PERLRUN(1) Perl Programmers Reference Guide PERLRUN(1)
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] ] [ -P ] [ -S ]
[ -x[dir] ] [ -i[extension] ]
[ -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:
1. Specified line by line via -e switches on the command
line.
2. Contained in the file specified by the first filename on
the command line. (Note that systems supporting the #!
notation invoke interpreters this way. See "Location of
Perl".)
3. Passed in implicitly via standard input. This works
only if there are no filename arguments--to pass argu-
ments 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 mes-
sage. (In this case you would indicate the end of the pro-
gram 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 recog-
nize the #! line, you still can get consistent switch
behavior regardless of how Perl was invoked, even if -x was
used to find the beginning of the program.
Because historically some operating systems silently chopped
off kernel interpretation of the #! line after 32 charac-
ters, some switches may be passed in on the command line,
and some may not; you could even get a "-" without its
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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 stan-
dard 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 men-
tioned in the line. The sequences "-*" and "- " are specifi-
cally ignored so that you could, if you were so inclined,
say
#!/bin/sh -- # -*- perl -*- -p
eval 'exec perl -wS $0 ${1+"$@"}'
if $running_under_some_shell;
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 inter-
preter, getting whatever version is first in the user's
path. If you want a specific version of Perl, say,
perl5.005_57, you should place that directly in the #!
line's path.
If the #! line does not contain the word "perl", the program
named after the #! is executed instead of the Perl inter-
preter. 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 pro-
gram 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.
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#! 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 associ-
ate 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 differ-
ence between an executable Perl program and a Perl
library file.
Macintosh
Under "Classic" MacOS, a perl program will have the
appropriate Creator and Type, so that double-clicking
them will invoke the MacPerl application. Under Mac OS
X, clickable apps can be made from any "#!" script using
Wil Sanchez' DropScript utility:
http://www.wsanchez.net/software/ .
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 dif-
ferent ideas on quoting than Unix shells. You'll need to
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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 sys-
tems. You might also have to change a single % to a %%.
For example:
# Unix
perl -e 'print "Hello world\n"'
# MS-DOS, etc.
perl -e "print \"Hello world\n\""
# Macintosh
print "Hello world\n"
(then Run "Myscript" or Shift-Command-R)
# VMS
perl -e "print ""Hello world\n"""
The problem is that none of this is reliable: it depends on
the command and it is entirely possible neither works. If
4DOS 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 func-
tionality in when nobody was looking, but just try to find
documentation for its quoting rules.
Under the Macintosh, it depends which environment you are
using. The MacPerl shell, or MPW, is much like Unix shells
in its support for several quoting variants, except that it
makes free use of the Macintosh's non-ASCII characters as
control characters.
There is no general solution to all of this. It's just a
mess.
Location of Perl
It may seem obvious to say, but Perl is useful only when
users can easily find it. When possible, it's good for both
/usr/bin/perl and /usr/local/bin/perl to be symlinks to the
actual binary. If that can't be done, system administrators
are strongly encouraged to put (symlinks to) perl and its
accompanying utilities into a directory typically found
along a user's PATH, or in some other obvious and convenient
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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/perl5.00554
or if you just want to be running at least version, place a
statement like this at the top of your program:
use 5.005_54;
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
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. The value 0777 will cause Perl to slurp
files whole because there is no legal byte with that
value.
If you want to specify any Unicode character, use the
hexadecimal format: "-0xHHH...", where the "H" are
valid hexadecimal digits. (This means that you cannot
use the "-x" with a directory name that consists of
hexadecimal digits.)
-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
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while (<>) {
@F = split(' ');
print pop(@F), "\n";
}
An alternate delimiter may be specified using -F.
-C [number/list]
The "-C" flag controls some Unicode 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_TYPE, and LANG, in the order
of decreasing precedence) -- if the variables indicate
UTF-8, then the selected "IOEioA" are in effect
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) 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 pro-
duced 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.
You can use "-C0" (or "0" for "PERL_UNICODE") to
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explicitly disable all the above Unicode features.
