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 #! notation invoke interpreters this
37 way. See L<Location of Perl>.)
41 Passed in implicitly via standard input. This works only if there are
42 no filename arguments--to pass arguments to a STDIN-read program you
43 must explicitly specify a "-" for the program name.
47 With methods 2 and 3, Perl starts parsing the input file from the
48 beginning, unless you've specified a B<-x> switch, in which case it
49 scans for the first line starting with #! and containing the word
50 "perl", and starts there instead. This is useful for running a program
51 embedded in a larger message. (In this case you would indicate the end
52 of the program using the C<__END__> token.)
54 The #! line is always examined for switches as the line is being
55 parsed. Thus, if you're on a machine that allows only one argument
56 with the #! line, or worse, doesn't even recognize the #! line, you
57 still can get consistent switch behavior regardless of how Perl was
58 invoked, even if B<-x> was used to find the beginning of the program.
60 Because historically some operating systems silently chopped off
61 kernel interpretation of the #! line after 32 characters, some
62 switches may be passed in on the command line, and some may not;
63 you could even get a "-" without its letter, if you're not careful.
64 You probably want to make sure that all your switches fall either
65 before or after that 32-character boundary. Most switches don't
66 actually care if they're processed redundantly, but getting a "-"
67 instead of a complete switch could cause Perl to try to execute
68 standard input instead of your program. And a partial B<-I> switch
69 could also cause odd results.
71 Some switches do care if they are processed twice, for instance
72 combinations of B<-l> and B<-0>. Either put all the switches after
73 the 32-character boundary (if applicable), or replace the use of
74 B<-0>I<digits> by C<BEGIN{ $/ = "\0digits"; }>.
76 Parsing of the #! switches starts wherever "perl" is mentioned in the line.
77 The sequences "-*" and "- " are specifically ignored so that you could,
78 if you were so inclined, say
82 eval 'exec perl -x -wS $0 ${1+"$@"}'
85 to let Perl see the B<-p> switch.
87 A similar trick involves the B<env> program, if you have it.
91 The examples above use a relative path to the perl interpreter,
92 getting whatever version is first in the user's path. If you want
93 a specific version of Perl, say, perl5.005_57, you should place
94 that directly in the #! line's path.
96 If the #! line does not contain the word "perl", the program named after
97 the #! is executed instead of the Perl interpreter. This is slightly
98 bizarre, but it helps people on machines that don't do #!, because they
99 can tell a program that their SHELL is F</usr/bin/perl>, and Perl will then
100 dispatch the program to the correct interpreter for them.
102 After locating your program, Perl compiles the entire program to an
103 internal form. If there are any compilation errors, execution of the
104 program is not attempted. (This is unlike the typical shell script,
105 which might run part-way through before finding a syntax error.)
107 If the program is syntactically correct, it is executed. If the program
108 runs off the end without hitting an exit() or die() operator, an implicit
109 C<exit(0)> is provided to indicate successful completion.
111 =head2 #! and quoting on non-Unix systems
114 Unix's #! 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
209 X<perl, location of interpreter>
211 It may seem obvious to say, but Perl is useful only when users can
212 easily find it. When possible, it's good for both F</usr/bin/perl>
213 and F</usr/local/bin/perl> to be symlinks to the actual binary. If
214 that can't be done, system administrators are strongly encouraged
215 to put (symlinks to) perl and its accompanying utilities into a
216 directory typically found along a user's PATH, or in some other
217 obvious and convenient place.
219 In this documentation, C<#!/usr/bin/perl> on the first line of the program
220 will stand in for whatever method works on your system. You are
221 advised to use a specific path if you care about a specific version.
223 #!/usr/local/bin/perl5.00554
225 or if you just want to be running at least version, place a statement
226 like this at the top of your program:
230 =head2 Command Switches
231 X<perl, command switches> X<command switches>
233 As with all standard commands, a single-character switch may be
234 clustered with the following switch, if any.
236 #!/usr/bin/perl -spi.orig # same as -s -p -i.orig
242 =item B<-0>[I<octal/hexadecimal>]
245 specifies the input record separator (C<$/>) as an octal or
246 hexadecimal number. If there are no digits, the null character is the
247 separator. Other switches may precede or follow the digits. For
248 example, if you have a version of B<find> which can print filenames
249 terminated by the null character, you can say this:
251 find . -name '*.orig' -print0 | perl -n0e unlink
253 The special value 00 will cause Perl to slurp files in paragraph mode.
254 The value 0777 will cause Perl to slurp files whole because there is no
255 legal byte with that value.
257 If you want to specify any Unicode character, use the hexadecimal
258 format: C<-0xHHH...>, where the C<H> are valid hexadecimal digits.
259 (This means that you cannot use the C<-x> with a directory name that
260 consists of hexadecimal digits.)
