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<ALTERNATIVE_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 in UTF-8
296 L 64 normally the "IOEioA" are unconditional,
297 the L makes them conditional on the locale environment
298 variables (the LC_ALL, LC_TYPE, and LANG, in the order
299 of decreasing precedence) -- if the variables indicate
300 UTF-8, then the selected "IOEioA" are in effect
302 For example, C<-COE> and C<-C6> will both turn on UTF-8-ness on both
303 STDOUT and STDERR. Repeating letters is just redundant, not cumulative
306 The C<io> options mean that any subsequent open() (or similar I/O
307 operations) will have the C<:utf8> PerlIO layer implicitly applied
308 to them, in other words, UTF-8 is expected from any input stream,
309 and UTF-8 is produced to any output stream. This is just the default,
310 with explicit layers in open() and with binmode() one can manipulate
313 C<-C> on its own (not followed by any number or option list), or the
314 empty string C<""> for the C<$ENV{PERL_UNICODE}, has the same effect
315 as <-CSDL>. In other words, the standard I/O handles and the default
316 C<open()> layer are UTF-8-fied B<but> only if the locale environment
317 variables indicate a UTF-8 locale. This behaviour follows the
318 I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
320 You can use C<-C0> (or C<"0"> for $ENV{PERL_UNICODE}) to explicitly
321 disable all the above Unicode features.
323 The read-only magic variable C<${^UNICODE}> reflects the numeric value
324 of this setting. This is variable is set during Perl startup and is
325 thereafter read-only. If you want runtime effects, use the three-arg
326 open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
327 and the C<open> pragma (see L<open>).
329 (In Perls earlier than 5.8.1 the C<-C> switch was a Win32-only switch
330 that enabled the use of Unicode-aware "wide system call" Win32 APIs.
331 This feature was practically unused, however, and the command line
332 switch was therefore "recycled".)
336 causes Perl to check the syntax of the program and then exit without
337 executing it. Actually, it I<will> execute C<BEGIN>, C<CHECK>, and
338 C<use> blocks, because these are considered as occurring outside the
339 execution of your program. C<INIT> and C<END> blocks, however, will
344 runs the program under the Perl debugger. See L<perldebug>.
346 =item B<-d:>I<foo[=bar,baz]>
348 runs the program under the control of a debugging, profiling, or
349 tracing module installed as Devel::foo. E.g., B<-d:DProf> executes
350 the program using the Devel::DProf profiler. As with the B<-M>
351 flag, options may be passed to the Devel::foo package where they
352 will be received and interpreted by the Devel::foo::import routine.
353 The comma-separated list of options must follow a C<=> character.
356 =item B<-D>I<letters>
360 sets debugging flags. To watch how it executes your program, use
361 B<-Dtls>. (This works only if debugging is compiled into your
362 Perl.) Another nice value is B<-Dx>, which lists your compiled
363 syntax tree. And B<-Dr> displays compiled regular expressions;
364 the format of the output is explained in L<perldebguts>.
366 As an alternative, specify a number instead of list of letters (e.g.,
367 B<-D14> is equivalent to B<-Dtls>):
369 1 p Tokenizing and parsing
371 with v, displays all stacks
372 4 l Context (loop) stack processing
374 16 o Method and overloading resolution
375 32 c String/numeric conversions
376 64 P Print profiling info, preprocessor command for -P, source file input state
377 128 m Memory allocation
378 256 f Format processing
379 512 r Regular expression parsing and execution
380 1024 x Syntax tree dump
381 2048 u Tainting checks
382 4096 (Obsolete, previously used for LEAKTEST)
383 8192 H Hash dump -- usurps values()
384 16384 X Scratchpad allocation
386 65536 S Thread synchronization
388 262144 R Include reference counts of dumped variables (eg when using -Ds)
389 524288 J Do not s,t,P-debug (Jump over) opcodes within package DB
390 1048576 v Verbose: use in conjunction with other flags
391 2097152 C Copy On Write
393 All these flags require B<-DDEBUGGING> when you compile the Perl
394 executable (but see L<Devel::Peek>, L<re> which may change this).