The read-only magic variable "${^UNICODE}" reflects the
numeric value of this setting. This is 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 "bin-
mode" 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 practi-
cally unused, however, and the command line switch was
therefore "recycled".)
-c causes Perl to check the syntax of the program and then
exit without executing it. Actually, it will execute
"BEGIN", "CHECK", and "use" blocks, because these are
considered as occurring outside the execution of your
program. "INIT" and "END" blocks, however, will be
skipped.
-d
-dt runs the program under the Perl debugger. See perlde-
bug. If t is specified, it indicates to the debugger
that threads will be used in the code being debugged.
-d:foo[=bar,baz]
-dt:foo[=bar,baz]
runs the program under the control of a debugging, pro-
filing, or tracing module installed as Devel::foo.
E.g., -d:DProf executes the program using the
Devel::DProf profiler. As with the -M flag, options
may be passed to the Devel::foo package where they will
be received and interpreted by the Devel::foo::import
routine. The comma-separated list of options must fol-
low 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. To watch how it executes your
program, use -Dtls. (This works only if debugging is
compiled into your Perl.) Another nice value is -Dx,
which lists your compiled syntax tree. And -Dr
displays compiled regular expressions; 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):
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1 p Tokenizing and parsing
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, preprocessor command for -P, source file input state
128 m Memory allocation
256 f Format processing
512 r Regular expression parsing and execution
1024 x Syntax tree dump
2048 u Tainting checks
4096 (Obsolete, previously used for LEAKTEST)
8192 H Hash dump -- usurps values()
16384 X Scratchpad allocation
32768 D Cleaning up
65536 S Thread synchronization
131072 T Tokenising
262144 R Include reference counts of dumped variables (eg when using -Ds)
524288 J Do not s,t,P-debug (Jump over) opcodes within package DB
1048576 v Verbose: use in conjunction with other flags
8388608 q quiet - currently only suppresses the "EXECUTING" message
All these flags require -DDEBUGGING when you compile
the Perl executable (but see Devel::Peek, re which may
change this). See the INSTALL file in the Perl source
distribution for how to do this. This flag is automat-
ically set if you include -g option when "Configure"
asks you about optimizer/debugger flags.
If you're just trying to get a print out of each line
of Perl code as it executes, the way that "sh -x" pro-
vides 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 argu-
ment 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.
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-f Disable executing $Config{sitelib}/sitecustomize.pl at
startup.
Perl can be built so that it by default will try to
execute $Config{sitelib}/sitecustomize.pl at startup.
This is a hook that allows the sysadmin to customize
how perl behaves. It can for instance be used to add
entries to the @INC array to make perl find modules in
non-standard locations.
-Fpattern
specifies the pattern to split on if -a is also in
effect. The pattern may be surrounded by "//", "", or
'', otherwise it will be put in single quotes. You
can't use literal whitespace in the pattern.
-h prints a summary of the options.
-i[extension]
specifies that files processed by the "<>" construct
are to be edited in-place. It does this by renaming
the input file, opening the output file by the original
name, and selecting that output file as the default for
print() statements. The extension, if supplied, is
used to modify the name of the old file to make a
backup copy, following these rules:
If no extension is supplied, no backup is made and the
current file is overwritten.
If the extension doesn't contain a "*", then it is
appended to the end of the current filename as a suf-
fix. If the extension does contain one or more "*"
characters, then each "*" is replaced with the current
filename. In Perl terms, you could think of this as:
($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:
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$ 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...
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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), and also tells the C prepro-
cessor where to search for include files. The C
preprocessor is invoked with -P; by default it searches
/usr/include and /usr/lib/perl.
-l[octnum]
enables automatic line-ending processing. It has two
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 state-
ments 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) = ""'
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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.
-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 gen-
erated by -Mmodule=foo,bar is "use module
split(/,/,q{foo,bar})". Note that the "=" form removes
the distinction between -m and -M.
A consequence of this is that -MFoo=number never does a
version check (unless "Foo::import()" itself is set up
to do a version check, which could happen for example
if Foo 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.
