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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
5f05dabc 10punctuation names have reasonable mnemonics, or analogues in one of
5a964f20 11the shells. Nevertheless, if you wish to use long variable names,
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12you just need to say
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,
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18generally borrowed from B<awk>.
19
20To go a step further, those variables that depend on the currently
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21selected filehandle may instead (and preferably) be set by calling an
22object method on the FileHandle object. (Summary lines below for this
23contain the word HANDLE.) First you must say
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24
25 use FileHandle;
26
27after which you may use either
28
29 method HANDLE EXPR
30
5a964f20 31or more safely,
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32
33 HANDLE->method(EXPR)
34
35Each of the methods returns the old value of the FileHandle attribute.
36The methods each take an optional EXPR, which if supplied specifies the
37new value for the FileHandle attribute in question. If not supplied,
38most of the methods do nothing to the current value, except for
39autoflush(), which will assume a 1 for you, just to be different.
40
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41A few of these variables are considered "read-only". This means that if
42you try to assign to this variable, either directly or indirectly through
43a reference, you'll raise a run-time exception.
a0d0e21e 44
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45The following list is ordered by scalar variables first, then the
46arrays, then the hashes (except $^M was added in the wrong place).
47This is somewhat obscured by the fact that %ENV and %SIG are listed as
48$ENV{expr} and $SIG{expr}.
49
50
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51=over 8
52
53=item $ARG
54
55=item $_
56
57The default input and pattern-searching space. The following pairs are
58equivalent:
59
5f05dabc 60 while (<>) {...} # equivalent in only while!
54310121 61 while (defined($_ = <>)) {...}
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62
63 /^Subject:/
64 $_ =~ /^Subject:/
65
66 tr/a-z/A-Z/
67 $_ =~ tr/a-z/A-Z/
68
69 chop
70 chop($_)
71
54310121 72Here are the places where Perl will assume $_ even if you
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73don't use it:
74
75=over 3
76
77=item *
78
79Various unary functions, including functions like ord() and int(), as well
80as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
81STDIN.
82
83=item *
84
85Various list functions like print() and unlink().
86
87=item *
88
89The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
90without an C<=~> operator.
91
54310121 92=item *
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93
94The default iterator variable in a C<foreach> loop if no other
95variable is supplied.
96
54310121 97=item *
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98
99The implicit iterator variable in the grep() and map() functions.
100
54310121 101=item *
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102
103The default place to put an input record when a C<E<lt>FHE<gt>>
104operation's result is tested by itself as the sole criterion of a C<while>
105test. Note that outside of a C<while> test, this will not happen.
106
107=back
108
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109(Mnemonic: underline is understood in certain operations.)
110
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111=back
112
113=over 8
114
5a964f20 115=item $E<lt>I<digits>E<gt>
a0d0e21e 116
54310121 117Contains the subpattern from the corresponding set of parentheses in
a0d0e21e 118the last pattern matched, not counting patterns matched in nested
5a964f20 119blocks that have been exited already. (Mnemonic: like \digits.)
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120These variables are all read-only.
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
128BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
129
130=item $PREMATCH
131
132=item $`
133
134The string preceding whatever was matched by the last successful
135pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 136enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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137string.) This variable is read-only.
138
139=item $POSTMATCH
140
141=item $'
142
143The string following whatever was matched by the last successful
144pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 145enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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146string.) Example:
147
148 $_ = 'abcdefghi';
149 /def/;
150 print "$`:$&:$'\n"; # prints abc:def:ghi
151
152This variable is read-only.
153
154=item $LAST_PAREN_MATCH
155
156=item $+
157
158The last bracket matched by the last search pattern. This is useful if
159you don't know which of a set of alternative patterns matched. For
160example:
161
162 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
163
164(Mnemonic: be positive and forward looking.)
165This variable is read-only.