265 turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
266 split command to the @F array is done as the first thing inside the
267 implicit while loop produced by the B<-n> or B<-p>.
269 perl -ane 'print pop(@F), "\n";'
278 An alternate delimiter may be specified using B<-F>.
280 =item B<-C [I<number/list>]>
283 The C<-C> flag controls some of the Perl Unicode features.
285 As of 5.8.1, the C<-C> can be followed either by a number or a list
286 of option letters. The letters, their numeric values, and effects
287 are as follows; listing the letters is equal to summing the numbers.
289 I 1 STDIN is assumed to be in UTF-8
290 O 2 STDOUT will be in UTF-8
291 E 4 STDERR will be in UTF-8
293 i 8 UTF-8 is the default PerlIO layer for input streams
294 o 16 UTF-8 is the default PerlIO layer for output streams
296 A 32 the @ARGV elements are expected to be strings encoded
298 L 64 normally the "IOEioA" are unconditional,
299 the L makes them conditional on the locale environment
300 variables (the LC_ALL, LC_TYPE, and LANG, in the order
301 of decreasing precedence) -- if the variables indicate
302 UTF-8, then the selected "IOEioA" are in effect
303 a 256 Set ${^UTF8CACHE} to -1, to run the UTF-8 caching code in
306 =for documenting_the_underdocumented
307 perl.h gives W/128 as PERL_UNICODE_WIDESYSCALLS "/* for Sarathy */"
310 perltodo mentions Unicode in %ENV and filenames. I guess that these will be
311 options e and f (or F).
313 For example, C<-COE> and C<-C6> will both turn on UTF-8-ness on both
314 STDOUT and STDERR. Repeating letters is just redundant, not cumulative
317 The C<io> options mean that any subsequent open() (or similar I/O
318 operations) will have the C<:utf8> PerlIO layer implicitly applied
319 to them, in other words, UTF-8 is expected from any input stream,
320 and UTF-8 is produced to any output stream. This is just the default,
321 with explicit layers in open() and with binmode() one can manipulate
324 C<-C> on its own (not followed by any number or option list), or the
325 empty string C<""> for the C<PERL_UNICODE> environment variable, has the
326 same effect as C<-CSDL>. In other words, the standard I/O handles and
327 the default C<open()> layer are UTF-8-fied B<but> only if the locale
328 environment variables indicate a UTF-8 locale. This behaviour follows
329 the I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
331 You can use C<-C0> (or C<"0"> for C<PERL_UNICODE>) to explicitly
332 disable all the above Unicode features.
334 The read-only magic variable C<${^UNICODE}> reflects the numeric value
335 of this setting. This is variable is set during Perl startup and is
336 thereafter read-only. If you want runtime effects, use the three-arg
337 open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
338 and the C<open> pragma (see L<open>).
340 (In Perls earlier than 5.8.1 the C<-C> switch was a Win32-only switch
341 that enabled the use of Unicode-aware "wide system call" Win32 APIs.
342 This feature was practically unused, however, and the command line
343 switch was therefore "recycled".)
345 B<Note:> Since perl 5.10.1, if the -C option is used on the #! line, it
346 must be specified on the command line as well, since the standard streams
347 are already set up at this point in the execution of the perl interpreter.
348 You can also use binmode() to set the encoding of an I/O stream.
353 causes Perl to check the syntax of the program and then exit without
354 executing it. Actually, it I<will> execute C<BEGIN>, C<UNITCHECK>,
355 C<CHECK>, and C<use> blocks, because these are considered as occurring
356 outside the execution of your program. C<INIT> and C<END> blocks,
357 however, will be skipped.
364 runs the program under the Perl debugger. See L<perldebug>.
365 If B<t> is specified, it indicates to the debugger that threads
366 will be used in the code being debugged.
368 =item B<-d:>I<foo[=bar,baz]>
371 =item B<-dt:>I<foo[=bar,baz]>
373 runs the program under the control of a debugging, profiling, or
374 tracing module installed as Devel::foo. E.g., B<-d:DProf> executes
375 the program using the Devel::DProf profiler. As with the B<-M>
376 flag, options may be passed to the Devel::foo package where they
377 will be received and interpreted by the Devel::foo::import routine.
378 The comma-separated list of options must follow a C<=> character.
379 If B<t> is specified, it indicates to the debugger that threads
380 will be used in the code being debugged.
383 =item B<-D>I<letters>
384 X<-D> X<DEBUGGING> X<-DDEBUGGING>
388 sets debugging flags. To watch how it executes your program, use
389 B<-Dtls>. (This works only if debugging is compiled into your
390 Perl.) Another nice value is B<-Dx>, which lists your compiled
391 syntax tree. And B<-Dr> displays compiled regular expressions;
392 the format of the output is explained in L<perldebguts>.
