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>[: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
345 runs the program under the Perl debugger. See L<perldebug>.
347 =item B<-d:>I<foo[=bar,baz]>
349 runs the program under the control of a debugging, profiling, or
350 tracing module installed as Devel::foo. E.g., B<-d:DProf> executes
351 the program using the Devel::DProf profiler. As with the B<-M>
352 flag, options may be passed to the Devel::foo package where they
353 will be received and interpreted by the Devel::foo::import routine.
354 The comma-separated list of options must follow a C<=> character.
357 =item B<-D>I<letters>
361 sets debugging flags. To watch how it executes your program, use
362 B<-Dtls>. (This works only if debugging is compiled into your
363 Perl.) Another nice value is B<-Dx>, which lists your compiled
364 syntax tree. And B<-Dr> displays compiled regular expressions;
365 the format of the output is explained in L<perldebguts>.
367 As an alternative, specify a number instead of list of letters (e.g.,
368 B<-D14> is equivalent to B<-Dtls>):
370 1 p Tokenizing and parsing
372 with v, displays all stacks
373 4 l Context (loop) stack processing
375 16 o Method and overloading resolution
376 32 c String/numeric conversions
377 64 P Print profiling info, preprocessor command for -P, source file input state
378 128 m Memory allocation
379 256 f Format processing
380 512 r Regular expression parsing and execution
381 1024 x Syntax tree dump
382 2048 u Tainting checks
383 4096 (Obsolete, previously used for LEAKTEST)
384 8192 H Hash dump -- usurps values()
385 16384 X Scratchpad allocation
387 65536 S Thread synchronization
389 262144 R Include reference counts of dumped variables (eg when using -Ds)
390 524288 J Do not s,t,P-debug (Jump over) opcodes within package DB
391 1048576 v Verbose: use in conjunction with other flags
392 2097152 C Copy On Write
394 All these flags require B<-DDEBUGGING> when you compile the Perl
395 executable (but see L<Devel::Peek>, L<re> which may change this).
396 See the F<INSTALL> file in the Perl source distribution
397 for how to do this. This flag is automatically set if you include B<-g>
398 option when C<Configure> asks you about optimizer/debugger flags.
400 If you're just trying to get a print out of each line of Perl code
401 as it executes, the way that C<sh -x> provides for shell scripts,
402 you can't use Perl's B<-D> switch. Instead do this
404 # If you have "env" utility
405 env=PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
407 # Bourne shell syntax
408 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
411 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
413 See L<perldebug> for details and variations.
415 =item B<-e> I<commandline>
417 may be used to enter one line of program. If B<-e> is given, Perl
418 will not look for a filename in the argument list. Multiple B<-e>
419 commands may be given to build up a multi-line script. Make sure
420 to use semicolons where you would in a normal program.
422 =item B<-F>I<pattern>
424 specifies the pattern to split on if B<-a> is also in effect. The
425 pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
426 put in single quotes.
430 prints a summary of the options.
432 =item B<-i>[I<extension>]
434 specifies that files processed by the C<E<lt>E<gt>> construct are to be
435 edited in-place. It does this by renaming the input file, opening the
436 output file by the original name, and selecting that output file as the
437 default for print() statements. The extension, if supplied, is used to
438 modify the name of the old file to make a backup copy, following these
441 If no extension is supplied, no backup is made and the current file is
444 If the extension doesn't contain a C<*>, then it is appended to the
445 end of the current filename as a suffix. If the extension does
446 contain one or more C<*> characters, then each C<*> is replaced
447 with the current filename. In Perl terms, you could think of this
450 ($backup = $extension) =~ s/\*/$file_name/g;
452 This allows you to add a prefix to the backup file, instead of (or in
453 addition to) a suffix:
455 $ perl -pi'orig_*' -e 's/bar/baz/' fileA # backup to 'orig_fileA'
457 Or even to place backup copies of the original files into another
458 directory (provided the directory already exists):
460 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'
462 These sets of one-liners are equivalent:
464 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
465 $ perl -pi'*' -e 's/bar/baz/' fileA # overwrite current file
467 $ perl -pi'.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
468 $ perl -pi'*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
470 From the shell, saying
472 $ perl -p -i.orig -e "s/foo/bar/; ... "
474 is the same as using the program:
476 #!/usr/bin/perl -pi.orig
479 which is equivalent to
482 $extension = '.orig';
484 if ($ARGV ne $oldargv) {
485 if ($extension !~ /\*/) {
486 $backup = $ARGV . $extension;
489 ($backup = $extension) =~ s/\*/$ARGV/g;
491 rename($ARGV, $backup);
492 open(ARGVOUT, ">$ARGV");
499 print; # this prints to original filename
503 except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
504 know when the filename has changed. It does, however, use ARGVOUT for
505 the selected filehandle. Note that STDOUT is restored as the default
506 output filehandle after the loop.
