3 perlrun - how to execute the Perl interpreter
7 B<perl> S<[ B<-sTtuUWX> ]>
8 S<[ B<-hv> ] [ B<-V>[:I<configvar>] ]>
9 S<[ B<-cw> ] [ B<-d>[B<t>][:I<debugger>] ] [ B<-D>[I<number/list>] ]>
10 S<[ B<-pna> ] [ B<-F>I<pattern> ] [ B<-l>[I<octal>] ] [ B<-0>[I<octal/hexadecimal>] ]>
11 S<[ B<-I>I<dir> ] [ B<-m>[B<->]I<module> ] [ B<-M>[B<->]I<'module...'> ] [ B<-f> ]>
12 S<[ B<-C [I<number/list>] >]>
15 S<[ B<-i>[I<extension>] ]>
16 S<[ B<-eE> I<'command'> ] [ B<--> ] [ I<programfile> ] [ I<argument> ]...>
20 The normal way to run a Perl program is by making it directly
21 executable, or else by passing the name of the source file as an
22 argument on the command line. (An interactive Perl environment
23 is also possible--see L<perldebug> for details on how to do that.)
24 Upon startup, Perl looks for your program in one of the following
31 Specified line by line via B<-e> or B<-E> switches on the command line.
35 Contained in the file specified by the first filename on the command line.
36 (Note that systems supporting the #! notation invoke interpreters this
37 way. See L<Location of Perl>.)
41 Passed in implicitly via standard input. This works only if there are
42 no filename arguments--to pass arguments to a STDIN-read program you
43 must explicitly specify a "-" for the program name.
47 With methods 2 and 3, Perl starts parsing the input file from the
48 beginning, unless you've specified a B<-x> switch, in which case it
49 scans for the first line starting with #! and containing the word
50 "perl", and starts there instead. This is useful for running a program
51 embedded in a larger message. (In this case you would indicate the end
52 of the program using the C<__END__> token.)
54 The #! line is always examined for switches as the line is being
55 parsed. Thus, if you're on a machine that allows only one argument
56 with the #! line, or worse, doesn't even recognize the #! line, you
57 still can get consistent switch behavior regardless of how Perl was
58 invoked, even if B<-x> was used to find the beginning of the program.
60 Because historically some operating systems silently chopped off
61 kernel interpretation of the #! line after 32 characters, some
62 switches may be passed in on the command line, and some may not;
63 you could even get a "-" without its letter, if you're not careful.
64 You probably want to make sure that all your switches fall either
65 before or after that 32-character boundary. Most switches don't
66 actually care if they're processed redundantly, but getting a "-"
67 instead of a complete switch could cause Perl to try to execute
68 standard input instead of your program. And a partial B<-I> switch
69 could also cause odd results.
71 Some switches do care if they are processed twice, for instance
72 combinations of B<-l> and B<-0>. Either put all the switches after
73 the 32-character boundary (if applicable), or replace the use of
74 B<-0>I<digits> by C<BEGIN{ $/ = "\0digits"; }>.
76 Parsing of the #! switches starts wherever "perl" is mentioned in the line.
77 The sequences "-*" and "- " are specifically ignored so that you could,
78 if you were so inclined, say
80 #!/bin/sh -- # -*- perl -*- -p
81 eval 'exec perl -wS $0 ${1+"$@"}'
82 if $running_under_some_shell;
84 to let Perl see the B<-p> switch.
86 A similar trick involves the B<env> program, if you have it.
90 The examples above use a relative path to the perl interpreter,
91 getting whatever version is first in the user's path. If you want
92 a specific version of Perl, say, perl5.005_57, you should place
93 that directly in the #! line's path.
95 If the #! line does not contain the word "perl", the program named after
96 the #! is executed instead of the Perl interpreter. This is slightly
97 bizarre, but it helps people on machines that don't do #!, because they
98 can tell a program that their SHELL is F</usr/bin/perl>, and Perl will then
99 dispatch the program to the correct interpreter for them.
101 After locating your program, Perl compiles the entire program to an
102 internal form. If there are any compilation errors, execution of the
103 program is not attempted. (This is unlike the typical shell script,
104 which might run part-way through before finding a syntax error.)
106 If the program is syntactically correct, it is executed. If the program
107 runs off the end without hitting an exit() or die() operator, an implicit
108 C<exit(0)> is provided to indicate successful completion.
110 =head2 #! and quoting on non-Unix systems
113 Unix's #! technique can be simulated on other systems:
121 extproc perl -S -your_switches
123 as the first line in C<*.cmd> file (B<-S> due to a bug in cmd.exe's
128 Create a batch file to run your program, and codify it in
129 C<ALTERNATE_SHEBANG> (see the F<dosish.h> file in the source
130 distribution for more information).
134 The Win95/NT installation, when using the ActiveState installer for Perl,
135 will modify the Registry to associate the F<.pl> extension with the perl
136 interpreter. If you install Perl by other means (including building from
137 the sources), you may have to modify the Registry yourself. Note that
138 this means you can no longer tell the difference between an executable
139 Perl program and a Perl library file.
143 Under "Classic" MacOS, a perl program will have the appropriate Creator and
144 Type, so that double-clicking them will invoke the MacPerl application.
145 Under Mac OS X, clickable apps can be made from any C<#!> script using Wil
146 Sanchez' DropScript utility: http://www.wsanchez.net/software/ .
152 $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
153 $ exit++ + ++$status != 0 and $exit = $status = undef;
155 at the top of your program, where B<-mysw> are any command line switches you
156 want to pass to Perl. You can now invoke the program directly, by saying
157 C<perl program>, or as a DCL procedure, by saying C<@program> (or implicitly
158 via F<DCL$PATH> by just using the name of the program).
