be defensive about setting {host,group,pass}cat (from Andy Dougherty)
[p5sagit/p5-mst-13.2.git] / pod / perlvar.pod
CommitLineData
a0d0e21e 1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
7=head2 Predefined Names
8
5a964f20 9The following names have special meaning to Perl. Most
14218588 10punctuation names have reasonable mnemonics, or analogs in the
11shells. Nevertheless, if you wish to use long variable names,
12you need only say
a0d0e21e 13
14 use English;
15
16at the top of your program. This will alias all the short names to the
5a964f20 17long names in the current package. Some even have medium names,
a0d0e21e 18generally borrowed from B<awk>.
19
19799a22 20If you don't mind the performance hit, variables that depend on the
21currently selected filehandle may instead be set by calling an
22appropriate object method on the IO::Handle object. (Summary lines
23below for this contain the word HANDLE.) First you must say
a0d0e21e 24
19799a22 25 use IO::Handle;
a0d0e21e 26
27after which you may use either
28
29 method HANDLE EXPR
30
5a964f20 31or more safely,
a0d0e21e 32
33 HANDLE->method(EXPR)
34
14218588 35Each method returns the old value of the IO::Handle attribute.
a0d0e21e 36The methods each take an optional EXPR, which if supplied specifies the
19799a22 37new value for the IO::Handle attribute in question. If not supplied,
14218588 38most methods do nothing to the current value--except for
a0d0e21e 39autoflush(), which will assume a 1 for you, just to be different.
14218588 40Because loading in the IO::Handle class is an expensive operation, you should
19799a22 41learn how to use the regular built-in variables.
a0d0e21e 42
748a9306 43A few of these variables are considered "read-only". This means that if
44you try to assign to this variable, either directly or indirectly through
45a reference, you'll raise a run-time exception.
a0d0e21e 46
fb73857a 47The following list is ordered by scalar variables first, then the
87275199 48arrays, then the hashes.
fb73857a 49
a0d0e21e 50=over 8
51
52=item $ARG
53
54=item $_
55
56The default input and pattern-searching space. The following pairs are
57equivalent:
58
19799a22 59 while (<>) {...} # equivalent only in while!
54310121 60 while (defined($_ = <>)) {...}
a0d0e21e 61
62 /^Subject:/
63 $_ =~ /^Subject:/
64
65 tr/a-z/A-Z/
66 $_ =~ tr/a-z/A-Z/
67
19799a22 68 chomp
69 chomp($_)
a0d0e21e 70
54310121 71Here are the places where Perl will assume $_ even if you
cb1a09d0 72don't use it:
73
74=over 3
75
76=item *
77
78Various unary functions, including functions like ord() and int(), as well
79as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
80STDIN.
81
82=item *
83
84Various list functions like print() and unlink().
85
86=item *
87
88The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
89without an C<=~> operator.
90
54310121 91=item *
cb1a09d0 92
93The default iterator variable in a C<foreach> loop if no other
94variable is supplied.
95
54310121 96=item *
cb1a09d0 97
98The implicit iterator variable in the grep() and map() functions.
99
54310121 100=item *
cb1a09d0 101
102The default place to put an input record when a C<E<lt>FHE<gt>>
103operation's result is tested by itself as the sole criterion of a C<while>
14218588 104test. Outside a C<while> test, this will not happen.
cb1a09d0 105
106=back
107
a0d0e21e 108(Mnemonic: underline is understood in certain operations.)
109
6e2995f4 110=back
111
112=over 8
113
5a964f20 114=item $E<lt>I<digits>E<gt>
a0d0e21e 115
19799a22 116Contains the subpattern from the corresponding set of capturing
117parentheses from the last pattern match, not counting patterns
118matched in nested blocks that have been exited already. (Mnemonic:
119like \digits.) These variables are all read-only and dynamically
120scoped to the current BLOCK.
a0d0e21e 121
122=item $MATCH
123
124=item $&
125
126The string matched by the last successful pattern match (not counting
127any matches hidden within a BLOCK or eval() enclosed by the current
19799a22 128BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
129and dynamically scoped to the current BLOCK.
a0d0e21e 130
19ddd453 131The use of this variable anywhere in a program imposes a considerable
19799a22 132performance penalty on all regular expression matches. See L<BUGS>.
19ddd453 133
a0d0e21e 134=item $PREMATCH
135
136=item $`
137
138The string preceding whatever was matched by the last successful
139pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 140enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
a0d0e21e 141string.) This variable is read-only.
142
19ddd453 143The use of this variable anywhere in a program imposes a considerable
19799a22 144performance penalty on all regular expression matches. See L<BUGS>.
