ch11_06.htm

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

<a name="INDEX-826" />timestamps, but
here they're represented in the system's timestamp
format: a 32-bit number telling how many seconds have passed since
the <em class="firstterm">Epoch</em>, an arbitrary starting point for
measuring <a name="INDEX-827" />system
time. On Unix systems and some others, the
<a name="INDEX-828" />Epoch is the beginning
of 1970 at midnight Universal Time, but the Epoch is different on
some machines. There's more information later in this chapter
on turning that timestamp number into something useful.
</p>
</dd>

</dl>

<p>Invoking <tt class="literal">stat</tt> on the name of a
<a name="INDEX-829" />symbolic link
returns information on what the symbolic link points at, not
information about the symbolic link itself (unless the link just
happens to be pointing at nothing currently accessible). If you need
the (mostly useless) information about the symbolic link itself, use
<tt class="literal">lstat</tt><a name="INDEX-830" /> rather than <tt class="literal">stat</tt>
(which returns the same information in the same order). If the
operand isn't a symbolic link, <tt class="literal">lstat</tt>
returns the same things that <tt class="literal">stat</tt> would.
</p>

<p>Like the file tests, the operand of <tt class="literal">stat</tt> or
<tt class="literal">lstat</tt> defaults to <tt class="literal">$_</tt>,
meaning that the underlying stat system call will be performed on the
file named by the scalar variable <tt class="literal">$_</tt>.
</p>

</div>
<a name="lperl3-CHP-11-SECT-6.2" /><div class="sect2">
<h3 class="sect2">11.6.2. The localtime Function</h3>

<p>When you have a <a name="INDEX-831" />
<a name="INDEX-832" />timestamp number (such as the ones
from <tt class="literal">stat</tt>), it will typically look something like
1080630098. That's not very useful for most humans, unless you
need to compare two timestamps by subtracting. You may need to
convert it to something human-readable, such as a string like
"<tt class="literal">Tue Mar 30 07:01:38 2004</tt>". Perl can
do that with the <tt class="literal">localtime</tt> function in a scalar
context:
</p>

<blockquote><pre class="code">my $timestamp = 1080630098;
my $date = localtime $timestamp;</pre></blockquote>

<p>In a list context, <tt class="literal">localtime</tt> returns a list of
numbers, several of which may not be quite what you'd expect:
</p>

<blockquote><pre class="code">my($sec, $min, $hour, $day, $mon, $year, $wday, $yday, $isdst)
  = localtime $timestamp;</pre></blockquote>

<p>The <tt class="literal">$mon</tt> is a month number, ranging from
<tt class="literal">0</tt> to <tt class="literal">11</tt>, which is handy as an
index into an array of month names. The <tt class="literal">$year</tt> is
the number of years since 1900, oddly enough, so add
<tt class="literal">1900</tt> to get the real year number. The
<tt class="literal">$wday</tt> ranges from <tt class="literal">0</tt> (for
Sunday) through <tt class="literal">6</tt> (for Saturday), and the
<tt class="literal">$yday</tt> is the day-of-the-year (ranging from 0 for
January 1, through 364 or 365 for December 31).
</p>

<p>There are two related functions that you'll also find useful.
The <tt class="literal">gmtime</tt><a name="INDEX-833" /> function is just the same as
<tt class="literal">localtime</tt>, except that it returns the time in
<a name="INDEX-834" />Universal
Time (what we once called <a name="INDEX-835" />Greenwich Mean Time). If you need the
current timestamp number from the system clock, just use the
<tt class="literal">time</tt><a name="INDEX-836" /> function. Both
<tt class="literal">localtime</tt> and <tt class="literal">gmtime</tt> default to
using the current <tt class="literal">time</tt> value if you don't
supply a parameter:
</p>

<blockquote><pre class="code">my $now = gmtime;  # Get the current universal timestamp as a string</pre></blockquote>

<p>For more information on manipulating date and time information, see
the information about some useful modules in <a href="appb_01.htm">Appendix B, "Beyond the Llama"</a>.
</p>

