perlrun - how to execute the Perl interpreter
perl [ -sTtuUWX ] [ -hv ] [ -V[:configvar] ] [ -cw ] [ -d[t][:debugger] ] [ -D[number/list] ] [ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal/hexadecimal] ] [ -Idir ] [ -m[-]module ] [ -M[-]'module...' ] [ -f ] [ -C [number/list] ] [ -S ] [ -x[dir] ] [ -i[extension] ] [ [-e|-E] 'command' ] [ -- ] [ programfile ] [ argument ]...
The normal way to run a Perl program is by making it directly executable, or else by passing the name of the source file as an argument on the command line. (An interactive Perl environment is also possible--see perldebug for details on how to do that.) Upon startup, Perl looks for your program in one of the following places:
Specified line by line via -e or -E switches on the command line.
Contained in the file specified by the first filename on the command line.
(Note that systems supporting the #!
notation invoke interpreters this
way. See Location of Perl.)
Passed in implicitly via standard input. This works only if there are no filename arguments--to pass arguments to a STDIN-read program you must explicitly specify a "-" for the program 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 program
embedded in a larger message. (In this case you would indicate the end
of the program 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 behaviour regardless of how Perl was
invoked, even if -x was used to find the beginning of the program.
Because historically some operating systems silently chopped 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 program. 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-*-
- eval 'exec perl -x -wS $0 ${1+"$@"}'
- if 0;
to let Perl see the -p switch.
A similar trick involves the env program, if you have it.
- #!/usr/bin/env perl
The examples above use a relative path to the perl interpreter,
getting whatever version is first in the user's path. If you want
a specific version of Perl, say, perl5.14.1, you should place
that directly in the #!
line's path.
If the #!
line does not contain the word "perl" nor the word "indir",
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 program, Perl compiles the entire program to an internal form. If there are any compilation errors, execution of the program is not attempted. (This is unlike the typical shell script, which might run part-way through before finding a syntax error.)
If the program is syntactically correct, it is executed. If the program
runs off the end without hitting an exit() or die() operator, an implicit
exit(0) is provided to indicate successful completion.
Unix's #!
technique can be simulated on other systems:
Put
- extproc perl -S -your_switches
as the first line in *.cmd file (-S due to a bug in cmd.exe's
`extproc' handling).
Create a batch file to run your program, and codify it in
ALTERNATE_SHEBANG
(see the dosish.h file in the source
distribution for more information).
The Win95/NT installation, when using the ActiveState installer for Perl, will modify the Registry to associate the .pl extension with the perl interpreter. If you install Perl by other means (including building from the sources), you may 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.
Put
at the top of your program, where -mysw are any command line switches you
want to pass to Perl. You can now invoke the program directly, by saying
perl program
, or as a DCL procedure, by saying @program
(or implicitly
via DCL$PATH by just using the name of the program).
This incantation is a bit much to remember, but Perl will display it for
you if you say perl "-V:startperl"
.
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 Plan 9 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\""
- # 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 were 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.
There is no general solution to all of this. It's just a mess.
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 into a directory typically found along a user's PATH, or in some other obvious and convenient place.
In this documentation, #!/usr/bin/perl
on the first line of the program
will stand in for whatever method works on your system. You are
advised to use a specific path if you care about a specific version.
- #!/usr/local/bin/perl5.14
or if you just want to be running at least version, place a statement like this at the top of your program:
- use 5.014;
As with all standard commands, a single-character switch may be clustered with the following switch, if any.
- #!/usr/bin/perl -spi.orig # same as -s -p -i.orig
A --
signals the end of options and disables further option processing. Any
arguments after the --
are treated as filenames and arguments.
Switches include:
specifies the input record separator ($/
) as an octal or
hexadecimal 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 '*.orig' -print0 | perl -n0e unlink
The special value 00 will cause Perl to slurp files in paragraph mode. Any value 0400 or above will cause Perl to slurp files whole, but by convention the value 0777 is the one normally used for this purpose.
