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PERLRUN

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index 

NAME

perlrun - how to execute the Perl interpreter 

SYNOPSIS

perl   [ -sTuU ]
        [ -hv ] [ -V[:configvar] ]
        [ -cw ] [ -d[:debugger] ] [ -D[number/list] ]
        [ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal] ]
        [ -Idir ] [ -m[-]module ] [ -M[-]'module...' ]
        [ -P ]
        [ -S ]
        [ -x[dir] ]
        [ -i[extension] ]
        [ -e 'command' ] [ -- ] [ programfile ] [ argument ]...
 

DESCRIPTION

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

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 works only if there areno filename arguments---to pass arguments to a STDIN script youmust explicitly specify a ``-'' for the script name.

With methods 2 and 3, Perl starts parsing the input file from thebeginning, unless you've specified a -x switch, in which case itscans for the first line starting with #! and containing the word``perl'', and starts there instead. This is useful for running a scriptembedded in a larger message. (In this case you would indicate the endof the script using the __END__ token.)

The #! line is always examined for switches as the line is beingparsed. Thus, if you're on a machine that allows only one argumentwith the #! line, or worse, doesn't even recognize the #! line, youstill can get consistent switch behavior regardless of how Perl wasinvoked, even if -x was used to find the beginning of the script.

Because many operating systems silently chop off kernel interpretation ofthe #! line after 32 characters, some switches may be passed in on thecommand line, and some may not; you could even get a ``-'' without itsletter, if you're not careful. You probably want to make sure that allyour switches fall either before or after that 32 character boundary.Most switches don't actually care if they're processed redundantly, butgetting a - instead of a complete switch could cause Perl to try toexecute standard input instead of your script. And a partial -I switchcould also cause odd results.

Some switches do care if they are processed twice, for instance combinationsof -l and -0. Either put all the switches after the 32 characterboundary (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 $0 -S ${1+"$@"}'        if $running_under_some_shell;

to let Perl see the -p switch.

If the #! line does not contain the word ``perl'', the program named afterthe #! is executed instead of the Perl interpreter. This is slightlybizarre, but it helps people on machines that don't do #!, because theycan tell a program that their SHELL is /usr/bin/perl, and Perl will thendispatch the program to the correct interpreter for them.

After locating your script, Perl compiles the entire script to aninternal form. If there are any compilation errors, execution of thescript 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 scriptruns off the end without hitting an exit() or die() operator, an implicitexit(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 inALTERNATIVE_SHEBANG (see the dosish.h file in the sourcedistribution 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 perlinterpreter. If you install another port of Perl, including the onein the Win32 directory of the Perl distribution, then you'll have tomodify the Registry yourself.

Macintosh

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

Command-interpreters on non-Unix systems have rather different ideason quoting than Unix shells. You'll need to learn the specialcharacters in your command-interpreter (*, \ and " arecommon) and how to protect whitespace and these characters to runone-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 alsohave 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 commandand it is entirely possible neither works. If 4DOS was the command shell, this wouldprobably work better:

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

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

Under the Macintosh, it depends which environment you are using. The MacPerlshell, or MPW, is much like Unix shells in its support for severalquoting variants, except that it makes free use of the Macintosh's non-ASCIIcharacters as control characters.

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

Switches

A single-character switch may be combined with the following switch, ifany.

    #!/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 areno digits, the null character is the separator. Other switches mayprecede or follow the digits. For example, if you have a version offind which can print filenames terminated by the null character, youcan 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 nolegal character with that value.

-a

turns on autosplit mode when used with a -n or -p. An implicitsplit command to the @F array is done as the first thing inside theimplicit 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 withoutexecuting it. Actually, it will execute BEGIN, END, and use blocks,because these are considered as occurring outside the execution ofyour program.

