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<-e>|B<-E>] 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 C<#!> 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 C<#!> 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 C<#!> 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 C<#!> line, or worse, doesn't even recognize the C<#!> line, you
57 still can get consistent switch behaviour 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 C<#!> 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 C<#!> 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
82 eval 'exec perl -x -wS $0 ${1+"$@"}'
85 to let Perl see the B<-p> switch.
87 A similar trick involves the I<env> program, if you have it.
91 The examples above use a relative path to the perl interpreter,
92 getting whatever version is first in the user's path. If you want
93 a specific version of Perl, say, perl5.005_57, you should place
94 that directly in the C<#!> line's path.
96 If the C<#!> line does not contain the word "perl", the program named after
97 the C<#!> is executed instead of the Perl interpreter. This is slightly
98 bizarre, but it helps people on machines that don't do C<#!>, because they
99 can tell a program that their SHELL is F</usr/bin/perl>, and Perl will then
100 dispatch the program to the correct interpreter for them.
102 After locating your program, Perl compiles the entire program to an
103 internal form. If there are any compilation errors, execution of the
104 program is not attempted. (This is unlike the typical shell script,
105 which might run part-way through before finding a syntax error.)
107 If the program is syntactically correct, it is executed. If the program
108 runs off the end without hitting an exit() or die() operator, an implicit
109 C<exit(0)> is provided to indicate successful completion.
111 =head2 #! and quoting on non-Unix systems
114 Unix's C<#!> technique can be simulated on other systems:
122 extproc perl -S -your_switches
124 as the first line in C<*.cmd> file (B<-S> due to a bug in cmd.exe's
129 Create a batch file to run your program, and codify it in
130 C<ALTERNATE_SHEBANG> (see the F<dosish.h> file in the source
131 distribution for more information).
135 The Win95/NT installation, when using the ActiveState installer for Perl,
136 will modify the Registry to associate the F<.pl> extension with the perl
137 interpreter. If you install Perl by other means (including building from
138 the sources), you may have to modify the Registry yourself. Note that
139 this means you can no longer tell the difference between an executable
140 Perl program and a Perl library file.
146 $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
147 $ exit++ + ++$status != 0 and $exit = $status = undef;
149 at the top of your program, where B<-mysw> are any command line switches you
150 want to pass to Perl. You can now invoke the program directly, by saying
151 C<perl program>, or as a DCL procedure, by saying C<@program> (or implicitly
152 via F<DCL$PATH> by just using the name of the program).
154 This incantation is a bit much to remember, but Perl will display it for
155 you if you say C<perl "-V:startperl">.
159 Command-interpreters on non-Unix systems have rather different ideas
160 on quoting than Unix shells. You'll need to learn the special
161 characters in your command-interpreter (C<*>, C<\> and C<"> are
162 common) and how to protect whitespace and these characters to run
163 one-liners (see L<-e|/-e commandline> below).
165 On some systems, you may have to change single-quotes to double ones,
166 which you must I<not> do on Unix or Plan 9 systems. You might also
167 have to change a single % to a %%.
172 perl -e 'print "Hello world\n"'
175 perl -e "print \"Hello world\n\""
178 perl -e "print ""Hello world\n"""
180 The problem is that none of this is reliable: it depends on the
181 command and it is entirely possible neither works. If I<4DOS> were
182 the command shell, this would probably work better:
184 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
186 B<CMD.EXE> in Windows NT slipped a lot of standard Unix functionality in
187 when nobody was looking, but just try to find documentation for its
190 There is no general solution to all of this. It's just a mess.
192 =head2 Location of Perl
193 X<perl, location of interpreter>
195 It may seem obvious to say, but Perl is useful only when users can
196 easily find it. When possible, it's good for both F</usr/bin/perl>
197 and F</usr/local/bin/perl> to be symlinks to the actual binary. If
198 that can't be done, system administrators are strongly encouraged
199 to put (symlinks to) perl and its accompanying utilities into a
200 directory typically found along a user's PATH, or in some other
201 obvious and convenient place.
203 In this documentation, C<#!/usr/bin/perl> on the first line of the program
204 will stand in for whatever method works on your system. You are
205 advised to use a specific path if you care about a specific version.
207 #!/usr/local/bin/perl5.00554
209 or if you just want to be running at least version, place a statement
210 like this at the top of your program:
214 =head2 Command Switches
215 X<perl, command switches> X<command switches>
217 As with all standard commands, a single-character switch may be
218 clustered with the following switch, if any.
220 #!/usr/bin/perl -spi.orig # same as -s -p -i.orig
226 =item B<-0>[I<octal/hexadecimal>]
229 specifies the input record separator (C<$/>) as an octal or
230 hexadecimal number. If there are no digits, the null character is the
231 separator. Other switches may precede or follow the digits. For
232 example, if you have a version of I<find> which can print filenames
233 terminated by the null character, you can say this:
235 find . -name '*.orig' -print0 | perl -n0e unlink
237 The special value 00 will cause Perl to slurp files in paragraph mode.
238 Any value 0400 or above will cause Perl to slurp files whole, but by convention
239 the value 0777 is the one normally used for this purpose.
