PERLRUN(1) Perl Programmers Reference Guide PERLRUN(1)
NAME
perlrun - how to execute the Perl interpreter
SYNOPSIS
perl [ -gsTtuUWX ] [ -h?v ] [ -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 ]...
DESCRIPTION
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:
1. Specified line by line via -e or -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. See "Location of Perl".)
3. 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 -*- -p
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.
#! 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 program, and codify it in
"ALTERNATE_SHEBANG" (see the dosish.h file in the source
distribution for more information).
Win95/NT
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.
VMS Put
$ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
$ exit++ + ++$status != 0 and $exit = $status = undef;
at the top of your program, where -mysw are any command line
switches you want to pass to Perl. You can now invoke the program
directly, by saying "perl program", or as a DCL procedure, by saying
@program (or implicitly via DCL$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.
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 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 v5.14;
Command Switches
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:
-0[octal/hexadecimal]
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. The "-g" flag is a simpler alias for it.
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.)
-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.
-a implicitly sets "-n".
-C [number/list]
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 ${^UTF8CACHE} to -1, to run the UTF-8 caching
code in debugging mode.
For example, -COE and -C6 will both turn on UTF-8-ness on both
STDOUT and STDERR. Repeating letters is just redundant, not
cumulative nor toggling.
The "io" options mean that any subsequent open() (or similar I/O
operations) in main program scope will have the ":utf8" PerlIO
layer implicitly applied to them, in other words, UTF-8 is expected
from any input stream, and UTF-8 is produced to any output stream.
This is just the default set via "${^OPEN}", with explicit layers
in open() and with binmode() one can manipulate streams as usual.
This has no effect on code run in modules.
-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" in perlfunc), the two-arg binmode()
(see "binmode" in perlfunc), 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.
-c 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.
If the syntax check is successful perl will exit with a status of
zero and report "yourprogram syntax OK". On failure perl will
print any detected errors and exit with a non-zero status.
-d
-dt 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.
-d:MOD[=bar,baz]
-dt:MOD[=bar,baz]
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.
-Dletters
-Dnumber
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 Show hash randomization debug output (changes to
PL_hash_rand_bits and their origin)
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 to
increase the verbosity of the output. Is a no-op on
many of the 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.
268435456 y trace y///, tr/// compilation and execution
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.
-e commandline
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.
-E commandline
behaves just like -e, except that it implicitly enables all
optional features and builtin functions (in the main compilation
unit). See feature and builtin.
-f 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:
BEGIN {
do { local $!; -f "$Config{sitelib}/sitecustomize.pl"; }
&& do "$Config{sitelib}/sitecustomize.pl";
}
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 $Config{usesitecustomize}.
-Fpattern
specifies the pattern to split on for "-a". The pattern may be
surrounded by "//", "", or '', otherwise it will be put in single
quotes. You can't use literal whitespace or NUL characters in the
pattern.
-F implicitly sets both "-a" and "-n".
-g undefines the input record separator ($/) and thus enables the
slurp mode. In other words, it causes Perl to read whole files at
once, instead of line by line.
This flag is a simpler alias for -0777.
Mnemonics: gobble, grab, gulp.
-h prints a summary of the options.
-? synonym for -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 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" in perlfunc).
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.
-Idirectory
Directories specified by -I are prepended to the search path for
modules (@INC).
-l[octnum]
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.
-m[-]module
-M[-]module
-M[-]'module ...'
-[mM][-]module=arg[,arg]...
-mmodule executes "use" module "();" before executing your program.
This loads the module, but does not call its "import" method, so
does not import subroutines and does not give effect to a pragma.
-Mmodule executes "use" module ";" before executing your program.
This loads the module and calls its "import" method, causing the
module to have its default effect, typically importing subroutines
or giving effect to a pragma. 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'. This makes no difference for -m.
Since Perl version 5.39.8, the "-M" switch and the module name can
now be passed in separate command-line arguments:
perl -M MODULE ... # like perl -MMODULE ...
perl -M MODULE=arg1,arg2,... # like perl -MMODULE=arg1,arg2,...
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 the "split" formulation 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.
-n 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" in
perlfunc, 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:
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 (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.
-p causes Perl to assume the following loop around your program, which
makes it iterate over filename arguments somewhat like sed:
LINE:
while (<>) {
... # your program goes here
} continue {
print or die "-p destination: $!\n";
}
If a file named by an argument cannot be opened for some reason,
Perl warns you about it, and moves on to the next file. Note that
the lines are printed automatically. An error occurring during
printing is treated as fatal. To suppress printing use the "-n"
switch. A -p overrides a -n switch.
"BEGIN" and "END" blocks may be used to capture control before or
after the implicit loop, just as in awk.