166
167=item $MULTILINE_MATCHING
168
169=item $*
170
4a6725af 171Set to 1 to do multi-line matching within a string, 0 to tell Perl
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172that it can assume that strings contain a single line, for the purpose
173of optimizing pattern matches. Pattern matches on strings containing
174multiple newlines can produce confusing results when "C<$*>" is 0. Default
175is 0. (Mnemonic: * matches multiple things.) Note that this variable
5f05dabc 176influences the interpretation of only "C<^>" and "C<$>". A literal newline can
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177be searched for even when C<$* == 0>.
178
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179Use of "C<$*>" is deprecated in modern Perls, supplanted by
180the C</s> and C</m> modifiers on pattern matching.
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181
182=item input_line_number HANDLE EXPR
183
184=item $INPUT_LINE_NUMBER
185
186=item $NR
187
188=item $.
189
6e2995f4 190The current input line number for the last file handle from
a8f8344d 191which you read (or performed a C<seek> or C<tell> on). An
5f05dabc 192explicit close on a filehandle resets the line number. Because
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193"C<E<lt>E<gt>>" never does an explicit close, line numbers increase
194across ARGV files (but see examples under eof()). Localizing C<$.> has
195the effect of also localizing Perl's notion of "the last read
196filehandle". (Mnemonic: many programs use "." to mean the current line
197number.)
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198
199=item input_record_separator HANDLE EXPR
200
201=item $INPUT_RECORD_SEPARATOR
202
203=item $RS
204
205=item $/
206
207The input record separator, newline by default. Works like B<awk>'s RS
303f2f76 208variable, including treating empty lines as delimiters if set to the
54310121 209null string. (Note: An empty line cannot contain any spaces or tabs.)
4a6725af 210You may set it to a multi-character string to match a multi-character
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211delimiter, or to C<undef> to read to end of file. Note that setting it
212to C<"\n\n"> means something slightly different than setting it to
213C<"">, if the file contains consecutive empty lines. Setting it to
214C<""> will treat two or more consecutive empty lines as a single empty
215line. Setting it to C<"\n\n"> will blindly assume that the next input
216character belongs to the next paragraph, even if it's a newline.
217(Mnemonic: / is used to delimit line boundaries when quoting poetry.)
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218
219 undef $/;
220 $_ = <FH>; # whole file now here
221 s/\n[ \t]+/ /g;
222
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223Remember: the value of $/ is a string, not a regexp. AWK has to be
224better for something :-)
225
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226Setting $/ to a reference to an integer, scalar containing an integer, or
227scalar that's convertable to an integer will attempt to read records
228instead of lines, with the maximum record size being the referenced
229integer. So this:
230
231 $/ = \32768; # or \"32768", or \$var_containing_32768
232 open(FILE, $myfile);
233 $_ = <FILE>;
234
235will read a record of no more than 32768 bytes from FILE. If you're not
236reading from a record-oriented file (or your OS doesn't have
237record-oriented files), then you'll likely get a full chunk of data with
238every read. If a record is larger than the record size you've set, you'll
239get the record back in pieces.
240
241On VMS, record reads are done with the equivalent of C<sysread>, so it's
242best not to mix record and non-record reads on the same file. (This is
243likely not a problem, as any file you'd want to read in record mode is
244proably usable in line mode) Non-VMS systems perform normal I/O, so
245it's safe to mix record and non-record reads of a file.
246
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247=item autoflush HANDLE EXPR
248
249=item $OUTPUT_AUTOFLUSH
250
251=item $|
252
54310121 253If set to nonzero, forces a flush right away and after every write or print on the
6e2995f4 254currently selected output channel. Default is 0 (regardless of whether
5f05dabc 255the channel is actually buffered by the system or not; C<$|> tells you
54310121 256only whether you've asked Perl explicitly to flush after each write).
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257Note that STDOUT will typically be line buffered if output is to the
258terminal and block buffered otherwise. Setting this variable is useful
259primarily when you are outputting to a pipe, such as when you are running
260a Perl script under rsh and want to see the output as it's happening. This
261has no effect on input buffering.
cb1a09d0 262(Mnemonic: when you want your pipes to be piping hot.)