395 See the F<INSTALL> file in the Perl source distribution
396 for how to do this. This flag is automatically set if you include B<-g>
397 option when C<Configure> asks you about optimizer/debugger flags.
399 If you're just trying to get a print out of each line of Perl code
400 as it executes, the way that C<sh -x> provides for shell scripts,
401 you can't use Perl's B<-D> switch. Instead do this
403 # If you have "env" utility
404 env=PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
406 # Bourne shell syntax
407 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
410 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
412 See L<perldebug> for details and variations.
414 =item B<-e> I<commandline>
416 may be used to enter one line of program. If B<-e> is given, Perl
417 will not look for a filename in the argument list. Multiple B<-e>
418 commands may be given to build up a multi-line script. Make sure
419 to use semicolons where you would in a normal program.
421 =item B<-F>I<pattern>
423 specifies the pattern to split on if B<-a> is also in effect. The
424 pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
425 put in single quotes.
429 prints a summary of the options.
431 =item B<-i>[I<extension>]
433 specifies that files processed by the C<E<lt>E<gt>> construct are to be
434 edited in-place. It does this by renaming the input file, opening the
435 output file by the original name, and selecting that output file as the
436 default for print() statements. The extension, if supplied, is used to
437 modify the name of the old file to make a backup copy, following these
440 If no extension is supplied, no backup is made and the current file is
443 If the extension doesn't contain a C<*>, then it is appended to the
444 end of the current filename as a suffix. If the extension does
445 contain one or more C<*> characters, then each C<*> is replaced
446 with the current filename. In Perl terms, you could think of this
449 ($backup = $extension) =~ s/\*/$file_name/g;
451 This allows you to add a prefix to the backup file, instead of (or in
452 addition to) a suffix:
454 $ perl -pi 'orig_*' -e 's/bar/baz/' fileA # backup to 'orig_fileA'
456 Or even to place backup copies of the original files into another
457 directory (provided the directory already exists):
459 $ perl -pi 'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'
461 These sets of one-liners are equivalent:
463 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
464 $ perl -pi '*' -e 's/bar/baz/' fileA # overwrite current file
466 $ perl -pi '.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
467 $ perl -pi '*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
469 From the shell, saying
471 $ perl -p -i.orig -e "s/foo/bar/; ... "
473 is the same as using the program:
475 #!/usr/bin/perl -pi.orig
478 which is equivalent to
481 $extension = '.orig';
483 if ($ARGV ne $oldargv) {
484 if ($extension !~ /\*/) {
485 $backup = $ARGV . $extension;
488 ($backup = $extension) =~ s/\*/$ARGV/g;
490 rename($ARGV, $backup);
491 open(ARGVOUT, ">$ARGV");
498 print; # this prints to original filename
502 except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
503 know when the filename has changed. It does, however, use ARGVOUT for
504 the selected filehandle. Note that STDOUT is restored as the default
505 output filehandle after the loop.
507 As shown above, Perl creates the backup file whether or not any output
508 is actually changed. So this is just a fancy way to copy files:
510 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
512 $ perl -p -i'.orig' -e 1 file1 file2 file3...
514 You can use C<eof> without parentheses to locate the end of each input
515 file, in case you want to append to each file, or reset line numbering
516 (see example in L<perlfunc/eof>).
518 If, for a given file, Perl is unable to create the backup file as
519 specified in the extension then it will skip that file and continue on
520 with the next one (if it exists).
522 For a discussion of issues surrounding file permissions and B<-i>,
523 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?>.
525 You cannot use B<-i> to create directories or to strip extensions from
528 Perl does not expand C<~> in filenames, which is good, since some
529 folks use it for their backup files:
531 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
533 Finally, the B<-i> switch does not impede execution when no
534 files are given on the command line. In this case, no backup is made
535 (the original file cannot, of course, be determined) and processing
536 proceeds from STDIN to STDOUT as might be expected.
538 =item B<-I>I<directory>
540 Directories specified by B<-I> are prepended to the search path for
541 modules (C<@INC>), and also tells the C preprocessor where to search for
542 include files. The C preprocessor is invoked with B<-P>; by default it
543 searches /usr/include and /usr/lib/perl.
