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<h1 class="chapter">Chapter 11. Filehandles and File Tests</h1>
<div class="htmltoc"><h4 class="tochead">Contents:</h4>
<p> <a href="#lperl3-CHP-11-SECT-1">What Is a Filehandle?</a><br />
<a href="ch11_02.htm">Opening a Filehandle</a><br />
<a href="ch11_03.htm">Fatal Errors with die</a><br />
<a href="ch11_04.htm">Using Filehandles</a><br />
<a href="ch11_05.htm">Reopening a Standard Filehandle</a><br />
<a href="ch11_06.htm">File Tests</a><br />
<a href="ch11_07.htm">Exercises</a><br /></p></div>
<div class="sect1"><a name="lperl3-CHP-11-SECT-1" /></a>
<h2 class="sect1">11.1. What Is a Filehandle?</h2>
<p><a name="INDEX-733" /></a>A
filehandle is the name in a Perl program for an
<a name="INDEX-734" /></a> <a name="INDEX-735" /></a>I/O
connection between your Perl process and the outside world. That is,
it's the name of a <em class="emphasis">connection</em>, not
necessarily the name of a file.
</p>
<p>Filehandles are named like other Perl identifiers (letters, digits,
and underscores, but they can't start with a digit), but since
they don't have any prefix character, they might be confused
with present or future reserved words, as we saw with labels. Once
again, as with labels, the recommendation from Larry is that you use
all uppercase letters in the name of your filehandle -- not only
will it stand out better, but it will also guarantee that your
program won't fail when a future (lowercase) reserved word is
introduced.
</p>
<p>But there are also six special filehandle names that Perl already
uses for its own purposes: <tt class="literal">STDIN</tt>,
<tt class="literal">STDOUT</tt>, <tt class="literal">STDERR</tt>,
<tt class="literal">DATA</tt>, <tt class="literal">ARGV</tt>, and
<tt class="literal">ARGVOUT</tt>.<a href="#FOOTNOTE-243">[243]</a> Although you may choose any filehandle name you'd
like, you shouldn't choose one of those six unless you intend
to use that one's special properties.<a href="#FOOTNOTE-244">[244]</a>
</p><blockquote class="footnote"> <a name="FOOTNOTE-243" /></a><p>[243]Some people hate typing
in all-caps, even for a moment, and will try spelling these in
lowercase, like <tt class="literal">stdin</tt>. Perl may even let you get
away with that from time to time, but not always. The details of when
these work and when they fail are beyond the scope of this book. But
the important thing is that programs that rely upon this kindness
will one day break, so it is best to avoid lowercase here.</p>
</blockquote><blockquote class="footnote"> <a name="FOOTNOTE-244" /></a><p>[244]In some
cases, you could (re-)use these names without a problem. But your
maintenance programmer may think that you're using the name for
its builtin features, and thus may be confused. </p> </blockquote>
<p>Maybe you recognized some of those names already. When your program
starts, <tt class="literal">STDIN</tt><a name="INDEX-736" /></a> is the filehandle naming the
connection between the Perl process and wherever the program should
get its input, known as the <em class="firstterm">standard input
stream</em><a name="INDEX-737" /></a>. This is generally the user's
keyboard unless the user asked for something else to be the source of
input, such as reading the input from a file or reading the output of
another program through a pipe.<a href="#FOOTNOTE-245">[245]</a>
</p><blockquote class="footnote"> <a name="FOOTNOTE-245" /></a><p>[245]The defaults we speak
of in this chapter for the three main I/O streams are what the Unix
shells do by default. But it's not just shells that launch
programs, of course. We'll see in <a href="ch14_01.htm">Chapter 14, "Process Management"</a>
what happens when you launch another program from Perl.</p>
</blockquote>
<p>There's also the <em class="firstterm">standard output
stream</em><a name="INDEX-738" /></a>, which is
<tt class="literal">STDOUT</tt><a name="INDEX-739" /></a>. By default, this one goes to the
user's display screen, but the user may send the output to a
file or to another program, as we'll see shortly. These
standard streams come to us from the
<a name="INDEX-740" /></a>Unix "standard I/O" library,
but they work in much the same way on most modern operating
