PERLRUN

NAME

perlrun - how to execute the Perl interpreter

SYNOPSIS

DESCRIPTION

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

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.)

The #! line is always examined for switches as the line is being parsed. Thus, if you’re on a machine that allows only 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.

Some switches do care if they are processed twice, for instance combinations of -l and -0. Either put all the switches after the 32 character boundary (if applicable), or replace the use of -0digits by BEGIN{ $/ = "\0digits"; }.

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 /usr/bin/perl -wS $0 ${1+"$@"}’
if $running_under_some_shell;

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 part-way 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.

#! and quoting on non-Unix systems

Unix’s #! technique can be simulated on other systems:
OS/2

Put

extproc perl -S -your_switches

as the first line in *.cmd file (-S due to a bug in cmd.exe’s ’extproc’ handling).

MS - DOS

Create a batch file to run your script, and codify it in ALTERNATIVE_SHEBANG (see the dosish.h file in the source distribution for more information).

Win95/ NT

The Win95/ NT installation, when using the Activeware port of Perl, will modify the Registry to associate the .pl extension with the perl interpreter. If you install another port of Perl, including the one in the Win32 directory of the Perl distribution, then you’ll have to modify the Registry yourself. Note that this means you can no longer tell the difference between an executable Perl program and a Perl library file.

Macintosh

Macintosh perl scripts will have the appropriate Creator and Type, so that double-clicking them will invoke the perl application.

Command-interpreters on non-Unix systems have rather different ideas on quoting than Unix shells. You’ll need to learn the special characters in your command-interpreter (*, \ and " are common) and how to protect whitespace and these characters to run one-liners (see -e below).

On some systems, you may have to change single-quotes to double ones, which you must NOT do on Unix or Plan9 systems. You might also have to change a single % to a %%.

For example:

# Unix
perl -e ’print "Hello world\n"’

# MS-DOS, etc.
perl -e "print \"Hello world\n\""

# Macintosh
print "Hello world\n"
(then Run "Myscript" or Shift-Command-R)

# VMS
perl -e "print ""Hello world\n"""

The problem is that none of this is reliable: it depends on the command and it is entirely possible neither works. If 4DOS was the command shell, this would probably work better:

perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""

CMD . EXE in Windows NT slipped a lot of standard Unix functionality in when nobody was looking, but just try to find documentation for its quoting rules.

Under the Macintosh, it depends which environment you are using. The MacPerl shell, or MPW , is much like Unix shells in its support for several quoting variants, except that it makes free use of the Macintosh’s non- ASCII characters as control characters.

There is no general solution to all of this. It’s just a mess.

Location of Perl

It may seem obvious to say, but Perl is useful only when users can easily find it. When possible, it’s good for both /usr/bin/perl and /usr/local/bin/perl to be symlinks to the actual binary. If that can’t be done, system administrators are strongly encouraged to put (symlinks to) perl and its accompanying utilities, such as perldoc, into a directory typically found along a user’s PATH , or in another obvious and convenient place.

In this documentation, #!/usr/bin/perl on the first line of the script will stand in for whatever method works on your system.

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:
-0[digits]

specifies the input 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 because there is no legal character with that value.

perl -ane ’print pop(@F), "\n";’

is equivalent to

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

An alternate delimiter may be specified using -F.

-d:foo

runs the script under the control of a debugging or tracing module installed as Devel::foo. E.g., -d:DProf executes the script using the Devel::DProf profiler. See the perldebug manpage.

-Dletters
-Dnumber

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

1 p Tokenizing and parsing
2 s Stack snapshots
4 l Context (loop) stack processing
8 t Trace execution
16 o Method and overloading resolution
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 and execution
1024 x Syntax tree dump
2048 u Tainting checks
4096 L Memory leaks (needs C<-DLEAKTEST> when compiling Perl)
8192 H Hash dump -- usurps values()
16384 X Scratchpad allocation
32768 D Cleaning up

All these flags require -DDEBUGGING when you compile the Perl executable. This flag is automatically set if you include -g option when Configure asks you about optimizer/debugger flags.

-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.

-Fpattern

specifies the pattern to split on if -a is also in effect. The pattern may be surrounded by //, "", or ’’, otherwise it will be put in single quotes.

-i[extension]

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 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 * then it is appended to the end of the current filename as a suffix.

If the extension does contain one or more * characters, then each * 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’bak_*’ -e ’s/bar/baz/’ fileA # backup to ’bak_fileA’

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

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

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’.bak’ -e ’s/bar/baz/’ fileA # backup to ’fileA.bak’
$ perl -pi’*.bak’ -e ’s/bar/baz/’ fileA # backup to ’fileA.bak’

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
$extension = ’.bak’;
while (<>) {
if ($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 -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 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 the eof entry in the perlfunc manpage).

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 -i, see the section on Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn’t this a bug in Perl? in the perlfaq5 manpage.

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

Perl does not expand ~, so don’t do that.

Finally, note that the -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.

-Idirectory

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

-l[octnum]

enables automatic line-ending processing. It has two effects: first, it automatically chomps "$/" (the input record separator) when used with -n or -p, and second, it assigns "$\" (the output record separator) to have the value of octnum so that any print statements will have that separator 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.

-m[-]module
-M[-]module
-M[-]’module ...’
-[mM][-]module=arg[,arg]...

-mmodule executes use module (); before executing your script.

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

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

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

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

Note that the lines are not printed by default. See -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 -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.

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 -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.

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

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 -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 ${1+"$@"} with $*, 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;

-V:name

Prints to STDOUT the value of the named configuration variable.

You can disable specific warnings using __WARN__ hooks, as described in the perlvar manpage and the warn entry in the perlfunc manpage. See also 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. If a directory name is specified, Perl will switch to that directory before running the script. The -x switch controls only 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).

ENVIRONMENT

use lib "/my/directory";

BEGIN { require ’perl5db.pl’ }

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 cmd.exe /x/c on WindowsNT and 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).

PERL_DEBUG_MSTATS

Relevant only if perl is compiled with the malloc included with the perl distribution (that is, if 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.

PERL_DESTRUCT_LEVEL

Relevant only if your perl executable was built with - DDEBUGGING , this controls the behavior of global destruction of objects and other references.

Perl also has environment variables that control how Perl handles data specific to particular natural languages. See the perllocale manpage.

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)};