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PERLRUN(1)							    PERLRUN(1)

NAME
       perlrun - how to execute the Perl interpreter

SYNOPSIS
       perl [switches] filename args

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

       1.  Specified line by line via -e switches on the command line.

       2.  Contained in the file specified by the first filename on the
	   command line.  (Note that systems supporting the #! notation invoke
	   interpreters this way.)

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

       With methods 2 and 3, Perl starts parsing the input file from the
       beginning, unless you've specified a -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 __END__ token.)

       As of Perl 5, the #! line is always examined for switches as the line
       is being parsed.	 Thus, if you're on a machine that only allows 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 -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 -I switch could also cause odd results.

       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 perl $0 -S ${1+"$@"}'
	       if 0;

       to let Perl see the -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 partway 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 exit(0) is provided to indicate successful completion.

       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:

       -0digits
	    specifies the record separator ($/) 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 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 since
	    there is no legal character with that value.

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

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

	    is equivalent to

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

	    An alternate delimiter may be specified using -F.

       -c   causes Perl to check the syntax of the script and then exit
	    without executing it.  Actually, it will execute BEGIN and use
	    blocks, since these are considered part of the compilation.

       -d   runs the script under the Perl debugger.  See the perldebug
	    manpage.

       -Dnumber

       -Dlist
	    sets debugging flags.  To watch how it executes your script, use
	    -D14.  (This only works if debugging is compiled into your Perl.)
	    Another nice value is -D1024, which lists your compiled syntax
	    tree.  And -D512 displays compiled regular expressions. As an
	    alternative specify a list of letters instead of numbers (e.g.
	    -D14 is equivalent to -Dtls):

		    1  p  Tokenizing and Parsing
		    2  s  Stack Snapshots
		    4  l  Label Stack Processing
		    8  t  Trace Execution
		   16  o  Operator Node Construction
		   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
		 1024  x  Syntax Tree Dump
		 2048  u  Tainting Checks
		 4096  L  Memory Leaks (not supported anymore)
		 8192  H  Hash Dump -- usurps values()
		16384  X  Scratchpad Allocation
		32768  D  Cleaning Up

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

       -Fregexp
	    specifies a regular expression to split on if -a is also in
	    effect.  If regexp has // around it, the slashes will be ignored.

       -iextension
	    specifies that files processed by the <> 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 added to the name of the old file to make a backup
	    copy.  If no extension is supplied, no backup is made.  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
		while (<>) {
		    if ($ARGV ne $oldargv) {
			rename($ARGV, $ARGV . '.bak');
			open(ARGVOUT, ">$ARGV");
			select(ARGVOUT);
			$oldargv = $ARGV;
		    }
		    s/foo/bar/;
		}
		continue {
		    print;  # this prints to original filename
		}
		select(STDOUT);

	    except that the -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.

	    You can use eof without parenthesis to locate the end of each
	    input file, in case you want to append to each file, or reset line
	    numbering (see example in the eof entry in the perlfunc manpage).

       -Idirectory
	    may be used in conjunction with -P to tell the C preprocessor
	    where to look for include files.  By default /usr/include and
	    /usr/lib/perl are searched.

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

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

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

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

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

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

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

	    Note that the lines are not printed by default.  See -p to have
	    lines printed.  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 -exec switch of find because you
	    don't have to start a process on every filename found.

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

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

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

	    Note that the lines are printed automatically.  To suppress
	    printing use the -n switch.	 A -p overrides a -n switch.

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

       -P   causes your script to be run through the C preprocessor before
	    compilation by Perl.  (Since 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".)

       -s   enables some rudimentary switch parsing for switches on the
	    command line after the script name but before any filename
	    arguments (or before a --).	 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 -xyz switch.

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

       -S   makes Perl use the PATH environment variable to search for the
	    script (unless the name of the script starts with a slash).
	    Typically this is used to emulate #! startup on machines that
	    don't support #!, in the following manner:

		#!/usr/bin/perl
		eval "exec /usr/bin/perl -S $0 $*"
			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 -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.  A better construct than
	    $* would be ${1+"$@"}, which handles embedded spaces and such in
	    the filenames, but doesn't work if the script is being interpreted
	    by csh.  In order 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 -S $0 ${1+"$@"}'
		    & eval 'exec /usr/bin/perl -S $0 $argv:q'
			    if 0;

       -T   forces "taint" checks to be turned on.  Ordinarily these checks
	    are done only when running setuid or setgid.  See the perlsec
	    manpage.

       -u   causes Perl to dump core after compiling your script.  You can
	    then take this core dump and turn it into an executable file by
	    using the 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 undump is platform specific and
	    may not be available for a specific port of Perl.

       -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.

       -v   prints the version and patchlevel of your Perl executable.

       -w   prints warnings about identifiers 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 readonly 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.  See the perldiag manpage and the
	    perltrap manpage.

       -x 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 (but only one group of switches, as with
	    normal #!  processing).  If a directory name is specified, Perl
	    will switch to that directory before running the script.  The -x
	    switch only controls the the disposal of leading garbage.  The
	    script must be terminated with __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).

3rd Berkeley Distribution					    PERLRUN(1)
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