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, perl5.14.1, you should place
94 that directly in the C<#!> line's path.
96 If the C<#!> line does not contain the word "perl" nor the word "indir"
97 the program named after the C<#!> is executed instead of the Perl
98 interpreter. This is slightly bizarre, but it helps people on machines
99 that don't do C<#!>, because they can tell a program that their SHELL is
100 F</usr/bin/perl>, and Perl will then dispatch the program to the correct
101 interpreter for them.
103 After locating your program, Perl compiles the entire program to an
104 internal form. If there are any compilation errors, execution of the
105 program is not attempted. (This is unlike the typical shell script,
106 which might run part-way through before finding a syntax error.)
108 If the program is syntactically correct, it is executed. If the program
109 runs off the end without hitting an exit() or die() operator, an implicit
110 C<exit(0)> is provided to indicate successful completion.
112 =head2 #! and quoting on non-Unix systems
115 Unix's C<#!> technique can be simulated on other systems:
123 extproc perl -S -your_switches
125 as the first line in C<*.cmd> file (B<-S> due to a bug in cmd.exe's
130 Create a batch file to run your program, and codify it in
131 C<ALTERNATE_SHEBANG> (see the F<dosish.h> file in the source
132 distribution for more information).
136 The Win95/NT installation, when using the ActiveState installer for Perl,
137 will modify the Registry to associate the F<.pl> extension with the perl
138 interpreter. If you install Perl by other means (including building from
139 the sources), you may have to modify the Registry yourself. Note that
140 this means you can no longer tell the difference between an executable
141 Perl program and a Perl library file.
147 $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
148 $ exit++ + ++$status != 0 and $exit = $status = undef;
150 at the top of your program, where B<-mysw> are any command line switches you
151 want to pass to Perl. You can now invoke the program directly, by saying
152 C<perl program>, or as a DCL procedure, by saying C<@program> (or implicitly
153 via F<DCL$PATH> by just using the name of the program).
155 This incantation is a bit much to remember, but Perl will display it for
156 you if you say C<perl "-V:startperl">.
160 Command-interpreters on non-Unix systems have rather different ideas
161 on quoting than Unix shells. You'll need to learn the special
162 characters in your command-interpreter (C<*>, C<\> and C<"> are
163 common) and how to protect whitespace and these characters to run
164 one-liners (see L<-e|/-e commandline> below).
166 On some systems, you may have to change single-quotes to double ones,
167 which you must I<not> do on Unix or Plan 9 systems. You might also
168 have to change a single % to a %%.
173 perl -e 'print "Hello world\n"'
176 perl -e "print \"Hello world\n\""
179 perl -e "print ""Hello world\n"""
181 The problem is that none of this is reliable: it depends on the
182 command and it is entirely possible neither works. If I<4DOS> were
183 the command shell, this would probably work better:
185 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
187 B<CMD.EXE> in Windows NT slipped a lot of standard Unix functionality in
188 when nobody was looking, but just try to find documentation for its
191 There is no general solution to all of this. It's just a mess.
193 =head2 Location of Perl
194 X<perl, location of interpreter>
196 It may seem obvious to say, but Perl is useful only when users can
197 easily find it. When possible, it's good for both F</usr/bin/perl>
198 and F</usr/local/bin/perl> to be symlinks to the actual binary. If
199 that can't be done, system administrators are strongly encouraged
200 to put (symlinks to) perl and its accompanying utilities into a
201 directory typically found along a user's PATH, or in some other
202 obvious and convenient place.
204 In this documentation, C<#!/usr/bin/perl> on the first line of the program
205 will stand in for whatever method works on your system. You are
206 advised to use a specific path if you care about a specific version.
208 #!/usr/local/bin/perl5.14
210 or if you just want to be running at least version, place a statement
211 like this at the top of your program:
215 =head2 Command Switches
216 X<perl, command switches> X<command switches>
218 As with all standard commands, a single-character switch may be
219 clustered with the following switch, if any.
221 #!/usr/bin/perl -spi.orig # same as -s -p -i.orig
223 A C<--> signals the end of options and disables further option processing. Any
224 arguments after the C<--> are treated as filenames and arguments.
230 =item B<-0>[I<octal/hexadecimal>]
233 specifies the input record separator (C<$/>) as an octal or
234 hexadecimal number. If there are no digits, the null character is the
235 separator. Other switches may precede or follow the digits. For
236 example, if you have a version of I<find> which can print filenames
237 terminated by the null character, you can say this:
239 find . -name '*.orig' -print0 | perl -n0e unlink
241 The special value 00 will cause Perl to slurp files in paragraph mode.
242 Any value 0400 or above will cause Perl to slurp files whole, but by convention
243 the value 0777 is the one normally used for this purpose.
245 You can also specify the separator character using hexadecimal notation:
246 B<-0xI<HHH...>>, where the C<I<H>> are valid hexadecimal digits. Unlike
247 the octal form, this one may be used to specify any Unicode character, even
248 those beyond 0xFF. So if you I<really> want a record separator of 0777,
249 specify it as B<-0x1FF>. (This means that you cannot use the B<-x> option
250 with a directory name that consists of hexadecimal digits, or else Perl
251 will think you have specified a hex number to B<-0>.)
256 turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
257 split command to the @F array is done as the first thing inside the
258 implicit while loop produced by the B<-n> or B<-p>.
260 perl -ane 'print pop(@F), "\n";'
269 An alternate delimiter may be specified using B<-F>.
271 B<-a> implicitly sets B<-n>.
273 =item B<-C [I<number/list>]>
276 The B<-C> flag controls some of the Perl Unicode features.
278 As of 5.8.1, the B<-C> can be followed either by a number or a list
279 of option letters. The letters, their numeric values, and effects
280 are as follows; listing the letters is equal to summing the numbers.
