ch24_01.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

You can't take the address of anything, although a similar operator in
Perl is the backslash, which creates a reference.</p>
</li>
<li>
<p>
<tt class="literal">ARGV</tt> must be
capitalized. <tt class="literal">$ARGV[0]</tt> is C's
<tt class="literal">argv[1]</tt>, and C's <tt class="literal">argv[0]</tt> ends up
in <tt class="literal">$0</tt>.
<a name="INDEX-4146"></a>
</p>
</li>
<li>
<p>
<a name="INDEX-4147"></a>
Syscalls such as <tt class="literal">link</tt>, <tt class="literal">unlink</tt>,
and <tt class="literal">rename</tt> return true for success, not
<tt class="literal">0</tt>.</p>
</li>
<li>
<p>
<a name="INDEX-4148"></a>
The signal handlers in <tt class="literal">%SIG</tt> deal with signal names,
not numbers.</p>
</li>
</ul>






<h3 class="sect2">24.1.4. Shell Traps</h3>

<p>
<a name="INDEX-4149"></a><a name="INDEX-4150"></a>
Sharp shell programmers should take note of the following:</p>

<ul>
<li>
<p>
<a name="INDEX-4151"></a>
Variables are prefixed with <tt class="literal">$</tt>,
<tt class="literal">@</tt>, or <tt class="literal">%</tt> on the left side of the
assignment as well as the right.  A shellish assignment like:
<blockquote>
<pre class="programlisting">camel='dromedary';      # WRONG</pre>
</blockquote>

won't be parsed the way you expect.  You need:
<blockquote>
<pre class="programlisting">$camel='dromedary';     # ok</pre>
</blockquote>
</p>
</li>
<li>
<p>
<a name="INDEX-4152"></a>
The loop variable of a <tt class="literal">foreach</tt> also requires a <tt class="literal">$</tt>. Although <em class="emphasis">csh</em>
likes:
<blockquote>
<pre class="programlisting">foreach hump (one two)
    stuff_it $hump
end</pre>
</blockquote>

in Perl, this is written as:
<blockquote>
<pre class="programlisting">foreach $hump ("one", "two") {
    stuff_it($hump);
}</pre>
</blockquote>
</p>
</li>
<li>
<p>
<a name="INDEX-4153"></a>
The backtick operator does variable interpolation without regard to the
presence of single quotes in the command.</p>
</li>
<li>
<p>The backtick operator does no translation of the return value. In Perl,
you have to trim the newline explicitly, like this:
<blockquote>
<pre class="programlisting">chomp($thishost = `hostname`);</pre>
</blockquote>
</p>
</li>
<li>
<p>
<a name="INDEX-4154"></a>
Shells (especially <em class="emphasis">csh</em>) do several levels of substitution on each
command line.  Perl does interpolation only within certain constructs
such as double quotes, backticks, angle brackets, and search patterns.</p>
</li>
<li>
<p>Shells tend to interpret scripts a little bit at a time.  Perl compiles
the entire program before executing it (except for <tt class="literal">BEGIN</tt> blocks,
which execute before the compilation is done).
<a name="INDEX-4155"></a>
</p>
</li>
<li>
<p>
<a name="INDEX-4156"></a>
Program arguments are available via <tt class="literal">@ARGV</tt>, not
<tt class="literal">$1</tt>, <tt class="literal">$2</tt>, and so on.</p>
</li>
<li>
<p>The environment is not automatically made available as individual scalar
variables.  Use the standard <tt class="literal">Env</tt> module if you want that to happen.
<a name="INDEX-4157"></a>
</p>
</li>
</ul>






<h3 class="sect2">24.1.5. Previous Perl Traps</h3>

<p>
<a name="INDEX-4158"></a><a name="INDEX-4159"></a>
Penitent Perl 4 (and Prior) Programmers should take note of the
following changes between release 4 and release 5 that might affect old
scripts:</p>

<ul>
<li>
<p>
<a name="INDEX-4160"></a><tt class="literal">@</tt> now always interpolates an array in double-quotish strings. Some
programs may now need to use backslashes to protect any <tt class="literal">@</tt> that
shouldn't interpolate.</p>
</li>
<li>
<p>
<a name="INDEX-4161"></a><a name="INDEX-4162"></a>
Barewords that used to look like strings to Perl will now look like
subroutine calls if a subroutine by that name is defined before the
compiler sees them.  For example:
<blockquote>
<pre class="programlisting">sub SeeYa { die "Hasta la vista, baby!" }
$SIG{'QUIT'} = SeeYa;</pre>
</blockquote>

