perlrun.pod

ARM上的如果你对底层感兴趣

    continue {
	print;	# this prints to original filename
    }
    select(STDOUT);

except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
know when the filename has changed.  It does, however, use ARGVOUT for
the selected filehandle.  Note that STDOUT is restored as the default
output filehandle after the loop.

As shown above, Perl creates the backup file whether or not any output
is actually changed.  So this is just a fancy way to copy files:

    $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
  or
    $ perl -p -i'.bak' -e 1 file1 file2 file3...

You can use C<eof> without parentheses to locate the end of each input
file, in case you want to append to each file, or reset line numbering
(see example in L<perlfunc/eof>).

If, for a given file, Perl is unable to create the backup file as
specified in the extension then it will skip that file and continue on
with the next one (if it exists).

For a discussion of issues surrounding file permissions and C<-i>, see
L<perlfaq5/Why does Perl let me delete read-only files?  Why does -i clobber protected files?  Isn't this a bug in Perl?>.

You cannot use B<-i> to create directories or to strip extensions from
files.

Perl does not expand C<~>, so don't do that.

Finally, note that the B<-i> switch does not impede execution when no
files are given on the command line.  In this case, no backup is made
(the original file cannot, of course, be determined) and processing
proceeds from STDIN to STDOUT as might be expected.

=item B<-I>I<directory>

Directories specified by B<-I> are prepended to the search path for
modules (C<@INC>), and also tells the C preprocessor where to search for
include files.  The C preprocessor is invoked with B<-P>; by default it
searches /usr/include and /usr/lib/perl.

=item B<-l>[I<octnum>]

enables automatic line-ending processing.  It has two effects:  first,
it automatically chomps "C<$/>" (the input record separator) when used
with B<-n> or B<-p>, and second, it assigns "C<$\>"
(the output record separator) to have the value of I<octnum> so that
any print statements will have that separator added back on.  If
I<octnum> is omitted, sets "C<$\>" to the current value of "C<$/>".  For
instance, to trim lines to 80 columns:

    perl -lpe 'substr($_, 80) = ""'

Note that the assignment C<$\ = $/> is done when the switch is processed,
so the input record separator can be different than the output record
separator if the B<-l> switch is followed by a B<-0> switch:

    gnufind / -print0 | perl -ln0e 'print "found $_" if -p'

This sets C<$\> to newline and then sets C<$/> to the null character.

=item B<-m>[B<->]I<module>

=item B<-M>[B<->]I<module>

=item B<-M>[B<->]I<'module ...'>

=item B<-[mM]>[B<->]I<module=arg[,arg]...>

C<-m>I<module> executes C<use> I<module> C<();> before executing your
script.

C<-M>I<module> executes C<use> I<module> C<;> before executing your
script.  You can use quotes to add extra code after the module name,
e.g., C<-M'module qw(foo bar)'>.

If the first character after the C<-M> or C<-m> is a dash (C<->)
then the 'use' is replaced with 'no'.

A little builtin syntactic sugar means you can also say
C<-mmodule=foo,bar> or C<-Mmodule=foo,bar> as a shortcut for
C<-M'module qw(foo bar)'>.  This avoids the need to use quotes when
importing symbols.  The actual code generated by C<-Mmodule=foo,bar> is
C<use module split(/,/,q{foo,bar})>.  Note that the C<=> form
removes the distinction between C<-m> and C<-M>.

=item B<-n>

causes Perl to assume the following loop around your script, which
makes it iterate over filename arguments somewhat like B<sed -n> or
B<awk>:

    while (<>) {
	...		# your script goes here
    }

Note that the lines are not printed by default.  See B<-p> to have
lines printed.  If a file named by an argument cannot be opened for
some reason, Perl warns you about it, and moves on to the next file.

Here is an efficient way to delete all files older than a week:

    find . -mtime +7 -print | perl -nle 'unlink;'

This is faster than using the C<-exec> switch of B<find> because you don't
have to start a process on every filename found.

C<BEGIN> and C<END> blocks may be used to capture control before or after
the implicit loop, just as in B<awk>.

=item B<-p>

causes Perl to assume the following loop around your script, which
makes it iterate over filename arguments somewhat like B<sed>:


    while (<>) {
	...		# your script goes here
    } continue {
	print or die "-p destination: $!\n";
    }

If a file named by an argument cannot be opened for some reason, Perl
warns you about it, and moves on to the next file.  Note that the
lines are printed automatically.  An error occuring during printing is
treated as fatal.  To suppress printing use the B<-n> switch.  A B<-p>
overrides a B<-n> switch.

C<BEGIN> and C<END> blocks may be used to capture control before or after
the implicit loop, just as in awk.

=item B<-P>

causes your script to be run through the C preprocessor before
compilation by Perl.  (Because both comments and cpp directives begin
with the # character, you should avoid starting comments with any words
recognized by the C preprocessor such as "if", "else", or "define".)

=item B<-s>

enables some rudimentary switch parsing for switches on the command
line after the script name but before any filename arguments (or before
a B<-->).  Any switch found there is removed from @ARGV and sets the
corresponding variable in the Perl script.  The following script
prints "true" if and only if the script is invoked with a B<-xyz> switch.

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

=item B<-S>

makes Perl use the PATH environment variable to search for the
script (unless the name of the script 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.

If the filename supplied contains directory separators (i.e. it is an
absolute or relative pathname), and if the 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 script 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
script will be searched for strictly on the PATH.

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 script to /bin/sh,
which proceeds to try to execute the Perl script 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
script if necessary.  After Perl locates the script, it parses the
lines and ignores them because the variable $running_under_some_shell
is never true. If the script will be interpreted by csh, you will need
to replace C<${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 csh, sh, or Perl, such as the following:

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

=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 run on another's behalf,
such as CGI programs.  See L<perlsec>.  Note that (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).

=item B<-u>

causes Perl to dump core after compiling your script.  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 script 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.  It has been superseded by the new perl-to-C compiler, which is more
portable, even though it's still only considered beta.

=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 B<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
value of @INC.

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

Prints to STDOUT the value of the named configuration variable.

=item B<-w>

prints warnings about variable names that are mentioned only once, and
scalar variables that are used before being set.  Also warns about
redefined subroutines, and references to undefined filehandles or
filehandles opened read-only that you are attempting to write on.  Also
warns you if you use values 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.

You can disable specific warnings using C<__WARN__> hooks, as described
in L<perlvar> and L<perlfunc/warn>. See also L<perldiag> and L<perltrap>.

=item B<-x> I<directory>

tells Perl that the script is embedded in a 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 script.  The B<-x> switch controls
only the disposal of leading garbage.  The script must be
terminated with C<__END__> if there is trailing garbage to be ignored (the
script 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 script 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.  If PERL5LIB is not defined, PERLLIB is used.  When running
taint checks (because the script was running setuid or setgid, or the
B<-T> switch was used), neither variable is used.  The script 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 script
was running setuid or setgid, or the B<-T> switch was used), this
variable is 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 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 delimited.  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.

=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 script being executed, and to child
processes.  However, scripts 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)};