Here is an efficient way to delete all files that
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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. It does suffer from the bug of mishan-
dling 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.
-P NOTE: Use of -P is strongly discouraged because of its
inherent problems, including poor portability.
This option causes your program to be run through the C
preprocessor before compilation by Perl. Because both
comments and cpp directives begin with the # character,
you should avoid starting comments with any words
recognized by the C preprocessor such as "if", "else",
or "define".
If you're considering using "-P", you might also want
to look at the Filter::cpp module from CPAN.
The problems of -P include, but are not limited to:
* The "#!" line is stripped, so any switches
there don't apply.
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* A "-P" on a "#!" line doesn't work.
* All lines that begin with (whitespace and) a
"#" but do not look like cpp commands, are
stripped, including anything inside Perl
strings, regular expressions, and here-docs .
* In some platforms the C preprocessor knows
too much: it knows about the C++ -style
until-end-of-line comments starting with
"//". This will cause problems with common
Perl constructs like
s/foo//;
because after -P this will became illegal
code
s/foo
The workaround is to use some other quoting
separator than "/", like for example "!":
s!foo!!;
* It requires not only a working C preprocessor
but also a working sed. If not on UNIX, you
are probably out of luck on this.
* Script line numbers are not preserved.
* The "-x" does not work with "-P".
-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. The fol-
lowing 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 "strict refs".
Also, when using this option on a script with warnings
enabled you may get a lot of spurious "used only once"
warnings.
-S makes Perl use the PATH environment variable to search
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for the program (unless the name of the program con-
tains directory separators).
On some platforms, this also makes Perl append suffixes
to the filename while searching for it. For example,
on Win32 platforms, the ".bat" and ".cmd" suffixes are
appended if a lookup for the original name fails, and
if the name does not already end in one of those suf-
fixes. 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 plat-
forms that don't support #!. Its 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:
eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
& eval 'exec /usr/bin/perl -wS $0 $argv:q'
if $running_under_some_shell;
If the filename supplied contains directory separators
(i.e., 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.
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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 be controlled
normally with "no warnings qw(taint)".
NOTE: this is not a substitute for -T. This is meant
only to be used 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.
-T forces "taint" checks to be turned on so you can test
them. Ordinarily these checks are done only when run-
ning setuid or setgid. It's a good idea to turn them
on explicitly for programs that run on behalf of some-
one 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 con-
struct.
-u This obsolete 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.
This switch has been superseded in favor of the new
Perl code generator backends to the compiler. See B
and B::Bytecode for details.
-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 the -w switch (or the $^W vari-
able) must be used along with this option to actually
generate the taint-check warnings.
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-v prints the version and patchlevel of your perl execut-
able.
-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 argu-
ment looks like a regex (has non-letters). For exam-
ple:
$ 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 for-
matting. 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 vari-
able names that are mentioned only once and scalar
variables that are used before being set, redefined
subroutines, references to undefined filehandles or
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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 innu-
merable other things.
This switch really just enables the internal $^W vari-
able. 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 new, fine-grained warning facility is
also available if you want to manipulate entire classes
of warnings; see warnings or perllexwarn.
-W Enables all warnings regardless of "no warnings" or
$^W. See perllexwarn.
-X Disables all warnings regardless of "use warnings" or
$^W. See perllexwarn.
-x
-x directory
tells Perl that the program is embedded in a larger
chunk of unrelated ASCII text, such as in a mail mes-
sage. Leading garbage will be discarded until the
first line that starts with #! and contains the string
"perl". Any meaningful switches on that line will be
applied. If a directory name is specified, Perl will
switch to that directory before running the 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).
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 and the current directory. Any
architecture-specific directories under the
specified locations are automatically included
if they exist. If PERL5LIB is not defined,
PERLLIB is used. Directories are separated
(like in PATH) by a colon on unixish platforms
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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
switch was used), neither variable is used. The
program should instead say:
use lib "/my/directory";
PERL5OPT Command-line options (switches). Switches in
this variable are taken as if they were on every
Perl command line. Only the -[DIMUdmtw]
switches are allowed. When running taint checks
(because the program was running setuid or set-
gid, or the -T switch was used), this variable
is ignored. If PERL5OPT begins with -T, taint-
ing will be enabled, and any subsequent options
ignored.