282 I 1 STDIN is assumed to be in UTF-8
283 O 2 STDOUT will be in UTF-8
284 E 4 STDERR will be in UTF-8
286 i 8 UTF-8 is the default PerlIO layer for input streams
287 o 16 UTF-8 is the default PerlIO layer for output streams
289 A 32 the @ARGV elements are expected to be strings encoded
291 L 64 normally the "IOEioA" are unconditional, the L makes
292 them conditional on the locale environment variables
293 (the LC_ALL, LC_TYPE, and LANG, in the order of
294 decreasing precedence) -- if the variables indicate
295 UTF-8, then the selected "IOEioA" are in effect
296 a 256 Set ${^UTF8CACHE} to -1, to run the UTF-8 caching
297 code in debugging mode.
299 =for documenting_the_underdocumented
300 perl.h gives W/128 as PERL_UNICODE_WIDESYSCALLS "/* for Sarathy */"
303 perltodo mentions Unicode in %ENV and filenames. I guess that these will be
304 options e and f (or F).
306 For example, B<-COE> and B<-C6> will both turn on UTF-8-ness on both
307 STDOUT and STDERR. Repeating letters is just redundant, not cumulative
310 The C<io> options mean that any subsequent open() (or similar I/O
311 operations) in the current file scope will have the C<:utf8> PerlIO layer
312 implicitly applied to them, in other words, UTF-8 is expected from any
313 input stream, and UTF-8 is produced to any output stream. This is just
314 the default, with explicit layers in open() and with binmode() one can
315 manipulate streams as usual.
317 B<-C> on its own (not followed by any number or option list), or the
318 empty string C<""> for the C<PERL_UNICODE> environment variable, has the
319 same effect as B<-CSDL>. In other words, the standard I/O handles and
320 the default C<open()> layer are UTF-8-fied I<but> only if the locale
321 environment variables indicate a UTF-8 locale. This behaviour follows
322 the I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
323 (See L<perl581delta/UTF-8 no longer default under UTF-8 locales>.)
325 You can use B<-C0> (or C<"0"> for C<PERL_UNICODE>) to explicitly
326 disable all the above Unicode features.
328 The read-only magic variable C<${^UNICODE}> reflects the numeric value
329 of this setting. This variable is set during Perl startup and is
330 thereafter read-only. If you want runtime effects, use the three-arg
331 open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
332 and the C<open> pragma (see L<open>).
334 (In Perls earlier than 5.8.1 the B<-C> switch was a Win32-only switch
335 that enabled the use of Unicode-aware "wide system call" Win32 APIs.
336 This feature was practically unused, however, and the command line
337 switch was therefore "recycled".)
339 B<Note:> Since perl 5.10.1, if the B<-C> option is used on the C<#!> line,
340 it must be specified on the command line as well, since the standard streams
341 are already set up at this point in the execution of the perl interpreter.
342 You can also use binmode() to set the encoding of an I/O stream.
347 causes Perl to check the syntax of the program and then exit without
348 executing it. Actually, it I<will> execute and C<BEGIN>, C<UNITCHECK>,
349 or C<CHECK> blocks and any C<use> statements: these are considered as
350 occurring outside the execution of your program. C<INIT> and C<END>
351 blocks, however, will be skipped.
358 runs the program under the Perl debugger. See L<perldebug>.
359 If B<t> is specified, it indicates to the debugger that threads
360 will be used in the code being debugged.
362 =item B<-d:>I<MOD[=bar,baz]>
365 =item B<-dt:>I<MOD[=bar,baz]>
367 runs the program under the control of a debugging, profiling, or tracing
368 module installed as C<Devel::I<MOD>>. E.g., B<-d:DProf> executes the
369 program using the C<Devel::DProf> profiler. As with the B<-M> flag, options
370 may be passed to the C<Devel::I<MOD>> package where they will be received
371 and interpreted by the C<Devel::I<MOD>::import> routine. Again, like B<-M>,
372 use -B<-d:-I<MOD>> to call C<Devel::I<MOD>::unimport> instead of import. The
373 comma-separated list of options must follow a C<=> character. If B<t> is
374 specified, it indicates to the debugger that threads will be used in the
375 code being debugged. See L<perldebug>.
377 =item B<-D>I<letters>
378 X<-D> X<DEBUGGING> X<-DDEBUGGING>
382 sets debugging flags. To watch how it executes your program, use
383 B<-Dtls>. (This works only if debugging is compiled into your
384 Perl.) Another nice value is B<-Dx>, which lists your compiled
385 syntax tree. And B<-Dr> displays compiled regular expressions;
386 the format of the output is explained in L<perldebguts>.
388 As an alternative, specify a number instead of list of letters (e.g.,
389 B<-D14> is equivalent to B<-Dtls>):
391 1 p Tokenizing and parsing (with v, displays parse stack)
392 2 s Stack snapshots (with v, displays all stacks)
393 4 l Context (loop) stack processing
395 16 o Method and overloading resolution
396 32 c String/numeric conversions
397 64 P Print profiling info, source file input state
398 128 m Memory and SV allocation
399 256 f Format processing
400 512 r Regular expression parsing and execution
401 1024 x Syntax tree dump
402 2048 u Tainting checks
403 4096 U Unofficial, User hacking (reserved for private,
405 8192 H Hash dump -- usurps values()
406 16384 X Scratchpad allocation
408 65536 S Op slab allocation
410 262144 R Include reference counts of dumped variables (eg when
412 524288 J show s,t,P-debug (don't Jump over) on opcodes within
414 1048576 v Verbose: use in conjunction with other flags
415 2097152 C Copy On Write
416 4194304 A Consistency checks on internal structures
417 8388608 q quiet - currently only suppresses the "EXECUTING"
419 16777216 M trace smart match resolution
420 33554432 B dump suBroutine definitions, including special Blocks
423 All these flags require B<-DDEBUGGING> when you compile the Perl
424 executable (but see C<:opd> in L<Devel::Peek> or L<re/'debug' mode>
425 which may change this).