545 =item B<-l>[I<octnum>]
547 enables automatic line-ending processing. It has two separate
548 effects. First, it automatically chomps C<$/> (the input record
549 separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
550 (the output record separator) to have the value of I<octnum> so
551 that any print statements will have that separator added back on.
552 If I<octnum> is omitted, sets C<$\> to the current value of
553 C<$/>. For instance, to trim lines to 80 columns:
555 perl -lpe 'substr($_, 80) = ""'
557 Note that the assignment C<$\ = $/> is done when the switch is processed,
558 so the input record separator can be different than the output record
559 separator if the B<-l> switch is followed by a B<-0> switch:
561 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
563 This sets C<$\> to newline and then sets C<$/> to the null character.
565 =item B<-m>[B<->]I<module>
567 =item B<-M>[B<->]I<module>
569 =item B<-M>[B<->]I<'module ...'>
571 =item B<-[mM]>[B<->]I<module=arg[,arg]...>
573 B<-m>I<module> executes C<use> I<module> C<();> before executing your
576 B<-M>I<module> executes C<use> I<module> C<;> before executing your
577 program. You can use quotes to add extra code after the module name,
578 e.g., C<'-Mmodule qw(foo bar)'>.
580 If the first character after the B<-M> or B<-m> is a dash (C<->)
581 then the 'use' is replaced with 'no'.
583 A little builtin syntactic sugar means you can also say
584 B<-mmodule=foo,bar> or B<-Mmodule=foo,bar> as a shortcut for
585 C<'-Mmodule qw(foo bar)'>. This avoids the need to use quotes when
586 importing symbols. The actual code generated by B<-Mmodule=foo,bar> is
587 C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
588 removes the distinction between B<-m> and B<-M>.
592 causes Perl to assume the following loop around your program, which
593 makes it iterate over filename arguments somewhat like B<sed -n> or
598 ... # your program goes here
601 Note that the lines are not printed by default. See B<-p> to have
602 lines printed. If a file named by an argument cannot be opened for
603 some reason, Perl warns you about it and moves on to the next file.
605 Here is an efficient way to delete all files older than a week:
607 find . -mtime +7 -print | perl -nle unlink
609 This is faster than using the B<-exec> switch of B<find> because you don't
610 have to start a process on every filename found. It does suffer from
611 the bug of mishandling newlines in pathnames, which you can fix if
612 you follow the example under B<-0>.
614 C<BEGIN> and C<END> blocks may be used to capture control before or after
615 the implicit program loop, just as in B<awk>.
619 causes Perl to assume the following loop around your program, which
620 makes it iterate over filename arguments somewhat like B<sed>:
625 ... # your program goes here
627 print or die "-p destination: $!\n";
630 If a file named by an argument cannot be opened for some reason, Perl
631 warns you about it, and moves on to the next file. Note that the
632 lines are printed automatically. An error occurring during printing is
633 treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
634 overrides a B<-n> switch.
636 C<BEGIN> and C<END> blocks may be used to capture control before or after
637 the implicit loop, just as in B<awk>.
641 B<NOTE: Use of -P is strongly discouraged because of its inherent
642 problems, including poor portability.>
644 This option causes your program to be run through the C preprocessor before
645 compilation by Perl. Because both comments and B<cpp> directives begin
646 with the # character, you should avoid starting comments with any words
647 recognized by the C preprocessor such as C<"if">, C<"else">, or C<"define">.
649 If you're considering using C<-P>, you might also want to look at the
650 Filter::cpp module from CPAN.
652 The problems of -P include, but are not limited to:
658 The C<#!> line is stripped, so any switches there don't apply.
662 A C<-P> on a C<#!> line doesn't work.
666 B<All> lines that begin with (whitespace and) a C<#> but
667 do not look like cpp commands, are stripped, including anything
668 inside Perl strings, regular expressions, and here-docs .
672 In some platforms the C preprocessor knows too much: it knows about
673 the C++ -style until-end-of-line comments starting with C<"//">.
