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
227 =item B<-0>[I<octal/hexadecimal>]
230 specifies the input record separator (C<$/>) as an octal or
231 hexadecimal number. If there are no digits, the null character is the
232 separator. Other switches may precede or follow the digits. For
233 example, if you have a version of I<find> which can print filenames
234 terminated by the null character, you can say this:
236 find . -name '*.orig' -print0 | perl -n0e unlink
238 The special value 00 will cause Perl to slurp files in paragraph mode.
239 Any value 0400 or above will cause Perl to slurp files whole, but by convention
240 the value 0777 is the one normally used for this purpose.
242 You can also specify the separator character using hexadecimal notation:
243 B<-0xI<HHH...>>, where the C<I<H>> are valid hexadecimal digits. Unlike
244 the octal form, this one may be used to specify any Unicode character, even
245 those beyond 0xFF. So if you I<really> want a record separator of 0777,
246 specify it as B<-0x1FF>. (This means that you cannot use the B<-x> option
247 with a directory name that consists of hexadecimal digits, or else Perl
248 will think you have specified a hex number to B<-0>.)
253 turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
254 split command to the @F array is done as the first thing inside the
255 implicit while loop produced by the B<-n> or B<-p>.
257 perl -ane 'print pop(@F), "\n";'
266 An alternate delimiter may be specified using B<-F>.
268 =item B<-C [I<number/list>]>
271 The B<-C> flag controls some of the Perl Unicode features.
273 As of 5.8.1, the B<-C> can be followed either by a number or a list
274 of option letters. The letters, their numeric values, and effects
275 are as follows; listing the letters is equal to summing the numbers.
277 I 1 STDIN is assumed to be in UTF-8
278 O 2 STDOUT will be in UTF-8
279 E 4 STDERR will be in UTF-8
281 i 8 UTF-8 is the default PerlIO layer for input streams
282 o 16 UTF-8 is the default PerlIO layer for output streams
284 A 32 the @ARGV elements are expected to be strings encoded
286 L 64 normally the "IOEioA" are unconditional, the L makes
287 them conditional on the locale environment variables
288 (the LC_ALL, LC_TYPE, and LANG, in the order of
289 decreasing precedence) -- if the variables indicate
290 UTF-8, then the selected "IOEioA" are in effect
291 a 256 Set ${^UTF8CACHE} to -1, to run the UTF-8 caching
292 code in debugging mode.
294 =for documenting_the_underdocumented
295 perl.h gives W/128 as PERL_UNICODE_WIDESYSCALLS "/* for Sarathy */"
298 perltodo mentions Unicode in %ENV and filenames. I guess that these will be
299 options e and f (or F).
301 For example, B<-COE> and B<-C6> will both turn on UTF-8-ness on both
302 STDOUT and STDERR. Repeating letters is just redundant, not cumulative
305 The C<io> options mean that any subsequent open() (or similar I/O
306 operations) in the current file scope will have the C<:utf8> PerlIO layer
307 implicitly applied to them, in other words, UTF-8 is expected from any
308 input stream, and UTF-8 is produced to any output stream. This is just
309 the default, with explicit layers in open() and with binmode() one can
310 manipulate streams as usual.
312 B<-C> on its own (not followed by any number or option list), or the
313 empty string C<""> for the C<PERL_UNICODE> environment variable, has the
314 same effect as B<-CSDL>. In other words, the standard I/O handles and
315 the default C<open()> layer are UTF-8-fied I<but> only if the locale
316 environment variables indicate a UTF-8 locale. This behaviour follows
317 the I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
318 (See L<perl581delta/UTF-8 no longer default under UTF-8 locales>.)
320 You can use B<-C0> (or C<"0"> for C<PERL_UNICODE>) to explicitly
321 disable all the above Unicode features.
323 The read-only magic variable C<${^UNICODE}> reflects the numeric value
324 of this setting. This variable is set during Perl startup and is
325 thereafter read-only. If you want runtime effects, use the three-arg
326 open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
327 and the C<open> pragma (see L<open>).
329 (In Perls earlier than 5.8.1 the B<-C> switch was a Win32-only switch
330 that enabled the use of Unicode-aware "wide system call" Win32 APIs.
331 This feature was practically unused, however, and the command line
332 switch was therefore "recycled".)
334 B<Note:> Since perl 5.10.1, if the B<-C> option is used on the C<#!> line,
335 it must be specified on the command line as well, since the standard streams
336 are already set up at this point in the execution of the perl interpreter.
337 You can also use binmode() to set the encoding of an I/O stream.
342 causes Perl to check the syntax of the program and then exit without
343 executing it. Actually, it I<will> execute and C<BEGIN>, C<UNITCHECK>,
344 or C<CHECK> blocks and any C<use> statements: these are considered as
345 occurring outside the execution of your program. C<INIT> and C<END>
346 blocks, however, will be skipped.