426 See the F<INSTALL> file in the Perl source distribution
427 for how to do this. This flag is automatically set if you include B<-g>
428 option when C<Configure> asks you about optimizer/debugger flags.
430 If you're just trying to get a print out of each line of Perl code
431 as it executes, the way that C<sh -x> provides for shell scripts,
432 you can't use Perl's B<-D> switch. Instead do this
434 # If you have "env" utility
435 env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
437 # Bourne shell syntax
438 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
441 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
443 See L<perldebug> for details and variations.
445 =item B<-e> I<commandline>
448 may be used to enter one line of program. If B<-e> is given, Perl
449 will not look for a filename in the argument list. Multiple B<-e>
450 commands may be given to build up a multi-line script. Make sure
451 to use semicolons where you would in a normal program.
453 =item B<-E> I<commandline>
456 behaves just like B<-e>, except that it implicitly enables all
457 optional features (in the main compilation unit). See L<feature>.
460 X<-f> X<sitecustomize> X<sitecustomize.pl>
462 Disable executing F<$Config{sitelib}/sitecustomize.pl> at startup.
464 Perl can be built so that it by default will try to execute
465 F<$Config{sitelib}/sitecustomize.pl> at startup (in a BEGIN block).
466 This is a hook that allows the sysadmin to customize how Perl behaves.
467 It can for instance be used to add entries to the @INC array to make Perl
468 find modules in non-standard locations.
470 Perl actually inserts the following code:
473 do { local $!; -f "$Config{sitelib}/sitecustomize.pl"; }
474 && do "$Config{sitelib}/sitecustomize.pl";
477 Since it is an actual C<do> (not a C<require>), F<sitecustomize.pl>
478 doesn't need to return a true value. The code is run in package C<main>,
479 in its own lexical scope. However, if the script dies, C<$@> will not
482 The value of C<$Config{sitelib}> is also determined in C code and not
483 read from C<Config.pm>, which is not loaded.
485 The code is executed I<very> early. For example, any changes made to
486 C<@INC> will show up in the output of `perl -V`. Of course, C<END>
487 blocks will be likewise executed very late.
489 To determine at runtime if this capability has been compiled in your
490 perl, you can check the value of C<$Config{usesitecustomize}>.
492 =item B<-F>I<pattern>
495 specifies the pattern to split on for B<-a>. The pattern may be
496 surrounded by C<//>, C<"">, or C<''>, otherwise it will be put in single
497 quotes. You can't use literal whitespace in the pattern.
499 B<-F> implicitly sets both B<-a> and B<-n>.
504 prints a summary of the options.
506 =item B<-i>[I<extension>]
509 specifies that files processed by the C<E<lt>E<gt>> construct are to be
510 edited in-place. It does this by renaming the input file, opening the
511 output file by the original name, and selecting that output file as the
512 default for print() statements. The extension, if supplied, is used to
513 modify the name of the old file to make a backup copy, following these
516 If no extension is supplied, and your system supports it, the original
517 I<file> is kept open without a name while the output is redirected to
518 a new file with the original I<filename>. When perl exits, cleanly or not,
519 the original I<file> is unlinked.
521 If the extension doesn't contain a C<*>, then it is appended to the
522 end of the current filename as a suffix. If the extension does
523 contain one or more C<*> characters, then each C<*> is replaced
524 with the current filename. In Perl terms, you could think of this
527 ($backup = $extension) =~ s/\*/$file_name/g;
529 This allows you to add a prefix to the backup file, instead of (or in
530 addition to) a suffix:
532 $ perl -pi'orig_*' -e 's/bar/baz/' fileA # backup to
535 Or even to place backup copies of the original files into another
536 directory (provided the directory already exists):
538 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to
541 These sets of one-liners are equivalent:
543 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
544 $ perl -pi'*' -e 's/bar/baz/' fileA # overwrite current file
546 $ perl -pi'.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
547 $ perl -pi'*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
549 From the shell, saying
551 $ perl -p -i.orig -e "s/foo/bar/; ... "
553 is the same as using the program:
555 #!/usr/bin/perl -pi.orig
558 which is equivalent to
561 $extension = '.orig';
563 if ($ARGV ne $oldargv) {
564 if ($extension !~ /\*/) {
565 $backup = $ARGV . $extension;
568 ($backup = $extension) =~ s/\*/$ARGV/g;
570 rename($ARGV, $backup);
571 open(ARGVOUT, ">$ARGV");
578 print; # this prints to original filename
582 except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
583 know when the filename has changed. It does, however, use ARGVOUT for
584 the selected filehandle. Note that STDOUT is restored as the default
585 output filehandle after the loop.
587 As shown above, Perl creates the backup file whether or not any output
588 is actually changed. So this is just a fancy way to copy files:
590 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
592 $ perl -p -i'.orig' -e 1 file1 file2 file3...
594 You can use C<eof> without parentheses to locate the end of each input
595 file, in case you want to append to each file, or reset line numbering
596 (see example in L<perlfunc/eof>).