You can also specify the separator character using hexadecimal notation:
-0xHHH..., where the H are valid hexadecimal digits. Unlike
the octal form, this one may be used to specify any Unicode character, even
those beyond 0xFF. So if you really want a record separator of 0777,
specify it as -0x1FF. (This means that you cannot use the -x option
with a directory name that consists of hexadecimal digits, or else Perl
will think you have specified a hex number to -0.)
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
An alternate delimiter may be specified using -F.
-a implicitly sets -n.
The -C flag controls some of the Perl Unicode features.
As of 5.8.1, the -C can be followed either by a number or a list of option letters. The letters, their numeric values, and effects are as follows; listing the letters is equal to summing the numbers.
- I 1 STDIN is assumed to be in UTF-8
- O 2 STDOUT will be in UTF-8
- E 4 STDERR will be in UTF-8
- S 7 I + O + E
- i 8 UTF-8 is the default PerlIO layer for input streams
- o 16 UTF-8 is the default PerlIO layer for output streams
- D 24 i + o
- A 32 the @ARGV elements are expected to be strings encoded
- in UTF-8
- L 64 normally the "IOEioA" are unconditional, the L makes
- them conditional on the locale environment variables
- (the LC_ALL, LC_CTYPE, and LANG, in the order of
- decreasing precedence) -- if the variables indicate
- UTF-8, then the selected "IOEioA" are in effect
- a 256 Set ${^UTF8CACHE} to -1, to run the UTF-8 caching
- code in debugging mode.
For example, -COE and -C6 will both turn on UTF-8-ness on both STDOUT and STDERR. Repeating letters is just redundant, not cumulative nor toggling.
The io
options mean that any subsequent open() (or similar I/O
operations) in the current file scope will have the :utf8
PerlIO layer
implicitly applied to them, in other words, UTF-8 is expected from any
input stream, and UTF-8 is produced to any output stream. This is just
the default, with explicit layers in open() and with binmode() one can
manipulate streams as usual.
-C on its own (not followed by any number or option list), or the
empty string ""
for the PERL_UNICODE
environment variable, has the
same effect as -CSDL. In other words, the standard I/O handles and
the default open() layer are UTF-8-fied but only if the locale
environment variables indicate a UTF-8 locale. This behaviour follows
the implicit (and problematic) UTF-8 behaviour of Perl 5.8.0.
(See UTF-8 no longer default under UTF-8 locales in perl581delta.)
You can use -C0 (or "0"
for PERL_UNICODE
) to explicitly
disable all the above Unicode features.
The read-only magic variable ${^UNICODE}
reflects the numeric value
of this setting. This variable is set during Perl startup and is
thereafter read-only. If you want runtime effects, use the three-arg
open() (see open), the two-arg binmode() (see binmode),
and the open pragma (see open).
(In Perls earlier than 5.8.1 the -C switch was a Win32-only switch that enabled the use of Unicode-aware "wide system call" Win32 APIs. This feature was practically unused, however, and the command line switch was therefore "recycled".)
Note: Since perl 5.10.1, if the -C option is used on the #!
line,
it must be specified on the command line as well, since the standard streams
are already set up at this point in the execution of the perl interpreter.
You can also use binmode() to set the encoding of an I/O stream.
causes Perl to check the syntax of the program and then exit without
executing it. Actually, it will execute any BEGIN
, UNITCHECK
,
or CHECK
blocks and any use statements: these are considered as
occurring outside the execution of your program. INIT
and END
blocks, however, will be skipped.
runs the program under the Perl debugger. See perldebug. If t is specified, it indicates to the debugger that threads will be used in the code being debugged.
runs the program under the control of a debugging, profiling, or tracing
module installed as Devel::MOD. E.g., -d:DProf executes the
program using the Devel::DProf
profiler. As with the -M flag, options
may be passed to the Devel::MOD package where they will be received
and interpreted by the Devel::MOD::import routine. Again, like -M,
use --d:-MOD to call Devel::MOD::unimport instead of import. The
comma-separated list of options must follow a =
character. If t is
specified, it indicates to the debugger that threads will be used in the
code being debugged. See perldebug.
sets debugging flags. This switch is enabled only if your perl binary has been built with debugging enabled: normal production perls won't have been.
