This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
Warn more about disclosing the hash seed, from Scott A. Crosby.
[perl5.git] / pod / perlrun.pod
CommitLineData
a0d0e21e
LW
1=head1 NAME
2
3perlrun - how to execute the Perl interpreter
4
5=head1 SYNOPSIS
6
672fde27 7B<perl> S<[ B<-sTtuUWX> ]>
e0ebc809
PP
8 S<[ B<-hv> ] [ B<-V>[:I<configvar>] ]>
9 S<[ B<-cw> ] [ B<-d>[:I<debugger>] ] [ B<-D>[I<number/list>] ]>
f2095865 10 S<[ B<-pna> ] [ B<-F>I<pattern> ] [ B<-l>[I<octal>] ] [ B<-0>[I<octal/hexadecimal>] ]>
e0ebc809
PP
11 S<[ B<-I>I<dir> ] [ B<-m>[B<->]I<module> ] [ B<-M>[B<->]I<'module...'> ]>
12 S<[ B<-P> ]>
13 S<[ B<-S> ]>
14 S<[ B<-x>[I<dir>] ]>
15 S<[ B<-i>[I<extension>] ]>
16 S<[ B<-e> I<'command'> ] [ B<--> ] [ I<programfile> ] [ I<argument> ]...>
702815ca
RGS
17 S<[ B<-A [I<assertions>] >]>
18 S<[ B<-C [I<number/list>] >]>
a0d0e21e
LW
19
20=head1 DESCRIPTION
21
19799a22
GS
22The normal way to run a Perl program is by making it directly
23executable, or else by passing the name of the source file as an
24argument on the command line. (An interactive Perl environment
25is also possible--see L<perldebug> for details on how to do that.)
26Upon startup, Perl looks for your program in one of the following
a0d0e21e
LW
27places:
28
29=over 4
30
31=item 1.
32
33Specified line by line via B<-e> switches on the command line.
34
35=item 2.
36
37Contained in the file specified by the first filename on the command line.
a3cb178b
GS
38(Note that systems supporting the #! notation invoke interpreters this
39way. See L<Location of Perl>.)
a0d0e21e
LW
40
41=item 3.
42
5f05dabc 43Passed in implicitly via standard input. This works only if there are
19799a22
GS
44no filename arguments--to pass arguments to a STDIN-read program you
45must explicitly specify a "-" for the program name.
a0d0e21e
LW
46
47=back
48
49With methods 2 and 3, Perl starts parsing the input file from the
50beginning, unless you've specified a B<-x> switch, in which case it
51scans for the first line starting with #! and containing the word
19799a22 52"perl", and starts there instead. This is useful for running a program
a0d0e21e 53embedded in a larger message. (In this case you would indicate the end
19799a22 54of the program using the C<__END__> token.)
a0d0e21e 55
5f05dabc
PP
56The #! line is always examined for switches as the line is being
57parsed. Thus, if you're on a machine that allows only one argument
58with the #! line, or worse, doesn't even recognize the #! line, you
59still can get consistent switch behavior regardless of how Perl was
19799a22
GS
60invoked, even if B<-x> was used to find the beginning of the program.
61
62Because historically some operating systems silently chopped off
63kernel interpretation of the #! line after 32 characters, some
64switches may be passed in on the command line, and some may not;
65you could even get a "-" without its letter, if you're not careful.
66You probably want to make sure that all your switches fall either
67before or after that 32-character boundary. Most switches don't
68actually care if they're processed redundantly, but getting a "-"
69instead of a complete switch could cause Perl to try to execute
70standard input instead of your program. And a partial B<-I> switch
a0d0e21e
LW
71could also cause odd results.
72
19799a22
GS
73Some switches do care if they are processed twice, for instance
74combinations of B<-l> and B<-0>. Either put all the switches after
75the 32-character boundary (if applicable), or replace the use of
76B<-0>I<digits> by C<BEGIN{ $/ = "\0digits"; }>.
fb73857a 77
a0d0e21e
LW
78Parsing of the #! switches starts wherever "perl" is mentioned in the line.
79The sequences "-*" and "- " are specifically ignored so that you could,
80if you were so inclined, say
81
82 #!/bin/sh -- # -*- perl -*- -p
19799a22 83 eval 'exec perl -wS $0 ${1+"$@"}'
5f05dabc 84 if $running_under_some_shell;
a0d0e21e 85
44a4342c 86to let Perl see the B<-p> switch.
19799a22
GS
87
88A similar trick involves the B<env> program, if you have it.
89
90 #!/usr/bin/env perl
91
92The examples above use a relative path to the perl interpreter,
93getting whatever version is first in the user's path. If you want
94a specific version of Perl, say, perl5.005_57, you should place
95that directly in the #! line's path.
a0d0e21e
LW
96
97If the #! line does not contain the word "perl", the program named after
98the #! is executed instead of the Perl interpreter. This is slightly
99bizarre, but it helps people on machines that don't do #!, because they
19799a22 100can tell a program that their SHELL is F</usr/bin/perl>, and Perl will then
a0d0e21e
LW
101dispatch the program to the correct interpreter for them.
102
19799a22 103After locating your program, Perl compiles the entire program to an
a0d0e21e 104internal form. If there are any compilation errors, execution of the
19799a22 105program is not attempted. (This is unlike the typical shell script,
54310121 106which might run part-way through before finding a syntax error.)
a0d0e21e 107
19799a22 108If the program is syntactically correct, it is executed. If the program
a0d0e21e
LW
109runs off the end without hitting an exit() or die() operator, an implicit
110C<exit(0)> is provided to indicate successful completion.
111
68dc0745
PP
112=head2 #! and quoting on non-Unix systems
113
114Unix's #! technique can be simulated on other systems:
115
116=over 4
117
118=item OS/2
119
120Put
121
122 extproc perl -S -your_switches
123
19799a22 124as the first line in C<*.cmd> file (B<-S> due to a bug in cmd.exe's
68dc0745
PP
125`extproc' handling).
126
54310121 127=item MS-DOS
68dc0745 128
19799a22 129Create a batch file to run your program, and codify it in
fd1adc71 130C<ALTERNATE_SHEBANG> (see the F<dosish.h> file in the source
68dc0745
PP
131distribution for more information).
132
133=item Win95/NT
134
6c6a61e2 135The Win95/NT installation, when using the ActiveState installer for Perl,
c8db1d39 136will modify the Registry to associate the F<.pl> extension with the perl
6c6a61e2
GS
137interpreter. If you install Perl by other means (including building from
138the sources), you may have to modify the Registry yourself. Note that
139this means you can no longer tell the difference between an executable
140Perl program and a Perl library file.