598 If, for a given file, Perl is unable to create the backup file as
599 specified in the extension then it will skip that file and continue on
600 with the next one (if it exists).
602 For a discussion of issues surrounding file permissions and B<-i>, see
603 L<perlfaq5/Why does Perl let me delete read-only files? Why does -i clobber
604 protected files? Isn't this a bug in Perl?>.
606 You cannot use B<-i> to create directories or to strip extensions from
609 Perl does not expand C<~> in filenames, which is good, since some
610 folks use it for their backup files:
612 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
614 Note that because B<-i> renames or deletes the original file before
615 creating a new file of the same name, Unix-style soft and hard links will
618 Finally, the B<-i> switch does not impede execution when no
619 files are given on the command line. In this case, no backup is made
620 (the original file cannot, of course, be determined) and processing
621 proceeds from STDIN to STDOUT as might be expected.
623 =item B<-I>I<directory>
626 Directories specified by B<-I> are prepended to the search path for
629 =item B<-l>[I<octnum>]
632 enables automatic line-ending processing. It has two separate
633 effects. First, it automatically chomps C<$/> (the input record
634 separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
635 (the output record separator) to have the value of I<octnum> so
636 that any print statements will have that separator added back on.
637 If I<octnum> is omitted, sets C<$\> to the current value of
638 C<$/>. For instance, to trim lines to 80 columns:
640 perl -lpe 'substr($_, 80) = ""'
642 Note that the assignment C<$\ = $/> is done when the switch is processed,
643 so the input record separator can be different than the output record
644 separator if the B<-l> switch is followed by a B<-0> switch:
646 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
648 This sets C<$\> to newline and then sets C<$/> to the null character.
650 =item B<-m>[B<->]I<module>
653 =item B<-M>[B<->]I<module>
655 =item B<-M>[B<->]I<'module ...'>
657 =item B<-[mM]>[B<->]I<module=arg[,arg]...>
659 B<-m>I<module> executes C<use> I<module> C<();> before executing your
662 B<-M>I<module> executes C<use> I<module> C<;> before executing your
663 program. You can use quotes to add extra code after the module name,
664 e.g., C<'-MI<MODULE> qw(foo bar)'>.
666 If the first character after the B<-M> or B<-m> is a dash (B<->)
667 then the 'use' is replaced with 'no'.
669 A little builtin syntactic sugar means you can also say
670 B<-mI<MODULE>=foo,bar> or B<-MI<MODULE>=foo,bar> as a shortcut for
671 B<'-MI<MODULE> qw(foo bar)'>. This avoids the need to use quotes when
672 importing symbols. The actual code generated by B<-MI<MODULE>=foo,bar> is
673 C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
674 removes the distinction between B<-m> and B<-M>; that is,
675 B<-mI<MODULE>=foo,bar> is the same as B<-MI<MODULE>=foo,bar>.
677 A consequence of this is that B<-MI<MODULE>=number> never does a version check,
678 unless C<I<MODULE>::import()> itself is set up to do a version check, which
679 could happen for example if I<MODULE> inherits from L<Exporter>.
684 causes Perl to assume the following loop around your program, which
685 makes it iterate over filename arguments somewhat like I<sed -n> or
690 ... # your program goes here
693 Note that the lines are not printed by default. See L</-p> to have
694 lines printed. If a file named by an argument cannot be opened for
695 some reason, Perl warns you about it and moves on to the next file.
697 Also note that C<< <> >> passes command line arguments to
698 L<perlfunc/open>, which doesn't necessarily interpret them as file names.
699 See L<perlop> for possible security implications.
701 Here is an efficient way to delete all files that haven't been modified for
704 find . -mtime +7 -print | perl -nle unlink
706 This is faster than using the B<-exec> switch of I<find> because you don't
707 have to start a process on every filename found. It does suffer from
708 the bug of mishandling newlines in pathnames, which you can fix if
709 you follow the example under B<-0>.
711 C<BEGIN> and C<END> blocks may be used to capture control before or after
712 the implicit program loop, just as in I<awk>.
717 causes Perl to assume the following loop around your program, which
718 makes it iterate over filename arguments somewhat like I<sed>:
723 ... # your program goes here
725 print or die "-p destination: $!\n";
728 If a file named by an argument cannot be opened for some reason, Perl
729 warns you about it, and moves on to the next file. Note that the
730 lines are printed automatically. An error occurring during printing is
731 treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
732 overrides a B<-n> switch.
734 C<BEGIN> and C<END> blocks may be used to capture control before or after
735 the implicit loop, just as in I<awk>.
740 enables rudimentary switch parsing for switches on the command
741 line after the program name but before any filename arguments (or before
742 an argument of B<-->). Any switch found there is removed from @ARGV and sets the
743 corresponding variable in the Perl program. The following program
744 prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
745 if it is invoked with B<-xyz=abc>.
748 if ($xyz) { print "$xyz\n" }
750 Do note that a switch like B<--help> creates the variable C<${-help}>, which is
751 not compliant with C<use strict "refs">. Also, when using this option on a
752 script with warnings enabled you may get a lot of spurious "used only once"
758 makes Perl use the PATH environment variable to search for the
759 program unless the name of the program contains path separators.
761 On some platforms, this also makes Perl append suffixes to the
762 filename while searching for it. For example, on Win32 platforms,
763 the ".bat" and ".cmd" suffixes are appended if a lookup for the
764 original name fails, and if the name does not already end in one
765 of those suffixes. If your Perl was compiled with C<DEBUGGING> turned
766 on, using the B<-Dp> switch to Perl shows how the search progresses.
