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...'> ]>
15 S<[ B<-i>[I<extension>] ]>
16 S<[ B<-e> I<'command'> ] [ B<--> ] [ I<programfile> ] [ I<argument> ]...>
17 S<[ B<-A [I<assertions>] >]>
18 S<[ B<-C [I<number/list>] >]>
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 A Macintosh perl program will have the appropriate Creator and
145 Type, so that double-clicking them will invoke the perl application.
151 $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
152 $ exit++ + ++$status != 0 and $exit = $status = undef;
154 at the top of your program, where B<-mysw> are any command line switches you
155 want to pass to Perl. You can now invoke the program directly, by saying
156 C<perl program>, or as a DCL procedure, by saying C<@program> (or implicitly
157 via F<DCL$PATH> by just using the name of the program).
159 This incantation is a bit much to remember, but Perl will display it for
160 you if you say C<perl "-V:startperl">.
164 Command-interpreters on non-Unix systems have rather different ideas
165 on quoting than Unix shells. You'll need to learn the special
166 characters in your command-interpreter (C<*>, C<\> and C<"> are
167 common) and how to protect whitespace and these characters to run
168 one-liners (see B<-e> below).
170 On some systems, you may have to change single-quotes to double ones,
171 which you must I<not> do on Unix or Plan 9 systems. You might also
172 have to change a single % to a %%.
177 perl -e 'print "Hello world\n"'
180 perl -e "print \"Hello world\n\""
183 print "Hello world\n"
184 (then Run "Myscript" or Shift-Command-R)
187 perl -e "print ""Hello world\n"""
189 The problem is that none of this is reliable: it depends on the
190 command and it is entirely possible neither works. If B<4DOS> were
191 the command shell, this would probably work better:
193 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
195 B<CMD.EXE> in Windows NT slipped a lot of standard Unix functionality in
196 when nobody was looking, but just try to find documentation for its
199 Under the Macintosh, it depends which environment you are using. The MacPerl
200 shell, or MPW, is much like Unix shells in its support for several
201 quoting variants, except that it makes free use of the Macintosh's non-ASCII
202 characters as control characters.
204 There is no general solution to all of this. It's just a mess.
206 =head2 Location of Perl
208 It may seem obvious to say, but Perl is useful only when users can
209 easily find it. When possible, it's good for both F</usr/bin/perl>
210 and F</usr/local/bin/perl> to be symlinks to the actual binary. If
211 that can't be done, system administrators are strongly encouraged
212 to put (symlinks to) perl and its accompanying utilities into a
213 directory typically found along a user's PATH, or in some other
214 obvious and convenient place.
216 In this documentation, C<#!/usr/bin/perl> on the first line of the program
217 will stand in for whatever method works on your system. You are
218 advised to use a specific path if you care about a specific version.
220 #!/usr/local/bin/perl5.00554
222 or if you just want to be running at least version, place a statement
223 like this at the top of your program:
227 =head2 Command Switches
229 As with all standard commands, a single-character switch may be
230 clustered with the following switch, if any.
232 #!/usr/bin/perl -spi.orig # same as -s -p -i.orig
238 =item B<-0>[I<octal/hexadecimal>]
240 specifies the input record separator (C<$/>) as an octal or
241 hexadecimal number. If there are no digits, the null character is the
242 separator. Other switches may precede or follow the digits. For
243 example, if you have a version of B<find> which can print filenames
244 terminated by the null character, you can say this:
246 find . -name '*.orig' -print0 | perl -n0e unlink
248 The special value 00 will cause Perl to slurp files in paragraph mode.
249 The value 0777 will cause Perl to slurp files whole because there is no
250 legal byte with that value.
252 If you want to specify any Unicode character, use the hexadecimal
253 format: C<-0xHHH...>, where the C<H> are valid hexadecimal digits.
254 (This means that you cannot use the C<-x> with a directory name that
255 consists of hexadecimal digits.)
257 =item B<-A [I<assertions>]>
259 Activates the assertions given after the switch as a comma-separated
260 list of assertion names. If no assertion name is given, activates all
261 assertions. See L<assertions>.
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>]>
282 The C<-C> flag controls some Unicode of the Perl Unicode features.
284 As of 5.8.1, the C<-C> can be followed either by a number or a list
285 of option letters. The letters, their numeric values, and effects
286 are as follows; listing the letters is equal to summing the numbers.