For example, to watch how perl executes your program, use -Dtls. Another nice value is -Dx, which lists your compiled syntax tree, and -Dr displays compiled regular expressions; the format of the output is explained in perldebguts.
As an alternative, specify a number instead of list of letters (e.g., -D14 is equivalent to -Dtls):
- 1 p Tokenizing and parsing (with v, displays parse
- stack)
- 2 s Stack snapshots (with v, displays all stacks)
- 4 l Context (loop) stack processing
- 8 t Trace execution
- 16 o Method and overloading resolution
- 32 c String/numeric conversions
- 64 P Print profiling info, source file input state
- 128 m Memory and SV allocation
- 256 f Format processing
- 512 r Regular expression parsing and execution
- 1024 x Syntax tree dump
- 2048 u Tainting checks
- 4096 U Unofficial, User hacking (reserved for private,
- unreleased use)
- 8192 H Hash dump -- usurps values()
- 16384 X Scratchpad allocation
- 32768 D Cleaning up
- 65536 S Op slab allocation
- 131072 T Tokenizing
- 262144 R Include reference counts of dumped variables
- (eg when using -Ds)
- 524288 J show s,t,P-debug (don't Jump over) on opcodes within
- package DB
- 1048576 v Verbose: use in conjunction with other flags
- 2097152 C Copy On Write
- 4194304 A Consistency checks on internal structures
- 8388608 q quiet - currently only suppresses the "EXECUTING"
- message
- 16777216 M trace smart match resolution
- 33554432 B dump suBroutine definitions, including special
- Blocks like BEGIN
- 67108864 L trace Locale-related info; what gets output is very
- subject to change
- 134217728 i trace PerlIO layer processing. Set PERLIO_DEBUG to
- the filename to trace to.
All these flags require -DDEBUGGING when you compile the Perl
executable (but see :opd
in Devel::Peek or 'debug' mode in re
which may change this).
See the INSTALL file in the Perl source distribution
for how to do this.
If you're just trying to get a print out of each line of Perl code
as it executes, the way that sh -x
provides for shell scripts,
you can't use Perl's -D switch. Instead do this
- # If you have "env" utility
- env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
- # Bourne shell syntax
- $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
- # csh syntax
- % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
See perldebug for details and variations.
may be used to enter one line of program. If -e is given, Perl will not look for a 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.
behaves just like -e, except that it implicitly enables all optional features (in the main compilation unit). See feature.
Disable executing $Config{sitelib}/sitecustomize.pl at startup.
Perl can be built so that it by default will try to execute $Config{sitelib}/sitecustomize.pl at startup (in a BEGIN block). This is a hook that allows the sysadmin to customize how Perl behaves. It can for instance be used to add entries to the @INC array to make Perl find modules in non-standard locations.
Perl actually inserts the following code:
Since it is an actual do (not a require), sitecustomize.pl
doesn't need to return a true value. The code is run in package main
,
in its own lexical scope. However, if the script dies, $@
will not
be set.
The value of $Config{sitelib}
is also determined in C code and not
read from Config.pm
, which is not loaded.
The code is executed very early. For example, any changes made to
@INC
will show up in the output of `perl -V`. Of course, END
blocks will be likewise executed very late.
To determine at runtime if this capability has been compiled in your
perl, you can check the value of $Config{usesitecustomize}
.
specifies the pattern to split on for -a. The pattern may be
surrounded by //
, ""
, or ''
, otherwise it will be put in single
quotes. You can't use literal whitespace or NUL characters in the pattern.
-F implicitly sets both -a and -n.
prints a summary of the options.
specifies that files processed by the <>
construct are to be
edited in-place. It does this by renaming the input file, opening the
output file by the original name, and selecting that output file as the
default for print() statements. The extension, if supplied, is used to
modify the name of the old file to make a backup copy, following these
rules:
If no extension is supplied, and your system supports it, the original file is kept open without a name while the output is redirected to a new file with the original filename. When perl exits, cleanly or not, the original file is unlinked.