394 As an alternative, specify a number instead of list of letters (e.g.,
395 B<-D14> is equivalent to B<-Dtls>):
397 1 p Tokenizing and parsing (with v, displays parse stack)
398 2 s Stack snapshots (with v, displays all stacks)
399 4 l Context (loop) stack processing
401 16 o Method and overloading resolution
402 32 c String/numeric conversions
403 64 P Print profiling info, source file input state
404 128 m Memory and SV allocation
405 256 f Format processing
406 512 r Regular expression parsing and execution
407 1024 x Syntax tree dump
408 2048 u Tainting checks
409 4096 U Unofficial, User hacking (reserved for private, unreleased use)
410 8192 H Hash dump -- usurps values()
411 16384 X Scratchpad allocation
414 262144 R Include reference counts of dumped variables (eg when using -Ds)
415 524288 J Do not s,t,P-debug (Jump over) opcodes within package DB
416 1048576 v Verbose: use in conjunction with other flags
417 2097152 C Copy On Write
418 4194304 A Consistency checks on internal structures
419 8388608 q quiet - currently only suppresses the "EXECUTING" message
420 16777216 M trace smart match resolution
421 33554432 B dump suBroutine definitions, including special Blocks like BEGIN
423 All these flags require B<-DDEBUGGING> when you compile the Perl
424 executable (but see L<Devel::Peek>, L<re> which may change this).
425 See the F<INSTALL> file in the Perl source distribution
426 for how to do this. This flag is automatically set if you include B<-g>
427 option when C<Configure> asks you about optimizer/debugger flags.
429 If you're just trying to get a print out of each line of Perl code
430 as it executes, the way that C<sh -x> provides for shell scripts,
431 you can't use Perl's B<-D> switch. Instead do this
433 # If you have "env" utility
434 env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
436 # Bourne shell syntax
437 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
440 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
442 See L<perldebug> for details and variations.
444 =item B<-e> I<commandline>
447 may be used to enter one line of program. If B<-e> is given, Perl
448 will not look for a filename in the argument list. Multiple B<-e>
449 commands may be given to build up a multi-line script. Make sure
450 to use semicolons where you would in a normal program.
452 =item B<-E> I<commandline>
455 behaves just like B<-e>, except that it implicitly enables all
456 optional features (in the main compilation unit). See L<feature>.
461 Disable executing F<$Config{sitelib}/sitecustomize.pl> at startup.
463 Perl can be built so that it by default will try to execute
464 F<$Config{sitelib}/sitecustomize.pl> at startup (in a BEGIN block).
465 This is a hook that allows the sysadmin to customize how perl behaves.
466 It can for instance be used to add entries to the @INC array to make perl
467 find modules in non-standard locations.
469 =item B<-F>I<pattern>
472 specifies the pattern to split on if B<-a> is also in effect. The
473 pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
474 put in single quotes. You can't use literal whitespace in the pattern.
479 prints a summary of the options.
481 =item B<-i>[I<extension>]
484 specifies that files processed by the C<E<lt>E<gt>> construct are to be
485 edited in-place. It does this by renaming the input file, opening the
486 output file by the original name, and selecting that output file as the
487 default for print() statements. The extension, if supplied, is used to
488 modify the name of the old file to make a backup copy, following these
491 If no extension is supplied, no backup is made and the current file is
494 If the extension doesn't contain a C<*>, then it is appended to the
495 end of the current filename as a suffix. If the extension does
496 contain one or more C<*> characters, then each C<*> is replaced
497 with the current filename. In Perl terms, you could think of this
500 ($backup = $extension) =~ s/\*/$file_name/g;
502 This allows you to add a prefix to the backup file, instead of (or in
503 addition to) a suffix:
505 $ perl -pi'orig_*' -e 's/bar/baz/' fileA # backup to 'orig_fileA'
507 Or even to place backup copies of the original files into another
508 directory (provided the directory already exists):
510 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'
512 These sets of one-liners are equivalent:
514 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
515 $ perl -pi'*' -e 's/bar/baz/' fileA # overwrite current file
517 $ perl -pi'.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
518 $ perl -pi'*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
520 From the shell, saying
522 $ perl -p -i.orig -e "s/foo/bar/; ... "
524 is the same as using the program:
526 #!/usr/bin/perl -pi.orig
529 which is equivalent to
532 $extension = '.orig';
534 if ($ARGV ne $oldargv) {
535 if ($extension !~ /\*/) {
536 $backup = $ARGV . $extension;
539 ($backup = $extension) =~ s/\*/$ARGV/g;
541 rename($ARGV, $backup);
542 open(ARGVOUT, ">$ARGV");
549 print; # this prints to original filename
553 except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
554 know when the filename has changed. It does, however, use ARGVOUT for
555 the selected filehandle. Note that STDOUT is restored as the default
556 output filehandle after the loop.