68dc0745
PP
141
142=item Macintosh
143
19799a22 144A Macintosh perl program will have the appropriate Creator and
68dc0745
PP
145Type, so that double-clicking them will invoke the perl application.
146
bd3fa61c
CB
147=item VMS
148
149Put
150
151 $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
152 $ exit++ + ++$status != 0 and $exit = $status = undef;
153
19799a22
GS
154at the top of your program, where B<-mysw> are any command line switches you
155want to pass to Perl. You can now invoke the program directly, by saying
156C<perl program>, or as a DCL procedure, by saying C<@program> (or implicitly
157via F<DCL$PATH> by just using the name of the program).
bd3fa61c
CB
158
159This incantation is a bit much to remember, but Perl will display it for
160you if you say C<perl "-V:startperl">.
161
68dc0745
PP
162=back
163
164Command-interpreters on non-Unix systems have rather different ideas
165on quoting than Unix shells. You'll need to learn the special
166characters in your command-interpreter (C<*>, C<\> and C<"> are
167common) and how to protect whitespace and these characters to run
19799a22 168one-liners (see B<-e> below).
68dc0745
PP
169
170On some systems, you may have to change single-quotes to double ones,
e6f03d26 171which you must I<not> do on Unix or Plan 9 systems. You might also
68dc0745
PP
172have to change a single % to a %%.
173
174For example:
175
176 # Unix
177 perl -e 'print "Hello world\n"'
178
54310121 179 # MS-DOS, etc.
68dc0745
PP
180 perl -e "print \"Hello world\n\""
181
54310121 182 # Macintosh
68dc0745
PP
183 print "Hello world\n"
184 (then Run "Myscript" or Shift-Command-R)
185
186 # VMS
187 perl -e "print ""Hello world\n"""
188
19799a22
GS
189The problem is that none of this is reliable: it depends on the
190command and it is entirely possible neither works. If B<4DOS> were
191the command shell, this would probably work better:
68dc0745
PP
192
193 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
194
19799a22 195B<CMD.EXE> in Windows NT slipped a lot of standard Unix functionality in
68dc0745
PP
196when nobody was looking, but just try to find documentation for its
197quoting rules.
198
54310121 199Under the Macintosh, it depends which environment you are using. The MacPerl
68dc0745 200shell, or MPW, is much like Unix shells in its support for several
54310121 201quoting variants, except that it makes free use of the Macintosh's non-ASCII
68dc0745
PP
202characters as control characters.
203
204There is no general solution to all of this. It's just a mess.
205
a3cb178b
GS
206=head2 Location of Perl
207
208It may seem obvious to say, but Perl is useful only when users can
19799a22
GS
209easily find it. When possible, it's good for both F</usr/bin/perl>
210and F</usr/local/bin/perl> to be symlinks to the actual binary. If
211that can't be done, system administrators are strongly encouraged
212to put (symlinks to) perl and its accompanying utilities into a
213directory typically found along a user's PATH, or in some other
214obvious and convenient place.
215
216In this documentation, C<#!/usr/bin/perl> on the first line of the program
217will stand in for whatever method works on your system. You are
218advised to use a specific path if you care about a specific version.
a3cb178b 219
19799a22 220 #!/usr/local/bin/perl5.00554
a3cb178b 221
19799a22
GS
222or if you just want to be running at least version, place a statement
223like this at the top of your program:
a0d0e21e 224
19799a22 225 use 5.005_54;
a0d0e21e 226
19799a22
GS
227=head2 Command Switches
228
229As with all standard commands, a single-character switch may be
230clustered with the following switch, if any.
231
232 #!/usr/bin/perl -spi.orig # same as -s -p -i.orig
a0d0e21e
LW
233
234Switches include:
235
236=over 5
237
f2095865 238=item B<-0>[I<octal/hexadecimal>]
a0d0e21e 239
f2095865
JH
240specifies the input record separator (C<$/>) as an octal or
241hexadecimal number. If there are no digits, the null character is the
242separator. Other switches may precede or follow the digits. For
243example, if you have a version of B<find> which can print filenames
244terminated by the null character, you can say this:
a0d0e21e 245
19799a22 246 find . -name '*.orig' -print0 | perl -n0e unlink
a0d0e21e
LW
247
248The special value 00 will cause Perl to slurp files in paragraph mode.
5f05dabc 249The value 0777 will cause Perl to slurp files whole because there is no
f2095865
JH
250legal byte with that value.
251
252If you want to specify any Unicode character, use the hexadecimal
253format: 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
255consists of hexadecimal digits.)
a0d0e21e 256
702815ca
RGS
257=item B<-A [I<assertions>]>
258
259Activates the assertions given after the switch as a comma-separated
260list of assertion names. If no assertion name is given, activates all
261assertions. See L<assertions>.
262
a0d0e21e
LW
263=item B<-a>
264
265turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
266split command to the @F array is done as the first thing inside the
267implicit while loop produced by the B<-n> or B<-p>.
268
269 perl -ane 'print pop(@F), "\n";'
270
271is equivalent to
272
273 while (<>) {
274 @F = split(' ');
275 print pop(@F), "\n";
276 }
277
278An alternate delimiter may be specified using B<-F>.
279
a05d7ebb 280=item B<-C [I<number/list>]>
46487f74 281
a05d7ebb
JH
282The C<-C> flag controls some Unicode of the Perl Unicode features.
283
284As of 5.8.1, the C<-C> can be followed either by a number or a list
f3f8427d 285of option letters. The letters, their numeric values, and effects
8aa8f774 286are as follows; listing the letters is equal to summing the numbers.
9f21530f
JH
287
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
291 S 7 I + O + E
44505768
JH
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
9f21530f 294 D 24 i + o
44505768 295 A 32 the @ARGV elements are expected to be strings encoded in UTF-8
ce81ff12 296 L 64 normally the "IOEioA" are unconditional,
9f21530f 297 the L makes them conditional on the locale environment
ce81ff12
JH
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
9f21530f
JH
301
302For example, C<-COE> and C<-C6> will both turn on UTF-8-ness on both
303STDOUT and STDERR. Repeating letters is just redundant, not cumulative
304nor toggling.
a05d7ebb 305
44505768
JH
306The C<io> options mean that any subsequent open() (or similar I/O
307operations) will have the C<:utf8> PerlIO layer implicitly applied
308to them, in other words, UTF-8 is expected from any input stream,
309and UTF-8 is produced to any output stream. This is just the default,
310with explicit layers in open() and with binmode() one can manipulate
311streams as usual.