19ddd453 145
a0d0e21e 146=item $POSTMATCH
147
148=item $'
149
150The string following whatever was matched by the last successful
151pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 152enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
a0d0e21e 153string.) Example:
154
155 $_ = 'abcdefghi';
156 /def/;
157 print "$`:$&:$'\n"; # prints abc:def:ghi
158
19799a22 159This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 160
19ddd453 161The use of this variable anywhere in a program imposes a considerable
19799a22 162performance penalty on all regular expression matches. See L<BUGS>.
19ddd453 163
a0d0e21e 164=item $LAST_PAREN_MATCH
165
166=item $+
167
168The last bracket matched by the last search pattern. This is useful if
19799a22 169you don't know which one of a set of alternative patterns matched. For
a0d0e21e 170example:
171
172 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
173
174(Mnemonic: be positive and forward looking.)
19799a22 175This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 176
6cef1e77 177=item @+
178
19799a22 179$+[0] is the offset of the end of the last successful match.
6cef1e77 180C<$+[>I<n>C<]> is the offset of the end of the substring matched by
8f580fb8 181I<n>-th subpattern, or undef if the subpattern did not match.
6cef1e77 182
183Thus after a match against $_, $& coincides with C<substr $_, $-[0],
8f580fb8 184$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
185$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
186C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#+> to find the number
14218588 187of subgroups in the last successful match. Contrast with
188C<$#->, the last I<matched> subgroup. Compare with C<@->.
6cef1e77 189
a0d0e21e 190=item $MULTILINE_MATCHING
191
192=item $*
193
4a6725af 194Set to 1 to do multi-line matching within a string, 0 to tell Perl
a0d0e21e 195that it can assume that strings contain a single line, for the purpose
196of optimizing pattern matches. Pattern matches on strings containing
19799a22 197multiple newlines can produce confusing results when C<$*> is 0. Default
198is 0. (Mnemonic: * matches multiple things.) This variable
199influences the interpretation of only C<^> and C<$>. A literal newline can
a0d0e21e 200be searched for even when C<$* == 0>.
201
19799a22 202Use of C<$*> is deprecated in modern Perl, supplanted by
5a964f20 203the C</s> and C</m> modifiers on pattern matching.
a0d0e21e 204
205=item input_line_number HANDLE EXPR
206
207=item $INPUT_LINE_NUMBER
208
209=item $NR
210
211=item $.
212
19799a22 213The current input record number for the last file handle from which
14218588 214you just read() (or called a C<seek> or C<tell> on). The value
883faa13 215may be different from the actual physical line number in the file,
19799a22 216depending on what notion of "line" is in effect--see C<$/> on how
217to change that. An explicit close on a filehandle resets the line
218number. Because C<E<lt>E<gt>> never does an explicit close, line
219numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
220Consider this variable read-only: setting it does not reposition
1e374101 221the seek pointer; you'll have to do that on your own. Localizing C<$.>
222has the effect of also localizing Perl's notion of "the last read
223filehandle". (Mnemonic: many programs use "." to mean the current line
224number.)
a0d0e21e 225
226=item input_record_separator HANDLE EXPR
227
228=item $INPUT_RECORD_SEPARATOR
229
230=item $RS
231
232=item $/
233
14218588 234The input record separator, newline by default. This
235influences Perl's idea of what a "line" is. Works like B<awk>'s RS
19799a22 236variable, including treating empty lines as a terminator if set to
14218588 237the null string. (An empty line cannot contain any spaces
238or tabs.) You may set it to a multi-character string to match a
19799a22 239multi-character terminator, or to C<undef> to read through the end
240of file. Setting it to C<"\n\n"> means something slightly
241different than setting to C<"">, if the file contains consecutive
242empty lines. Setting to C<""> will treat two or more consecutive
243empty lines as a single empty line. Setting to C<"\n\n"> will
244blindly assume that the next input character belongs to the next
14218588 245paragraph, even if it's a newline. (Mnemonic: / delimits
19799a22 246line boundaries when quoting poetry.)
a0d0e21e 247
fbad3eb5 248 undef $/; # enable "slurp" mode
249 $_ = <FH>; # whole file now here
a0d0e21e 250 s/\n[ \t]+/ /g;
251
19799a22 252Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
253better for something. :-)
68dc0745 254
19799a22 255Setting C<$/> to a reference to an integer, scalar containing an integer, or
256scalar that's convertible to an integer will attempt to read records
5b2b9c68 257instead of lines, with the maximum record size being the referenced
19799a22 258integer. So this:
5b2b9c68 259
260 $/ = \32768; # or \"32768", or \$var_containing_32768
261 open(FILE, $myfile);
262 $_ = <FILE>;
263
19799a22 264will read a record of no more than 32768 bytes from FILE. If you're
265not reading from a record-oriented file (or your OS doesn't have
266record-oriented files), then you'll likely get a full chunk of data
267with every read. If a record is larger than the record size you've
268set, you'll get the record back in pieces.