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263
264=item output_field_separator HANDLE EXPR
265
266=item $OUTPUT_FIELD_SEPARATOR
267
268=item $OFS
269
270=item $,
271
272The output field separator for the print operator. Ordinarily the
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273print operator simply prints out the comma-separated fields you
274specify. To get behavior more like B<awk>, set this variable
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275as you would set B<awk>'s OFS variable to specify what is printed
276between fields. (Mnemonic: what is printed when there is a , in your
277print statement.)
278
279=item output_record_separator HANDLE EXPR
280
281=item $OUTPUT_RECORD_SEPARATOR
282
283=item $ORS
284
285=item $\
286
287The output record separator for the print operator. Ordinarily the
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288print operator simply prints out the comma-separated fields you
289specify, with no trailing newline or record separator assumed.
290To get behavior more like B<awk>, set this variable as you would
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291set B<awk>'s ORS variable to specify what is printed at the end of the
292print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
a8f8344d 293print. Also, it's just like C<$/>, but it's what you get "back" from
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294Perl.)
295
296=item $LIST_SEPARATOR
297
298=item $"
299
300This is like "C<$,>" except that it applies to array values interpolated
301into a double-quoted string (or similar interpreted string). Default
302is a space. (Mnemonic: obvious, I think.)
303
304=item $SUBSCRIPT_SEPARATOR
305
306=item $SUBSEP
307
308=item $;
309
54310121 310The subscript separator for multidimensional array emulation. If you
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311refer to a hash element as
312
313 $foo{$a,$b,$c}
314
315it really means
316
317 $foo{join($;, $a, $b, $c)}
318
319But don't put
320
321 @foo{$a,$b,$c} # a slice--note the @
322
323which means
324
325 ($foo{$a},$foo{$b},$foo{$c})
326
327Default is "\034", the same as SUBSEP in B<awk>. Note that if your
328keys contain binary data there might not be any safe value for "C<$;>".
329(Mnemonic: comma (the syntactic subscript separator) is a
330semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
331taken for something more important.)
332
54310121 333Consider using "real" multidimensional arrays.
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334
335=item $OFMT
336
337=item $#
338
339The output format for printed numbers. This variable is a half-hearted
340attempt to emulate B<awk>'s OFMT variable. There are times, however,
341when B<awk> and Perl have differing notions of what is in fact
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342numeric. The initial value is %.I<n>g, where I<n> is the value
343of the macro DBL_DIG from your system's F<float.h>. This is different from
344B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
345explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
a0d0e21e 346
5f05dabc 347Use of "C<$#>" is deprecated.
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348
349=item format_page_number HANDLE EXPR
350
351=item $FORMAT_PAGE_NUMBER
352
353=item $%
354
355The current page number of the currently selected output channel.
356(Mnemonic: % is page number in B<nroff>.)
357
358=item format_lines_per_page HANDLE EXPR
359
360=item $FORMAT_LINES_PER_PAGE
361
362=item $=
363
364The current page length (printable lines) of the currently selected
365output channel. Default is 60. (Mnemonic: = has horizontal lines.)
366
367=item format_lines_left HANDLE EXPR
368
369=item $FORMAT_LINES_LEFT
370
371=item $-
372
373The number of lines left on the page of the currently selected output
374channel. (Mnemonic: lines_on_page - lines_printed.)
375
376=item format_name HANDLE EXPR
377
378=item $FORMAT_NAME
379
380=item $~
381
382The name of the current report format for the currently selected output
383channel. Default is name of the filehandle. (Mnemonic: brother to
384"C<$^>".)
385
386=item format_top_name HANDLE EXPR
387
388=item $FORMAT_TOP_NAME
389
390=item $^
391
392The name of the current top-of-page format for the currently selected
393output channel. Default is name of the filehandle with _TOP
394appended. (Mnemonic: points to top of page.)
