ch02_11.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

</blockquote>

except that it isn't so cumbersome to say, and will actually work.
It really does shift array <tt class="literal">@ARGV</tt> and put the current filename
into the global variable <tt class="literal">$ARGV</tt>.  It also uses the special
filehandle <tt class="literal">ARGV</tt> internally--<tt class="literal">&lt;&gt;</tt>
is just a synonym for
the more explicitly written <tt class="literal">&lt;ARGV&gt;</tt>, which is a magical
filehandle.  (The pseudocode above doesn't work because it treats
<tt class="literal">&lt;ARGV&gt;</tt> as nonmagical.)</p>

<p>You can modify <tt class="literal">@ARGV</tt> before the first
<tt class="literal">&lt;&gt;</tt> as long as the array ends up containing
the list of filenames you really want.  Because Perl uses its normal
<tt class="literal">open</tt> function here, a filename of
"<tt class="literal">-</tt>" counts as standard input wherever it is
encountered, and the more esoteric features of <tt class="literal">open</tt>
are automatically available to you (such as opening a "file" named
"<tt class="literal">gzip -dc &lt; file.gz|</tt>").  Line numbers
(<tt class="literal">$.</tt>) continue as if the input were one big happy
file.  (But see the example under <tt class="literal">eof</tt> in
<a href="ch29_01.htm">Chapter 29, "Functions"</a> for how to
reset line numbers on each file.)</p>

<p>If you want to set <tt class="literal">@ARGV</tt> to your own list of files, go right ahead:
<blockquote>
<pre class="programlisting"># default to README file if no args given
@ARGV = ("README") unless @ARGV;</pre>
</blockquote>

If you want to pass switches into your script, you can use one of the
<tt class="literal">Getopt::*</tt> modules or put a loop on the front like this:
<blockquote>
<pre class="programlisting">while (@ARGV and $ARGV[0] =~ /^-/) {
    $_ = shift;
    last if /^--$/;
    if (/^-D(.*)/) { $debug = $1 }
    if (/^-v/)     { $verbose++  }
    ...             # other switches
}
while (&lt;&gt;) {
    ...             # code for each line
}</pre>
</blockquote>

The <tt class="literal">&lt;&gt;</tt> symbol will return false only once.  If you call it
again after this, it will assume you are processing another <tt class="literal">@ARGV</tt>
list, and if you haven't set <tt class="literal">@ARGV</tt>, it will input from <tt class="literal">STDIN</tt>.</p>

<p>
<a name="INDEX-754"></a>
If the string inside the angle brackets is a scalar variable (for
example, <tt class="literal">&lt;$foo&gt;</tt>), that variable contains an <em class="emphasis">indirect</em>
filehandle, either the name of the filehandle to input from or a
reference to such a filehandle.  For example:
<blockquote>
<pre class="programlisting">$fh = \*STDIN;
$line = &lt;$fh&gt;;</pre>
</blockquote>

or:
<blockquote>
<pre class="programlisting">open($fh, "&lt;data.txt");
$line = &lt;$fh&gt;;</pre>
</blockquote>
</p>






<h3 class="sect2">2.11.3. Filename Globbing Operator</h3>

<a name="INDEX-755"></a><a name="INDEX-756"></a><a name="INDEX-757"></a><a name="INDEX-758"></a><a name="INDEX-759"></a><a name="INDEX-760"></a>
<p>You might wonder what happens to a line input operator if you put
something fancier inside the angle brackets.  What happens is that it
mutates into a different operator.  If the string inside the angle
brackets is anything other than a filehandle name or a scalar variable
(even if there are just extra spaces), it is interpreted as a filename
pattern to be "globbed".<a href="#FOOTNOTE-19">[19]</a> The pattern is matched against the files in the
current directory (or the directory specified as part of the fileglob
pattern), and the filenames so matched are returned by the operator.  As
with line input, names are returned one at a time in scalar context,
or all at once in list context.  The latter usage is more
common; you often see things like:
<blockquote>
<pre class="programlisting">@files = &lt;*.xml&gt;;</pre>
</blockquote>