288 I 1 STDIN is assumed to be in UTF-8
289 O 2 STDOUT will be in UTF-8
290 E 4 STDERR will be in UTF-8
292 i 8 UTF-8 is the default PerlIO layer for input streams
293 o 16 UTF-8 is the default PerlIO layer for output streams
295 A 32 the @ARGV elements are expected to be strings encoded
297 L 64 normally the "IOEioA" are unconditional,
298 the L makes them conditional on the locale environment
299 variables (the LC_ALL, LC_TYPE, and LANG, in the order
300 of decreasing precedence) -- if the variables indicate
301 UTF-8, then the selected "IOEioA" are in effect
303 For example, C<-COE> and C<-C6> will both turn on UTF-8-ness on both
304 STDOUT and STDERR. Repeating letters is just redundant, not cumulative
307 The C<io> options mean that any subsequent open() (or similar I/O
308 operations) will have the C<:utf8> PerlIO layer implicitly applied
309 to them, in other words, UTF-8 is expected from any input stream,
310 and UTF-8 is produced to any output stream. This is just the default,
311 with explicit layers in open() and with binmode() one can manipulate
314 C<-C> on its own (not followed by any number or option list), or the
315 empty string C<""> for the C<PERL_UNICODE> environment variable, has the
316 same effect as C<-CSDL>. In other words, the standard I/O handles and
317 the default C<open()> layer are UTF-8-fied B<but> only if the locale
318 environment variables indicate a UTF-8 locale. This behaviour follows
319 the I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
321 You can use C<-C0> (or C<"0"> for C<PERL_UNICODE>) to explicitly
322 disable all the above Unicode features.
324 The read-only magic variable C<${^UNICODE}> reflects the numeric value
325 of this setting. This is variable is set during Perl startup and is
326 thereafter read-only. If you want runtime effects, use the three-arg
327 open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
328 and the C<open> pragma (see L<open>).
330 (In Perls earlier than 5.8.1 the C<-C> switch was a Win32-only switch
331 that enabled the use of Unicode-aware "wide system call" Win32 APIs.
332 This feature was practically unused, however, and the command line
333 switch was therefore "recycled".)
337 causes Perl to check the syntax of the program and then exit without
338 executing it. Actually, it I<will> execute C<BEGIN>, C<CHECK>, and
339 C<use> blocks, because these are considered as occurring outside the
340 execution of your program. C<INIT> and C<END> blocks, however, will
347 runs the program under the Perl debugger. See L<perldebug>.
348 If B<t> is specified, it indicates to the debugger that threads
349 will be used in the code being debugged.
351 =item B<-d:>I<foo[=bar,baz]>
353 =item B<-dt:>I<foo[=bar,baz]>
355 runs the program under the control of a debugging, profiling, or
356 tracing module installed as Devel::foo. E.g., B<-d:DProf> executes
357 the program using the Devel::DProf profiler. As with the B<-M>
358 flag, options may be passed to the Devel::foo package where they
359 will be received and interpreted by the Devel::foo::import routine.
360 The comma-separated list of options must follow a C<=> character.
361 If B<t> is specified, it indicates to the debugger that threads
362 will be used in the code being debugged.
365 =item B<-D>I<letters>
369 sets debugging flags. To watch how it executes your program, use
370 B<-Dtls>. (This works only if debugging is compiled into your
371 Perl.) Another nice value is B<-Dx>, which lists your compiled
372 syntax tree. And B<-Dr> displays compiled regular expressions;
373 the format of the output is explained in L<perldebguts>.
375 As an alternative, specify a number instead of list of letters (e.g.,
376 B<-D14> is equivalent to B<-Dtls>):
378 1 p Tokenizing and parsing (with v, displays parse stack)
379 2 s Stack snapshots (with v, displays all stacks)
380 4 l Context (loop) stack processing
382 16 o Method and overloading resolution
383 32 c String/numeric conversions
384 64 P Print profiling info, preprocessor command for -P, source file input state
385 128 m Memory allocation
386 256 f Format processing
387 512 r Regular expression parsing and execution
388 1024 x Syntax tree dump
389 2048 u Tainting checks
390 4096 (Obsolete, previously used for LEAKTEST)
391 8192 H Hash dump -- usurps values()
392 16384 X Scratchpad allocation
394 65536 S Thread synchronization
396 262144 R Include reference counts of dumped variables (eg when using -Ds)
397 524288 J Do not s,t,P-debug (Jump over) opcodes within package DB
398 1048576 v Verbose: use in conjunction with other flags
399 2097152 C Copy On Write
400 4194304 A Consistency checks on internal structures
401 8388608 q quiet - currently only suppresses the "EXECUTING" message
403 All these flags require B<-DDEBUGGING> when you compile the Perl
404 executable (but see L<Devel::Peek>, L<re> which may change this).