674 This will cause problems with common Perl constructs like
678 because after -P this will became illegal code
682 The workaround is to use some other quoting separator than C<"/">,
683 like for example C<"!">:
691 It requires not only a working C preprocessor but also a working
692 F<sed>. If not on UNIX, you are probably out of luck on this.
696 Script line numbers are not preserved.
700 The C<-x> does not work with C<-P>.
706 enables rudimentary switch parsing for switches on the command
707 line after the program name but before any filename arguments (or before
708 an argument of B<-->). This means you can have switches with two leading
709 dashes (B<--help>). Any switch found there is removed from @ARGV and sets the
710 corresponding variable in the Perl program. The following program
711 prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
712 if it is invoked with B<-xyz=abc>.
715 if ($xyz) { print "$xyz\n" }
717 Do note that B<--help> creates the variable ${-help}, which is not compliant
722 makes Perl use the PATH environment variable to search for the
723 program (unless the name of the program contains directory separators).
725 On some platforms, this also makes Perl append suffixes to the
726 filename while searching for it. For example, on Win32 platforms,
727 the ".bat" and ".cmd" suffixes are appended if a lookup for the
728 original name fails, and if the name does not already end in one
729 of those suffixes. If your Perl was compiled with DEBUGGING turned
730 on, using the -Dp switch to Perl shows how the search progresses.
732 Typically this is used to emulate #! startup on platforms that
733 don't support #!. This example works on many platforms that
734 have a shell compatible with Bourne shell:
737 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
738 if $running_under_some_shell;
740 The system ignores the first line and feeds the program to F</bin/sh>,
741 which proceeds to try to execute the Perl program as a shell script.
742 The shell executes the second line as a normal shell command, and thus
743 starts up the Perl interpreter. On some systems $0 doesn't always
744 contain the full pathname, so the B<-S> tells Perl to search for the
745 program if necessary. After Perl locates the program, it parses the
746 lines and ignores them because the variable $running_under_some_shell
747 is never true. If the program will be interpreted by csh, you will need
748 to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
749 embedded spaces (and such) in the argument list. To start up sh rather
750 than csh, some systems may have to replace the #! line with a line
751 containing just a colon, which will be politely ignored by Perl. Other
752 systems can't control that, and need a totally devious construct that
753 will work under any of B<csh>, B<sh>, or Perl, such as the following:
755 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
756 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
757 if $running_under_some_shell;
759 If the filename supplied contains directory separators (i.e., is an
760 absolute or relative pathname), and if that file is not found,
761 platforms that append file extensions will do so and try to look
762 for the file with those extensions added, one by one.
764 On DOS-like platforms, if the program does not contain directory
765 separators, it will first be searched for in the current directory
766 before being searched for on the PATH. On Unix platforms, the
767 program will be searched for strictly on the PATH.
771 Like B<-T>, but taint checks will issue warnings rather than fatal
772 errors. These warnings can be controlled normally with C<no warnings
775 B<NOTE: this is not a substitute for -T.> This is meant only to be
776 used as a temporary development aid while securing legacy code:
777 for real production code and for new secure code written from scratch
778 always use the real B<-T>.
782 forces "taint" checks to be turned on so you can test them. Ordinarily
783 these checks are done only when running setuid or setgid. It's a
784 good idea to turn them on explicitly for programs that run on behalf
785 of someone else whom you might not necessarily trust, such as CGI
786 programs or any internet servers you might write in Perl. See
787 L<perlsec> for details. For security reasons, this option must be
788 seen by Perl quite early; usually this means it must appear early
789 on the command line or in the #! line for systems which support
794 This obsolete switch causes Perl to dump core after compiling your
795 program. You can then in theory take this core dump and turn it
796 into an executable file by using the B<undump> program (not supplied).
797 This speeds startup at the expense of some disk space (which you
798 can minimize by stripping the executable). (Still, a "hello world"
799 executable comes out to about 200K on my machine.) If you want to
800 execute a portion of your program before dumping, use the dump()
801 operator instead. Note: availability of B<undump> is platform
802 specific and may not be available for a specific port of Perl.