5b2b9c68 269
19799a22 270On VMS, record reads are done with the equivalent of C<sysread>,
271so it's best not to mix record and non-record reads on the same
272file. (This is unlikely to be a problem, because any file you'd
273want to read in record mode is probably usable in line mode.)
14218588 274Non-VMS systems do normal I/O, so it's safe to mix record and
19799a22 275non-record reads of a file.
5b2b9c68 276
14218588 277See also L<perlport/"Newlines">. Also see C<$.>.
883faa13 278
a0d0e21e 279=item autoflush HANDLE EXPR
280
281=item $OUTPUT_AUTOFLUSH
282
283=item $|
284
19799a22 285If set to nonzero, forces a flush right away and after every write
286or print on the currently selected output channel. Default is 0
14218588 287(regardless of whether the channel is really buffered by the
19799a22 288system or not; C<$|> tells you only whether you've asked Perl
289explicitly to flush after each write). STDOUT will
290typically be line buffered if output is to the terminal and block
291buffered otherwise. Setting this variable is useful primarily when
292you are outputting to a pipe or socket, such as when you are running
293a Perl program under B<rsh> and want to see the output as it's
294happening. This has no effect on input buffering. See L<perlfunc/getc>
295for that. (Mnemonic: when you want your pipes to be piping hot.)
a0d0e21e 296
297=item output_field_separator HANDLE EXPR
298
299=item $OUTPUT_FIELD_SEPARATOR
300
301=item $OFS
302
303=item $,
304
305The output field separator for the print operator. Ordinarily the
19799a22 306print operator simply prints out its arguments without further
307adornment. To get behavior more like B<awk>, set this variable as
308you would set B<awk>'s OFS variable to specify what is printed
309between fields. (Mnemonic: what is printed when there is a "," in
310your print statement.)
a0d0e21e 311
312=item output_record_separator HANDLE EXPR
313
314=item $OUTPUT_RECORD_SEPARATOR
315
316=item $ORS
317
318=item $\
319
320The output record separator for the print operator. Ordinarily the
19799a22 321print operator simply prints out its arguments as is, with no
322trailing newline or other end-of-record string added. To get
323behavior more like B<awk>, set this variable as you would set
324B<awk>'s ORS variable to specify what is printed at the end of the
325print. (Mnemonic: you set C<$\> instead of adding "\n" at the
326end of the print. Also, it's just like C<$/>, but it's what you
327get "back" from Perl.)
a0d0e21e 328
329=item $LIST_SEPARATOR
330
331=item $"
332
19799a22 333This is like C<$,> except that it applies to array and slice values
334interpolated into a double-quoted string (or similar interpreted
335string). Default is a space. (Mnemonic: obvious, I think.)
a0d0e21e 336
337=item $SUBSCRIPT_SEPARATOR
338
339=item $SUBSEP
340
341=item $;
342
54310121 343The subscript separator for multidimensional array emulation. If you
a0d0e21e 344refer to a hash element as
345
346 $foo{$a,$b,$c}
347
348it really means
349
350 $foo{join($;, $a, $b, $c)}
351
352But don't put
353
354 @foo{$a,$b,$c} # a slice--note the @
355
356which means
357
358 ($foo{$a},$foo{$b},$foo{$c})
359
19799a22 360Default is "\034", the same as SUBSEP in B<awk>. If your
361keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 362(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 363semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
a0d0e21e 364taken for something more important.)
365
19799a22 366Consider using "real" multidimensional arrays as described
367in L<perllol>.
a0d0e21e 368
369=item $OFMT
370
371=item $#
372
373The output format for printed numbers. This variable is a half-hearted
374attempt to emulate B<awk>'s OFMT variable. There are times, however,
14218588 375when B<awk> and Perl have differing notions of what counts as
19799a22 376numeric. The initial value is "%.I<n>g", where I<n> is the value
6e2995f4 377of the macro DBL_DIG from your system's F<float.h>. This is different from
19799a22 378B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
6e2995f4 379explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
a0d0e21e 380
19799a22 381Use of C<$#> is deprecated.
a0d0e21e 382
383=item format_page_number HANDLE EXPR
384
385=item $FORMAT_PAGE_NUMBER
386
387=item $%
388
389The current page number of the currently selected output channel.
19799a22 390Used with formats.
a0d0e21e 391(Mnemonic: % is page number in B<nroff>.)
392
393=item format_lines_per_page HANDLE EXPR
394
395=item $FORMAT_LINES_PER_PAGE
396
397=item $=
398
399The current page length (printable lines) of the currently selected
19799a22 400output channel. Default is 60.
401Used with formats.
402(Mnemonic: = has horizontal lines.)
a0d0e21e 403
404=item format_lines_left HANDLE EXPR
405
406=item $FORMAT_LINES_LEFT
407
408=item $-
409
410The number of lines left on the page of the currently selected output
19799a22 411channel.
412Used with formats.
413(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 414
6cef1e77 415=item @-
416
19799a22 417$-[0] is the offset of the start of the last successful match.
