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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
7=head2 Predefined Names
8
9The following names have special meaning to Perl. Most of the
5f05dabc 10punctuation names have reasonable mnemonics, or analogues in one of
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11the shells. Nevertheless, if you wish to use the long variable names,
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
17long names in the current package. Some of them even have medium names,
18generally borrowed from B<awk>.
19
20To go a step further, those variables that depend on the currently
21selected filehandle may instead be set by calling an object method on
22the FileHandle object. (Summary lines below for this contain the word
23HANDLE.) First you must say
24
25 use FileHandle;
26
27after which you may use either
28
29 method HANDLE EXPR
30
31or
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.
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44
45=over 8
46
47=item $ARG
48
49=item $_
50
51The default input and pattern-searching space. The following pairs are
52equivalent:
53
5f05dabc 54 while (<>) {...} # equivalent in only while!
54310121 55 while (defined($_ = <>)) {...}
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56
57 /^Subject:/
58 $_ =~ /^Subject:/
59
60 tr/a-z/A-Z/
61 $_ =~ tr/a-z/A-Z/
62
63 chop
64 chop($_)
65
54310121 66Here are the places where Perl will assume $_ even if you
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67don't use it:
68
69=over 3
70
71=item *
72
73Various unary functions, including functions like ord() and int(), as well
74as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
75STDIN.
76
77=item *
78
79Various list functions like print() and unlink().
80
81=item *
82
83The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
84without an C<=~> operator.
85
54310121 86=item *
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87
88The default iterator variable in a C<foreach> loop if no other
89variable is supplied.
90
54310121 91=item *
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92
93The implicit iterator variable in the grep() and map() functions.
94
54310121 95=item *
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96
97The default place to put an input record when a C<E<lt>FHE<gt>>
98operation's result is tested by itself as the sole criterion of a C<while>
99test. Note that outside of a C<while> test, this will not happen.
100
101=back
102
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103(Mnemonic: underline is understood in certain operations.)
104
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105=back
106
107=over 8
108
a8f8344d 109=item $E<lt>I<digit>E<gt>
a0d0e21e 110
54310121 111Contains the subpattern from the corresponding set of parentheses in
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112the last pattern matched, not counting patterns matched in nested
113blocks that have been exited already. (Mnemonic: like \digit.)
114These variables are all read-only.
115
116=item $MATCH
117
118=item $&
119
120The string matched by the last successful pattern match (not counting
121any matches hidden within a BLOCK or eval() enclosed by the current
122BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
123
124=item $PREMATCH
125
126=item $`
127
128The string preceding whatever was matched by the last successful
129pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 130enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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131string.) This variable is read-only.
132
133=item $POSTMATCH
134
135=item $'
136
137The string following whatever was matched by the last successful
138pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 139enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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140string.) Example:
141
142 $_ = 'abcdefghi';
143 /def/;
144 print "$`:$&:$'\n"; # prints abc:def:ghi
145
146This variable is read-only.
147
148=item $LAST_PAREN_MATCH
149
150=item $+
151
152The last bracket matched by the last search pattern. This is useful if
153you don't know which of a set of alternative patterns matched. For
154example:
155
156 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
157
158(Mnemonic: be positive and forward looking.)
159This variable is read-only.
160
161=item $MULTILINE_MATCHING
162
163=item $*
164
4a6725af 165Set to 1 to do multi-line matching within a string, 0 to tell Perl
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166that it can assume that strings contain a single line, for the purpose
167of optimizing pattern matches. Pattern matches on strings containing
168multiple newlines can produce confusing results when "C<$*>" is 0. Default
169is 0. (Mnemonic: * matches multiple things.) Note that this variable
5f05dabc 170influences the interpretation of only "C<^>" and "C<$>". A literal newline can
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171be searched for even when C<$* == 0>.
172
5f05dabc 173Use of "C<$*>" is deprecated in modern perls.
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174
175=item input_line_number HANDLE EXPR
176
177=item $INPUT_LINE_NUMBER
178
179=item $NR
180
181=item $.
182
6e2995f4 183The current input line number for the last file handle from
a8f8344d 184which you read (or performed a C<seek> or C<tell> on). An
5f05dabc 185explicit close on a filehandle resets the line number. Because
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186"C<E<lt>E<gt>>" never does an explicit close, line numbers increase
187across ARGV files (but see examples under eof()). Localizing C<$.> has
188the effect of also localizing Perl's notion of "the last read
189filehandle". (Mnemonic: many programs use "." to mean the current line
190number.)