558 As shown above, Perl creates the backup file whether or not any output
559 is actually changed. So this is just a fancy way to copy files:
561 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
563 $ perl -p -i'.orig' -e 1 file1 file2 file3...
565 You can use C<eof> without parentheses to locate the end of each input
566 file, in case you want to append to each file, or reset line numbering
567 (see example in L<perlfunc/eof>).
569 If, for a given file, Perl is unable to create the backup file as
570 specified in the extension then it will skip that file and continue on
571 with the next one (if it exists).
573 For a discussion of issues surrounding file permissions and B<-i>,
574 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?>.
576 You cannot use B<-i> to create directories or to strip extensions from
579 Perl does not expand C<~> in filenames, which is good, since some
580 folks use it for their backup files:
582 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
584 Note that because B<-i> renames or deletes the original file before
585 creating a new file of the same name, UNIX-style soft and hard links will
588 Finally, the B<-i> switch does not impede execution when no
589 files are given on the command line. In this case, no backup is made
590 (the original file cannot, of course, be determined) and processing
591 proceeds from STDIN to STDOUT as might be expected.
593 =item B<-I>I<directory>
596 Directories specified by B<-I> are prepended to the search path for
599 =item B<-l>[I<octnum>]
602 enables automatic line-ending processing. It has two separate
603 effects. First, it automatically chomps C<$/> (the input record
604 separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
605 (the output record separator) to have the value of I<octnum> so
606 that any print statements will have that separator added back on.
607 If I<octnum> is omitted, sets C<$\> to the current value of
608 C<$/>. For instance, to trim lines to 80 columns:
610 perl -lpe 'substr($_, 80) = ""'
612 Note that the assignment C<$\ = $/> is done when the switch is processed,
613 so the input record separator can be different than the output record
614 separator if the B<-l> switch is followed by a B<-0> switch:
616 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
618 This sets C<$\> to newline and then sets C<$/> to the null character.
620 =item B<-m>[B<->]I<module>
623 =item B<-M>[B<->]I<module>
625 =item B<-M>[B<->]I<'module ...'>
627 =item B<-[mM]>[B<->]I<module=arg[,arg]...>
629 B<-m>I<module> executes C<use> I<module> C<();> before executing your
632 B<-M>I<module> executes C<use> I<module> C<;> before executing your
633 program. You can use quotes to add extra code after the module name,
634 e.g., C<'-Mmodule qw(foo bar)'>.
636 If the first character after the B<-M> or B<-m> is a dash (C<->)
637 then the 'use' is replaced with 'no'.
639 A little builtin syntactic sugar means you can also say
640 B<-mmodule=foo,bar> or B<-Mmodule=foo,bar> as a shortcut for
641 C<'-Mmodule qw(foo bar)'>. This avoids the need to use quotes when
642 importing symbols. The actual code generated by B<-Mmodule=foo,bar> is
643 C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
644 removes the distinction between B<-m> and B<-M>.
646 A consequence of this is that B<-MFoo=number> never does a version check
647 (unless C<Foo::import()> itself is set up to do a version check, which
648 could happen for example if Foo inherits from Exporter.)
653 causes Perl to assume the following loop around your program, which
654 makes it iterate over filename arguments somewhat like B<sed -n> or
659 ... # your program goes here
662 Note that the lines are not printed by default. See B<-p> to have
663 lines printed. If a file named by an argument cannot be opened for
664 some reason, Perl warns you about it and moves on to the next file.
666 Also note that C<< <> >> passes command line arguments to
667 L<perlfunc/open>, which doesn't necessarily interpret them as file names.
668 See L<perlop> for possible security implications.
670 Here is an efficient way to delete all files that haven't been modified for
673 find . -mtime +7 -print | perl -nle unlink
675 This is faster than using the B<-exec> switch of B<find> because you don't
676 have to start a process on every filename found. It does suffer from
677 the bug of mishandling newlines in pathnames, which you can fix if
678 you follow the example under B<-0>.
680 C<BEGIN> and C<END> blocks may be used to capture control before or after
681 the implicit program loop, just as in B<awk>.
686 causes Perl to assume the following loop around your program, which
687 makes it iterate over filename arguments somewhat like B<sed>:
692 ... # your program goes here
694 print or die "-p destination: $!\n";
697 If a file named by an argument cannot be opened for some reason, Perl
698 warns you about it, and moves on to the next file. Note that the
699 lines are printed automatically. An error occurring during printing is
700 treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
701 overrides a B<-n> switch.
703 C<BEGIN> and C<END> blocks may be used to capture control before or after
704 the implicit loop, just as in B<awk>.
709 enables rudimentary switch parsing for switches on the command
710 line after the program name but before any filename arguments (or before
711 an argument of B<-->). Any switch found there is removed from @ARGV and sets the
712 corresponding variable in the Perl program. The following program
713 prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
714 if it is invoked with B<-xyz=abc>.
