ch16.htm

《Perl 5 Unreleased》

You can execute Perl statements at the command line with the <TT><FONT FACE="Courier">-e</FONT></TT>
option. Here is an example of a command that prints a string:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">$ <B>perl -e 'print (&quot;Kamran Wuz
Here\n&quot;);'<BR>
</B>Kamran Wuz Here</FONT></TT>
</BLOCKQUOTE>
<P>
Don't forget to type the semicolon (<TT><FONT FACE="Courier">;</FONT></TT>)
at the end of each statement. You can specify more than one statement
by using either semicolons to separate them or using multiple
<TT><FONT FACE="Courier">-e</FONT></TT> options. For example,
the following two statements both print the string <TT><FONT FACE="Courier">Howdy
folks</FONT></TT>:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">$ <B>perl -e 'print (&quot;Howdy\n&quot;);'
-e 'print (&quot; folks\n&quot;);'<BR>
</B>Howdy folks<BR>
$ <B>perl -e 'print (&quot;Howdy\n&quot;); print (&quot; folks\n&quot;);'
<BR>
</B>Howdy folks</FONT></TT>
</BLOCKQUOTE>
<P>
In the case of multiple <TT><FONT FACE="Courier">-e</FONT></TT>
options, the Perl interpreter executes them from left to right.
Here's an example:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">$ perl -e 'print (&quot;Donald&quot;);'
-e 'print (&quot; Duck&quot;);'<BR>
Donald Duck</FONT></TT>
</BLOCKQUOTE>
<H3><A NAME="ThesOptiontoSupplyCustomCommandl">The <TT><FONT SIZE=4 FACE="Courier">-s</FONT></TT><FONT SIZE=4>
Option to Supply Custom Command-line Options</FONT></A></H3>
<P>
Generally, you'll specify the command line in a Perl script with
execute permissions in the first line of a script file, as follows:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#!/usr/bin/perl </FONT></TT>
</BLOCKQUOTE>
<P>
The first line is the complete pathname to the Perl interpreter.
<P>
You can run the same script using the following command at the
command line, as follows: 
<BLOCKQUOTE>
<TT><FONT FACE="Courier">perl <I>scriptFile</I></FONT></TT>
</BLOCKQUOTE>
<P>
where <TT><I><FONT FACE="Courier">scriptFile</FONT></I></TT> is
the name of the script file. Any command-line options specified
before the script file's name will be passed to the Perl interpreter
and not to your script file. 
<P>
To pass options to the script that you run, you have to use the
<TT><FONT FACE="Courier">-s</FONT></TT> option. 
<BLOCKQUOTE>
<TT><FONT FACE="Courier">perl -s scriptFile -w</FONT></TT>
</BLOCKQUOTE>
<P>
This command starts the Perl program <TT><FONT FACE="Courier">scriptFile</FONT></TT>
and passes it the <TT><FONT FACE="Courier">-w </FONT></TT>option.
If you do not specify the <TT><FONT FACE="Courier">-s</FONT></TT>
option, your <TT><FONT FACE="Courier">-w</FONT></TT> will be sent
as part of the <TT><FONT FACE="Courier">@ARGV</FONT></TT> array
to the program being run. For programs that are run from the command
line with the Perl command, it's best to include <TT><FONT FACE="Courier">-s</FONT></TT>
as part of your header comment:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#!perl -s</FONT></TT>
</BLOCKQUOTE>
<P>
This way you are guaranteed that the program always will check
for options provided that no other Perl options were specified
on the command line when you invoked the program.
<P>
A scalar variable with the same name as the name of any specified
option is created and automatically set to <TT><FONT FACE="Courier">1</FONT></TT>
before the Perl interpreter executes a program. For example, if
a Perl program named <TT><FONT FACE="Courier">scriptFile</FONT></TT><I>
</I>is called with the <TT><FONT FACE="Courier">-x</FONT></TT>
option, as in
<BLOCKQUOTE>
<TT><FONT FACE="Courier">perl -s scriptFile -x</FONT></TT>
</BLOCKQUOTE>
<P>
the scalar variable <TT><FONT FACE="Courier">$x </FONT></TT>is
automatically set to <TT><FONT FACE="Courier">1</FONT></TT>. This
lets you test the <TT><FONT FACE="Courier">$x</FONT></TT> variable
in a conditional expression to see whether the option has been
set. The named variable will not appear in the <TT><FONT FACE="Courier">@ARGV</FONT></TT>
array. Options do not have to be a single character. For example,
the following command sets the value of the scalar variable <TT><FONT FACE="Courier">$surge</FONT></TT>
to <TT><FONT FACE="Courier">1</FONT></TT>:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">perl -s scriptFile -surge</FONT></TT>
</BLOCKQUOTE>
<P>
Options can be set to a value other than <TT><FONT FACE="Courier">1</FONT></TT>
by simply assigning a value at the command line. For example:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">perl -s scriptFile -surge=&quot;power&quot;</FONT></TT>
</BLOCKQUOTE>
<P>
This command sets the value of <TT><FONT FACE="Courier">$surge</FONT></TT>
to <TT><FONT FACE="Courier">power</FONT></TT> in the program specified
in <TT><FONT FACE="Courier">scriptFile</FONT></TT>.
