ch11_06.htm

来自「by Randal L. Schwartz and Tom Phoenix I」· HTM 代码 · 共 924 行 · 第 1/3 页

</tr>
<tr>
<td>
<blockquote><pre class="code">-A</pre></blockquote>
</td>
<td>
<p>Access age (measured in days)</p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">-C</pre></blockquote>
</td>
<td>
<p>Inode-modification age (measured in days)</p>
</td>
</tr>

</table><p>

<p>The tests <tt class="literal">-r</tt>, <tt class="literal">-w</tt>,
<tt class="literal">-x</tt>, and <tt class="literal">-o</tt> tell whether the
given attribute is true for the effective user or group ID,<a href="#FOOTNOTE-266">[266]</a> which essentially refers to the person who is "in
charge of" running the program.<a href="#FOOTNOTE-267">[267]</a> These tests look at
the "permission bits" on the file to see what is
permitted. If your system uses Access Control Lists (ACLs), the tests
will use those as well. These tests generally tell whether the system
would <em class="emphasis">try</em> to permit something, but it
doesn't mean that it really would be possible. For example,
<tt class="literal">-w</tt> may be true for a file on a CD-ROM, even though
you can't write to it, or <tt class="literal">-x</tt> may be true on
an empty file, which can't truly be executed.
</p><blockquote class="footnote">
<a name="FOOTNOTE-266" /><p>[266]The <tt class="literal">-o</tt> and <tt class="literal">-O</tt> tests
relate only to the user ID and not to the group ID.</p>
</blockquote><blockquote class="footnote"> <a name="FOOTNOTE-267" /><p>[267]Note for
advanced students: the corresponding <tt class="literal">-R</tt>,
<tt class="literal">-W</tt>, <tt class="literal">-X</tt>, and
<tt class="literal">-O</tt> tests use the real user or group ID, which
becomes important if your program may be running set-ID; in that
case, it's generally the ID of the person who requested running
it. See any good book about advanced Unix programming for a
discussion of set-ID programs.</p> </blockquote>

<p>The <tt class="literal">-s</tt> test does return true if the file is
nonempty, but it's a special kind of true. It's the
length of the file, measured in bytes, which evaluates as true for a
nonzero number.
</p>

<p>On a Unix filesystem,<a href="#FOOTNOTE-268">[268]</a> there are
just seven types of items, represented by the seven file tests
<tt class="literal">-f</tt>, <tt class="literal">-d</tt>, <tt class="literal">-l</tt>,
<tt class="literal">-S</tt>, <tt class="literal">-p</tt>, <tt class="literal">-b</tt>,
and <tt class="literal">-c</tt>. Any item should be one of those. But if
you have a <a name="INDEX-802" />symbolic link pointing to a file, that
will report true for both <tt class="literal">-f</tt> and
<tt class="literal">-l</tt>. So if you want to know whether something is a
symbolic link, you should generally test that first. (We'll
learn more about symbolic links in <a href="ch13_01.htm">Chapter 13, "Manipulating Files and Directories"</a>.)
</p><blockquote class="footnote"> <a name="FOOTNOTE-268" /><p>[268]This is the case on many
non-Unix filesystems, but not all of the file tests are meaningful
everywhere. For example, you aren't likely to have block
special files on your non-Unix system.</p> </blockquote>

<p>The age tests, <tt class="literal">-M</tt>, <tt class="literal">-A</tt>, and
<tt class="literal">-C</tt> (yes, they're uppercase), return the
number of days since the file was last modified, accessed, or had its
inode changed.<a href="#FOOTNOTE-269">[269]</a> (The inode contains all of the information about the file
except for its contents -- see the <tt class="literal">stat</tt> system
call manpage or a good book on Unix internals for details.) This age
value is a full floating-point number, so you might get a value of
<tt class="literal">2.00001</tt> if a file were modified two days and one
second ago. (These "days" aren't necessarily the
same as a human would count; for example, if it's one thirty in
the morning when you check a file modified at about an hour before
midnight, the value of <tt class="literal">-M</tt> for this file would be
around <tt class="literal">0.1</tt>, even though it was modified
"yesterday.")
</p><blockquote class="footnote"> <a name="FOOTNOTE-269" /><p>[269]This information will be somewhat
different on non-Unix systems, since not all keep track of the same
times that Unix does. For example, on some systems, the ctime field
(which the <tt class="literal">-C</tt> test looks at) is the file creation
time (which Unix doesn't keep track of), rather than the inode
change time; see the <em class="emphasis">perlport</em> manpage.</p>
</blockquote>