717 if ($xyz) { print "$xyz\n" }
719 Do note that a switch like B<--help> creates the variable ${-help}, which is not compliant
720 with C<strict refs>. Also, when using this option on a script with
721 warnings enabled you may get a lot of spurious "used only once" warnings.
726 makes Perl use the PATH environment variable to search for the
727 program (unless the name of the program contains directory separators).
729 On some platforms, this also makes Perl append suffixes to the
730 filename while searching for it. For example, on Win32 platforms,
731 the ".bat" and ".cmd" suffixes are appended if a lookup for the
732 original name fails, and if the name does not already end in one
733 of those suffixes. If your Perl was compiled with DEBUGGING turned
734 on, using the -Dp switch to Perl shows how the search progresses.
736 Typically this is used to emulate #! startup on platforms that don't
737 support #!. Its also convenient when debugging a script that uses #!,
738 and is thus normally found by the shell's $PATH search mechanism.
740 This example works on many platforms that have a shell compatible with
744 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
745 if $running_under_some_shell;
747 The system ignores the first line and feeds the program to F</bin/sh>,
748 which proceeds to try to execute the Perl program as a shell script.
749 The shell executes the second line as a normal shell command, and thus
750 starts up the Perl interpreter. On some systems $0 doesn't always
751 contain the full pathname, so the B<-S> tells Perl to search for the
752 program if necessary. After Perl locates the program, it parses the
753 lines and ignores them because the variable $running_under_some_shell
754 is never true. If the program will be interpreted by csh, you will need
755 to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
756 embedded spaces (and such) in the argument list. To start up sh rather
757 than csh, some systems may have to replace the #! line with a line
758 containing just a colon, which will be politely ignored by Perl. Other
759 systems can't control that, and need a totally devious construct that
760 will work under any of B<csh>, B<sh>, or Perl, such as the following:
762 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
763 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
764 if $running_under_some_shell;
766 If the filename supplied contains directory separators (i.e., is an
767 absolute or relative pathname), and if that file is not found,
768 platforms that append file extensions will do so and try to look
769 for the file with those extensions added, one by one.
771 On DOS-like platforms, if the program does not contain directory
772 separators, it will first be searched for in the current directory
773 before being searched for on the PATH. On Unix platforms, the
774 program will be searched for strictly on the PATH.
779 Like B<-T>, but taint checks will issue warnings rather than fatal
780 errors. These warnings can be controlled normally with C<no warnings
783 B<NOTE: this is not a substitute for -T.> This is meant only to be
784 used as a temporary development aid while securing legacy code:
785 for real production code and for new secure code written from scratch
786 always use the real B<-T>.
791 forces "taint" checks to be turned on so you can test them. Ordinarily
792 these checks are done only when running setuid or setgid. It's a
793 good idea to turn them on explicitly for programs that run on behalf
794 of someone else whom you might not necessarily trust, such as CGI
795 programs or any internet servers you might write in Perl. See
796 L<perlsec> for details. For security reasons, this option must be
797 seen by Perl quite early; usually this means it must appear early
798 on the command line or in the #! line for systems which support
804 This obsolete switch causes Perl to dump core after compiling your
805 program. You can then in theory take this core dump and turn it
806 into an executable file by using the B<undump> program (not supplied).
807 This speeds startup at the expense of some disk space (which you
808 can minimize by stripping the executable). (Still, a "hello world"
809 executable comes out to about 200K on my machine.) If you want to
810 execute a portion of your program before dumping, use the dump()
811 operator instead. Note: availability of B<undump> is platform
812 specific and may not be available for a specific port of Perl.
817 allows Perl to do unsafe operations. Currently the only "unsafe"
818 operations are attempting to unlink directories while running as
819 superuser, and running setuid programs with fatal taint checks turned
820 into warnings. Note that the B<-w> switch (or the C<$^W> variable)
821 must be used along with this option to actually I<generate> the
822 taint-check warnings.
827 prints the version and patchlevel of your perl executable.
832 prints summary of the major perl configuration values and the current
835 =item B<-V:>I<configvar>
837 Prints to STDOUT the value of the named configuration variable(s),
838 with multiples when your configvar argument looks like a regex (has
839 non-letters). For example:
842 libc='/lib/libc-2.2.4.so';
844 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
845 libc='/lib/libc-2.2.4.so';
847 libpth='/usr/local/lib /lib /usr/lib';
848 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
850 libc='/lib/libc-2.2.4.so';
854 Additionally, extra colons can be used to control formatting. A
855 trailing colon suppresses the linefeed and terminator ';', allowing
856 you to embed queries into shell commands. (mnemonic: PATH separator
859 $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
860 compression-vars: zcat='' zip='zip' are here !