241 You can also specify the separator character using hexadecimal notation:
242 B<-0xI<HHH...>>, where the C<I<H>> are valid hexadecimal digits. Unlike
243 the octal form, this one may be used to specify any Unicode character, even
244 those beyond 0xFF. So if you I<really> want a record separator of 0777,
245 specify it as B<-0x1FF>. (This means that you cannot use the B<-x> option
246 with a directory name that consists of hexadecimal digits, or else Perl
247 will think you have specified a hex number to B<-0>.)
252 turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
253 split command to the @F array is done as the first thing inside the
254 implicit while loop produced by the B<-n> or B<-p>.
256 perl -ane 'print pop(@F), "\n";'
265 An alternate delimiter may be specified using B<-F>.
267 =item B<-C [I<number/list>]>
270 The B<-C> flag controls some of the Perl Unicode features.
272 As of 5.8.1, the B<-C> can be followed either by a number or a list
273 of option letters. The letters, their numeric values, and effects
274 are as follows; listing the letters is equal to summing the numbers.
276 I 1 STDIN is assumed to be in UTF-8
277 O 2 STDOUT will be in UTF-8
278 E 4 STDERR will be in UTF-8
280 i 8 UTF-8 is the default PerlIO layer for input streams
281 o 16 UTF-8 is the default PerlIO layer for output streams
283 A 32 the @ARGV elements are expected to be strings encoded
285 L 64 normally the "IOEioA" are unconditional, the L makes
286 them conditional on the locale environment variables
287 (the LC_ALL, LC_TYPE, and LANG, in the order of
288 decreasing precedence) -- if the variables indicate
289 UTF-8, then the selected "IOEioA" are in effect
290 a 256 Set ${^UTF8CACHE} to -1, to run the UTF-8 caching
291 code in debugging mode.
293 =for documenting_the_underdocumented
294 perl.h gives W/128 as PERL_UNICODE_WIDESYSCALLS "/* for Sarathy */"
297 perltodo mentions Unicode in %ENV and filenames. I guess that these will be
298 options e and f (or F).
300 For example, B<-COE> and B<-C6> will both turn on UTF-8-ness on both
301 STDOUT and STDERR. Repeating letters is just redundant, not cumulative
304 The C<io> options mean that any subsequent open() (or similar I/O
305 operations) in the current file scope will have the C<:utf8> PerlIO layer
306 implicitly applied to them, in other words, UTF-8 is expected from any
307 input stream, and UTF-8 is produced to any output stream. This is just
308 the default, with explicit layers in open() and with binmode() one can
309 manipulate streams as usual.
311 B<-C> on its own (not followed by any number or option list), or the
312 empty string C<""> for the C<PERL_UNICODE> environment variable, has the
313 same effect as B<-CSDL>. In other words, the standard I/O handles and
314 the default C<open()> layer are UTF-8-fied I<but> only if the locale
315 environment variables indicate a UTF-8 locale. This behaviour follows
316 the I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
317 (See L<perl581delta/UTF-8 no longer default under UTF-8 locales>.)
319 You can use B<-C0> (or C<"0"> for C<PERL_UNICODE>) to explicitly
320 disable all the above Unicode features.
322 The read-only magic variable C<${^UNICODE}> reflects the numeric value
323 of this setting. This variable is set during Perl startup and is
324 thereafter read-only. If you want runtime effects, use the three-arg
325 open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
326 and the C<open> pragma (see L<open>).
328 (In Perls earlier than 5.8.1 the B<-C> switch was a Win32-only switch
329 that enabled the use of Unicode-aware "wide system call" Win32 APIs.
330 This feature was practically unused, however, and the command line
331 switch was therefore "recycled".)
333 B<Note:> Since perl 5.10.1, if the B<-C> option is used on the C<#!> line,
334 it must be specified on the command line as well, since the standard streams
335 are already set up at this point in the execution of the perl interpreter.
336 You can also use binmode() to set the encoding of an I/O stream.
341 causes Perl to check the syntax of the program and then exit without
342 executing it. Actually, it I<will> execute and C<BEGIN>, C<UNITCHECK>,
343 or C<CHECK> blocks and any C<use> statements: these are considered as
344 occurring outside the execution of your program. C<INIT> and C<END>
345 blocks, however, will be skipped.
352 runs the program under the Perl debugger. See L<perldebug>.
353 If B<t> is specified, it indicates to the debugger that threads
354 will be used in the code being debugged.
356 =item B<-d:>I<MOD[=bar,baz]>
359 =item B<-dt:>I<MOD[=bar,baz]>
361 runs the program under the control of a debugging, profiling, or tracing
362 module installed as C<Devel::I<MOD>>. E.g., B<-d:DProf> executes the
363 program using the C<Devel::DProf> profiler. As with the B<-M> flag, options
364 may be passed to the C<Devel::I<MOD>> package where they will be received
365 and interpreted by the C<Devel::I<MOD>::import> routine. Again, like B<-M>,
366 use -B<-d:-I<MOD>> to call C<Devel::I<MOD>::unimport> instead of import. The
367 comma-separated list of options must follow a C<=> character. If B<t> is
368 specified, it indicates to the debugger that threads will be used in the
369 code being debugged. See L<perldebug>.
371 =item B<-D>I<letters>
372 X<-D> X<DEBUGGING> X<-DDEBUGGING>
376 sets debugging flags. To watch how it executes your program, use
377 B<-Dtls>. (This works only if debugging is compiled into your
378 Perl.) Another nice value is B<-Dx>, which lists your compiled
379 syntax tree. And B<-Dr> displays compiled regular expressions;
380 the format of the output is explained in L<perldebguts>.