As with other kinds of pseudoliterals, one level of variable
interpolation is done first, but you can't say <tt class="literal">&lt;$foo&gt;</tt> because
that's an indirect filehandle as explained earlier.  In older versions of
Perl, programmers would insert braces to force interpretation as a
fileglob: <tt class="literal">&lt;${foo}&gt;</tt>.  These days, it's considered cleaner
to call the internal function directly as <tt class="literal">glob($foo)</tt>, which is
probably the right way to have invented it in the first place.
So instead you'd write
<blockquote>
<pre class="programlisting">@files = glob("*.xml");</pre>
</blockquote>

if you despise overloading the angle operator for this.
Which you're allowed to do.</p>
<blockquote class="footnote">

<a name="FOOTNOTE-19"></a>
<p>[19]Fileglobs have nothing to do with the
previously mentioned typeglobs, other than that they both use the <tt class="literal">*</tt>
character in a wildcard fashion.  The <tt class="literal">*</tt> character has the nickname
"glob" when used like this.  With typeglobs, you're globbing symbols with
the same name from the symbol table.  With a fileglob, you're doing
wildcard matching on the filenames in a directory, just as the various
shells do.</p>

</blockquote>

<p>
<a name="INDEX-761"></a><a name="INDEX-762"></a> Whether you use the
<tt class="literal">glob</tt> function or the old angle-bracket form, the
fileglob operator also does <tt class="literal">while</tt> magic like the
line input operator, assigning the result to <tt class="literal">$_</tt>.
(That was the rationale for overloading the angle operator in the
first place.)  For example, if you wanted to change the permissions on
all your C code files, you might say:
<blockquote>
<pre class="programlisting">while (glob "*.c") {
    chmod 0644, $_;
}</pre>
</blockquote>

which is equivalent to:
<blockquote>
<pre class="programlisting">while (&lt;*.c&gt;) {
    chmod 0644, $_;
}</pre>
</blockquote>

The <tt class="literal">glob</tt> function was originally implemented as a
shell command in older versions of Perl (and in even older versions of
Unix), which meant it was comparatively expensive to execute and,
worse still, wouldn't work exactly the same everywhere.  Nowadays it's
a built-in, so it's more reliable and a lot faster.  See the
description of the <tt class="literal">File::Glob</tt> module in
<a href="ch32_01.htm">Chapter 32, "Standard Modules"</a> for how to alter the default
behavior of this operator, such as whether to treat spaces in its
operand (argument) as pathname separators, whether to expand tildes or
braces, whether to be case insensitive, and whether to sort the return
values--amongst other things.</p>

<p>Of course, the shortest and arguably the most readable way to do the
<tt class="literal">chmod</tt> command above is to use the fileglob as a list operator:
<blockquote>
<pre class="programlisting">chmod 0644, &lt;*.c&gt;;</pre>
</blockquote>

A fileglob evaluates its (embedded) operand only when starting a new
list.  All values must be read before the operator will start over.
In a list context, this isn't important because you automatically get
them all anyway.  In a scalar context, however, the operator returns
the next value each time it is called, or a false value if you've just
run out.  Again, false is returned only once.  So if you're expecting
a single value from a fileglob, it is much better to say:
<blockquote>
<pre class="programlisting">($file) = &lt;blurch*&gt;;  # list context</pre>
</blockquote>
than to say:
<blockquote>
<pre class="programlisting">$file = &lt;blurch*&gt;;    # scalar context</pre>
</blockquote>

because the former returns all matched filenames and resets the
operator, whereas the latter alternates between returning filenames
and returning false.</p>

<p>
<a name="INDEX-763"></a><a name="INDEX-764"></a>
If you're trying to do variable interpolation, it's definitely better
to use the <tt class="literal">glob</tt> operator because the older notation can cause
confusion with the indirect filehandle notation.
This is where it
becomes apparent that the borderline between terms and operators is a
bit mushy:
<blockquote>
<pre class="programlisting">@files = &lt;$dir/*.[ch]&gt;;         # Works, but avoid.
@files = glob("$dir/*.[ch]");   # Call glob as function.
@files = glob $some_pattern;    # Call glob as operator.</pre>
</blockquote>
<a name="INDEX-765"></a><a name="INDEX-766"></a>
</p>

<p>We left the parentheses off of the last example to illustrate that
<tt class="literal">glob</tt> can be used either as a function (a term) or as a <em class="emphasis">unary</em>
operator; that is, a prefix operator that takes a single argument.  The
<tt class="literal">glob</tt> operator is an example of a <em class="emphasis">named unary operator</em>, which is
just one kind of operator we'll talk about in the next chapter.  Later,
we'll talk about pattern-matching operators, which also parse like
terms but behave like operators.</p>



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