862 A leading colon removes the 'name=' part of the response, this allows
863 you to map to the name you need. (mnemonic: empty label)
865 $ echo "goodvfork="`./perl -Ilib -V::usevfork`
868 Leading and trailing colons can be used together if you need
869 positional parameter values without the names. Note that in the case
870 below, the PERL_API params are returned in alphabetical order.
872 $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
873 building_on 'linux' '5' '1' '9' now
878 prints warnings about dubious constructs, such as variable names
879 that are mentioned only once and scalar variables that are used
880 before being set, redefined subroutines, references to undefined
881 filehandles or filehandles opened read-only that you are attempting
882 to write on, values used as a number that don't look like numbers,
883 using an array as though it were a scalar, if your subroutines
884 recurse more than 100 deep, and innumerable other things.
886 This switch really just enables the internal C<$^W> variable. You
887 can disable or promote into fatal errors specific warnings using
888 C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
889 See also L<perldiag> and L<perltrap>. A new, fine-grained warning
890 facility is also available if you want to manipulate entire classes
891 of warnings; see L<warnings> or L<perllexwarn>.
896 Enables all warnings regardless of C<no warnings> or C<$^W>.
902 Disables all warnings regardless of C<use warnings> or C<$^W>.
908 =item B<-x>I<directory>
910 tells Perl that the program is embedded in a larger chunk of unrelated
911 ASCII text, such as in a mail message. Leading garbage will be
912 discarded until the first line that starts with #! and contains the
913 string "perl". Any meaningful switches on that line will be applied.
915 All references to line numbers by the program (warnings, errors, ...)
916 will treat the #! line as the first line.
917 Thus a warning on the 2nd line of the program (which is on the 100th
918 line in the file) will be reported as line 2, and not as line 100.
919 This can be overridden by using the #line directive.
920 (See L<perlsyn/"Plain-Old-Comments-(Not!)">)
922 If a directory name is specified, Perl will switch to that directory
923 before running the program. The B<-x> switch controls only the
924 disposal of leading garbage. The program must be terminated with
925 C<__END__> if there is trailing garbage to be ignored (the program
926 can process any or all of the trailing garbage via the DATA filehandle
929 The directory, if specified, must appear immediately following the B<-x>
930 with no intervening whitespace.
935 X<perl, environment variables>
942 Used if chdir has no argument.
947 Used if chdir has no argument and HOME is not set.
952 Used in executing subprocesses, and in finding the program if B<-S> is
958 A list of directories in which to look for Perl library
959 files before looking in the standard library and the current
960 directory. Any architecture-specific directories under the specified
961 locations are automatically included if they exist (this lookup
962 being done at interpreter startup time.)
964 If PERL5LIB is not defined, PERLLIB is used. Directories are separated
965 (like in PATH) by a colon on unixish platforms and by a semicolon on
966 Windows (the proper path separator being given by the command C<perl
969 When running taint checks (either because the program was running setuid
970 or setgid, or the B<-T> or B<-t> switch was specified), neither variable
971 is used. The program should instead say:
973 use lib "/my/directory";
978 Command-line options (switches). Switches in this variable are taken
979 as if they were on every Perl command line. Only the B<-[CDIMUdmtwW]>
980 switches are allowed. When running taint checks (because the program
981 was running setuid or setgid, or the B<-T> switch was used), this
982 variable is ignored. If PERL5OPT begins with B<-T>, tainting will be
983 enabled, and any subsequent options ignored.
988 A space (or colon) separated list of PerlIO layers. If perl is built
989 to use PerlIO system for IO (the default) these layers effect perl's IO.
991 It is conventional to start layer names with a colon e.g. C<:perlio> to
992 emphasise their similarity to variable "attributes". But the code that parses
993 layer specification strings (which is also used to decode the PERLIO
994 environment variable) treats the colon as a separator.
996 An unset or empty PERLIO is equivalent to the default set of layers for
997 your platform, for example C<:unix:perlio> on UNIX-like systems
998 and C<:unix:crlf> on Windows and other DOS-like systems.
1000 The list becomes the default for I<all> perl's IO. Consequently only built-in
1001 layers can appear in this list, as external layers (such as :encoding()) need
1002 IO in order to load them!. See L<"open pragma"|open> for how to add external
1003 encodings as defaults.
1005 The layers that it makes sense to include in the PERLIO environment
1006 variable are briefly summarised below. For more details see L<PerlIO>.
1013 A pseudolayer that turns I<off> the C<:utf8> flag for the layer below.
1014 Unlikely to be useful on its own in the global PERLIO environment variable.
1015 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.)
1028 A layer which implements "reading" of files by using C<mmap()> to
1029 make (whole) file appear in the process's address space, and then
1030 using that as PerlIO's "buffer".
1035 This is a re-implementation of "stdio-like" buffering written as a
1036 PerlIO "layer". As such it will call whatever layer is below it for
1037 its operations (typically C<:unix>).