768 Typically this is used to emulate C<#!> startup on platforms that don't
769 support C<#!>. It's also convenient when debugging a script that uses C<#!>,
770 and is thus normally found by the shell's $PATH search mechanism.
772 This example works on many platforms that have a shell compatible with
776 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
777 if $running_under_some_shell;
779 The system ignores the first line and feeds the program to F</bin/sh>,
780 which proceeds to try to execute the Perl program as a shell script.
781 The shell executes the second line as a normal shell command, and thus
782 starts up the Perl interpreter. On some systems $0 doesn't always
783 contain the full pathname, so the B<-S> tells Perl to search for the
784 program if necessary. After Perl locates the program, it parses the
785 lines and ignores them because the variable $running_under_some_shell
786 is never true. If the program will be interpreted by csh, you will need
787 to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
788 embedded spaces (and such) in the argument list. To start up I<sh> rather
789 than I<csh>, some systems may have to replace the C<#!> line with a line
790 containing just a colon, which will be politely ignored by Perl. Other
791 systems can't control that, and need a totally devious construct that
792 will work under any of I<csh>, I<sh>, or Perl, such as the following:
794 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
795 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
796 if $running_under_some_shell;
798 If the filename supplied contains directory separators (and so is an
799 absolute or relative pathname), and if that file is not found,
800 platforms that append file extensions will do so and try to look
801 for the file with those extensions added, one by one.
803 On DOS-like platforms, if the program does not contain directory
804 separators, it will first be searched for in the current directory
805 before being searched for on the PATH. On Unix platforms, the
806 program will be searched for strictly on the PATH.
811 Like B<-T>, but taint checks will issue warnings rather than fatal
812 errors. These warnings can now be controlled normally with C<no warnings
815 B<Note: This is not a substitute for C<-T>!> This is meant to be
816 used I<only> as a temporary development aid while securing legacy code:
817 for real production code and for new secure code written from scratch,
818 always use the real B<-T>.
823 turns on "taint" so you can test them. Ordinarily
824 these checks are done only when running setuid or setgid. It's a
825 good idea to turn them on explicitly for programs that run on behalf
826 of someone else whom you might not necessarily trust, such as CGI
827 programs or any internet servers you might write in Perl. See
828 L<perlsec> for details. For security reasons, this option must be
829 seen by Perl quite early; usually this means it must appear early
830 on the command line or in the C<#!> line for systems which support
836 This switch causes Perl to dump core after compiling your
837 program. You can then in theory take this core dump and turn it
838 into an executable file by using the I<undump> program (not supplied).
839 This speeds startup at the expense of some disk space (which you
840 can minimize by stripping the executable). (Still, a "hello world"
841 executable comes out to about 200K on my machine.) If you want to
842 execute a portion of your program before dumping, use the dump()
843 operator instead. Note: availability of I<undump> is platform
844 specific and may not be available for a specific port of Perl.
849 allows Perl to do unsafe operations. Currently the only "unsafe"
850 operations are attempting to unlink directories while running as superuser
851 and running setuid programs with fatal taint checks turned into warnings.
852 Note that warnings must be enabled along with this option to actually
853 I<generate> the taint-check warnings.
858 prints the version and patchlevel of your perl executable.
863 prints summary of the major perl configuration values and the current
866 =item B<-V:>I<configvar>
868 Prints to STDOUT the value of the named configuration variable(s),
869 with multiples when your C<I<configvar>> argument looks like a regex (has
870 non-letters). For example:
873 libc='/lib/libc-2.2.4.so';
875 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
876 libc='/lib/libc-2.2.4.so';
878 libpth='/usr/local/lib /lib /usr/lib';
879 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
881 libc='/lib/libc-2.2.4.so';
885 Additionally, extra colons can be used to control formatting. A
886 trailing colon suppresses the linefeed and terminator ";", allowing
887 you to embed queries into shell commands. (mnemonic: PATH separator
890 $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
891 compression-vars: zcat='' zip='zip' are here !
893 A leading colon removes the "name=" part of the response, this allows
894 you to map to the name you need. (mnemonic: empty label)
896 $ echo "goodvfork="`./perl -Ilib -V::usevfork`
899 Leading and trailing colons can be used together if you need
900 positional parameter values without the names. Note that in the case
901 below, the C<PERL_API> params are returned in alphabetical order.
903 $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
904 building_on 'linux' '5' '1' '9' now
909 prints warnings about dubious constructs, such as variable names
910 mentioned only once and scalar variables used
911 before being set; redefined subroutines; references to undefined
912 filehandles; filehandles opened read-only that you are attempting
913 to write on; values used as a number that don't I<look> like numbers;
914 using an array as though it were a scalar; if your subroutines
915 recurse more than 100 deep; and innumerable other things.
917 This switch really just enables the global C<$^W> variable; normally,
918 the lexically scoped C<use warnings> pragma is preferred. You
919 can disable or promote into fatal errors specific warnings using
920 C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
921 See also L<perldiag> and L<perltrap>. A fine-grained warning
922 facility is also available if you want to manipulate entire classes
923 of warnings; see L<warnings>.
928 Enables all warnings regardless of C<no warnings> or C<$^W>.
934 Disables all warnings regardless of C<use warnings> or C<$^W>.
940 =item B<-x>I<directory>
942 tells Perl that the program is embedded in a larger chunk of unrelated
943 text, such as in a mail message. Leading garbage will be
944 discarded until the first line that starts with C<#!> and contains the
945 string "perl". Any meaningful switches on that line will be applied.
947 All references to line numbers by the program (warnings, errors, ...)
948 will treat the C<#!> line as the first line.
949 Thus a warning on the 2nd line of the program, which is on the 100th
950 line in the file will be reported as line 2, not as line 100.