-d

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

-d:foo

runs the script under the control of a debugging or tracing moduleinstalled as Devel::foo. E.g., -d:DProf executes the script using theDevel::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 yourPerl.) Another nice value is -Dx, which lists your compiledsyntax tree. And -Dr displays compiled regular expressions. As analternative, specify a number instead of list of letters (e.g., -D14 isequivalent 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 (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, Perlwill not look for a script filename in the argument list.Multiple -e commands maybe 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. Thepattern may be surrounded by //, "", or '', otherwise it will beput in single quotes.

-h

prints a summary of the options.

-i[extension]

specifies that files processed by the <> construct are to be editedin-place. It does this by renaming the input file, opening the outputfile by the original name, and selecting that output file as the defaultfor print() statements. The extension, if supplied, is added to the nameof the old file to make a backup copy. If no extension is supplied, nobackup 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 toknow when the filename has changed. It does, however, use ARGVOUT forthe selected filehandle. Note that STDOUT is restored as thedefault 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 (seeexample in the eof entry in the perlfunc manpage).

-Idirectory

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

-l[octnum]

enables automatic line-ending processing. It has two effects: first,it automatically chomps ``$/'' (the input record separator) when usedwith -n or -p, and second, it assigns ``$\''(the output record separator) to have the value of octnum so thatany print statements will have that separator added back on. Ifoctnum is omitted, sets ``$\'' to the current value of ``$/''. Forinstance, 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 recordseparator 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 yourscript.

-Mmodule executes use module ; before executing yourscript. 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 whenimporting symbols. The actual code generated by -Mmodule=foo,bar isuse module split(/,/,q{foo,bar}). Note that the = formremoves the distinction between -m and -M.

-n

causes Perl to assume the following loop around your script, whichmakes it iterate over filename arguments somewhat like sed -n orawk:

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

Note that the lines are not printed by default. See -p to havelines printed. If a file named by an argument cannot be opened forsome 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'thave to start a process on every filename found.

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

-p

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

    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, Perlwarns you about it, and moves on to the next file. Note that thelines are printed automatically. An error occuring during printing istreated as fatal. To suppress printing use the -n switch. A -poverrides a -n switch.

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

-P

causes your script to be run through the C preprocessor beforecompilation by Perl. (Because both comments and cpp directives beginwith the # character, you should avoid starting comments with any wordsrecognized by the C preprocessor such as ``if'', ``else'', or ``define''.)

-s

enables some rudimentary switch parsing for switches on the commandline after the script name but before any filename arguments (or beforea --). Any switch found there is removed from @ARGV and sets thecorresponding variable in the Perl script. The following scriptprints ``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 thescript (unless the name of the script contains directory separators).On some platforms, this also makes Perl append suffixes to thefilename while searching for it. For example, on Win32 platforms,the ``.bat'' and ``.cmd'' suffixes are appended if a lookup for theoriginal name fails, and if the name does not already end in oneof those suffixes. If your Perl was compiled with DEBUGGING turnedon, using the -Dp switch to Perl shows how the search progresses.

If the file supplied contains directory separators (i.e. it is anabsolute or relative pathname), and if the file is not found,platforms that append file extensions will do so and try to lookfor the file with those extensions added, one by one.

On DOS-like platforms, if the script does not contain directoryseparators, it will first be searched for in the current directorybefore being searched for on the PATH. On Unix platforms, thescript will be searched for strictly on the PATH.

Typically this is used to emulate #! startup on platforms thatdon't support #!. This example works on many platforms thathave a shell compatible with Bourne shell:

    #!/usr/bin/perl    eval 'exec /usr/bin/perl -S $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 thusstarts up the Perl interpreter. On some systems $0 doesn't alwayscontain the full pathname, so the -S tells Perl to search for thescript if necessary. After Perl locates the script, it parses thelines and ignores them because the variable $running_under_some_shellis never true. A better construct than $* would be ${1+"$@"}, whichhandles embedded spaces and such in the filenames, but doesn't work ifthe script is being interpreted by csh. To start up sh ratherthan csh, some systems may have to replace the #! line with a linecontaining just a colon, which will be politely ignored by Perl. Othersystems can't control that, and need a totally devious construct thatwill 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 $running_under_some_shell;

-T

forces ``taint'' checks to be turned on so you can test them. Ordinarily these checks aredone only when running setuid or setgid. It's a good idea to turnthem on explicitly for programs run on another's behalf, such as CGIprograms. See the perlsec manpage.