If the extension doesn't contain a *
, then it is appended to the
end of the current filename as a suffix. If the extension does
contain one or more *
characters, then each *
is replaced
with the current filename. In Perl terms, you could think of this
as:
- ($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'orig_*' -e 's/bar/baz/' fileA # backup to
- # 'orig_fileA'
Or even to place backup copies of the original files into another directory (provided the directory already exists):
- $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to
- # 'old/fileA.orig'
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'.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
- $ perl -pi'*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
From the shell, saying
- $ perl -p -i.orig -e "s/foo/bar/; ... "
is the same as using the program:
- #!/usr/bin/perl -pi.orig
- s/foo/bar/;
which is equivalent to
- #!/usr/bin/perl
- $extension = '.orig';
- LINE: while (<>) {
- if ($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'.orig' -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 eof).
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 Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl? in perlfaq5.
You cannot use -i to create directories or to strip extensions from files.
Perl does not expand ~
in filenames, which is good, since some
folks use it for their backup files:
- $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
Note that because -i renames or deletes the original file before creating a new file of the same name, Unix-style soft and hard links will not be preserved.
Finally, 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.
Directories specified by -I are prepended to the search path for
modules (@INC
).
enables automatic line-ending processing. It has two separate
effects. First, it automatically chomps $/
(the input record
separator) when used with -n or -p. 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.
-mmodule executes use module ();
before executing your
program.
-Mmodule executes use module ;
before executing your
program. You can use quotes to add extra code after the module name,
e.g., '-MMODULE 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
'-MMODULE 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; that is,
-mMODULE=foo,bar is the same as -MMODULE=foo,bar.
A consequence of this is that -MMODULE=number never does a version check,
unless MODULE::import() itself is set up to do a version check, which
could happen for example if MODULE inherits from Exporter.
causes Perl to assume the following loop around your program, which makes it iterate over filename arguments somewhat like sed -n or awk:
- LINE:
- while (<>) {
- ... # your program 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.
Also note that <>
passes command line arguments to
open, which doesn't necessarily interpret them as file names.
See perlop for possible security implications.
Here is an efficient way to delete all files that haven't been modified for at least a week:
This is faster than using the -exec switch of find because you don't have to start a process on every filename found (but it's not faster than using the -delete switch available in newer versions of find. It does suffer from the bug of mishandling newlines in pathnames, which you can fix if you follow the example under -0.
BEGIN
and END
blocks may be used to capture control before or after
the implicit program loop, just as in awk.
causes Perl to assume the following loop around your program, which makes it iterate over filename arguments somewhat like sed:
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 occurring 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.
enables rudimentary switch parsing for switches on the command line after the program name but before any filename arguments (or before an argument of --). Any switch found there is removed from @ARGV and sets the corresponding variable in the Perl program. The following program prints "1" if the program is invoked with a -xyz switch, and "abc" if it is invoked with -xyz=abc.
- #!/usr/bin/perl -s
- if ($xyz) { print "$xyz\n" }
Do note that a switch like --help creates the variable ${-help}
, which is
not compliant with use strict "refs"
. Also, when using this option on a
script with warnings enabled you may get a lot of spurious "used only once"
warnings.
makes Perl use the PATH environment variable to search for the program unless the name of the program contains path 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.
Typically this is used to emulate #!
startup on platforms that don't
support #!
. It's also convenient when debugging a script that uses #!
,
and is thus normally found by the shell's $PATH search mechanism.
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 program to /bin/sh,
which proceeds to try to execute the Perl program 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
program if necessary. After Perl locates the program, it parses the
lines and ignores them because the variable $running_under_some_shell
is never true. If the program 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:
If the filename supplied contains directory separators (and so is an absolute or relative pathname), and if that 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 program 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 program will be searched for strictly on the PATH.
Like -T, but taint checks will issue warnings rather than fatal
errors. These warnings can now be controlled normally with no warnings
qw(taint)
.
