perlrun.pod

MSYS在windows下模拟了一个类unix的终端

Also, in some platforms the C preprocessor knows too much: it knows
about the C++ -style until-end-of-line comments starting with C<"//">.
This will cause problems with common Perl constructs like

    s/foo//;

because after -P this will became illegal code

    s/foo

The workaround is to use some other quoting separator than C<"/">,
like for example C<"!">:

    s!foo!!;

=item B<-s>

enables rudimentary switch parsing for switches on the command
line after the program name but before any filename arguments (or before
an argument of B<-->).  This means you can have switches with two leading
dashes (B<--help>).  Any switch found there is removed from @ARGV and sets the
corresponding variable in the Perl program.  The following program
prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
if it is invoked with B<-xyz=abc>.

    #!/usr/bin/perl -s
    if ($xyz) { print "$xyz\n" }

Do note that B<--help> creates the variable ${-help}, which is not compliant
with C<strict refs>.

=item B<-S>

makes Perl use the PATH environment variable to search for the
program (unless the name of the program contains directory separators).

On some platforms, this also makes Perl append suffixes to the
filename while searching for it.  For example, on Win32 platforms,
the ".bat" and ".cmd" suffixes are appended if a lookup for the
original name fails, and if the name does not already end in one
of those suffixes.  If your Perl was compiled with DEBUGGING turned
on, using the -Dp switch to Perl shows how the search progresses.

Typically this is used to emulate #! startup on platforms that
don't support #!.  This example works on many platforms that
have a shell compatible with Bourne shell:

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

The system ignores the first line and feeds the program to F</bin/sh>,
which proceeds to try to execute the Perl program as a shell script.
The shell executes the second line as a normal shell command, and thus
starts up the Perl interpreter.  On some systems $0 doesn't always
contain the full pathname, so the B<-S> tells Perl to search for the
program if necessary.  After Perl locates the program, it parses the
lines and ignores them because the variable $running_under_some_shell
is never true.  If the program will be interpreted by csh, you will need
to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
embedded spaces (and such) in the argument list.  To start up sh rather
than csh, some systems may have to replace the #! line with a line
containing just a colon, which will be politely ignored by Perl.  Other
systems can't control that, and need a totally devious construct that
will work under any of B<csh>, B<sh>, or Perl, such as the following:

	eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
	& eval 'exec /usr/bin/perl -wS $0 $argv:q'
		if $running_under_some_shell;

If the filename supplied contains directory separators (i.e., is an
absolute or relative pathname), and if that file is not found,
platforms that append file extensions will do so and try to look
for the file with those extensions added, one by one.

On DOS-like platforms, if the program does not contain directory
separators, it will first be searched for in the current directory
before being searched for on the PATH.  On Unix platforms, the
program will be searched for strictly on the PATH.

=item B<-T>

forces "taint" checks to be turned on so you can test them.  Ordinarily
these checks are done only when running setuid or setgid.  It's a
good idea to turn them on explicitly for programs that run on behalf
of someone else whom you might not necessarily trust, such as CGI
programs or any internet servers you might write in Perl.  See
L<perlsec> for details.  For security reasons, this option must be
seen by Perl quite early; usually this means it must appear early
on the command line or in the #! line for systems which support
that construct.

=item B<-u>

This obsolete switch causes Perl to dump core after compiling your
program.  You can then in theory take this core dump and turn it
into an executable file by using the B<undump> program (not supplied).
This speeds startup at the expense of some disk space (which you
can minimize by stripping the executable).  (Still, a "hello world"
executable comes out to about 200K on my machine.)  If you want to
execute a portion of your program before dumping, use the dump()
operator instead.  Note: availability of B<undump> is platform
specific and may not be available for a specific port of Perl.

This switch has been superseded in favor of the new Perl code
generator backends to the compiler.  See L<B> and L<B::Bytecode>
for details.

=item B<-U>

allows Perl to do unsafe operations.  Currently the only "unsafe"
operations are the unlinking of directories while running as superuser,
and running setuid programs with fatal taint checks turned into
warnings.  Note that the B<-w> switch (or the C<$^W> variable) must
be used along with this option to actually I<generate> the
taint-check warnings.

=item B<-v>

prints the version and patchlevel of your perl executable.

=item B<-V>

prints summary of the major perl configuration values and the current
values of @INC.

=item B<-V:>I<name>

Prints to STDOUT the value of the named configuration variable.
For example, 

    $ perl -V:man.dir

will provide strong clues about what your MANPATH variable should
be set to in order to access the Perl documentation.

