ch28_02.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

handle under the <span class="option">-i</span> switch or the <tt class="literal">$^I</tt> variable.  See the
<span class="option">-i</span> switch in <a href="ch19_01.htm">Chapter 19, "The Command-Line Interface"</a>.</p>
</dd>


<dt>
<b><tt class="literal">$b</tt></b>
</dt>
<dd>
<p>[PKG] The variable, companion to <tt class="literal">$a</tt>, used in <tt class="literal">sort</tt>
comparisons.  See <tt class="literal">$a</tt> and the <tt class="literal">sort</tt> function for details.</p>
</dd>


<dt>
<b><tt class="literal">$BASETIME</tt></b>
</dt>
<dt>
<b><tt class="literal">$^T</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4414"></a><a name="INDEX-4415"></a><a name="INDEX-4416"></a><a name="INDEX-4417"></a>
[ALL] The time at which the script began running, in seconds since the epoch
(the beginning of 1970, for Unix systems).  The values returned by the
<tt class="literal">-M</tt>, <tt class="literal">-A</tt>, and <tt class="literal">-C</tt> file tests are relative to this moment.</p>
</dd>


<dt>
<b><tt class="literal">$CHILD_ERROR</tt></b>
</dt>
<dt>
<b><tt class="literal">$?</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4418"></a><a name="INDEX-4419"></a><a name="INDEX-4420"></a><a name="INDEX-4421"></a><a name="INDEX-4422"></a><a name="INDEX-4423"></a>
[ALL] The status returned by the last pipe close, backtick
(<tt class="literal">``</tt>) command, or <tt class="literal">wait</tt>,
<tt class="literal">waitpid</tt>, or <tt class="literal">system</tt> functions.
Note that this is not just the simple exit code, but the entire 16-bit
status word returned by the underlying
<em class="emphasis">wait</em>(2) or
<em class="emphasis">waitpid</em>(2) syscall (or equivalent).  Thus,
the exit value of the subprocess is in the high byte, that is,
<tt class="literal">$? &gt;&gt; 8</tt>; in the low byte, <tt class="literal">$? &amp;
127</tt> says which signal (if any) the process died from, while
<tt class="literal">$? &amp; 128</tt> reports whether its demise produced a
core dump.  (Mnemonic: similar to <tt class="literal">$?</tt> in the
<em class="emphasis">sh</em> and its offspring.)</p>

<p>Inside an <tt class="literal">END</tt> block, <tt class="literal">$?</tt> contains the value that is going to be
given to <tt class="literal">exit</tt>.  You can modify <tt class="literal">$?</tt> in an <tt class="literal">END</tt> to
change the exit status of the script.</p>

<p>Under VMS, the pragma <tt class="literal">use vmsish 'status'</tt> makes <tt class="literal">$?</tt> reflect
the true VMS exit status, instead of the default emulation of POSIX
status.</p>

<p>If the <tt class="literal">h_errno</tt> variable is supported in C, its numeric value is
returned via <tt class="literal">$?</tt> if any of the <tt class="literal">gethost*()</tt> functions fail.</p>
</dd>


<dt>
<b><tt class="literal">$COMPILING</tt></b>
</dt>
<dt>
<b><tt class="literal">$^C</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4424"></a><a name="INDEX-4425"></a><a name="INDEX-4426"></a><a name="INDEX-4427"></a>
[ALL] The current value of the internal flag associated with the
<span class="option">-c</span> switch, mainly of use with
<span class="option">-MO</span> and the
<em class="emphasis">perlcc</em>(1) tool to let code alter its
behavior when being compiled for code generation.  For example, you
might want to <tt class="literal">AUTOLOAD</tt> at compile time instead of
using the normal, deferred loading so that code can be generated right
away.  See <a href="ch18_01.htm">Chapter 18, "Compiling"</a>.</p>
</dd>


<dt>
<b><tt class="literal">DATA</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4428"></a><a name="INDEX-4429"></a><a name="INDEX-4430"></a>
[PKG] This special filehandle refers to anything following
either the <tt class="literal">__END__</tt> token or the <tt class="literal">__DATA__</tt> token in the current file.
The <tt class="literal">__END__</tt> token always opens the <tt class="literal">main::DATA</tt> filehandle, and so is
used in the main program.  The <tt class="literal">__DATA__</tt> token opens the <tt class="literal">DATA</tt> handle
in whichever package is in effect at the time, so different modules can
each have their own <tt class="literal">DATA</tt> filehandle, since they (presumably) have
different package names.</p>
</dd>


<dt>
<b><tt class="literal">$DEBUGGING</tt></b>
</dt>
<dt>
<b><tt class="literal">$^D</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4431"></a><a name="INDEX-4432"></a><a name="INDEX-4433"></a>
[ALL] The current value of the internal debugging flags, set from the
<span class="option">-D</span> switch on the command line; see <a href="ch19_01.htm">Chapter 19, "The Command-Line Interface"</a> for the bit values.  (Mnemonic:  value of the <span class="option">-D</span> switch.)</p>
</dd>