312
8aa8f774
JH
313C<-C> on its own (not followed by any number or option list), or the
314empty string C<""> for the C<$ENV{PERL_UNICODE}, has the same effect
ce81ff12
JH
315as <-CSDL>. In other words, the standard I/O handles and the default
316C<open()> layer are UTF-8-fied B<but> only if the locale environment
8aa8f774 317variables indicate a UTF-8 locale. This behaviour follows the
5b4f334e 318I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
a05d7ebb 319
ab9e1bb7 320You can use C<-C0> (or C<"0"> for $ENV{PERL_UNICODE}) to explicitly
5b4f334e 321disable all the above Unicode features.
fde18df1 322
8aa8f774 323The read-only magic variable C<${^UNICODE}> reflects the numeric value
ab9e1bb7
JH
324of this setting. This is variable is set during Perl startup and is
325thereafter read-only. If you want runtime effects, use the three-arg
2307c6d0 326open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
ab9e1bb7 327and the C<open> pragma (see L<open>).
fde18df1
JH
328
329(In Perls earlier than 5.8.1 the C<-C> switch was a Win32-only switch
330that enabled the use of Unicode-aware "wide system call" Win32 APIs.
331This feature was practically unused, however, and the command line
332switch was therefore "recycled".)
46487f74 333
a0d0e21e
LW
334=item B<-c>
335
19799a22 336causes Perl to check the syntax of the program and then exit without
7d30b5c4 337executing it. Actually, it I<will> execute C<BEGIN>, C<CHECK>, and
4f25aa18
GS
338C<use> blocks, because these are considered as occurring outside the
339execution of your program. C<INIT> and C<END> blocks, however, will
340be skipped.
a0d0e21e
LW
341
342=item B<-d>
343
19799a22 344runs the program under the Perl debugger. See L<perldebug>.
a0d0e21e 345
70c94a19 346=item B<-d:>I<foo[=bar,baz]>
3c81428c 347
19799a22
GS
348runs the program under the control of a debugging, profiling, or
349tracing module installed as Devel::foo. E.g., B<-d:DProf> executes
70c94a19
RR
350the program using the Devel::DProf profiler. As with the B<-M>
351flag, options may be passed to the Devel::foo package where they
352will be received and interpreted by the Devel::foo::import routine.
353The comma-separated list of options must follow a C<=> character.
354See L<perldebug>.
3c81428c 355
db2ba183 356=item B<-D>I<letters>
a0d0e21e 357
db2ba183 358=item B<-D>I<number>
a0d0e21e 359
19799a22 360sets debugging flags. To watch how it executes your program, use
db2ba183
TB
361B<-Dtls>. (This works only if debugging is compiled into your
362Perl.) Another nice value is B<-Dx>, which lists your compiled
4197b13f 363syntax tree. And B<-Dr> displays compiled regular expressions;
44a4342c 364the format of the output is explained in L<perldebguts>.
4197b13f
MJD
365
366As an alternative, specify a number instead of list of letters (e.g.,
367B<-D14> is equivalent to B<-Dtls>):
a0d0e21e 368
db2ba183
TB
369 1 p Tokenizing and parsing
370 2 s Stack snapshots
d6721266 371 with v, displays all stacks
db2ba183
TB
372 4 l Context (loop) stack processing
373 8 t Trace execution
374 16 o Method and overloading resolution
375 32 c String/numeric conversions
1045810a 376 64 P Print profiling info, preprocessor command for -P, source file input state
db2ba183
TB
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
7bab3ede 382 4096 (Obsolete, previously used for LEAKTEST)
db2ba183
TB
383 8192 H Hash dump -- usurps values()
384 16384 X Scratchpad allocation
385 32768 D Cleaning up
8b73bbec 386 65536 S Thread synchronization
607df283 387 131072 T Tokenising
04932ac8 388 262144 R Include reference counts of dumped variables (eg when using -Ds)
1045810a 389 524288 J Do not s,t,P-debug (Jump over) opcodes within package DB
d6721266 390 1048576 v Verbose: use in conjunction with other flags
46187eeb 391 2097152 C Copy On Write
a0d0e21e 392
19799a22 393All these flags require B<-DDEBUGGING> when you compile the Perl
1045810a 394executable (but see L<Devel::Peek>, L<re> which may change this).
44a4342c 395See the F<INSTALL> file in the Perl source distribution
19799a22 396for how to do this. This flag is automatically set if you include B<-g>
8c52afec
IZ
397option when C<Configure> asks you about optimizer/debugger flags.
398
19799a22
GS
399If you're just trying to get a print out of each line of Perl code
400as it executes, the way that C<sh -x> provides for shell scripts,
44a4342c 401you can't use Perl's B<-D> switch. Instead do this
19799a22 402
c406981e
JH
403 # If you have "env" utility
404 env=PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
405
19799a22
GS
406 # Bourne shell syntax
407 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
408
409 # csh syntax
410 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
411
412See L<perldebug> for details and variations.
413
a0d0e21e
LW
414=item B<-e> I<commandline>
415
19799a22
GS
416may be used to enter one line of program. If B<-e> is given, Perl
417will not look for a filename in the argument list. Multiple B<-e>
418commands may be given to build up a multi-line script. Make sure
419to use semicolons where you would in a normal program.
a0d0e21e 420
e0ebc809 421=item B<-F>I<pattern>
a0d0e21e 422
e0ebc809 423specifies the pattern to split on if B<-a> is also in effect. The
5f05dabc 424pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
e0ebc809 425put in single quotes.
a0d0e21e 426
e0ebc809
PP
427=item B<-h>
428
429prints a summary of the options.
430
431=item B<-i>[I<extension>]
a0d0e21e 432
2d259d92
CK
433specifies that files processed by the C<E<lt>E<gt>> construct are to be
434edited in-place. It does this by renaming the input file, opening the
435output file by the original name, and selecting that output file as the
436default for print() statements. The extension, if supplied, is used to
437modify the name of the old file to make a backup copy, following these
438rules:
439
440If no extension is supplied, no backup is made and the current file is
441overwritten.