=item B<-w>

prints warnings about dubious constructs, such as variable names
that are mentioned only once and scalar variables that are used
before being set, redefined subroutines, references to undefined
filehandles or filehandles opened read-only that you are attempting
to write on, values used as a number that doesn't look like numbers,
using an array as though it were a scalar, if your subroutines
recurse more than 100 deep, and innumerable other things.

This switch really just enables the internal C<^$W> variable.  You
can disable or promote into fatal errors specific warnings using
C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
See also L<perldiag> and L<perltrap>.  A new, fine-grained warning
facility is also available if you want to manipulate entire classes
of warnings; see L<warnings> or L<perllexwarn>.

=item B<-W>

Enables all warnings regardless of C<no warnings> or C<$^W>.
See L<perllexwarn>.

=item B<-X>

Disables all warnings regardless of C<use warnings> or C<$^W>.
See L<perllexwarn>.

=item B<-x> I<directory>

tells Perl that the program is embedded in a larger chunk of unrelated
ASCII text, such as in a mail message.  Leading garbage will be
discarded until the first line that starts with #! and contains the
string "perl".  Any meaningful switches on that line will be applied.
If a directory name is specified, Perl will switch to that directory
before running the program.  The B<-x> switch controls only the
disposal of leading garbage.  The program must be terminated with
C<__END__> if there is trailing garbage to be ignored (the program
can process any or all of the trailing garbage via the DATA filehandle
if desired).

=back

=head1 ENVIRONMENT

=over 12

=item HOME

Used if chdir has no argument.

=item LOGDIR

Used if chdir has no argument and HOME is not set.

=item PATH

Used in executing subprocesses, and in finding the program if B<-S> is
used.

=item PERL5LIB

A colon-separated list of directories in which to look for Perl library
files before looking in the standard library and the current
directory.  Any architecture-specific directories under the specified
locations are automatically included if they exist.  If PERL5LIB is not
defined, PERLLIB is used.

When running taint checks (either because the program was running setuid
or setgid, or the B<-T> switch was used), neither variable is used.
The program should instead say:

    use lib "/my/directory";

=item PERL5OPT

Command-line options (switches).  Switches in this variable are taken
as if they were on every Perl command line.  Only the B<-[DIMUdmw]>
switches are allowed.  When running taint checks (because the program
was running setuid or setgid, or the B<-T> switch was used), this
variable is ignored.  If PERL5OPT begins with B<-T>, tainting will be
enabled, and any subsequent options ignored.

=item PERLLIB

A colon-separated list of directories in which to look for Perl library
files before looking in the standard library and the current directory.
If PERL5LIB is defined, PERLLIB is not used.

=item PERL5DB

The command used to load the debugger code.  The default is:

	BEGIN { require 'perl5db.pl' }

=item PERL5SHELL (specific to the Win32 port)

May be set to an alternative shell that perl must use internally for
executing "backtick" commands or system().  Default is C<cmd.exe /x/c>
on WindowsNT and C<command.com /c> on Windows95.  The value is considered
to be space-separated.  Precede any character that needs to be protected
(like a space or backslash) with a backslash.

Note that Perl doesn't use COMSPEC for this purpose because
COMSPEC has a high degree of variability among users, leading to
portability concerns.  Besides, perl can use a shell that may not be
fit for interactive use, and setting COMSPEC to such a shell may
interfere with the proper functioning of other programs (which usually
look in COMSPEC to find a shell fit for interactive use).

=item PERL_DEBUG_MSTATS

Relevant only if perl is compiled with the malloc included with the perl
distribution (that is, if C<perl -V:d_mymalloc> is 'define').
If set, this causes memory statistics to be dumped after execution.  If set
to an integer greater than one, also causes memory statistics to be dumped
after compilation.

=item PERL_DESTRUCT_LEVEL

Relevant only if your perl executable was built with B<-DDEBUGGING>,
this controls the behavior of global destruction of objects and other
references.

=item PERL_ROOT (specific to the VMS port)

A translation concealed rooted logical name that contains perl and the
logical device for the @INC path on VMS only.  Other logical names that
affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and 
SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in 
L<perlvms> and in F<README.vms> in the Perl source distribution.

=item SYS$LOGIN (specific to the VMS port)

Used if chdir has no argument and HOME and LOGDIR are not set.

=back

Perl also has environment variables that control how Perl handles data
specific to particular natural languages.  See L<perllocale>.

Apart from these, Perl uses no other environment variables, except
to make them available to the program being executed, and to child
processes.  However, programs running setuid would do well to execute
the following lines before doing anything else, just to keep people
honest:

    $ENV{PATH}  = '/bin:/usr/bin';    # or whatever you need
    $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
    delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};