395
396=item format_line_break_characters HANDLE EXPR
397
398=item $FORMAT_LINE_BREAK_CHARACTERS
399
400=item $:
401
402The current set of characters after which a string may be broken to
54310121 403fill continuation fields (starting with ^) in a format. Default is
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404S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
405poetry is a part of a line.)
406
407=item format_formfeed HANDLE EXPR
408
409=item $FORMAT_FORMFEED
410
411=item $^L
412
5f05dabc 413What formats output to perform a form feed. Default is \f.
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414
415=item $ACCUMULATOR
416
417=item $^A
418
419The current value of the write() accumulator for format() lines. A format
420contains formline() commands that put their result into C<$^A>. After
421calling its format, write() prints out the contents of C<$^A> and empties.
422So you never actually see the contents of C<$^A> unless you call
423formline() yourself and then look at it. See L<perlform> and
424L<perlfunc/formline()>.
425
426=item $CHILD_ERROR
427
428=item $?
429
54310121 430The status returned by the last pipe close, backtick (C<``>) command,
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431or system() operator. Note that this is the status word returned by the
432wait() system call (or else is made up to look like it). Thus, the exit
433value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and C<$? & 127>
434gives which signal, if any, the process died from, and C<$? & 128> reports
435whether there was a core dump. (Mnemonic: similar to B<sh> and B<ksh>.)
a0d0e21e 436
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437Additionally, if the C<h_errno> variable is supported in C, its value
438is returned via $? if any of the C<gethost*()> functions fail.
439
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440Note that if you have installed a signal handler for C<SIGCHLD>, the
441value of C<$?> will usually be wrong outside that handler.
442
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443Inside an C<END> subroutine C<$?> contains the value that is going to be
444given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
445change the exit status of the script.
446
aa689395 447Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
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448actual VMS exit status, instead of the default emulation of POSIX
449status.
f86702cc 450
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451=item $OS_ERROR
452
453=item $ERRNO
454
455=item $!
456
457If used in a numeric context, yields the current value of errno, with
458all the usual caveats. (This means that you shouldn't depend on the
22fae026 459value of C<$!> to be anything in particular unless you've gotten a
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460specific error return indicating a system error.) If used in a string
461context, yields the corresponding system error string. You can assign
22fae026 462to C<$!> to set I<errno> if, for instance, you want C<"$!"> to return the
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463string for error I<n>, or you want to set the exit value for the die()
464operator. (Mnemonic: What just went bang?)
465
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466=item $EXTENDED_OS_ERROR
467
468=item $^E
469
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470Error information specific to the current operating system. At
471the moment, this differs from C<$!> under only VMS, OS/2, and Win32
472(and for MacPerl). On all other platforms, C<$^E> is always just
473the same as C<$!>.
474
475Under VMS, C<$^E> provides the VMS status value from the last
476system error. This is more specific information about the last
477system error than that provided by C<$!>. This is particularly
d516a115 478important when C<$!> is set to B<EVMSERR>.
22fae026 479
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480Under OS/2, C<$^E> is set to the error code of the last call to
481OS/2 API either via CRT, or directly from perl.
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482
483Under Win32, C<$^E> always returns the last error information
484reported by the Win32 call C<GetLastError()> which describes
485the last error from within the Win32 API. Most Win32-specific
486code will report errors via C<$^E>. ANSI C and UNIX-like calls
487set C<errno> and so most portable Perl code will report errors
488via C<$!>.
489
490Caveats mentioned in the description of C<$!> generally apply to
491C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 492
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493=item $EVAL_ERROR
494
495=item $@
496
497The Perl syntax error message from the last eval() command. If null, the
498last eval() parsed and executed correctly (although the operations you
499invoked may have failed in the normal fashion). (Mnemonic: Where was
500the syntax error "at"?)