405 See the F<INSTALL> file in the Perl source distribution
406 for how to do this. This flag is automatically set if you include B<-g>
407 option when C<Configure> asks you about optimizer/debugger flags.
409 If you're just trying to get a print out of each line of Perl code
410 as it executes, the way that C<sh -x> provides for shell scripts,
411 you can't use Perl's B<-D> switch. Instead do this
413 # If you have "env" utility
414 env=PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
416 # Bourne shell syntax
417 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
420 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
422 See L<perldebug> for details and variations.
424 =item B<-e> I<commandline>
426 may be used to enter one line of program. If B<-e> is given, Perl
427 will not look for a filename in the argument list. Multiple B<-e>
428 commands may be given to build up a multi-line script. Make sure
429 to use semicolons where you would in a normal program.
431 =item B<-F>I<pattern>
433 specifies the pattern to split on if B<-a> is also in effect. The
434 pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
435 put in single quotes.
439 prints a summary of the options.
441 =item B<-i>[I<extension>]
443 specifies that files processed by the C<E<lt>E<gt>> construct are to be
444 edited in-place. It does this by renaming the input file, opening the
445 output file by the original name, and selecting that output file as the
446 default for print() statements. The extension, if supplied, is used to
447 modify the name of the old file to make a backup copy, following these
450 If no extension is supplied, no backup is made and the current file is
453 If the extension doesn't contain a C<*>, then it is appended to the
454 end of the current filename as a suffix. If the extension does
455 contain one or more C<*> characters, then each C<*> is replaced
456 with the current filename. In Perl terms, you could think of this
459 ($backup = $extension) =~ s/\*/$file_name/g;
461 This allows you to add a prefix to the backup file, instead of (or in
462 addition to) a suffix:
464 $ perl -pi'orig_*' -e 's/bar/baz/' fileA # backup to 'orig_fileA'
466 Or even to place backup copies of the original files into another
467 directory (provided the directory already exists):
469 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'
471 These sets of one-liners are equivalent:
473 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
474 $ perl -pi'*' -e 's/bar/baz/' fileA # overwrite current file
476 $ perl -pi'.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
477 $ perl -pi'*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
479 From the shell, saying
481 $ perl -p -i.orig -e "s/foo/bar/; ... "
483 is the same as using the program:
485 #!/usr/bin/perl -pi.orig
488 which is equivalent to
491 $extension = '.orig';
493 if ($ARGV ne $oldargv) {
494 if ($extension !~ /\*/) {
495 $backup = $ARGV . $extension;
498 ($backup = $extension) =~ s/\*/$ARGV/g;
500 rename($ARGV, $backup);
501 open(ARGVOUT, ">$ARGV");
508 print; # this prints to original filename
512 except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
513 know when the filename has changed. It does, however, use ARGVOUT for
514 the selected filehandle. Note that STDOUT is restored as the default
515 output filehandle after the loop.
517 As shown above, Perl creates the backup file whether or not any output
518 is actually changed. So this is just a fancy way to copy files:
520 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
522 $ perl -p -i'.orig' -e 1 file1 file2 file3...
524 You can use C<eof> without parentheses to locate the end of each input
525 file, in case you want to append to each file, or reset line numbering
526 (see example in L<perlfunc/eof>).
528 If, for a given file, Perl is unable to create the backup file as
529 specified in the extension then it will skip that file and continue on
530 with the next one (if it exists).
532 For a discussion of issues surrounding file permissions and B<-i>,
533 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?>.
535 You cannot use B<-i> to create directories or to strip extensions from
538 Perl does not expand C<~> in filenames, which is good, since some
539 folks use it for their backup files:
541 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
543 Note that because B<-i> renames or deletes the original file before
544 creating a new file of the same name, UNIX-style soft and hard links will
547 Finally, the B<-i> switch does not impede execution when no
548 files are given on the command line. In this case, no backup is made
549 (the original file cannot, of course, be determined) and processing
550 proceeds from STDIN to STDOUT as might be expected.
552 =item B<-I>I<directory>
554 Directories specified by B<-I> are prepended to the search path for
555 modules (C<@INC>), and also tells the C preprocessor where to search for
556 include files. The C preprocessor is invoked with B<-P>; by default it
557 searches /usr/include and /usr/lib/perl.
559 =item B<-l>[I<octnum>]
561 enables automatic line-ending processing. It has two separate
562 effects. First, it automatically chomps C<$/> (the input record
563 separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
564 (the output record separator) to have the value of I<octnum> so
565 that any print statements will have that separator added back on.
