ch03_10.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

otherwise documented, it returns <tt class="literal">1</tt> for true and
<tt class="literal">""</tt> for false, or the undefined value if the file
doesn't exist or is otherwise inaccessible. Currently implemented file
test operators are listed in <a href="ch03_10.htm#perl3-tab-filetest">Table 3-4</a>.</p>

<a name="perl3-tab-filetest"></a>
<h4 class="objtitle">Table 3.4. File Test Operators</h4>
<table border="1">

<tr>
<th>Operator</th>
<th>Meaning</th>
</tr>




<tr>
<td><tt class="literal">-r</tt></td>
<td>File is readable by effective UID/GID.</td>
</tr>

<tr>
<td><tt class="literal">-w</tt></td>
<td>File is writable by effective UID/GID.</td>
</tr>

<tr>
<td><tt class="literal">-x</tt></td>
<td>File is executable by effective UID/GID.</td>
</tr>

<tr>
<td><tt class="literal">-o</tt></td>
<td>File is owned by effective UID.</td>
</tr>

<tr>
<td><tt class="literal">-R</tt></td>
<td>File is readable by real UID/GID.</td>
</tr>

<tr>
<td><tt class="literal">-W</tt></td>
<td>File is writable by real UID/GID.</td>
</tr>

<tr>
<td><tt class="literal">-X</tt></td>
<td>File is executable by real UID/GID.</td>
</tr>

<tr>
<td><tt class="literal">-O</tt></td>
<td>File is owned by real UID.</td>
</tr>

<tr>
<td><tt class="literal">-e</tt></td>
<td>File exists.</td>
</tr>

<tr>
<td><tt class="literal">-z</tt></td>
<td>File has zero size.</td>
</tr>

<tr>
<td><tt class="literal">-s</tt></td>
<td>File has nonzero size (returns size).</td>
</tr>

<tr>
<td><tt class="literal">-f</tt></td>
<td>File is a plain file.</td>
</tr>

<tr>
<td><tt class="literal">-d</tt></td>
<td>File is a directory.</td>
</tr>

<tr>
<td><tt class="literal">-l</tt></td>
<td>File is a symbolic link.</td>
</tr>

<tr>
<td><tt class="literal">-p</tt></td>
<td>File is a named pipe (FIFO).</td>
</tr>

<tr>
<td><tt class="literal">-S</tt></td>
<td>File is a socket.</td>
</tr>

<tr>
<td><tt class="literal">-b</tt></td>
<td>File is a block special file.</td>
</tr>

<tr>
<td><tt class="literal">-c</tt></td>
<td>File is a character special file.</td>
</tr>

<tr>
<td><tt class="literal">-t</tt></td>
<td>Filehandle is opened to a tty.</td>
</tr>

<tr>
<td><tt class="literal">-u</tt></td>
<td>File has setuid bit set.</td>
</tr>

<tr>
<td><tt class="literal">-g</tt></td>
<td>File has setgid bit set.</td>
</tr>

<tr>
<td><tt class="literal">-k</tt></td>
<td>File has sticky bit set.</td>
</tr>

<tr>
<td><tt class="literal">-T</tt></td>
<td>File is a text file.</td>
</tr>

<tr>
<td><tt class="literal">-B</tt></td>
<td>File is a binary file (opposite of <tt class="literal">-T</tt>).</td>
</tr>

<tr>
<td><tt class="literal">-M</tt></td>
<td>Age of file (at startup) in days since modification.</td>
</tr>

<tr>
<td><tt class="literal">-A</tt></td>
<td>Age of file (at startup) in days since last access.</td>
</tr>

<tr>
<td><tt class="literal">-C</tt></td>
<td>Age of file (at startup) in days since inode change.</td>
</tr>


</table>

<p>Note that <tt class="literal">-s/a/b/</tt> does not do a negated substitution.  Saying
<tt class="literal">-exp($foo)</tt> still works as expected, however--only single letters
following a minus are interpreted as file tests.</p>