160 This incantation is a bit much to remember, but Perl will display it for
161 you if you say C<perl "-V:startperl">.
165 Command-interpreters on non-Unix systems have rather different ideas
166 on quoting than Unix shells. You'll need to learn the special
167 characters in your command-interpreter (C<*>, C<\> and C<"> are
168 common) and how to protect whitespace and these characters to run
169 one-liners (see B<-e> below).
171 On some systems, you may have to change single-quotes to double ones,
172 which you must I<not> do on Unix or Plan 9 systems. You might also
173 have to change a single % to a %%.
178 perl -e 'print "Hello world\n"'
181 perl -e "print \"Hello world\n\""
184 print "Hello world\n"
185 (then Run "Myscript" or Shift-Command-R)
188 perl -e "print ""Hello world\n"""
190 The problem is that none of this is reliable: it depends on the
191 command and it is entirely possible neither works. If B<4DOS> were
192 the command shell, this would probably work better:
194 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
196 B<CMD.EXE> in Windows NT slipped a lot of standard Unix functionality in
197 when nobody was looking, but just try to find documentation for its
200 Under the Macintosh, it depends which environment you are using. The MacPerl
201 shell, or MPW, is much like Unix shells in its support for several
202 quoting variants, except that it makes free use of the Macintosh's non-ASCII
203 characters as control characters.
205 There is no general solution to all of this. It's just a mess.
207 =head2 Location of Perl
208 X<perl, location of interpreter>
210 It may seem obvious to say, but Perl is useful only when users can
211 easily find it. When possible, it's good for both F</usr/bin/perl>
212 and F</usr/local/bin/perl> to be symlinks to the actual binary. If
213 that can't be done, system administrators are strongly encouraged
214 to put (symlinks to) perl and its accompanying utilities into a
215 directory typically found along a user's PATH, or in some other
216 obvious and convenient place.
218 In this documentation, C<#!/usr/bin/perl> on the first line of the program
219 will stand in for whatever method works on your system. You are
220 advised to use a specific path if you care about a specific version.
222 #!/usr/local/bin/perl5.00554
224 or if you just want to be running at least version, place a statement
225 like this at the top of your program:
229 =head2 Command Switches
230 X<perl, command switches> X<command switches>
232 As with all standard commands, a single-character switch may be
233 clustered with the following switch, if any.
235 #!/usr/bin/perl -spi.orig # same as -s -p -i.orig
241 =item B<-0>[I<octal/hexadecimal>]
244 specifies the input record separator (C<$/>) as an octal or
245 hexadecimal number. If there are no digits, the null character is the
246 separator. Other switches may precede or follow the digits. For
247 example, if you have a version of B<find> which can print filenames
248 terminated by the null character, you can say this:
250 find . -name '*.orig' -print0 | perl -n0e unlink
252 The special value 00 will cause Perl to slurp files in paragraph mode.
253 The value 0777 will cause Perl to slurp files whole because there is no
254 legal byte with that value.
256 If you want to specify any Unicode character, use the hexadecimal
257 format: C<-0xHHH...>, where the C<H> are valid hexadecimal digits.
258 (This means that you cannot use the C<-x> with a directory name that
259 consists of hexadecimal digits.)
264 turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
265 split command to the @F array is done as the first thing inside the
266 implicit while loop produced by the B<-n> or B<-p>.
268 perl -ane 'print pop(@F), "\n";'
277 An alternate delimiter may be specified using B<-F>.
279 =item B<-C [I<number/list>]>
282 The C<-C> flag controls some of the Perl Unicode features.
284 As of 5.8.1, the C<-C> can be followed either by a number or a list
285 of option letters. The letters, their numeric values, and effects
286 are as follows; listing the letters is equal to summing the numbers.
288 I 1 STDIN is assumed to be in UTF-8
289 O 2 STDOUT will be in UTF-8
290 E 4 STDERR will be in UTF-8
292 i 8 UTF-8 is the default PerlIO layer for input streams
293 o 16 UTF-8 is the default PerlIO layer for output streams
295 A 32 the @ARGV elements are expected to be strings encoded
297 L 64 normally the "IOEioA" are unconditional,
298 the L makes them conditional on the locale environment
299 variables (the LC_ALL, LC_TYPE, and LANG, in the order
300 of decreasing precedence) -- if the variables indicate
301 UTF-8, then the selected "IOEioA" are in effect
302 a 256 Set ${^UTF8CACHE} to -1, to run the UTF-8 caching code in
305 =for documenting_the_underdocumented
306 perl.h gives W/128 as PERL_UNICODE_WIDESYSCALLS "/* for Sarathy */"
309 perltodo mentions Unicode in %ENV and filenames. I guess that these will be
310 options e and f (or F).