-s enables rudimentary switch parsing for switches on the command line
after the program name but before any filename arguments (or before
an argument of --). Any switch found there is removed from @ARGV
and sets the corresponding variable in the Perl program, in the
main package. The following program prints "1" if the program is
invoked with a -xyz switch, and "abc" if it is invoked with
-xyz=abc.
#!/usr/bin/perl -s
if ($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. For these reasons, use of -s is
discouraged. See Getopt::Long for much more flexible switch
parsing.
-S 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 0; # ^ Run only under a 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
$0 doesn't always contain the full pathname, so the "-S" tells Perl
to search for the program if necessary. After Perl locates the
program, it parses the lines and ignores them because the check 'if
0' is never true. If the program will be interpreted by csh, you
will need to replace "${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:
eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
& eval 'exec /usr/bin/perl -wS $0 $argv:q'
if 0; # ^ Run only under a shell
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.
-t 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".
This has no effect if your perl was built without taint support.
-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.
-u 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
CORE::dump() function 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 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.
-v prints the version and patchlevel of your perl executable.
-V prints summary of the major perl configuration values and the
current values of @INC.
-V:configvar
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
-w 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" in perlfunc.
See also perldiag and perltrap. A fine-grained warning facility is
also available if you want to manipulate entire classes of
warnings; see warnings.
-W Enables all warnings regardless of "no warnings" or $^W. See
warnings.
-X Disables all warnings regardless of "use warnings" or $^W. See
warnings.
Forbidden in "PERL5OPT".
-x
-xdirectory
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.
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 program
if "-S" is used.
PERL5LIB A list of directories in which to look for Perl library
files before looking in the standard library. 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";
PERL5OPT Command-line options (switches). Switches in this variable
are treated as if they were on every Perl command line.
Only the -[CDIMTUWdmtw] 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.
PERLIO 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.
:crlf A layer which does CRLF to "\n" translation
distinguishing "text" and "binary" files in the
manner of MS-DOS and similar operating systems, and
also provides buffering similar to ":perlio" on
these architectures.
:perlio 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".
:stdio 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.
:unix Low-level layer that calls "read", "write", "lseek",
etc.
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 (not common on modern
architectures); otherwise, it uses the ":unix:perlio"
implementation.
On Win32 the default in this release (5.30) 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.
This release (5.30) uses ":unix" as the bottom layer on
Win32, and so still uses the C compiler's numeric file
descriptor routines.
The PERLIO environment variable is completely ignored when
Perl is run in taint mode.
PERLIO_DEBUG
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.
PERLLIB A list of directories in which to look for Perl library
files before looking in the standard library. If "PERL5LIB"
is defined, PERLLIB is not used.
The PERLLIB environment variable is completely ignored when
Perl is run in taint mode.
PERL5DB 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.
PERL5DB_THREADED
If set to a true value, indicates to the debugger that the
code being debugged uses threads.
PERL5SHELL (specific to the Win32 port)
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.
PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
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.
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 dumps out memory statistics after execution.
If set to an integer greater than one, also dumps out memory
statistics after compilation.
PERL_DESTRUCT_LEVEL
Controls the behaviour of global destruction of objects and
other references. See "PERL_DESTRUCT_LEVEL" in perlhacktips
for more information.
PERL_DL_NONLAZY
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.
PERL_ENCODING
If using the "use encoding" pragma without an explicit
encoding name, the PERL_ENCODING environment variable is
consulted for an encoding name.
PERL_HASH_SEED
(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"/"PERL_PERTURB_KEYS=NO" and any other
value implies
"PERL_PERTURB_KEYS=2"/"PERL_PERTURB_KEYS=DETERMINISTIC". See
the documentation for PERL_PERTURB_KEYS for important
caveats regarding the "DETERMINISTIC" mode.
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.
PERL_PERTURB_KEYS
(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 no PERL_HASH_SEED has been explicitly
provided.
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, provided that
the same hash seed is used, and that the code does not
itself perform any non-deterministic operations and also
provided exactly the same environment context. Adding or
removing an environment variable may and likely will change
the key order. If any part of the code builds a hash using
non- deterministic keys, for instance a hash keyed by the
stringified form of a reference, or the address of the
objects it contains, then this may and likely will have a
global effect on the key order of *every* hash in the
process. To work properly this setting MUST be coupled with
the PERL_HASH_SEED to produce deterministic results, and in
fact, if you do set the "PERL_HASH_SEED" explicitly you do
not need to set this as well, it will be automatically set
to this mode.
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.
PERL_HASH_SEED_DEBUG
(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 hash_traversal_mask().
An example output might be:
HASH_FUNCTION = ONE_AT_A_TIME_HARD HASH_SEED = 0x652e9b9349a7a032 PERTURB_KEYS = 1 (RANDOM)
PERL_MEM_LOG
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 ...
PERL_ROOT (specific to the VMS port)
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$TIMEZONE_DIFFERENTIAL, but are
optional and discussed further in perlvms and in README.vms
in the Perl source distribution.