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191
192=item input_record_separator HANDLE EXPR
193
194=item $INPUT_RECORD_SEPARATOR
195
196=item $RS
197
198=item $/
199
200The input record separator, newline by default. Works like B<awk>'s RS
303f2f76 201variable, including treating empty lines as delimiters if set to the
54310121 202null string. (Note: An empty line cannot contain any spaces or tabs.)
4a6725af 203You may set it to a multi-character string to match a multi-character
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204delimiter, or to C<undef> to read to end of file. Note that setting it
205to C<"\n\n"> means something slightly different than setting it to
206C<"">, if the file contains consecutive empty lines. Setting it to
207C<""> will treat two or more consecutive empty lines as a single empty
208line. Setting it to C<"\n\n"> will blindly assume that the next input
209character belongs to the next paragraph, even if it's a newline.
210(Mnemonic: / is used to delimit line boundaries when quoting poetry.)
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211
212 undef $/;
213 $_ = <FH>; # whole file now here
214 s/\n[ \t]+/ /g;
215
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216Remember: the value of $/ is a string, not a regexp. AWK has to be
217better for something :-)
218
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219=item autoflush HANDLE EXPR
220
221=item $OUTPUT_AUTOFLUSH
222
223=item $|
224
54310121 225If set to nonzero, forces a flush right away and after every write or print on the
6e2995f4 226currently selected output channel. Default is 0 (regardless of whether
5f05dabc 227the channel is actually buffered by the system or not; C<$|> tells you
54310121 228only whether you've asked Perl explicitly to flush after each write).
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229Note that STDOUT will typically be line buffered if output is to the
230terminal and block buffered otherwise. Setting this variable is useful
231primarily when you are outputting to a pipe, such as when you are running
232a Perl script under rsh and want to see the output as it's happening. This
233has no effect on input buffering.
cb1a09d0 234(Mnemonic: when you want your pipes to be piping hot.)
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235
236=item output_field_separator HANDLE EXPR
237
238=item $OUTPUT_FIELD_SEPARATOR
239
240=item $OFS
241
242=item $,
243
244The output field separator for the print operator. Ordinarily the
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245print operator simply prints out the comma-separated fields you
246specify. To get behavior more like B<awk>, set this variable
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247as you would set B<awk>'s OFS variable to specify what is printed
248between fields. (Mnemonic: what is printed when there is a , in your
249print statement.)
250
251=item output_record_separator HANDLE EXPR
252
253=item $OUTPUT_RECORD_SEPARATOR
254
255=item $ORS
256
257=item $\
258
259The output record separator for the print operator. Ordinarily the
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260print operator simply prints out the comma-separated fields you
261specify, with no trailing newline or record separator assumed.
262To get behavior more like B<awk>, set this variable as you would
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263set B<awk>'s ORS variable to specify what is printed at the end of the
264print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
a8f8344d 265print. Also, it's just like C<$/>, but it's what you get "back" from
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266Perl.)
267
268=item $LIST_SEPARATOR
269
270=item $"
271
272This is like "C<$,>" except that it applies to array values interpolated
273into a double-quoted string (or similar interpreted string). Default
274is a space. (Mnemonic: obvious, I think.)
275
276=item $SUBSCRIPT_SEPARATOR
277
278=item $SUBSEP
279
280=item $;
281
54310121 282The subscript separator for multidimensional array emulation. If you
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283refer to a hash element as
284
285 $foo{$a,$b,$c}
286
287it really means
288
289 $foo{join($;, $a, $b, $c)}
290
291But don't put
292
293 @foo{$a,$b,$c} # a slice--note the @
294
295which means
296
297 ($foo{$a},$foo{$b},$foo{$c})
298
299Default is "\034", the same as SUBSEP in B<awk>. Note that if your
300keys contain binary data there might not be any safe value for "C<$;>".
301(Mnemonic: comma (the syntactic subscript separator) is a
302semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
303taken for something more important.)
304
54310121 305Consider using "real" multidimensional arrays.
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306
307=item $OFMT
308
309=item $#
310
311The output format for printed numbers. This variable is a half-hearted
312attempt to emulate B<awk>'s OFMT variable. There are times, however,
313when B<awk> and Perl have differing notions of what is in fact
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314numeric. The initial value is %.I<n>g, where I<n> is the value
315of the macro DBL_DIG from your system's F<float.h>. This is different from
316B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
317explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
a0d0e21e 318
5f05dabc 319Use of "C<$#>" is deprecated.
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320
321=item format_page_number HANDLE EXPR
322
323=item $FORMAT_PAGE_NUMBER
324
325=item $%
326
327The current page number of the currently selected output channel.