6cef1e77 418C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 419I<n>-th subpattern, or undef if the subpattern did not match.
6cef1e77 420
421Thus after a match against $_, $& coincides with C<substr $_, $-[0],
8f580fb8 422$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
423$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
424C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
14218588 425matched subgroup in the last successful match. Contrast with
426C<$#+>, the number of subgroups in the regular expression. Compare
19799a22 427with C<@+>.
6cef1e77 428
a0d0e21e 429=item format_name HANDLE EXPR
430
431=item $FORMAT_NAME
432
433=item $~
434
435The name of the current report format for the currently selected output
14218588 436channel. Default is the name of the filehandle. (Mnemonic: brother to
19799a22 437C<$^>.)
a0d0e21e 438
439=item format_top_name HANDLE EXPR
440
441=item $FORMAT_TOP_NAME
442
443=item $^
444
445The name of the current top-of-page format for the currently selected
14218588 446output channel. Default is the name of the filehandle with _TOP
a0d0e21e 447appended. (Mnemonic: points to top of page.)
448
449=item format_line_break_characters HANDLE EXPR
450
451=item $FORMAT_LINE_BREAK_CHARACTERS
452
453=item $:
454
455The current set of characters after which a string may be broken to
54310121 456fill continuation fields (starting with ^) in a format. Default is
a0d0e21e 457S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
458poetry is a part of a line.)
459
460=item format_formfeed HANDLE EXPR
461
462=item $FORMAT_FORMFEED
463
464=item $^L
465
14218588 466What formats output as a form feed. Default is \f.
a0d0e21e 467
468=item $ACCUMULATOR
469
470=item $^A
471
472The current value of the write() accumulator for format() lines. A format
19799a22 473contains formline() calls that put their result into C<$^A>. After
a0d0e21e 474calling its format, write() prints out the contents of C<$^A> and empties.
14218588 475So you never really see the contents of C<$^A> unless you call
a0d0e21e 476formline() yourself and then look at it. See L<perlform> and
477L<perlfunc/formline()>.
478
479=item $CHILD_ERROR
480
481=item $?
482
54310121 483The status returned by the last pipe close, backtick (C<``>) command,
19799a22 484successful call to wait() or waitpid(), or from the system()
485operator. This is just the 16-bit status word returned by the
486wait() system call (or else is made up to look like it). Thus, the
14218588 487exit value of the subprocess is really (C<$? E<gt>E<gt> 8>), and
19799a22 488C<$? & 127> gives which signal, if any, the process died from, and
489C<$? & 128> reports whether there was a core dump. (Mnemonic:
490similar to B<sh> and B<ksh>.)
a0d0e21e 491
7b8d334a 492Additionally, if the C<h_errno> variable is supported in C, its value
14218588 493is returned via $? if any C<gethost*()> function fails.
7b8d334a 494
19799a22 495If you have installed a signal handler for C<SIGCHLD>, the
aa689395 496value of C<$?> will usually be wrong outside that handler.
497
a8f8344d 498Inside an C<END> subroutine C<$?> contains the value that is going to be
499given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
19799a22 500change the exit status of your program. For example:
501
502 END {
503 $? = 1 if $? == 255; # die would make it 255
504 }
a8f8344d 505
aa689395 506Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
ff0cee69 507actual VMS exit status, instead of the default emulation of POSIX
508status.
f86702cc 509
55602bd2 510Also see L<Error Indicators>.
511
a0d0e21e 512=item $OS_ERROR
513
514=item $ERRNO
515
516=item $!
517
19799a22 518If used numerically, yields the current value of the C C<errno>
519variable, with all the usual caveats. (This means that you shouldn't
520depend on the value of C<$!> to be anything in particular unless
521you've gotten a specific error return indicating a system error.)
522If used an a string, yields the corresponding system error string.
523You can assign a number to C<$!> to set I<errno> if, for instance,
524you want C<"$!"> to return the string for error I<n>, or you want
525to set the exit value for the die() operator. (Mnemonic: What just
526went bang?)
a0d0e21e 527
55602bd2 528Also see L<Error Indicators>.
529
5c055ba3 530=item $EXTENDED_OS_ERROR
531
532=item $^E
533
22fae026 534Error information specific to the current operating system. At
535the moment, this differs from C<$!> under only VMS, OS/2, and Win32
536(and for MacPerl). On all other platforms, C<$^E> is always just
537the same as C<$!>.
538
539Under VMS, C<$^E> provides the VMS status value from the last
540system error. This is more specific information about the last
541system error than that provided by C<$!>. This is particularly
d516a115 542important when C<$!> is set to B<EVMSERR>.
22fae026 543
1c1c7f20 544Under OS/2, C<$^E> is set to the error code of the last call to
545OS/2 API either via CRT, or directly from perl.