951 This can be overridden by using the C<#line> directive.
952 (See L<perlsyn/"Plain Old Comments (Not!)">)
954 If a directory name is specified, Perl will switch to that directory
955 before running the program. The B<-x> switch controls only the
956 disposal of leading garbage. The program must be terminated with
957 C<__END__> if there is trailing garbage to be ignored; the program
958 can process any or all of the trailing garbage via the C<DATA> filehandle
961 The directory, if specified, must appear immediately following the B<-x>
962 with no intervening whitespace.
967 X<perl, environment variables>
974 Used if C<chdir> has no argument.
979 Used if C<chdir> has no argument and HOME is not set.
984 Used in executing subprocesses, and in finding the program if B<-S> is
990 A list of directories in which to look for Perl library
991 files before looking in the standard library and the current
992 directory. Any architecture-specific and version-specific directories,
993 such as F<version/archname/>, F<version/>, or F<archname/> under the
994 specified locations are automatically included if they exist, with this
995 lookup done at interpreter startup time. In addition, any directories
996 matching the entries in C<$Config{inc_version_list}> are added.
997 (These typically would be for older compatible perl versions installed
998 in the same directory tree.)
1000 If PERL5LIB is not defined, PERLLIB is used. Directories are separated
1001 (like in PATH) by a colon on Unixish platforms and by a semicolon on
1002 Windows (the proper path separator being given by the command C<perl
1005 When running taint checks, either because the program was running setuid or
1006 setgid, or the B<-T> or B<-t> switch was specified, neither PERL5LIB nor
1007 PERLLIB is consulted. The program should instead say:
1009 use lib "/my/directory";
1014 Command-line options (switches). Switches in this variable are treated
1015 as if they were on every Perl command line. Only the B<-[CDIMUdmtwW]>
1016 switches are allowed. When running taint checks (either because the
1017 program was running setuid or setgid, or because the B<-T> or B<-t>
1018 switch was used), this variable is ignored. If PERL5OPT begins with
1019 B<-T>, tainting will be enabled and subsequent options ignored. If
1020 PERL5OPT begins with B<-t>, tainting will be enabled, a writable dot
1021 removed from @INC, and subsequent options honored.
1026 A space (or colon) separated list of PerlIO layers. If perl is built
1027 to use PerlIO system for IO (the default) these layers affect Perl's IO.
1029 It is conventional to start layer names with a colon (for example, C<:perlio>) to
1030 emphasize their similarity to variable "attributes". But the code that parses
1031 layer specification strings, which is also used to decode the PERLIO
1032 environment variable, treats the colon as a separator.
1034 An unset or empty PERLIO is equivalent to the default set of layers for
1035 your platform; for example, C<:unix:perlio> on Unix-like systems
1036 and C<:unix:crlf> on Windows and other DOS-like systems.
1038 The list becomes the default for I<all> Perl's IO. Consequently only built-in
1039 layers can appear in this list, as external layers (such as C<:encoding()>) need
1040 IO in order to load them! See L<"open pragma"|open> for how to add external
1041 encodings as defaults.
1043 Layers it makes sense to include in the PERLIO environment
1044 variable are briefly summarized below. For more details see L<PerlIO>.
1051 A pseudolayer that turns the C<:utf8> flag I<off> for the layer below;
1052 unlikely to be useful on its own in the global PERLIO environment variable.
1053 You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
1058 A layer which does CRLF to C<"\n"> translation distinguishing "text" and
1059 "binary" files in the manner of MS-DOS and similar operating systems.
1060 (It currently does I<not> mimic MS-DOS as far as treating of Control-Z
1061 as being an end-of-file marker.)
1066 A layer that implements "reading" of files by using I<mmap>(2) to
1067 make an entire file appear in the process's address space, and then
1068 using that as PerlIO's "buffer".
1073 This is a re-implementation of stdio-like buffering written as a
1074 PerlIO layer. As such it will call whatever layer is below it for
1075 its operations, typically C<:unix>.
1080 An experimental pseudolayer that removes the topmost layer.
1081 Use with the same care as is reserved for nitroglycerine.
1086 A pseudolayer that manipulates other layers. Applying the C<:raw>
1087 layer is equivalent to calling C<binmode($fh)>. It makes the stream
1088 pass each byte as-is without translation. In particular, both CRLF
1089 translation and intuiting C<:utf8> from the locale are disabled.
1091 Unlike in earlier versions of Perl, C<:raw> is I<not>
1092 just the inverse of C<:crlf>: other layers which would affect the
1093 binary nature of the stream are also removed or disabled.
1098 This layer provides a PerlIO interface by wrapping system's ANSI C "stdio"
1099 library calls. The layer provides both buffering and IO.
1100 Note that the C<:stdio> layer does I<not> do CRLF translation even if that
1101 is the platform's normal behaviour. You will need a C<:crlf> layer above it
1107 Low-level layer that calls C<read>, C<write>, C<lseek>, etc.
1112 A pseudolayer that enables a flag in the layer below to tell Perl
1113 that output should be in utf8 and that input should be regarded as
1114 already in valid utf8 form. B<WARNING: It does not check for validity and as such
1115 should be handled with extreme caution for input, because security violations
1116 can occur with non-shortest UTF-8 encodings, etc.> Generally C<:encoding(utf8)> is
1117 the best option when reading UTF-8 encoded data.
1122 On Win32 platforms this I<experimental> layer uses native "handle" IO
1123 rather than a Unix-like numeric file descriptor layer. Known to be
1124 buggy in this release (5.14).