328(Mnemonic: % is page number in B<nroff>.)
329
330=item format_lines_per_page HANDLE EXPR
331
332=item $FORMAT_LINES_PER_PAGE
333
334=item $=
335
336The current page length (printable lines) of the currently selected
337output channel. Default is 60. (Mnemonic: = has horizontal lines.)
338
339=item format_lines_left HANDLE EXPR
340
341=item $FORMAT_LINES_LEFT
342
343=item $-
344
345The number of lines left on the page of the currently selected output
346channel. (Mnemonic: lines_on_page - lines_printed.)
347
348=item format_name HANDLE EXPR
349
350=item $FORMAT_NAME
351
352=item $~
353
354The name of the current report format for the currently selected output
355channel. Default is name of the filehandle. (Mnemonic: brother to
356"C<$^>".)
357
358=item format_top_name HANDLE EXPR
359
360=item $FORMAT_TOP_NAME
361
362=item $^
363
364The name of the current top-of-page format for the currently selected
365output channel. Default is name of the filehandle with _TOP
366appended. (Mnemonic: points to top of page.)
367
368=item format_line_break_characters HANDLE EXPR
369
370=item $FORMAT_LINE_BREAK_CHARACTERS
371
372=item $:
373
374The current set of characters after which a string may be broken to
54310121 375fill continuation fields (starting with ^) in a format. Default is
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376S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
377poetry is a part of a line.)
378
379=item format_formfeed HANDLE EXPR
380
381=item $FORMAT_FORMFEED
382
383=item $^L
384
5f05dabc 385What formats output to perform a form feed. Default is \f.
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386
387=item $ACCUMULATOR
388
389=item $^A
390
391The current value of the write() accumulator for format() lines. A format
392contains formline() commands that put their result into C<$^A>. After
393calling its format, write() prints out the contents of C<$^A> and empties.
394So you never actually see the contents of C<$^A> unless you call
395formline() yourself and then look at it. See L<perlform> and
396L<perlfunc/formline()>.
397
398=item $CHILD_ERROR
399
400=item $?
401
54310121 402The status returned by the last pipe close, backtick (C<``>) command,
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403or system() operator. Note that this is the status word returned by
404the wait() system call (or else is made up to look like it). Thus,
405the exit value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and
406C<$? & 255> gives which signal, if any, the process died from, and
407whether there was a core dump. (Mnemonic: similar to B<sh> and
408B<ksh>.)
a0d0e21e 409
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410Note that if you have installed a signal handler for C<SIGCHLD>, the
411value of C<$?> will usually be wrong outside that handler.
412
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413Inside an C<END> subroutine C<$?> contains the value that is going to be
414given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
415change the exit status of the script.
416
aa689395 417Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
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418actual VMS exit status, instead of the default emulation of POSIX
419status.
f86702cc 420
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421=item $OS_ERROR
422
423=item $ERRNO
424
425=item $!
426
427If used in a numeric context, yields the current value of errno, with
428all the usual caveats. (This means that you shouldn't depend on the
429value of "C<$!>" to be anything in particular unless you've gotten a
430specific error return indicating a system error.) If used in a string
431context, yields the corresponding system error string. You can assign
5f05dabc 432to "C<$!>" to set I<errno> if, for instance, you want "C<$!>" to return the
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433string for error I<n>, or you want to set the exit value for the die()
434operator. (Mnemonic: What just went bang?)
435
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436=item $EXTENDED_OS_ERROR
437
438=item $^E
439
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440More specific information about the last system error than that provided by
441C<$!>, if available. (If not, it's just C<$!> again, except under OS/2.)
5f05dabc 442At the moment, this differs from C<$!> under only VMS and OS/2, where it
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443provides the VMS status value from the last system error, and OS/2 error
444code of the last call to OS/2 API which was not directed via CRT. The
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445caveats mentioned in the description of C<$!> apply here, too.
446(Mnemonic: Extra error explanation.)
447
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448Note that under OS/2 C<$!> and C<$^E> do not track each other, so if an
449OS/2-specific call is performed, you may need to check both.
5c055ba3 450
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451=item $EVAL_ERROR
452
453=item $@
454
455The Perl syntax error message from the last eval() command. If null, the
456last eval() parsed and executed correctly (although the operations you
457invoked may have failed in the normal fashion). (Mnemonic: Where was
458the syntax error "at"?)
459
748a9306 460Note that warning messages are not collected in this variable. You can,
a8f8344d 461however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 462as described below.
748a9306 463
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464=item $PROCESS_ID
465
466=item $PID
467
468=item $$
469
470The process number of the Perl running this script. (Mnemonic: same
471as shells.)
