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<html><head><title>Filehandles (Perl in a Nutshell, 2nd Edition)</title><link rel="stylesheet" type="text/css" href="../style/style1.css" /><meta name="DC.Creator" content="Stephen Spainhour" /><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="0596002416L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="Perl in a Nutshell, 2nd Edition" /><meta name="DC.Type" content="Text.Monograph" /></head><body bgcolor="#ffffff"><img src="gifs/smbanner.gif" usemap="#banner-map" border="0" alt="Book Home" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Java and XSLT" /><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="ch04_08.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td align="center" valign="top" width="228" /><td align="right" valign="top" width="228"><a href="ch04_10.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr></table></div><h2 class="sect1">4.9. Filehandles</h2><p><a name="INDEX-767" />A filehandle isthe name for an I/O connection between your Perl process and theoperating system. Filehandle names are like label names but use theirown namespace. Like label names, the convention is to use alluppercase letters for filehandle names.</p><p><a name="INDEX-768" /><a name="INDEX-769" /><a name="INDEX-770" />Every Perl program has threefilehandles that are automatically opened: STDIN, STDOUT, and STDERR.By default, the <tt class="literal">print</tt> and <tt class="literal">write</tt>functions write to STDOUT. Additional filehandles are created usingthe <tt class="literal">open</tt> function:</p><blockquote><pre class="code">open (DATA, "numbers.txt");</pre></blockquote><p>DATA is the new filehandle that is attached to the external file,which is now opened for reading. You can open filehandles forreading, writing, and appending to external files and devices. Toopen a file for writing, prefix the filename with a greater-thansign:</p><blockquote><pre class="code">open(OUT, ">outfile");</pre></blockquote><p><a name="INDEX-771" /><a name="INDEX-772" />To open a file for appending, prefixthe filename with two greater-than signs:</p><blockquote><pre class="code">open(LOGFILE, ">>error_log");</pre></blockquote><p>The <tt class="literal">open</tt> function returns true if the file issuccessfully opened, and false if it failed to open. Opening a filecan fail for any number of reasons, e.g., a file does not exist, iswrite-protected, or you don't have permission for afile or directory. However, a filehandle that has not beensuccessfully opened can still be read from (giving you an immediateEOF) or written to with no noticeable effects.</p><p><a name="INDEX-773" /><a name="INDEX-774" />You should always check theresult of <tt class="literal">open</tt> immediately and report an error ifthe operation does not succeed. The <tt class="literal">warn</tt> functioncan report an error to standard error if something goes wrong, and<tt class="literal">die</tt> can terminate your program and tell you whatwent wrong. For example:</p><blockquote><pre class="code">open(LOGFILE, "/usr/httpd/error_log") || warn "Could not open /usr/httpd/error_log.\n";open(DATA, ">/tmp/data") || die "Could not create /tmp/data\n.";</pre></blockquote><p><a name="INDEX-775" /><a name="INDEX-776" /><a name="INDEX-777" /><a name="INDEX-778" />Once the file is opened,you can access the data using the diamond operator,<tt class="literal"><</tt><em class="replaceable"><tt>filehandle</tt></em><tt class="literal">></tt>.This is the line-input operator. When used on a filehandle in ascalar context, it returns a line from a filehandle as a string. Eachtime it is called, it returns the next line from the filehandle untilit reaches the end-of-file. The operator keeps track of the line itis on in the file, unless the filehandle is closed and reopened,which resets the operator to the top-of-file.</p><p>For example, the following prints any line containing the word"secret.html" from the LOGFILEfilehandle:</p><blockquote><pre class="code">while (<LOGFILE>) { print "$_\n" if /secret\.html/;}</pre></blockquote><p><a name="INDEX-779" />In a listcontext, the line-input operator returns a list in which each line isan element. The empty <tt class="literal"><></tt> operator reads fromthe ARGV filehandle, which reads the array of filenames from the Perlcommand line. If <tt class="literal">@ARGV</tt> is empty, the operatorresorts to standard input.</p><p>A number of functions send output to a filehandle. The filehandlemust already be opened for writing, of course. In the previousexample, <tt class="literal">print</tt> wrote to the STDOUT filehandle,even though it wasn't specified. Without afilehandle, <tt class="literal">print</tt> defaults to the currentlyselected output filehandle, which will be STDOUT until you open andselect another one in your program. See the <tt class="literal">select</tt>function (filehandle version) for more information.</p><p>If your program involves more than a couple of open filehandles, youshould specify the filehandles for all of your I/O functions to besafe:</p><blockquote><pre class="code">print LOGFILE "======= Generated report $date ======="</pre></blockquote><p><a name="INDEX-780" />Toclose a filehandle, use the <tt class="literal">close</tt> function.Filehandles are also closed when the program exits.</p><a name="perlnut2-CHP-4-SECT-9.1" /><div class="sect2"><h3 class="sect2">4.9.1. Perl 5.8 and PerlIO</h3><p>Perl 5.8 does <a name="INDEX-781" /> <a name="INDEX-782" /><a name="INDEX-783" />I/O viaPerlIO instead of through your system's I/O (STDIO).By implementing <tt class="literal">open()</tt><a name="INDEX-784" />with PerlIO, the default behavior of <tt class="literal">open</tt> ischanged to support a three-argument format. For example:</p><blockquote><pre class="code">open($fh, '>:utf-8', $filename) or die("..."); # Open $filename and support utf-8</pre></blockquote><p>In this example, the filehandle is marked with<tt class="literal">utf-8</tt> (or <tt class="literal">utf8</tt> for EBCDICusers) to accept Perl's internal Unicode encoding.</p><p>The PerlIO layers are:</p><dl><dt><i>unix</i></dt><dd>Low-level read/write </p></dd><dt><i>stdio</i></dt><dd>Standard I/O</p></dd><dt><i>perlio</i></dt><dd>Portable implementation of buffering</p></dd><dt><i>crlf</i></dt><dd>Win32</p></dd></dl><p>Also in Perl 5.8, you are no longer required to name a filehandle in<tt class="literal">open( )</tt> because Perl will handle the filehandlesinternally:</p><blockquote><pre class="code">open($fh, ...) or ...</pre></blockquote><p>You can also use anonymous temporary files with the new form of<tt class="literal">open( )</tt>:</p><blockquote><pre class="code">open($fh, ">", undef) or ...</pre></blockquote><p>Pipes can also be used with a multiple-argument form of<tt class="literal">open</tt>. The following code is roughly equivalent tothe Unix command <tt class="literal">'ls -al'</tt>:</p><blockquote><pre class="code">open($fh, "-|", 'ls -al', '/users') or ...</pre></blockquote></div><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch04_08.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0" /></a></td><td align="right" valign="top" width="228"><a href="ch04_10.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr><tr><td align="left" valign="top" width="228">4.8. References and Complex Data Structures</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0" /></a></td><td align="right" valign="top" width="228">4.10. 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