353 runs the program under the Perl debugger. See L<perldebug>.
354 If B<t> is specified, it indicates to the debugger that threads
355 will be used in the code being debugged.
357 =item B<-d:>I<MOD[=bar,baz]>
360 =item B<-dt:>I<MOD[=bar,baz]>
362 runs the program under the control of a debugging, profiling, or tracing
363 module installed as C<Devel::I<MOD>>. E.g., B<-d:DProf> executes the
364 program using the C<Devel::DProf> profiler. As with the B<-M> flag, options
365 may be passed to the C<Devel::I<MOD>> package where they will be received
366 and interpreted by the C<Devel::I<MOD>::import> routine. Again, like B<-M>,
367 use -B<-d:-I<MOD>> to call C<Devel::I<MOD>::unimport> instead of import. The
368 comma-separated list of options must follow a C<=> character. If B<t> is
369 specified, it indicates to the debugger that threads will be used in the
370 code being debugged. See L<perldebug>.
372 =item B<-D>I<letters>
373 X<-D> X<DEBUGGING> X<-DDEBUGGING>
377 sets debugging flags. To watch how it executes your program, use
378 B<-Dtls>. (This works only if debugging is compiled into your
379 Perl.) Another nice value is B<-Dx>, which lists your compiled
380 syntax tree. And B<-Dr> displays compiled regular expressions;
381 the format of the output is explained in L<perldebguts>.
383 As an alternative, specify a number instead of list of letters (e.g.,
384 B<-D14> is equivalent to B<-Dtls>):
386 1 p Tokenizing and parsing (with v, displays parse stack)
387 2 s Stack snapshots (with v, displays all stacks)
388 4 l Context (loop) stack processing
390 16 o Method and overloading resolution
391 32 c String/numeric conversions
392 64 P Print profiling info, source file input state
393 128 m Memory and SV allocation
394 256 f Format processing
395 512 r Regular expression parsing and execution
396 1024 x Syntax tree dump
397 2048 u Tainting checks
398 4096 U Unofficial, User hacking (reserved for private,
400 8192 H Hash dump -- usurps values()
401 16384 X Scratchpad allocation
403 65536 S Op slab allocation
405 262144 R Include reference counts of dumped variables (eg when
407 524288 J show s,t,P-debug (don't Jump over) on opcodes within
409 1048576 v Verbose: use in conjunction with other flags
410 2097152 C Copy On Write
411 4194304 A Consistency checks on internal structures
412 8388608 q quiet - currently only suppresses the "EXECUTING"
414 16777216 M trace smart match resolution
415 33554432 B dump suBroutine definitions, including special Blocks
418 All these flags require B<-DDEBUGGING> when you compile the Perl
419 executable (but see C<:opd> in L<Devel::Peek> or L<re/'debug' mode>
420 which may change this).
421 See the F<INSTALL> file in the Perl source distribution
422 for how to do this. This flag is automatically set if you include B<-g>
423 option when C<Configure> asks you about optimizer/debugger flags.
425 If you're just trying to get a print out of each line of Perl code
426 as it executes, the way that C<sh -x> provides for shell scripts,
427 you can't use Perl's B<-D> switch. Instead do this
429 # If you have "env" utility
430 env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
432 # Bourne shell syntax
433 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
436 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
438 See L<perldebug> for details and variations.
440 =item B<-e> I<commandline>
443 may be used to enter one line of program. If B<-e> is given, Perl
444 will not look for a filename in the argument list. Multiple B<-e>
445 commands may be given to build up a multi-line script. Make sure
446 to use semicolons where you would in a normal program.
448 =item B<-E> I<commandline>
451 behaves just like B<-e>, except that it implicitly enables all
452 optional features (in the main compilation unit). See L<feature>.
455 X<-f> X<sitecustomize> X<sitecustomize.pl>
457 Disable executing F<$Config{sitelib}/sitecustomize.pl> at startup.
459 Perl can be built so that it by default will try to execute
460 F<$Config{sitelib}/sitecustomize.pl> at startup (in a BEGIN block).
461 This is a hook that allows the sysadmin to customize how Perl behaves.
462 It can for instance be used to add entries to the @INC array to make Perl
463 find modules in non-standard locations.
465 Perl actually inserts the following code:
468 do { local $!; -f "$Config{sitelib}/sitecustomize.pl"; }
469 && do "$Config{sitelib}/sitecustomize.pl";
472 Since it is an actual C<do> (not a C<require>), F<sitecustomize.pl>
473 doesn't need to return a true value. The code is run in package C<main>,
474 in its own lexical scope. However, if the script dies, C<$@> will not
477 The value of C<$Config{sitelib}> is also determined in C code and not
478 read from C<Config.pm>, which is not loaded.