systems.<a href="#FOOTNOTE-246">[246]</a> The general idea is that your program
should blindly read from <tt class="literal">STDIN</tt> and blindly write
to <tt class="literal">STDOUT</tt>, trusting in the user (or generally
whichever program is starting your program) to have set those up. In
that way, the user can type a command like this one at the shell
prompt:
</p><blockquote class="footnote"> <a name="FOOTNOTE-246" /></a><p>[246]If you're not already familiar with
how your non-Unix system provides standard input and output, see
the<a name="INDEX-741" /></a> <em class="emphasis">perlport</em> manpage and
the documentation for that system's equivalent to the Unix
shell (the program that runs programs based upon your keyboard
input).</p> </blockquote>
<blockquote><pre class="code">$ <tt class="userinput"><b>./your_program <dino >wilma</b></tt></pre></blockquote>
<p>That command tells the shell that the program's input should be
read from the file <em class="filename">dino</em>, and the output should
go to the file <em class="filename">wilma</em>. As long as the program
blindly reads its input from <tt class="literal">STDIN</tt>, processes it
(in whatever way we need), and blindly writes its output to
<tt class="literal">STDOUT</tt>, this will work just fine.
</p>
<p>And at no extra charge, the program will work in a
<a name="INDEX-742" /></a>
<a name="INDEX-743" /></a><em class="firstterm">pipeline</em>. This is
another concept from Unix, which lets us write command lines like
this one:
</p>
<blockquote><pre class="code">$ <tt class="userinput"><b>cat fred barney | sort | ./your_program | grep something | lpr</b></tt></pre></blockquote>
<p>Now, if you're not familiar with these Unix commands,
that's okay. This line says that the <i class="command">cat</i>
command should print out all of the lines of file
<em class="filename">fred</em> followed by all of the lines of file
<em class="filename">barney</em>. Then that output should be the input of
the <i class="command">sort</i> command, which sorts those lines and
passes them on to <i class="command">your_program</i>. After it has done
its processing, <i class="command">your_program</i> will send the data on
to <i class="command">grep</i>, which discards certain lines in the data,
sending the others on to the <i class="command">lpr</i> command, which
should print everything that it gets on a printer. Whew!
</p>
<p>But pipelines like that are common in Unix and many other systems
today because they let you put together a powerful, complex command
out of simple, standard building blocks.
</p>
<p>There's one more standard I/O stream. If (in the previous
example) <i class="command">your_program</i> had to emit any warnings or
other diagnostic messages, those shouldn't go down the
pipeline. The <i class="command">grep</i> command is set to discard
anything that it hasn't specifically been told to look for, and
so it will most likely discard the warnings. Even if it did keep the
warnings, we probably don't want those to be passed downstream
to the other programs in the pipeline. So that's why
there's also the <em class="firstterm">standard error
stream</em><a name="INDEX-744" /></a>:
<tt class="literal">STDERR</tt><a name="INDEX-745" /></a>. Even if the standard output is
going to another program or file, the errors will go to wherever the
user desires. By default, the errors will generally go to the
user's display screen,<a href="#FOOTNOTE-247">[247]</a> but the
user may send the errors to a file with a shell command like this
one:
</p><blockquote class="footnote"> <a name="FOOTNOTE-247" /></a><p>[247]Also, generally, errors
aren't buffered. That means that if the standard error and
standard output streams are both going to the same place (such as the
monitor), the errors may appear earlier than the normal output. For
example, if your program prints a line of ordinary text, then tries
to divide by zero, the output may show the message about dividing by
zero first, and the ordinary text second.</p> </blockquote>
<blockquote><pre class="code">$ <tt class="userinput"><b>netstat | ./your_program 2>/tmp/my_errors</b></tt></pre></blockquote>
</div>
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