382 As an alternative, specify a number instead of list of letters (e.g.,
383 B<-D14> is equivalent to B<-Dtls>):
385 1 p Tokenizing and parsing (with v, displays parse stack)
386 2 s Stack snapshots (with v, displays all stacks)
387 4 l Context (loop) stack processing
389 16 o Method and overloading resolution
390 32 c String/numeric conversions
391 64 P Print profiling info, source file input state
392 128 m Memory and SV allocation
393 256 f Format processing
394 512 r Regular expression parsing and execution
395 1024 x Syntax tree dump
396 2048 u Tainting checks
397 4096 U Unofficial, User hacking (reserved for private,
399 8192 H Hash dump -- usurps values()
400 16384 X Scratchpad allocation
403 262144 R Include reference counts of dumped variables (eg when
405 524288 J show s,t,P-debug (don't Jump over) on opcodes within
407 1048576 v Verbose: use in conjunction with other flags
408 2097152 C Copy On Write
409 4194304 A Consistency checks on internal structures
410 8388608 q quiet - currently only suppresses the "EXECUTING"
412 16777216 M trace smart match resolution
413 33554432 B dump suBroutine definitions, including special Blocks
416 All these flags require B<-DDEBUGGING> when you compile the Perl
417 executable (but see C<:opd> in L<Devel::Peek> or L<re/'debug' mode>
418 which may change this).
419 See the F<INSTALL> file in the Perl source distribution
420 for how to do this. This flag is automatically set if you include B<-g>
421 option when C<Configure> asks you about optimizer/debugger flags.
423 If you're just trying to get a print out of each line of Perl code
424 as it executes, the way that C<sh -x> provides for shell scripts,
425 you can't use Perl's B<-D> switch. Instead do this
427 # If you have "env" utility
428 env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
430 # Bourne shell syntax
431 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
434 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
436 See L<perldebug> for details and variations.
438 =item B<-e> I<commandline>
441 may be used to enter one line of program. If B<-e> is given, Perl
442 will not look for a filename in the argument list. Multiple B<-e>
443 commands may be given to build up a multi-line script. Make sure
444 to use semicolons where you would in a normal program.
446 =item B<-E> I<commandline>
449 behaves just like B<-e>, except that it implicitly enables all
450 optional features (in the main compilation unit). See L<feature>.
453 X<-f> X<sitecustomize> X<sitecustomize.pl>
455 Disable executing F<$Config{sitelib}/sitecustomize.pl> at startup.
457 Perl can be built so that it by default will try to execute
458 F<$Config{sitelib}/sitecustomize.pl> at startup (in a BEGIN block).
459 This is a hook that allows the sysadmin to customize how Perl behaves.
460 It can for instance be used to add entries to the @INC array to make Perl
461 find modules in non-standard locations.
463 Perl actually inserts the following code:
466 do { local $!; -f "$Config{sitelib}/sitecustomize.pl"; }
467 && do "$Config{sitelib}/sitecustomize.pl";
470 Since it is an actual C<do> (not a C<require>), F<sitecustomize.pl>
471 doesn't need to return a true value. The code is run in package C<main>,
472 in its own lexical scope. However, if the script dies, C<$@> will not
475 The value of C<$Config{sitelib}> is also determined in C code and not
476 read from C<Config.pm>, which is not loaded.
478 The code is executed I<very> early. For example, any changes made to
479 C<@INC> will show up in the output of `perl -V`. Of course, C<END>
480 blocks will be likewise executed very late.
482 To determine at runtime if this capability has been compiled in your
483 perl, you can check the value of C<$Config{usesitecustomize}>.
485 =item B<-F>I<pattern>
488 specifies the pattern to split on if B<-a> is also in effect. The
489 pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
490 put in single quotes. You can't use literal whitespace in the pattern.
495 prints a summary of the options.
497 =item B<-i>[I<extension>]
500 specifies that files processed by the C<E<lt>E<gt>> construct are to be
501 edited in-place. It does this by renaming the input file, opening the
502 output file by the original name, and selecting that output file as the
503 default for print() statements. The extension, if supplied, is used to
504 modify the name of the old file to make a backup copy, following these
507 If no extension is supplied, no backup is made and the current file is
510 If the extension doesn't contain a C<*>, then it is appended to the
511 end of the current filename as a suffix. If the extension does
512 contain one or more C<*> characters, then each C<*> is replaced
513 with the current filename. In Perl terms, you could think of this
516 ($backup = $extension) =~ s/\*/$file_name/g;
518 This allows you to add a prefix to the backup file, instead of (or in
519 addition to) a suffix:
521 $ perl -pi'orig_*' -e 's/bar/baz/' fileA # backup to
524 Or even to place backup copies of the original files into another
525 directory (provided the directory already exists):
527 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to
530 These sets of one-liners are equivalent:
532 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
533 $ perl -pi'*' -e 's/bar/baz/' fileA # overwrite current file
535 $ perl -pi'.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
536 $ perl -pi'*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
538 From the shell, saying
540 $ perl -p -i.orig -e "s/foo/bar/; ... "
542 is the same as using the program:
544 #!/usr/bin/perl -pi.orig
547 which is equivalent to
550 $extension = '.orig';
552 if ($ARGV ne $oldargv) {
553 if ($extension !~ /\*/) {
554 $backup = $ARGV . $extension;
557 ($backup = $extension) =~ s/\*/$ARGV/g;
559 rename($ARGV, $backup);
560 open(ARGVOUT, ">$ARGV");
567 print; # this prints to original filename
571 except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
572 know when the filename has changed. It does, however, use ARGVOUT for
573 the selected filehandle. Note that STDOUT is restored as the default
574 output filehandle after the loop.