480 The code is executed I<very> early. For example, any changes made to
481 C<@INC> will show up in the output of `perl -V`. Of course, C<END>
482 blocks will be likewise executed very late.
484 To determine at runtime if this capability has been compiled in your
485 perl, you can check the value of C<$Config{usesitecustomize}>.
487 =item B<-F>I<pattern>
490 specifies the pattern to split on if B<-a> is also in effect. The
491 pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
492 put in single quotes. You can't use literal whitespace in the pattern.
497 prints a summary of the options.
499 =item B<-i>[I<extension>]
502 specifies that files processed by the C<E<lt>E<gt>> construct are to be
503 edited in-place. It does this by renaming the input file, opening the
504 output file by the original name, and selecting that output file as the
505 default for print() statements. The extension, if supplied, is used to
506 modify the name of the old file to make a backup copy, following these
509 If no extension is supplied, no backup is made and the current file is
512 If the extension doesn't contain a C<*>, then it is appended to the
513 end of the current filename as a suffix. If the extension does
514 contain one or more C<*> characters, then each C<*> is replaced
515 with the current filename. In Perl terms, you could think of this
518 ($backup = $extension) =~ s/\*/$file_name/g;
520 This allows you to add a prefix to the backup file, instead of (or in
521 addition to) a suffix:
523 $ perl -pi'orig_*' -e 's/bar/baz/' fileA # backup to
526 Or even to place backup copies of the original files into another
527 directory (provided the directory already exists):
529 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to
532 These sets of one-liners are equivalent:
534 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
535 $ perl -pi'*' -e 's/bar/baz/' fileA # overwrite current file
537 $ perl -pi'.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
538 $ perl -pi'*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
540 From the shell, saying
542 $ perl -p -i.orig -e "s/foo/bar/; ... "
544 is the same as using the program:
546 #!/usr/bin/perl -pi.orig
549 which is equivalent to
552 $extension = '.orig';
554 if ($ARGV ne $oldargv) {
555 if ($extension !~ /\*/) {
556 $backup = $ARGV . $extension;
559 ($backup = $extension) =~ s/\*/$ARGV/g;
561 rename($ARGV, $backup);
562 open(ARGVOUT, ">$ARGV");
569 print; # this prints to original filename
573 except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
574 know when the filename has changed. It does, however, use ARGVOUT for
575 the selected filehandle. Note that STDOUT is restored as the default
576 output filehandle after the loop.
578 As shown above, Perl creates the backup file whether or not any output
579 is actually changed. So this is just a fancy way to copy files:
581 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
583 $ perl -p -i'.orig' -e 1 file1 file2 file3...
585 You can use C<eof> without parentheses to locate the end of each input
586 file, in case you want to append to each file, or reset line numbering
587 (see example in L<perlfunc/eof>).
589 If, for a given file, Perl is unable to create the backup file as
590 specified in the extension then it will skip that file and continue on
591 with the next one (if it exists).
593 For a discussion of issues surrounding file permissions and B<-i>,
594 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?>.
596 You cannot use B<-i> to create directories or to strip extensions from
599 Perl does not expand C<~> in filenames, which is good, since some
600 folks use it for their backup files:
602 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
604 Note that because B<-i> renames or deletes the original file before
605 creating a new file of the same name, Unix-style soft and hard links will
608 Finally, the B<-i> switch does not impede execution when no
609 files are given on the command line. In this case, no backup is made
610 (the original file cannot, of course, be determined) and processing
611 proceeds from STDIN to STDOUT as might be expected.
613 =item B<-I>I<directory>
616 Directories specified by B<-I> are prepended to the search path for
619 =item B<-l>[I<octnum>]
622 enables automatic line-ending processing. It has two separate
623 effects. First, it automatically chomps C<$/> (the input record
624 separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
625 (the output record separator) to have the value of I<octnum> so
626 that any print statements will have that separator added back on.
627 If I<octnum> is omitted, sets C<$\> to the current value of
628 C<$/>. For instance, to trim lines to 80 columns:
630 perl -lpe 'substr($_, 80) = ""'
632 Note that the assignment C<$\ = $/> is done when the switch is processed,
633 so the input record separator can be different than the output record
634 separator if the B<-l> switch is followed by a B<-0> switch:
636 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
638 This sets C<$\> to newline and then sets C<$/> to the null character.
640 =item B<-m>[B<->]I<module>
643 =item B<-M>[B<->]I<module>
645 =item B<-M>[B<->]I<'module ...'>
647 =item B<-[mM]>[B<->]I<module=arg[,arg]...>
649 B<-m>I<module> executes C<use> I<module> C<();> before executing your
652 B<-M>I<module> executes C<use> I<module> C<;> before executing your
653 program. You can use quotes to add extra code after the module name,
654 e.g., C<'-MI<MODULE> qw(foo bar)'>.
