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perlrun - how to execute the Perl interpreter
perl [ -CsTtuUWX ] [ -hv ] [ -V[:configvar] ]
[ -cw ] [ -d[:debugger] ] [ -D[number/list] ]
[ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal] ]
[ -Idir ] [ -m[-]module ] [ -M[-]'module...' ]
[ -P ] [ -S ] [ -x[dir] ] [ -i[extension] ]
[ -e 'command' ] [ -- ] [ programfile ] [ argument ]...
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 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 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 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 -wS $0 ${1+"$@"}'
if $running_under_some_shell;
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to let Perl see the -p switch.
A similar trick involves the env program, if you have it.
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, perl5.005_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 interpreter. This is slightly bizarre, but it helps people on
machines that don't do #!, because they can tell a program that their SHELL is /usr/bin/perl,
and Perl will then dispatch the program to the correct interpreter for them.
After locating your program, Perl compiles the entire program to an internal form. If there
are any compilation errors, execution of the program is not attempted. (This is unlike the
typical shell script, which might run part-way through before finding a syntax error.)
If the program is syntactically correct, it is executed. If the program runs off the end
without hitting an exit() or die() operator, an implicit exit(0) is provided to
indicate successful completion.
Unix's #! technique can be simulated on other systems:
- OS/2
-
Put
extproc perl -S -your_switches
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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
ALTERNATIVE_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.
- Macintosh
- A Macintosh perl program will have the appropriate Creator and Type, so that
double-clicking them will invoke the perl application.
- VMS
-
Put
$ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
$ exit++ + ++$status != 0 and $exit = $status = undef;
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at the top of your program, where -mysw are any command line switches you want to
pass to Perl. You can now invoke the program directly, by saying perl program,
or as a DCL procedure, by saying @program (or implicitly via DCL$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\""
# Macintosh
print "Hello world\n"
(then Run "Myscript" or Shift-Command-R)
# VMS
perl -e "print ""Hello world\n"""
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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>""
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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.
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.
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/perl5.00554
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or if you just want to be running at least version, place a statement like this at the top of
your program:
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
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Switches include:
- -0[digits]
-
specifies the input record separator ($/) as an octal number. If there are
no digits, the null character is the separator. Other switches may precede or follow the
digits. For example, if you have a version of find which can print filenames
terminated by the null character, you can say this:
find . -name '*.orig' -print0 | perl -n0e unlink
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The special value 00 will cause Perl to slurp files in paragraph mode. The value 0777
will cause Perl to slurp files whole because there is no legal character with that value.
- -a
-
turns on autosplit mode when used with a -n or -p. An implicit split
command to the @F array is done as the first thing inside the implicit while loop produced
by the -n or -p.
perl -ane 'print pop(@F), "\n";'
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is equivalent to
while (<>) {
@F = split(' ');
print pop(@F), "\n";
}
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An alternate delimiter may be specified using -F.
- -C
-
enables Perl to use the native wide character APIs on the target system. The magic
variable ${^WIDE_SYSTEM_CALLS} reflects the state of this switch. See perlvar/"${^WIDE_SYSTEM_CALLS}".
This feature is currently only implemented on the Win32 platform.
- -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
- runs the program under the Perl debugger. See perldebug.
- -d:foo[=bar,baz]
- runs the program under the control of a debugging, profiling, 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 follow a
= character. 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):
1 p Tokenizing and parsing
2 s Stack snapshots
4 l Context (loop) stack processing
8 t Trace execution
16 o Method and overloading resolution
32 c String/numeric conversions
64 P Print 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 L Memory leaks (needs -DLEAKTEST when compiling Perl)
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
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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 automatically 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 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)
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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.
- -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.
- -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 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;
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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'
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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'
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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'
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From the shell, saying
$ perl -p -i.orig -e "s/foo/bar/; ... "
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is the same as using the program:
#!/usr/bin/perl -pi.orig
s/foo/bar/;
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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);
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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 perlfunc/eof).
If, for a given file, Perl is unable to create the backup file as specified in the
extension then it will skip that file and continue on with the next one (if it exists).
For a discussion of issues surrounding file permissions and -i, see perlfaq5/Why
does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a
bug in Perl?.
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...
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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 preprocessor 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 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) = ""'
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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'
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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 generated by -Mmodule=foo,bar is use
module split(/,/,q{foo,bar}). Note that the = form removes the
distinction between -m and -M.
- -n
-
causes Perl to assume the following loop around your program, which makes it iterate over
filename arguments somewhat like sed -n or awk:
LINE:
while (<>) {
... # your program goes here
}
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Note that the lines are not printed by default. See -p to have lines printed. If a
file named by an argument cannot be opened for some reason, Perl warns you about it and
moves on to the next file.
Here is an efficient way to delete all files older than a week:
find . -mtime +7 -print | perl -nle unlink
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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 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";
}
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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.
- 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
because after -P this will became illegal code
The workaround is to use some other quoting separator than "/",
like for example "!":
- 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 --). This means you
can have switches with two leading dashes (--help). Any switch found there is removed
from @ARGV and sets the corresponding variable in the Perl program. The following program
prints "1" if the program is invoked with a -xyz switch, and "abc"
if it is invoked with -xyz=abc.
#!/usr/bin/perl -s
if ($xyz) { print "$xyz\n" }
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Do note that --help creates the variable ${-help}, which is not compliant with strict
refs.
- -S
-
makes Perl use the PATH environment variable to search for the program (unless the name
of the program contains directory separators).