312 For example, C<-COE> and C<-C6> will both turn on UTF-8-ness on both
313 STDOUT and STDERR. Repeating letters is just redundant, not cumulative
316 The C<io> options mean that any subsequent open() (or similar I/O
317 operations) will have the C<:utf8> PerlIO layer implicitly applied
318 to them, in other words, UTF-8 is expected from any input stream,
319 and UTF-8 is produced to any output stream. This is just the default,
320 with explicit layers in open() and with binmode() one can manipulate
323 C<-C> on its own (not followed by any number or option list), or the
324 empty string C<""> for the C<PERL_UNICODE> environment variable, has the
325 same effect as C<-CSDL>. In other words, the standard I/O handles and
326 the default C<open()> layer are UTF-8-fied B<but> only if the locale
327 environment variables indicate a UTF-8 locale. This behaviour follows
328 the I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
330 You can use C<-C0> (or C<"0"> for C<PERL_UNICODE>) to explicitly
331 disable all the above Unicode features.
333 The read-only magic variable C<${^UNICODE}> reflects the numeric value
334 of this setting. This is variable is set during Perl startup and is
335 thereafter read-only. If you want runtime effects, use the three-arg
336 open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
337 and the C<open> pragma (see L<open>).
339 (In Perls earlier than 5.8.1 the C<-C> switch was a Win32-only switch
340 that enabled the use of Unicode-aware "wide system call" Win32 APIs.
341 This feature was practically unused, however, and the command line
342 switch was therefore "recycled".)
344 B<Note:> Since perl 5.10.0, the -C option can no longer be used
345 on the #! line. It wasn't working there anyway, since the standard streams
346 are already set up at this point in the execution of the perl interpreter.
347 You can use binmode() instead to get the desired behaviour.
352 causes Perl to check the syntax of the program and then exit without
353 executing it. Actually, it I<will> execute C<BEGIN>, C<UNITCHECK>,
354 C<CHECK>, and C<use> blocks, because these are considered as occurring
355 outside the execution of your program. C<INIT> and C<END> blocks,
356 however, will be skipped.
363 runs the program under the Perl debugger. See L<perldebug>.
364 If B<t> is specified, it indicates to the debugger that threads
365 will be used in the code being debugged.
367 =item B<-d:>I<foo[=bar,baz]>
370 =item B<-dt:>I<foo[=bar,baz]>
372 runs the program under the control of a debugging, profiling, or
373 tracing module installed as Devel::foo. E.g., B<-d:DProf> executes
374 the program using the Devel::DProf profiler. As with the B<-M>
375 flag, options may be passed to the Devel::foo package where they
376 will be received and interpreted by the Devel::foo::import routine.
377 The comma-separated list of options must follow a C<=> character.
378 If B<t> is specified, it indicates to the debugger that threads
379 will be used in the code being debugged.
382 =item B<-D>I<letters>
383 X<-D> X<DEBUGGING> X<-DDEBUGGING>
387 sets debugging flags. To watch how it executes your program, use
388 B<-Dtls>. (This works only if debugging is compiled into your
389 Perl.) Another nice value is B<-Dx>, which lists your compiled
390 syntax tree. And B<-Dr> displays compiled regular expressions;
391 the format of the output is explained in L<perldebguts>.
393 As an alternative, specify a number instead of list of letters (e.g.,
394 B<-D14> is equivalent to B<-Dtls>):
396 1 p Tokenizing and parsing (with v, displays parse stack)
397 2 s Stack snapshots (with v, displays all stacks)
398 4 l Context (loop) stack processing
400 16 o Method and overloading resolution
401 32 c String/numeric conversions
402 64 P Print profiling info, source file input state
403 128 m Memory and SV allocation
404 256 f Format processing
405 512 r Regular expression parsing and execution
406 1024 x Syntax tree dump
407 2048 u Tainting checks
408 4096 U Unofficial, User hacking (reserved for private, unreleased use)
409 8192 H Hash dump -- usurps values()
410 16384 X Scratchpad allocation
413 262144 R Include reference counts of dumped variables (eg when using -Ds)
414 524288 J Do not s,t,P-debug (Jump over) opcodes within package DB
415 1048576 v Verbose: use in conjunction with other flags
416 2097152 C Copy On Write
417 4194304 A Consistency checks on internal structures
418 8388608 q quiet - currently only suppresses the "EXECUTING" message
420 All these flags require B<-DDEBUGGING> when you compile the Perl
421 executable (but see L<Devel::Peek>, L<re> which may change this).
422 See the F<INSTALL> file in the Perl source distribution
423 for how to do this. This flag is automatically set if you include B<-g>
424 option when C<Configure> asks you about optimizer/debugger flags.
426 If you're just trying to get a print out of each line of Perl code
427 as it executes, the way that C<sh -x> provides for shell scripts,
428 you can't use Perl's B<-D> switch. Instead do this
430 # If you have "env" utility
431 env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
433 # Bourne shell syntax
434 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
437 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
439 See L<perldebug> for details and variations.
441 =item B<-e> I<commandline>
444 may be used to enter one line of program. If B<-e> is given, Perl
445 will not look for a filename in the argument list. Multiple B<-e>
446 commands may be given to build up a multi-line script. Make sure
447 to use semicolons where you would in a normal program.
449 =item B<-E> I<commandline>
452 behaves just like B<-e>, except that it implicitly enables all
453 optional features (in the main compilation unit). See L<feature>.
458 Disable executing F<$Config{sitelib}/sitecustomize.pl> at startup.
460 Perl can be built so that it by default will try to execute
461 F<$Config{sitelib}/sitecustomize.pl> at startup (in a BEGIN block).
462 This is a hook that allows the sysadmin to customize how perl behaves.
463 It can for instance be used to add entries to the @INC array to make perl
464 find modules in non-standard locations.