472
473=item $REAL_USER_ID
474
475=item $UID
476
477=item $<
478
479The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
480if you're running setuid.)
481
482=item $EFFECTIVE_USER_ID
483
484=item $EUID
485
486=item $>
487
488The effective uid of this process. Example:
489
490 $< = $>; # set real to effective uid
491 ($<,$>) = ($>,$<); # swap real and effective uid
492
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493(Mnemonic: it's the uid you went I<TO>, if you're running setuid.)
494Note: "C<$E<lt>>" and "C<$E<gt>>" can be swapped only on machines
495supporting setreuid().
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496
497=item $REAL_GROUP_ID
498
499=item $GID
500
501=item $(
502
503The real gid of this process. If you are on a machine that supports
504membership in multiple groups simultaneously, gives a space separated
505list of groups you are in. The first number is the one returned by
506getgid(), and the subsequent ones by getgroups(), one of which may be
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507the same as the first number.
508
509However, a value assigned to "C<$(>" must be a single number used to
510set the real gid. So the value given by "C<$(>" should I<not> be assigned
511back to "C<$(>" without being forced numeric, such as by adding zero.
512
513(Mnemonic: parentheses are used to I<GROUP> things. The real gid is the
514group you I<LEFT>, if you're running setgid.)
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515
516=item $EFFECTIVE_GROUP_ID
517
518=item $EGID
519
520=item $)
521
522The effective gid of this process. If you are on a machine that
523supports membership in multiple groups simultaneously, gives a space
524separated list of groups you are in. The first number is the one
525returned by getegid(), and the subsequent ones by getgroups(), one of
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526which may be the same as the first number.
527
528Similarly, a value assigned to "C<$)>" must also be a space-separated
529list of numbers. The first number is used to set the effective gid, and
530the rest (if any) are passed to setgroups(). To get the effect of an
531empty list for setgroups(), just repeat the new effective gid; that is,
532to force an effective gid of 5 and an effectively empty setgroups()
533list, say C< $) = "5 5" >.
534
535(Mnemonic: parentheses are used to I<GROUP> things. The effective gid
536is the group that's I<RIGHT> for you, if you're running setgid.)
a0d0e21e 537
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538Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
539machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
8cc95fdb 540and "C<$)>" can be swapped only on machines supporting setregid().
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541
542=item $PROGRAM_NAME
543
544=item $0
545
546Contains the name of the file containing the Perl script being
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547executed. On some operating systems
548assigning to "C<$0>" modifies the argument area that the ps(1)
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549program sees. This is more useful as a way of indicating the
550current program state than it is for hiding the program you're running.
551(Mnemonic: same as B<sh> and B<ksh>.)
552
553=item $[
554
555The index of the first element in an array, and of the first character
556in a substring. Default is 0, but you could set it to 1 to make
557Perl behave more like B<awk> (or Fortran) when subscripting and when
558evaluating the index() and substr() functions. (Mnemonic: [ begins
559subscripts.)
560
561As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
562and cannot influence the behavior of any other file. Its use is
563discouraged.
564
565=item $PERL_VERSION
566
567=item $]
568
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569The version + patchlevel / 1000 of the Perl interpreter. This variable
570can be used to determine whether the Perl interpreter executing a
571script is in the right range of versions. (Mnemonic: Is this version
572of perl in the right bracket?) Example:
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573
574 warn "No checksumming!\n" if $] < 3.019;
575
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576See also the documentation of C<use VERSION> and C<require VERSION>
577for a convenient way to fail if the Perl interpreter is too old.
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578
579=item $DEBUGGING
580
581=item $^D
582
583The current value of the debugging flags. (Mnemonic: value of B<-D>
584switch.)
585
586=item $SYSTEM_FD_MAX
587
588=item $^F
589
590The maximum system file descriptor, ordinarily 2. System file
591descriptors are passed to exec()ed processes, while higher file
592descriptors are not. Also, during an open(), system file descriptors are
593preserved even if the open() fails. (Ordinary file descriptors are
594closed before the open() is attempted.) Note that the close-on-exec
595status of a file descriptor will be decided according to the value of
596C<$^F> at the time of the open, not the time of the exec.
597
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598=item $^H
599
600The current set of syntax checks enabled by C<use strict>. See the
601documentation of C<strict> for more details.
602
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603=item $INPLACE_EDIT
604
605=item $^I
606
607The current value of the inplace-edit extension. Use C<undef> to disable
608inplace editing. (Mnemonic: value of B<-i> switch.)