566 If I<octnum> is omitted, sets C<$\> to the current value of
567 C<$/>. For instance, to trim lines to 80 columns:
569 perl -lpe 'substr($_, 80) = ""'
571 Note that the assignment C<$\ = $/> is done when the switch is processed,
572 so the input record separator can be different than the output record
573 separator if the B<-l> switch is followed by a B<-0> switch:
575 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
577 This sets C<$\> to newline and then sets C<$/> to the null character.
579 =item B<-m>[B<->]I<module>
581 =item B<-M>[B<->]I<module>
583 =item B<-M>[B<->]I<'module ...'>
585 =item B<-[mM]>[B<->]I<module=arg[,arg]...>
587 B<-m>I<module> executes C<use> I<module> C<();> before executing your
590 B<-M>I<module> executes C<use> I<module> C<;> before executing your
591 program. You can use quotes to add extra code after the module name,
592 e.g., C<'-Mmodule qw(foo bar)'>.
594 If the first character after the B<-M> or B<-m> is a dash (C<->)
595 then the 'use' is replaced with 'no'.
597 A little builtin syntactic sugar means you can also say
598 B<-mmodule=foo,bar> or B<-Mmodule=foo,bar> as a shortcut for
599 C<'-Mmodule qw(foo bar)'>. This avoids the need to use quotes when
600 importing symbols. The actual code generated by B<-Mmodule=foo,bar> is
601 C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
602 removes the distinction between B<-m> and B<-M>.
606 causes Perl to assume the following loop around your program, which
607 makes it iterate over filename arguments somewhat like B<sed -n> or
612 ... # your program goes here
615 Note that the lines are not printed by default. See B<-p> to have
616 lines printed. If a file named by an argument cannot be opened for
617 some reason, Perl warns you about it and moves on to the next file.
619 Here is an efficient way to delete all files that haven't been modified for
622 find . -mtime +7 -print | perl -nle unlink
624 This is faster than using the B<-exec> switch of B<find> because you don't
625 have to start a process on every filename found. It does suffer from
626 the bug of mishandling newlines in pathnames, which you can fix if
627 you follow the example under B<-0>.
629 C<BEGIN> and C<END> blocks may be used to capture control before or after
630 the implicit program loop, just as in B<awk>.
634 causes Perl to assume the following loop around your program, which
635 makes it iterate over filename arguments somewhat like B<sed>:
640 ... # your program goes here
642 print or die "-p destination: $!\n";
645 If a file named by an argument cannot be opened for some reason, Perl
646 warns you about it, and moves on to the next file. Note that the
647 lines are printed automatically. An error occurring during printing is
648 treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
649 overrides a B<-n> switch.
651 C<BEGIN> and C<END> blocks may be used to capture control before or after
652 the implicit loop, just as in B<awk>.
656 B<NOTE: Use of -P is strongly discouraged because of its inherent
657 problems, including poor portability.>
659 This option causes your program to be run through the C preprocessor before
660 compilation by Perl. Because both comments and B<cpp> directives begin
661 with the # character, you should avoid starting comments with any words
662 recognized by the C preprocessor such as C<"if">, C<"else">, or C<"define">.
664 If you're considering using C<-P>, you might also want to look at the
665 Filter::cpp module from CPAN.
667 The problems of -P include, but are not limited to:
673 The C<#!> line is stripped, so any switches there don't apply.
677 A C<-P> on a C<#!> line doesn't work.
681 B<All> lines that begin with (whitespace and) a C<#> but
682 do not look like cpp commands, are stripped, including anything
683 inside Perl strings, regular expressions, and here-docs .
687 In some platforms the C preprocessor knows too much: it knows about
688 the C++ -style until-end-of-line comments starting with C<"//">.
689 This will cause problems with common Perl constructs like
693 because after -P this will became illegal code
697 The workaround is to use some other quoting separator than C<"/">,
698 like for example C<"!">:
706 It requires not only a working C preprocessor but also a working
707 F<sed>. If not on UNIX, you are probably out of luck on this.
711 Script line numbers are not preserved.
715 The C<-x> does not work with C<-P>.
721 enables rudimentary switch parsing for switches on the command
722 line after the program name but before any filename arguments (or before
723 an argument of B<-->). This means you can have switches with two leading
724 dashes (B<--help>). Any switch found there is removed from @ARGV and sets the
725 corresponding variable in the Perl program. The following program
726 prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
727 if it is invoked with B<-xyz=abc>.