</div>
<a name="lperl3-CHP-11-SECT-6.3" /><div class="sect2">
<h3 class="sect2">11.6.3. Bitwise Operators</h3>

<p><a name="INDEX-837" />When you need to work with numbers
bit-by-bit, as when working with the mode bits returned by
<tt class="literal">stat</tt>, you'll need to use the bitwise
operators. The bitwise-and operator (<tt class="literal">&amp;</tt>)
reports which bits are set in the left argument
<em class="emphasis">and</em> in the right argument. For example, the
expression <tt class="literal">10 &amp; 12</tt> has the value
<tt class="literal">8</tt>. The bitwise-and needs to have a one-bit in both
operands to produce a one-bit in the result. That means that the
logical-and operation on ten (which is <tt class="literal">1010</tt> in
binary) and twelve (which is <tt class="literal">1100</tt>) gives eight
(which is <tt class="literal">1000</tt>, with a one-bit only where the left
operand has a one-bit <em class="emphasis">and</em> the right operand also
has a one-bit). See <a href="ch11_06.htm#lperl3-CHP-11-FIG-1">Figure 11-1</a>.
</p>

<a name="lperl3-CHP-11-FIG-1" /><div class="figure"><img height="88" alt="Figure 11-1" src="figs/lrnp_1101.gif" width="99" /></div><h4 class="objtitle">Figure 11-1. Bitwise-and addition</h4>

<p>The different bitwise operators and their meanings are shown in this
table:
</p>

<a name="ch11-9-fm2xml" /><table border="1">



<tr>
<th>
<p>Expression</p>
</th>
<th>
<p>Meaning</p>
</th>
</tr>


<tr>
<td>
<blockquote><pre class="code">10 &amp; 12</pre></blockquote>
</td>
<td>
<p>Bitwise-and -- which bits are true in both operands (this gives
<tt class="literal">8</tt>)
</p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">10 | 12</pre></blockquote>
</td>
<td>
<p>Bitwise-or -- which bits are true in one operand or the other
(this gives <tt class="literal">14</tt>)
</p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">10 ^ 12</pre></blockquote>
</td>
<td>
<p>Bitwise-xor -- which bits are true in one operand or the other but
not both (this gives <tt class="literal">6</tt>)
</p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">6 &lt;&lt; 2</pre></blockquote>
</td>
<td>
<p>Bitwise shift left -- shift the left operand the number of bits
shown by the right operand, adding zero-bits at the least-significant
places (this gives <tt class="literal">24</tt>)
</p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">25 &gt;&gt; 2</pre></blockquote>
</td>
<td>
<p>Bitwise shift right -- shift the left operand the number of bits
shown by the right operand, discarding the least-significant bits
(this gives <tt class="literal">6</tt>)
</p>
</td>
</tr>
<tr>
<td>
<blockquote><pre class="code">~ 10</pre></blockquote>
</td>
<td>
<p>Bitwise negation, also called unary bit complement -- return the
number with the opposite bit for each bit in the operand (this gives
<tt class="literal">0xFFFFFFF5</tt>, but see the text)
</p>
</td>
</tr>

</table><p>

<p>So, here's an example of some things you could do with the
<tt class="literal">$mode</tt> returned by <tt class="literal">stat</tt>. The
results of these bit manipulations could be useful with
<tt class="literal">chmod</tt>, which we'll see in <a href="ch13_01.htm">Chapter 13, "Manipulating Files and Directories"</a>: 
</p>

<blockquote><pre class="code"># $mode is the mode value returned from a stat of CONFIG
warn "Hey, the configuration file is world-writable!\n"
  if $mode &amp; 0002;                                # configuration security problem
my $classical_mode = 0777 &amp; $mode;                # mask off extra high-bits
my $u_plus_x = $classical_mode | 0100;            # turn one bit on
my $go_minus_r = $classical_mode &amp; (~ 0044);      # turn two bits off</pre></blockquote>

</div>
<a name="lperl3-CHP-11-SECT-6.4" /><div class="sect2">
<h3 class="sect2">11.6.4. Using Bitstrings</h3>