576 As shown above, Perl creates the backup file whether or not any output
577 is actually changed. So this is just a fancy way to copy files:
579 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
581 $ perl -p -i'.orig' -e 1 file1 file2 file3...
583 You can use C<eof> without parentheses to locate the end of each input
584 file, in case you want to append to each file, or reset line numbering
585 (see example in L<perlfunc/eof>).
587 If, for a given file, Perl is unable to create the backup file as
588 specified in the extension then it will skip that file and continue on
589 with the next one (if it exists).
591 For a discussion of issues surrounding file permissions and B<-i>,
592 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?>.
594 You cannot use B<-i> to create directories or to strip extensions from
597 Perl does not expand C<~> in filenames, which is good, since some
598 folks use it for their backup files:
600 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
602 Note that because B<-i> renames or deletes the original file before
603 creating a new file of the same name, Unix-style soft and hard links will
606 Finally, the B<-i> switch does not impede execution when no
607 files are given on the command line. In this case, no backup is made
608 (the original file cannot, of course, be determined) and processing
609 proceeds from STDIN to STDOUT as might be expected.
611 =item B<-I>I<directory>
614 Directories specified by B<-I> are prepended to the search path for
617 =item B<-l>[I<octnum>]
620 enables automatic line-ending processing. It has two separate
621 effects. First, it automatically chomps C<$/> (the input record
622 separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
623 (the output record separator) to have the value of I<octnum> so
624 that any print statements will have that separator added back on.
625 If I<octnum> is omitted, sets C<$\> to the current value of
626 C<$/>. For instance, to trim lines to 80 columns:
628 perl -lpe 'substr($_, 80) = ""'
630 Note that the assignment C<$\ = $/> is done when the switch is processed,
631 so the input record separator can be different than the output record
632 separator if the B<-l> switch is followed by a B<-0> switch:
634 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
636 This sets C<$\> to newline and then sets C<$/> to the null character.
638 =item B<-m>[B<->]I<module>
641 =item B<-M>[B<->]I<module>
643 =item B<-M>[B<->]I<'module ...'>
645 =item B<-[mM]>[B<->]I<module=arg[,arg]...>
647 B<-m>I<module> executes C<use> I<module> C<();> before executing your
650 B<-M>I<module> executes C<use> I<module> C<;> before executing your
651 program. You can use quotes to add extra code after the module name,
652 e.g., C<'-MI<MODULE> qw(foo bar)'>.
654 If the first character after the B<-M> or B<-m> is a dash (B<->)
655 then the 'use' is replaced with 'no'.
657 A little builtin syntactic sugar means you can also say
658 B<-mI<MODULE>=foo,bar> or B<-MI<MODULE>=foo,bar> as a shortcut for
659 B<'-MI<MODULE> qw(foo bar)'>. This avoids the need to use quotes when
660 importing symbols. The actual code generated by B<-MI<MODULE>=foo,bar> is
661 C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
662 removes the distinction between B<-m> and B<-M>.
664 A consequence of this is that B<-MI<MODULE>=number> never does a version check,
665 unless C<I<MODULE>::import()> itself is set up to do a version check, which
666 could happen for example if I<MODULE> inherits from L<Exporter>.
671 causes Perl to assume the following loop around your program, which
672 makes it iterate over filename arguments somewhat like I<sed -n> or
677 ... # your program goes here
680 Note that the lines are not printed by default. See L</-p> to have
681 lines printed. If a file named by an argument cannot be opened for
682 some reason, Perl warns you about it and moves on to the next file.
684 Also note that C<< <> >> passes command line arguments to
685 L<perlfunc/open>, which doesn't necessarily interpret them as file names.
686 See L<perlop> for possible security implications.
688 Here is an efficient way to delete all files that haven't been modified for
691 find . -mtime +7 -print | perl -nle unlink
693 This is faster than using the B<-exec> switch of I<find> because you don't
694 have to start a process on every filename found. It does suffer from
695 the bug of mishandling newlines in pathnames, which you can fix if
696 you follow the example under B<-0>.
698 C<BEGIN> and C<END> blocks may be used to capture control before or after
699 the implicit program loop, just as in I<awk>.
704 causes Perl to assume the following loop around your program, which
705 makes it iterate over filename arguments somewhat like I<sed>:
710 ... # your program goes here
712 print or die "-p destination: $!\n";
715 If a file named by an argument cannot be opened for some reason, Perl
716 warns you about it, and moves on to the next file. Note that the
717 lines are printed automatically. An error occurring during printing is
718 treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
719 overrides a B<-n> switch.
721 C<BEGIN> and C<END> blocks may be used to capture control before or after
722 the implicit loop, just as in I<awk>.
727 enables rudimentary switch parsing for switches on the command
728 line after the program name but before any filename arguments (or before
729 an argument of B<-->). Any switch found there is removed from @ARGV and sets the
730 corresponding variable in the Perl program. The following program
731 prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
732 if it is invoked with B<-xyz=abc>.