730 if ($xyz) { print "$xyz\n" }
732 Do note that B<--help> creates the variable ${-help}, which is not compliant
733 with C<strict refs>. Also, when using this option on a script with
734 warnings enabled you may get a lot of spurious "used only once" warnings.
738 makes Perl use the PATH environment variable to search for the
739 program (unless the name of the program contains directory separators).
741 On some platforms, this also makes Perl append suffixes to the
742 filename while searching for it. For example, on Win32 platforms,
743 the ".bat" and ".cmd" suffixes are appended if a lookup for the
744 original name fails, and if the name does not already end in one
745 of those suffixes. If your Perl was compiled with DEBUGGING turned
746 on, using the -Dp switch to Perl shows how the search progresses.
748 Typically this is used to emulate #! startup on platforms that don't
749 support #!. Its also convenient when debugging a script that uses #!,
750 and is thus normally found by the shell's $PATH search mechanism.
752 This example works on many platforms that have a shell compatible with
756 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
757 if $running_under_some_shell;
759 The system ignores the first line and feeds the program to F</bin/sh>,
760 which proceeds to try to execute the Perl program as a shell script.
761 The shell executes the second line as a normal shell command, and thus
762 starts up the Perl interpreter. On some systems $0 doesn't always
763 contain the full pathname, so the B<-S> tells Perl to search for the
764 program if necessary. After Perl locates the program, it parses the
765 lines and ignores them because the variable $running_under_some_shell
766 is never true. If the program will be interpreted by csh, you will need
767 to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
768 embedded spaces (and such) in the argument list. To start up sh rather
769 than csh, some systems may have to replace the #! line with a line
770 containing just a colon, which will be politely ignored by Perl. Other
771 systems can't control that, and need a totally devious construct that
772 will work under any of B<csh>, B<sh>, or Perl, such as the following:
774 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
775 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
776 if $running_under_some_shell;
778 If the filename supplied contains directory separators (i.e., is an
779 absolute or relative pathname), and if that file is not found,
780 platforms that append file extensions will do so and try to look
781 for the file with those extensions added, one by one.
783 On DOS-like platforms, if the program does not contain directory
784 separators, it will first be searched for in the current directory
785 before being searched for on the PATH. On Unix platforms, the
786 program will be searched for strictly on the PATH.
790 Like B<-T>, but taint checks will issue warnings rather than fatal
791 errors. These warnings can be controlled normally with C<no warnings
794 B<NOTE: this is not a substitute for -T.> This is meant only to be
795 used as a temporary development aid while securing legacy code:
796 for real production code and for new secure code written from scratch
797 always use the real B<-T>.
801 forces "taint" checks to be turned on so you can test them. Ordinarily
802 these checks are done only when running setuid or setgid. It's a
803 good idea to turn them on explicitly for programs that run on behalf
804 of someone else whom you might not necessarily trust, such as CGI
805 programs or any internet servers you might write in Perl. See
806 L<perlsec> for details. For security reasons, this option must be
807 seen by Perl quite early; usually this means it must appear early
808 on the command line or in the #! line for systems which support
813 This obsolete switch causes Perl to dump core after compiling your
814 program. You can then in theory take this core dump and turn it
815 into an executable file by using the B<undump> program (not supplied).
816 This speeds startup at the expense of some disk space (which you
817 can minimize by stripping the executable). (Still, a "hello world"
818 executable comes out to about 200K on my machine.) If you want to
819 execute a portion of your program before dumping, use the dump()
820 operator instead. Note: availability of B<undump> is platform
821 specific and may not be available for a specific port of Perl.
823 This switch has been superseded in favor of the new Perl code
824 generator backends to the compiler. See L<B> and L<B::Bytecode>
829 allows Perl to do unsafe operations. Currently the only "unsafe"
830 operations are the unlinking of directories while running as superuser,
831 and running setuid programs with fatal taint checks turned into
832 warnings. Note that the B<-w> switch (or the C<$^W> variable) must
833 be used along with this option to actually I<generate> the
834 taint-check warnings.
838 prints the version and patchlevel of your perl executable.
842 prints summary of the major perl configuration values and the current
845 =item B<-V:>I<configvar>
847 Prints to STDOUT the value of the named configuration variable(s),
848 with multiples when your configvar argument looks like a regex (has
849 non-letters). For example:
852 libc='/lib/libc-2.2.4.so';
854 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
855 libc='/lib/libc-2.2.4.so';
857 libpth='/usr/local/lib /lib /usr/lib';
858 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
860 libc='/lib/libc-2.2.4.so';
864 Additionally, extra colons can be used to control formatting. A
865 trailing colon suppresses the linefeed and terminator ';', allowing
866 you to embed queries into shell commands. (mnemonic: PATH separator
869 $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
870 compression-vars: zcat='' zip='zip' are here !
