perlrun(3)
PERLRUN(1) Perl Programmers Reference Guide PERLRUN(1)
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
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 works
only 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.)
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
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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 $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 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
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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.
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.
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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.
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.
-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, END, and use blocks, because these are
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considered as occurring outside the execution of your
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 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 (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.
-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.
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-h prints a summary of the options.
-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
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 $oldargv to know when the filename has changed. It
does, however, use ARGVOUT for the selected filehandle.
Note that STDOUT is restored as the default output
filehandle after the loop.
You can use eof without parenthesis to locate the end
of each input file, in case you want to append to each
file, or reset line numbering (see example in the eof
entry in the perlfunc manpage).
-Idirectory
Directories specified by -I are prepended to the search
path for modules (@INC), and also tells the C
preprocessor where to search for include files. The C
preprocessor is invoked with -P; by default it searches
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/usr/include and /usr/lib/perl.
-l[octnum]
enables automatic line-ending processing. It has two
effects: first, it automatically chomps "$/" (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.
-n causes Perl to assume the following loop around your
script, which makes it iterate over filename arguments
somewhat like sed -n or awk:
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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.
-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 or die "-p destination: $!\n";
}
If a file named by an argument cannot be opened for
some reason, Perl warns you about it, and moves on to
the next file. Note that the lines are printed
automatically. An error occuring during printing is
treated as fatal. To suppress printing use the -n
switch. A -p overrides a -n switch.
BEGIN and END blocks may be used to capture control
before or after the implicit loop, just as in awk.
-P causes your script to be run through the C preprocessor
before compilation by Perl. (Because both comments and
cpp directives begin with the # character, you should
avoid starting comments with any words recognized by
the C preprocessor such as "if", "else", or "define".)
-s enables some rudimentary switch parsing for switches on
the command line after the script name but before any
filename arguments (or before a --). Any switch found
there is removed from @ARGV and sets the corresponding
variable in the Perl script. The following script
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prints "true" if and only if the script is invoked with
a -xyz switch.
#!/usr/bin/perl -s
if ($xyz) { print "true\n"; }
-S makes Perl use the PATH environment variable to search
for the script (unless the name of the script contains
directory separators). On some platforms, this also
makes Perl append suffixes to the filename while
searching for it. For example, on Win32 platforms, the
".bat" and ".cmd" suffixes are appended if a lookup for
the original name fails, and if the name does not
already end in one of those suffixes. If your Perl was
compiled with DEBUGGING turned on, using the -Dp switch
to Perl shows how the search progresses.
If the file 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 -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 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. To start up sh rather than csh,
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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 $running_under_some_shell;
-T forces "taint" checks to be turned on so you can test
them. Ordinarily these checks are done only when
running setuid or setgid. It's a good idea to turn
them on explicitly for programs run on another's
behalf, such as CGI programs. 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. Note that the -w switch (or the $^W variable)
must be used along with this option to actually
generate the taint-check warnings.
-v prints the version and patchlevel of your Perl
executable.
-V prints summary of the major perl configuration values
and the current value 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, and scalar variables that are used before
being set. Also warns about redefined subroutines, and
references to undefined filehandles or filehandles
opened read-only that you are attempting to write on.
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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.
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
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 is used.
PERL5LIB A colon-separated list of directories in which
to look for Perl library files before looking in
the standard library and the current directory.
If PERL5LIB is not defined, PERLLIB is used.
When running taint checks (because the script
was running setuid or setgid, or the -T switch
was used), neither variable is used. The script
should instead say
use lib "/my/directory";
PERL5OPT Command-line options (switches). Switches in
this variable are taken as if they were on every
Perl command line. Only the -[DIMUdmw] switches
are allowed. When running taint checks (because
the script was running setuid or setgid, or the
-T switch was used), this variable is ignored.
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PERLLIB A colon-separated list of directories in which
to look for Perl library files 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 for executing "backtick"
commands or system(). 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 your perl executable was built
with -DDEBUGGING_MSTATS, 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)};
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