Note: This is not a substitute for -T
! This is meant to be
used only as a temporary development aid while securing legacy code:
for real production code and for new secure code written from scratch,
always use the real -T.
turns on "taint" 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 that run on behalf
of someone else whom you might not necessarily trust, such as CGI
programs or any internet servers you might write in Perl. See
perlsec for details. For security reasons, this option must be
seen by Perl quite early; usually this means it must appear early
on the command line or in the #!
line for systems which support
that construct.
This switch causes Perl to dump core after compiling your program. You can then in theory 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 program 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.
allows Perl to do unsafe operations. Currently the only "unsafe" operations are attempting to unlink directories while running as superuser and running setuid programs with fatal taint checks turned into warnings. Note that warnings must be enabled along with this option to actually generate the taint-check warnings.
prints the version and patchlevel of your perl executable.
prints summary of the major perl configuration values and the current values of @INC.
Prints to STDOUT the value of the named configuration variable(s),
with multiples when your configvar argument looks like a regex (has
non-letters). For example:
- $ perl -V:libc
- libc='/lib/libc-2.2.4.so';
- $ perl -V:lib.
- libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
- libc='/lib/libc-2.2.4.so';
- $ perl -V:lib.*
- libpth='/usr/local/lib /lib /usr/lib';
- libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
- lib_ext='.a';
- libc='/lib/libc-2.2.4.so';
- libperl='libperl.a';
- ....
Additionally, extra colons can be used to control formatting. A trailing colon suppresses the linefeed and terminator ";", allowing you to embed queries into shell commands. (mnemonic: PATH separator ":".)
- $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
- compression-vars: zcat='' zip='zip' are here !
A leading colon removes the "name=" part of the response, this allows you to map to the name you need. (mnemonic: empty label)
- $ echo "goodvfork="`./perl -Ilib -V::usevfork`
- goodvfork=false;
Leading and trailing colons can be used together if you need
positional parameter values without the names. Note that in the case
below, the PERL_API
params are returned in alphabetical order.
- $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
- building_on 'linux' '5' '1' '9' now
prints warnings about dubious constructs, such as variable names mentioned only once and scalar variables used before being set; redefined subroutines; references to undefined filehandles; filehandles opened read-only that you are attempting to write on; values used as a number that don'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.
This switch really just enables the global $^W
variable; normally,
the lexically scoped use warnings
pragma is preferred. You
can disable or promote into fatal errors specific warnings using
__WARN__
hooks, as described in perlvar and warn.
See also perldiag and perltrap. A fine-grained warning
facility is also available if you want to manipulate entire classes
of warnings; see warnings.
Enables all warnings regardless of no warnings
or $^W
.
See warnings.
Disables all warnings regardless of use warnings
or $^W
.
See warnings.
tells Perl that the program is embedded in a larger chunk of unrelated
text, such as in a mail 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.
All references to line numbers by the program (warnings, errors, ...)
will treat the #!
line as the first line.
Thus a warning on the 2nd line of the program, which is on the 100th
line in the file will be reported as line 2, not as line 100.
This can be overridden by using the #line
directive.
(See Plain Old Comments (Not!) in perlsyn)
If a directory name is specified, Perl will switch to that directory
before running the program. The -x switch controls only the
disposal of leading garbage. The program must be terminated with
__END__
if there is trailing garbage to be ignored; the program
can process any or all of the trailing garbage via the DATA
filehandle
if desired.
The directory, if specified, must appear immediately following the -x with no intervening whitespace.
Used if chdir has no argument.
Used if chdir has no argument and HOME is not set.
Used in executing subprocesses, and in finding the program if -S is used.
A list of directories in which to look for Perl library
files before looking in the standard library and the current
directory. Any architecture-specific and version-specific directories,
such as version/archname/, version/, or archname/ under the
specified locations are automatically included if they exist, with this
lookup done at interpreter startup time. In addition, any directories
matching the entries in $Config{inc_version_list}
are added.
(These typically would be for older compatible perl versions installed
in the same directory tree.)