508 As shown above, Perl creates the backup file whether or not any output
509 is actually changed. So this is just a fancy way to copy files:
511 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
513 $ perl -p -i'.orig' -e 1 file1 file2 file3...
515 You can use C<eof> without parentheses to locate the end of each input
516 file, in case you want to append to each file, or reset line numbering
517 (see example in L<perlfunc/eof>).
519 If, for a given file, Perl is unable to create the backup file as
520 specified in the extension then it will skip that file and continue on
521 with the next one (if it exists).
523 For a discussion of issues surrounding file permissions and B<-i>,
524 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?>.
526 You cannot use B<-i> to create directories or to strip extensions from
529 Perl does not expand C<~> in filenames, which is good, since some
530 folks use it for their backup files:
532 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
534 Finally, the B<-i> switch does not impede execution when no
535 files are given on the command line. In this case, no backup is made
536 (the original file cannot, of course, be determined) and processing
537 proceeds from STDIN to STDOUT as might be expected.
539 =item B<-I>I<directory>
541 Directories specified by B<-I> are prepended to the search path for
542 modules (C<@INC>), and also tells the C preprocessor where to search for
543 include files. The C preprocessor is invoked with B<-P>; by default it
544 searches /usr/include and /usr/lib/perl.
546 =item B<-l>[I<octnum>]
548 enables automatic line-ending processing. It has two separate
549 effects. First, it automatically chomps C<$/> (the input record
550 separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
551 (the output record separator) to have the value of I<octnum> so
552 that any print statements will have that separator added back on.
553 If I<octnum> is omitted, sets C<$\> to the current value of
554 C<$/>. For instance, to trim lines to 80 columns:
556 perl -lpe 'substr($_, 80) = ""'
558 Note that the assignment C<$\ = $/> is done when the switch is processed,
559 so the input record separator can be different than the output record
560 separator if the B<-l> switch is followed by a B<-0> switch:
562 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
564 This sets C<$\> to newline and then sets C<$/> to the null character.
566 =item B<-m>[B<->]I<module>
568 =item B<-M>[B<->]I<module>
570 =item B<-M>[B<->]I<'module ...'>
572 =item B<-[mM]>[B<->]I<module=arg[,arg]...>
574 B<-m>I<module> executes C<use> I<module> C<();> before executing your
577 B<-M>I<module> executes C<use> I<module> C<;> before executing your
578 program. You can use quotes to add extra code after the module name,
579 e.g., C<'-Mmodule qw(foo bar)'>.
581 If the first character after the B<-M> or B<-m> is a dash (C<->)
582 then the 'use' is replaced with 'no'.
584 A little builtin syntactic sugar means you can also say
585 B<-mmodule=foo,bar> or B<-Mmodule=foo,bar> as a shortcut for
586 C<'-Mmodule qw(foo bar)'>. This avoids the need to use quotes when
587 importing symbols. The actual code generated by B<-Mmodule=foo,bar> is
588 C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
589 removes the distinction between B<-m> and B<-M>.
593 causes Perl to assume the following loop around your program, which
594 makes it iterate over filename arguments somewhat like B<sed -n> or
599 ... # your program goes here
602 Note that the lines are not printed by default. See B<-p> to have
603 lines printed. If a file named by an argument cannot be opened for
604 some reason, Perl warns you about it and moves on to the next file.