<dt>
<b><tt class="literal">$EFFECTIVE_GROUP_ID</tt></b>
</dt>
<dt>
<b><tt class="literal">$EGID</tt></b>
</dt>
<dt>
<b><tt class="literal">$)</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4434"></a><a name="INDEX-4435"></a><a name="INDEX-4436"></a><a name="INDEX-4437"></a>
[ALL] The effective GID (group ID) of this process.  If you are on a
machine that supports membership in multiple groups simultaneously,
<tt class="literal">$)</tt> gives a space-separated list of groups you are
in.  The first number is the one returned by
<em class="emphasis">getegid</em>(2), and the subsequent ones by
<em class="emphasis">getgroups</em>(2), one of which may be the same
as the first number.</p>

<p>Similarly, a value assigned to <tt class="literal">$)</tt> must also be a
space-separated list of numbers.  The first number is used to set the
effective GID, and the rest (if any) are passed to the
<em class="emphasis">setgroups</em>(2) syscall.  To get the effect
of an empty list for <tt class="literal">setgroups</tt>, just repeat the new
effective GID; for example,

to force an effective GID of 5 and an
effectively empty <tt class="literal">setgroups</tt> list, say:
<blockquote>
<pre class="programlisting">$) = "5 5";</pre>
</blockquote>
</p>

<p>(Mnemonic: parentheses are used to <em class="emphasis">group</em> things.  The effective
GID is the group that's <em class="emphasis">right</em> for you, if you're running setgid.)  Note: <tt class="literal">$&lt;</tt>, <tt class="literal">$&gt;</tt>, <tt class="literal">$(</tt>, and <tt class="literal">$)</tt> can only be set on
machines that support the corresponding system set-id routine.
<tt class="literal">$(</tt> and <tt class="literal">$)</tt> can be swapped only on machines supporting
<em class="emphasis">setregid</em>(2).</p>
</dd>


<dt>
<b><tt class="literal">$EFFECTIVE_USER_ID</tt></b>
</dt>
<dt>
<b><tt class="literal">$EUID</tt></b>
</dt>
<dt>
<b><tt class="literal">$&gt;</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4438"></a><a name="INDEX-4439"></a><a name="INDEX-4440"></a><a name="INDEX-4441"></a><a name="INDEX-4442"></a>
[ALL] The effective UID of this process as returned by the
<em class="emphasis">geteuid</em>(2) syscall.  Example:
<blockquote>
<pre class="programlisting">$&lt; = $&gt;;            # set real to effective uid
($&lt;,$&gt;) = ($&gt;,$&lt;);  # swap real and effective uid</pre>
</blockquote>

(Mnemonic: it's the UID you went <em class="emphasis">to</em>, if you're
running setuid.)  Note: <tt class="literal">$&lt;</tt> and
<tt class="literal">$&gt;</tt> can only be swapped on machines supporting
<em class="emphasis">setreuid</em>(2).  And sometimes not even then.</p>
</dd>


<dt>
<b><tt class="literal">%ENV</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4443"></a><a name="INDEX-4444"></a><a name="INDEX-4445"></a>
[ALL] The hash containing your current environment variables.  Setting
a value in <tt class="literal">%ENV</tt> changes the environment for both your process and child processes launched after the assignment.  (It cannot change
a parent process's environment on any system resembling Unix.)
<blockquote>
<pre class="programlisting">$ENV{PATH}  = "/bin:/usr/bin";
$ENV{PAGER} = "less";
$ENV{LESS}  = "MQeicsnf";  # our favorite switches to less(1)
system "man perl";         # picks up new settings</pre>
</blockquote>

To remove something from your environment, make sure to use the <tt class="literal">delete</tt>
function instead of <tt class="literal">undef</tt> on the hash value.</p>

<p>Note that processes running as <em class="emphasis">crontab</em>(5)
entries inherit a particularly impoverished set of environment
variables. (If your program runs fine from the command line but not
under <em class="emphasis">cron</em>, this is probably why.)  Also note that
you should set <tt class="literal">$ENV{PATH}</tt>,
<tt class="literal">$ENV{SHELL}</tt>, <tt class="literal">$ENV{BASH_ENV}</tt>, and
<tt class="literal">$ENV{IFS}</tt> if you are running as a setuid script.
See <a href="ch23_01.htm">Chapter 23, "Security"</a>.</p>
</dd>


<dt>
<b><tt class="literal">$EVAL_ERROR</tt></b>
</dt>
<dt>
<b><tt class="literal">$@</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4446"></a><a name="INDEX-4447"></a><a name="INDEX-4448"></a><a name="INDEX-4449"></a>
[ALL] The currently raised exception or the Perl syntax error message
from the last <tt class="literal">eval</tt> operation. (Mnemonic: where was
the syntax error "at"?)  Unlike <tt class="literal">$!</tt>
(<tt class="literal">$OS_ERROR</tt>), which is set on failure but not cleared
on success, <tt class="literal">$@</tt> is guaranteed to be set (to a true
value) if the last <tt class="literal">eval</tt> had a compilation error or
run-time exception, and guaranteed to be cleared (to a false value) if
no such problem occurred.</p>

