perlrun.pod

ARM上的如果你对底层感兴趣

=head1 NAME

perlrun - how to execute the Perl interpreter

=head1 SYNOPSIS

B<perl>	S<[ B<-sTuU> ]>
	S<[ B<-hv> ] [ B<-V>[:I<configvar>] ]>
	S<[ B<-cw> ] [ B<-d>[:I<debugger>] ] [ B<-D>[I<number/list>] ]>
	S<[ B<-pna> ] [ B<-F>I<pattern> ] [ B<-l>[I<octal>] ] [ B<-0>[I<octal>] ]>
	S<[ B<-I>I<dir> ] [ B<-m>[B<->]I<module> ] [ B<-M>[B<->]I<'module...'> ]>
	S<[ B<-P> ]>
	S<[ B<-S> ]>
	S<[ B<-x>[I<dir>] ]>
	S<[ B<-i>[I<extension>] ]>
	S<[ B<-e> I<'command'> ] [ B<--> ] [ I<programfile> ] [ I<argument> ]...>

=head1 DESCRIPTION

Upon startup, Perl looks for your script in one of the following
places:

=over 4

=item 1.

Specified line by line via B<-e> switches on the command line.

=item 2.

Contained in the file specified by the first filename on the command line.
(Note that systems supporting the #! notation invoke interpreters this
way. See L<Location of Perl>.)

=item 3.

Passed in implicitly via standard input.  This works only if there are
no filename arguments--to pass arguments to a STDIN script you
must explicitly specify a "-" for the script name.

=back

With methods 2 and 3, Perl starts parsing the input file from the
beginning, unless you've specified a B<-x> switch, in which case it
scans for the first line starting with #! and containing the word
"perl", and starts there instead.  This is useful for running a script
embedded in a larger message.  (In this case you would indicate the end
of the script using the C<__END__> token.)

The #! line is always examined for switches as the line is being
parsed.  Thus, if you're on a machine that allows only one argument
with the #! line, or worse, doesn't even recognize the #! line, you
still can get consistent switch behavior regardless of how Perl was
invoked, even if B<-x> was used to find the beginning of the script.

Because many operating systems silently chop off kernel interpretation of
the #! line after 32 characters, some switches may be passed in on the
command line, and some may not; you could even get a "-" without its
letter, if you're not careful.  You probably want to make sure that all
your switches fall either before or after that 32 character boundary.
Most switches don't actually care if they're processed redundantly, but
getting a - instead of a complete switch could cause Perl to try to
execute standard input instead of your script.  And a partial B<-I> switch
could also cause odd results.

Some switches do care if they are processed twice, for instance combinations
of B<-l> and B<-0>.  Either put all the switches after the 32 character
boundary (if applicable), or replace the use of B<-0>I<digits> by 
C<BEGIN{ $/ = "\0digits"; }>.

Parsing of the #! switches starts wherever "perl" is mentioned in the line.
The sequences "-*" and "- " are specifically ignored so that you could,
if you were so inclined, say

    #!/bin/sh -- # -*- perl -*- -p
    eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
        if $running_under_some_shell;

to let Perl see the B<-p> switch.

If the #! line does not contain the word "perl", the program named after
the #! is executed instead of the Perl interpreter.  This is slightly
bizarre, but it helps people on machines that don't do #!, because they
can tell a program that their SHELL is /usr/bin/perl, and Perl will then
dispatch the program to the correct interpreter for them.

After locating your script, Perl compiles the entire script to an
internal form.  If there are any compilation errors, execution of the
script is not attempted.  (This is unlike the typical shell script,
which might run part-way through before finding a syntax error.)

If the script is syntactically correct, it is executed.  If the script
runs off the end without hitting an exit() or die() operator, an implicit
C<exit(0)> is provided to indicate successful completion.

=head2 #! and quoting on non-Unix systems

Unix's #! technique can be simulated on other systems:

=over 4

=item OS/2

Put

    extproc perl -S -your_switches

as the first line in C<*.cmd> file (C<-S> due to a bug in cmd.exe's
`extproc' handling).