On some platforms, this also makes Perl append suffixes to the filename while searching
for it. For example, on Win32 platforms, the ".bat" and ".cmd" suffixes
are appended if a lookup for the original name fails, and if the name does not already end
in one of those suffixes. If your Perl was compiled with DEBUGGING turned on, using the -Dp
switch to Perl shows how the search progresses.
Typically this is used to emulate #! startup on platforms that don't support #!. This
example works on many platforms that have a shell compatible with Bourne shell:
#!/usr/bin/perl
eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
if $running_under_some_shell;
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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;
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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.
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 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 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
the unlinking of directories while running as superuser, and running setuid programs with
fatal taint checks turned into warnings. Note that the -w switch (or the
$^W
variable) must be used along with this option to actually generate the taint-check
warnings.
- -v
- prints the version and patchlevel of your perl executable.
- -V
- prints summary of the major perl configuration values and the current values of @INC.
- -V:name
-
Prints to STDOUT the value of the named configuration variable. For example,
will provide strong clues about what your MANPATH variable should be set to in order to
access the Perl documentation.
- -w
-
prints warnings about dubious constructs, such as variable names that are mentioned only
once and scalar variables that are used before being set, redefined subroutines, references
to undefined filehandles or filehandles opened read-only that you are attempting to write
on, values used as a number that doesn't look like numbers, using an array as though it were
a scalar, if your subroutines recurse more than 100 deep, and innumerable other things.
This switch really just enables the internal ^$W variable. You can disable
or promote into fatal errors specific warnings using __WARN__ hooks, as
described in perlvar and perlfunc/warn. 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 directory
- tells Perl that the program is embedded in a larger chunk of unrelated ASCII 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. 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 colon-separated 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.
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:
- 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 setgid, or the -T switch was
used), this variable is ignored. If PERL5OPT begins with -T, tainting 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 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.
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.
The layers that it makes sense to include in the PERLIO environment variable are
summarised below. For more details see PerlIO.
- :bytes
- Turns off the
:utf8 flag for the layer below. Unlikely to be
useful in global PERLIO environment variable.
- :crlf
- A layer that implements DOS/Windows like CRLF line endings. On read converts pairs of
CR,LF to a single "\n" newline character. On write converts each
"\n" to a CR,LF pair. Based on the
:perlio layer.
- :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". This may be faster in certain circumstances for large files,
and may result in less physical memory use when multiple processes are reading the same
file.
Files which are not mmap()-able revert to behaving like the :perlio
layer. Writes also behave like :perlio layer as mmap() for
write needs extra house-keeping (to extend the file) which negates any advantage.
The :mmap layer will not exist if platform does not support mmap().
- :perlio
-
A from scratch implementation of buffering for PerlIO. Provides fast access to the
buffer for sv_gets which implements perl's readline/<> and in general
attempts to minimize data copying.
:perlio will insert a :unix layer below itself to do low
level IO.
- :raw
-
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 inuited from locale are disabled.
Arranges for all accesses go straight to the lowest buffered layer provided by the
configration. That is it strips off any layers above that layer.
In Perl 5.6 and some books the :raw layer (previously sometimes also
referred to as a "discipline") is documented as the inverse of the :crlf
layer. That is no longer the case - other layers which would alter binary nature of the
stream are also disabled. If you want UNIX line endings on a platform that normally does
CRLF translation, but still want UTF-8 or encoding defaults the appropriate thing to do
is to add :perlio to PERLIO environment variable.
- :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
- Lowest level layer which provides basic PerlIO operations in terms of UNIX/POSIX
numeric file descriptor calls
open(), read(), write(), lseek(), close()
- :utf8
- Turns on a flag on the layer below to tell perl that data sent to the stream should be
converted to perl internal "utf8" form and that data from the stream should be
considered as so encoded. On ASCII based platforms the encoding is UTF-8 and on EBCDIC
platforms UTF-EBCDIC. 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 provides
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 experimental native win32
layer which is expected to be enhanced and should eventually replace the unix
layer.
- PERLIO_DEBUG
-
If set to the name of a file or device then certain operations of PerlIO sub-system will
be logged to that file (opened as append). Typical uses are UNIX:
PERLIO_DEBUG=/dev/tty perl script ...
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and Win32 approximate equivalent:
set PERLIO_DEBUG=CON
perl script ...
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- PERLLIB
- A colon-separated list of directories in which to look for Perl library files before
looking in the standard library and the current directory. If PERL5LIB is defined, PERLLIB
is not used.
- PERL5DB
-
The command used to load the debugger code. The default is:
BEGIN { require 'perl5db.pl' }
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- 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/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 purpose because COMSPEC has a high degree of
variability among users, leading to portability concerns. Besides, perl can use a shell that
may not be fit for interactive use, and setting COMSPEC to such a shell may interfere with
the proper functioning of other programs (which usually look in COMSPEC to find a shell fit
for interactive use).
- PERL_DEBUG_MSTATS
- Relevant only if perl is compiled with the malloc 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 references. See perlhack/PERL_DESTRUCT_LEVEL
for more information.
- PERL_ENCODING
- If using the
encoding pragma without an explicit encoding name, the
PERL_ENCODING environment variable is consulted for an encoding name.
- 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$TIMEZONE_DIFFERENTIAL but are optional and discussed further in perlvms and in README.vms
in the Perl source distribution.
- SYS$LOGIN (specific to the VMS port)
- Used if chdir has no argument and HOME and LOGDIR are not set.
Perl also has environment variables that control how Perl handles data specific to particular
natural languages. See perllocale.
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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