735 if ($xyz) { print "$xyz\n" }
737 Do note that a switch like B<--help> creates the variable C<${-help}>, which is not compliant
738 with C<use strict "refs">. Also, when using this option on a script with
739 warnings enabled you may get a lot of spurious "used only once" warnings.
744 makes Perl use the PATH environment variable to search for the
745 program unless the name of the program contains path separators.
747 On some platforms, this also makes Perl append suffixes to the
748 filename while searching for it. For example, on Win32 platforms,
749 the ".bat" and ".cmd" suffixes are appended if a lookup for the
750 original name fails, and if the name does not already end in one
751 of those suffixes. If your Perl was compiled with C<DEBUGGING> turned
752 on, using the B<-Dp> switch to Perl shows how the search progresses.
754 Typically this is used to emulate C<#!> startup on platforms that don't
755 support C<#!>. It's also convenient when debugging a script that uses C<#!>,
756 and is thus normally found by the shell's $PATH search mechanism.
758 This example works on many platforms that have a shell compatible with
762 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
763 if $running_under_some_shell;
765 The system ignores the first line and feeds the program to F</bin/sh>,
766 which proceeds to try to execute the Perl program as a shell script.
767 The shell executes the second line as a normal shell command, and thus
768 starts up the Perl interpreter. On some systems $0 doesn't always
769 contain the full pathname, so the B<-S> tells Perl to search for the
770 program if necessary. After Perl locates the program, it parses the
771 lines and ignores them because the variable $running_under_some_shell
772 is never true. If the program will be interpreted by csh, you will need
773 to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
774 embedded spaces (and such) in the argument list. To start up I<sh> rather
775 than I<csh>, some systems may have to replace the C<#!> line with a line
776 containing just a colon, which will be politely ignored by Perl. Other
777 systems can't control that, and need a totally devious construct that
778 will work under any of I<csh>, I<sh>, or Perl, such as the following:
780 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
781 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
782 if $running_under_some_shell;
784 If the filename supplied contains directory separators (and so is an
785 absolute or relative pathname), and if that file is not found,
786 platforms that append file extensions will do so and try to look
787 for the file with those extensions added, one by one.
789 On DOS-like platforms, if the program does not contain directory
790 separators, it will first be searched for in the current directory
791 before being searched for on the PATH. On Unix platforms, the
792 program will be searched for strictly on the PATH.
797 Like B<-T>, but taint checks will issue warnings rather than fatal
798 errors. These warnings can now be controlled normally with C<no warnings
801 B<Note: This is not a substitute for C<-T>!> This is meant to be
802 used I<only> as a temporary development aid while securing legacy code:
803 for real production code and for new secure code written from scratch,
804 always use the real B<-T>.
809 turns on "taint" so you can test them. Ordinarily
810 these checks are done only when running setuid or setgid. It's a
811 good idea to turn them on explicitly for programs that run on behalf
812 of someone else whom you might not necessarily trust, such as CGI
813 programs or any internet servers you might write in Perl. See
814 L<perlsec> for details. For security reasons, this option must be
815 seen by Perl quite early; usually this means it must appear early
816 on the command line or in the C<#!> line for systems which support
822 This switch causes Perl to dump core after compiling your
823 program. You can then in theory take this core dump and turn it
824 into an executable file by using the I<undump> program (not supplied).
825 This speeds startup at the expense of some disk space (which you
826 can minimize by stripping the executable). (Still, a "hello world"
827 executable comes out to about 200K on my machine.) If you want to
828 execute a portion of your program before dumping, use the dump()
829 operator instead. Note: availability of I<undump> is platform
830 specific and may not be available for a specific port of Perl.
835 allows Perl to do unsafe operations. Currently the only "unsafe"
836 operations are attempting to unlink directories while running as superuser
837 and running setuid programs with fatal taint checks turned into warnings.
838 Note that warnings must be enabled along with this option to actually
839 I<generate> the taint-check warnings.
844 prints the version and patchlevel of your perl executable.
849 prints summary of the major perl configuration values and the current
852 =item B<-V:>I<configvar>
854 Prints to STDOUT the value of the named configuration variable(s),
855 with multiples when your C<I<configvar>> argument looks like a regex (has
856 non-letters). For example:
859 libc='/lib/libc-2.2.4.so';
861 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
862 libc='/lib/libc-2.2.4.so';
864 libpth='/usr/local/lib /lib /usr/lib';
865 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
867 libc='/lib/libc-2.2.4.so';
871 Additionally, extra colons can be used to control formatting. A
872 trailing colon suppresses the linefeed and terminator ";", allowing
873 you to embed queries into shell commands. (mnemonic: PATH separator
876 $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
877 compression-vars: zcat='' zip='zip' are here !
879 A leading colon removes the "name=" part of the response, this allows
880 you to map to the name you need. (mnemonic: empty label)
882 $ echo "goodvfork="`./perl -Ilib -V::usevfork`
885 Leading and trailing colons can be used together if you need
886 positional parameter values without the names. Note that in the case
887 below, the C<PERL_API> params are returned in alphabetical order.
889 $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
890 building_on 'linux' '5' '1' '9' now
895 prints warnings about dubious constructs, such as variable names
896 mentioned only once and scalar variables used
897 before being set; redefined subroutines; references to undefined
898 filehandles; filehandles opened read-only that you are attempting
899 to write on; values used as a number that don't I<look> like numbers;
900 using an array as though it were a scalar; if your subroutines
901 recurse more than 100 deep; and innumerable other things.