<P>
The <TT><FONT FACE="Courier">-s</FONT></TT> option lets you supply
both options and command-line arguments based on these rules:
<UL>
<LI><FONT COLOR=#000000>All arguments that start with a dash (</FONT><TT><FONT FACE="Courier">-</FONT></TT>)
and immediately follow the program name are assumed to be options.
<LI><FONT COLOR=#000000>Any argument not starting with a dash
(</FONT><TT><FONT FACE="Courier">-</FONT></TT>) is assumed to
be an ordinary argument. All subsequent arguments, even if they
start with a <TT><FONT FACE="Courier">-</FONT></TT> , are then
assumed to be ordinary arguments and not options.
<LI><FONT COLOR=#000000>A double dash (</FONT><TT><FONT FACE="Courier">--</FONT></TT>)
will end Perl's parsing of command-line switches.
</UL>
<P>
This means, for example, that the command
<BLOCKQUOTE>
<TT><FONT FACE="Courier">perl -s scriptFile -arg1 -arg2 -arg3</FONT></TT>
</BLOCKQUOTE>
<P>
treats <TT><FONT FACE="Courier">-arg1</FONT></TT> as an option
to <TT><FONT FACE="Courier">scriptFile</FONT></TT>, and <TT><FONT FACE="Courier">-arg2</FONT></TT>
and <TT><FONT FACE="Courier">-arg3</FONT></TT> are ordinary arguments
that are placed in <TT><FONT FACE="Courier">@ARGV</FONT></TT>.
<H3><A NAME="TheIOptiontoIncludeOtherFiles">The <TT><FONT SIZE=4 FACE="Courier">-I</FONT></TT><FONT SIZE=4>
Option to Include Other Files</FONT></A></H3>
<P>
The <TT><FONT FACE="Courier">-I</FONT></TT> option is used with
the <TT><FONT FACE="Courier">-P</FONT></TT> option (which is described
in the next section). The <TT><FONT FACE="Courier">-I</FONT></TT>
option lets you specify the pathnames to search for include files
to be processed by the C preprocessor. For example:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">perl -P -I /usr/local/other <I>scriptFile</I></FONT></TT>
</BLOCKQUOTE>
<P>
This command tells the Perl interpreter to search the directory
<TT><FONT FACE="Courier">/usr/local/other</FONT></TT> for include
files if the file is not found in its default paths. The default
path is the current directory and, if the file is not found, the
<TT><FONT FACE="Courier">/usr/local/lib/perl</FONT></TT> directory.
The <TT><FONT FACE="Courier">-I</FONT></TT> option can be repeated
on the same command line to specify more than one include-file
path. 
<P>
Using the <TT><FONT FACE="Courier">-I</FONT></TT> option also
adds the path or paths to the <TT><FONT FACE="Courier">@Inc</FONT></TT>
variable. The paths are then made available to the Perl interpreter
when it uses the <TT><FONT FACE="Courier">require</FONT></TT>
function to find its modules. 