=item MS-DOS

Create a batch file to run your script, and codify it in
C<ALTERNATIVE_SHEBANG> (see the F<dosish.h> file in the source
distribution for more information).

=item Win95/NT

The Win95/NT installation, when using the Activeware port of Perl,
will modify the Registry to associate the F<.pl> extension with the perl
interpreter.  If you install another port of Perl, including the one
in the Win32 directory of the Perl distribution, then you'll have to
modify the Registry yourself.  Note that this means you can no 
longer tell the difference between an executable Perl program
and a Perl library file.

=item Macintosh

Macintosh perl scripts will have the appropriate Creator and
Type, so that double-clicking them will invoke the perl application.

=back

Command-interpreters on non-Unix systems have rather different ideas
on quoting than Unix shells.  You'll need to learn the special
characters in your command-interpreter (C<*>, C<\> and C<"> are
common) and how to protect whitespace and these characters to run
one-liners (see C<-e> below).

On some systems, you may have to change single-quotes to double ones,
which you must I<NOT> do on Unix or Plan9 systems.  You might also
have to change a single % to a %%.

For example:

    # Unix
    perl -e 'print "Hello world\n"'

    # MS-DOS, etc.
    perl -e "print \"Hello world\n\""

    # Macintosh
    print "Hello world\n"
     (then Run "Myscript" or Shift-Command-R)

    # VMS
    perl -e "print ""Hello world\n"""

The problem is that none of this is reliable: it depends on the command
and it is entirely possible neither works.  If 4DOS was the command shell, this would
probably work better:

    perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""

CMD.EXE in Windows NT slipped a lot of standard Unix functionality in
when nobody was looking, but just try to find documentation for its
quoting rules.

Under the Macintosh, it depends which environment you are using.  The MacPerl
shell, or MPW, is much like Unix shells in its support for several
quoting variants, except that it makes free use of the Macintosh's non-ASCII
characters as control characters.

There is no general solution to all of this.  It's just a mess.

=head2 Location of Perl

It may seem obvious to say, but Perl is useful only when users can
easily find it. When possible, it's good for both B</usr/bin/perl> and
B</usr/local/bin/perl> to be symlinks to the actual binary. If that
can't be done, system administrators are strongly encouraged to put
(symlinks to) perl and its accompanying utilities, such as perldoc, into
a directory typically found along a user's PATH, or in another obvious
and convenient place.

In this documentation, C<#!/usr/bin/perl> on the first line of the script
will stand in for whatever method works on your system.

=head2 Switches

A single-character switch may be combined with the following switch, if
any.

    #!/usr/bin/perl -spi.bak	# same as -s -p -i.bak

Switches include:

=over 5

=item B<-0>[I<digits>]

specifies the input record separator (C<$/>) as an octal number.  If there are
no digits, the null character is the separator.  Other switches may
precede or follow the digits.  For example, if you have a version of
B<find> which can print filenames terminated by the null character, you
can say this:

    find . -name '*.bak' -print0 | perl -n0e unlink

The special value 00 will cause Perl to slurp files in paragraph mode.
The value 0777 will cause Perl to slurp files whole because there is no
legal character with that value.

=item B<-a>

turns on autosplit mode when used with a B<-n> or B<-p>.  An implicit
split command to the @F array is done as the first thing inside the
implicit while loop produced by the B<-n> or B<-p>.

    perl -ane 'print pop(@F), "\n";'

is equivalent to

    while (<>) {
	@F = split(' ');
	print pop(@F), "\n";
    }

An alternate delimiter may be specified using B<-F>.

=item B<-c>

causes Perl to check the syntax of the script and then exit without
executing it.  Actually, it I<will> execute C<BEGIN>, C<END>, and C<use> blocks,
because these are considered as occurring outside the execution of
your program.

=item B<-d>

runs the script under the Perl debugger.  See L<perldebug>.