1128 The default set of layers should give acceptable results on all platforms
1130 For Unix platforms that will be the equivalent of "unix perlio" or "stdio".
1131 Configure is set up to prefer the "stdio" implementation if the system's library
1132 provides for fast access to the buffer; otherwise, it uses the "unix perlio"
1135 On Win32 the default in this release (5.14) is "unix crlf". Win32's "stdio"
1136 has a number of bugs/mis-features for Perl IO which are somewhat depending
1137 on the version and vendor of the C compiler. Using our own C<crlf> layer as
1138 the buffer avoids those issues and makes things more uniform. The C<crlf>
1139 layer provides CRLF conversion as well as buffering.
1141 This release (5.14) uses C<unix> as the bottom layer on Win32, and so still
1142 uses the C compiler's numeric file descriptor routines. There is an
1143 experimental native C<win32> layer, which is expected to be enhanced and
1144 should eventually become the default under Win32.
1146 The PERLIO environment variable is completely ignored when Perl
1147 is run in taint mode.
1152 If set to the name of a file or device, certain operations of PerlIO
1153 subsystem will be logged to that file, which is opened in append mode.
1154 Typical uses are in Unix:
1156 % env PERLIO_DEBUG=/dev/tty perl script ...
1158 and under Win32, the approximately equivalent:
1160 > set PERLIO_DEBUG=CON
1163 This functionality is disabled for setuid scripts and for scripts run
1169 A list of directories in which to look for Perl library
1170 files before looking in the standard library and the current directory.
1171 If PERL5LIB is defined, PERLLIB is not used.
1173 The PERLLIB environment variable is completely ignored when Perl
1174 is run in taint mode.
1179 The command used to load the debugger code. The default is:
1181 BEGIN { require "perl5db.pl" }
1183 The PERL5DB environment variable is only used when Perl is started with
1184 a bare B<-d> switch.
1186 =item PERL5DB_THREADED
1189 If set to a true value, indicates to the debugger that the code being
1190 debugged uses threads.
1192 =item PERL5SHELL (specific to the Win32 port)
1195 On Win32 ports only, may be set to an alternative shell that Perl must use
1196 internally for executing "backtick" commands or system(). Default is
1197 C<cmd.exe /x/d/c> on WindowsNT and C<command.com /c> on Windows95. The
1198 value is considered space-separated. Precede any character that
1199 needs to be protected, like a space or backslash, with another backslash.
1201 Note that Perl doesn't use COMSPEC for this purpose because
1202 COMSPEC has a high degree of variability among users, leading to
1203 portability concerns. Besides, Perl can use a shell that may not be
1204 fit for interactive use, and setting COMSPEC to such a shell may
1205 interfere with the proper functioning of other programs (which usually
1206 look in COMSPEC to find a shell fit for interactive use).
1208 Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint checked
1209 when running external commands. It is recommended that
1210 you explicitly set (or delete) C<$ENV{PERL5SHELL}> when running
1211 in taint mode under Windows.
1213 =item PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
1214 X<PERL_ALLOW_NON_IFS_LSP>
1216 Set to 1 to allow the use of non-IFS compatible LSPs (Layered Service Providers).
1217 Perl normally searches for an IFS-compatible LSP because this is required
1218 for its emulation of Windows sockets as real filehandles. However, this may
1219 cause problems if you have a firewall such as I<McAfee Guardian>, which requires
1220 that all applications use its LSP but which is not IFS-compatible, because clearly
1221 Perl will normally avoid using such an LSP.
1223 Setting this environment variable to 1 means that Perl will simply use the
1224 first suitable LSP enumerated in the catalog, which keeps I<McAfee Guardian>
1225 happy--and in that particular case Perl still works too because I<McAfee
1226 Guardian>'s LSP actually plays other games which allow applications
1227 requiring IFS compatibility to work.
1229 =item PERL_DEBUG_MSTATS
1230 X<PERL_DEBUG_MSTATS>
1232 Relevant only if Perl is compiled with the C<malloc> included with the Perl
1233 distribution; that is, if C<perl -V:d_mymalloc> is "define".
1235 If set, this dumps out memory statistics after execution. If set
1236 to an integer greater than one, also dumps out memory statistics
1239 =item PERL_DESTRUCT_LEVEL
1240 X<PERL_DESTRUCT_LEVEL>
1242 Relevant only if your Perl executable was built with B<-DDEBUGGING>,
1243 this controls the behaviour of global destruction of objects and other
1244 references. See L<perlhacktips/PERL_DESTRUCT_LEVEL> for more information.
1246 =item PERL_DL_NONLAZY
1249 Set to C<"1"> to have Perl resolve I<all> undefined symbols when it loads
1250 a dynamic library. The default behaviour is to resolve symbols when
1251 they are used. Setting this variable is useful during testing of
1252 extensions, as it ensures that you get an error on misspelled function
1253 names even if the test suite doesn't call them.
1258 If using the C<use encoding> pragma without an explicit encoding name, the
1259 PERL_ENCODING environment variable is consulted for an encoding name.
1261 =item PERL_HASH_SEED
1264 (Since Perl 5.8.1, new semantics in Perl 5.18.0) Used to override
1265 the randomization of Perl's internal hash function. The value is expressed
1266 in hexadecimal, and may include a leading 0x. Truncated patterns
1267 are treated as though they are suffixed with sufficient 0's as required.
1269 If the option is provided, and C<PERL_PERTURB_KEYS> is NOT set, then
1270 a value of '0' implies C<PERL_PERTURB_KEYS=0> and any other value
1271 implies C<PERL_PERTURB_KEYS=2>.