501
748a9306 502Note that warning messages are not collected in this variable. You can,
a8f8344d 503however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 504as described below.
748a9306 505
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506=item $PROCESS_ID
507
508=item $PID
509
510=item $$
511
512The process number of the Perl running this script. (Mnemonic: same
513as shells.)
514
515=item $REAL_USER_ID
516
517=item $UID
518
519=item $<
520
521The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
522if you're running setuid.)
523
524=item $EFFECTIVE_USER_ID
525
526=item $EUID
527
528=item $>
529
530The effective uid of this process. Example:
531
532 $< = $>; # set real to effective uid
533 ($<,$>) = ($>,$<); # swap real and effective uid
534
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535(Mnemonic: it's the uid you went I<TO>, if you're running setuid.)
536Note: "C<$E<lt>>" and "C<$E<gt>>" can be swapped only on machines
537supporting setreuid().
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538
539=item $REAL_GROUP_ID
540
541=item $GID
542
543=item $(
544
545The real gid of this process. If you are on a machine that supports
546membership in multiple groups simultaneously, gives a space separated
547list of groups you are in. The first number is the one returned by
548getgid(), and the subsequent ones by getgroups(), one of which may be
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549the same as the first number.
550
551However, a value assigned to "C<$(>" must be a single number used to
552set the real gid. So the value given by "C<$(>" should I<not> be assigned
553back to "C<$(>" without being forced numeric, such as by adding zero.
554
555(Mnemonic: parentheses are used to I<GROUP> things. The real gid is the
556group you I<LEFT>, if you're running setgid.)
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557
558=item $EFFECTIVE_GROUP_ID
559
560=item $EGID
561
562=item $)
563
564The effective gid of this process. If you are on a machine that
565supports membership in multiple groups simultaneously, gives a space
566separated list of groups you are in. The first number is the one
567returned by getegid(), and the subsequent ones by getgroups(), one of
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568which may be the same as the first number.
569
570Similarly, a value assigned to "C<$)>" must also be a space-separated
571list of numbers. The first number is used to set the effective gid, and
572the rest (if any) are passed to setgroups(). To get the effect of an
573empty list for setgroups(), just repeat the new effective gid; that is,
574to force an effective gid of 5 and an effectively empty setgroups()
575list, say C< $) = "5 5" >.
576
577(Mnemonic: parentheses are used to I<GROUP> things. The effective gid
578is the group that's I<RIGHT> for you, if you're running setgid.)
a0d0e21e 579
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580Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
581machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
8cc95fdb 582and "C<$)>" can be swapped only on machines supporting setregid().
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583
584=item $PROGRAM_NAME
585
586=item $0
587
588Contains the name of the file containing the Perl script being
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589executed. On some operating systems
590assigning to "C<$0>" modifies the argument area that the ps(1)
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591program sees. This is more useful as a way of indicating the
592current program state than it is for hiding the program you're running.
593(Mnemonic: same as B<sh> and B<ksh>.)
594
595=item $[
596
597The index of the first element in an array, and of the first character
598in a substring. Default is 0, but you could set it to 1 to make
599Perl behave more like B<awk> (or Fortran) when subscripting and when
600evaluating the index() and substr() functions. (Mnemonic: [ begins
601subscripts.)
602
603As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
604and cannot influence the behavior of any other file. Its use is
605discouraged.
606
607=item $PERL_VERSION
608
609=item $]
610
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611The version + patchlevel / 1000 of the Perl interpreter. This variable
612can be used to determine whether the Perl interpreter executing a
613script is in the right range of versions. (Mnemonic: Is this version
614of perl in the right bracket?) Example:
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615
616 warn "No checksumming!\n" if $] < 3.019;
617
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618See also the documentation of C<use VERSION> and C<require VERSION>
619for a convenient way to fail if the Perl interpreter is too old.