466 =item B<-F>I<pattern>
469 specifies the pattern to split on if B<-a> is also in effect. The
470 pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
471 put in single quotes. You can't use literal whitespace in the pattern.
476 prints a summary of the options.
478 =item B<-i>[I<extension>]
481 specifies that files processed by the C<E<lt>E<gt>> construct are to be
482 edited in-place. It does this by renaming the input file, opening the
483 output file by the original name, and selecting that output file as the
484 default for print() statements. The extension, if supplied, is used to
485 modify the name of the old file to make a backup copy, following these
488 If no extension is supplied, no backup is made and the current file is
491 If the extension doesn't contain a C<*>, then it is appended to the
492 end of the current filename as a suffix. If the extension does
493 contain one or more C<*> characters, then each C<*> is replaced
494 with the current filename. In Perl terms, you could think of this
497 ($backup = $extension) =~ s/\*/$file_name/g;
499 This allows you to add a prefix to the backup file, instead of (or in
500 addition to) a suffix:
502 $ perl -pi'orig_*' -e 's/bar/baz/' fileA # backup to 'orig_fileA'
504 Or even to place backup copies of the original files into another
505 directory (provided the directory already exists):
507 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'
509 These sets of one-liners are equivalent:
511 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
512 $ perl -pi'*' -e 's/bar/baz/' fileA # overwrite current file
514 $ perl -pi'.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
515 $ perl -pi'*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
517 From the shell, saying
519 $ perl -p -i.orig -e "s/foo/bar/; ... "
521 is the same as using the program:
523 #!/usr/bin/perl -pi.orig
526 which is equivalent to
529 $extension = '.orig';
531 if ($ARGV ne $oldargv) {
532 if ($extension !~ /\*/) {
533 $backup = $ARGV . $extension;
536 ($backup = $extension) =~ s/\*/$ARGV/g;
538 rename($ARGV, $backup);
539 open(ARGVOUT, ">$ARGV");
546 print; # this prints to original filename
550 except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
551 know when the filename has changed. It does, however, use ARGVOUT for
552 the selected filehandle. Note that STDOUT is restored as the default
553 output filehandle after the loop.
555 As shown above, Perl creates the backup file whether or not any output
556 is actually changed. So this is just a fancy way to copy files:
558 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
560 $ perl -p -i'.orig' -e 1 file1 file2 file3...
562 You can use C<eof> without parentheses to locate the end of each input
563 file, in case you want to append to each file, or reset line numbering
564 (see example in L<perlfunc/eof>).
566 If, for a given file, Perl is unable to create the backup file as
567 specified in the extension then it will skip that file and continue on
568 with the next one (if it exists).
570 For a discussion of issues surrounding file permissions and B<-i>,
571 see L<perlfaq5/Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl?>.
573 You cannot use B<-i> to create directories or to strip extensions from
576 Perl does not expand C<~> in filenames, which is good, since some
577 folks use it for their backup files:
579 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
581 Note that because B<-i> renames or deletes the original file before
582 creating a new file of the same name, UNIX-style soft and hard links will
585 Finally, the B<-i> switch does not impede execution when no
586 files are given on the command line. In this case, no backup is made
587 (the original file cannot, of course, be determined) and processing
588 proceeds from STDIN to STDOUT as might be expected.
590 =item B<-I>I<directory>
593 Directories specified by B<-I> are prepended to the search path for
596 =item B<-l>[I<octnum>]
599 enables automatic line-ending processing. It has two separate
600 effects. First, it automatically chomps C<$/> (the input record
601 separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
602 (the output record separator) to have the value of I<octnum> so
603 that any print statements will have that separator added back on.
604 If I<octnum> is omitted, sets C<$\> to the current value of
605 C<$/>. For instance, to trim lines to 80 columns:
607 perl -lpe 'substr($_, 80) = ""'
609 Note that the assignment C<$\ = $/> is done when the switch is processed,
610 so the input record separator can be different than the output record
611 separator if the B<-l> switch is followed by a B<-0> switch:
613 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
615 This sets C<$\> to newline and then sets C<$/> to the null character.
617 =item B<-m>[B<->]I<module>
620 =item B<-M>[B<->]I<module>
622 =item B<-M>[B<->]I<'module ...'>
624 =item B<-[mM]>[B<->]I<module=arg[,arg]...>
626 B<-m>I<module> executes C<use> I<module> C<();> before executing your
629 B<-M>I<module> executes C<use> I<module> C<;> before executing your
630 program. You can use quotes to add extra code after the module name,
631 e.g., C<'-Mmodule qw(foo bar)'>.
633 If the first character after the B<-M> or B<-m> is a dash (C<->)
634 then the 'use' is replaced with 'no'.
636 A little builtin syntactic sugar means you can also say
637 B<-mmodule=foo,bar> or B<-Mmodule=foo,bar> as a shortcut for
638 C<'-Mmodule qw(foo bar)'>. This avoids the need to use quotes when
639 importing symbols. The actual code generated by B<-Mmodule=foo,bar> is
640 C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
641 removes the distinction between B<-m> and B<-M>.
643 A consequence of this is that B<-MFoo=number> never does a version check
644 (unless C<Foo::import()> itself is set up to do a version check, which
645 could happen for example if Foo inherits from Exporter.)