804 This switch has been superseded in favor of the new Perl code
805 generator backends to the compiler. See L<B> and L<B::Bytecode>
810 allows Perl to do unsafe operations. Currently the only "unsafe"
811 operations are the unlinking of directories while running as superuser,
812 and running setuid programs with fatal taint checks turned into
813 warnings. Note that the B<-w> switch (or the C<$^W> variable) must
814 be used along with this option to actually I<generate> the
815 taint-check warnings.
819 prints the version and patchlevel of your perl executable.
823 prints summary of the major perl configuration values and the current
828 Prints to STDOUT the value of the named configuration variable.
833 will provide strong clues about what your MANPATH variable should
834 be set to in order to access the Perl documentation.
838 prints warnings about dubious constructs, such as variable names
839 that are mentioned only once and scalar variables that are used
840 before being set, redefined subroutines, references to undefined
841 filehandles or filehandles opened read-only that you are attempting
842 to write on, values used as a number that doesn't look like numbers,
843 using an array as though it were a scalar, if your subroutines
844 recurse more than 100 deep, and innumerable other things.
846 This switch really just enables the internal C<$^W> variable. You
847 can disable or promote into fatal errors specific warnings using
848 C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
849 See also L<perldiag> and L<perltrap>. A new, fine-grained warning
850 facility is also available if you want to manipulate entire classes
851 of warnings; see L<warnings> or L<perllexwarn>.
855 Enables all warnings regardless of C<no warnings> or C<$^W>.
860 Disables all warnings regardless of C<use warnings> or C<$^W>.
863 =item B<-x> I<directory>
865 tells Perl that the program is embedded in a larger chunk of unrelated
866 ASCII text, such as in a mail message. Leading garbage will be
867 discarded until the first line that starts with #! and contains the
868 string "perl". Any meaningful switches on that line will be applied.
869 If a directory name is specified, Perl will switch to that directory
870 before running the program. The B<-x> switch controls only the
871 disposal of leading garbage. The program must be terminated with
872 C<__END__> if there is trailing garbage to be ignored (the program
873 can process any or all of the trailing garbage via the DATA filehandle
884 Used if chdir has no argument.
888 Used if chdir has no argument and HOME is not set.
892 Used in executing subprocesses, and in finding the program if B<-S> is
897 A colon-separated list of directories in which to look for Perl library
898 files before looking in the standard library and the current
899 directory. Any architecture-specific directories under the specified
900 locations are automatically included if they exist. If PERL5LIB is not
901 defined, PERLLIB is used.
903 When running taint checks (either because the program was running setuid
904 or setgid, or the B<-T> switch was used), neither variable is used.
905 The program should instead say:
907 use lib "/my/directory";
911 Command-line options (switches). Switches in this variable are taken
912 as if they were on every Perl command line. Only the B<-[DIMUdmtw]>
913 switches are allowed. When running taint checks (because the program
914 was running setuid or setgid, or the B<-T> switch was used), this
915 variable is ignored. If PERL5OPT begins with B<-T>, tainting will be
916 enabled, and any subsequent options ignored.
920 A space (or colon) separated list of PerlIO layers. If perl is built
921 to use PerlIO system for IO (the default) these layers effect perl's IO.
923 It is conventional to start layer names with a colon e.g. C<:perlio> to
924 emphasise their similarity to variable "attributes". But the code that parses
925 layer specification strings (which is also used to decode the PERLIO
926 environment variable) treats the colon as a separator.
928 An unset or empty PERLIO is equivalent to C<:stdio>.
930 The list becomes the default for I<all> perl's IO. Consequently only built-in
931 layers can appear in this list, as external layers (such as :encoding()) need
932 IO in order to load them!. See L<"open pragma"|open> for how to add external
933 encodings as defaults.
935 The layers that it makes sense to include in the PERLIO environment
936 variable are summarised below. For more details see L<PerlIO>.