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620
621=item $DEBUGGING
622
623=item $^D
624
625The current value of the debugging flags. (Mnemonic: value of B<-D>
626switch.)
627
628=item $SYSTEM_FD_MAX
629
630=item $^F
631
632The maximum system file descriptor, ordinarily 2. System file
633descriptors are passed to exec()ed processes, while higher file
634descriptors are not. Also, during an open(), system file descriptors are
635preserved even if the open() fails. (Ordinary file descriptors are
636closed before the open() is attempted.) Note that the close-on-exec
637status of a file descriptor will be decided according to the value of
638C<$^F> at the time of the open, not the time of the exec.
639
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640=item $^H
641
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642The current set of syntax checks enabled by C<use strict> and other block
643scoped compiler hints. See the documentation of C<strict> for more details.
6e2995f4 644
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645=item $INPLACE_EDIT
646
647=item $^I
648
649The current value of the inplace-edit extension. Use C<undef> to disable
650inplace editing. (Mnemonic: value of B<-i> switch.)
651
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652=item $^M
653
654By default, running out of memory it is not trappable. However, if
655compiled for this, Perl may use the contents of C<$^M> as an emergency
656pool after die()ing with this message. Suppose that your Perl were
657compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. Then
658
659 $^M = 'a' x (1<<16);
660
661would allocate a 64K buffer for use when in emergency. See the F<INSTALL>
662file for information on how to enable this option. As a disincentive to
663casual use of this advanced feature, there is no L<English> long name for
664this variable.
665
5c055ba3 666=item $OSNAME
6e2995f4 667
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668=item $^O
669
670The name of the operating system under which this copy of Perl was
671built, as determined during the configuration process. The value
672is identical to C<$Config{'osname'}>.
673
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674=item $PERLDB
675
676=item $^P
677
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678The internal variable for debugging support. Different bits mean the
679following (subject to change):
680
681=over 6
682
683=item 0x01
684
685Debug subroutine enter/exit.
686
687=item 0x02
688
689Line-by-line debugging.
690
691=item 0x04
692
693Switch off optimizations.
694
695=item 0x08
696
697Preserve more data for future interactive inspections.
698
699=item 0x10
700
701Keep info about source lines on which a subroutine is defined.
702
703=item 0x20
704
705Start with single-step on.
706
707=back
708
709Note that some bits may be relevent at compile-time only, some at
710run-time only. This is a new mechanism and the details may change.
a0d0e21e 711
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712=item $^S
713
714Current state of the interpreter. Undefined if parsing of the current
715module/eval is not finished (may happen in $SIG{__DIE__} and
a3cb178b 716$SIG{__WARN__} handlers). True if inside an eval, otherwise false.
fb73857a 717
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718=item $BASETIME
719
720=item $^T
721
722The time at which the script began running, in seconds since the
5f05dabc 723epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
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724and B<-C> filetests are
725based on this value.
726
727=item $WARNING
728
729=item $^W
730
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731The current value of the warning switch, either TRUE or FALSE.
732(Mnemonic: related to the B<-w> switch.)
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733
734=item $EXECUTABLE_NAME
735
736=item $^X
737
738The name that the Perl binary itself was executed as, from C's C<argv[0]>.
739
740=item $ARGV
741
a8f8344d 742contains the name of the current file when reading from E<lt>E<gt>.
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743
744=item @ARGV
745
746The array @ARGV contains the command line arguments intended for the
747script. Note that C<$#ARGV> is the generally number of arguments minus
5f05dabc 748one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
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749"C<$0>" for the command name.
750
751=item @INC
752
753The array @INC contains the list of places to look for Perl scripts to
754be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
755initially consists of the arguments to any B<-I> command line switches,
6e2995f4 756followed by the default Perl library, probably F</usr/local/lib/perl>,
cb1a09d0 757followed by ".", to represent the current directory. If you need to
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758modify this at runtime, you should use the C<use lib> pragma
759to get the machine-dependent library properly loaded also:
a0d0e21e 760
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761 use lib '/mypath/libdir/';
762 use SomeMod;
303f2f76 763
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764=item @_
765
766Within a subroutine the array @_ contains the parameters passed to that
767subroutine. See L<perlsub>.