PERL_SIGNALS
Available in Perls 5.8.1 and later. If set to "unsafe", the
pre-Perl-5.8.0 signal behaviour (which is immediate but
unsafe) is restored. If set to "safe", then safe (but
deferred) signals are used. See "Deferred Signals (Safe
Signals)" in perlipc.
PERL_UNICODE
Equivalent to the -C command-line switch. Note that this is
not a boolean variable. Setting this to "1" is not the right
way to "enable Unicode" (whatever that would mean). You can
use "0" to "disable Unicode", though (or alternatively unset
PERL_UNICODE in your shell before starting Perl). See the
description of the -C switch for more information.
PERL_USE_UNSAFE_INC
If perl has been configured to not have the current
directory in @INC by default, this variable can be set to
"1" to reinstate it. It's primarily intended for use while
building and testing modules that have not been updated to
deal with "." not being in @INC and should not be set in the
environment for day-to-day use.
SYS$LOGIN (specific to the VMS port)
Used if chdir has no argument and "HOME" and "LOGDIR" are
not set.
PERL_INTERNAL_RAND_SEED
Set to a non-negative integer to seed the random number
generator used internally by perl for a variety of purposes.
Ignored if perl is run setuid or setgid. Used only for some
limited startup randomization (hash keys) if "-T" or "-t"
perl is started with tainting enabled.
Perl may be built to ignore this variable.
PERL_RAND_SEED
When set to an integer value this value will be used to seed
the perl internal random number generator used for rand()
when it is used without an explicit srand() call or for when
an explicit no-argument srand() call is made.
Normally calling rand() prior to calling srand() or calling
srand() explicitly with no arguments should result in the
random number generator using "best efforts" to seed the
generator state with a relatively high quality random seed.
When this environment variable is set then the seeds used
will be deterministically computed from the value provided
in the env var in such a way that the application process
and any forks or threads should continue to have their own
unique seed but that the program may be run twice with
identical results as far as rand() goes (assuming all else
is equal).
PERL_RAND_SEED is intended for performance measurements and
debugging and is explicitly NOT intended for stable testing.
The only guarantee is that a specific perl executable will
produce the same results twice in a row, there is no
guarantee that the results will be the same between perl
releases or on different architectures.
Ignored if perl is run setuid or setgid.
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:
$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)};
ORDER OF APPLICATION
Some options, in particular "-I", "-M", "PERL5LIB" and "PERL5OPT" can
interact, and the order in which they are applied is important.
Note that this section does not document what actually happens inside
the perl interpreter, it documents what effectively happens.
-I The effect of multiple "-I" options is to "unshift" them onto @INC
from right to left. So for example:
perl -I 1 -I 2 -I 3
will first prepend 3 onto the front of @INC, then prepend 2, and
then prepend 1. The result is that @INC begins with:
qw(1 2 3)
-M Multiple "-M" options are processed from left to right. So this:
perl -Mlib=1 -Mlib=2 -Mlib=3
will first use the lib pragma to prepend 1 to @INC, then it will
prepend 2, then it will prepend 3, resulting in an @INC that begins
with:
qw(3 2 1)
the PERL5LIB environment variable
This contains a list of directories, separated by colons. The entire
list is prepended to @INC in one go. This:
PERL5LIB=1:2:3 perl
will result in an @INC that begins with:
qw(1 2 3)
combinations of -I, -M and PERL5LIB
"PERL5LIB" is applied first, then all the "-I" arguments, then all
the "-M" arguments. This:
PERL5LIB=e1:e2 perl -I i1 -Mlib=m1 -I i2 -Mlib=m2
will result in an @INC that begins with:
qw(m2 m1 i1 i2 e1 e2)
the PERL5OPT environment variable
This contains a space separated list of switches. We only consider
the effects of "-M" and "-I" in this section.
After normal processing of "-I" switches from the command line, all
the "-I" switches in "PERL5OPT" are extracted. They are processed
from left to right instead of from right to left. Also note that
while whitespace is allowed between a "-I" and its directory on the
command line, it is not allowed in "PERL5OPT".
After normal processing of "-M" switches from the command line, all
the "-M" switches in "PERL5OPT" are extracted. They are processed
from left to right, i.e. the same as those on the command line.
An example may make this clearer:
export PERL5OPT="-Mlib=optm1 -Iopti1 -Mlib=optm2 -Iopti2"
export PERL5LIB=e1:e2
perl -I i1 -Mlib=m1 -I i2 -Mlib=m2
will result in an @INC that begins with:
qw(
optm2
optm1
m2
m1
opti2
opti1
i1
i2
e1
e2
)
Other complications
There are some complications that are ignored in the examples above:
arch and version subdirs
All of "-I", "PERL5LIB" and "use lib" will also prepend arch and
version subdirs if they are present
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