942 A pseudolayer that turns I<off> the C<:utf8> flag for the layer below.
943 Unlikely to be useful on its own in the global PERLIO environment variable.
944 You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
948 A layer that implements DOS/Windows like CRLF line endings.
949 On read converts pairs of CR,LF to a single "\n" newline character.
950 On write converts each "\n" to a CR,LF pair.
951 Based on the C<:perlio> layer.
955 A layer which implements "reading" of files by using C<mmap()> to
956 make (whole) file appear in the process's address space, and then
957 using that as PerlIO's "buffer". This I<may> be faster in certain
958 circumstances for large files, and may result in less physical memory
959 use when multiple processes are reading the same file.
961 Files which are not C<mmap()>-able revert to behaving like the C<:perlio>
962 layer. Writes also behave like C<:perlio> layer as C<mmap()> for write
963 needs extra house-keeping (to extend the file) which negates any advantage.
965 The C<:mmap> layer will not exist if platform does not support C<mmap()>.
969 A from scratch implementation of buffering for PerlIO. Provides fast
970 access to the buffer for C<sv_gets> which implements perl's readline/E<lt>E<gt>
971 and in general attempts to minimize data copying.
973 C<:perlio> will insert a C<:unix> layer below itself to do low level IO.
977 An experimental pseudolayer that removes the topmost layer.
978 Use with the same care as is reserved for nitroglyserin.
982 A pseudolayer that manipulates other layers. Applying the <:raw>
983 layer is equivalent to calling C<binmode($fh)>. It makes the stream
984 pass each byte as-is without any translation. In particular CRLF
985 translation, and/or :utf8 intuited from locale are disabled.
987 Arranges for all accesses go straight to the lowest buffered layer provided
988 by the configration. That is it strips off any layers above that layer.
990 In Perl 5.6 and some books the C<:raw> layer (previously sometimes also
991 referred to as a "discipline") is documented as the inverse of the
992 C<:crlf> layer. That is no longer the case - other layers which would
993 alter binary nature of the stream are also disabled. If you want UNIX
994 line endings on a platform that normally does CRLF translation, but still
995 want UTF-8 or encoding defaults the appropriate thing to do is to add
996 C<:perlio> to PERLIO environment variable.
1000 This layer provides PerlIO interface by wrapping system's ANSI C "stdio"
1001 library calls. The layer provides both buffering and IO.
1002 Note that C<:stdio> layer does I<not> do CRLF translation even if that
1003 is platforms normal behaviour. You will need a C<:crlf> layer above it
1008 Lowest level layer which provides basic PerlIO operations in terms of
1009 UNIX/POSIX numeric file descriptor calls
1010 C<open(), read(), write(), lseek(), close()>
1014 A pseudolayer that turns on a flag on the layer below to tell perl
1015 that data sent to the stream should be converted to perl internal
1016 "utf8" form and that data from the stream should be considered as so
1017 encoded. On ASCII based platforms the encoding is UTF-8 and on EBCDIC
1018 platforms UTF-EBCDIC. May be useful in PERLIO environment variable to
1019 make UTF-8 the default. (To turn off that behaviour use C<:bytes>
1024 On Win32 platforms this I<experimental> layer uses native "handle" IO
1025 rather than unix-like numeric file descriptor layer. Known to be
1026 buggy in this release.
1030 On all platforms the default set of layers should give acceptable results.
1032 For UNIX platforms that will equivalent of "unix perlio" or "stdio".
1033 Configure is setup to prefer "stdio" implementation if system's library
1034 provides for fast access to the buffer, otherwise it uses the "unix perlio"
1037 On Win32 the default in this release is "unix crlf". Win32's "stdio"
1038 has a number of bugs/mis-features for perl IO which are somewhat
1039 C compiler vendor/version dependent. Using our own C<crlf> layer as
1040 the buffer avoids those issues and makes things more uniform.
1041 The C<crlf> layer provides CRLF to/from "\n" conversion as well as
1044 This release uses C<unix> as the bottom layer on Win32 and so still uses C
1045 compiler's numeric file descriptor routines. There is an experimental native
1046 C<win32> layer which is expected to be enhanced and should eventually replace
1051 If set to the name of a file or device then certain operations of PerlIO
1052 sub-system will be logged to that file (opened as append). Typical uses
1055 PERLIO_DEBUG=/dev/tty perl script ...
1057 and Win32 approximate equivalent:
1059 set PERLIO_DEBUG=CON
1065 A colon-separated list of directories in which to look for Perl library
1066 files before looking in the standard library and the current directory.