PERLIO A space (or colon) separated list of PerlIO
layers. If perl is built to use PerlIO system
for IO (the default) these layers effect perl's
IO.
It is conventional to start layer names with a
colon e.g. ":perlio" to emphasise their similar-
ity to variable "attributes". But the code that
parses layer specification strings (which is
also used to decode the PERLIO environment vari-
able) treats the colon as a separator.
An unset or empty PERLIO is equivalent to
":stdio".
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 :encod-
ing()) need IO in order to load them!. See
"open pragma" for how to add external encodings
as defaults.
The layers that it makes sense to include in the
PERLIO environment variable are briefly summar-
ised below. For more details see PerlIO.
:bytes A pseudolayer that turns off the ":utf8"
flag 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
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":perlio:bytes".
:crlf A layer which does CRLF to "\n" transla-
tion distinguishing "text" and "binary"
files in the manner of MS-DOS and simi-
lar operating systems. (It currently
does not mimic MS-DOS as far as treating
of Control-Z as being an end-of-file
marker.)
:mmap A layer which implements "reading" of
files by using "mmap()" to make (whole)
file appear in the process's address
space, and then using that as PerlIO's
"buffer".
:perlio This is a re-implementation of
"stdio-like" buffering written as a Per-
lIO "layer". As such it will call what-
ever layer is below it for its opera-
tions (typically ":unix").
:pop An experimental pseudolayer that removes
the topmost layer. Use with the same
care as is reserved for nitroglycerin.
:raw 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 any translation. In particular
CRLF translation, and/or :utf8 intuited
from locale are disabled.
Unlike in the 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.
:stdio This layer provides PerlIO interface by
wrapping system's ANSI C "stdio" library
calls. The layer provides both buffering
and IO. Note that ":stdio" layer does
not do CRLF translation even if that is
platforms normal behaviour. You will
need a ":crlf" layer above it to do
that.
:unix Low level layer which calls "read",
"write" and "lseek" etc.
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:utf8 A pseudolayer that turns on a flag on
the layer below to tell perl that output
should be in utf8 and that input should
be regarded as already in utf8 form.
May be useful in PERLIO environment
variable to make UTF-8 the default. (To
turn off that behaviour use ":bytes"
layer.)
:win32 On Win32 platforms this experimental
layer uses native "handle" IO rather
than unix-like numeric file descriptor
layer. Known to be buggy in this
release.
On all platforms the default set of layers
should give acceptable results.
For UNIX platforms that will equivalent of "unix
perlio" or "stdio". Configure is setup to prefer
"stdio" implementation if system's library pro-
vides for fast access to the buffer, otherwise
it uses the "unix perlio" implementation.
On Win32 the default in this release is "unix
crlf". Win32's "stdio" has a number of
bugs/mis-features for perl IO which are somewhat
C compiler vendor/version dependent. Using our
own "crlf" layer as the buffer avoids those
issues and makes things more uniform. The "crlf"
layer provides CRLF to/from "\n" conversion as
well as buffering.
This release uses "unix" as the bottom layer on
Win32 and so still uses C compiler's numeric
file descriptor routines. There is an experimen-
tal native "win32" layer which is expected to be
enhanced and should eventually be the default
under Win32.
PERLIO_DEBUG
If set to the name of a file or device then cer-
tain operations of PerlIO sub-system will be
logged to that file (opened as append). Typical
uses are UNIX:
PERLIO_DEBUG=/dev/tty perl script ...
and Win32 approximate equivalent:
set PERLIO_DEBUG=CON
perl script ...
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This functionality is disabled for setuid
scripts and for scripts run with -T.
PERLLIB 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.