If PERL5LIB is not defined, PERLLIB is used. Directories are separated
(like in PATH) by a colon on Unixish platforms and by a semicolon on
Windows (the proper path separator being given by the command perl
-V:path_sep).
When running taint checks, either because the program was running setuid or setgid, or the -T or -t switch was specified, neither PERL5LIB nor PERLLIB is consulted. The program should instead say:
- use lib "/my/directory";
Command-line options (switches). Switches in this variable are treated as if they were on every Perl command line. Only the -[CDIMUdmtwW] switches are allowed. When running taint checks (either because the program was running setuid or setgid, or because the -T or -t switch was used), this variable is ignored. If PERL5OPT begins with -T, tainting will be enabled and subsequent options ignored. If PERL5OPT begins with -t, tainting will be enabled, a writable dot removed from @INC, and subsequent options honored.
A space (or colon) separated list of PerlIO layers. If perl is built to use PerlIO system for IO (the default) these layers affect Perl's IO.
It is conventional to start layer names with a colon (for example, :perlio
) to
emphasize their similarity to variable "attributes". But the code that parses
layer specification strings, which is also used to decode the PERLIO
environment variable, treats the colon as a separator.
An unset or empty PERLIO is equivalent to the default set of layers for
your platform; for example, :unix:perlio on Unix-like systems
and :unix:crlf on Windows and other DOS-like systems.
The list becomes the default for all Perl's IO. Consequently only built-in
layers can appear in this list, as external layers (such as :encoding()
) need
IO in order to load them! See open pragma for how to add external
encodings as defaults.
Layers it makes sense to include in the PERLIO environment variable are briefly summarized below. For more details see PerlIO.
A pseudolayer that turns the :utf8
flag off for the layer below;
unlikely to be useful on its own in the global PERLIO environment variable.
You perhaps were thinking of :crlf:bytes or :perlio:bytes.
A layer which does CRLF to "\n"
translation distinguishing "text" and
"binary" files in the manner of MS-DOS and similar operating systems.
(It currently does not mimic MS-DOS as far as treating of Control-Z
as being an end-of-file marker.)
A layer that implements "reading" of files by using mmap(2) to make an entire file appear in the process's address space, and then using that as PerlIO's "buffer".
This is a re-implementation of stdio-like buffering written as a
PerlIO layer. As such it will call whatever layer is below it for
its operations, typically :unix
.
An experimental pseudolayer that removes the topmost layer. Use with the same care as is reserved for nitroglycerine.
A pseudolayer that manipulates other layers. Applying the :raw
layer is equivalent to calling binmode($fh). It makes the stream
pass each byte as-is without translation. In particular, both CRLF
translation and intuiting :utf8
from the locale are disabled.
Unlike in earlier versions of Perl, :raw
is not
just the inverse of :crlf
: other layers which would affect the
binary nature of the stream are also removed or disabled.
This layer provides a PerlIO interface by wrapping system's ANSI C "stdio"
library calls. The layer provides both buffering and IO.
Note that the :stdio
layer does not do CRLF translation even if that
is the platform's normal behaviour. You will need a :crlf
layer above it
to do that.
A pseudolayer that enables a flag in the layer below to tell Perl
that output should be in utf8 and that input should be regarded as
already in valid utf8 form. WARNING: It does not check for validity and as such
should be handled with extreme caution for input, because security violations
can occur with non-shortest UTF-8 encodings, etc. Generally :encoding(UTF-8)
is
the best option when reading UTF-8 encoded data.
On Win32 platforms this experimental layer uses native "handle" IO rather than a Unix-like numeric file descriptor layer. Known to be buggy in this release (5.14).
The default set of layers should give acceptable results on all platforms
For Unix platforms that will be the equivalent of "unix perlio" or "stdio". Configure is set up to prefer the "stdio" implementation if the system's library provides for fast access to the buffer; otherwise, it uses the "unix perlio" implementation.
On Win32 the default in this release (5.14) is "unix crlf". Win32's "stdio"
has a number of bugs/mis-features for Perl IO which are somewhat depending
on the version and vendor of the C compiler. Using our own crlf
layer as
the buffer avoids those issues and makes things more uniform. The crlf
layer provides CRLF conversion as well as buffering.