22fae026 546
547Under Win32, C<$^E> always returns the last error information
548reported by the Win32 call C<GetLastError()> which describes
549the last error from within the Win32 API. Most Win32-specific
19799a22 550code will report errors via C<$^E>. ANSI C and Unix-like calls
22fae026 551set C<errno> and so most portable Perl code will report errors
552via C<$!>.
553
554Caveats mentioned in the description of C<$!> generally apply to
555C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 556
55602bd2 557Also see L<Error Indicators>.
558
a0d0e21e 559=item $EVAL_ERROR
560
561=item $@
562
19799a22 563The Perl syntax error message from the last eval() operator. If null, the
a0d0e21e 564last eval() parsed and executed correctly (although the operations you
565invoked may have failed in the normal fashion). (Mnemonic: Where was
566the syntax error "at"?)
567
19799a22 568Warning messages are not collected in this variable. You can,
a8f8344d 569however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 570as described below.
748a9306 571
55602bd2 572Also see L<Error Indicators>.
573
a0d0e21e 574=item $PROCESS_ID
575
576=item $PID
577
578=item $$
579
19799a22 580The process number of the Perl running this script. You should
581consider this variable read-only, although it will be altered
582across fork() calls. (Mnemonic: same as shells.)
a0d0e21e 583
584=item $REAL_USER_ID
585
586=item $UID
587
588=item $<
589
19799a22 590The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a0d0e21e 591if you're running setuid.)
592
593=item $EFFECTIVE_USER_ID
594
595=item $EUID
596
597=item $>
598
599The effective uid of this process. Example:
600
601 $< = $>; # set real to effective uid
602 ($<,$>) = ($>,$<); # swap real and effective uid
603
19799a22 604(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
14218588 605C<$E<lt>> and C<$E<gt>> can be swapped only on machines
8cc95fdb 606supporting setreuid().
a0d0e21e 607
608=item $REAL_GROUP_ID
609
610=item $GID
611
612=item $(
613
614The real gid of this process. If you are on a machine that supports
615membership in multiple groups simultaneously, gives a space separated
616list of groups you are in. The first number is the one returned by
617getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb 618the same as the first number.
619
19799a22 620However, a value assigned to C<$(> must be a single number used to
621set the real gid. So the value given by C<$(> should I<not> be assigned
622back to C<$(> without being forced numeric, such as by adding zero.
8cc95fdb 623
19799a22 624(Mnemonic: parentheses are used to I<group> things. The real gid is the
625group you I<left>, if you're running setgid.)
a0d0e21e 626
627=item $EFFECTIVE_GROUP_ID
628
629=item $EGID
630
631=item $)
632
633The effective gid of this process. If you are on a machine that
634supports membership in multiple groups simultaneously, gives a space
635separated list of groups you are in. The first number is the one
636returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb 637which may be the same as the first number.
638
19799a22 639Similarly, a value assigned to C<$)> must also be a space-separated
14218588 640list of numbers. The first number sets the effective gid, and
8cc95fdb 641the rest (if any) are passed to setgroups(). To get the effect of an
642empty list for setgroups(), just repeat the new effective gid; that is,
643to force an effective gid of 5 and an effectively empty setgroups()
644list, say C< $) = "5 5" >.
645
19799a22 646(Mnemonic: parentheses are used to I<group> things. The effective gid
647is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 648
14218588 649C<$E<lt>>, C<$E<gt>>, C<$(> and C<$)> can be set only on
19799a22 650machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
651and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e 652
653=item $PROGRAM_NAME
654
655=item $0
656
19799a22 657Contains the name of the program being executed. On some operating
658systems assigning to C<$0> modifies the argument area that the B<ps>
659program sees. This is more useful as a way of indicating the current
660program state than it is for hiding the program you're running.
a0d0e21e 661(Mnemonic: same as B<sh> and B<ksh>.)
662
663=item $[
664
665The index of the first element in an array, and of the first character
19799a22 666in a substring. Default is 0, but you could theoretically set it
667to 1 to make Perl behave more like B<awk> (or Fortran) when
668subscripting and when evaluating the index() and substr() functions.
669(Mnemonic: [ begins subscripts.)
a0d0e21e 670
19799a22 671As of release 5 of Perl, assignment to C<$[> is treated as a compiler
672directive, and cannot influence the behavior of any other file.
673Its use is highly discouraged.
a0d0e21e 674
675=item $PERL_VERSION
676
677=item $]
678
54310121 679The version + patchlevel / 1000 of the Perl interpreter. This variable
680can be used to determine whether the Perl interpreter executing a
681script is in the right range of versions. (Mnemonic: Is this version
682of perl in the right bracket?) Example:
a0d0e21e 683
684 warn "No checksumming!\n" if $] < 3.019;
685
54310121 686See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 687for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 688
305aace0 689=item $COMPILING
690
691=item $^C
692
19799a22 693The current value of the flag associated with the B<-c> switch.