606 Here is an efficient way to delete all files older than a week:
608 find . -mtime +7 -print | perl -nle unlink
610 This is faster than using the B<-exec> switch of B<find> because you don't
611 have to start a process on every filename found. It does suffer from
612 the bug of mishandling newlines in pathnames, which you can fix if
613 you follow the example under B<-0>.
615 C<BEGIN> and C<END> blocks may be used to capture control before or after
616 the implicit program loop, just as in B<awk>.
620 causes Perl to assume the following loop around your program, which
621 makes it iterate over filename arguments somewhat like B<sed>:
626 ... # your program goes here
628 print or die "-p destination: $!\n";
631 If a file named by an argument cannot be opened for some reason, Perl
632 warns you about it, and moves on to the next file. Note that the
633 lines are printed automatically. An error occurring during printing is
634 treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
635 overrides a B<-n> switch.
637 C<BEGIN> and C<END> blocks may be used to capture control before or after
638 the implicit loop, just as in B<awk>.
642 B<NOTE: Use of -P is strongly discouraged because of its inherent
643 problems, including poor portability.>
645 This option causes your program to be run through the C preprocessor before
646 compilation by Perl. Because both comments and B<cpp> directives begin
647 with the # character, you should avoid starting comments with any words
648 recognized by the C preprocessor such as C<"if">, C<"else">, or C<"define">.
650 If you're considering using C<-P>, you might also want to look at the
651 Filter::cpp module from CPAN.
653 The problems of -P include, but are not limited to:
659 The C<#!> line is stripped, so any switches there don't apply.
663 A C<-P> on a C<#!> line doesn't work.
667 B<All> lines that begin with (whitespace and) a C<#> but
668 do not look like cpp commands, are stripped, including anything
669 inside Perl strings, regular expressions, and here-docs .
673 In some platforms the C preprocessor knows too much: it knows about
674 the C++ -style until-end-of-line comments starting with C<"//">.
675 This will cause problems with common Perl constructs like
679 because after -P this will became illegal code
683 The workaround is to use some other quoting separator than C<"/">,
684 like for example C<"!">:
692 It requires not only a working C preprocessor but also a working
693 F<sed>. If not on UNIX, you are probably out of luck on this.
697 Script line numbers are not preserved.
701 The C<-x> does not work with C<-P>.
707 enables rudimentary switch parsing for switches on the command
708 line after the program name but before any filename arguments (or before
709 an argument of B<-->). This means you can have switches with two leading
710 dashes (B<--help>). Any switch found there is removed from @ARGV and sets the
711 corresponding variable in the Perl program. The following program
712 prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
713 if it is invoked with B<-xyz=abc>.
716 if ($xyz) { print "$xyz\n" }
718 Do note that B<--help> creates the variable ${-help}, which is not compliant
723 makes Perl use the PATH environment variable to search for the
724 program (unless the name of the program contains directory separators).
726 On some platforms, this also makes Perl append suffixes to the
727 filename while searching for it. For example, on Win32 platforms,
728 the ".bat" and ".cmd" suffixes are appended if a lookup for the
729 original name fails, and if the name does not already end in one
730 of those suffixes. If your Perl was compiled with DEBUGGING turned
731 on, using the -Dp switch to Perl shows how the search progresses.
733 Typically this is used to emulate #! startup on platforms that
734 don't support #!. This example works on many platforms that
735 have a shell compatible with Bourne shell:
738 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
739 if $running_under_some_shell;
741 The system ignores the first line and feeds the program to F</bin/sh>,
742 which proceeds to try to execute the Perl program as a shell script.
743 The shell executes the second line as a normal shell command, and thus
744 starts up the Perl interpreter. On some systems $0 doesn't always
745 contain the full pathname, so the B<-S> tells Perl to search for the
746 program if necessary. After Perl locates the program, it parses the
747 lines and ignores them because the variable $running_under_some_shell
748 is never true. If the program will be interpreted by csh, you will need
749 to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
750 embedded spaces (and such) in the argument list. To start up sh rather
751 than csh, some systems may have to replace the #! line with a line
752 containing just a colon, which will be politely ignored by Perl. Other
753 systems can't control that, and need a totally devious construct that
754 will work under any of B<csh>, B<sh>, or Perl, such as the following:
756 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
757 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
758 if $running_under_some_shell;
760 If the filename supplied contains directory separators (i.e., is an
761 absolute or relative pathname), and if that file is not found,
762 platforms that append file extensions will do so and try to look
763 for the file with those extensions added, one by one.