1042 An experimental pseudolayer that removes the topmost layer.
1043 Use with the same care as is reserved for nitroglycerin.
1048 A pseudolayer that manipulates other layers. Applying the C<:raw>
1049 layer is equivalent to calling C<binmode($fh)>. It makes the stream
1050 pass each byte as-is without any translation. In particular CRLF
1051 translation, and/or :utf8 intuited from locale are disabled.
1053 Unlike in the earlier versions of Perl C<:raw> is I<not>
1054 just the inverse of C<:crlf> - other layers which would affect the
1055 binary nature of the stream are also removed or disabled.
1060 This layer provides PerlIO interface by wrapping system's ANSI C "stdio"
1061 library calls. The layer provides both buffering and IO.
1062 Note that C<:stdio> layer does I<not> do CRLF translation even if that
1063 is platforms normal behaviour. You will need a C<:crlf> layer above it
1069 Low level layer which calls C<read>, C<write> and C<lseek> etc.
1074 A pseudolayer that turns on a flag on the layer below to tell perl
1075 that output should be in utf8 and that input should be regarded as
1076 already in valid utf8 form. It does not check for validity and as such
1077 should be handled with caution for input. Generally C<:encoding(utf8)> is
1078 the best option when reading UTF-8 encoded data.
1083 On Win32 platforms this I<experimental> layer uses native "handle" IO
1084 rather than unix-like numeric file descriptor layer. Known to be
1085 buggy in this release.
1089 On all platforms the default set of layers should give acceptable results.
1091 For UNIX platforms that will equivalent of "unix perlio" or "stdio".
1092 Configure is setup to prefer "stdio" implementation if system's library
1093 provides for fast access to the buffer, otherwise it uses the "unix perlio"
1096 On Win32 the default in this release is "unix crlf". Win32's "stdio"
1097 has a number of bugs/mis-features for perl IO which are somewhat
1098 C compiler vendor/version dependent. Using our own C<crlf> layer as
1099 the buffer avoids those issues and makes things more uniform.
1100 The C<crlf> layer provides CRLF to/from "\n" conversion as well as
1103 This release uses C<unix> as the bottom layer on Win32 and so still uses C
1104 compiler's numeric file descriptor routines. There is an experimental native
1105 C<win32> layer which is expected to be enhanced and should eventually be
1106 the default under Win32.
1108 The PERLIO environment variable is completely ignored when perl
1109 is run in taint mode.
1114 If set to the name of a file or device then certain operations of PerlIO
1115 sub-system will be logged to that file (opened as append). Typical uses
1118 PERLIO_DEBUG=/dev/tty perl script ...
1120 and Win32 approximate equivalent:
1122 set PERLIO_DEBUG=CON
1125 This functionality is disabled for setuid scripts and for scripts run
1131 A list of directories in which to look for Perl library
1132 files before looking in the standard library and the current directory.
1133 If PERL5LIB is defined, PERLLIB is not used.
1135 The PERLLIB environment variable is completely ignored when perl
1136 is run in taint mode.
1141 The command used to load the debugger code. The default is:
1143 BEGIN { require 'perl5db.pl' }
1145 The PERL5DB environment variable only used when perl is started with
1146 a bare B<-d> switch.
1148 =item PERL5DB_THREADED
1151 If set to a true value, indicates to the debugger that the code being
1152 debugged uses threads.
1154 =item PERL5SHELL (specific to the Win32 port)
1157 May be set to an alternative shell that perl must use internally for
1158 executing "backtick" commands or system(). Default is C<cmd.exe /x/d/c>
1159 on WindowsNT and C<command.com /c> on Windows95. The value is considered
1160 to be space-separated. Precede any character that needs to be protected
1161 (like a space or backslash) with a backslash.
1163 Note that Perl doesn't use COMSPEC for this purpose because
1164 COMSPEC has a high degree of variability among users, leading to
1165 portability concerns. Besides, perl can use a shell that may not be
1166 fit for interactive use, and setting COMSPEC to such a shell may
1167 interfere with the proper functioning of other programs (which usually
1168 look in COMSPEC to find a shell fit for interactive use).
1170 Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint checked
1171 when running external commands. It is recommended that
1172 you explicitly set (or delete) C<$ENV{PERL5SHELL}> when running
1173 in taint mode under Windows.
1175 =item PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
1176 X<PERL_ALLOW_NON_IFS_LSP>
1178 Set to 1 to allow the use of non-IFS compatible LSP's.
1179 Perl normally searches for an IFS-compatible LSP because this is required
1180 for its emulation of Windows sockets as real filehandles. However, this may
1181 cause problems if you have a firewall such as McAfee Guardian which requires
1182 all applications to use its LSP which is not IFS-compatible, because clearly
1183 Perl will normally avoid using such an LSP.
1184 Setting this environment variable to 1 means that Perl will simply use the
1185 first suitable LSP enumerated in the catalog, which keeps McAfee Guardian
1186 happy (and in that particular case Perl still works too because McAfee
1187 Guardian's LSP actually plays some other games which allow applications
1188 requiring IFS compatibility to work).
