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<html><head><title>Directory Handles (Learning Perl, 3rd Edition)</title><link rel="stylesheet" type="text/css" href="../style/style1.css" /><meta name="DC.Creator" content="Randal L. Schwartz and Tom Phoenix" /><meta name="DC.Format" content="text/xml" scheme="MIME" /><meta name="DC.Language" content="en-US" /><meta name="DC.Publisher" content="O'Reilly & Associates, Inc." /><meta name="DC.Source" scheme="ISBN" content="0596001320L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="Learning Perl, 3rd Edition" /><meta name="DC.Type" content="Text.Monograph" /></head><body bgcolor="#ffffff"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Learning Perl, 3rd Edition" /><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch12_03.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"></a></td><td align="right" valign="top" width="228"><a href="ch12_05.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">12.4. Directory Handles</h2><p><a name="INDEX-868" />Another way to get a list of names froma given directory is with a <em class="emphasis">directory handle</em>. Adirectory handle looks and acts like a filehandle. You open it (with<tt class="literal">opendir</tt><a name="INDEX-869" /> instead of <tt class="literal">open</tt>),you read from it (with<tt class="literal">readdir</tt><a name="INDEX-870" /> instead of<tt class="literal">readline</tt>), and you close it (with<tt class="literal">closedir</tt><a name="INDEX-871" /> instead of<tt class="literal">close</tt>). But instead of reading the<em class="emphasis">contents</em> of a file, you're reading the<em class="emphasis">names</em> of<a name="INDEX-872" />files (and other things)in a directory. For example:</p><blockquote><pre class="code">my $dir_to_process = "/etc";opendir DH, $dir or die "Cannot open $dir: $!";foreach $file (readdir DH) { print "one file in $dir is $file\n";}closedir DH;</pre></blockquote><p>Like filehandles, directory handles are automatically closed at theend of the program or if the directory handle is reopened ontoanother directory.</p><p>Unlike <a name="INDEX-873" />globbing, which in older versions ofPerl fired off a separate process, a directory handle never fires offanother process. So it makes them more efficient for applicationsthat demand every ounce of power from the machine. However,it's also a lower-level operation, meaning that we have to domore of the work ourselves.</p><p>For example, the names are returned in no particular order.<a href="#FOOTNOTE-280">[280]</a> And the listincludes all files, not just those matching a particular pattern(like <tt class="literal">*.pm</tt> from our globbing examples). And thelist includes all files, especially the dot files, and particularlythe dot and dot-dot entries.<a href="#FOOTNOTE-281">[281]</a></p><blockquote class="footnote"><a name="FOOTNOTE-280" /><p>[280]It's actually the unsorted order of the directoryentries, similar to the order you get from <i class="command">ls -f</i>or <i class="command">find</i>.</p> </blockquote><blockquote class="footnote"> <a name="FOOTNOTE-281" /><p>[281]Do not make the mistakeof many old Unix programs and presume that dot and dot-dot are alwaysreturned as the first two entries (sorted or not). If thathadn't even occurred to you, pretend we never said it, becauseit's a false presumption. In fact, we're now sorry foreven bringing it up.</p> </blockquote><p>So, if we wanted only the <em class="emphasis">pm</em>-ending files, wecould use a skip-over function inside the loop:</p><blockquote><pre class="code">while ($name = readdir DIR) { next unless $name =~ /\.pm$/; ... more processing ...}</pre></blockquote><p>Note here that the syntax is that of a regular expression, not aglob. And if we wanted all the non-dot files, we could say that:</p><blockquote><pre class="code">next if $name =~ /^\./;</pre></blockquote><p>Or if we wanted everything but the common dot (current directory) anddot-dot (parent directory) entries, we could explicitly say that:</p><blockquote><pre class="code">next if $name eq "." or $name eq "..";</pre></blockquote><p>Now we'll look at the part that gets most people mixed up, sopay close attention. The filenames returned by the<tt class="literal">readdir</tt> operator have <em class="emphasis">no</em>pathname component. It's just the <em class="emphasis">name</em>within the directory. So, we're not looking at<em class="emphasis">/etc/passwd</em>, we're just looking at<em class="emphasis">passwd</em>. (And because this is another differencefrom the globbing operation, it's easy to see how people getconfused.)</p><p>So you'll need to patch up the name to get the full name:</p><blockquote><pre class="code">opendir SOMEDIR, $dirname or die "Cannot open $dirname: $!";while (my $name = readdir SOMEDIR) { next if $name =~ /^\./; # skip over dot files $name = "$dirname/$name"; # patch up the path next unless -f $name and -r $name; # only readable files ...}</pre></blockquote><p>Without the patch, the file tests would have been checking files inthe current directory, rather than in the directory named in<tt class="literal">$dirname</tt>. This is the single most-common mistakewhen using directory handles.<a name="INDEX-874" /></p><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch12_03.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img alt="Home" border="0" src="../gifs/txthome.gif" /></a></td><td align="right" valign="top" width="228"><a href="ch12_05.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">12.3. An Alternate Syntax for Globbing</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img alt="Book Index" border="0" src="../gifs/index.gif" /></a></td><td align="right" valign="top" width="228">12.5. Recursive Directory Listing</td></tr></table></div><hr width="684" align="left" /><img alt="Library Navigation Links" border="0" src="../gifs/navbar.gif" usemap="#library-map" /><p><p><font size="-1"><a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. All rights reserved.</font></p><map name="library-map"><area shape="rect" coords="1,0,85,94" href="../index.htm"><area shape="rect" coords="86,1,178,103" href="../lwp/index.htm"><area shape="rect" coords="180,0,265,103" href="../lperl/index.htm"><area shape="rect" coords="267,0,353,105" href="../perlnut/index.htm"><area shape="rect" coords="354,1,446,115" href="../prog/index.htm"><area shape="rect" coords="448,0,526,132" href="../tk/index.htm"><area shape="rect" coords="528,1,615,119" href="../cookbook/index.htm"><area shape="rect" coords="617,0,690,135" href="../pxml/index.htm"></map></body></html>⌨️ 快捷键说明
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