694Mainly of use with B<-MO=...> to allow code to alter its behavior
695when being compiled, such as for example to AUTOLOAD at compile
696time rather than normal, deferred loading. See L<perlcc>. Setting
697C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 698
a0d0e21e 699=item $DEBUGGING
700
701=item $^D
702
703The current value of the debugging flags. (Mnemonic: value of B<-D>
704switch.)
705
706=item $SYSTEM_FD_MAX
707
708=item $^F
709
710The maximum system file descriptor, ordinarily 2. System file
711descriptors are passed to exec()ed processes, while higher file
712descriptors are not. Also, during an open(), system file descriptors are
713preserved even if the open() fails. (Ordinary file descriptors are
19799a22 714closed before the open() is attempted.) The close-on-exec
a0d0e21e 715status of a file descriptor will be decided according to the value of
4771b018 716C<$^F> when the open() or pipe() was called, not the time of the exec().
a0d0e21e 717
6e2995f4 718=item $^H
719
fb73857a 720The current set of syntax checks enabled by C<use strict> and other block
721scoped compiler hints. See the documentation of C<strict> for more details.
6e2995f4 722
a0d0e21e 723=item $INPLACE_EDIT
724
725=item $^I
726
727The current value of the inplace-edit extension. Use C<undef> to disable
728inplace editing. (Mnemonic: value of B<-i> switch.)
729
fb73857a 730=item $^M
731
19799a22 732By default, running out of memory is an untrappable, fatal error.
733However, if suitably built, Perl can use the contents of C<$^M>
734as an emergency memory pool after die()ing. Suppose that your Perl
735were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
736Then
fb73857a 737
19799a22 738 $^M = 'a' x (1 << 16);
fb73857a 739
19799a22 740would allocate a 64K buffer for use when in emergency. See the
741F<INSTALL> file in the Perl distribution for information on how to
742enable this option. To discourage casual use of this advanced
743feature, there is no L<English> long name for this variable.
fb73857a 744
5c055ba3 745=item $OSNAME
6e2995f4 746
5c055ba3 747=item $^O
748
749The name of the operating system under which this copy of Perl was
750built, as determined during the configuration process. The value
19799a22 751is identical to C<$Config{'osname'}>. See also L<Config> and the
752B<-V> command-line switch documented in L<perlrun>.
5c055ba3 753
a0d0e21e 754=item $PERLDB
755
756=item $^P
757
19799a22 758The internal variable for debugging support. The meanings of the
759various bits are subject to change, but currently indicate:
84902520 760
761=over 6
762
763=item 0x01
764
765Debug subroutine enter/exit.
766
767=item 0x02
768
769Line-by-line debugging.
770
771=item 0x04
772
773Switch off optimizations.
774
775=item 0x08
776
777Preserve more data for future interactive inspections.
778
779=item 0x10
780
781Keep info about source lines on which a subroutine is defined.
782
783=item 0x20
784
785Start with single-step on.
786
787=back
788
19799a22 789Some bits may be relevant at compile-time only, some at
790run-time only. This is a new mechanism and the details may change.
a0d0e21e 791
b9ac3b5b 792=item $^R
793
19799a22 794The result of evaluation of the last successful C<(?{ code })>
795regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 796
fb73857a 797=item $^S
798
799Current state of the interpreter. Undefined if parsing of the current
800module/eval is not finished (may happen in $SIG{__DIE__} and
19799a22 801$SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
fb73857a 802
a0d0e21e 803=item $BASETIME
804
805=item $^T
806
19799a22 807The time at which the program began running, in seconds since the
5f05dabc 808epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 809and B<-C> filetests are based on this value.
a0d0e21e 810
811=item $WARNING
812
813=item $^W
814
19799a22 815The current value of the warning switch, initially true if B<-w>
816was used, false otherwise, but directly modifiable. (Mnemonic:
4438c4b7 817related to the B<-w> switch.) See also L<warnings>.
818
819=item ${^Warnings}
820
821The current set of warning checks enabled by the C<use warnings> pragma.
822See the documentation of C<warnings> for more details.
a0d0e21e 823
824=item $EXECUTABLE_NAME
825
826=item $^X
827
828The name that the Perl binary itself was executed as, from C's C<argv[0]>.