903 This switch really just enables the global C<$^W> variable; normally,
904 the lexically scoped C<use warnings> pragma is preferred. You
905 can disable or promote into fatal errors specific warnings using
906 C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
907 See also L<perldiag> and L<perltrap>. A fine-grained warning
908 facility is also available if you want to manipulate entire classes
909 of warnings; see L<warnings> or L<perllexwarn>.
914 Enables all warnings regardless of C<no warnings> or C<$^W>.
920 Disables all warnings regardless of C<use warnings> or C<$^W>.
926 =item B<-x>I<directory>
928 tells Perl that the program is embedded in a larger chunk of unrelated
929 text, such as in a mail message. Leading garbage will be
930 discarded until the first line that starts with C<#!> and contains the
931 string "perl". Any meaningful switches on that line will be applied.
933 All references to line numbers by the program (warnings, errors, ...)
934 will treat the C<#!> line as the first line.
935 Thus a warning on the 2nd line of the program, which is on the 100th
936 line in the file will be reported as line 2, not as line 100.
937 This can be overridden by using the C<#line> directive.
938 (See L<perlsyn/"Plain Old Comments (Not!)">)
940 If a directory name is specified, Perl will switch to that directory
941 before running the program. The B<-x> switch controls only the
942 disposal of leading garbage. The program must be terminated with
943 C<__END__> if there is trailing garbage to be ignored; the program
944 can process any or all of the trailing garbage via the C<DATA> filehandle
947 The directory, if specified, must appear immediately following the B<-x>
948 with no intervening whitespace.
953 X<perl, environment variables>
960 Used if C<chdir> has no argument.
965 Used if C<chdir> has no argument and HOME is not set.
970 Used in executing subprocesses, and in finding the program if B<-S> is
976 A list of directories in which to look for Perl library
977 files before looking in the standard library and the current
978 directory. Any architecture-specific directories under the specified
979 locations are automatically included if they exist, with this lookup
980 done at interpreter startup time.
982 If PERL5LIB is not defined, PERLLIB is used. Directories are separated
983 (like in PATH) by a colon on Unixish platforms and by a semicolon on
984 Windows (the proper path separator being given by the command C<perl
987 When running taint checks, either because the program was running setuid or
988 setgid, or the B<-T> or B<-t> switch was specified, neither PERL5LIB nor
989 PERLLIB is consulted. The program should instead say:
991 use lib "/my/directory";
996 Command-line options (switches). Switches in this variable are treated
997 as if they were on every Perl command line. Only the B<-[CDIMUdmtwW]>
998 switches are allowed. When running taint checks (either because the
999 program was running setuid or setgid, or because the B<-T> or B<-t>
1000 switch was used), this variable is ignored. If PERL5OPT begins with
1001 B<- T>, tainting will be enabled and subsequent options ignored. If
1002 PERL5OPT begins with B<-t>, tainting will be enabled, a writable dot
1003 removed from @INC, and subsequent options honored.
1008 A space (or colon) separated list of PerlIO layers. If perl is built
1009 to use PerlIO system for IO (the default) these layers affect Perl's IO.
1011 It is conventional to start layer names with a colon (for example, C<:perlio>) to
1012 emphasize their similarity to variable "attributes". But the code that parses
1013 layer specification strings, which is also used to decode the PERLIO
1014 environment variable, treats the colon as a separator.
1016 An unset or empty PERLIO is equivalent to the default set of layers for
1017 your platform; for example, C<:unix:perlio> on Unix-like systems
1018 and C<:unix:crlf> on Windows and other DOS-like systems.
1020 The list becomes the default for I<all> Perl's IO. Consequently only built-in
1021 layers can appear in this list, as external layers (such as C<:encoding()>) need
1022 IO in order to load them!. See L<"open pragma"|open> for how to add external
1023 encodings as defaults.
1025 Layers it makes sense to include in the PERLIO environment
1026 variable are briefly summarized below. For more details see L<PerlIO>.
1033 A pseudolayer that turns the C<:utf8> flag I<off> for the layer below;
1034 unlikely to be useful on its own in the global PERLIO environment variable.
1035 You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
1040 A layer which does CRLF to C<"\n"> translation distinguishing "text" and
1041 "binary" files in the manner of MS-DOS and similar operating systems.
1042 (It currently does I<not> mimic MS-DOS as far as treating of Control-Z
1043 as being an end-of-file marker.)
1048 A layer that implements "reading" of files by using I<mmap>(2) to
1049 make an entire file appear in the process's address space, and then
1050 using that as PerlIO's "buffer".
1055 This is a re-implementation of stdio-like buffering written as a
1056 PerlIO layer. As such it will call whatever layer is below it for
1057 its operations, typically C<:unix>.
1062 An experimental pseudolayer that removes the topmost layer.
1063 Use with the same care as is reserved for nitroglycerine.
1068 A pseudolayer that manipulates other layers. Applying the C<:raw>
1069 layer is equivalent to calling C<binmode($fh)>. It makes the stream
1070 pass each byte as-is without translation. In particular, both CRLF
1071 translation and intuiting C<:utf8> from the locale are disabled.
1073 Unlike in earlier versions of Perl, C<:raw> is I<not>
1074 just the inverse of C<:crlf>: other layers which would affect the
1075 binary nature of the stream are also removed or disabled.