650 causes Perl to assume the following loop around your program, which
651 makes it iterate over filename arguments somewhat like B<sed -n> or
656 ... # your program goes here
659 Note that the lines are not printed by default. See B<-p> to have
660 lines printed. If a file named by an argument cannot be opened for
661 some reason, Perl warns you about it and moves on to the next file.
663 Also note that C<< <> >> passes command line arguments to
664 L<perlfunc/open>, which doesn't necessarily interpret them as file names.
665 See L<perlop> for possible security implications.
667 Here is an efficient way to delete all files that haven't been modified for
670 find . -mtime +7 -print | perl -nle unlink
672 This is faster than using the B<-exec> switch of B<find> because you don't
673 have to start a process on every filename found. It does suffer from
674 the bug of mishandling newlines in pathnames, which you can fix if
675 you follow the example under B<-0>.
677 C<BEGIN> and C<END> blocks may be used to capture control before or after
678 the implicit program loop, just as in B<awk>.
683 causes Perl to assume the following loop around your program, which
684 makes it iterate over filename arguments somewhat like B<sed>:
689 ... # your program goes here
691 print or die "-p destination: $!\n";
694 If a file named by an argument cannot be opened for some reason, Perl
695 warns you about it, and moves on to the next file. Note that the
696 lines are printed automatically. An error occurring during printing is
697 treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
698 overrides a B<-n> switch.
700 C<BEGIN> and C<END> blocks may be used to capture control before or after
701 the implicit loop, just as in B<awk>.
706 enables rudimentary switch parsing for switches on the command
707 line after the program name but before any filename arguments (or before
708 an argument of B<-->). Any switch found there is removed from @ARGV and sets the
709 corresponding variable in the Perl program. The following program
710 prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
711 if it is invoked with B<-xyz=abc>.
714 if ($xyz) { print "$xyz\n" }
716 Do note that a switch like B<--help> creates the variable ${-help}, which is not compliant
717 with C<strict refs>. Also, when using this option on a script with
718 warnings enabled you may get a lot of spurious "used only once" warnings.
723 makes Perl use the PATH environment variable to search for the
724 program (unless the name of the program contains directory separators).
726 On some platforms, this also makes Perl append suffixes to the
727 filename while searching for it. For example, on Win32 platforms,
728 the ".bat" and ".cmd" suffixes are appended if a lookup for the
729 original name fails, and if the name does not already end in one
730 of those suffixes. If your Perl was compiled with DEBUGGING turned
731 on, using the -Dp switch to Perl shows how the search progresses.
733 Typically this is used to emulate #! startup on platforms that don't
734 support #!. Its also convenient when debugging a script that uses #!,
735 and is thus normally found by the shell's $PATH search mechanism.
737 This example works on many platforms that have a shell compatible with
741 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
742 if $running_under_some_shell;
744 The system ignores the first line and feeds the program to F</bin/sh>,
745 which proceeds to try to execute the Perl program as a shell script.
746 The shell executes the second line as a normal shell command, and thus
747 starts up the Perl interpreter. On some systems $0 doesn't always
748 contain the full pathname, so the B<-S> tells Perl to search for the
749 program if necessary. After Perl locates the program, it parses the
750 lines and ignores them because the variable $running_under_some_shell
751 is never true. If the program will be interpreted by csh, you will need
752 to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
753 embedded spaces (and such) in the argument list. To start up sh rather
754 than csh, some systems may have to replace the #! line with a line
755 containing just a colon, which will be politely ignored by Perl. Other
756 systems can't control that, and need a totally devious construct that
757 will work under any of B<csh>, B<sh>, or Perl, such as the following:
759 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
760 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
761 if $running_under_some_shell;
763 If the filename supplied contains directory separators (i.e., is an
764 absolute or relative pathname), and if that file is not found,
765 platforms that append file extensions will do so and try to look
766 for the file with those extensions added, one by one.
768 On DOS-like platforms, if the program does not contain directory
769 separators, it will first be searched for in the current directory
770 before being searched for on the PATH. On Unix platforms, the
771 program will be searched for strictly on the PATH.
776 Like B<-T>, but taint checks will issue warnings rather than fatal
777 errors. These warnings can be controlled normally with C<no warnings
780 B<NOTE: this is not a substitute for -T.> This is meant only to be
781 used as a temporary development aid while securing legacy code:
782 for real production code and for new secure code written from scratch
783 always use the real B<-T>.
788 forces "taint" checks to be turned on so you can test them. Ordinarily
789 these checks are done only when running setuid or setgid. It's a
790 good idea to turn them on explicitly for programs that run on behalf
791 of someone else whom you might not necessarily trust, such as CGI
792 programs or any internet servers you might write in Perl. See
793 L<perlsec> for details. For security reasons, this option must be
794 seen by Perl quite early; usually this means it must appear early
795 on the command line or in the #! line for systems which support
801 This obsolete switch causes Perl to dump core after compiling your
802 program. You can then in theory take this core dump and turn it
803 into an executable file by using the B<undump> program (not supplied).
804 This speeds startup at the expense of some disk space (which you
805 can minimize by stripping the executable). (Still, a "hello world"
806 executable comes out to about 200K on my machine.) If you want to
807 execute a portion of your program before dumping, use the dump()
808 operator instead. Note: availability of B<undump> is platform
809 specific and may not be available for a specific port of Perl.
