctness of each regular expression, youshould always check them from the shell prompt with the relevant toolbefore committing to a script, so I won't belabour the specifics.</P><P>Shell globbing however is much more regular (no pun intended), andprovides a reasonable and sometimes more cpu efficient solution to manyshell matching problems. The key is to make good use of the<CODE>case</CODE> command, which is easier to use (because it uses globbingrules) and doesn't require additional processes to be spawned.Unfortunately, GNU Bash doesn't handle backslashes correctly inglob character classes -- the backslash must be the first character inthe class, or else it will never match. For example, if you want todetect absolute directory paths on Unix and Windows using<CODE>case</CODE>, you should write the code like this:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre>case $dir in [\\/]* | ?:[\\/]* ) echo absolute ;; * ) echo relative ;;esac</pre></td></tr></table></P><P>Even though <CODE>expr</CODE> uses regular expressions rather than shellglobbing, it is often<A NAME="DOCF52" HREF="autobook_fot.html#FOOT52">(52)</A> a shell builtin, so usingit to extract sections of strings can be faster than spawning a sedprocess to do the same. As with <CODE>echo</CODE> and <CODE>set</CODE>, forexample, you must be careful that variable or command expansions for thefirst argument to <CODE>expr</CODE> are not accidentally interpreted asreserved keywords. As with <CODE>echo</CODE>, you can work around thisproblem by prefixing any expansions with a literal <SAMP>`x'</SAMP>, as follows:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre>$ foo=substr$ expr $foo : '.*\(str\)'expr: syntax error$ expr x$foo : '.*\(str\)'str</pre></td></tr></table></P><P><A NAME="Environment"></A></TR></TABLE><BR> <FONT SIZE="-1">This document was generatedby <I>Gary V. Vaughan</I> on <I>September, 12 2004</I>using <A HREF="http://www.mathematik.uni-kl.de/~obachman/Texi2html"><I>texi2html</I></A></BODY></HTML>⌨️ 快捷键说明
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