<p>
<a name="INDEX-915"></a><a name="INDEX-916"></a>
The interpretation of the file permission operators
<tt class="literal">-r</tt>, <tt class="literal">-R</tt>, <tt class="literal">-w</tt>,
<tt class="literal">-W</tt>, <tt class="literal">-x</tt>, and
<tt class="literal">-X</tt> is based solely on the mode of the file and the
user and group IDs of the user.  There may be other reasons you can't
actually read, write, or execute the file, such as Andrew File System
(AFS) access control lists.<a href="#FOOTNOTE-3">[3]</a>
Also note
that for the superuser, <tt class="literal">-r</tt>, <tt class="literal">-R</tt>,
<tt class="literal">-w</tt>, and <tt class="literal">-W</tt> always return 1, and
<tt class="literal">-x</tt> and <tt class="literal">-X</tt> return 1 if any
execute bit is set in the mode.  Thus, scripts run by the superuser
may need to do a <tt class="literal">stat</tt> in order to determine the
actual mode of the file or temporarily set the UID to something else.</p>
<blockquote class="footnote">

<a name="FOOTNOTE-3"></a>
<p>[3]You may, however, override
the built-in semantics with the <tt class="literal">use filetest</tt>
pragma. See <a href="ch31_01.htm">Chapter 31, "Pragmatic Modules"</a>.</p>

</blockquote>

<p>The other file test operators don't care who you are.  Anybody can use
the test for "regular" files:
<blockquote>
<pre class="programlisting">while (&lt;&gt;) {
    chomp;
    next unless -f $_;      # ignore "special" files
    ...
}</pre>
</blockquote>
<a name="INDEX-917"></a><a name="INDEX-918"></a>
</p>

<p>The <tt class="literal">-T</tt> and <tt class="literal">-B</tt> switches work as
follows.  The first block or so of the file is examined for strange
characters such as control codes or bytes with the high bit set (that
don't look like UTF-8).  If more than a third of the bytes appear to
be strange, it's a binary file; otherwise, it's a text file.  Also,
any file containing ASCII NUL (<tt class="literal">\0</tt>) in the first
block is considered a binary file.  If <tt class="literal">-T</tt> or
<tt class="literal">-B</tt> is used on a filehandle, the current input
(standard I/O or "stdio") buffer is examined rather than the first
block of the file.  Both <tt class="literal">-T</tt> and
<tt class="literal">-B</tt> return true on an empty file, or on a file at
EOF (end-of-file) when testing a filehandle.  Because Perl has to read
a file to do the <tt class="literal">-T</tt> test, you don't want to use
<tt class="literal">-T</tt> on special files that might hang or give you
other kinds of grief.  So on most occasions you'll want to test with a
<tt class="literal">-f</tt> first, as in:
<blockquote>
<pre class="programlisting">next unless -f $file &amp;&amp; -T $file;</pre>
</blockquote>

If any of the file tests (or either the <tt class="literal">stat</tt> or <tt class="literal">lstat</tt> operator)
are given the special filehandle consisting of a solitary underline,
then the <em class="emphasis">stat</em> structure of the previous file test (or <tt class="literal">stat</tt>
operator) is used, thereby saving a system call.  (This doesn't work
with <tt class="literal">-t</tt>, and you need to remember that <tt class="literal">lstat</tt> and <tt class="literal">-l</tt> will leave
values in the <em class="emphasis">stat</em> structure for the symbolic link, not the real
file.  Likewise, <tt class="literal">-l _</tt> will always be false after a normal
<tt class="literal">stat</tt>.)</p>

<p>Here are a couple of examples:
<blockquote>
<pre class="programlisting">print "Can do.\n" if -r $a || -w _ || -x _;

stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;

print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;</pre>
</blockquote>
<a name="INDEX-919"></a><a name="INDEX-920"></a><a name="INDEX-921"></a><a name="INDEX-922"></a><a name="INDEX-923"></a><a name="INDEX-924"></a><a name="INDEX-925"></a><a name="INDEX-926"></a>
</p>

<p>File ages for <tt class="literal">-M</tt>, <tt class="literal">-A</tt>, and
<tt class="literal">-C</tt> are returned in days (including fractional days)
since the script started running. This time is stored in the special
variable <tt class="literal">$^T</tt> (<tt class="literal">$BASETIME</tt>).  Thus,
if the file changed after the script started, you would get a negative
time. Note that most time values (86,399 out of 86,400, on average)
are fractional, so testing for equality with an integer without using
the <tt class="literal">int</tt> function is usually futile.  Examples:
<blockquote>
<pre class="programlisting">next unless -M $file &gt; .5;      # files are older than 12 hours
&amp;newfile if -M $file &lt; 0;       # file is newer than process
&amp;mailwarning if int(-A) == 90;  # file ($_) was accessed 90 days ago today</pre>
</blockquote>

To reset the script's start time to the current time, say this:
<blockquote>
<pre class="programlisting">$^T = time;</pre>
</blockquote>
</p>

<a name="INDEX-927"></a>


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