In prior versions of Perl, that code would set the signal handler.  Now,
it actually calls the function!  You may use the <span class="option">-w</span> switch to find
such risky usage or <tt class="literal">use strict</tt> to outlaw it.</p>
</li>
<li>
<p>
<a name="INDEX-4163"></a><a name="INDEX-4164"></a>
Identifiers starting with "<tt class="literal">_</tt>" are no longer forced
into package <tt class="literal">main</tt>, except for the bare underscore
itself (as in <tt class="literal">$_</tt>, <tt class="literal">@_</tt>, and so on).</p>
</li>
<li>
<p>
<a name="INDEX-4165"></a>
A double colon is now a valid package separator in an identifier.  Thus,
the statement:
<blockquote>
<pre class="programlisting">print "$a::$b::$c\n";</pre>
</blockquote>

now parses <tt class="literal">$a::</tt> as the variable reference, where in
prior versions only the <tt class="literal">$a</tt> was considered to be the
variable reference.  Similarly:
<blockquote>
<pre class="programlisting">print "$var::abc::xyz\n";</pre>
</blockquote>

is now interpreted as a single variable
<tt class="literal">$var::abc::xyz</tt>, whereas in prior versions, the
variable <tt class="literal">$var</tt> would have been followed by the
constant text <tt class="literal">::abc::xyz</tt>.</p>
</li>
<li>
<p>
<tt class="literal">s'$pattern'replacement'</tt> now performs no interpolation on
<tt class="literal">$pattern</tt>.  (The <tt class="literal">$</tt> would be
interpreted as an end-of-line assertion.) This behavior occurs only
when using single quotes as the substitution delimiter; in other
substitutions, <tt class="literal">$pattern</tt> is always interpolated.
<a name="INDEX-4166"></a>
</p>
</li>
<li>
<p>
<a name="INDEX-4167"></a>
The second and third arguments of <tt class="literal">splice</tt> are now
evaluated in scalar context rather than in list context.</p>
</li>
<li>
<p>
<a name="INDEX-4168"></a>
These are now semantic errors because of precedence:
<blockquote>
<pre class="programlisting">shift @list + 20;    # Now parses like shift(@list + 20), illegal!
$n = keys %map + 20; # Now parses like keys(%map + 20), illegal!</pre>
</blockquote>

Because if those were to work, then this couldn't:
<blockquote>
<pre class="programlisting">sleep $dormancy + 20;</pre>
</blockquote>
</p>
</li>
<li>
<p>
<a name="INDEX-4169"></a>
The precedence of assignment operators is now the same as the precedence
of assignment.  Previous versions of Perl mistakenly gave them the
precedence of the associated operator.  So you now must parenthesize
them in expressions like:
<blockquote>
<pre class="programlisting">/foo/ ? ($a += 2) : ($a -= 2);</pre>
</blockquote>

Otherwise:
<blockquote>
<pre class="programlisting">/foo/ ? $a += 2 : $a -= 2;</pre>
</blockquote>

would be erroneously parsed as:
<blockquote>
<pre class="programlisting">(/foo/ ? $a += 2 : $a) -= 2;</pre>
</blockquote>

On the other hand:
<blockquote>
<pre class="programlisting">$a += /foo/ ? 1 : 2;</pre>
</blockquote>