442
19799a22
GS
443If the extension doesn't contain a C<*>, then it is appended to the
444end of the current filename as a suffix. If the extension does
445contain one or more C<*> characters, then each C<*> is replaced
446with the current filename. In Perl terms, you could think of this
447as:
2d259d92 448
66606d78 449 ($backup = $extension) =~ s/\*/$file_name/g;
2d259d92
CK
450
451This allows you to add a prefix to the backup file, instead of (or in
452addition to) a suffix:
453
19799a22 454 $ perl -pi 'orig_*' -e 's/bar/baz/' fileA # backup to 'orig_fileA'
2d259d92
CK
455
456Or even to place backup copies of the original files into another
457directory (provided the directory already exists):
458
19799a22 459 $ perl -pi 'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'
2d259d92 460
66606d78
CK
461These sets of one-liners are equivalent:
462
463 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
19799a22 464 $ perl -pi '*' -e 's/bar/baz/' fileA # overwrite current file
66606d78 465
19799a22
GS
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'
66606d78 468
2d259d92 469From the shell, saying
a0d0e21e 470
19799a22 471 $ perl -p -i.orig -e "s/foo/bar/; ... "
a0d0e21e 472
19799a22 473is the same as using the program:
a0d0e21e 474
19799a22 475 #!/usr/bin/perl -pi.orig
a0d0e21e
LW
476 s/foo/bar/;
477
478which is equivalent to
479
480 #!/usr/bin/perl
19799a22
GS
481 $extension = '.orig';
482 LINE: while (<>) {
a0d0e21e 483 if ($ARGV ne $oldargv) {
66606d78
CK
484 if ($extension !~ /\*/) {
485 $backup = $ARGV . $extension;
486 }
487 else {
488 ($backup = $extension) =~ s/\*/$ARGV/g;
489 }
490 rename($ARGV, $backup);
a0d0e21e
LW
491 open(ARGVOUT, ">$ARGV");
492 select(ARGVOUT);
493 $oldargv = $ARGV;
494 }
495 s/foo/bar/;
496 }
497 continue {
498 print; # this prints to original filename
499 }
500 select(STDOUT);
501
502except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
503know when the filename has changed. It does, however, use ARGVOUT for
66606d78
CK
504the selected filehandle. Note that STDOUT is restored as the default
505output filehandle after the loop.
506
507As shown above, Perl creates the backup file whether or not any output
508is actually changed. So this is just a fancy way to copy files:
509
cd2d1bac 510 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
19799a22 511or
cd2d1bac 512 $ perl -p -i'.orig' -e 1 file1 file2 file3...
66606d78
CK
513
514You can use C<eof> without parentheses to locate the end of each input
515file, in case you want to append to each file, or reset line numbering
516(see example in L<perlfunc/eof>).
517
518If, for a given file, Perl is unable to create the backup file as
519specified in the extension then it will skip that file and continue on
520with the next one (if it exists).
521
19799a22 522For a discussion of issues surrounding file permissions and B<-i>,
cea6626f 523see L<perlfaq5/Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl?>.
66606d78
CK
524
525You cannot use B<-i> to create directories or to strip extensions from
526files.
a0d0e21e 527
19799a22
GS
528Perl does not expand C<~> in filenames, which is good, since some
529folks use it for their backup files:
a0d0e21e 530
19799a22
GS
531 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
532
533Finally, the B<-i> switch does not impede execution when no
a2008d6d
GS
534files are given on the command line. In this case, no backup is made
535(the original file cannot, of course, be determined) and processing
536proceeds from STDIN to STDOUT as might be expected.
537
a0d0e21e
LW
538=item B<-I>I<directory>
539
e0ebc809 540Directories specified by B<-I> are prepended to the search path for
1fef88e7 541modules (C<@INC>), and also tells the C preprocessor where to search for
e0ebc809
PP
542include files. The C preprocessor is invoked with B<-P>; by default it
543searches /usr/include and /usr/lib/perl.
a0d0e21e 544
e0ebc809 545=item B<-l>[I<octnum>]
a0d0e21e 546
19799a22
GS
547enables automatic line-ending processing. It has two separate
548effects. First, it automatically chomps C<$/> (the input record
549separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
550(the output record separator) to have the value of I<octnum> so
551that any print statements will have that separator added back on.
552If I<octnum> is omitted, sets C<$\> to the current value of
553C<$/>. For instance, to trim lines to 80 columns:
a0d0e21e
LW
554
555 perl -lpe 'substr($_, 80) = ""'
556
557Note that the assignment C<$\ = $/> is done when the switch is processed,
558so the input record separator can be different than the output record
559separator if the B<-l> switch is followed by a B<-0> switch:
560
561 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
562
1fef88e7 563This sets C<$\> to newline and then sets C<$/> to the null character.
a0d0e21e 564
e0ebc809
PP
565=item B<-m>[B<->]I<module>
566
567=item B<-M>[B<->]I<module>
c07a80fd 568
e0ebc809
PP
569=item B<-M>[B<->]I<'module ...'>
570
571=item B<-[mM]>[B<->]I<module=arg[,arg]...>
3c81428c 572
19799a22
GS
573B<-m>I<module> executes C<use> I<module> C<();> before executing your
574program.
3c81428c 575
19799a22
GS
576B<-M>I<module> executes C<use> I<module> C<;> before executing your
577program. You can use quotes to add extra code after the module name,
578e.g., C<'-Mmodule qw(foo bar)'>.
3c81428c 579
19799a22 580If the first character after the B<-M> or B<-m> is a dash (C<->)
a5f75d66
AD
581then the 'use' is replaced with 'no'.
582
54310121 583A little builtin syntactic sugar means you can also say
19799a22
GS
584B<-mmodule=foo,bar> or B<-Mmodule=foo,bar> as a shortcut for
585C<'-Mmodule qw(foo bar)'>. This avoids the need to use quotes when
586importing symbols. The actual code generated by B<-Mmodule=foo,bar> is
e0ebc809 587C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
19799a22 588removes the distinction between B<-m> and B<-M>.
3c81428c 589
a0d0e21e
LW
590=item B<-n>
591
19799a22 592causes Perl to assume the following loop around your program, which
a0d0e21e
LW
593makes it iterate over filename arguments somewhat like B<sed -n> or
594B<awk>:
595
19799a22 596 LINE:
a0d0e21e 597 while (<>) {
19799a22 598 ... # your program goes here
a0d0e21e
LW
599 }
600
601Note that the lines are not printed by default. See B<-p> to have
08e9d68e 602lines printed. If a file named by an argument cannot be opened for
19799a22 603some reason, Perl warns you about it and moves on to the next file.