-u

causes Perl to dump core after compiling your script. You can thentake this core dump and turn it into an executable file by using theundump program (not supplied). This speeds startup at the expense ofsome disk space (which you can minimize by stripping the executable).(Still, a ``hello world'' executable comes out to about 200K on mymachine.) If you want to execute a portion of your script before dumping,use the dump() operator instead. Note: availability of undump isplatform specific and may not be available for a specific port ofPerl.

-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 intowarnings. Note that the -w switch (or the $^W variable) mustbe used along with this option to actually generate thetaint-check warnings.

-v

prints the version and patchlevel of your Perl executable.

-V

prints summary of the major perl configuration values and the currentvalue of @INC.

-V:name

Prints to STDOUT the value of the named configuration variable.

-w

prints warnings about variable names that are mentioned only once, andscalar variables that are used before being set. Also warns aboutredefined subroutines, and references to undefined filehandles orfilehandles opened read-only that you are attempting to write on. Alsowarns 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 recursemore than 100 deep, and innumerable other things.

You can disable specific warnings using __WARN__ hooks, as describedin 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. Leadinggarbage will be discarded until the first line that starts with #! andcontains the string ``perl''. Any meaningful switches on that line willbe applied. If a directory name is specified, Perl will switch tothat directory before running the script. The -x switch controlsonly the disposal of leading garbage. The script must beterminated with __END__ if there is trailing garbage to be ignored (thescript can process any or all of the trailing garbage via the DATAfilehandle if desired).

 

ENVIRONMENT

HOME

Used if chdir has no argument.

LOGDIR

Used if chdir has no argument and HOME is not set.

PATH

Used in executing subprocesses, and in finding the script if -S isused.

PERL5LIB

A colon-separated list of directories in which to look for Perl libraryfiles before looking in the standard library and the currentdirectory. If PERL5LIB is not defined, PERLLIB is used. When runningtaint checks (because the script was running setuid or setgid, or the-T switch was used), neither variable is used. The script shouldinstead say

    use lib "/my/directory";

PERL5OPT

Command-line options (switches). Switches in this variable are takenas if they were on every Perl command line. Only the -[DIMUdmw]switches are allowed. When running taint checks (because the scriptwas running setuid or setgid, or the -T switch was used), thisvariable is ignored.

PERLLIB

A colon-separated list of directories in which to look for Perl libraryfiles before looking in the standard library and the current directory.If PERL5LIB is defined, PERLLIB is not used.

PERL5DB

The command used to load the debugger code. The default is:

        BEGIN { require 'perl5db.pl' }

PERL5SHELL (specific to WIN32 port)

May be set to an alternative shell that perl must use internally forexecuting ``backtick'' commands or system(). Perl doesn't use COMSPECfor this purpose because COMSPEC has a high degree of variabilityamong users, leading to portability concerns. Besides, perl can usea shell that may not be fit for interactive use, and setting COMSPECto such a shell may interfere with the proper functioning of otherprograms (which usually look in COMSPEC to find a shell fit forinteractive use).

PERL_DEBUG_MSTATS

Relevant only if your perl executable was built with -DDEBUGGING_MSTATS,if set, this causes memory statistics to be dumped after execution. If setto an integer greater than one, also causes memory statistics to be dumpedafter compilation.

PERL_DESTRUCT_LEVEL

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

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

Apart from these, Perl uses no other environment variables, exceptto make them available to the script being executed, and to childprocesses. However, scripts running setuid would do well to executethe following lines before doing anything else, just to keep peoplehonest:

    $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)};

---

 

Index

NAME

SYNOPSIS

DESCRIPTION

#! and quoting on non-Unix systems

Switches

ENVIRONMENT

---

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