814 allows Perl to do unsafe operations. Currently the only "unsafe"
815 operations are attempting to unlink directories while running as
816 superuser, and running setuid programs with fatal taint checks turned
817 into warnings. Note that the B<-w> switch (or the C<$^W> variable)
818 must be used along with this option to actually I<generate> the
819 taint-check warnings.
824 prints the version and patchlevel of your perl executable.
829 prints summary of the major perl configuration values and the current
832 =item B<-V:>I<configvar>
834 Prints to STDOUT the value of the named configuration variable(s),
835 with multiples when your configvar argument looks like a regex (has
836 non-letters). For example:
839 libc='/lib/libc-2.2.4.so';
841 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
842 libc='/lib/libc-2.2.4.so';
844 libpth='/usr/local/lib /lib /usr/lib';
845 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
847 libc='/lib/libc-2.2.4.so';
851 Additionally, extra colons can be used to control formatting. A
852 trailing colon suppresses the linefeed and terminator ';', allowing
853 you to embed queries into shell commands. (mnemonic: PATH separator
856 $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
857 compression-vars: zcat='' zip='zip' are here !
859 A leading colon removes the 'name=' part of the response, this allows
860 you to map to the name you need. (mnemonic: empty label)
862 $ echo "goodvfork="`./perl -Ilib -V::usevfork`
865 Leading and trailing colons can be used together if you need
866 positional parameter values without the names. Note that in the case
867 below, the PERL_API params are returned in alphabetical order.
869 $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
870 building_on 'linux' '5' '1' '9' now
875 prints warnings about dubious constructs, such as variable names
876 that are mentioned only once and scalar variables that are used
877 before being set, redefined subroutines, references to undefined
878 filehandles or filehandles opened read-only that you are attempting
879 to write on, values used as a number that don't look like numbers,
880 using an array as though it were a scalar, if your subroutines
881 recurse more than 100 deep, and innumerable other things.
883 This switch really just enables the internal C<$^W> variable. You
884 can disable or promote into fatal errors specific warnings using
885 C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
886 See also L<perldiag> and L<perltrap>. A new, fine-grained warning
887 facility is also available if you want to manipulate entire classes
888 of warnings; see L<warnings> or L<perllexwarn>.
893 Enables all warnings regardless of C<no warnings> or C<$^W>.
899 Disables all warnings regardless of C<use warnings> or C<$^W>.
905 =item B<-x>I<directory>
907 tells Perl that the program is embedded in a larger chunk of unrelated
908 ASCII text, such as in a mail message. Leading garbage will be
909 discarded until the first line that starts with #! and contains the
910 string "perl". Any meaningful switches on that line will be applied.
912 All references to line numbers by the program (warnings, errors, ...)
913 will treat the #! line as the first line.
914 Thus a warning on the 2nd line of the program (which is on the 100th
915 line in the file) will be reported as line 2, and not as line 100.
916 This can be overridden by using the #line directive.
917 (See L<perlsyn/"Plain-Old-Comments-(Not!)">)
919 If a directory name is specified, Perl will switch to that directory
920 before running the program. The B<-x> switch controls only the
921 disposal of leading garbage. The program must be terminated with
922 C<__END__> if there is trailing garbage to be ignored (the program
923 can process any or all of the trailing garbage via the DATA filehandle
926 The directory, if specified, must appear immediately following the B<-x>
927 with no intervening whitespace.
932 X<perl, environment variables>
939 Used if chdir has no argument.
944 Used if chdir has no argument and HOME is not set.
949 Used in executing subprocesses, and in finding the program if B<-S> is
955 A list of directories in which to look for Perl library
956 files before looking in the standard library and the current
957 directory. Any architecture-specific directories under the specified
958 locations are automatically included if they exist (this lookup
959 being done at interpreter startup time.)
961 If PERL5LIB is not defined, PERLLIB is used. Directories are separated
962 (like in PATH) by a colon on unixish platforms and by a semicolon on
963 Windows (the proper path separator being given by the command C<perl
966 When running taint checks (either because the program was running setuid
967 or setgid, or the B<-T> or B<-t> switch was specified), neither variable
968 is used. The program should instead say:
970 use lib "/my/directory";
975 Command-line options (switches). Switches in this variable are taken
976 as if they were on every Perl command line. Only the B<-[CDIMUdmtw]>
977 switches are allowed. When running taint checks (because the program
978 was running setuid or setgid, or the B<-T> switch was used), this
979 variable is ignored. If PERL5OPT begins with B<-T>, tainting will be
980 enabled, and any subsequent options ignored.
985 A space (or colon) separated list of PerlIO layers. If perl is built
986 to use PerlIO system for IO (the default) these layers effect perl's IO.
988 It is conventional to start layer names with a colon e.g. C<:perlio> to
989 emphasise their similarity to variable "attributes". But the code that parses
990 layer specification strings (which is also used to decode the PERLIO
991 environment variable) treats the colon as a separator.
993 An unset or empty PERLIO is equivalent to the default set of layers for
994 your platform, for example C<:unix:perlio> on UNIX-like systems
995 and C<:unix:crlf> on Windows and other DOS-like systems.
997 The list becomes the default for I<all> perl's IO. Consequently only built-in
998 layers can appear in this list, as external layers (such as :encoding()) need
999 IO in order to load them!. See L<"open pragma"|open> for how to add external
1000 encodings as defaults.