<p>All of the bitwise operators can work with
<a name="INDEX-838" />bitstrings, as
well as with integers. If the operands are integers, the result will
be an integer. (The integer will be at least a 32-bit integer, but
may be larger if your machine supports that. That is, if you have a
64-bit machine, <tt class="literal">~10</tt> may give the 64-bit result
<tt class="literal">0xFFFFFFFFFFFFFFF5</tt>, rather than the 32-bit result
<tt class="literal">0xFFFFFFF5</tt>.) 
</p>

<p>But if any operand of a bitwise operator is a string, Perl will
perform the operation on that bitstring. That is, <tt class="literal">"\xAA" |
"\x55"</tt> will give the string <tt class="literal">"\xFF"</tt>. Note
that these values are single-byte strings; the result is a byte with
all eight bits set. Bitstrings may be arbitrarily long.
</p>

<p>This is one of the very few places where Perl distinguishes between
strings and numbers. See the
<em class="emphasis">perlop</em><a name="INDEX-839" /> <a name="INDEX-840" />
manpage for more information on using bitwise operators on
strings.<a name="INDEX-841" />
</p>

</div>
<a name="lperl3-CHP-11-SECT-6.5" /><div class="sect2">
<h3 class="sect2">11.6.5. Using the Special Underscore Filehandle</h3>

<p>Every time you use <tt class="literal">stat</tt>,
<tt class="literal">lstat</tt>, or a file test in a program, Perl has to go
out to the system to ask for a <a name="INDEX-842" />
<a name="INDEX-843" />stat
buffer on the file (that is, the return buffer from the stat system
call). That means if you want to know whether a file is both readable
and writable, you've essentially asked the system twice for the
same information (which isn't likely to change in a fairly
nonhostile environment).
</p>

<p>This looks like a waste of time,<a href="#FOOTNOTE-274">[274]</a> and in fact, it can be avoided. Doing a file test,
<tt class="literal">stat</tt><a name="INDEX-844" />, or
<tt class="literal">lstat</tt><a name="INDEX-845" /> on the special
<tt class="literal">_</tt><a name="INDEX-846" /> <a name="INDEX-847" />
<a name="INDEX-848" />
filehandle (that is, the operand is nothing but a single underscore)
tells Perl to use whatever happened to be lounging around in memory
from the previous file test, <tt class="literal">stat</tt>, or
<tt class="literal">lstat</tt> function, rather than going out to the
operating system again. Sometimes this is dangerous: a subroutine
call can invoke <tt class="literal">stat</tt> without your knowledge,
blowing your buffer away. But if you're careful, you can save
yourself a few unneeded system calls, thereby making your program
considerably faster. Here's that example of finding files to
put on the backup tapes again, using the new tricks we've
learned:
</p><blockquote class="footnote"> <a name="FOOTNOTE-274" /><p>[274]Because it is.
Asking the system for information is relatively slow.</p>
</blockquote>

<blockquote><pre class="code">my @original_files = qw/ fred barney betty wilma pebbles dino bamm-bamm /;
my @big_old_files;                       # The ones we want to put on backup tapes
foreach (@original_files) {
  push @big_old_files, $_
    if (-s) &gt; 100_000 and -A _ &gt; 90;     # More efficient than before
}</pre></blockquote>

<p>Note that we used the default of <tt class="literal">$_</tt> for the first
test -- this is no more efficient (except perhaps for the
programmer), but it gets the data from the operating system. The
second test uses the magic <tt class="literal">_</tt> filehandle; for this
test, the data left around after getting the file's size is
used, which is exactly what we want.
</p>

<p>Note that testing the <tt class="literal">_</tt> filehandle is not the same
as allowing the operand of a file test, <tt class="literal">stat</tt>, or
<tt class="literal">lstat</tt> to default to testing
<tt class="literal">$_</tt>; using <tt class="literal">$_</tt> would be a fresh
test each time on the current file named by the contents of
<tt class="literal">$_</tt>, but using <tt class="literal">_</tt> saves the
trouble of calling the system again. Here is another case where
similar names were chosen for radically different functions. By now,
you are probably used to<a name="INDEX-849" /> it.<a name="INDEX-850" />
</p>

</div>


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