656 If the first character after the B<-M> or B<-m> is a dash (B<->)
657 then the 'use' is replaced with 'no'.
659 A little builtin syntactic sugar means you can also say
660 B<-mI<MODULE>=foo,bar> or B<-MI<MODULE>=foo,bar> as a shortcut for
661 B<'-MI<MODULE> qw(foo bar)'>. This avoids the need to use quotes when
662 importing symbols. The actual code generated by B<-MI<MODULE>=foo,bar> is
663 C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
664 removes the distinction between B<-m> and B<-M>.
666 A consequence of this is that B<-MI<MODULE>=number> never does a version check,
667 unless C<I<MODULE>::import()> itself is set up to do a version check, which
668 could happen for example if I<MODULE> inherits from L<Exporter>.
673 causes Perl to assume the following loop around your program, which
674 makes it iterate over filename arguments somewhat like I<sed -n> or
679 ... # your program goes here
682 Note that the lines are not printed by default. See L</-p> to have
683 lines printed. If a file named by an argument cannot be opened for
684 some reason, Perl warns you about it and moves on to the next file.
686 Also note that C<< <> >> passes command line arguments to
687 L<perlfunc/open>, which doesn't necessarily interpret them as file names.
688 See L<perlop> for possible security implications.
690 Here is an efficient way to delete all files that haven't been modified for
693 find . -mtime +7 -print | perl -nle unlink
695 This is faster than using the B<-exec> switch of I<find> because you don't
696 have to start a process on every filename found. It does suffer from
697 the bug of mishandling newlines in pathnames, which you can fix if
698 you follow the example under B<-0>.
700 C<BEGIN> and C<END> blocks may be used to capture control before or after
701 the implicit program loop, just as in I<awk>.
706 causes Perl to assume the following loop around your program, which
707 makes it iterate over filename arguments somewhat like I<sed>:
712 ... # your program goes here
714 print or die "-p destination: $!\n";
717 If a file named by an argument cannot be opened for some reason, Perl
718 warns you about it, and moves on to the next file. Note that the
719 lines are printed automatically. An error occurring during printing is
720 treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
721 overrides a B<-n> switch.
723 C<BEGIN> and C<END> blocks may be used to capture control before or after
724 the implicit loop, just as in I<awk>.
729 enables rudimentary switch parsing for switches on the command
730 line after the program name but before any filename arguments (or before
731 an argument of B<-->). Any switch found there is removed from @ARGV and sets the
732 corresponding variable in the Perl program. The following program
733 prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
734 if it is invoked with B<-xyz=abc>.
737 if ($xyz) { print "$xyz\n" }
739 Do note that a switch like B<--help> creates the variable C<${-help}>, which is not compliant
740 with C<use strict "refs">. Also, when using this option on a script with
741 warnings enabled you may get a lot of spurious "used only once" warnings.
746 makes Perl use the PATH environment variable to search for the
747 program unless the name of the program contains path separators.
749 On some platforms, this also makes Perl append suffixes to the
750 filename while searching for it. For example, on Win32 platforms,
751 the ".bat" and ".cmd" suffixes are appended if a lookup for the
752 original name fails, and if the name does not already end in one
753 of those suffixes. If your Perl was compiled with C<DEBUGGING> turned
754 on, using the B<-Dp> switch to Perl shows how the search progresses.
756 Typically this is used to emulate C<#!> startup on platforms that don't
757 support C<#!>. It's also convenient when debugging a script that uses C<#!>,
758 and is thus normally found by the shell's $PATH search mechanism.
760 This example works on many platforms that have a shell compatible with
764 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
765 if $running_under_some_shell;
767 The system ignores the first line and feeds the program to F</bin/sh>,
768 which proceeds to try to execute the Perl program as a shell script.
769 The shell executes the second line as a normal shell command, and thus
770 starts up the Perl interpreter. On some systems $0 doesn't always
771 contain the full pathname, so the B<-S> tells Perl to search for the
772 program if necessary. After Perl locates the program, it parses the
773 lines and ignores them because the variable $running_under_some_shell
774 is never true. If the program will be interpreted by csh, you will need
775 to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
776 embedded spaces (and such) in the argument list. To start up I<sh> rather
777 than I<csh>, some systems may have to replace the C<#!> line with a line
778 containing just a colon, which will be politely ignored by Perl. Other
779 systems can't control that, and need a totally devious construct that
780 will work under any of I<csh>, I<sh>, or Perl, such as the following:
782 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
783 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
784 if $running_under_some_shell;
786 If the filename supplied contains directory separators (and so is an
787 absolute or relative pathname), and if that file is not found,
788 platforms that append file extensions will do so and try to look
789 for the file with those extensions added, one by one.
791 On DOS-like platforms, if the program does not contain directory
792 separators, it will first be searched for in the current directory
793 before being searched for on the PATH. On Unix platforms, the
794 program will be searched for strictly on the PATH.
799 Like B<-T>, but taint checks will issue warnings rather than fatal
800 errors. These warnings can now be controlled normally with C<no warnings
803 B<Note: This is not a substitute for C<-T>!> This is meant to be
804 used I<only> as a temporary development aid while securing legacy code:
805 for real production code and for new secure code written from scratch,
806 always use the real B<-T>.
811 turns on "taint" so you can test them. Ordinarily
812 these checks are done only when running setuid or setgid. It's a
813 good idea to turn them on explicitly for programs that run on behalf
814 of someone else whom you might not necessarily trust, such as CGI
815 programs or any internet servers you might write in Perl. See
816 L<perlsec> for details. For security reasons, this option must be
817 seen by Perl quite early; usually this means it must appear early
818 on the command line or in the C<#!> line for systems which support
824 This switch causes Perl to dump core after compiling your
825 program. You can then in theory take this core dump and turn it
826 into an executable file by using the I<undump> program (not supplied).
827 This speeds startup at the expense of some disk space (which you
828 can minimize by stripping the executable). (Still, a "hello world"
829 executable comes out to about 200K on my machine.) If you want to
830 execute a portion of your program before dumping, use the dump()
831 operator instead. Note: availability of I<undump> is platform
832 specific and may not be available for a specific port of Perl.
837 allows Perl to do unsafe operations. Currently the only "unsafe"
838 operations are attempting to unlink directories while running as superuser
839 and running setuid programs with fatal taint checks turned into warnings.
840 Note that warnings must be enabled along with this option to actually
841 I<generate> the taint-check warnings.
846 prints the version and patchlevel of your perl executable.
851 prints summary of the major perl configuration values and the current
854 =item B<-V:>I<configvar>
856 Prints to STDOUT the value of the named configuration variable(s),
857 with multiples when your C<I<configvar>> argument looks like a regex (has
858 non-letters). For example:
861 libc='/lib/libc-2.2.4.so';
863 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
864 libc='/lib/libc-2.2.4.so';
866 libpth='/usr/local/lib /lib /usr/lib';
867 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
869 libc='/lib/libc-2.2.4.so';
873 Additionally, extra colons can be used to control formatting. A
874 trailing colon suppresses the linefeed and terminator ";", allowing
875 you to embed queries into shell commands. (mnemonic: PATH separator
878 $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
879 compression-vars: zcat='' zip='zip' are here !
881 A leading colon removes the "name=" part of the response, this allows
882 you to map to the name you need. (mnemonic: empty label)
884 $ echo "goodvfork="`./perl -Ilib -V::usevfork`
887 Leading and trailing colons can be used together if you need
888 positional parameter values without the names. Note that in the case
889 below, the C<PERL_API> params are returned in alphabetical order.
891 $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
892 building_on 'linux' '5' '1' '9' now
897 prints warnings about dubious constructs, such as variable names
898 mentioned only once and scalar variables used
899 before being set; redefined subroutines; references to undefined
900 filehandles; filehandles opened read-only that you are attempting
901 to write on; values used as a number that don't I<look> like numbers;
902 using an array as though it were a scalar; if your subroutines
903 recurse more than 100 deep; and innumerable other things.
905 This switch really just enables the global C<$^W> variable; normally,
906 the lexically scoped C<use warnings> pragma is preferred. You
907 can disable or promote into fatal errors specific warnings using
908 C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
909 See also L<perldiag> and L<perltrap>. A fine-grained warning
910 facility is also available if you want to manipulate entire classes
911 of warnings; see L<warnings> or L<perllexwarn>.
916 Enables all warnings regardless of C<no warnings> or C<$^W>.
922 Disables all warnings regardless of C<use warnings> or C<$^W>.
928 =item B<-x>I<directory>
930 tells Perl that the program is embedded in a larger chunk of unrelated
931 text, such as in a mail message. Leading garbage will be
932 discarded until the first line that starts with C<#!> and contains the
933 string "perl". Any meaningful switches on that line will be applied.
935 All references to line numbers by the program (warnings, errors, ...)
936 will treat the C<#!> line as the first line.
937 Thus a warning on the 2nd line of the program, which is on the 100th
938 line in the file will be reported as line 2, not as line 100.
939 This can be overridden by using the C<#line> directive.
940 (See L<perlsyn/"Plain Old Comments (Not!)">)
942 If a directory name is specified, Perl will switch to that directory
943 before running the program. The B<-x> switch controls only the
944 disposal of leading garbage. The program must be terminated with
945 C<__END__> if there is trailing garbage to be ignored; the program
946 can process any or all of the trailing garbage via the C<DATA> filehandle
949 The directory, if specified, must appear immediately following the B<-x>
950 with no intervening whitespace.
955 X<perl, environment variables>
962 Used if C<chdir> has no argument.
967 Used if C<chdir> has no argument and HOME is not set.
972 Used in executing subprocesses, and in finding the program if B<-S> is
978 A list of directories in which to look for Perl library
979 files before looking in the standard library and the current
980 directory. Any architecture-specific directories under the specified
981 locations are automatically included if they exist, with this lookup
982 done at interpreter startup time.
984 If PERL5LIB is not defined, PERLLIB is used. Directories are separated
985 (like in PATH) by a colon on Unixish platforms and by a semicolon on
986 Windows (the proper path separator being given by the command C<perl
989 When running taint checks, either because the program was running setuid or
990 setgid, or the B<-T> or B<-t> switch was specified, neither PERL5LIB nor
991 PERLLIB is consulted. The program should instead say:
993 use lib "/my/directory";
998 Command-line options (switches). Switches in this variable are treated
999 as if they were on every Perl command line. Only the B<-[CDIMUdmtwW]>
1000 switches are allowed. When running taint checks (either because the
1001 program was running setuid or setgid, or because the B<-T> or B<-t>
1002 switch was used), this variable is ignored. If PERL5OPT begins with
1003 B<-T>, tainting will be enabled and subsequent options ignored. If
1004 PERL5OPT begins with B<-t>, tainting will be enabled, a writable dot
1005 removed from @INC, and subsequent options honored.
1010 A space (or colon) separated list of PerlIO layers. If perl is built
1011 to use PerlIO system for IO (the default) these layers affect Perl's IO.
1013 It is conventional to start layer names with a colon (for example, C<:perlio>) to
1014 emphasize their similarity to variable "attributes". But the code that parses
1015 layer specification strings, which is also used to decode the PERLIO
1016 environment variable, treats the colon as a separator.
1018 An unset or empty PERLIO is equivalent to the default set of layers for
1019 your platform; for example, C<:unix:perlio> on Unix-like systems
1020 and C<:unix:crlf> on Windows and other DOS-like systems.
1022 The list becomes the default for I<all> Perl's IO. Consequently only built-in
1023 layers can appear in this list, as external layers (such as C<:encoding()>) need
1024 IO in order to load them! See L<"open pragma"|open> for how to add external
1025 encodings as defaults.
1027 Layers it makes sense to include in the PERLIO environment
1028 variable are briefly summarized below. For more details see L<PerlIO>.
1035 A pseudolayer that turns the C<:utf8> flag I<off> for the layer below;
1036 unlikely to be useful on its own in the global PERLIO environment variable.
1037 You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
1042 A layer which does CRLF to C<"\n"> translation distinguishing "text" and
1043 "binary" files in the manner of MS-DOS and similar operating systems.
1044 (It currently does I<not> mimic MS-DOS as far as treating of Control-Z
1045 as being an end-of-file marker.)
1050 A layer that implements "reading" of files by using I<mmap>(2) to
1051 make an entire file appear in the process's address space, and then
1052 using that as PerlIO's "buffer".
1057 This is a re-implementation of stdio-like buffering written as a
1058 PerlIO layer. As such it will call whatever layer is below it for
1059 its operations, typically C<:unix>.
1064 An experimental pseudolayer that removes the topmost layer.
1065 Use with the same care as is reserved for nitroglycerine.
1070 A pseudolayer that manipulates other layers. Applying the C<:raw>
1071 layer is equivalent to calling C<binmode($fh)>. It makes the stream
1072 pass each byte as-is without translation. In particular, both CRLF
1073 translation and intuiting C<:utf8> from the locale are disabled.
1075 Unlike in earlier versions of Perl, C<:raw> is I<not>
1076 just the inverse of C<:crlf>: other layers which would affect the
1077 binary nature of the stream are also removed or disabled.
1082 This layer provides a PerlIO interface by wrapping system's ANSI C "stdio"
1083 library calls. The layer provides both buffering and IO.
1084 Note that the C<:stdio> layer does I<not> do CRLF translation even if that
1085 is the platform's normal behaviour. You will need a C<:crlf> layer above it
1091 Low-level layer that calls C<read>, C<write>, C<lseek>, etc.
1096 A pseudolayer that enables a flag in the layer below to tell Perl
1097 that output should be in utf8 and that input should be regarded as
1098 already in valid utf8 form. B<WARNING: It does not check for validity and as such
1099 should be handled with extreme caution for input, because security violations
1100 can occur with non-shortest UTF-8 encodings, etc.> Generally C<:encoding(utf8)> is
1101 the best option when reading UTF-8 encoded data.
1106 On Win32 platforms this I<experimental> layer uses native "handle" IO
1107 rather than a Unix-like numeric file descriptor layer. Known to be
1108 buggy in this release (5.14).
1112 The default set of layers should give acceptable results on all platforms
1114 For Unix platforms that will be the equivalent of "unix perlio" or "stdio".
1115 Configure is set up to prefer the "stdio" implementation if the system's library
1116 provides for fast access to the buffer; otherwise, it uses the "unix perlio"
1119 On Win32 the default in this release (5.14) is "unix crlf". Win32's "stdio"
1120 has a number of bugs/mis-features for Perl IO which are somewhat depending
1121 on the version and vendor of the C compiler. Using our own C<crlf> layer as
1122 the buffer avoids those issues and makes things more uniform. The C<crlf>
1123 layer provides CRLF conversion as well as buffering.
1125 This release (5.14) uses C<unix> as the bottom layer on Win32, and so still
1126 uses the C compiler's numeric file descriptor routines. There is an
1127 experimental native C<win32> layer, which is expected to be enhanced and
1128 should eventually become the default under Win32.
1130 The PERLIO environment variable is completely ignored when Perl
1131 is run in taint mode.
1136 If set to the name of a file or device, certain operations of PerlIO
1137 subsystem will be logged to that file, which is opened in append mode.
1138 Typical uses are in Unix:
1140 % env PERLIO_DEBUG=/dev/tty perl script ...
1142 and under Win32, the approximately equivalent:
1144 > set PERLIO_DEBUG=CON
1147 This functionality is disabled for setuid scripts and for scripts run
1153 A list of directories in which to look for Perl library
1154 files before looking in the standard library and the current directory.
1155 If PERL5LIB is defined, PERLLIB is not used.
1157 The PERLLIB environment variable is completely ignored when Perl
1158 is run in taint mode.
1163 The command used to load the debugger code. The default is:
1165 BEGIN { require "perl5db.pl" }
1167 The PERL5DB environment variable is only used when Perl is started with
1168 a bare B<-d> switch.
1170 =item PERL5DB_THREADED
1173 If set to a true value, indicates to the debugger that the code being
1174 debugged uses threads.
1176 =item PERL5SHELL (specific to the Win32 port)
1179 On Win32 ports only, may be set to an alternative shell that Perl must use
1180 internally for executing "backtick" commands or system(). Default is
1181 C<cmd.exe /x/d/c> on WindowsNT and C<command.com /c> on Windows95. The
1182 value is considered space-separated. Precede any character that
1183 needs to be protected, like a space or backslash, with another backslash.
1185 Note that Perl doesn't use COMSPEC for this purpose because
1186 COMSPEC has a high degree of variability among users, leading to
1187 portability concerns. Besides, Perl can use a shell that may not be
1188 fit for interactive use, and setting COMSPEC to such a shell may
1189 interfere with the proper functioning of other programs (which usually
1190 look in COMSPEC to find a shell fit for interactive use).
1192 Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint checked
1193 when running external commands. It is recommended that
1194 you explicitly set (or delete) C<$ENV{PERL5SHELL}> when running
1195 in taint mode under Windows.
1197 =item PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
1198 X<PERL_ALLOW_NON_IFS_LSP>
1200 Set to 1 to allow the use of non-IFS compatible LSPs (Layered Service Providers).
1201 Perl normally searches for an IFS-compatible LSP because this is required
1202 for its emulation of Windows sockets as real filehandles. However, this may
1203 cause problems if you have a firewall such as I<McAfee Guardian>, which requires
1204 that all applications use its LSP but which is not IFS-compatible, because clearly
1205 Perl will normally avoid using such an LSP.
1207 Setting this environment variable to 1 means that Perl will simply use the
1208 first suitable LSP enumerated in the catalog, which keeps I<McAfee Guardian>
1209 happy--and in that particular case Perl still works too because I<McAfee
1210 Guardian>'s LSP actually plays other games which allow applications
1211 requiring IFS compatibility to work.
1213 =item PERL_DEBUG_MSTATS
1214 X<PERL_DEBUG_MSTATS>
1216 Relevant only if Perl is compiled with the C<malloc> included with the Perl
1217 distribution; that is, if C<perl -V:d_mymalloc> is "define".
1219 If set, this dumps out memory statistics after execution. If set
1220 to an integer greater than one, also dumps out memory statistics
1223 =item PERL_DESTRUCT_LEVEL
1224 X<PERL_DESTRUCT_LEVEL>
1226 Relevant only if your Perl executable was built with B<-DDEBUGGING>,
1227 this controls the behaviour of global destruction of objects and other
1228 references. See L<perlhacktips/PERL_DESTRUCT_LEVEL> for more information.
1230 =item PERL_DL_NONLAZY
1233 Set to C<"1"> to have Perl resolve I<all> undefined symbols when it loads
1234 a dynamic library. The default behaviour is to resolve symbols when
1235 they are used. Setting this variable is useful during testing of
1236 extensions, as it ensures that you get an error on misspelled function
1237 names even if the test suite doesn't call them.
1242 If using the C<use encoding> pragma without an explicit encoding name, the
1243 PERL_ENCODING environment variable is consulted for an encoding name.
1245 =item PERL_HASH_SEED
1248 (Since Perl 5.8.1.) Used to randomize Perl's internal hash function.
1249 To emulate the pre-5.8.1 behaviour, set to an integer; C<"0"> means
1250 exactly the same order as in 5.8.0. "Pre-5.8.1" means, among other
1251 things, that hash keys will always have the same ordering between
1252 different runs of Perl.
1254 Most hashes by default return elements in the same order as in Perl 5.8.0.
1255 On a hash by hash basis, if pathological data is detected during a hash
1256 key insertion, then that hash will switch to an alternative random hash
1259 The default behaviour is to randomize unless the PERL_HASH_SEED is set.
1260 If Perl has been compiled with B<-DUSE_HASH_SEED_EXPLICIT>, the default
1261 behaviour is I<not> to randomize unless the PERL_HASH_SEED is set.
1263 If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses
1264 the pseudorandom seed supplied by the operating system and libraries.
1266 B<PLEASE NOTE: The hash seed is sensitive information>. Hashes are
1267 randomized to protect against local and remote attacks against Perl
1268 code. By manually setting a seed, this protection may be partially or
1271 See L<perlsec/"Algorithmic Complexity Attacks"> and
1272 L</PERL_HASH_SEED_DEBUG> for more information.
1274 =item PERL_HASH_SEED_DEBUG
1275 X<PERL_HASH_SEED_DEBUG>
1277 (Since Perl 5.8.1.) Set to C<"1"> to display (to STDERR) the value of
1278 the hash seed at the beginning of execution. This, combined with
1279 L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
1280 behaviour caused by hash randomization.
1282 B<Note that the hash seed is sensitive information>: by knowing it, one
1283 can craft a denial-of-service attack against Perl code, even remotely;
1284 see L<perlsec/"Algorithmic Complexity Attacks"> for more information.
1285 B<Do not disclose the hash seed> to people who don't need to know it.
1286 See also hash_seed() in L<Hash::Util>.
1291 If your Perl was configured with B<-Accflags=-DPERL_MEM_LOG>, setting
1292 the environment variable C<PERL_MEM_LOG> enables logging debug
1293 messages. The value has the form C<< <I<number>>[m][s][t] >>, where
1294 C<I<number>> is the file descriptor number you want to write to (2 is
1295 default), and the combination of letters specifies that you want
1296 information about (m)emory and/or (s)v, optionally with
1297 (t)imestamps. For example, C<PERL_MEM_LOG=1mst> logs all
1298 information to stdout. You can write to other opened file descriptors
1299 in a variety of ways:
1301 $ 3>foo3 PERL_MEM_LOG=3m perl ...
1303 =item PERL_ROOT (specific to the VMS port)
1306 A translation-concealed rooted logical name that contains Perl and the
1307 logical device for the @INC path on VMS only. Other logical names that
1308 affect Perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1309 SYS$TIMEZONE_DIFFERENTIAL, but are optional and discussed further in
1310 L<perlvms> and in F<README.vms> in the Perl source distribution.
1315 Available in Perls 5.8.1 and later. If set to C<"unsafe">, the pre-Perl-5.8.0
1316 signal behaviour (which is immediate but unsafe) is restored. If set
1317 to C<safe>, then safe (but deferred) signals are used. See
1318 L<perlipc/"Deferred Signals (Safe Signals)">.
1323 Equivalent to the B<-C> command-line switch. Note that this is not
1324 a boolean variable. Setting this to C<"1"> is not the right way to
1325 "enable Unicode" (whatever that would mean). You can use C<"0"> to
1326 "disable Unicode", though (or alternatively unset PERL_UNICODE in
1327 your shell before starting Perl). See the description of the B<-C>
1328 switch for more information.
1330 =item SYS$LOGIN (specific to the VMS port)
1333 Used if chdir has no argument and HOME and LOGDIR are not set.
1337 Perl also has environment variables that control how Perl handles data
1338 specific to particular natural languages; see L<perllocale>.
1340 Perl and its various modules and components, including its test frameworks,
1341 may sometimes make use of certain other environment variables. Some of
1342 these are specific to a particular platform. Please consult the
1343 appropriate module documentation and any documentation for your platform
1344 (like L<perlsolaris>, L<perllinux>, L<perlmacosx>, L<perlwin32>, etc) for
1345 variables peculiar to those specific situations.
1347 Perl makes all environment variables available to the program being
1348 executed, and passes these along to any child processes it starts.
1349 However, programs running setuid would do well to execute the following
1350 lines before doing anything else, just to keep people honest:
1352 $ENV{PATH} = "/bin:/usr/bin"; # or whatever you need
1353 $ENV{SHELL} = "/bin/sh" if exists $ENV{SHELL};
1354 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};