This release (5.14) uses unix
as the bottom layer on Win32, and so still
uses the C compiler's numeric file descriptor routines. There is an
experimental native win32
layer, which is expected to be enhanced and
should eventually become the default under Win32.
The PERLIO environment variable is completely ignored when Perl is run in taint mode.
If set to the name of a file or device when Perl is run with the -Di command-line switch, the logging of certain operations of the PerlIO subsystem will be redirected to the specified file rather than going to stderr, which is the default. The file is opened in append mode. Typical uses are in Unix:
- % env PERLIO_DEBUG=/tmp/perlio.log perl -Di script ...
and under Win32, the approximately equivalent:
- > set PERLIO_DEBUG=CON
- perl -Di script ...
This functionality is disabled for setuid scripts, for scripts run
with -T, and for scripts run on a Perl built without -DDEBUGGING
support.
A 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.
The PERLLIB environment variable is completely ignored when Perl is run in taint mode.
The command used to load the debugger code. The default is:
- BEGIN { require "perl5db.pl" }
The PERL5DB environment variable is only used when Perl is started with a bare -d switch.
If set to a true value, indicates to the debugger that the code being debugged uses threads.
On Win32 ports only, may be set to an alternative shell that Perl must use
internally for executing "backtick" commands or system(). Default is
cmd.exe /x/d/c
on WindowsNT and command.com /c
on Windows95. The
value is considered space-separated. Precede any character that
needs to be protected, like a space or backslash, with another 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).
Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint checked
when running external commands. It is recommended that
you explicitly set (or delete) $ENV{PERL5SHELL}
when running
in taint mode under Windows.
Set to 1 to allow the use of non-IFS compatible LSPs (Layered Service Providers). Perl normally searches for an IFS-compatible LSP because this is required for its emulation of Windows sockets as real filehandles. However, this may cause problems if you have a firewall such as McAfee Guardian, which requires that all applications use its LSP but which is not IFS-compatible, because clearly Perl will normally avoid using such an LSP.
Setting this environment variable to 1 means that Perl will simply use the first suitable LSP enumerated in the catalog, which keeps McAfee Guardian happy--and in that particular case Perl still works too because McAfee Guardian's LSP actually plays other games which allow applications requiring IFS compatibility to work.
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 dumps out memory statistics after execution. If set to an integer greater than one, also dumps out memory statistics after compilation.
Relevant only if your Perl executable was built with -DDEBUGGING, this controls the behaviour of global destruction of objects and other references. See PERL_DESTRUCT_LEVEL in perlhacktips for more information.
Set to "1"
to have Perl resolve all undefined symbols when it loads
a dynamic library. The default behaviour is to resolve symbols when
they are used. Setting this variable is useful during testing of
extensions, as it ensures that you get an error on misspelled function
names even if the test suite doesn't call them.
If using the use encoding
pragma without an explicit encoding name, the
PERL_ENCODING environment variable is consulted for an encoding name.
(Since Perl 5.8.1, new semantics in Perl 5.18.0) Used to override the randomization of Perl's internal hash function. The value is expressed in hexadecimal, and may include a leading 0x. Truncated patterns are treated as though they are suffixed with sufficient 0's as required.
If the option is provided, and PERL_PERTURB_KEYS
is NOT set, then
a value of '0' implies PERL_PERTURB_KEYS=0
and any other value
implies PERL_PERTURB_KEYS=2
.
PLEASE NOTE: The hash seed is sensitive information. Hashes are randomized to protect against local and remote attacks against Perl code. By manually setting a seed, this protection may be partially or completely lost.
See Algorithmic Complexity Attacks in perlsec, PERL_PERTURB_KEYS, and PERL_HASH_SEED_DEBUG for more information.
(Since Perl 5.18.0) Set to "0"
or "NO"
then traversing keys
will be repeatable from run to run for the same PERL_HASH_SEED.