1067 If PERL5LIB is defined, PERLLIB is not used.
1071 The command used to load the debugger code. The default is:
1073 BEGIN { require 'perl5db.pl' }
1075 =item PERL5SHELL (specific to the Win32 port)
1077 May be set to an alternative shell that perl must use internally for
1078 executing "backtick" commands or system(). Default is C<cmd.exe /x/d/c>
1079 on WindowsNT and C<command.com /c> on Windows95. The value is considered
1080 to be space-separated. Precede any character that needs to be protected
1081 (like a space or backslash) with a backslash.
1083 Note that Perl doesn't use COMSPEC for this purpose because
1084 COMSPEC has a high degree of variability among users, leading to
1085 portability concerns. Besides, perl can use a shell that may not be
1086 fit for interactive use, and setting COMSPEC to such a shell may
1087 interfere with the proper functioning of other programs (which usually
1088 look in COMSPEC to find a shell fit for interactive use).
1090 =item PERL_DEBUG_MSTATS
1092 Relevant only if perl is compiled with the malloc included with the perl
1093 distribution (that is, if C<perl -V:d_mymalloc> is 'define').
1094 If set, this causes memory statistics to be dumped after execution. If set
1095 to an integer greater than one, also causes memory statistics to be dumped
1098 =item PERL_DESTRUCT_LEVEL
1100 Relevant only if your perl executable was built with B<-DDEBUGGING>,
1101 this controls the behavior of global destruction of objects and other
1102 references. See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.
1106 If using the C<encoding> pragma without an explicit encoding name, the
1107 PERL_ENCODING environment variable is consulted for an encoding name.
1109 =item PERL_HASH_SEED
1113 Used to randomise Perl's internal hash function. To emulate the
1114 pre-5.8.1 behaviour, set to an integer (zero means exactly the same
1115 order as 5.8.0). "Pre-5.8.1" means, among other things, that hash
1116 keys will be ordered the same between different runs of Perl.
1118 The default behaviour is to randomise unless the PERL_HASH_SEED is set.
1119 If Perl has been compiled with the -DUSE_HASH_SEED_EXPLICIT the default
1120 behaviour is B<not> to randomise unless the PERL_HASH_SEED is set.
1122 If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses
1123 the pseudorandom seed supplied by the operating system and libraries.
1124 If unset, each different run of Perl will have different ordering of
1125 the outputs of keys(), values, and each().
1127 See L<perlsec/"Algorithmic Complexity Attacks"> for more information.
1129 =item PERL_ROOT (specific to the VMS port)
1131 A translation concealed rooted logical name that contains perl and the
1132 logical device for the @INC path on VMS only. Other logical names that
1133 affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1134 SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in
1135 L<perlvms> and in F<README.vms> in the Perl source distribution.
1139 In Perls 5.8.1 and later. If set to C<unsafe> the pre-Perl-5.8.0
1140 signals behaviour (immediate but unsafe) is restored. If set to
1141 C<safe> the safe (or deferred) signals are used. See L<perlipc>.
1145 Equivalent to the B<-C> command-line switch. Note that this is not
1146 a boolean variable-- setting this to C<"1"> is not the right way to
1147 "enable Unicode" (whatever that would mean). You can use C<"0"> to
1148 "disable Unicode", though (or alternatively unset PERL_UNICODE in
1149 your shell before starting Perl). See the description of the C<-C>
1150 switch for more information.
1152 =item SYS$LOGIN (specific to the VMS port)
1154 Used if chdir has no argument and HOME and LOGDIR are not set.
1158 Perl also has environment variables that control how Perl handles data
1159 specific to particular natural languages. See L<perllocale>.
1161 Apart from these, Perl uses no other environment variables, except
1162 to make them available to the program being executed, and to child
1163 processes. However, programs running setuid would do well to execute
1164 the following lines before doing anything else, just to keep people
1167 $ENV{PATH} = '/bin:/usr/bin'; # or whatever you need
1168 $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
1169 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};