perlrun.pod

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and the C<open> pragma (see L<open>).

(In Perls earlier than 5.8.1 the C<-C> switch was a Win32-only switch
that enabled the use of Unicode-aware "wide system call" Win32 APIs.
This feature was practically unused, however, and the command line
switch was therefore "recycled".)

=item B<-c>
X<-c>

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

=item B<-d>
X<-d> X<-dt>

=item B<-dt>

runs the program under the Perl debugger.  See L<perldebug>.
If B<t> is specified, it indicates to the debugger that threads
will be used in the code being debugged.

=item B<-d:>I<foo[=bar,baz]>
X<-d> X<-dt>

=item B<-dt:>I<foo[=bar,baz]>

runs the program under the control of a debugging, profiling, or
tracing module installed as Devel::foo. E.g., B<-d:DProf> executes
the program using the Devel::DProf profiler.  As with the B<-M>
flag, options may be passed to the Devel::foo package where they
will be received and interpreted by the Devel::foo::import routine.
The comma-separated list of options must follow a C<=> character.
If B<t> is specified, it indicates to the debugger that threads
will be used in the code being debugged.
See L<perldebug>.

=item B<-D>I<letters>
X<-D> X<DEBUGGING> X<-DDEBUGGING>

=item B<-D>I<number>

sets debugging flags.  To watch how it executes your program, 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;
the format of the output is explained in L<perldebguts>.

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 (with v, displays parse stack)
        2  s  Stack snapshots (with v, displays all stacks)
        4  l  Context (loop) stack processing
        8  t  Trace execution
       16  o  Method and overloading resolution
       32  c  String/numeric conversions
       64  P  Print profiling info, preprocessor command for -P, source file input state
      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  U  Unofficial, User hacking (reserved for private, unreleased use)
     8192  H  Hash dump -- usurps values()
    16384  X  Scratchpad allocation
    32768  D  Cleaning up
    65536  S  Thread synchronization
   131072  T  Tokenising
   262144  R  Include reference counts of dumped variables (eg when using -Ds)
   524288  J  Do not s,t,P-debug (Jump over) opcodes within package DB
  1048576  v  Verbose: use in conjunction with other flags
  2097152  C  Copy On Write
  4194304  A  Consistency checks on internal structures
  8388608  q  quiet - currently only suppresses the "EXECUTING" message

All these flags require B<-DDEBUGGING> when you compile the Perl
executable (but see L<Devel::Peek>, L<re> which may change this).
See the F<INSTALL> file in the Perl source distribution
for how to do this.  This flag is automatically set if you include B<-g>
option when C<Configure> asks you about optimizer/debugger flags.

If you're just trying to get a print out of each line of Perl code
as it executes, the way that C<sh -x> provides for shell scripts,
you can't use Perl's B<-D> switch.  Instead do this

  # If you have "env" utility
  env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program

  # Bourne shell syntax
  $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program

  # csh syntax
  % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)

See L<perldebug> for details and variations.

=item B<-e> I<commandline>
X<-e>

may be used to enter one line of program.  If B<-e> is given, Perl
will not look for a 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<-E> I<commandline>
X<-E>

behaves just like B<-e>, except that it implicitly enables all
optional features (in the main compilation unit). See L<feature>.

=item B<-f>
X<-f>

Disable executing F<$Config{sitelib}/sitecustomize.pl> at startup.

Perl can be built so that it by default will try to execute
F<$Config{sitelib}/sitecustomize.pl> at startup.  This is a hook that
allows the sysadmin to customize how perl behaves.  It can for
instance be used to add entries to the @INC array to make perl find
modules in non-standard locations.

=item B<-F>I<pattern>
X<-F>

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. You can't use literal whitespace in the pattern.

=item B<-h>
X<-h>

prints a summary of the options.

=item B<-i>[I<extension>]
X<-i> X<in-place>

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'orig_*' -e 's/bar/baz/' fileA	# backup to 'orig_fileA'

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

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

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

From the shell, saying

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

is the same as using the program:

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

which is equivalent to

    #!/usr/bin/perl
    $extension = '.orig';
    LINE: while (<>) {
	if ($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/;
    }
    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'.orig' -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 B<-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<~> in filenames, which is good, since some
folks use it for their backup files:

    $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...

Note that because B<-i> renames or deletes the original file before
creating a new file of the same name, UNIX-style soft and hard links will
not be preserved.

Finally, 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>
X<-I> X<@INC>

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>]
X<-l> X<$/> X<$\>

enables automatic line-ending processing.  It has two separate
effects.  First, it automatically chomps C<$/> (the input record
separator) when used with B<-n> or B<-p>.  Second, it assigns C<$\>
(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>
X<-m> X<-M>

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

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

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

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

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

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

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

A consequence of this is that B<-MFoo=number> never does a version check
(unless C<Foo::import()> itself is set up to do a version check, which
could happen for example if Foo inherits from Exporter.)

=item B<-n>
X<-n>

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

  LINE:
    while (<>) {
	...		# your program 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 that haven't been modified for
at least a week:

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

This is faster than using the B<-exec> switch of B<find> because you don't
have to start a process on every filename found.  It does suffer from
the bug of mishandling newlines in pathnames, which you can fix if
you follow the example under B<-0>.

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