ch13_08.htm

by Randal L. Schwartz and Tom Phoenix ISBN 0-596-00132-0 Third Edition, published July 2001. (See

<p>Simply give <tt class="literal">File::Basename</tt>, in your
<tt class="literal">use</tt> declaration, an <em class="firstterm">import
list</em><a name="INDEX-938" /> showing exactly which function names it
should give you, and it'll supply those and no others. Here,
we'll get nothing but <tt class="literal">basename</tt>:
</p>

<blockquote><pre class="code">use File::Basename qw/ basename /;</pre></blockquote>

<p>And here, we'll ask for no new functions at all:</p>

<blockquote><pre class="code">use File::Basename qw/ /;</pre></blockquote>

<p>Why would you want to do that? Well, this directive tells Perl to
load up <tt class="literal">File::Basename</tt>, just as before, but not to
<em class="firstterm">import</em> any function names. Importing lets us
use the short, simple function names like <tt class="literal">basename</tt>
and <tt class="literal">dirname</tt>. But even if we don't import
those names, we can still use the functions. When they're not
imported, though, we have to call them by their full names:
</p>

<blockquote><pre class="code">use File::Basename qw/ /;                     # import no function names

my $betty = &amp;dirname($wilma);                 # uses our own subroutine &amp;dirname (not shown)

my $name = "/usr/local/bin/perl";
my $dirname = File::Basename::dirname $name;  # dirname from the module</pre></blockquote>

<p>As you see, the full name of the <tt class="literal">dirname</tt> function
from the module is <tt class="literal">File::Basename::dirname</tt>. We can
always use the function's full name (once we've loaded
the module) whether we've imported the short name
<tt class="literal">dirname</tt> or not.
</p>

<p>Most of the time, you'll want to use a module's default
import list. But you can always override that with a list of your
own, if you want to leave out some of the default items. Another
reason to supply your own list would be if you wanted to import some
function not on the default list, since most modules include some
(infrequently needed) functions that are not on the default import
list.
</p>

<p>As you'd guess, some modules will, by default, import more
symbols than others. Each module's documentation should make it
clear which symbols it imports, if any, but you are always free to
override the default import list by specifying one of your own, just
as we did with <tt class="literal">File::Basename</tt>. Supplying an empty
list imports no<a name="INDEX-939" /> symbols.<a name="INDEX-940" /> <a name="INDEX-941" />
</p>

</div>
<a name="lperl3-CHP-13-SECT-8.3" /><div class="sect2">
<h3 class="sect2">13.8.3. The File::Spec Module</h3>

<p><a name="INDEX-942" />Now you can find out a file's
basename. That's useful, but you'll often want to put
that together with a directory name to get a full filename. For
example, here we want to take a filename like
<em class="filename">/home/rootbeer/ice-2.1.txt</em> and add a prefix to
the basename:
</p>

<blockquote><pre class="code">use File::Basename;

print "Please enter a filename: ";
chomp(my $old_name = &lt;STDIN&gt;);

my $dirname = dirname $old_name;
my $basename = basename $old_name;

$basename =~ s/^/not/;  # Add a prefix to the basename
my $new_name = "$dirname/$basename";

rename($old_name, $new_name)
  or warn "Can't rename '$old_name' to '$new_name': $!";</pre></blockquote>

<p>Do you see the problem here? Once again, we're making the
assumption that filenames will follow the Unix conventions and use a
forward slash between the directory name and the basename.
Fortunately, Perl comes with a module to help with this problem, too.
</p>

<p>The <tt class="literal">File::Spec</tt> module is used for manipulating
<em class="firstterm">file
specifications</em><a name="INDEX-943" />,
which are the names of files, directories, and the other things that
are stored on filesystems. Like <tt class="literal">File::Basename</tt>, it
understands what kind of system it's running on, and it chooses
the right set of rules every time. But unlike
<tt class="literal">File::Basename</tt>, <tt class="literal">File::Spec</tt> is
an object-oriented (often abbreviated "OO") module.
</p>

<p>If you've never caught the fever of OO, don't let that
bother you. If you understand objects, that's great; you can
use this OO module. If you don't understand objects,
that's okay, too. You just type the symbols as we show you, and
it works just as if you knew what you were doing.
</p>

<p>In this case, we learn from reading the documentation for
<tt class="literal">File::Spec</tt> that we want to use a
<em class="firstterm">method</em><a name="INDEX-944" />
called <tt class="literal">catfile</tt>. What's a method?
It's just a different kind of function, as far as we're
concerned here. The difference is that you'll always call the
methods from <tt class="literal">File::Spec</tt> with their full names,
like this:
</p>

<blockquote><pre class="code">use File::Spec;

.
.  # Get the values for $dirname and $basename as above
.

my $new_name = File::Spec-&gt;catfile($dirname, $basename);

rename($old_name, $new_name)
  or warn "Can't rename '$old_name' to '$new_name': $!";</pre></blockquote>

<p>As you can see, the full name of a method is the name of the module
(called a
<em class="firstterm">class</em><a name="INDEX-945" />,
here), a <a name="INDEX-946" />
<a name="INDEX-947" />small arrow, and the short name
of the method. It is important to use the small arrow, rather than
the double-colon that we used with <tt class="literal">File::Basename</tt>.
</p>

<p>Since we're calling the method by its full name, though, what
symbols does the module import? None of them. That's normal for
OO modules. So you don't have to worry about having a
subroutine with the same name as one of the many methods of
<tt class="literal">File::Spec</tt>.
</p>

<p>Should you bother using modules like these? It's up to you, as
always. If you're sure your program will never be run anywhere
but on a Unix machine, say, and you're sure you completely
understand the rules for filenames on Unix,<a href="#FOOTNOTE-303">[303]</a> then you may prefer to hardcode your assumptions into
your programs. But these modules give you an easy way to make your
programs more robust in less time -- and more
<a name="INDEX-948" />portable at no
extra<a name="INDEX-949" /> charge.<a name="INDEX-950" />
</p><blockquote class="footnote"> <a name="FOOTNOTE-303" /><p>[303]If you
didn't know that filenames and directory names could contain
newline characters, as we mentioned earlier in this section, then you
<em class="emphasis">don't</em> know all the rules, do you? </p>
</blockquote>

</div>


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