765 On DOS-like platforms, if the program does not contain directory
766 separators, it will first be searched for in the current directory
767 before being searched for on the PATH. On Unix platforms, the
768 program will be searched for strictly on the PATH.
772 Like B<-T>, but taint checks will issue warnings rather than fatal
773 errors. These warnings can be controlled normally with C<no warnings
776 B<NOTE: this is not a substitute for -T.> This is meant only to be
777 used as a temporary development aid while securing legacy code:
778 for real production code and for new secure code written from scratch
779 always use the real B<-T>.
783 forces "taint" checks to be turned on so you can test them. Ordinarily
784 these checks are done only when running setuid or setgid. It's a
785 good idea to turn them on explicitly for programs that run on behalf
786 of someone else whom you might not necessarily trust, such as CGI
787 programs or any internet servers you might write in Perl. See
788 L<perlsec> for details. For security reasons, this option must be
789 seen by Perl quite early; usually this means it must appear early
790 on the command line or in the #! line for systems which support
795 This obsolete switch causes Perl to dump core after compiling your
796 program. You can then in theory take this core dump and turn it
797 into an executable file by using the B<undump> program (not supplied).
798 This speeds startup at the expense of some disk space (which you
799 can minimize by stripping the executable). (Still, a "hello world"
800 executable comes out to about 200K on my machine.) If you want to
801 execute a portion of your program before dumping, use the dump()
802 operator instead. Note: availability of B<undump> is platform
803 specific and may not be available for a specific port of Perl.
805 This switch has been superseded in favor of the new Perl code
806 generator backends to the compiler. See L<B> and L<B::Bytecode>
811 allows Perl to do unsafe operations. Currently the only "unsafe"
812 operations are the unlinking of directories while running as superuser,
813 and running setuid programs with fatal taint checks turned into
814 warnings. Note that the B<-w> switch (or the C<$^W> variable) must
815 be used along with this option to actually I<generate> the
816 taint-check warnings.
820 prints the version and patchlevel of your perl executable.
824 prints summary of the major perl configuration values and the current
829 Prints to STDOUT the value of the named configuration variable.
834 will provide strong clues about what your MANPATH variable should
835 be set to in order to access the Perl documentation.
839 prints warnings about dubious constructs, such as variable names
840 that are mentioned only once and scalar variables that are used
841 before being set, redefined subroutines, references to undefined
842 filehandles or filehandles opened read-only that you are attempting
843 to write on, values used as a number that doesn't look like numbers,
844 using an array as though it were a scalar, if your subroutines
845 recurse more than 100 deep, and innumerable other things.
847 This switch really just enables the internal C<$^W> variable. You
848 can disable or promote into fatal errors specific warnings using
849 C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
850 See also L<perldiag> and L<perltrap>. A new, fine-grained warning
851 facility is also available if you want to manipulate entire classes
852 of warnings; see L<warnings> or L<perllexwarn>.
856 Enables all warnings regardless of C<no warnings> or C<$^W>.
861 Disables all warnings regardless of C<use warnings> or C<$^W>.
864 =item B<-x> I<directory>
866 tells Perl that the program is embedded in a larger chunk of unrelated
867 ASCII text, such as in a mail message. Leading garbage will be
868 discarded until the first line that starts with #! and contains the
869 string "perl". Any meaningful switches on that line will be applied.