=item B<-d:>I<foo>

runs the script under the control of a debugging or tracing module
installed as Devel::foo. E.g., B<-d:DProf> executes the script using the
Devel::DProf profiler.  See L<perldebug>.

=item B<-D>I<letters>

=item B<-D>I<number>

sets debugging flags.  To watch how it executes your script, use
B<-Dtls>.  (This works only if debugging is compiled into your
Perl.)  Another nice value is B<-Dx>, which lists your compiled
syntax tree.  And B<-Dr> displays compiled regular expressions. As an
alternative, specify a number instead of list of letters (e.g., B<-D14> is
equivalent to B<-Dtls>):

        1  p  Tokenizing and parsing
        2  s  Stack snapshots
        4  l  Context (loop) stack processing
        8  t  Trace execution
       16  o  Method and overloading resolution
       32  c  String/numeric conversions
       64  P  Print preprocessor command for -P
      128  m  Memory allocation
      256  f  Format processing
      512  r  Regular expression parsing and execution
     1024  x  Syntax tree dump
     2048  u  Tainting checks
     4096  L  Memory leaks (needs C<-DLEAKTEST> when compiling Perl)
     8192  H  Hash dump -- usurps values()
    16384  X  Scratchpad allocation
    32768  D  Cleaning up
    65536  S  Thread synchronization

All these flags require C<-DDEBUGGING> when you compile the Perl
executable.  This flag is automatically set if you include C<-g>
option when C<Configure> asks you about optimizer/debugger flags.

=item B<-e> I<commandline>

may be used to enter one line of script.
If B<-e> is given, Perl
will not look for a script filename in the argument list.
Multiple B<-e> commands may
be given to build up a multi-line script.
Make sure to use semicolons where you would in a normal program.

=item B<-F>I<pattern>

specifies the pattern to split on if B<-a> is also in effect.  The
pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
put in single quotes.

=item B<-h>

prints a summary of the options.

=item B<-i>[I<extension>]

specifies that files processed by the C<E<lt>E<gt>> construct are to be
edited in-place.  It does this by renaming the input file, opening the
output file by the original name, and selecting that output file as the
default for print() statements.  The extension, if supplied, is used to
modify the name of the old file to make a backup copy, following these
rules:

If no extension is supplied, no backup is made and the current file is
overwritten.

If the extension doesn't contain a C<*> then it is appended to the end
of the current filename as a suffix.

If the extension does contain one or more C<*> characters, then each C<*>
is replaced with the current filename.  In perl terms you could think of
this as:

    ($backup = $extension) =~ s/\*/$file_name/g;

This allows you to add a prefix to the backup file, instead of (or in
addition to) a suffix:

    $ perl -pi'bak_*' -e 's/bar/baz/' fileA	# backup to 'bak_fileA'

Or even to place backup copies of the original files into another
directory (provided the directory already exists):

    $ perl -pi'old/*.bak' -e 's/bar/baz/' fileA # backup to 'old/fileA.bak'

These sets of one-liners are equivalent:

    $ perl -pi -e 's/bar/baz/' fileA		# overwrite current file
    $ perl -pi'*' -e 's/bar/baz/' fileA		# overwrite current file

    $ perl -pi'.bak' -e 's/bar/baz/' fileA	# backup to 'fileA.bak'
    $ perl -pi'*.bak' -e 's/bar/baz/' fileA	# backup to 'fileA.bak'

From the shell, saying

    $ perl -p -i.bak -e "s/foo/bar/; ... "

is the same as using the script:

    #!/usr/bin/perl -pi.bak
    s/foo/bar/;

which is equivalent to

    #!/usr/bin/perl
    $extension = '.bak';
    while (<>) {
	if ($ARGV ne $oldargv) {
	    if ($extension !~ /\*/) {
		$backup = $ARGV . $extension;
	    }
	    else {
		($backup = $extension) =~ s/\*/$ARGV/g;
	    }
	    rename($ARGV, $backup);
	    open(ARGVOUT, ">$ARGV");
	    select(ARGVOUT);
	    $oldargv = $ARGV;
	}
	s/foo/bar/;
    }