now works as a C programmer would expect.</p>
</li>
<li>
<p>
<tt class="literal">open FOO || die</tt> is incorrect.  You need parentheses around the
filehandle, because <tt class="literal">open</tt> has the precedence of a list operator.</p>
</li>
<li>
<p>
<a name="INDEX-4170"></a>
The elements of argument lists for formats are now evaluated in list
context.  This means you can interpolate list values now.</p>
</li>
<li>
<p>You can't do a <tt class="literal">goto</tt> into a block that is optimized away.  Darn.</p>
</li>
<a name="INDEX-4171"></a>
<li>
<p>It is no longer legal to use whitespace as the name of a variable or
as a delimiter for any kind of quote construct. Double darn.
<a name="INDEX-4172"></a>
</p>
</li>
<li>
<p>
<a name="INDEX-4173"></a>
The <tt class="literal">caller</tt> function now returns a false value in scalar context
if there is no caller.  This lets modules determine whether they're
being required or run directly.</p>
</li>
<li>
<p>
<tt class="literal">m//g</tt> now attaches its state to the searched string rather than the
regular expression.  See <a href="ch05_01.htm">Chapter 5, "Pattern Matching"</a>, for further
details.
<a name="INDEX-4174"></a>
</p>
</li>
<li>
<p>
<tt class="literal">reverse</tt> is no longer allowed as the name of a <tt class="literal">sort</tt> subroutine.
<a name="INDEX-4175"></a><a name="INDEX-4176"></a>
</p>
</li>
<li>
<p>
<a name="INDEX-4177"></a><em class="emphasis">taintperl</em> is no longer a separate executable.   There is now a <span class="option">-T</span>
switch to turn on tainting when it isn't turned on automatically.</p>
</li>
<li>
<p>
<a name="INDEX-4178"></a>
Double-quoted strings may no longer end with an unescaped <tt class="literal">$</tt> or <tt class="literal">@</tt>.</p>
</li>
<li>
<p>The archaic <tt class="literal">if</tt><em class="replaceable">BLOCK BLOCK</em> syntax is no longer supported.</p>
</li>
<li>
<p>Negative array subscripts now count from the end of the array.
<a name="INDEX-4179"></a><a name="INDEX-4180"></a>
</p>
</li>
<li>
<p>The comma operator in a scalar context is now guaranteed to give a
scalar context to its arguments.
<a name="INDEX-4181"></a><a name="INDEX-4182"></a>
</p>
</li>
<li>
<p>The <tt class="literal">**</tt> operator now binds more tightly than unary minus.</p>
</li>
<li>
<p>Setting <tt class="literal">$#array</tt> lower now discards array elements immediately.</p>
</li>
<li>
<p>
<tt class="literal">delete</tt> is not guaranteed to return the deleted value for <tt class="literal">tie</tt>d
arrays, since this capability may be onerous for some modules to
implement.
<a name="INDEX-4183"></a><a name="INDEX-4184"></a>
</p>
</li>
<li>
<p>The construct <tt class="literal">"this is $$x"</tt>, which used to interpolate the process
ID at that point, now tries to dereference <tt class="literal">$x</tt>.  <tt class="literal">$$</tt> by itself
still works fine, however.</p>
</li>
<li>
<p>
<a name="INDEX-4185"></a>
The behavior of <tt class="literal">foreach</tt> when it iterates over a
list that is not an array has changed slightly.  It used to assign the
list to a temporary array but now, for efficiency, no longer does so.
This means that you'll now be iterating over the actual values, not
copies of the values.  Modifications to the loop variable can change
the original values, even after the <tt class="literal">grep</tt>!
For instance:
<blockquote>
<pre class="programlisting">% <tt class="userinput"><b>perl4 -e '@a = (1,2,3); for (grep(/./, @a)) { $_++ }; print "@a\n"'</b></tt>
1 2 3
% <tt class="userinput"><b>perl5 -e '@a = (1,2,3); for (grep(/./, @a)) { $_++ }; print "@a\n"'</b></tt>
2 3 4</pre>
</blockquote>

To retain prior Perl semantics, you'd need to explicitly assign your list to a temporary array and then iterate over that.  For
example, you might need to change:
<blockquote>
<pre class="programlisting">foreach $var (grep /x/, @list) { ... }</pre>
</blockquote>

to:
<blockquote>
<pre class="programlisting">foreach $var (my @tmp = grep /x/, @list) { ... }</pre>
</blockquote>

Otherwise changing <tt class="literal">$var</tt> will clobber the values of <tt class="literal">@list</tt>.  (This
most often happens when you use <tt class="literal">$_</tt> for the loop variable and call
subroutines in the loop that don't properly localize <tt class="literal">$_</tt>.)</p>
</li>
<li>
<p>Some error messages and warnings will be different.</p>
</li>
<li>
<p>Some bugs may have been inadvertently removed.
<a name="INDEX-4186"></a><a name="INDEX-4187"></a>
</p>
</li>
</ul>
<a name="INDEX-4188"></a><a name="INDEX-4189"></a><a name="INDEX-4190"></a><a name="INDEX-4191"></a>














































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