19799a22 829This may not be a full pathname, nor even necessarily in your path.
a0d0e21e 830
831=item $ARGV
832
a8f8344d 833contains the name of the current file when reading from E<lt>E<gt>.
a0d0e21e 834
835=item @ARGV
836
19799a22 837The array @ARGV contains the command-line arguments intended for
14218588 838the script. C<$#ARGV> is generally the number of arguments minus
19799a22 839one, because C<$ARGV[0]> is the first argument, I<not> the program's
840command name itself. See C<$0> for the command name.
a0d0e21e 841
842=item @INC
843
19799a22 844The array @INC contains the list of places that the C<do EXPR>,
845C<require>, or C<use> constructs look for their library files. It
846initially consists of the arguments to any B<-I> command-line
847switches, followed by the default Perl library, probably
848F</usr/local/lib/perl>, followed by ".", to represent the current
849directory. If you need to modify this at runtime, you should use
850the C<use lib> pragma to get the machine-dependent library properly
851loaded also:
a0d0e21e 852
cb1a09d0 853 use lib '/mypath/libdir/';
854 use SomeMod;
303f2f76 855
fb73857a 856=item @_
857
858Within a subroutine the array @_ contains the parameters passed to that
19799a22 859subroutine. See L<perlsub>.
fb73857a 860
a0d0e21e 861=item %INC
862
19799a22 863The hash %INC contains entries for each filename included via the
864C<do>, C<require>, or C<use> operators. The key is the filename
865you specified (with module names converted to pathnames), and the
14218588 866value is the location of the file found. The C<require>
87275199 867operator uses this hash to determine whether a particular file has
19799a22 868already been included.
a0d0e21e 869
b687b08b 870=item %ENV
871
872=item $ENV{expr}
a0d0e21e 873
874The hash %ENV contains your current environment. Setting a
19799a22 875value in C<ENV> changes the environment for any child processes
876you subsequently fork() off.
a0d0e21e 877
b687b08b 878=item %SIG
879
880=item $SIG{expr}
a0d0e21e 881
14218588 882The hash %SIG contains signal handlers for signals. For example:
a0d0e21e 883
884 sub handler { # 1st argument is signal name
fb73857a 885 my($sig) = @_;
a0d0e21e 886 print "Caught a SIG$sig--shutting down\n";
887 close(LOG);
888 exit(0);
889 }
890
fb73857a 891 $SIG{'INT'} = \&handler;
892 $SIG{'QUIT'} = \&handler;
a0d0e21e 893 ...
19799a22 894 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e 895 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
896
f648820c 897Using a value of C<'IGNORE'> usually has the effect of ignoring the
898signal, except for the C<CHLD> signal. See L<perlipc> for more about
899this special case.
900
19799a22 901Here are some other examples:
a0d0e21e 902
fb73857a 903 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 904 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 905 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e 906 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
907
19799a22 908Be sure not to use a bareword as the name of a signal handler,
909lest you inadvertently call it.
748a9306 910
44a8e56a 911If your system has the sigaction() function then signal handlers are
912installed using it. This means you get reliable signal handling. If
913your system has the SA_RESTART flag it is used when signals handlers are
19799a22 914installed. This means that system calls for which restarting is supported
44a8e56a 915continue rather than returning when a signal arrives. If you want your
916system calls to be interrupted by signal delivery then do something like
917this:
918
919 use POSIX ':signal_h';
920
921 my $alarm = 0;
922 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
923 or die "Error setting SIGALRM handler: $!\n";
924
925See L<POSIX>.
926
748a9306 927Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 928routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306 929about to be printed. The warning message is passed as the first
930argument. The presence of a __WARN__ hook causes the ordinary printing
931of warnings to STDERR to be suppressed. You can use this to save warnings
932in a variable, or turn warnings into fatal errors, like this:
933
934 local $SIG{__WARN__} = sub { die $_[0] };
935 eval $proggie;
936
a8f8344d 937The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306 938is about to be thrown. The error message is passed as the first
939argument. When a __DIE__ hook routine returns, the exception
940processing continues as it would have in the absence of the hook,
cb1a09d0 941unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
774d564b 942The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a 943can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
944
19799a22 945Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
946even inside an eval(). Do not use this to rewrite a pending exception
947in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
948This strange action at a distance may be fixed in a future release
949so that C<$SIG{__DIE__}> is only called if your program is about
950to exit, as was the original intent. Any other use is deprecated.
951
952C<__DIE__>/C<__WARN__> handlers are very special in one respect:
953they may be called to report (probable) errors found by the parser.
954In such a case the parser may be in inconsistent state, so any
955attempt to evaluate Perl code from such a handler will probably
956result in a segfault. This means that warnings or errors that
957result from parsing Perl should be used with extreme caution, like
958this:
fb73857a 959
960 require Carp if defined $^S;
961 Carp::confess("Something wrong") if defined &Carp::confess;
962 die "Something wrong, but could not load Carp to give backtrace...
963 To see backtrace try starting Perl with -MCarp switch";
964
965Here the first line will load Carp I<unless> it is the parser who
966called the handler. The second line will print backtrace and die if
967Carp was available. The third line will be executed only if Carp was
968not available.
969
19799a22 970See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
4438c4b7 971L<warnings> for additional information.