870 If a directory name is specified, Perl will switch to that directory
871 before running the program. The B<-x> switch controls only the
872 disposal of leading garbage. The program must be terminated with
873 C<__END__> if there is trailing garbage to be ignored (the program
874 can process any or all of the trailing garbage via the DATA filehandle
885 Used if chdir has no argument.
889 Used if chdir has no argument and HOME is not set.
893 Used in executing subprocesses, and in finding the program if B<-S> is
898 A list of directories in which to look for Perl library
899 files before looking in the standard library and the current
900 directory. Any architecture-specific directories under the specified
901 locations are automatically included if they exist. If PERL5LIB is not
902 defined, PERLLIB is used. Directories are separated (like in PATH) by
903 a colon on unixish platforms and by a semicolon on Windows (the proper
904 path separator being given by the command C<perl -V:path_sep>).
906 When running taint checks (either because the program was running setuid
907 or setgid, or the B<-T> switch was used), neither variable is used.
908 The program should instead say:
910 use lib "/my/directory";
914 Command-line options (switches). Switches in this variable are taken
915 as if they were on every Perl command line. Only the B<-[DIMUdmtw]>
916 switches are allowed. When running taint checks (because the program
917 was running setuid or setgid, or the B<-T> switch was used), this
918 variable is ignored. If PERL5OPT begins with B<-T>, tainting will be
919 enabled, and any subsequent options ignored.
923 A space (or colon) separated list of PerlIO layers. If perl is built
924 to use PerlIO system for IO (the default) these layers effect perl's IO.
926 It is conventional to start layer names with a colon e.g. C<:perlio> to
927 emphasise their similarity to variable "attributes". But the code that parses
928 layer specification strings (which is also used to decode the PERLIO
929 environment variable) treats the colon as a separator.
931 An unset or empty PERLIO is equivalent to C<:stdio>.
933 The list becomes the default for I<all> perl's IO. Consequently only built-in
934 layers can appear in this list, as external layers (such as :encoding()) need
935 IO in order to load them!. See L<"open pragma"|open> for how to add external
936 encodings as defaults.
938 The layers that it makes sense to include in the PERLIO environment
939 variable are summarised below. For more details see L<PerlIO>.
945 A pseudolayer that turns I<off> the C<:utf8> flag for the layer below.
946 Unlikely to be useful on its own in the global PERLIO environment variable.
947 You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
951 A layer that implements DOS/Windows like CRLF line endings. On read
952 converts pairs of CR,LF to a single "\n" newline character. On write
953 converts each "\n" to a CR,LF pair. Note that this layer likes to be
954 one of its kind: it silently ignores attempts to be pushed into the
955 layer stack more than once.
957 (Gory details follow) To be more exact what happens is this: after
958 pushing itself to the stack, the C<:crlf> layer checks all the layers
959 below itself to find the first layer that is capable of being a CRLF
960 layer but is not yet enabled to be a CRLF layer. If it finds such a
961 layer, it enables the CRLFness of that other deeper layer, and then
962 pops itself off the stack. If not, fine, use the one we just pushed.
964 The end result is that a C<:crlf> means "please enable the first CRLF
965 layer you can find, and if you can't find one, here would be a good
966 spot to place a new one."
968 Based on the C<:perlio> layer.
972 A layer which implements "reading" of files by using C<mmap()> to
973 make (whole) file appear in the process's address space, and then
974 using that as PerlIO's "buffer". This I<may> be faster in certain
975 circumstances for large files, and may result in less physical memory
976 use when multiple processes are reading the same file.
978 Files which are not C<mmap()>-able revert to behaving like the C<:perlio>
979 layer. Writes also behave like C<:perlio> layer as C<mmap()> for write
980 needs extra house-keeping (to extend the file) which negates any advantage.
982 The C<:mmap> layer will not exist if platform does not support C<mmap()>.
986 A from scratch implementation of buffering for PerlIO. Provides fast
987 access to the buffer for C<sv_gets> which implements perl's readline/E<lt>E<gt>
988 and in general attempts to minimize data copying.
990 C<:perlio> will insert a C<:unix> layer below itself to do low level IO.
994 An experimental pseudolayer that removes the topmost layer.
995 Use with the same care as is reserved for nitroglyserin.
999 A pseudolayer that manipulates other layers. Applying the <:raw>
1000 layer is equivalent to calling C<binmode($fh)>. It makes the stream
1001 pass each byte as-is without any translation. In particular CRLF
1002 translation, and/or :utf8 intuited from locale are disabled.