1273 B<PLEASE NOTE: The hash seed is sensitive information>. Hashes are
1274 randomized to protect against local and remote attacks against Perl
1275 code. By manually setting a seed, this protection may be partially or
1278 See L<perlsec/"Algorithmic Complexity Attacks">, L</PERL_PERTURB_KEYS>, and
1279 L</PERL_HASH_SEED_DEBUG> for more information.
1281 =item PERL_PERTURB_KEYS
1282 X<PERL_PERTURB_KEYS>
1284 (Since Perl 5.18.0) Set to C<"0"> or C<"NO"> then traversing keys
1285 will be repeatable from run to run for the same PERL_HASH_SEED.
1286 Insertion into a hash will not change the order, except to provide
1287 for more space in the hash. When combined with setting PERL_HASH_SEED
1288 this mode is as close to pre 5.18 behavior as you can get.
1290 When set to C<"1"> or C<"RANDOM"> then traversing keys will be randomized.
1291 Every time a hash is inserted into the key order will change in a random
1292 fashion. The order may not be repeatable in a following program run
1293 even if the PERL_HASH_SEED has been specified. This is the default
1296 When set to C<"2"> or C<"DETERMINISTIC"> then inserting keys into a hash
1297 will cause the key order to change, but in a way that is repeatable
1298 from program run to program run.
1300 B<NOTE:> Use of this option is considered insecure, and is intended only
1301 for debugging non-deterministic behavior in Perl's hash function. Do
1302 not use it in production.
1304 See L<perlsec/"Algorithmic Complexity Attacks"> and L</PERL_HASH_SEED>
1305 and L</PERL_HASH_SEED_DEBUG> for more information. You can get and set the
1306 key traversal mask for a specific hash by using the C<hash_traversal_mask()>
1307 function from L<Hash::Util>.
1309 =item PERL_HASH_SEED_DEBUG
1310 X<PERL_HASH_SEED_DEBUG>
1312 (Since Perl 5.8.1.) Set to C<"1"> to display (to STDERR) information
1313 about the hash function, seed, and what type of key traversal
1314 randomization is in effect at the beginning of execution. This, combined
1315 with L</PERL_HASH_SEED> and L</PERL_PERTURB_KEYS> is intended to aid in
1316 debugging nondeterministic behaviour caused by hash randomization.
1318 B<Note> that any information about the hash function, especially the hash
1319 seed is B<sensitive information>: by knowing it, one can craft a denial-of-service
1320 attack against Perl code, even remotely; see L<perlsec/"Algorithmic Complexity Attacks">
1321 for more information. B<Do not disclose the hash seed> to people who
1322 don't need to know it. See also C<hash_seed()> and
1323 C<key_traversal_mask()> in L<Hash::Util>.
1325 An example output might be:
1327 HASH_FUNCTION = ONE_AT_A_TIME_HARD HASH_SEED = 0x652e9b9349a7a032 PERTURB_KEYS = 1 (RANDOM)
1332 If your Perl was configured with B<-Accflags=-DPERL_MEM_LOG>, setting
1333 the environment variable C<PERL_MEM_LOG> enables logging debug
1334 messages. The value has the form C<< <I<number>>[m][s][t] >>, where
1335 C<I<number>> is the file descriptor number you want to write to (2 is
1336 default), and the combination of letters specifies that you want
1337 information about (m)emory and/or (s)v, optionally with
1338 (t)imestamps. For example, C<PERL_MEM_LOG=1mst> logs all
1339 information to stdout. You can write to other opened file descriptors
1340 in a variety of ways:
1342 $ 3>foo3 PERL_MEM_LOG=3m perl ...
1344 =item PERL_ROOT (specific to the VMS port)
1347 A translation-concealed rooted logical name that contains Perl and the
1348 logical device for the @INC path on VMS only. Other logical names that
1349 affect Perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1350 SYS$TIMEZONE_DIFFERENTIAL, but are optional and discussed further in
1351 L<perlvms> and in F<README.vms> in the Perl source distribution.
1356 Available in Perls 5.8.1 and later. If set to C<"unsafe">, the pre-Perl-5.8.0
1357 signal behaviour (which is immediate but unsafe) is restored. If set
1358 to C<safe>, then safe (but deferred) signals are used. See
1359 L<perlipc/"Deferred Signals (Safe Signals)">.
1364 Equivalent to the B<-C> command-line switch. Note that this is not
1365 a boolean variable. Setting this to C<"1"> is not the right way to
1366 "enable Unicode" (whatever that would mean). You can use C<"0"> to
1367 "disable Unicode", though (or alternatively unset PERL_UNICODE in
1368 your shell before starting Perl). See the description of the B<-C>
1369 switch for more information.
1371 =item SYS$LOGIN (specific to the VMS port)
1374 Used if chdir has no argument and HOME and LOGDIR are not set.
1378 Perl also has environment variables that control how Perl handles data
1379 specific to particular natural languages; see L<perllocale>.
1381 Perl and its various modules and components, including its test frameworks,
1382 may sometimes make use of certain other environment variables. Some of
1383 these are specific to a particular platform. Please consult the
1384 appropriate module documentation and any documentation for your platform
1385 (like L<perlsolaris>, L<perllinux>, L<perlmacosx>, L<perlwin32>, etc) for
1386 variables peculiar to those specific situations.
1388 Perl makes all environment variables available to the program being
1389 executed, and passes these along to any child processes it starts.
1390 However, programs running setuid would do well to execute the following
1391 lines before doing anything else, just to keep people honest:
1393 $ENV{PATH} = "/bin:/usr/bin"; # or whatever you need
1394 $ENV{SHELL} = "/bin/sh" if exists $ENV{SHELL};
1395 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};