<p>When checking the age of a file, you might even get a negative value
like <tt class="literal">-1.2</tt>, which means that the file's
last-access timestamp is set at about thirty hours in the future! The
zero point on this timescale is the moment your program started
running,<a href="#FOOTNOTE-270">[270]</a> so that value might mean
that a long-running program was looking at a file that had just been
accessed. Or a timestamp could be set (accidentally or intentionally)
to a time in the future.
</p><blockquote class="footnote"> <a name="FOOTNOTE-270" /><p>[270]As recorded in the <tt class="literal">$^T</tt>
variable, which you could update (with a statement like <tt class="literal">$^T
= time;</tt>) if you needed to get the ages relative to a
different starting time.</p> </blockquote>

<p>The tests <tt class="literal">-T</tt> and <tt class="literal">-B</tt> take a try
at telling whether a file is text or binary. But people who know a
lot about filesystems know that there's no bit (at least in
Unix-like operating systems) to indicate that a file is a binary or
text file -- so how can Perl tell? The answer is that Perl cheats:
it opens the file, looks at the first few thousand bytes, and makes
an educated guess. If it sees a lot of null bytes, unusual control
characters, and bytes with the high bit set, then that looks like a
binary file. If there's not much weird stuff then it looks like
text. As you might guess, it sometimes guesses wrong. If a text file
has a lot of Swedish or French words (which may have characters
represented with the high bit set, as some ISO-8859-something
variant, or perhaps even a Unicode version), it may fool Perl into
declaring it binary. So it's not perfect, but if you just need
to separate your source code from compiled files, or HTML files from
PNGs, these tests should do the trick.
</p>

<p>You'd think that <tt class="literal">-T</tt> and
<tt class="literal">-B</tt> would always disagree, since a text file
isn't a binary and vice versa, but there are two special cases
where they're in complete agreement. If the file doesn't
exist, both are false, since it's neither a text file nor a
binary. Alternatively, if the file is empty, it's an empty text
file and an empty binary file at the same time, so they're both
true.
</p>

<p>The <tt class="literal">-t</tt> file test returns true if the given
filehandle is a TTY -- in short, if it's able to be
interactive because it's not a simple file or pipe. When
<tt class="literal">-t STDIN</tt> returns true, it generally means that you
can interactively ask the user questions. If it's false, your
program is probably getting input from a file or pipe, rather than a
keyboard.
</p>

<p>Don't worry if you don't know what some of the other file
tests mean -- if you've never heard of them, you won't
be needing them. But if you're curious, get a good book about
programming for <a name="INDEX-803" />Unix.
(On non-Unix systems, these tests all try to give results analogous
to what they do on Unix. Usually you'll be able to guess
correctly what they'll do.)
</p>

<p>If you omit the filename or filehandle parameter to a file test (that
is, if you have just <tt class="literal">-r</tt> or just
<tt class="literal">-s</tt>, say), the default operand is the file named in
<tt class="literal">$_</tt>.<a href="#FOOTNOTE-271">[271]</a> So, to test a list of
filenames to see which ones are readable, you simply type:
</p><blockquote class="footnote"> <a name="FOOTNOTE-271" /><p>[271]The <tt class="literal">-t</tt> file
test is an exception; since that test isn't useful with
filenames (they're never TTYs). By default it tests
<tt class="literal">STDIN</tt>. </p> </blockquote>

<blockquote><pre class="code">foreach (@lots_of_filenames) {
  print "$_ is readable\n" if -r;  # same as -r $_
}</pre></blockquote>

<p>But if you omit the parameter, be careful that whatever follows the
file test doesn't look like it <em class="emphasis">could</em> be a
parameter. For example, if you wanted to find out the size of a file
in K rather than in bytes, you might be tempted to divide the result
of <tt class="literal">-s</tt> by <tt class="literal">1000</tt> (or
<tt class="literal">1024</tt>), like this:
</p>

<blockquote><pre class="code"># The filename is in $_
my $size_in_K = -s / 1000;  # Oops!</pre></blockquote>

<p>When the Perl parser sees the slash, it doesn't think about
division; since it's looking for the optional operand for
<tt class="literal">-s</tt>, it sees what looks like the start of a regular
expression in forward slashes. One simple way to prevent this kind of
confusion is to put
<a name="INDEX-804" />parentheses around the file test:
</p>

<blockquote><pre class="code">my $size_in_k = (-s) / 1024;  # Uses $_ by default</pre></blockquote>

<p>Of course, it's always safe to explicitly give a file test a
parameter.
</p>

<a name="lperl3-CHP-11-SECT-6.1" /><div class="sect2">
<h3 class="sect2">11.6.1. The stat and lstat Functions</h3>