08e9d68e
DD
604
605Here is an efficient way to delete all files older than a week:
a0d0e21e 606
19799a22 607 find . -mtime +7 -print | perl -nle unlink
a0d0e21e 608
19799a22
GS
609This is faster than using the B<-exec> switch of B<find> because you don't
610have to start a process on every filename found. It does suffer from
611the bug of mishandling newlines in pathnames, which you can fix if
44a4342c 612you follow the example under B<-0>.
a0d0e21e
LW
613
614C<BEGIN> and C<END> blocks may be used to capture control before or after
19799a22 615the implicit program loop, just as in B<awk>.
a0d0e21e
LW
616
617=item B<-p>
618
19799a22 619causes Perl to assume the following loop around your program, which
a0d0e21e
LW
620makes it iterate over filename arguments somewhat like B<sed>:
621
622
19799a22 623 LINE:
a0d0e21e 624 while (<>) {
19799a22 625 ... # your program goes here
a0d0e21e 626 } continue {
08e9d68e 627 print or die "-p destination: $!\n";
a0d0e21e
LW
628 }
629
08e9d68e
DD
630If a file named by an argument cannot be opened for some reason, Perl
631warns you about it, and moves on to the next file. Note that the
c2611fb3 632lines are printed automatically. An error occurring during printing is
08e9d68e
DD
633treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
634overrides a B<-n> switch.
a0d0e21e
LW
635
636C<BEGIN> and C<END> blocks may be used to capture control before or after
19799a22 637the implicit loop, just as in B<awk>.
a0d0e21e
LW
638
639=item B<-P>
640
079a94c4
JH
641B<NOTE: Use of -P is strongly discouraged because of its inherent
642problems, including poor portability.>
643
644This option causes your program to be run through the C preprocessor before
efdf3af0 645compilation by Perl. Because both comments and B<cpp> directives begin
a0d0e21e 646with the # character, you should avoid starting comments with any words
efdf3af0 647recognized by the C preprocessor such as C<"if">, C<"else">, or C<"define">.
079a94c4
JH
648
649If you're considering using C<-P>, you might also want to look at the
650Filter::cpp module from CPAN.
651
652The problems of -P include, but are not limited to:
653
654=over 10
655
656=item *
657
658The C<#!> line is stripped, so any switches there don't apply.
659
660=item *
661
662A C<-P> on a C<#!> line doesn't work.
663
664=item *
665
666B<All> lines that begin with (whitespace and) a C<#> but
667do not look like cpp commands, are stripped, including anything
44a4342c 668inside Perl strings, regular expressions, and here-docs .
079a94c4
JH
669
670=item *
671
672In some platforms the C preprocessor knows too much: it knows about
673the C++ -style until-end-of-line comments starting with C<"//">.
efdf3af0
JH
674This will cause problems with common Perl constructs like
675
676 s/foo//;
677
678because after -P this will became illegal code
679
680 s/foo
681
682The workaround is to use some other quoting separator than C<"/">,
683like for example C<"!">:
684
685 s!foo!!;
a0d0e21e 686
079a94c4
JH
687
688
689=item *
690
691It requires not only a working C preprocessor but also a working
692F<sed>. If not on UNIX, you are probably out of luck on this.
693
694=item *
695
696Script line numbers are not preserved.
697
698=item *
699
700The C<-x> does not work with C<-P>.
701
702=back
9a1f07e7 703
a0d0e21e
LW
704=item B<-s>
705
19799a22
GS
706enables rudimentary switch parsing for switches on the command
707line after the program name but before any filename arguments (or before
3bbcc830
JP
708an argument of B<-->). This means you can have switches with two leading
709dashes (B<--help>). Any switch found there is removed from @ARGV and sets the
19799a22 710corresponding variable in the Perl program. The following program
3c0facb2
GS
711prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
712if it is invoked with B<-xyz=abc>.
a0d0e21e
LW
713
714 #!/usr/bin/perl -s
3c0facb2 715 if ($xyz) { print "$xyz\n" }
a0d0e21e 716
3bbcc830
JP
717Do note that B<--help> creates the variable ${-help}, which is not compliant
718with C<strict refs>.
719
a0d0e21e
LW
720=item B<-S>
721
722makes Perl use the PATH environment variable to search for the
19799a22
GS
723program (unless the name of the program contains directory separators).
724
2a92aaa0
GS
725On some platforms, this also makes Perl append suffixes to the
726filename while searching for it. For example, on Win32 platforms,
727the ".bat" and ".cmd" suffixes are appended if a lookup for the
728original name fails, and if the name does not already end in one
729of those suffixes. If your Perl was compiled with DEBUGGING turned
730on, using the -Dp switch to Perl shows how the search progresses.
731
2a92aaa0
GS
732Typically this is used to emulate #! startup on platforms that
733don't support #!. This example works on many platforms that
734have a shell compatible with Bourne shell:
a0d0e21e
LW
735
736 #!/usr/bin/perl
a3cb178b 737 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
a0d0e21e
LW
738 if $running_under_some_shell;
739
19799a22
GS
740The system ignores the first line and feeds the program to F</bin/sh>,
741which proceeds to try to execute the Perl program as a shell script.
a0d0e21e
LW
742The shell executes the second line as a normal shell command, and thus
743starts up the Perl interpreter. On some systems $0 doesn't always
744contain the full pathname, so the B<-S> tells Perl to search for the
19799a22 745program if necessary. After Perl locates the program, it parses the
a0d0e21e 746lines and ignores them because the variable $running_under_some_shell
19799a22 747is never true. If the program will be interpreted by csh, you will need
a3cb178b
GS
748to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
749embedded spaces (and such) in the argument list. To start up sh rather
a0d0e21e
LW
750than csh, some systems may have to replace the #! line with a line
751containing just a colon, which will be politely ignored by Perl. Other
752systems can't control that, and need a totally devious construct that
19799a22 753will work under any of B<csh>, B<sh>, or Perl, such as the following:
a0d0e21e 754
19799a22 755 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
a3cb178b 756 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
5f05dabc 757 if $running_under_some_shell;
a0d0e21e 758
19799a22
GS
759If the filename supplied contains directory separators (i.e., is an
760absolute or relative pathname), and if that file is not found,
761platforms that append file extensions will do so and try to look
762for the file with those extensions added, one by one.
763
764On DOS-like platforms, if the program does not contain directory
765separators, it will first be searched for in the current directory
766before being searched for on the PATH. On Unix platforms, the
767program will be searched for strictly on the PATH.
768
6537fe72
MS
769=item B<-t>
770
771Like B<-T>, but taint checks will issue warnings rather than fatal
317ea90d
MS
772errors. These warnings can be controlled normally with C<no warnings
773qw(taint)>.