768
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769=item %INC
770
771The hash %INC contains entries for each filename that has
772been included via C<do> or C<require>. The key is the filename you
773specified, and the value is the location of the file actually found.
774The C<require> command uses this array to determine whether a given file
775has already been included.
776
fb73857a 777=item %ENV $ENV{expr}
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778
779The hash %ENV contains your current environment. Setting a
780value in C<ENV> changes the environment for child processes.
781
fb73857a 782=item %SIG $SIG{expr}
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783
784The hash %SIG is used to set signal handlers for various
785signals. Example:
786
787 sub handler { # 1st argument is signal name
fb73857a 788 my($sig) = @_;
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789 print "Caught a SIG$sig--shutting down\n";
790 close(LOG);
791 exit(0);
792 }
793
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794 $SIG{'INT'} = \&handler;
795 $SIG{'QUIT'} = \&handler;
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796 ...
797 $SIG{'INT'} = 'DEFAULT'; # restore default action
798 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
799
5f05dabc 800The %SIG array contains values for only the signals actually set within
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801the Perl script. Here are some other examples:
802
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803 $SIG{"PIPE"} = Plumber; # SCARY!!
804 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
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805 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
806 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
807
808The one marked scary is problematic because it's a bareword, which means
54310121 809sometimes it's a string representing the function, and sometimes it's
a0d0e21e 810going to call the subroutine call right then and there! Best to be sure
a8f8344d 811and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
748a9306 812
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813If your system has the sigaction() function then signal handlers are
814installed using it. This means you get reliable signal handling. If
815your system has the SA_RESTART flag it is used when signals handlers are
816installed. This means that system calls for which it is supported
817continue rather than returning when a signal arrives. If you want your
818system calls to be interrupted by signal delivery then do something like
819this:
820
821 use POSIX ':signal_h';
822
823 my $alarm = 0;
824 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
825 or die "Error setting SIGALRM handler: $!\n";
826
827See L<POSIX>.
828
748a9306 829Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 830routine indicated by C<$SIG{__WARN__}> is called when a warning message is
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831about to be printed. The warning message is passed as the first
832argument. The presence of a __WARN__ hook causes the ordinary printing
833of warnings to STDERR to be suppressed. You can use this to save warnings
834in a variable, or turn warnings into fatal errors, like this:
835
836 local $SIG{__WARN__} = sub { die $_[0] };
837 eval $proggie;
838
a8f8344d 839The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
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840is about to be thrown. The error message is passed as the first
841argument. When a __DIE__ hook routine returns, the exception
842processing continues as it would have in the absence of the hook,
cb1a09d0 843unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
774d564b 844The C<__DIE__> handler is explicitly disabled during the call, so that you
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845can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
846
847Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
7b8d334a 848blocks/strings. See L<perlfunc/die> and L<perlvar/$^S> for how to
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849circumvent this.
850
851Note that C<__DIE__>/C<__WARN__> handlers are very special in one
852respect: they may be called to report (probable) errors found by the
853parser. In such a case the parser may be in inconsistent state, so
854any attempt to evaluate Perl code from such a handler will probably
855result in a segfault. This means that calls which result/may-result
856in parsing Perl should be used with extreme causion, like this:
857
858 require Carp if defined $^S;
859 Carp::confess("Something wrong") if defined &Carp::confess;
860 die "Something wrong, but could not load Carp to give backtrace...
861 To see backtrace try starting Perl with -MCarp switch";
862
863Here the first line will load Carp I<unless> it is the parser who
864called the handler. The second line will print backtrace and die if
865Carp was available. The third line will be executed only if Carp was
866not available.
867
868See L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval> for
869additional info.
68dc0745 870
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