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/ perlrun

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 perldebug.

-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 "eof" in perlfunc).

-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 perlsec.

-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 perldiag and perltrap.

-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).