1080 This layer provides a PerlIO interface by wrapping system's ANSI C "stdio"
1081 library calls. The layer provides both buffering and IO.
1082 Note that the C<:stdio> layer does I<not> do CRLF translation even if that
1083 is the platform's normal behaviour. You will need a C<:crlf> layer above it
1089 Low-level layer that calls C<read>, C<write>, C<lseek>, etc.
1094 A pseudolayer that enables a flag in the layer below to tell Perl
1095 that output should be in utf8 and that input should be regarded as
1096 already in valid utf8 form. B<WARNING: It does not check for validity and as such
1097 should be handled with extreme caution for input, because security violations
1098 can occur with non-shortest UTF-8 encodings, etc.> Generally C<:encoding(utf8)> is
1099 the best option when reading UTF-8 encoded data.
1104 On Win32 platforms this I<experimental> layer uses native "handle" IO
1105 rather than a Unix-like numeric file descriptor layer. Known to be
1106 buggy in this release (5.14).
1110 The default set of layers should give acceptable results on all platforms
1112 For Unix platforms that will be the equivalent of "unix perlio" or "stdio".
1113 Configure is set up to prefer the "stdio" implementation if the system's library
1114 provides for fast access to the buffer; otherwise, it uses the "unix perlio"
1117 On Win32 the default in this release (5.14) is "unix crlf". Win32's "stdio"
1118 has a number of bugs/mis-features for Perl IO which are somewhat depending
1119 on the version and vendor of the C compiler. Using our own C<crlf> layer as
1120 the buffer avoids those issues and makes things more uniform. The C<crlf>
1121 layer provides CRLF conversion as well as buffering.
1123 This release (5.14) uses C<unix> as the bottom layer on Win32, and so still
1124 uses the C compiler's numeric file descriptor routines. There is an
1125 experimental native C<win32> layer, which is expected to be enhanced and
1126 should eventually become the default under Win32.
1128 The PERLIO environment variable is completely ignored when Perl
1129 is run in taint mode.
1134 If set to the name of a file or device, certain operations of PerlIO
1135 subsystem will be logged to that file, which is opened in append mode
1136 Typical uses are in Unix:
1138 % env PERLIO_DEBUG=/dev/tty perl script ...
1140 and under Win32, the approximately equivalent:
1142 > set PERLIO_DEBUG=CON
1145 This functionality is disabled for setuid scripts and for scripts run
1151 A list of directories in which to look for Perl library
1152 files before looking in the standard library and the current directory.
1153 If PERL5LIB is defined, PERLLIB is not used.
1155 The PERLLIB environment variable is completely ignored when Perl
1156 is run in taint mode.
1161 The command used to load the debugger code. The default is:
1163 BEGIN { require "perl5db.pl" }
1165 The PERL5DB environment variable is only used when Perl is started with
1166 a bare B<-d> switch.
1168 =item PERL5DB_THREADED
1171 If set to a true value, indicates to the debugger that the code being
1172 debugged uses threads.
1174 =item PERL5SHELL (specific to the Win32 port)
1177 On Win32 ports only, may be set to an alternative shell that Perl must use
1178 internally for executing "backtick" commands or system(). Default is
1179 C<cmd.exe /x/d/c> on WindowsNT and C<command.com /c> on Windows95. The
1180 value is considered space-separated. Precede any character that
1181 needs to be protected, like a space or backslash, with another backslash.
1183 Note that Perl doesn't use COMSPEC for this purpose because
1184 COMSPEC has a high degree of variability among users, leading to
1185 portability concerns. Besides, Perl can use a shell that may not be
1186 fit for interactive use, and setting COMSPEC to such a shell may
1187 interfere with the proper functioning of other programs (which usually
1188 look in COMSPEC to find a shell fit for interactive use).
1190 Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint checked
1191 when running external commands. It is recommended that
1192 you explicitly set (or delete) C<$ENV{PERL5SHELL}> when running
1193 in taint mode under Windows.
1195 =item PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
1196 X<PERL_ALLOW_NON_IFS_LSP>
1198 Set to 1 to allow the use of non-IFS compatible LSPs (Layered Service Providers).
1199 Perl normally searches for an IFS-compatible LSP because this is required
1200 for its emulation of Windows sockets as real filehandles. However, this may
1201 cause problems if you have a firewall such as I<McAfee Guardian>, which requires
1202 that all applications use its LSP but which is not IFS-compatible, because clearly
1203 Perl will normally avoid using such an LSP.
1205 Setting this environment variable to 1 means that Perl will simply use the
1206 first suitable LSP enumerated in the catalog, which keeps I<McAfee Guardian>
1207 happy--and in that particular case Perl still works too because I<McAfee
1208 Guardian>'s LSP actually plays other games which allow applications
1209 requiring IFS compatibility to work.
1211 =item PERL_DEBUG_MSTATS
1212 X<PERL_DEBUG_MSTATS>
1214 Relevant only if Perl is compiled with the C<malloc> included with the Perl
1215 distribution; that is, if C<perl -V:d_mymalloc> is "define".
1217 If set, this dumps out memory statistics after execution. If set
1218 to an integer greater than one, also dumps out memory statistics
1221 =item PERL_DESTRUCT_LEVEL
1222 X<PERL_DESTRUCT_LEVEL>
1224 Relevant only if your Perl executable was built with B<-DDEBUGGING>,
1225 this controls the behaviour of global destruction of objects and other
1226 references. See L<perlhacktips/PERL_DESTRUCT_LEVEL> for more information.