1004 Arranges for all accesses go straight to the lowest buffered layer provided
1005 by the configration. That is it strips off any layers above that layer.
1007 In Perl 5.6 and some books the C<:raw> layer (previously sometimes also
1008 referred to as a "discipline") is documented as the inverse of the
1009 C<:crlf> layer. That is no longer the case - other layers which would
1010 alter binary nature of the stream are also disabled. If you want UNIX
1011 line endings on a platform that normally does CRLF translation, but still
1012 want UTF-8 or encoding defaults the appropriate thing to do is to add
1013 C<:perlio> to PERLIO environment variable.
1017 This layer provides PerlIO interface by wrapping system's ANSI C "stdio"
1018 library calls. The layer provides both buffering and IO.
1019 Note that C<:stdio> layer does I<not> do CRLF translation even if that
1020 is platforms normal behaviour. You will need a C<:crlf> layer above it
1025 Lowest level layer which provides basic PerlIO operations in terms of
1026 UNIX/POSIX numeric file descriptor calls
1027 C<open(), read(), write(), lseek(), close()>
1031 A pseudolayer that turns on a flag on the layer below to tell perl
1032 that data sent to the stream should be converted to perl internal
1033 "utf8" form and that data from the stream should be considered as so
1034 encoded. On ASCII based platforms the encoding is UTF-8 and on EBCDIC
1035 platforms UTF-EBCDIC. May be useful in PERLIO environment variable to
1036 make UTF-8 the default. (To turn off that behaviour use C<:bytes>
1041 On Win32 platforms this I<experimental> layer uses native "handle" IO
1042 rather than unix-like numeric file descriptor layer. Known to be
1043 buggy in this release.
1047 On all platforms the default set of layers should give acceptable results.
1049 For UNIX platforms that will equivalent of "unix perlio" or "stdio".
1050 Configure is setup to prefer "stdio" implementation if system's library
1051 provides for fast access to the buffer, otherwise it uses the "unix perlio"
1054 On Win32 the default in this release is "unix crlf". Win32's "stdio"
1055 has a number of bugs/mis-features for perl IO which are somewhat
1056 C compiler vendor/version dependent. Using our own C<crlf> layer as
1057 the buffer avoids those issues and makes things more uniform.
1058 The C<crlf> layer provides CRLF to/from "\n" conversion as well as
1061 This release uses C<unix> as the bottom layer on Win32 and so still uses C
1062 compiler's numeric file descriptor routines. There is an experimental native
1063 C<win32> layer which is expected to be enhanced and should eventually replace
1068 If set to the name of a file or device then certain operations of PerlIO
1069 sub-system will be logged to that file (opened as append). Typical uses
1072 PERLIO_DEBUG=/dev/tty perl script ...
1074 and Win32 approximate equivalent:
1076 set PERLIO_DEBUG=CON
1082 A list of directories in which to look for Perl library
1083 files before looking in the standard library and the current directory.
1084 If PERL5LIB is defined, PERLLIB is not used.
1088 The command used to load the debugger code. The default is:
1090 BEGIN { require 'perl5db.pl' }
1092 =item PERL5SHELL (specific to the Win32 port)
1094 May be set to an alternative shell that perl must use internally for
1095 executing "backtick" commands or system(). Default is C<cmd.exe /x/d/c>
1096 on WindowsNT and C<command.com /c> on Windows95. The value is considered
1097 to be space-separated. Precede any character that needs to be protected
1098 (like a space or backslash) with a backslash.
1100 Note that Perl doesn't use COMSPEC for this purpose because
1101 COMSPEC has a high degree of variability among users, leading to
1102 portability concerns. Besides, perl can use a shell that may not be
1103 fit for interactive use, and setting COMSPEC to such a shell may
1104 interfere with the proper functioning of other programs (which usually
1105 look in COMSPEC to find a shell fit for interactive use).
1107 =item PERL_DEBUG_MSTATS
1109 Relevant only if perl is compiled with the malloc included with the perl
1110 distribution (that is, if C<perl -V:d_mymalloc> is 'define').