1002 The layers that it makes sense to include in the PERLIO environment
1003 variable are briefly summarised below. For more details see L<PerlIO>.
1010 A pseudolayer that turns I<off> the C<:utf8> flag for the layer below.
1011 Unlikely to be useful on its own in the global PERLIO environment variable.
1012 You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
1017 A layer which does CRLF to "\n" translation distinguishing "text" and
1018 "binary" files in the manner of MS-DOS and similar operating systems.
1019 (It currently does I<not> mimic MS-DOS as far as treating of Control-Z
1020 as being an end-of-file marker.)
1025 A layer which implements "reading" of files by using C<mmap()> to
1026 make (whole) file appear in the process's address space, and then
1027 using that as PerlIO's "buffer".
1032 This is a re-implementation of "stdio-like" buffering written as a
1033 PerlIO "layer". As such it will call whatever layer is below it for
1034 its operations (typically C<:unix>).
1039 An experimental pseudolayer that removes the topmost layer.
1040 Use with the same care as is reserved for nitroglycerin.
1045 A pseudolayer that manipulates other layers. Applying the C<:raw>
1046 layer is equivalent to calling C<binmode($fh)>. It makes the stream
1047 pass each byte as-is without any translation. In particular CRLF
1048 translation, and/or :utf8 intuited from locale are disabled.
1050 Unlike in the earlier versions of Perl C<:raw> is I<not>
1051 just the inverse of C<:crlf> - other layers which would affect the
1052 binary nature of the stream are also removed or disabled.
1057 This layer provides PerlIO interface by wrapping system's ANSI C "stdio"
1058 library calls. The layer provides both buffering and IO.
1059 Note that C<:stdio> layer does I<not> do CRLF translation even if that
1060 is platforms normal behaviour. You will need a C<:crlf> layer above it
1066 Low level layer which calls C<read>, C<write> and C<lseek> etc.
1071 A pseudolayer that turns on a flag on the layer below to tell perl
1072 that output should be in utf8 and that input should be regarded as
1073 already in valid utf8 form. It does not check for validity and as such
1074 should be handled with caution for input. Generally C<:encoding(utf8)> is
1075 the best option when reading UTF-8 encoded data.
1080 On Win32 platforms this I<experimental> layer uses native "handle" IO
1081 rather than unix-like numeric file descriptor layer. Known to be
1082 buggy in this release.
1086 On all platforms the default set of layers should give acceptable results.
1088 For UNIX platforms that will equivalent of "unix perlio" or "stdio".
1089 Configure is setup to prefer "stdio" implementation if system's library
1090 provides for fast access to the buffer, otherwise it uses the "unix perlio"
1093 On Win32 the default in this release is "unix crlf". Win32's "stdio"
1094 has a number of bugs/mis-features for perl IO which are somewhat
1095 C compiler vendor/version dependent. Using our own C<crlf> layer as
1096 the buffer avoids those issues and makes things more uniform.
1097 The C<crlf> layer provides CRLF to/from "\n" conversion as well as
1100 This release uses C<unix> as the bottom layer on Win32 and so still uses C
1101 compiler's numeric file descriptor routines. There is an experimental native
1102 C<win32> layer which is expected to be enhanced and should eventually be
1103 the default under Win32.
1105 The PERLIO environment variable is completely ignored when perl
1106 is run in taint mode.
1111 If set to the name of a file or device then certain operations of PerlIO
1112 sub-system will be logged to that file (opened as append). Typical uses
1115 PERLIO_DEBUG=/dev/tty perl script ...
1117 and Win32 approximate equivalent:
1119 set PERLIO_DEBUG=CON
1122 This functionality is disabled for setuid scripts and for scripts run
1128 A list of directories in which to look for Perl library
1129 files before looking in the standard library and the current directory.
1130 If PERL5LIB is defined, PERLLIB is not used.
1132 The PERLLIB environment variable is completely ignored when perl
1133 is run in taint mode.
1138 The command used to load the debugger code. The default is:
1140 BEGIN { require 'perl5db.pl' }
1142 The PERL5DB environment variable only used when perl is started with
1143 a bare B<-d> switch.
1145 =item PERL5DB_THREADED
1148 If set to a true value, indicates to the debugger that the code being
1149 debugged uses threads.
1151 =item PERL5SHELL (specific to the Win32 port)
1154 May be set to an alternative shell that perl must use internally for
1155 executing "backtick" commands or system(). Default is C<cmd.exe /x/d/c>
1156 on WindowsNT and C<command.com /c> on Windows95. The value is considered
1157 to be space-separated. Precede any character that needs to be protected
1158 (like a space or backslash) with a backslash.
1160 Note that Perl doesn't use COMSPEC for this purpose because
1161 COMSPEC has a high degree of variability among users, leading to
1162 portability concerns. Besides, perl can use a shell that may not be
1163 fit for interactive use, and setting COMSPEC to such a shell may
1164 interfere with the proper functioning of other programs (which usually
1165 look in COMSPEC to find a shell fit for interactive use).
1167 Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint checked
1168 when running external commands. It is recommended that
1169 you explicitly set (or delete) C<$ENV{PERL5SHELL}> when running
1170 in taint mode under Windows.