<H3><A NAME="ThePOptionforUsingtheCPreprocess">The <TT><FONT SIZE=4 FACE="Courier">-P</FONT></TT><FONT SIZE=4>
Option for Using the C Preprocessor</FONT></A></H3>
<P>
The <TT><FONT FACE="Courier">-P</FONT></TT> option is helpful
only if you have a C compiler on your system. Although all UNIX
systems come with a C compiler, DOS and Windows NT systems don't;
you have to purchase your own. The <TT><FONT FACE="Courier">cpp</FONT></TT>
preprocessor is the default C preprocessor in UNIX. The C preprocessor
is a program that takes code written in C which does basic string
substitution based on the values of variables. To enable the use
of <TT><FONT FACE="Courier">cpp</FONT></TT> with the <TT><FONT FACE="Courier">-P</FONT></TT>
option, use the following statement to start your Perl program:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">perl -P <I>scriptFile</I></FONT></TT>
</BLOCKQUOTE>
<P>
The Perl program <TT><I><FONT FACE="Courier">scriptFile</FONT></I></TT>
is first run through the C preprocessor, and then the resulting
output is executed by the Perl interpreter. You can also specify
the use of the C preprocessor in the header comment like this:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#!perl -P</FONT></TT>
</BLOCKQUOTE>
<P>
All C preprocessor statements have the following syntax:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#<I>command</I> <I>value</I></FONT></TT>
</BLOCKQUOTE>
<P>
The hash (<TT><FONT FACE="Courier">#</FONT></TT>) is interpreted
by Perl as a comment, and so any statements intended for the C
preprocessor are ignored by Perl even if the <TT><FONT FACE="Courier">-P</FONT></TT>
option is not used. The <TT><I><FONT FACE="Courier">command</FONT></I></TT>
is the preprocessor operation to perform, and <TT><I><FONT FACE="Courier">value</FONT></I></TT>,
which is optional, is associated with this operation.
<P>
The <TT><FONT FACE="Courier">#define</FONT></TT> operator is the
most common preprocessor statement. It tells the preprocessor
to replace every occurrence of a particular character string with
a specified value. The syntax for <TT><FONT FACE="Courier">#define</FONT></TT>
is
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#define<I> item value</I></FONT></TT>
</BLOCKQUOTE>
<P>
This statement replaces all occurrences of the character string
<TT><I><FONT FACE="Courier">item</FONT></I></TT> with <TT><I><FONT FACE="Courier">value</FONT></I></TT>.
This substitution operation is known as <I>macro substitution</I>.
The item being substituted can contain a combination of letters,
digits, and underscores. The value specified in a <TT><FONT FACE="Courier">#define</FONT></TT>
statement can be any character string or number. For example,
the following statement will replace all occurrences of <TT><FONT FACE="Courier">DOCTOR</FONT></TT>
with <TT><FONT FACE="Courier">quack</FONT></TT>, and <TT><FONT FACE="Courier">Donald</FONT></TT>
with <TT><FONT FACE="Courier">&quot;Duck&quot;</FONT></TT> including
the quotation marks:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#define DOCTOR  QUACK<BR>
#define Donald &quot;Duck&quot;</FONT></TT>
</BLOCKQUOTE>
<P>
Any expressions are treated as strings. For example, the following
statement: 
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#define AREA  (3.141 * 2)</FONT></TT>
</BLOCKQUOTE>
<P>
replaces <TT><FONT FACE="Courier">AREA</FONT></TT> with the string
<TT><FONT FACE="Courier">(3.141 * 2)</FONT></TT>, including the
parentheses and not the value of 6.282. 
<P>
When using <TT><FONT FACE="Courier">#define</FONT></TT> with expressions,
don't forget to enclose the value in parentheses. For example,
consider the following Perl statement:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">$result = ITEM * 10;</FONT></TT>
</BLOCKQUOTE>
<P>
If the statement you use in your preprocessor command is this:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#define ITEM  1 + 2</FONT></TT>
</BLOCKQUOTE>
<P>
the resulting Perl statement is this:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">$result = 1 + 2 * 10;</FONT></TT>
</BLOCKQUOTE>
<P>
This statement assigns 21 to <TT><FONT FACE="Courier">$result</FONT></TT>
instead of 30, which would be the result if you used this expression
instead:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#define ITEM  (1 + 2)</FONT></TT>
</BLOCKQUOTE>
<P>
to get this statement as a result:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">$result = (1 + 2) * 10;</FONT></TT>
</BLOCKQUOTE>
<P>
You can even specify multiple parameters with a <TT><FONT FACE="Courier">#define</FONT></TT>
statement thus enabling you to use a preprocessor command like
a simple function that also accepts arguments. For example, this
preprocessor statement:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#define SQUARE(val)  ((val) * (val))</FONT></TT>
</BLOCKQUOTE>
<P>
will return the square of a passed value. This statement:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">$result = SQUARE(4) </FONT></TT>
</BLOCKQUOTE>
<P>
will generate the following statement: 
<BLOCKQUOTE>
<TT><FONT FACE="Courier">$result = ((4) * (4));</FONT></TT>
</BLOCKQUOTE>