872 A leading colon removes the 'name=' part of the response, this allows
873 you to map to the name you need. (mnemonic: empty label)
875 $ echo "goodvfork="`./perl -Ilib -V::usevfork`
878 Leading and trailing colons can be used together if you need
879 positional parameter values without the names. Note that in the case
880 below, the PERL_API params are returned in alphabetical order.
882 $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
883 building_on 'linux' '5' '1' '9' now
887 prints warnings about dubious constructs, such as variable names
888 that are mentioned only once and scalar variables that are used
889 before being set, redefined subroutines, references to undefined
890 filehandles or filehandles opened read-only that you are attempting
891 to write on, values used as a number that doesn't look like numbers,
892 using an array as though it were a scalar, if your subroutines
893 recurse more than 100 deep, and innumerable other things.
895 This switch really just enables the internal C<$^W> variable. You
896 can disable or promote into fatal errors specific warnings using
897 C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
898 See also L<perldiag> and L<perltrap>. A new, fine-grained warning
899 facility is also available if you want to manipulate entire classes
900 of warnings; see L<warnings> or L<perllexwarn>.
904 Enables all warnings regardless of C<no warnings> or C<$^W>.
909 Disables all warnings regardless of C<use warnings> or C<$^W>.
914 =item B<-x> I<directory>
916 tells Perl that the program is embedded in a larger chunk of unrelated
917 ASCII text, such as in a mail message. Leading garbage will be
918 discarded until the first line that starts with #! and contains the
919 string "perl". Any meaningful switches on that line will be applied.
920 If a directory name is specified, Perl will switch to that directory
921 before running the program. The B<-x> switch controls only the
922 disposal of leading garbage. The program must be terminated with
923 C<__END__> if there is trailing garbage to be ignored (the program
924 can process any or all of the trailing garbage via the DATA filehandle
935 Used if chdir has no argument.
939 Used if chdir has no argument and HOME is not set.
943 Used in executing subprocesses, and in finding the program if B<-S> is
948 A list of directories in which to look for Perl library
949 files before looking in the standard library and the current
950 directory. Any architecture-specific directories under the specified
951 locations are automatically included if they exist. If PERL5LIB is not
952 defined, PERLLIB is used. Directories are separated (like in PATH) by
953 a colon on unixish platforms and by a semicolon on Windows (the proper
954 path separator being given by the command C<perl -V:path_sep>).
956 When running taint checks (either because the program was running setuid
957 or setgid, or the B<-T> switch was used), neither variable is used.
958 The program should instead say:
960 use lib "/my/directory";
964 Command-line options (switches). Switches in this variable are taken
965 as if they were on every Perl command line. Only the B<-[DIMUdmtw]>
966 switches are allowed. When running taint checks (because the program
967 was running setuid or setgid, or the B<-T> switch was used), this
968 variable is ignored. If PERL5OPT begins with B<-T>, tainting will be
969 enabled, and any subsequent options ignored.
973 A space (or colon) separated list of PerlIO layers. If perl is built
974 to use PerlIO system for IO (the default) these layers effect perl's IO.
976 It is conventional to start layer names with a colon e.g. C<:perlio> to
977 emphasise their similarity to variable "attributes". But the code that parses
978 layer specification strings (which is also used to decode the PERLIO
979 environment variable) treats the colon as a separator.
981 An unset or empty PERLIO is equivalent to C<:stdio>.
983 The list becomes the default for I<all> perl's IO. Consequently only built-in
984 layers can appear in this list, as external layers (such as :encoding()) need
985 IO in order to load them!. See L<"open pragma"|open> for how to add external
986 encodings as defaults.
988 The layers that it makes sense to include in the PERLIO environment
989 variable are briefly summarised below. For more details see L<PerlIO>.
995 A pseudolayer that turns I<off> the C<:utf8> flag for the layer below.
996 Unlikely to be useful on its own in the global PERLIO environment variable.
997 You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
1001 A layer which does CRLF to "\n" translation distinguishing "text" and
1002 "binary" files in the manner of MS-DOS and similar operating systems.
1003 (It currently does I<not> mimic MS-DOS as far as treating of Control-Z
1004 as being an end-of-file marker.)
1008 A layer which implements "reading" of files by using C<mmap()> to
1009 make (whole) file appear in the process's address space, and then
1010 using that as PerlIO's "buffer".