68dc0745 972
a0d0e21e 973=back
55602bd2 974
975=head2 Error Indicators
976
19799a22 977The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
978about different types of error conditions that may appear during
979execution of a Perl program. The variables are shown ordered by
980the "distance" between the subsystem which reported the error and
981the Perl process. They correspond to errors detected by the Perl
982interpreter, C library, operating system, or an external program,
983respectively.
55602bd2 984
985To illustrate the differences between these variables, consider the
19799a22 986following Perl expression, which uses a single-quoted string:
55602bd2 987
19799a22 988 eval q{
989 open PIPE, "/cdrom/install |";
990 @res = <PIPE>;
991 close PIPE or die "bad pipe: $?, $!";
992 };
55602bd2 993
994After execution of this statement all 4 variables may have been set.
995
19799a22 996C<$@> is set if the string to be C<eval>-ed did not compile (this
997may happen if C<open> or C<close> were imported with bad prototypes),
998or if Perl code executed during evaluation die()d . In these cases
999the value of $@ is the compile error, or the argument to C<die>
1000(which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1001though.)
1002
1003When the eval() expression above is executed, open(), C<<PIPEE<gt>>,
1004and C<close> are translated to calls in the C run-time library and
1005thence to the operating system kernel. C<$!> is set to the C library's
1006C<errno> if one of these calls fails.
1007
1008Under a few operating systems, C<$^E> may contain a more verbose
1009error indicator, such as in this case, "CDROM tray not closed."
14218588 1010Systems that do not support extended error messages leave C<$^E>
19799a22 1011the same as C<$!>.
1012
1013Finally, C<$?> may be set to non-0 value if the external program
1014F</cdrom/install> fails. The upper eight bits reflect specific
1015error conditions encountered by the program (the program's exit()
1016value). The lower eight bits reflect mode of failure, like signal
1017death and core dump information See wait(2) for details. In
1018contrast to C<$!> and C<$^E>, which are set only if error condition
1019is detected, the variable C<$?> is set on each C<wait> or pipe
1020C<close>, overwriting the old value. This is more like C<$@>, which
1021on every eval() is always set on failure and cleared on success.
2b92dfce 1022
19799a22 1023For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1024and C<$?>.
2b92dfce 1025
1026=head2 Technical Note on the Syntax of Variable Names
1027
19799a22 1028Variable names in Perl can have several formats. Usually, they
1029must begin with a letter or underscore, in which case they can be
1030arbitrarily long (up to an internal limit of 251 characters) and
1031may contain letters, digits, underscores, or the special sequence
1032C<::> or C<'>. In this case, the part before the last C<::> or
1033C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce 1034
1035Perl variable names may also be a sequence of digits or a single
1036punctuation or control character. These names are all reserved for
19799a22 1037special uses by Perl; for example, the all-digits names are used
1038to hold data captured by backreferences after a regular expression
1039match. Perl has a special syntax for the single-control-character
1040names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1041character. For example, the notation C<$^W> (dollar-sign caret
1042C<W>) is the scalar variable whose name is the single character
1043control-C<W>. This is better than typing a literal control-C<W>
1044into your program.
2b92dfce 1045
87275199 1046Finally, new in Perl 5.6, Perl variable names may be alphanumeric
19799a22 1047strings that begin with control characters (or better yet, a caret).
1048These variables must be written in the form C<${^Foo}>; the braces
1049are not optional. C<${^Foo}> denotes the scalar variable whose
1050name is a control-C<F> followed by two C<o>'s. These variables are
1051reserved for future special uses by Perl, except for the ones that
1052begin with C<^_> (control-underscore or caret-underscore). No
1053control-character name that begins with C<^_> will acquire a special
1054meaning in any future version of Perl; such names may therefore be
1055used safely in programs. C<$^_> itself, however, I<is> reserved.
1056
1057Perl identifiers that begin with digits, control characters, or
2b92dfce 1058punctuation characters are exempt from the effects of the C<package>
1059declaration and are always forced to be in package C<main>. A few
1060other names are also exempt:
1061
1062 ENV STDIN
1063 INC STDOUT
1064 ARGV STDERR
1065 ARGVOUT
1066 SIG
1067
1068In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1069to be in package C<main>, regardless of any C<package> declarations
2b92dfce 1070presently in scope.
1071
19799a22 1072=head1 BUGS
1073
1074Due to an unfortunate accident of Perl's implementation, C<use
1075English> imposes a considerable performance penalty on all regular
1076expression matches in a program, regardless of whether they occur
1077in the scope of C<use English>. For that reason, saying C<use
1078English> in libraries is strongly discouraged. See the
1079Devel::SawAmpersand module documentation from CPAN
1080(http://www.perl.com/CPAN/modules/by-module/Devel/Devel-SawAmpersand-0.10.readme)
1081for more information.
2b92dfce 1082
19799a22 1083Having to even think about the C<$^S> variable in your exception
1084handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1085invites grievous and difficult to track down errors. Avoid it
1086and use an C<END{}> or CORE::GLOBAL::die override instead.