Insertion into a hash will not change the order, except to provide
for more space in the hash. When combined with setting PERL_HASH_SEED
this mode is as close to pre 5.18 behavior as you can get.
When set to "1"
or "RANDOM"
then traversing keys will be randomized.
Every time a hash is inserted into the key order will change in a random
fashion. The order may not be repeatable in a following program run
even if the PERL_HASH_SEED has been specified. This is the default
mode for perl.
When set to "2"
or "DETERMINISTIC"
then inserting keys into a hash
will cause the key order to change, but in a way that is repeatable
from program run to program run.
NOTE: Use of this option is considered insecure, and is intended only for debugging non-deterministic behavior in Perl's hash function. Do not use it in production.
See Algorithmic Complexity Attacks in perlsec and PERL_HASH_SEED
and PERL_HASH_SEED_DEBUG for more information. You can get and set the
key traversal mask for a specific hash by using the hash_traversal_mask()
function from Hash::Util.
(Since Perl 5.8.1.) Set to "1"
to display (to STDERR) information
about the hash function, seed, and what type of key traversal
randomization is in effect at the beginning of execution. This, combined
with PERL_HASH_SEED and PERL_PERTURB_KEYS is intended to aid in
debugging nondeterministic behaviour caused by hash randomization.
Note that any information about the hash function, especially the hash
seed is sensitive information: by knowing it, one can craft a denial-of-service
attack against Perl code, even remotely; see Algorithmic Complexity Attacks in perlsec
for more information. Do not disclose the hash seed to people who
don't need to know it. See also hash_seed()
and
key_traversal_mask()
in Hash::Util.
An example output might be:
- HASH_FUNCTION = ONE_AT_A_TIME_HARD HASH_SEED = 0x652e9b9349a7a032 PERTURB_KEYS = 1 (RANDOM)
If your Perl was configured with -Accflags=-DPERL_MEM_LOG, setting
the environment variable PERL_MEM_LOG
enables logging debug
messages. The value has the form <number>[m][s][t], where
number is the file descriptor number you want to write to (2 is
default), and the combination of letters specifies that you want
information about (m)emory and/or (s)v, optionally with
(t)imestamps. For example, PERL_MEM_LOG=1mst logs all
information to stdout. You can write to other opened file descriptors
in a variety of ways:
- $ 3>foo3 PERL_MEM_LOG=3m perl ...
A translation-concealed rooted logical name that contains Perl and the logical device for the @INC path on VMS only. Other logical names that affect Perl on VMS include PERLSHR, PERL_ENV_TABLES, and SYS$TIMEZONE_DIFFERENTIAL, but are optional and discussed further in perlvms and in README.vms in the Perl source distribution.
Available in Perls 5.8.1 and later. If set to "unsafe"
, the pre-Perl-5.8.0
signal behaviour (which is immediate but unsafe) is restored. If set
to safe
, then safe (but deferred) signals are used. See
Deferred Signals (Safe Signals) in perlipc.
Equivalent to the -C command-line switch. Note that this is not
a boolean variable. Setting this to "1"
is not the right way to
"enable Unicode" (whatever that would mean). You can use "0"
to
"disable Unicode", though (or alternatively unset PERL_UNICODE in
your shell before starting Perl). See the description of the -C
switch for more information.
If perl has been configured to not have the current directory in
@INC by default, this variable can be set to "1"
to reinstate it. It's primarily intended for use while building and
testing modules that have not been updated to deal with "." not being in
@INC
and should not be set in the environment for day-to-day use.
Used if chdir has no argument and HOME and LOGDIR are not set.
Perl also has environment variables that control how Perl handles data specific to particular natural languages; see perllocale.
Perl and its various modules and components, including its test frameworks, may sometimes make use of certain other environment variables. Some of these are specific to a particular platform. Please consult the appropriate module documentation and any documentation for your platform (like perlsolaris, perllinux, perlmacosx, perlwin32, etc) for variables peculiar to those specific situations.
Perl makes all environment variables available to the program being executed, and passes these along to any child processes it starts. However, programs running setuid would do well to execute the following lines before doing anything else, just to keep people honest: