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<-A>[I<module>][=I<assertions>] ]>
13 S<[ B<-C [I<number/list>] >]>
17 S<[ B<-i>[I<extension>] ]>
18 S<[ B<-e> I<'command'> ] [ B<--> ] [ I<programfile> ] [ I<argument> ]...>
22 The normal way to run a Perl program is by making it directly
23 executable, or else by passing the name of the source file as an
24 argument on the command line. (An interactive Perl environment
25 is also possible--see L<perldebug> for details on how to do that.)
26 Upon startup, Perl looks for your program in one of the following
33 Specified line by line via B<-e> switches on the command line.
37 Contained in the file specified by the first filename on the command line.
38 (Note that systems supporting the #! notation invoke interpreters this
39 way. See L<Location of Perl>.)
43 Passed in implicitly via standard input. This works only if there are
44 no filename arguments--to pass arguments to a STDIN-read program you
45 must explicitly specify a "-" for the program name.
49 With methods 2 and 3, Perl starts parsing the input file from the
50 beginning, unless you've specified a B<-x> switch, in which case it
51 scans for the first line starting with #! and containing the word
52 "perl", and starts there instead. This is useful for running a program
53 embedded in a larger message. (In this case you would indicate the end
54 of the program using the C<__END__> token.)
56 The #! line is always examined for switches as the line is being
57 parsed. Thus, if you're on a machine that allows only one argument
58 with the #! line, or worse, doesn't even recognize the #! line, you
59 still can get consistent switch behavior regardless of how Perl was
60 invoked, even if B<-x> was used to find the beginning of the program.
62 Because historically some operating systems silently chopped off
63 kernel interpretation of the #! line after 32 characters, some
64 switches may be passed in on the command line, and some may not;
65 you could even get a "-" without its letter, if you're not careful.
66 You probably want to make sure that all your switches fall either
67 before or after that 32-character boundary. Most switches don't
68 actually care if they're processed redundantly, but getting a "-"
69 instead of a complete switch could cause Perl to try to execute
70 standard input instead of your program. And a partial B<-I> switch
71 could also cause odd results.
73 Some switches do care if they are processed twice, for instance
74 combinations of B<-l> and B<-0>. Either put all the switches after
75 the 32-character boundary (if applicable), or replace the use of
76 B<-0>I<digits> by C<BEGIN{ $/ = "\0digits"; }>.
78 Parsing of the #! switches starts wherever "perl" is mentioned in the line.
79 The sequences "-*" and "- " are specifically ignored so that you could,
80 if you were so inclined, say
82 #!/bin/sh -- # -*- perl -*- -p
83 eval 'exec perl -wS $0 ${1+"$@"}'
84 if $running_under_some_shell;
86 to let Perl see the B<-p> switch.
88 A similar trick involves the B<env> program, if you have it.
92 The examples above use a relative path to the perl interpreter,
93 getting whatever version is first in the user's path. If you want
94 a specific version of Perl, say, perl5.005_57, you should place
95 that directly in the #! line's path.
97 If the #! line does not contain the word "perl", the program named after
98 the #! is executed instead of the Perl interpreter. This is slightly
99 bizarre, but it helps people on machines that don't do #!, because they
100 can tell a program that their SHELL is F</usr/bin/perl>, and Perl will then
101 dispatch the program to the correct 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
114 Unix's #! technique can be simulated on other systems:
122 extproc perl -S -your_switches
124 as the first line in C<*.cmd> file (B<-S> due to a bug in cmd.exe's
129 Create a batch file to run your program, and codify it in
130 C<ALTERNATE_SHEBANG> (see the F<dosish.h> file in the source
131 distribution for more information).
135 The Win95/NT installation, when using the ActiveState installer for Perl,
136 will modify the Registry to associate the F<.pl> extension with the perl
137 interpreter. If you install Perl by other means (including building from
138 the sources), you may have to modify the Registry yourself. Note that
139 this means you can no longer tell the difference between an executable
140 Perl program and a Perl library file.
144 Under "Classic" MacOS, a perl program will have the appropriate Creator and
145 Type, so that double-clicking them will invoke the MacPerl application.
146 Under Mac OS X, clickable apps can be made from any C<#!> script using Wil
147 Sanchez' DropScript utility: http://www.wsanchez.net/software/ .
153 $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
154 $ exit++ + ++$status != 0 and $exit = $status = undef;
156 at the top of your program, where B<-mysw> are any command line switches you
157 want to pass to Perl. You can now invoke the program directly, by saying
158 C<perl program>, or as a DCL procedure, by saying C<@program> (or implicitly
159 via F<DCL$PATH> by just using the name of the program).
161 This incantation is a bit much to remember, but Perl will display it for
162 you if you say C<perl "-V:startperl">.
166 Command-interpreters on non-Unix systems have rather different ideas
167 on quoting than Unix shells. You'll need to learn the special
168 characters in your command-interpreter (C<*>, C<\> and C<"> are
169 common) and how to protect whitespace and these characters to run
170 one-liners (see B<-e> below).
172 On some systems, you may have to change single-quotes to double ones,
173 which you must I<not> do on Unix or Plan 9 systems. You might also
174 have to change a single % to a %%.
179 perl -e 'print "Hello world\n"'
182 perl -e "print \"Hello world\n\""
185 print "Hello world\n"
186 (then Run "Myscript" or Shift-Command-R)
189 perl -e "print ""Hello world\n"""
191 The problem is that none of this is reliable: it depends on the
192 command and it is entirely possible neither works. If B<4DOS> were
193 the command shell, this would probably work better:
195 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
197 B<CMD.EXE> in Windows NT slipped a lot of standard Unix functionality in
198 when nobody was looking, but just try to find documentation for its
201 Under the Macintosh, it depends which environment you are using. The MacPerl
202 shell, or MPW, is much like Unix shells in its support for several
203 quoting variants, except that it makes free use of the Macintosh's non-ASCII
204 characters as control characters.
206 There is no general solution to all of this. It's just a mess.
208 =head2 Location of Perl
210 It may seem obvious to say, but Perl is useful only when users can
211 easily find it. When possible, it's good for both F</usr/bin/perl>
212 and F</usr/local/bin/perl> to be symlinks to the actual binary. If
213 that can't be done, system administrators are strongly encouraged
214 to put (symlinks to) perl and its accompanying utilities into a
215 directory typically found along a user's PATH, or in some other
216 obvious and convenient place.
218 In this documentation, C<#!/usr/bin/perl> on the first line of the program
219 will stand in for whatever method works on your system. You are
220 advised to use a specific path if you care about a specific version.
222 #!/usr/local/bin/perl5.00554
224 or if you just want to be running at least version, place a statement
225 like this at the top of your program:
229 =head2 Command Switches
231 As with all standard commands, a single-character switch may be
232 clustered with the following switch, if any.
234 #!/usr/bin/perl -spi.orig # same as -s -p -i.orig
240 =item B<-0>[I<octal/hexadecimal>]
242 specifies the input record separator (C<$/>) as an octal or
243 hexadecimal number. If there are no digits, the null character is the
244 separator. Other switches may precede or follow the digits. For
245 example, if you have a version of B<find> which can print filenames
246 terminated by the null character, you can say this:
248 find . -name '*.orig' -print0 | perl -n0e unlink
250 The special value 00 will cause Perl to slurp files in paragraph mode.
251 The value 0777 will cause Perl to slurp files whole because there is no
252 legal byte with that value.
254 If you want to specify any Unicode character, use the hexadecimal
255 format: C<-0xHHH...>, where the C<H> are valid hexadecimal digits.
256 (This means that you cannot use the C<-x> with a directory name that
257 consists of hexadecimal digits.)
259 =item B<-A[I<module>][=I<assertions>]>
261 Activates the assertions given after the equal sign as a comma-separated
262 list of assertion names or regular expressions. If no assertion name
263 is given, activates all assertions.
265 The module L<assertions::activate> is used by default to activate the
266 selected assertions. An alternate module may be specified including
267 its name between the switch and the equal sign.
269 See L<assertions> and L<assertions::activate>.
273 turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
274 split command to the @F array is done as the first thing inside the
275 implicit while loop produced by the B<-n> or B<-p>.
277 perl -ane 'print pop(@F), "\n";'
286 An alternate delimiter may be specified using B<-F>.
288 =item B<-C [I<number/list>]>
290 The C<-C> flag controls some Unicode of the Perl Unicode features.
292 As of 5.8.1, the C<-C> can be followed either by a number or a list
293 of option letters. The letters, their numeric values, and effects
294 are as follows; listing the letters is equal to summing the numbers.
296 I 1 STDIN is assumed to be in UTF-8
297 O 2 STDOUT will be in UTF-8
298 E 4 STDERR will be in UTF-8
300 i 8 UTF-8 is the default PerlIO layer for input streams
301 o 16 UTF-8 is the default PerlIO layer for output streams
303 A 32 the @ARGV elements are expected to be strings encoded
305 L 64 normally the "IOEioA" are unconditional,
306 the L makes them conditional on the locale environment
307 variables (the LC_ALL, LC_TYPE, and LANG, in the order
308 of decreasing precedence) -- if the variables indicate
309 UTF-8, then the selected "IOEioA" are in effect
311 For example, C<-COE> and C<-C6> will both turn on UTF-8-ness on both
312 STDOUT and STDERR. Repeating letters is just redundant, not cumulative
315 The C<io> options mean that any subsequent open() (or similar I/O
316 operations) will have the C<:utf8> PerlIO layer implicitly applied
317 to them, in other words, UTF-8 is expected from any input stream,
318 and UTF-8 is produced to any output stream. This is just the default,
319 with explicit layers in open() and with binmode() one can manipulate
322 C<-C> on its own (not followed by any number or option list), or the
323 empty string C<""> for the C<PERL_UNICODE> environment variable, has the
324 same effect as C<-CSDL>. In other words, the standard I/O handles and
325 the default C<open()> layer are UTF-8-fied B<but> only if the locale
326 environment variables indicate a UTF-8 locale. This behaviour follows
327 the I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
329 You can use C<-C0> (or C<"0"> for C<PERL_UNICODE>) to explicitly
330 disable all the above Unicode features.
332 The read-only magic variable C<${^UNICODE}> reflects the numeric value
333 of this setting. This is variable is set during Perl startup and is
334 thereafter read-only. If you want runtime effects, use the three-arg
335 open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
336 and the C<open> pragma (see L<open>).
338 (In Perls earlier than 5.8.1 the C<-C> switch was a Win32-only switch
339 that enabled the use of Unicode-aware "wide system call" Win32 APIs.
340 This feature was practically unused, however, and the command line
341 switch was therefore "recycled".)
345 causes Perl to check the syntax of the program and then exit without
346 executing it. Actually, it I<will> execute C<BEGIN>, C<CHECK>, and
347 C<use> blocks, because these are considered as occurring outside the
348 execution of your program. C<INIT> and C<END> blocks, however, will
355 runs the program under the Perl debugger. See L<perldebug>.
356 If B<t> is specified, it indicates to the debugger that threads
357 will be used in the code being debugged.
359 =item B<-d:>I<foo[=bar,baz]>
361 =item B<-dt:>I<foo[=bar,baz]>
363 runs the program under the control of a debugging, profiling, or
364 tracing module installed as Devel::foo. E.g., B<-d:DProf> executes
365 the program using the Devel::DProf profiler. As with the B<-M>
366 flag, options may be passed to the Devel::foo package where they
367 will be received and interpreted by the Devel::foo::import routine.
368 The comma-separated list of options must follow a C<=> character.
369 If B<t> is specified, it indicates to the debugger that threads
370 will be used in the code being debugged.
373 =item B<-D>I<letters>
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, preprocessor command for -P, source file input state
393 128 m Memory 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 (Obsolete, previously used for LEAKTEST)
399 8192 H Hash dump -- usurps values()
400 16384 X Scratchpad allocation
402 65536 S Thread synchronization
404 262144 R Include reference counts of dumped variables (eg when using -Ds)
405 524288 J Do not s,t,P-debug (Jump over) opcodes within package DB
406 1048576 v Verbose: use in conjunction with other flags
407 2097152 C Copy On Write
408 4194304 A Consistency checks on internal structures
409 8388608 q quiet - currently only suppresses the "EXECUTING" message
411 All these flags require B<-DDEBUGGING> when you compile the Perl
412 executable (but see L<Devel::Peek>, L<re> which may change this).
413 See the F<INSTALL> file in the Perl source distribution
414 for how to do this. This flag is automatically set if you include B<-g>
415 option when C<Configure> asks you about optimizer/debugger flags.
417 If you're just trying to get a print out of each line of Perl code
418 as it executes, the way that C<sh -x> provides for shell scripts,
419 you can't use Perl's B<-D> switch. Instead do this
421 # If you have "env" utility
422 env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
424 # Bourne shell syntax
425 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
428 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
430 See L<perldebug> for details and variations.
432 =item B<-e> I<commandline>
434 may be used to enter one line of program. If B<-e> is given, Perl
435 will not look for a filename in the argument list. Multiple B<-e>
436 commands may be given to build up a multi-line script. Make sure
437 to use semicolons where you would in a normal program.
441 Disable executing F<$Config{siteperl}/sitecustomize.pl> at
444 Perl can be built so that it by default will try to execute
445 F<$Config{siteperl}/sitecustomize.pl> at startup. This is a hook that
446 allows the sysadmin to customize how perl behaves. It can for
447 instance be used to add entries to the @INC array to make perl find
448 modules in non-standard locations.
450 =item B<-F>I<pattern>
452 specifies the pattern to split on if B<-a> is also in effect. The
453 pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
454 put in single quotes.
458 prints a summary of the options.
460 =item B<-i>[I<extension>]
462 specifies that files processed by the C<E<lt>E<gt>> construct are to be
463 edited in-place. It does this by renaming the input file, opening the
464 output file by the original name, and selecting that output file as the
465 default for print() statements. The extension, if supplied, is used to
466 modify the name of the old file to make a backup copy, following these
469 If no extension is supplied, no backup is made and the current file is
472 If the extension doesn't contain a C<*>, then it is appended to the
473 end of the current filename as a suffix. If the extension does
474 contain one or more C<*> characters, then each C<*> is replaced
475 with the current filename. In Perl terms, you could think of this
478 ($backup = $extension) =~ s/\*/$file_name/g;
480 This allows you to add a prefix to the backup file, instead of (or in
481 addition to) a suffix:
483 $ perl -pi'orig_*' -e 's/bar/baz/' fileA # backup to 'orig_fileA'
485 Or even to place backup copies of the original files into another
486 directory (provided the directory already exists):
488 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'
490 These sets of one-liners are equivalent:
492 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
493 $ perl -pi'*' -e 's/bar/baz/' fileA # overwrite current file
495 $ perl -pi'.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
496 $ perl -pi'*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
498 From the shell, saying
500 $ perl -p -i.orig -e "s/foo/bar/; ... "
502 is the same as using the program:
504 #!/usr/bin/perl -pi.orig
507 which is equivalent to
510 $extension = '.orig';
512 if ($ARGV ne $oldargv) {
513 if ($extension !~ /\*/) {
514 $backup = $ARGV . $extension;
517 ($backup = $extension) =~ s/\*/$ARGV/g;
519 rename($ARGV, $backup);
520 open(ARGVOUT, ">$ARGV");
527 print; # this prints to original filename
531 except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
532 know when the filename has changed. It does, however, use ARGVOUT for
533 the selected filehandle. Note that STDOUT is restored as the default
534 output filehandle after the loop.
536 As shown above, Perl creates the backup file whether or not any output
537 is actually changed. So this is just a fancy way to copy files:
539 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
541 $ perl -p -i'.orig' -e 1 file1 file2 file3...
543 You can use C<eof> without parentheses to locate the end of each input
544 file, in case you want to append to each file, or reset line numbering
545 (see example in L<perlfunc/eof>).
547 If, for a given file, Perl is unable to create the backup file as
548 specified in the extension then it will skip that file and continue on
549 with the next one (if it exists).
551 For a discussion of issues surrounding file permissions and B<-i>,
552 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?>.
554 You cannot use B<-i> to create directories or to strip extensions from
557 Perl does not expand C<~> in filenames, which is good, since some
558 folks use it for their backup files:
560 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
562 Note that because B<-i> renames or deletes the original file before
563 creating a new file of the same name, UNIX-style soft and hard links will
566 Finally, the B<-i> switch does not impede execution when no
567 files are given on the command line. In this case, no backup is made
568 (the original file cannot, of course, be determined) and processing
569 proceeds from STDIN to STDOUT as might be expected.
571 =item B<-I>I<directory>
573 Directories specified by B<-I> are prepended to the search path for
574 modules (C<@INC>), and also tells the C preprocessor where to search for
575 include files. The C preprocessor is invoked with B<-P>; by default it
576 searches /usr/include and /usr/lib/perl.
578 =item B<-l>[I<octnum>]
580 enables automatic line-ending processing. It has two separate
581 effects. First, it automatically chomps C<$/> (the input record
582 separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
583 (the output record separator) to have the value of I<octnum> so
584 that any print statements will have that separator added back on.
585 If I<octnum> is omitted, sets C<$\> to the current value of
586 C<$/>. For instance, to trim lines to 80 columns:
588 perl -lpe 'substr($_, 80) = ""'
590 Note that the assignment C<$\ = $/> is done when the switch is processed,
591 so the input record separator can be different than the output record
592 separator if the B<-l> switch is followed by a B<-0> switch:
594 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
596 This sets C<$\> to newline and then sets C<$/> to the null character.
598 =item B<-m>[B<->]I<module>
600 =item B<-M>[B<->]I<module>
602 =item B<-M>[B<->]I<'module ...'>
604 =item B<-[mM]>[B<->]I<module=arg[,arg]...>
606 B<-m>I<module> executes C<use> I<module> C<();> before executing your
609 B<-M>I<module> executes C<use> I<module> C<;> before executing your
610 program. You can use quotes to add extra code after the module name,
611 e.g., C<'-Mmodule qw(foo bar)'>.
613 If the first character after the B<-M> or B<-m> is a dash (C<->)
614 then the 'use' is replaced with 'no'.
616 A little builtin syntactic sugar means you can also say
617 B<-mmodule=foo,bar> or B<-Mmodule=foo,bar> as a shortcut for
618 C<'-Mmodule qw(foo bar)'>. This avoids the need to use quotes when
619 importing symbols. The actual code generated by B<-Mmodule=foo,bar> is
620 C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
621 removes the distinction between B<-m> and B<-M>.
625 causes Perl to assume the following loop around your program, which
626 makes it iterate over filename arguments somewhat like B<sed -n> or
631 ... # your program goes here
634 Note that the lines are not printed by default. See B<-p> to have
635 lines printed. If a file named by an argument cannot be opened for
636 some reason, Perl warns you about it and moves on to the next file.
638 Here is an efficient way to delete all files that haven't been modified for
641 find . -mtime +7 -print | perl -nle unlink
643 This is faster than using the B<-exec> switch of B<find> because you don't
644 have to start a process on every filename found. It does suffer from
645 the bug of mishandling newlines in pathnames, which you can fix if
646 you follow the example under B<-0>.
648 C<BEGIN> and C<END> blocks may be used to capture control before or after
649 the implicit program loop, just as in B<awk>.
653 causes Perl to assume the following loop around your program, which
654 makes it iterate over filename arguments somewhat like B<sed>:
659 ... # your program goes here
661 print or die "-p destination: $!\n";
664 If a file named by an argument cannot be opened for some reason, Perl
665 warns you about it, and moves on to the next file. Note that the
666 lines are printed automatically. An error occurring during printing is
667 treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
668 overrides a B<-n> switch.
670 C<BEGIN> and C<END> blocks may be used to capture control before or after
671 the implicit loop, just as in B<awk>.
675 B<NOTE: Use of -P is strongly discouraged because of its inherent
676 problems, including poor portability.>
678 This option causes your program to be run through the C preprocessor before
679 compilation by Perl. Because both comments and B<cpp> directives begin
680 with the # character, you should avoid starting comments with any words
681 recognized by the C preprocessor such as C<"if">, C<"else">, or C<"define">.
683 If you're considering using C<-P>, you might also want to look at the
684 Filter::cpp module from CPAN.
686 The problems of -P include, but are not limited to:
692 The C<#!> line is stripped, so any switches there don't apply.
696 A C<-P> on a C<#!> line doesn't work.
700 B<All> lines that begin with (whitespace and) a C<#> but
701 do not look like cpp commands, are stripped, including anything
702 inside Perl strings, regular expressions, and here-docs .
706 In some platforms the C preprocessor knows too much: it knows about
707 the C++ -style until-end-of-line comments starting with C<"//">.
708 This will cause problems with common Perl constructs like
712 because after -P this will became illegal code
716 The workaround is to use some other quoting separator than C<"/">,
717 like for example C<"!">:
725 It requires not only a working C preprocessor but also a working
726 F<sed>. If not on UNIX, you are probably out of luck on this.
730 Script line numbers are not preserved.
734 The C<-x> does not work with C<-P>.
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<-->). This means you can have switches with two leading
743 dashes (B<--help>). Any switch found there is removed from @ARGV and sets the
744 corresponding variable in the Perl program. The following program
745 prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
746 if it is invoked with B<-xyz=abc>.
749 if ($xyz) { print "$xyz\n" }
751 Do note that B<--help> creates the variable ${-help}, which is not compliant
752 with C<strict refs>. Also, when using this option on a script with
753 warnings enabled you may get a lot of spurious "used only once" warnings.
757 makes Perl use the PATH environment variable to search for the
758 program (unless the name of the program contains directory separators).
760 On some platforms, this also makes Perl append suffixes to the
761 filename while searching for it. For example, on Win32 platforms,
762 the ".bat" and ".cmd" suffixes are appended if a lookup for the
763 original name fails, and if the name does not already end in one
764 of those suffixes. If your Perl was compiled with DEBUGGING turned
765 on, using the -Dp switch to Perl shows how the search progresses.
767 Typically this is used to emulate #! startup on platforms that don't
768 support #!. Its also convenient when debugging a script that uses #!,
769 and is thus normally found by the shell's $PATH search mechanism.
771 This example works on many platforms that have a shell compatible with
775 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
776 if $running_under_some_shell;
778 The system ignores the first line and feeds the program to F</bin/sh>,
779 which proceeds to try to execute the Perl program as a shell script.
780 The shell executes the second line as a normal shell command, and thus
781 starts up the Perl interpreter. On some systems $0 doesn't always
782 contain the full pathname, so the B<-S> tells Perl to search for the
783 program if necessary. After Perl locates the program, it parses the
784 lines and ignores them because the variable $running_under_some_shell
785 is never true. If the program will be interpreted by csh, you will need
786 to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
787 embedded spaces (and such) in the argument list. To start up sh rather
788 than csh, some systems may have to replace the #! line with a line
789 containing just a colon, which will be politely ignored by Perl. Other
790 systems can't control that, and need a totally devious construct that
791 will work under any of B<csh>, B<sh>, or Perl, such as the following:
793 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
794 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
795 if $running_under_some_shell;
797 If the filename supplied contains directory separators (i.e., is an
798 absolute or relative pathname), and if that file is not found,
799 platforms that append file extensions will do so and try to look
800 for the file with those extensions added, one by one.
802 On DOS-like platforms, if the program does not contain directory
803 separators, it will first be searched for in the current directory
804 before being searched for on the PATH. On Unix platforms, the
805 program will be searched for strictly on the PATH.
809 Like B<-T>, but taint checks will issue warnings rather than fatal
810 errors. These warnings can be controlled normally with C<no warnings
813 B<NOTE: this is not a substitute for -T.> This is meant only to be
814 used as a temporary development aid while securing legacy code:
815 for real production code and for new secure code written from scratch
816 always use the real B<-T>.
820 forces "taint" checks to be turned on so you can test them. Ordinarily
821 these checks are done only when running setuid or setgid. It's a
822 good idea to turn them on explicitly for programs that run on behalf
823 of someone else whom you might not necessarily trust, such as CGI
824 programs or any internet servers you might write in Perl. See
825 L<perlsec> for details. For security reasons, this option must be
826 seen by Perl quite early; usually this means it must appear early
827 on the command line or in the #! line for systems which support
832 This obsolete switch causes Perl to dump core after compiling your
833 program. You can then in theory take this core dump and turn it
834 into an executable file by using the B<undump> program (not supplied).
835 This speeds startup at the expense of some disk space (which you
836 can minimize by stripping the executable). (Still, a "hello world"
837 executable comes out to about 200K on my machine.) If you want to
838 execute a portion of your program before dumping, use the dump()
839 operator instead. Note: availability of B<undump> is platform
840 specific and may not be available for a specific port of Perl.
842 This switch has been superseded in favor of the new Perl code
843 generator backends to the compiler. See L<B> and L<B::Bytecode>
848 allows Perl to do unsafe operations. Currently the only "unsafe"
849 operations are the unlinking of directories while running as superuser,
850 and running setuid programs with fatal taint checks turned into
851 warnings. Note that the B<-w> switch (or the C<$^W> variable) must
852 be used along with this option to actually I<generate> the
853 taint-check warnings.
857 prints the version and patchlevel of your perl executable.
861 prints summary of the major perl configuration values and the current
864 =item B<-V:>I<configvar>
866 Prints to STDOUT the value of the named configuration variable(s),
867 with multiples when your configvar argument looks like a regex (has
868 non-letters). For example:
871 libc='/lib/libc-2.2.4.so';
873 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
874 libc='/lib/libc-2.2.4.so';
876 libpth='/usr/local/lib /lib /usr/lib';
877 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
879 libc='/lib/libc-2.2.4.so';
883 Additionally, extra colons can be used to control formatting. A
884 trailing colon suppresses the linefeed and terminator ';', allowing
885 you to embed queries into shell commands. (mnemonic: PATH separator
888 $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
889 compression-vars: zcat='' zip='zip' are here !
891 A leading colon removes the 'name=' part of the response, this allows
892 you to map to the name you need. (mnemonic: empty label)
894 $ echo "goodvfork="`./perl -Ilib -V::usevfork`
897 Leading and trailing colons can be used together if you need
898 positional parameter values without the names. Note that in the case
899 below, the PERL_API params are returned in alphabetical order.
901 $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
902 building_on 'linux' '5' '1' '9' now
906 prints warnings about dubious constructs, such as variable names
907 that are mentioned only once and scalar variables that are used
908 before being set, redefined subroutines, references to undefined
909 filehandles or filehandles opened read-only that you are attempting
910 to write on, values used as a number that don't look like numbers,
911 using an array as though it were a scalar, if your subroutines
912 recurse more than 100 deep, and innumerable other things.
914 This switch really just enables the internal C<$^W> variable. You
915 can disable or promote into fatal errors specific warnings using
916 C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
917 See also L<perldiag> and L<perltrap>. A new, fine-grained warning
918 facility is also available if you want to manipulate entire classes
919 of warnings; see L<warnings> or L<perllexwarn>.
923 Enables all warnings regardless of C<no warnings> or C<$^W>.
928 Disables all warnings regardless of C<use warnings> or C<$^W>.
933 =item B<-x> I<directory>
935 tells Perl that the program is embedded in a larger chunk of unrelated
936 ASCII text, such as in a mail message. Leading garbage will be
937 discarded until the first line that starts with #! and contains the
938 string "perl". Any meaningful switches on that line will be applied.
939 If a directory name is specified, Perl will switch to that directory
940 before running the program. The B<-x> switch controls only the
941 disposal of leading garbage. The program must be terminated with
942 C<__END__> if there is trailing garbage to be ignored (the program
943 can process any or all of the trailing garbage via the DATA filehandle
954 Used if chdir has no argument.
958 Used if chdir has no argument and HOME is not set.
962 Used in executing subprocesses, and in finding the program if B<-S> is
967 A list of directories in which to look for Perl library
968 files before looking in the standard library and the current
969 directory. Any architecture-specific directories under the specified
970 locations are automatically included if they exist. If PERL5LIB is not
971 defined, PERLLIB is used. Directories are separated (like in PATH) by
972 a colon on unixish platforms and by a semicolon on Windows (the proper
973 path separator being given by the command C<perl -V:path_sep>).
975 When running taint checks (either because the program was running setuid
976 or setgid, or the B<-T> switch was used), neither variable is used.
977 The program should instead say:
979 use lib "/my/directory";
983 Command-line options (switches). Switches in this variable are taken
984 as if they were on every Perl command line. Only the B<-[DIMUdmtw]>
985 switches are allowed. When running taint checks (because the program
986 was running setuid or setgid, or the B<-T> switch was used), this
987 variable is ignored. If PERL5OPT begins with B<-T>, tainting will be
988 enabled, and any subsequent options ignored.
992 A space (or colon) separated list of PerlIO layers. If perl is built
993 to use PerlIO system for IO (the default) these layers effect perl's IO.
995 It is conventional to start layer names with a colon e.g. C<:perlio> to
996 emphasise their similarity to variable "attributes". But the code that parses
997 layer specification strings (which is also used to decode the PERLIO
998 environment variable) treats the colon as a separator.
1000 An unset or empty PERLIO is equivalent to C<:stdio>.
1002 The list becomes the default for I<all> perl's IO. Consequently only built-in
1003 layers can appear in this list, as external layers (such as :encoding()) need
1004 IO in order to load them!. See L<"open pragma"|open> for how to add external
1005 encodings as defaults.
1007 The layers that it makes sense to include in the PERLIO environment
1008 variable are briefly summarised below. For more details see L<PerlIO>.
1014 A pseudolayer that turns I<off> the C<:utf8> flag for the layer below.
1015 Unlikely to be useful on its own in the global PERLIO environment variable.
1016 You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
1020 A layer which does CRLF to "\n" translation distinguishing "text" and
1021 "binary" files in the manner of MS-DOS and similar operating systems.
1022 (It currently does I<not> mimic MS-DOS as far as treating of Control-Z
1023 as being an end-of-file marker.)
1027 A layer which implements "reading" of files by using C<mmap()> to
1028 make (whole) file appear in the process's address space, and then
1029 using that as PerlIO's "buffer".
1033 This is a re-implementation of "stdio-like" buffering written as a
1034 PerlIO "layer". As such it will call whatever layer is below it for
1035 its operations (typically C<:unix>).
1039 An experimental pseudolayer that removes the topmost layer.
1040 Use with the same care as is reserved for nitroglycerin.
1044 A pseudolayer that manipulates other layers. Applying the C<:raw>
1045 layer is equivalent to calling C<binmode($fh)>. It makes the stream
1046 pass each byte as-is without any translation. In particular CRLF
1047 translation, and/or :utf8 intuited from locale are disabled.
1049 Unlike in the earlier versions of Perl C<:raw> is I<not>
1050 just the inverse of C<:crlf> - other layers which would affect the
1051 binary nature of the stream are also removed or disabled.
1055 This layer provides PerlIO interface by wrapping system's ANSI C "stdio"
1056 library calls. The layer provides both buffering and IO.
1057 Note that C<:stdio> layer does I<not> do CRLF translation even if that
1058 is platforms normal behaviour. You will need a C<:crlf> layer above it
1063 Low level layer which calls C<read>, C<write> and C<lseek> etc.
1067 A pseudolayer that turns on a flag on the layer below to tell perl
1068 that output should be in utf8 and that input should be regarded as
1069 already in utf8 form. May be useful in PERLIO environment
1070 variable to make UTF-8 the default. (To turn off that behaviour
1071 use C<:bytes> layer.)
1075 On Win32 platforms this I<experimental> layer uses native "handle" IO
1076 rather than unix-like numeric file descriptor layer. Known to be
1077 buggy in this release.
1081 On all platforms the default set of layers should give acceptable results.
1083 For UNIX platforms that will equivalent of "unix perlio" or "stdio".
1084 Configure is setup to prefer "stdio" implementation if system's library
1085 provides for fast access to the buffer, otherwise it uses the "unix perlio"
1088 On Win32 the default in this release is "unix crlf". Win32's "stdio"
1089 has a number of bugs/mis-features for perl IO which are somewhat
1090 C compiler vendor/version dependent. Using our own C<crlf> layer as
1091 the buffer avoids those issues and makes things more uniform.
1092 The C<crlf> layer provides CRLF to/from "\n" conversion as well as
1095 This release uses C<unix> as the bottom layer on Win32 and so still uses C
1096 compiler's numeric file descriptor routines. There is an experimental native
1097 C<win32> layer which is expected to be enhanced and should eventually be
1098 the default under Win32.
1102 If set to the name of a file or device then certain operations of PerlIO
1103 sub-system will be logged to that file (opened as append). Typical uses
1106 PERLIO_DEBUG=/dev/tty perl script ...
1108 and Win32 approximate equivalent:
1110 set PERLIO_DEBUG=CON
1113 This functionality is disabled for setuid scripts and for scripts run
1118 A list of directories in which to look for Perl library
1119 files before looking in the standard library and the current directory.
1120 If PERL5LIB is defined, PERLLIB is not used.
1124 The command used to load the debugger code. The default is:
1126 BEGIN { require 'perl5db.pl' }
1128 =item PERL5DB_THREADED
1130 If set to a true value, indicates to the debugger that the code being
1131 debugged uses threads.
1133 =item PERL5SHELL (specific to the Win32 port)
1135 May be set to an alternative shell that perl must use internally for
1136 executing "backtick" commands or system(). Default is C<cmd.exe /x/d/c>
1137 on WindowsNT and C<command.com /c> on Windows95. The value is considered
1138 to be space-separated. Precede any character that needs to be protected
1139 (like a space or backslash) with a backslash.
1141 Note that Perl doesn't use COMSPEC for this purpose because
1142 COMSPEC has a high degree of variability among users, leading to
1143 portability concerns. Besides, perl can use a shell that may not be
1144 fit for interactive use, and setting COMSPEC to such a shell may
1145 interfere with the proper functioning of other programs (which usually
1146 look in COMSPEC to find a shell fit for interactive use).
1148 =item PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
1150 Set to 1 to allow the use of non-IFS compatible LSP's.
1151 Perl normally searches for an IFS-compatible LSP because this is required
1152 for its emulation of Windows sockets as real filehandles. However, this may
1153 cause problems if you have a firewall such as McAfee Guardian which requires
1154 all applications to use its LSP which is not IFS-compatible, because clearly
1155 Perl will normally avoid using such an LSP.
1156 Setting this environment variable to 1 means that Perl will simply use the
1157 first suitable LSP enumerated in the catalog, which keeps McAfee Guardian
1158 happy (and in that particular case Perl still works too because McAfee
1159 Guardian's LSP actually plays some other games which allow applications
1160 requiring IFS compatibility to work).
1162 =item PERL_DEBUG_MSTATS
1164 Relevant only if perl is compiled with the malloc included with the perl
1165 distribution (that is, if C<perl -V:d_mymalloc> is 'define').
1166 If set, this causes memory statistics to be dumped after execution. If set
1167 to an integer greater than one, also causes memory statistics to be dumped
1170 =item PERL_DESTRUCT_LEVEL
1172 Relevant only if your perl executable was built with B<-DDEBUGGING>,
1173 this controls the behavior of global destruction of objects and other
1174 references. See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.
1176 =item PERL_DL_NONLAZY
1178 Set to one to have perl resolve B<all> undefined symbols when it loads
1179 a dynamic library. The default behaviour is to resolve symbols when
1180 they are used. Setting this variable is useful during testing of
1181 extensions as it ensures that you get an error on misspelled function
1182 names even if the test suite doesn't call it.
1186 If using the C<encoding> pragma without an explicit encoding name, the
1187 PERL_ENCODING environment variable is consulted for an encoding name.
1189 =item PERL_HASH_SEED
1191 (Since Perl 5.8.1.) Used to randomise Perl's internal hash function.
1192 To emulate the pre-5.8.1 behaviour, set to an integer (zero means
1193 exactly the same order as 5.8.0). "Pre-5.8.1" means, among other
1194 things, that hash keys will be ordered the same between different runs
1197 The default behaviour is to randomise unless the PERL_HASH_SEED is set.
1198 If Perl has been compiled with C<-DUSE_HASH_SEED_EXPLICIT>, the default
1199 behaviour is B<not> to randomise unless the PERL_HASH_SEED is set.
1201 If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses
1202 the pseudorandom seed supplied by the operating system and libraries.
1203 This means that each different run of Perl will have a different
1204 ordering of the results of keys(), values(), and each().
1206 B<Please note that the hash seed is sensitive information>. Hashes are
1207 randomized to protect against local and remote attacks against Perl
1208 code. By manually setting a seed this protection may be partially or
1211 See L<perlsec/"Algorithmic Complexity Attacks"> and
1212 L</PERL_HASH_SEED_DEBUG> for more information.
1214 =item PERL_HASH_SEED_DEBUG
1216 (Since Perl 5.8.1.) Set to one to display (to STDERR) the value of
1217 the hash seed at the beginning of execution. This, combined with
1218 L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
1219 behavior caused by hash randomization.
1221 B<Note that the hash seed is sensitive information>: by knowing it one
1222 can craft a denial-of-service attack against Perl code, even remotely,
1223 see L<perlsec/"Algorithmic Complexity Attacks"> for more information.
1224 B<Do not disclose the hash seed> to people who don't need to know it.
1225 See also hash_seed() of L<Hash::Util>.
1227 =item PERL_ROOT (specific to the VMS port)
1229 A translation concealed rooted logical name that contains perl and the
1230 logical device for the @INC path on VMS only. Other logical names that
1231 affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1232 SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in
1233 L<perlvms> and in F<README.vms> in the Perl source distribution.
1237 In Perls 5.8.1 and later. If set to C<unsafe> the pre-Perl-5.8.0
1238 signals behaviour (immediate but unsafe) is restored. If set to
1239 C<safe> the safe (or deferred) signals are used.
1240 See L<perlipc/"Deferred Signals (Safe Signals)">.
1244 Equivalent to the B<-C> command-line switch. Note that this is not
1245 a boolean variable-- setting this to C<"1"> is not the right way to
1246 "enable Unicode" (whatever that would mean). You can use C<"0"> to
1247 "disable Unicode", though (or alternatively unset PERL_UNICODE in
1248 your shell before starting Perl). See the description of the C<-C>
1249 switch for more information.
1251 =item SYS$LOGIN (specific to the VMS port)
1253 Used if chdir has no argument and HOME and LOGDIR are not set.
1257 Perl also has environment variables that control how Perl handles data
1258 specific to particular natural languages. See L<perllocale>.
1260 Apart from these, Perl uses no other environment variables, except
1261 to make them available to the program being executed, and to child
1262 processes. However, programs running setuid would do well to execute
1263 the following lines before doing anything else, just to keep people
1266 $ENV{PATH} = '/bin:/usr/bin'; # or whatever you need
1267 $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
1268 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};