1228 =item PERL_DL_NONLAZY
1231 Set to C<"1"> to have Perl resolve I<all> undefined symbols when it loads
1232 a dynamic library. The default behaviour is to resolve symbols when
1233 they are used. Setting this variable is useful during testing of
1234 extensions, as it ensures that you get an error on misspelled function
1235 names even if the test suite doesn't call them.
1240 If using the C<use encoding> pragma without an explicit encoding name, the
1241 PERL_ENCODING environment variable is consulted for an encoding name.
1243 =item PERL_HASH_SEED
1246 (Since Perl 5.8.1.) Used to randomize Perl's internal hash function.
1247 To emulate the pre-5.8.1 behaviour, set to an integer; C<"0"> means
1248 exactly the same order as in 5.8.0. "Pre-5.8.1" means, among other
1249 things, that hash keys will always have the same ordering between
1250 different runs of Perl.
1252 Most hashes by default return elements in the same order as in Perl 5.8.0.
1253 On a hash by hash basis, if pathological data is detected during a hash
1254 key insertion, then that hash will switch to an alternative random hash
1257 The default behaviour is to randomize unless the PERL_HASH_SEED is set.
1258 If Perl has been compiled with B<-DUSE_HASH_SEED_EXPLICIT>, the default
1259 behaviour is I<not> to randomize unless the PERL_HASH_SEED is set.
1261 If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses
1262 the pseudorandom seed supplied by the operating system and libraries.
1264 B<PLEASE NOTE: The hash seed is sensitive information>. Hashes are
1265 randomized to protect against local and remote attacks against Perl
1266 code. By manually setting a seed, this protection may be partially or
1269 See L<perlsec/"Algorithmic Complexity Attacks"> and
1270 L</PERL_HASH_SEED_DEBUG> for more information.
1272 =item PERL_HASH_SEED_DEBUG
1273 X<PERL_HASH_SEED_DEBUG>
1275 (Since Perl 5.8.1.) Set to C<"1"> to display (to STDERR) the value of
1276 the hash seed at the beginning of execution. This, combined with
1277 L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
1278 behaviour caused by hash randomization.
1280 B<Note that the hash seed is sensitive information>: by knowing it, one
1281 can craft a denial-of-service attack against Perl code, even remotely;
1282 see L<perlsec/"Algorithmic Complexity Attacks"> for more information.
1283 B<Do not disclose the hash seed> to people who don't need to know it.
1284 See also hash_seed() in L<Hash::Util>.
1289 If your Perl was configured with B<-Accflags=-DPERL_MEM_LOG>, setting
1290 the environment variable C<PERL_MEM_LOG> enables logging debug
1291 messages. The value has the form C<< <I<number>>[m][s][t] >>, where
1292 C<I<number>> is the file descriptor number you want to write to (2 is
1293 default), and the combination of letters specifies that you want
1294 information about (m)emory and/or (s)v, optionally with
1295 (t)imestamps. For example, C<PERL_MEM_LOG=1mst> logs all
1296 information to stdout. You can write to other opened file descriptors
1297 in a variety of ways:
1299 $ 3>foo3 PERL_MEM_LOG=3m perl ...
1301 =item PERL_ROOT (specific to the VMS port)
1304 A translation-concealed rooted logical name that contains Perl and the
1305 logical device for the @INC path on VMS only. Other logical names that
1306 affect Perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1307 SYS$TIMEZONE_DIFFERENTIAL, but are optional and discussed further in
1308 L<perlvms> and in F<README.vms> in the Perl source distribution.
1313 Available in Perls 5.8.1 and later. If set to C<"unsafe">, the pre-Perl-5.8.0
1314 signal behaviour (which is immediate but unsafe) is restored. If set
1315 to C<safe>, then safe (but deferred) signals are used. See
1316 L<perlipc/"Deferred Signals (Safe Signals)">.
1321 Equivalent to the B<-C> command-line switch. Note that this is not
1322 a boolean variable. Setting this to C<"1"> is not the right way to
1323 "enable Unicode" (whatever that would mean). You can use C<"0"> to
1324 "disable Unicode", though (or alternatively unset PERL_UNICODE in
1325 your shell before starting Perl). See the description of the B<-C>
1326 switch for more information.
1328 =item SYS$LOGIN (specific to the VMS port)
1331 Used if chdir has no argument and HOME and LOGDIR are not set.
1335 Perl also has environment variables that control how Perl handles data
1336 specific to particular natural languages; see L<perllocale>.
1338 Perl and its various modules and components, including its test frameworks,
1339 may sometimes make use of certain other environment variables. Some of
1340 these are specific to a particular platform. Please consult the
1341 appropriate module documentation and any documentation for your platform
1342 (like L<perlsolaris>, L<perllinux>, L<perlmacosx>, L<perlwin32>, etc) for
1343 variables peculiar to those specific situations.
1345 Perl makes all environment variables available to the program being
1346 executed, and passes these along to any child processes it starts.
1347 However, programs running setuid would do well to execute the following
1348 lines before doing anything else, just to keep people honest:
1350 $ENV{PATH} = "/bin:/usr/bin"; # or whatever you need
1351 $ENV{SHELL} = "/bin/sh" if exists $ENV{SHELL};
1352 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};