1172 =item PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
1173 X<PERL_ALLOW_NON_IFS_LSP>
1175 Set to 1 to allow the use of non-IFS compatible LSP's.
1176 Perl normally searches for an IFS-compatible LSP because this is required
1177 for its emulation of Windows sockets as real filehandles. However, this may
1178 cause problems if you have a firewall such as McAfee Guardian which requires
1179 all applications to use its LSP which is not IFS-compatible, because clearly
1180 Perl will normally avoid using such an LSP.
1181 Setting this environment variable to 1 means that Perl will simply use the
1182 first suitable LSP enumerated in the catalog, which keeps McAfee Guardian
1183 happy (and in that particular case Perl still works too because McAfee
1184 Guardian's LSP actually plays some other games which allow applications
1185 requiring IFS compatibility to work).
1187 =item PERL_DEBUG_MSTATS
1188 X<PERL_DEBUG_MSTATS>
1190 Relevant only if perl is compiled with the malloc included with the perl
1191 distribution (that is, if C<perl -V:d_mymalloc> is 'define').
1192 If set, this causes memory statistics to be dumped after execution. If set
1193 to an integer greater than one, also causes memory statistics to be dumped
1196 =item PERL_DESTRUCT_LEVEL
1197 X<PERL_DESTRUCT_LEVEL>
1199 Relevant only if your perl executable was built with B<-DDEBUGGING>,
1200 this controls the behavior of global destruction of objects and other
1201 references. See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.
1203 =item PERL_DL_NONLAZY
1206 Set to one to have perl resolve B<all> undefined symbols when it loads
1207 a dynamic library. The default behaviour is to resolve symbols when
1208 they are used. Setting this variable is useful during testing of
1209 extensions as it ensures that you get an error on misspelled function
1210 names even if the test suite doesn't call it.
1215 If using the C<encoding> pragma without an explicit encoding name, the
1216 PERL_ENCODING environment variable is consulted for an encoding name.
1218 =item PERL_HASH_SEED
1221 (Since Perl 5.8.1.) Used to randomise perl's internal hash function.
1222 To emulate the pre-5.8.1 behaviour, set to an integer (zero means
1223 exactly the same order as 5.8.0). "Pre-5.8.1" means, among other
1224 things, that hash keys will always have the same ordering between
1225 different runs of perl.
1227 Most hashes return elements in the same order as Perl 5.8.0 by default.
1228 On a hash by hash basis, if pathological data is detected during a hash
1229 key insertion, then that hash will switch to an alternative random hash
1232 The default behaviour is to randomise unless the PERL_HASH_SEED is set.
1233 If perl has been compiled with C<-DUSE_HASH_SEED_EXPLICIT>, the default
1234 behaviour is B<not> to randomise unless the PERL_HASH_SEED is set.
1236 If PERL_HASH_SEED is unset or set to a non-numeric string, perl uses
1237 the pseudorandom seed supplied by the operating system and libraries.
1239 B<Please note that the hash seed is sensitive information>. Hashes are
1240 randomized to protect against local and remote attacks against Perl
1241 code. By manually setting a seed this protection may be partially or
1244 See L<perlsec/"Algorithmic Complexity Attacks"> and
1245 L</PERL_HASH_SEED_DEBUG> for more information.
1247 =item PERL_HASH_SEED_DEBUG
1248 X<PERL_HASH_SEED_DEBUG>
1250 (Since Perl 5.8.1.) Set to one to display (to STDERR) the value of
1251 the hash seed at the beginning of execution. This, combined with
1252 L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
1253 behavior caused by hash randomization.
1255 B<Note that the hash seed is sensitive information>: by knowing it one
1256 can craft a denial-of-service attack against Perl code, even remotely,
1257 see L<perlsec/"Algorithmic Complexity Attacks"> for more information.
1258 B<Do not disclose the hash seed> to people who don't need to know it.
1259 See also hash_seed() of L<Hash::Util>.
1261 =item PERL_ROOT (specific to the VMS port)
1264 A translation concealed rooted logical name that contains perl and the
1265 logical device for the @INC path on VMS only. Other logical names that
1266 affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1267 SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in
1268 L<perlvms> and in F<README.vms> in the Perl source distribution.
1273 In Perls 5.8.1 and later. If set to C<unsafe> the pre-Perl-5.8.0
1274 signals behaviour (immediate but unsafe) is restored. If set to
1275 C<safe> the safe (or deferred) signals are used.
1276 See L<perlipc/"Deferred Signals (Safe Signals)">.
1281 Equivalent to the B<-C> command-line switch. Note that this is not
1282 a boolean variable-- setting this to C<"1"> is not the right way to
1283 "enable Unicode" (whatever that would mean). You can use C<"0"> to
1284 "disable Unicode", though (or alternatively unset PERL_UNICODE in
1285 your shell before starting Perl). See the description of the C<-C>
1286 switch for more information.
1288 =item SYS$LOGIN (specific to the VMS port)
1291 Used if chdir has no argument and HOME and LOGDIR are not set.
1295 Perl also has environment variables that control how Perl handles data
1296 specific to particular natural languages. See L<perllocale>.
1298 Apart from these, Perl uses no other environment variables, except
1299 to make them available to the program being executed, and to child
1300 processes. However, programs running setuid would do well to execute
1301 the following lines before doing anything else, just to keep people
1304 $ENV{PATH} = '/bin:/usr/bin'; # or whatever you need
1305 $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
1306 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};