1dbad523
JH
774
775B<NOTE: this is not a substitute for -T.> This is meant only to be
776used as a temporary development aid while securing legacy code:
777for real production code and for new secure code written from scratch
778always use the real B<-T>.
6537fe72 779
a0d0e21e
LW
780=item B<-T>
781
a3cb178b 782forces "taint" checks to be turned on so you can test them. Ordinarily
19799a22
GS
783these checks are done only when running setuid or setgid. It's a
784good idea to turn them on explicitly for programs that run on behalf
785of someone else whom you might not necessarily trust, such as CGI
786programs or any internet servers you might write in Perl. See
787L<perlsec> for details. For security reasons, this option must be
788seen by Perl quite early; usually this means it must appear early
789on the command line or in the #! line for systems which support
790that construct.
a0d0e21e
LW
791
792=item B<-u>
793
19799a22
GS
794This obsolete switch causes Perl to dump core after compiling your
795program. You can then in theory take this core dump and turn it
796into an executable file by using the B<undump> program (not supplied).
797This speeds startup at the expense of some disk space (which you
798can minimize by stripping the executable). (Still, a "hello world"
799executable comes out to about 200K on my machine.) If you want to
800execute a portion of your program before dumping, use the dump()
801operator instead. Note: availability of B<undump> is platform
802specific and may not be available for a specific port of Perl.
803
804This switch has been superseded in favor of the new Perl code
805generator backends to the compiler. See L<B> and L<B::Bytecode>
806for details.
a0d0e21e
LW
807
808=item B<-U>
809
810allows Perl to do unsafe operations. Currently the only "unsafe"
811operations are the unlinking of directories while running as superuser,
812and running setuid programs with fatal taint checks turned into
19799a22
GS
813warnings. Note that the B<-w> switch (or the C<$^W> variable) must
814be used along with this option to actually I<generate> the
fb73857a 815taint-check warnings.
a0d0e21e
LW
816
817=item B<-v>
818
19799a22 819prints the version and patchlevel of your perl executable.
a0d0e21e 820
3c81428c
PP
821=item B<-V>
822
823prints summary of the major perl configuration values and the current
19799a22 824values of @INC.
3c81428c 825
e0ebc809 826=item B<-V:>I<name>
3c81428c
PP
827
828Prints to STDOUT the value of the named configuration variable.
44a4342c 829For example,
3c81428c 830
19799a22
GS
831 $ perl -V:man.dir
832
833will provide strong clues about what your MANPATH variable should
834be set to in order to access the Perl documentation.
a0d0e21e 835
19799a22 836=item B<-w>
774d564b 837
19799a22
GS
838prints warnings about dubious constructs, such as variable names
839that are mentioned only once and scalar variables that are used
840before being set, redefined subroutines, references to undefined
841filehandles or filehandles opened read-only that you are attempting
842to write on, values used as a number that doesn't look like numbers,
843using an array as though it were a scalar, if your subroutines
844recurse more than 100 deep, and innumerable other things.
845
b40da996 846This switch really just enables the internal C<$^W> variable. You
19799a22
GS
847can disable or promote into fatal errors specific warnings using
848C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
849See also L<perldiag> and L<perltrap>. A new, fine-grained warning
850facility is also available if you want to manipulate entire classes
9f1b1f2d 851of warnings; see L<warnings> or L<perllexwarn>.
a0d0e21e 852
0453d815
PM
853=item B<-W>
854
3c0facb2 855Enables all warnings regardless of C<no warnings> or C<$^W>.
0453d815
PM
856See L<perllexwarn>.
857
858=item B<-X>
859
3c0facb2 860Disables all warnings regardless of C<use warnings> or C<$^W>.
0453d815
PM
861See L<perllexwarn>.
862
a0d0e21e
LW
863=item B<-x> I<directory>
864
19799a22
GS
865tells Perl that the program is embedded in a larger chunk of unrelated
866ASCII text, such as in a mail message. Leading garbage will be
867discarded until the first line that starts with #! and contains the
868string "perl". Any meaningful switches on that line will be applied.
869If a directory name is specified, Perl will switch to that directory
870before running the program. The B<-x> switch controls only the
871disposal of leading garbage. The program must be terminated with
872C<__END__> if there is trailing garbage to be ignored (the program
873can process any or all of the trailing garbage via the DATA filehandle
874if desired).
a0d0e21e 875
1e422769
PP
876=back
877
878=head1 ENVIRONMENT
879
880=over 12
881
882=item HOME
883
884Used if chdir has no argument.
885
886=item LOGDIR
887
888Used if chdir has no argument and HOME is not set.
889
890=item PATH
891
19799a22 892Used in executing subprocesses, and in finding the program if B<-S> is
1e422769
PP
893used.
894
895=item PERL5LIB
896
48b971ca 897A list of directories in which to look for Perl library
1e422769 898files before looking in the standard library and the current
951ba7fe
GS
899directory. Any architecture-specific directories under the specified
900locations are automatically included if they exist. If PERL5LIB is not
48b971ca
RGS
901defined, PERLLIB is used. Directories are separated (like in PATH) by
902a colon on unixish platforms and by a semicolon on Windows (the proper
903path separator being given by the command C<perl -V:path_sep>).
951ba7fe
GS
904
905When running taint checks (either because the program was running setuid
906or setgid, or the B<-T> switch was used), neither variable is used.
907The program should instead say:
1e422769
PP
908
909 use lib "/my/directory";
910
54310121
PP
911=item PERL5OPT
912
913Command-line options (switches). Switches in this variable are taken
1c4db469 914as if they were on every Perl command line. Only the B<-[DIMUdmtw]>
19799a22 915switches are allowed. When running taint checks (because the program
54310121 916was running setuid or setgid, or the B<-T> switch was used), this
74288ac8
GS
917variable is ignored. If PERL5OPT begins with B<-T>, tainting will be
918enabled, and any subsequent options ignored.
54310121 919
16537909
JH
920=item PERLIO
921
44a4342c 922A space (or colon) separated list of PerlIO layers. If perl is built
03d9e98a 923to use PerlIO system for IO (the default) these layers effect perl's IO.
44a4342c
NIS
924
925It is conventional to start layer names with a colon e.g. C<:perlio> to
926emphasise their similarity to variable "attributes". But the code that parses
927layer specification strings (which is also used to decode the PERLIO
928environment variable) treats the colon as a separator.