1190 =item PERL_DEBUG_MSTATS
1191 X<PERL_DEBUG_MSTATS>
1193 Relevant only if perl is compiled with the malloc included with the perl
1194 distribution (that is, if C<perl -V:d_mymalloc> is 'define').
1195 If set, this causes memory statistics to be dumped after execution. If set
1196 to an integer greater than one, also causes memory statistics to be dumped
1199 =item PERL_DESTRUCT_LEVEL
1200 X<PERL_DESTRUCT_LEVEL>
1202 Relevant only if your perl executable was built with B<-DDEBUGGING>,
1203 this controls the behavior of global destruction of objects and other
1204 references. See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.
1206 =item PERL_DL_NONLAZY
1209 Set to one to have perl resolve B<all> undefined symbols when it loads
1210 a dynamic library. The default behaviour is to resolve symbols when
1211 they are used. Setting this variable is useful during testing of
1212 extensions as it ensures that you get an error on misspelled function
1213 names even if the test suite doesn't call it.
1218 If using the C<encoding> pragma without an explicit encoding name, the
1219 PERL_ENCODING environment variable is consulted for an encoding name.
1221 =item PERL_HASH_SEED
1224 (Since Perl 5.8.1.) Used to randomise perl's internal hash function.
1225 To emulate the pre-5.8.1 behaviour, set to an integer (zero means
1226 exactly the same order as 5.8.0). "Pre-5.8.1" means, among other
1227 things, that hash keys will always have the same ordering between
1228 different runs of perl.
1230 Most hashes return elements in the same order as Perl 5.8.0 by default.
1231 On a hash by hash basis, if pathological data is detected during a hash
1232 key insertion, then that hash will switch to an alternative random hash
1235 The default behaviour is to randomise unless the PERL_HASH_SEED is set.
1236 If perl has been compiled with C<-DUSE_HASH_SEED_EXPLICIT>, the default
1237 behaviour is B<not> to randomise unless the PERL_HASH_SEED is set.
1239 If PERL_HASH_SEED is unset or set to a non-numeric string, perl uses
1240 the pseudorandom seed supplied by the operating system and libraries.
1242 B<Please note that the hash seed is sensitive information>. Hashes are
1243 randomized to protect against local and remote attacks against Perl
1244 code. By manually setting a seed this protection may be partially or
1247 See L<perlsec/"Algorithmic Complexity Attacks"> and
1248 L</PERL_HASH_SEED_DEBUG> for more information.
1250 =item PERL_HASH_SEED_DEBUG
1251 X<PERL_HASH_SEED_DEBUG>
1253 (Since Perl 5.8.1.) Set to one to display (to STDERR) the value of
1254 the hash seed at the beginning of execution. This, combined with
1255 L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
1256 behavior caused by hash randomization.
1258 B<Note that the hash seed is sensitive information>: by knowing it one
1259 can craft a denial-of-service attack against Perl code, even remotely,
1260 see L<perlsec/"Algorithmic Complexity Attacks"> for more information.
1261 B<Do not disclose the hash seed> to people who don't need to know it.
1262 See also hash_seed() of L<Hash::Util>.
1264 =item PERL_ROOT (specific to the VMS port)
1267 A translation concealed rooted logical name that contains perl and the
1268 logical device for the @INC path on VMS only. Other logical names that
1269 affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1270 SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in
1271 L<perlvms> and in F<README.vms> in the Perl source distribution.
1276 In Perls 5.8.1 and later. If set to C<unsafe> the pre-Perl-5.8.0
1277 signals behaviour (immediate but unsafe) is restored. If set to
1278 C<safe> the safe (or deferred) signals are used.
1279 See L<perlipc/"Deferred Signals (Safe Signals)">.
1284 Equivalent to the B<-C> command-line switch. Note that this is not
1285 a boolean variable-- setting this to C<"1"> is not the right way to
1286 "enable Unicode" (whatever that would mean). You can use C<"0"> to
1287 "disable Unicode", though (or alternatively unset PERL_UNICODE in
1288 your shell before starting Perl). See the description of the C<-C>
1289 switch for more information.
1291 =item SYS$LOGIN (specific to the VMS port)
1294 Used if chdir has no argument and HOME and LOGDIR are not set.
1298 Perl also has environment variables that control how Perl handles data
1299 specific to particular natural languages. See L<perllocale>.
1301 Apart from these, Perl uses no other environment variables, except
1302 to make them available to the program being executed, and to child
1303 processes. However, programs running setuid would do well to execute
1304 the following lines before doing anything else, just to keep people
1307 $ENV{PATH} = '/bin:/usr/bin'; # or whatever you need
1308 $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
1309 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};