PERL5DB The command used to load the debugger code. The
default is:
BEGIN { require 'perl5db.pl' }
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)
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 to be
space-separated. Precede any character that
needs to be protected (like a space or
backslash) with a backslash.
Note that Perl doesn't use COMSPEC for this pur-
pose because COMSPEC has a high degree of varia-
bility among users, leading to portability con-
cerns. Besides, perl can use a shell that may
not be fit for interactive use, and setting COM-
SPEC to such a shell may interfere with the
proper functioning of other programs (which usu-
ally look in COMSPEC to find a shell fit for
interactive use).
PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
Set to 1 to allow the use of non-IFS compatible
LSP's. Perl normally searches for an IFS-
compatible LSP because this is required for its
emulation of Windows sockets as real filehan-
dles. However, this may cause problems if you
have a firewall such as McAfee Guardian which
requires all applications to use its LSP 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
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too because McAfee Guardian's LSP actually plays
some other games which allow applications
requiring IFS compatibility to work).
PERL_DEBUG_MSTATS
Relevant only if perl is compiled with the mal-
loc included with the perl distribution (that
is, if "perl -V:d_mymalloc" is 'define'). If
set, this causes memory statistics to be dumped
after execution. If set to an integer greater
than one, also causes memory statistics to be
dumped after compilation.
PERL_DESTRUCT_LEVEL
Relevant only if your perl executable was built
with -DDEBUGGING, this controls the behavior of
global destruction of objects and other refer-
ences. See "PERL_DESTRUCT_LEVEL" in perlhack
for more information.
PERL_DL_NONLAZY
Set to one 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 it.
PERL_ENCODING
If using the "encoding" pragma without an expli-
cit encoding name, the PERL_ENCODING environment
variable is consulted for an encoding name.
PERL_HASH_SEED
(Since Perl 5.8.1.) Used to randomise Perl's
internal hash function. To emulate the pre-5.8.1
behaviour, set to an integer (zero means exactly
the same order as 5.8.0). "Pre-5.8.1" means,
among other things, that hash keys will be
ordered the same between different runs of Perl.
The default behaviour is to randomise unless the
PERL_HASH_SEED is set. If Perl has been compiled
with "-DUSE_HASH_SEED_EXPLICIT", the default
behaviour is not to randomise unless the
PERL_HASH_SEED is set.
If PERL_HASH_SEED is unset or set to a non-
numeric string, Perl uses the pseudorandom seed
supplied by the operating system and libraries.
This means that each different run of Perl will
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have a different ordering of the results of
keys(), values(), and each().
Please note that 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
and "PERL_HASH_SEED_DEBUG" for more information.
PERL_HASH_SEED_DEBUG
(Since Perl 5.8.1.) Set to one to display (to
STDERR) the value of the hash seed at the begin-
ning of execution. This, combined with
"PERL_HASH_SEED" is intended to aid in debugging
nondeterministic behavior caused by hash random-
ization.
Note that the hash seed is sensitive informa-
tion: 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() of Hash::Util.
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$TIMEZONE_DIFFERENTIAL
but are optional and discussed further in
perlvms and in README.vms in the Perl source
distribution.
PERL_SIGNALS
In Perls 5.8.1 and later. If set to "unsafe"
the pre-Perl-5.8.0 signals behaviour (immediate
but unsafe) is restored. If set to "safe" the
safe (or deferred) signals are used. See
"Deferred Signals (Safe Signals)" in perlipc.
PERL_UNICODE
Equivalent to the -C command-line switch. Note
that this is not a boolean variable-- setting
this to "1" is not the right way to "enable
Unicode" (whatever that would mean). You can
use "0" to "disable Unicode", though (or alter-
natively unset PERL_UNICODE in your shell before
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starting Perl). See the description of the "-C"
switch for more information.
SYS$LOGIN (specific to the VMS port)
Used if chdir has no argument and HOME and LOG-
DIR are not set.
Perl also has environment variables that control how Perl
handles data specific to particular natural languages. See
perllocale.
Apart from these, Perl uses no other environment variables,
except to make them available to the program being executed,
and to child processes. 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)};
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