929
3b0db4f9
JH
930An unset or empty PERLIO is equivalent to C<:stdio>.
931
44a4342c
NIS
932The list becomes the default for I<all> perl's IO. Consequently only built-in
933layers can appear in this list, as external layers (such as :encoding()) need
934IO in order to load them!. See L<"open pragma"|open> for how to add external
935encodings as defaults.
936
937The layers that it makes sense to include in the PERLIO environment
938variable are summarised below. For more details see L<PerlIO>.
16537909
JH
939
940=over 8
941
942=item :bytes
943
18aba96f
JH
944A pseudolayer that turns I<off> the C<:utf8> flag for the layer below.
945Unlikely to be useful on its own in the global PERLIO environment variable.
946You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
16537909
JH
947
948=item :crlf
949
8229d19f
JH
950A layer that implements DOS/Windows like CRLF line endings. On read
951converts pairs of CR,LF to a single "\n" newline character. On write
952converts each "\n" to a CR,LF pair. Note that this layer likes to be
953one of its kind: it silently ignores attempts to be pushed into the
954layer stack more than once.
955
956(Gory details follow) To be more exact what happens is this: after
957pushing itself to the stack, the C<:crlf> layer checks all the layers
958below itself to find the first layer that is capable of being a CRLF
959layer but is not yet enabled to be a CRLF layer. If it finds such a
960layer, it enables the CRLFness of that other deeper layer, and then
961pops itself off the stack. If not, fine, use the one we just pushed.
962
963The end result is that a C<:crlf> means "please enable the first CRLF
964layer you can find, and if you can't find one, here would be a good
965spot to place a new one."
966
44a4342c
NIS
967Based on the C<:perlio> layer.
968
969=item :mmap
970
971A layer which implements "reading" of files by using C<mmap()> to
972make (whole) file appear in the process's address space, and then
973using that as PerlIO's "buffer". This I<may> be faster in certain
974circumstances for large files, and may result in less physical memory
975use when multiple processes are reading the same file.
16537909 976
44a4342c
NIS
977Files which are not C<mmap()>-able revert to behaving like the C<:perlio>
978layer. Writes also behave like C<:perlio> layer as C<mmap()> for write
979needs extra house-keeping (to extend the file) which negates any advantage.
16537909 980
44a4342c 981The C<:mmap> layer will not exist if platform does not support C<mmap()>.
16537909 982
44a4342c 983=item :perlio
16537909 984
44a4342c
NIS
985A from scratch implementation of buffering for PerlIO. Provides fast
986access to the buffer for C<sv_gets> which implements perl's readline/E<lt>E<gt>
987and in general attempts to minimize data copying.
16537909 988
44a4342c 989C<:perlio> will insert a C<:unix> layer below itself to do low level IO.
16537909 990
18aba96f
JH
991=item :pop
992
993An experimental pseudolayer that removes the topmost layer.
994Use with the same care as is reserved for nitroglyserin.
995
44a4342c 996=item :raw
16537909 997
18aba96f
JH
998A pseudolayer that manipulates other layers. Applying the <:raw>
999layer is equivalent to calling C<binmode($fh)>. It makes the stream
1000pass each byte as-is without any translation. In particular CRLF
1001translation, and/or :utf8 intuited from locale are disabled.
1cbfc93d 1002
0226bbdb 1003Arranges for all accesses go straight to the lowest buffered layer provided
44a4342c 1004by the configration. That is it strips off any layers above that layer.
16537909 1005
fae2c0fb
RGS
1006In Perl 5.6 and some books the C<:raw> layer (previously sometimes also
1007referred to as a "discipline") is documented as the inverse of the
1008C<:crlf> layer. That is no longer the case - other layers which would
1009alter binary nature of the stream are also disabled. If you want UNIX
1010line endings on a platform that normally does CRLF translation, but still
1011want UTF-8 or encoding defaults the appropriate thing to do is to add
1012C<:perlio> to PERLIO environment variable.
16537909 1013
44a4342c
NIS
1014=item :stdio
1015
1016This layer provides PerlIO interface by wrapping system's ANSI C "stdio"
1017library calls. The layer provides both buffering and IO.
1018Note that C<:stdio> layer does I<not> do CRLF translation even if that
1019is platforms normal behaviour. You will need a C<:crlf> layer above it
1020to do that.
1021
1022=item :unix
1023
1024Lowest level layer which provides basic PerlIO operations in terms of
1025UNIX/POSIX numeric file descriptor calls
1026C<open(), read(), write(), lseek(), close()>
16537909
JH
1027
1028=item :utf8
1029
18aba96f
JH
1030A pseudolayer that turns on a flag on the layer below to tell perl
1031that data sent to the stream should be converted to perl internal
1032"utf8" form and that data from the stream should be considered as so
1033encoded. On ASCII based platforms the encoding is UTF-8 and on EBCDIC
1034platforms UTF-EBCDIC. May be useful in PERLIO environment variable to
1035make UTF-8 the default. (To turn off that behaviour use C<:bytes>
1036layer.)
44a4342c
NIS
1037
1038=item :win32
1039
ab4f7683 1040On Win32 platforms this I<experimental> layer uses native "handle" IO
44a4342c
NIS
1041rather than unix-like numeric file descriptor layer. Known to be
1042buggy in this release.
16537909
JH
1043
1044=back
1045
44a4342c
NIS
1046On all platforms the default set of layers should give acceptable results.
1047
ab4f7683 1048For UNIX platforms that will equivalent of "unix perlio" or "stdio".
44a4342c
NIS
1049Configure is setup to prefer "stdio" implementation if system's library
1050provides for fast access to the buffer, otherwise it uses the "unix perlio"
1051implementation.
1052
1053On Win32 the default in this release is "unix crlf". Win32's "stdio"
1054has a number of bugs/mis-features for perl IO which are somewhat
99366417 1055C compiler vendor/version dependent. Using our own C<crlf> layer as
44a4342c
NIS
1056the buffer avoids those issues and makes things more uniform.
1057The C<crlf> layer provides CRLF to/from "\n" conversion as well as
1058buffering.
1059
1060This release uses C<unix> as the bottom layer on Win32 and so still uses C
1061compiler's numeric file descriptor routines. There is an experimental native
1062C<win32> layer which is expected to be enhanced and should eventually replace
1063the C<unix> layer.
1064
1065=item PERLIO_DEBUG
1066
1067If set to the name of a file or device then certain operations of PerlIO
1068sub-system will be logged to that file (opened as append). Typical uses
1069are UNIX:
1070
1071 PERLIO_DEBUG=/dev/tty perl script ...
1072
1073and Win32 approximate equivalent:
1074
1075 set PERLIO_DEBUG=CON
1076 perl script ...