1111 If set, this causes memory statistics to be dumped after execution. If set
1112 to an integer greater than one, also causes memory statistics to be dumped
1115 =item PERL_DESTRUCT_LEVEL
1117 Relevant only if your perl executable was built with B<-DDEBUGGING>,
1118 this controls the behavior of global destruction of objects and other
1119 references. See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.
1121 =item PERL_DL_NONLAZY
1123 Set to one to have perl resolve B<all> undefined symbols when it loads
1124 a dynamic library. The default behaviour is to resolve symbols when
1125 they are used. Setting this variable is useful during testing of
1126 extensions as it ensures that you get an error on misspelled function
1127 names even if the test suite doesn't call it.
1131 If using the C<encoding> pragma without an explicit encoding name, the
1132 PERL_ENCODING environment variable is consulted for an encoding name.
1134 =item PERL_HASH_SEED
1136 (Since Perl 5.8.1.) Used to randomise Perl's internal hash function.
1137 To emulate the pre-5.8.1 behaviour, set to an integer (zero means
1138 exactly the same order as 5.8.0). "Pre-5.8.1" means, among other
1139 things, that hash keys will be ordered the same between different runs
1142 The default behaviour is to randomise unless the PERL_HASH_SEED is set.
1143 If Perl has been compiled with C<-DUSE_HASH_SEED_EXPLICIT>, the default
1144 behaviour is B<not> to randomise unless the PERL_HASH_SEED is set.
1146 If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses
1147 the pseudorandom seed supplied by the operating system and libraries.
1148 This means that each different run of Perl will have a different
1149 ordering of the results of keys(), values(), and each().
1151 B<Please note that the hash seed is sensitive information>. Hashes are
1152 randomized to protect against local and remote attacks against Perl
1153 code. By manually setting a seed this protection may be partially or
1156 See L<perlsec/"Algorithmic Complexity Attacks"> and
1157 L</PERL_HASH_SEED_DEBUG> for more information.
1159 =item PERL_HASH_SEED_DEBUG
1161 (Since Perl 5.8.1.) Set to one to display (to STDERR) the value of
1162 the hash seed at the beginning of execution. This, combined with
1163 L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
1164 behavior caused by hash randomization.
1166 B<Note that the hash seed is sensitive information>: by knowing it one
1167 can craft a denial-of-service attack against Perl code, even remotely,
1168 see L<perlsec/"Algorithmic Complexity Attacks"> for more information.
1169 B<Do not disclose the hash seed> to people who don't need to know it.
1170 See also hash_seed() of L<Hash::Util>.
1172 =item PERL_ROOT (specific to the VMS port)
1174 A translation concealed rooted logical name that contains perl and the
1175 logical device for the @INC path on VMS only. Other logical names that
1176 affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1177 SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in
1178 L<perlvms> and in F<README.vms> in the Perl source distribution.
1182 In Perls 5.8.1 and later. If set to C<unsafe> the pre-Perl-5.8.0
1183 signals behaviour (immediate but unsafe) is restored. If set to
1184 C<safe> the safe (or deferred) signals are used.
1185 See L<perlipc/"Deferred Signals (Safe signals)">.
1189 Equivalent to the B<-C> command-line switch. Note that this is not
1190 a boolean variable-- setting this to C<"1"> is not the right way to
1191 "enable Unicode" (whatever that would mean). You can use C<"0"> to
1192 "disable Unicode", though (or alternatively unset PERL_UNICODE in
1193 your shell before starting Perl). See the description of the C<-C>
1194 switch for more information.
1196 =item SYS$LOGIN (specific to the VMS port)
1198 Used if chdir has no argument and HOME and LOGDIR are not set.
1202 Perl also has environment variables that control how Perl handles data
1203 specific to particular natural languages. See L<perllocale>.
1205 Apart from these, Perl uses no other environment variables, except
1206 to make them available to the program being executed, and to child
1207 processes. However, programs running setuid would do well to execute
1208 the following lines before doing anything else, just to keep people
1211 $ENV{PATH} = '/bin:/usr/bin'; # or whatever you need
1212 $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
1213 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};