<p>While these file tests are fine for testing various attributes
regarding a particular file or filehandle, they don't tell the
whole story. For example, there's no file test that returns the
number of links to a
<a name="INDEX-805" />file or the owner's
<a name="INDEX-806" />
<a name="INDEX-807" />user-ID (uid).
To get at the remaining information about a file, merely call the
<tt class="literal">stat</tt><a name="INDEX-808" /> function, which returns pretty much
everything that the stat Unix system call returns (hopefully more
than you want to know).<a href="#FOOTNOTE-272">[272]</a>
</p><blockquote class="footnote"> <a name="FOOTNOTE-272" /><p>[272]On a non-Unix system, both
<tt class="literal">stat</tt> and <tt class="literal">lstat</tt>, as well as
the file tests, should return "the closest thing
available." For example, a system that doesn't have user
IDs (that is, a system that has just one "user," in the
Unix sense) might return zero for the user and group IDs, as if the
one and only user is the system administrator. If
<tt class="literal">stat</tt> or <tt class="literal">lstat</tt> fails, it will
return an empty list. If the system call underlying a file test fails
(or isn't available on the given system), that test will
generally return <tt class="literal">undef</tt>. See the
<em class="emphasis">perlport</em> manpage for the latest about what to
expect on different systems.</p> </blockquote>

<p>The operand to <tt class="literal">stat</tt> is a filehandle, or an
expression that evaluates to a filename. The return value is either
the empty list, indicating that the <tt class="literal">stat</tt> failed
(usually because the file doesn't exist), or a 13-element list
of numbers, most easily described using the following list of
<a name="INDEX-809" />scalar variables:
</p>

<blockquote><pre class="code">my($dev, $ino, $mode, $nlink, $uid, $gid, $rdev,
  $size, $atime, $mtime, $ctime, $blksize, $blocks)
    = stat($filename);</pre></blockquote>

<p>The names here refer to the parts of the stat structure, described in
detail in the <em class="emphasis">stat(2)</em><a name="INDEX-810" /> manpage. You should probably look there
for the detailed descriptions. But in short, here's a quick
summary of the important ones:
</p>

<dl>
<dt><b><tt class="literal">$dev</tt><a name="INDEX-811" /> and <tt class="literal">$ino</tt><a name="INDEX-812" /> </b></dt>
<dd>
The device number and inode number of the file. Together they make up
a "license plate" for the file. Even if it has more than
one name (hard link), the combination of device and inode numbers
should always be unique.
</p>
</dd>

</dl>

<dl>
<dt><b><tt class="literal">$mode</tt><a name="INDEX-813" /></b></dt>
<dd>
The set of permission bits for the file, and some other bits. If
you've ever used the Unix command <i class="command">ls -l</i> to
get a detailed (long) file listing, you'll see that each line
of output starts with something like <tt class="literal">-rwxr-xr-x</tt>.
The nine letters and hyphens of file permissions<a href="#FOOTNOTE-273">[273]</a> correspond to the nine least-significant bits of
<tt class="literal">$mode</tt>, which would in this case give the octal
number <tt class="literal">0755</tt>. The other bits, beyond the lowest
nine, indicate other details about the file. So if you need to work
with the mode, you'll generally want to use the bitwise
operators covered later in this chapter.
</p><blockquote class="footnote"> <a name="FOOTNOTE-273" /><p>[273]The
first character in that string isn't a permission bit; it
indicates the type of entry: a hyphen for an ordinary file,
<tt class="literal">d</tt> for directory, or <tt class="literal">l</tt> for
symbolic link, among others. The <i class="command">ls </i>command
determines this from the other bits past the least-significant nine.
</p> </blockquote>
</dd>

</dl>

<dl>
<dt><b><tt class="literal">$nlink</tt><a name="INDEX-814" />
<a name="INDEX-815" /> </b></dt>
<dd>
The number of (hard) links to the file or directory. This is the
number of true names that the item has. This number is always
<tt class="literal">2</tt> or more for directories and (usually)
<tt class="literal">1</tt> for files. We'll see more about this when
we talk about creating links to files in <a href="ch13_01.htm">Chapter 13, "Manipulating Files and Directories"</a>. In the listing from <i class="command">ls -l</i>,
this is the number just after the permission-bits string.
</p>
</dd>

</dl>

<dl>
<dt><b><tt class="literal">$uid</tt><a name="INDEX-816" />
<a name="INDEX-817" />
<a name="INDEX-818" /> and<a name="INDEX-819" />
<a name="INDEX-820" /> <tt class="literal">$gid</tt></b></dt>
<dd>
The numeric user-ID and group-ID showing the file's ownership.</p>
</dd>

</dl>

<dl>
<dt><b><tt class="literal">$size</tt><a name="INDEX-821" />
<a name="INDEX-822" /></b></dt>
<dd>
The size in bytes, as returned by the <tt class="literal">-s</tt> file test.</p>
</dd>

</dl>

<dl>
<dt><b><tt class="literal">$atime</tt><a name="INDEX-823" />
<a name="INDEX-824" />
<a name="INDEX-825" />, <tt class="literal">$mtime</tt>, and <tt class="literal">$ctime</tt> </b></dt>
<dd>
The three