1077
16537909 1078
1e422769
PP
1079=item PERLLIB
1080
48b971ca 1081A list of directories in which to look for Perl library
1e422769
PP
1082files before looking in the standard library and the current directory.
1083If PERL5LIB is defined, PERLLIB is not used.
1084
1085=item PERL5DB
1086
1087The command used to load the debugger code. The default is:
1088
1089 BEGIN { require 'perl5db.pl' }
1090
19799a22 1091=item PERL5SHELL (specific to the Win32 port)
174c211a
GS
1092
1093May be set to an alternative shell that perl must use internally for
11998fdb 1094executing "backtick" commands or system(). Default is C<cmd.exe /x/d/c>
ce1da67e 1095on WindowsNT and C<command.com /c> on Windows95. The value is considered
19799a22 1096to be space-separated. Precede any character that needs to be protected
ce1da67e
GS
1097(like a space or backslash) with a backslash.
1098
1099Note that Perl doesn't use COMSPEC for this purpose because
1100COMSPEC has a high degree of variability among users, leading to
1101portability concerns. Besides, perl can use a shell that may not be
1102fit for interactive use, and setting COMSPEC to such a shell may
1103interfere with the proper functioning of other programs (which usually
1104look in COMSPEC to find a shell fit for interactive use).
174c211a 1105
1e422769
PP
1106=item PERL_DEBUG_MSTATS
1107
67ce8856 1108Relevant only if perl is compiled with the malloc included with the perl
a3cb178b
GS
1109distribution (that is, if C<perl -V:d_mymalloc> is 'define').
1110If set, this causes memory statistics to be dumped after execution. If set
1e422769
PP
1111to an integer greater than one, also causes memory statistics to be dumped
1112after compilation.
1113
1114=item PERL_DESTRUCT_LEVEL
1115
1116Relevant only if your perl executable was built with B<-DDEBUGGING>,
1117this controls the behavior of global destruction of objects and other
64cea5fd 1118references. See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.
a0d0e21e 1119
5d170f3a
JH
1120=item PERL_ENCODING
1121
1122If using the C<encoding> pragma without an explicit encoding name, the
1123PERL_ENCODING environment variable is consulted for an encoding name.
1124
504f80c1
JH
1125=item PERL_HASH_SEED
1126
183c3da1 1127(Since Perl 5.8.1.) Used to randomise Perl's internal hash function.
4546b9e6
JH
1128To emulate the pre-5.8.1 behaviour, set to an integer (zero means
1129exactly the same order as 5.8.0). "Pre-5.8.1" means, among other
1130things, that hash keys will be ordered the same between different runs
1131of Perl.
504f80c1 1132
4546b9e6
JH
1133The default behaviour is to randomise unless the PERL_HASH_SEED is set.
1134If Perl has been compiled with C<-DUSE_HASH_SEED_EXPLICIT>, the default
1135behaviour is B<not> to randomise unless the PERL_HASH_SEED is set.
504f80c1
JH
1136
1137If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses
1138the pseudorandom seed supplied by the operating system and libraries.
4546b9e6
JH
1139This means that each different run of Perl will have a different
1140ordering of the results of keys(), values(), and each().
504f80c1 1141
26a2d347
JH
1142B<Please note that the hash seed is sensitive information>. Hashes are
1143randomized to protect against local and remote attacks against Perl
1144code. By manually setting a seed this protection may be partially or
1145completely lost.
1146
1147See L<perlsec/"Algorithmic Complexity Attacks"> and
1148L</PERL_HASH_SEED_DEBUG> for more information.
504f80c1 1149
2191697e
JH
1150=item PERL_HASH_SEED_DEBUG
1151
e67b9e52 1152(Since Perl 5.8.1.) Set to one to display (to STDERR) the value of
26a2d347
JH
1153the hash seed at the beginning of execution. This, combined with
1154L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
1155behavior caused by hash randomization.
1156
1157B<Note that the hash seed is sensitive information>: by knowing it one
1158can craft a denial-of-service attack against Perl code, even remotely,
1159see L<perlsec/"Algorithmic Complexity Attacks"> for more information.
e67b9e52 1160B<Do not disclose the hash seed> to people who don't need to know it.
9a7034eb 1161See also hash_seed() of L<Hash::Util>.
2191697e 1162
3d0ae7ba
GS
1163=item PERL_ROOT (specific to the VMS port)
1164
1165A translation concealed rooted logical name that contains perl and the
1166logical device for the @INC path on VMS only. Other logical names that
44a4342c
NIS
1167affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1168SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in
3d0ae7ba
GS
1169L<perlvms> and in F<README.vms> in the Perl source distribution.
1170
4ffa73a3
JH
1171=item PERL_SIGNALS
1172
1173In Perls 5.8.1 and later. If set to C<unsafe> the pre-Perl-5.8.0
1174signals behaviour (immediate but unsafe) is restored. If set to
ec488bcf
JH
1175C<safe> the safe (or deferred) signals are used.
1176See L<perlipc/"Deferred Signals (Safe signals)">.
4ffa73a3 1177
a05d7ebb 1178=item PERL_UNICODE
acae81db 1179
bf61ac64
JH
1180Equivalent to the B<-C> command-line switch. Note that this is not
1181a boolean variable-- setting this to C<"1"> is not the right way to
5b4f334e 1182"enable Unicode" (whatever that would mean). You can use C<"0"> to
e654d908
JH
1183"disable Unicode", though (or alternatively unset PERL_UNICODE in
1184your shell before starting Perl). See the description of the C<-C>
1185switch for more information.
acae81db 1186
3d0ae7ba
GS
1187=item SYS$LOGIN (specific to the VMS port)
1188
1189Used if chdir has no argument and HOME and LOGDIR are not set.
1190
a0d0e21e 1191=back
1e422769
PP
1192
1193Perl also has environment variables that control how Perl handles data
1194specific to particular natural languages. See L<perllocale>.
1195
1196Apart from these, Perl uses no other environment variables, except
19799a22
GS
1197to make them available to the program being executed, and to child
1198processes. However, programs running setuid would do well to execute
1e422769
PP
1199the following lines before doing anything else, just to keep people
1200honest:
1201
19799a22 1202 $ENV{PATH} = '/bin:/usr/bin'; # or whatever you need
7bac28a0 1203 $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
c90c0ff4 1204 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};