<p>Warning messages are not collected in this variable.  You
can, however, set up a routine to process warnings by setting
<tt class="literal">$SIG{__WARN__}</tt> as described later in this section.</p>

<p>Note that the value of <tt class="literal">$@</tt> may be an exception object rather than a string.
If so, you can still probably treat it as a string if the exception object
has stringification overloading defined for its class.  If you propagate
an exception by saying:
<blockquote>
<pre class="programlisting">die if $@;</pre>
</blockquote>

then an exception object will call <tt class="literal">$@-&gt;PROPAGATE</tt> to see what to do.
(A string exception merely adds a "propagated at" line to the string.)</p>
</dd>


<dt>
<b><tt class="literal">$EXCEPTIONS_BEING_CAUGHT</tt></b>
</dt>
<dt>
<b><tt class="literal">$^S</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4450"></a><a name="INDEX-4451"></a>
[ALL] This variable reflects the current state of the interpreter,
returning true if inside an <tt class="literal">eval</tt>, false otherwise.  It's undefined if
parsing of the current compilation unit hasn't finished yet, which may
be the case in <tt class="literal">$SIG{__DIE__}</tt> and <tt class="literal">$SIG{__WARN__}</tt> handlers.  (Mnemonic: state of <tt class="literal">eval</tt>.)</p>
</dd>


<dt>
<b><tt class="literal">$EXECUTABLE_NAME</tt></b>
</dt>
<dt>
<b><tt class="literal">$^X</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4452"></a><a name="INDEX-4453"></a><a name="INDEX-4454"></a>
[ALL] The name that the <em class="emphasis">perl</em> binary itself was executed as, from C's
<tt class="literal">argv[0]</tt>.</p>
</dd>


<dt>
<b><tt class="literal">@EXPORT</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4455"></a>
[PKG] This array variable is consulted by the
<tt class="literal">Exporter</tt> module's <tt class="literal">import</tt> method
to find the list of other package variables and subroutines to be
exported by default when the module is <tt class="literal">use</tt>d, or
when the <tt class="literal">:DEFAULT</tt> import tag is used.  It is not
exempt from <tt class="literal">use strict</tt> complaints, so it must be
declared with <tt class="literal">our</tt> or fully qualified by package
name if you've enabled that pragma.  However, all variables whose
names begin with the string "<tt class="literal">EXPORT</tt>" are exempt
from warnings about being used only once.  See <a href="ch11_01.htm">Chapter 11, "Modules"</a>.
</p>
</dd>


<dt>
<b><tt class="literal">@EXPORT_OK</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4456"></a>
[PKG] This array variable is consulted by the <tt class="literal">Exporter</tt>
module's <tt class="literal">import</tt> method to determine whether a requested import
is legal.  It is not exempt from <tt class="literal">use strict</tt>.  See <a href="ch11_01.htm">Chapter 11, "Modules"</a>.</p>
</dd>


<dt>
<b><tt class="literal">%EXPORT_TAGS</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4457"></a>
[PKG] This hash variable is consulted by the <tt class="literal">Exporter</tt>
module's <tt class="literal">import</tt> method when an import symbol with a leading colon
is requested, as in <tt class="literal">use POSIX ":sys_wait_h"</tt>.  The keys are the
colon tags, but without the loading colon.  The values should be
references to arrays containing symbols to import when the
colon tag is requested, all of which must also appear in either
<tt class="literal">@EXPORT</tt> or <tt class="literal">@EXPORT_OK</tt>.  It is not exempt from <tt class="literal">use strict</tt>.
See <a href="ch11_01.htm">Chapter 11, "Modules"</a>.</p>
</dd>


<dt>
<b><tt class="literal">$EXTENDED_OS_ERROR</tt></b>
</dt>
<dt>
<b><tt class="literal">$^E</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4458"></a><a name="INDEX-4459"></a>
</p>

<p>[ALL] Error information specific to the current operating system.  Under
Unix, <tt class="literal">$^E</tt> is identical to <tt class="literal">$!</tt> (<tt class="literal">$OS_ERROR</tt>), but it differs under OS/2, VMS,
and Microsoft systems, and on MacPerl.  See your port's information for
specifics.  Caveats mentioned in the description of <tt class="literal">$!</tt> generally
apply to <tt class="literal">$^E</tt> as well.  (Mnemonic: extra error explanation.)</p>
</dd>


<dt>
<b><tt class="literal">@F</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4460"></a><a name="INDEX-4461"></a><a name="INDEX-4462"></a><a name="INDEX-4463"></a>
[PKG] The array into which the input line's fields are split when the
<span class="option">-a</span>