1014 This is a re-implementation of "stdio-like" buffering written as a
1015 PerlIO "layer". As such it will call whatever layer is below it for
1016 its operations (typically C<:unix>).
1020 An experimental pseudolayer that removes the topmost layer.
1021 Use with the same care as is reserved for nitroglycerin.
1025 A pseudolayer that manipulates other layers. Applying the C<:raw>
1026 layer is equivalent to calling C<binmode($fh)>. It makes the stream
1027 pass each byte as-is without any translation. In particular CRLF
1028 translation, and/or :utf8 intuited from locale are disabled.
1030 Unlike in the earlier versions of Perl C<:raw> is I<not>
1031 just the inverse of C<:crlf> - other layers which would affect the
1032 binary nature of the stream are also removed or disabled.
1036 This layer provides PerlIO interface by wrapping system's ANSI C "stdio"
1037 library calls. The layer provides both buffering and IO.
1038 Note that C<:stdio> layer does I<not> do CRLF translation even if that
1039 is platforms normal behaviour. You will need a C<:crlf> layer above it
1044 Low level layer which calls C<read>, C<write> and C<lseek> etc.
1048 A pseudolayer that turns on a flag on the layer below to tell perl
1049 that output should be in utf8 and that input should be regarded as
1050 already in utf8 form. May be useful in PERLIO environment
1051 variable to make UTF-8 the default. (To turn off that behaviour
1052 use C<:bytes> layer.)
1056 On Win32 platforms this I<experimental> layer uses native "handle" IO
1057 rather than unix-like numeric file descriptor layer. Known to be
1058 buggy in this release.
1062 On all platforms the default set of layers should give acceptable results.
1064 For UNIX platforms that will equivalent of "unix perlio" or "stdio".
1065 Configure is setup to prefer "stdio" implementation if system's library
1066 provides for fast access to the buffer, otherwise it uses the "unix perlio"
1069 On Win32 the default in this release is "unix crlf". Win32's "stdio"
1070 has a number of bugs/mis-features for perl IO which are somewhat
1071 C compiler vendor/version dependent. Using our own C<crlf> layer as
1072 the buffer avoids those issues and makes things more uniform.
1073 The C<crlf> layer provides CRLF to/from "\n" conversion as well as
1076 This release uses C<unix> as the bottom layer on Win32 and so still uses C
1077 compiler's numeric file descriptor routines. There is an experimental native
1078 C<win32> layer which is expected to be enhanced and should eventually be
1079 the default under Win32.
1083 If set to the name of a file or device then certain operations of PerlIO
1084 sub-system will be logged to that file (opened as append). Typical uses
1087 PERLIO_DEBUG=/dev/tty perl script ...
1089 and Win32 approximate equivalent:
1091 set PERLIO_DEBUG=CON
1094 This functionality is disabled for setuid scripts and for scripts run
1099 A list of directories in which to look for Perl library
1100 files before looking in the standard library and the current directory.
1101 If PERL5LIB is defined, PERLLIB is not used.
1105 The command used to load the debugger code. The default is:
1107 BEGIN { require 'perl5db.pl' }
1109 =item PERL5DB_THREADED
1111 If set to a true value, indicates to the debugger that the code being
1112 debugged uses threads.
1114 =item PERL5SHELL (specific to the Win32 port)
1116 May be set to an alternative shell that perl must use internally for
1117 executing "backtick" commands or system(). Default is C<cmd.exe /x/d/c>
1118 on WindowsNT and C<command.com /c> on Windows95. The value is considered
1119 to be space-separated. Precede any character that needs to be protected
1120 (like a space or backslash) with a backslash.
1122 Note that Perl doesn't use COMSPEC for this purpose because
1123 COMSPEC has a high degree of variability among users, leading to
1124 portability concerns. Besides, perl can use a shell that may not be
1125 fit for interactive use, and setting COMSPEC to such a shell may
1126 interfere with the proper functioning of other programs (which usually
1127 look in COMSPEC to find a shell fit for interactive use).
1129 =item PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
1131 Set to 1 to allow the use of non-IFS compatible LSP's.
1132 Perl normally searches for an IFS-compatible LSP because this is required
1133 for its emulation of Windows sockets as real filehandles. However, this may
1134 cause problems if you have a firewall such as McAfee Guardian which requires
1135 all applications to use its LSP which is not IFS-compatible, because clearly
1136 Perl will normally avoid using such an LSP.
1137 Setting this environment variable to 1 means that Perl will simply use the
1138 first suitable LSP enumerated in the catalog, which keeps McAfee Guardian
1139 happy (and in that particular case Perl still works too because McAfee
1140 Guardian's LSP actually plays some other games which allow applications
1141 requiring IFS compatibility to work).
