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Man Page for PERLRUN

     perlrun - how to execute the Perl interpreter

     perl [switches] filename args

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

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

     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

     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.


     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:

          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.

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


          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.

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

          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

          which is equivalent to

              while (<>) {
                  if ($ARGV ne $oldargv) {
                      rename($ARGV, $ARGV . '.bak');
                      open(ARGVOUT, ">$ARGV");
                      $oldargv = $ARGV;
              continue {
                  print;  # this prints to original filename

          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).

          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.

          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 "$\" to have the value of octnum so that any
          print statements will have that line terminator added
          back on.  If octnum is omitted, sets "$
          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

     -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 {

          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:

              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

                  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

     -v   prints the version and patchlevel of your Perl

     -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 a an array as though
          it were a scalar, if your subroutines recurse more than
          100 deep, and innumeriable 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).



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