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<html><head><title>Using Filehandles (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="ch11_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="ch11_05.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">11.4. Using Filehandles</h2><p>Once a<a name="INDEX-784" />filehandleis open for reading, you can read lines from it just like you canread from standard input with <tt class="literal">STDIN</tt>. So, forexample, to read lines from the Unix password file:</p><blockquote><pre class="code">open PASSWD, "/etc/passwd" or die "How did you get logged in? ($!)";while (<PASSWD>) { chomp; if (/^root:/) { # found root entry... ...; }}</pre></blockquote><p>In this example, the <tt class="literal">die</tt> message usesparentheses around <tt class="literal">$!</tt>. Those are merelyparentheses around the message in the output. (Sometimes apunctuation mark is just a punctuation mark.) As you can see, whatwe've been calling the "<a name="INDEX-785" />line-input operator" is reallymade of two components; the angle brackets (the<em class="emphasis">real</em> line-input operator) are around an inputfilehandle. Each line of input is then tested to see if it beginswith <tt class="literal">root</tt> followed by a colon, triggering unseenactions.</p><p>A filehandle open for writing or appending may be used with<tt class="literal">print</tt><a name="INDEX-786" />or <tt class="literal">printf</tt><a name="INDEX-787" />,appearing immediately after the keyword but before the list ofarguments:</p><blockquote><pre class="code">print LOG "Captain's log, stardate 3.14159\n"; # output goes to LOGprintf STDERR "%d percent complete.\n", $done/$total * 100;</pre></blockquote><p>Did you notice that there's no comma between the filehandle andthe items to be printed?<a href="#FOOTNOTE-259">[259]</a> This looks especiallyweird if you use parentheses. Either of these forms is correct:</p><blockquote class="footnote"> <a name="FOOTNOTE-259" /><p>[259]If you got straightA's in freshman English or Linguistics, when we say that thisis called "indirect object syntax," you may say"Ah, of course! I see why there's no comma after thefilehandle name -- it's an indirect object!" Wedidn't get straight A's; we don't understand whythere's no comma; we merely omit it because Larry told us thatwe should omit the comma.</p> </blockquote><blockquote><pre class="code">printf (STDERR "%d percent complete.\n", $done/$total * 100);printf STDERR ("%d percent complete.\n", $done/$total * 100);</pre></blockquote><a name="lperl3-CHP-11-SECT-4.1" /><div class="sect2"><h3 class="sect2">11.4.1. Changing the Default Output Filehandle</h3><p>By default, if you don't give a<a name="INDEX-788" /><a name="INDEX-789" />filehandle to<tt class="literal">print</tt> (or to <tt class="literal">printf</tt>, aseverything we say here about one applies equally well to the other),the output will go to<tt class="literal">STDOUT</tt><a name="INDEX-790" />. But that default may be changedwith the <tt class="literal">select</tt><a name="INDEX-791" /> operator. Here we'll send someoutput lines to <tt class="literal">BEDROCK</tt>:</p><blockquote><pre class="code">select BEDROCK;print "I hope Mr. Slate doesn't find out about this.\n";print "Wilma!\n";</pre></blockquote><p>Once you've selected a filehandle as the default for output, itwill stay that way. But it's generally a bad idea to confusethe rest of the program, so you should generally set it back to<tt class="literal">STDOUT</tt> when you're done.<a href="#FOOTNOTE-260">[260]</a></p><blockquote class="footnote"> <a name="FOOTNOTE-260" /><p>[260]Inthe unlikely case that <tt class="literal">STDOUT</tt> might not be theselected filehandle, you could save and restore the filehandle, usingthe technique shown in the documentation for<tt class="literal">select</tt> in the <em class="emphasis">perlfunc</em>manpage. And as long as we're sending you to that manpage, wemay as well tell you that there are actually <em class="emphasis">two</em>builtin functions in Perl named <tt class="literal">select</tt>,and both covered in the <em class="emphasis">perlfunc</em> manpage. Theother <tt class="literal">select</tt> always has four arguments, soit's sometimes called "four-argument<tt class="literal">select</tt>". </p> </blockquote><p>Also by default, the output to each filehandle is buffered. Settingthe special <tt class="literal">$|</tt> variable to <tt class="literal">1</tt>will set the currently selected filehandle (that is, the one selectedat the time that the variable is modified) to always flush the<a name="INDEX-792" />buffer after each output operation. So ifyou wanted to be sure that the <a name="INDEX-793" />logfile gets its entries atonce, in case you might be reading the log to monitor progress ofyour long-running program, you could use something like this:</p><blockquote><pre class="code">select LOG;$| = 1; # don't keep LOG entries sitting in the bufferselect STDOUT;# ... time passes, babies learn to walk, tectonic plates shift, and then...print LOG "This gets written to the LOG at once!\n";</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="ch11_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="ch11_05.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">11.3. Fatal Errors with die</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">11.5. Reopening a Standard Filehandle</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. 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