1143 =item PERL_DEBUG_MSTATS
1145 Relevant only if perl is compiled with the malloc included with the perl
1146 distribution (that is, if C<perl -V:d_mymalloc> is 'define').
1147 If set, this causes memory statistics to be dumped after execution. If set
1148 to an integer greater than one, also causes memory statistics to be dumped
1151 =item PERL_DESTRUCT_LEVEL
1153 Relevant only if your perl executable was built with B<-DDEBUGGING>,
1154 this controls the behavior of global destruction of objects and other
1155 references. See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.
1157 =item PERL_DL_NONLAZY
1159 Set to one to have perl resolve B<all> undefined symbols when it loads
1160 a dynamic library. The default behaviour is to resolve symbols when
1161 they are used. Setting this variable is useful during testing of
1162 extensions as it ensures that you get an error on misspelled function
1163 names even if the test suite doesn't call it.
1167 If using the C<encoding> pragma without an explicit encoding name, the
1168 PERL_ENCODING environment variable is consulted for an encoding name.
1170 =item PERL_HASH_SEED
1172 (Since Perl 5.8.1.) Used to randomise Perl's internal hash function.
1173 To emulate the pre-5.8.1 behaviour, set to an integer (zero means
1174 exactly the same order as 5.8.0). "Pre-5.8.1" means, among other
1175 things, that hash keys will be ordered the same between different runs
1178 The default behaviour is to randomise unless the PERL_HASH_SEED is set.
1179 If Perl has been compiled with C<-DUSE_HASH_SEED_EXPLICIT>, the default
1180 behaviour is B<not> to randomise unless the PERL_HASH_SEED is set.
1182 If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses
1183 the pseudorandom seed supplied by the operating system and libraries.
1184 This means that each different run of Perl will have a different
1185 ordering of the results of keys(), values(), and each().
1187 B<Please note that the hash seed is sensitive information>. Hashes are
1188 randomized to protect against local and remote attacks against Perl
1189 code. By manually setting a seed this protection may be partially or
1192 See L<perlsec/"Algorithmic Complexity Attacks"> and
1193 L</PERL_HASH_SEED_DEBUG> for more information.
1195 =item PERL_HASH_SEED_DEBUG
1197 (Since Perl 5.8.1.) Set to one to display (to STDERR) the value of
1198 the hash seed at the beginning of execution. This, combined with
1199 L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
1200 behavior caused by hash randomization.
1202 B<Note that the hash seed is sensitive information>: by knowing it one
1203 can craft a denial-of-service attack against Perl code, even remotely,
1204 see L<perlsec/"Algorithmic Complexity Attacks"> for more information.
1205 B<Do not disclose the hash seed> to people who don't need to know it.
1206 See also hash_seed() of L<Hash::Util>.
1208 =item PERL_ROOT (specific to the VMS port)
1210 A translation concealed rooted logical name that contains perl and the
1211 logical device for the @INC path on VMS only. Other logical names that
1212 affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1213 SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in
1214 L<perlvms> and in F<README.vms> in the Perl source distribution.
1218 In Perls 5.8.1 and later. If set to C<unsafe> the pre-Perl-5.8.0
1219 signals behaviour (immediate but unsafe) is restored. If set to
1220 C<safe> the safe (or deferred) signals are used.
1221 See L<perlipc/"Deferred Signals (Safe Signals)">.
1225 Equivalent to the B<-C> command-line switch. Note that this is not
1226 a boolean variable-- setting this to C<"1"> is not the right way to
1227 "enable Unicode" (whatever that would mean). You can use C<"0"> to
1228 "disable Unicode", though (or alternatively unset PERL_UNICODE in
1229 your shell before starting Perl). See the description of the C<-C>
1230 switch for more information.
1232 =item SYS$LOGIN (specific to the VMS port)
1234 Used if chdir has no argument and HOME and LOGDIR are not set.
1238 Perl also has environment variables that control how Perl handles data
1239 specific to particular natural languages. See L<perllocale>.
1241 Apart from these, Perl uses no other environment variables, except
1242 to make them available to the program being executed, and to child
1243 processes. However, programs running setuid would do well to execute
1244 the following lines before doing anything else, just to keep people
1247 $ENV{PATH} = '/bin:/usr/bin'; # or whatever you need
1248 $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
1249 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};