609
5c055ba3 610=item $OSNAME
6e2995f4 611
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612=item $^O
613
614The name of the operating system under which this copy of Perl was
615built, as determined during the configuration process. The value
616is identical to C<$Config{'osname'}>.
617
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618=item $PERLDB
619
620=item $^P
621
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622The internal variable for debugging support. Different bits mean the
623following (subject to change):
624
625=over 6
626
627=item 0x01
628
629Debug subroutine enter/exit.
630
631=item 0x02
632
633Line-by-line debugging.
634
635=item 0x04
636
637Switch off optimizations.
638
639=item 0x08
640
641Preserve more data for future interactive inspections.
642
643=item 0x10
644
645Keep info about source lines on which a subroutine is defined.
646
647=item 0x20
648
649Start with single-step on.
650
651=back
652
653Note that some bits may be relevent at compile-time only, some at
654run-time only. This is a new mechanism and the details may change.
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655
656=item $BASETIME
657
658=item $^T
659
660The time at which the script began running, in seconds since the
5f05dabc 661epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
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662and B<-C> filetests are
663based on this value.
664
665=item $WARNING
666
667=item $^W
668
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669The current value of the warning switch, either TRUE or FALSE.
670(Mnemonic: related to the B<-w> switch.)
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671
672=item $EXECUTABLE_NAME
673
674=item $^X
675
676The name that the Perl binary itself was executed as, from C's C<argv[0]>.
677
678=item $ARGV
679
a8f8344d 680contains the name of the current file when reading from E<lt>E<gt>.
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681
682=item @ARGV
683
684The array @ARGV contains the command line arguments intended for the
685script. Note that C<$#ARGV> is the generally number of arguments minus
5f05dabc 686one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
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687"C<$0>" for the command name.
688
689=item @INC
690
691The array @INC contains the list of places to look for Perl scripts to
692be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
693initially consists of the arguments to any B<-I> command line switches,
6e2995f4 694followed by the default Perl library, probably F</usr/local/lib/perl>,
cb1a09d0 695followed by ".", to represent the current directory. If you need to
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696modify this at runtime, you should use the C<use lib> pragma
697to get the machine-dependent library properly loaded also:
a0d0e21e 698
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699 use lib '/mypath/libdir/';
700 use SomeMod;
303f2f76 701
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702=item %INC
703
704The hash %INC contains entries for each filename that has
705been included via C<do> or C<require>. The key is the filename you
706specified, and the value is the location of the file actually found.
707The C<require> command uses this array to determine whether a given file
708has already been included.
709
710=item $ENV{expr}
711
712The hash %ENV contains your current environment. Setting a
713value in C<ENV> changes the environment for child processes.
714
715=item $SIG{expr}
716
717The hash %SIG is used to set signal handlers for various
718signals. Example:
719
720 sub handler { # 1st argument is signal name
721 local($sig) = @_;
722 print "Caught a SIG$sig--shutting down\n";
723 close(LOG);
724 exit(0);
725 }
726
727 $SIG{'INT'} = 'handler';
728 $SIG{'QUIT'} = 'handler';
729 ...
730 $SIG{'INT'} = 'DEFAULT'; # restore default action
731 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
732
5f05dabc 733The %SIG array contains values for only the signals actually set within
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734the Perl script. Here are some other examples:
735
736 $SIG{PIPE} = Plumber; # SCARY!!
737 $SIG{"PIPE"} = "Plumber"; # just fine, assumes main::Plumber
738 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
739 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
740
741The one marked scary is problematic because it's a bareword, which means
54310121 742sometimes it's a string representing the function, and sometimes it's
a0d0e21e 743going to call the subroutine call right then and there! Best to be sure
a8f8344d 744and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
748a9306 745
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746If your system has the sigaction() function then signal handlers are
747installed using it. This means you get reliable signal handling. If
748your system has the SA_RESTART flag it is used when signals handlers are
749installed. This means that system calls for which it is supported
750continue rather than returning when a signal arrives. If you want your
751system calls to be interrupted by signal delivery then do something like
752this:
753
754 use POSIX ':signal_h';
755
756 my $alarm = 0;
757 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
758 or die "Error setting SIGALRM handler: $!\n";
759
760See L<POSIX>.
761
748a9306 762Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 763routine indicated by C<$SIG{__WARN__}> is called when a warning message is
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764about to be printed. The warning message is passed as the first
765argument. The presence of a __WARN__ hook causes the ordinary printing
766of warnings to STDERR to be suppressed. You can use this to save warnings
767in a variable, or turn warnings into fatal errors, like this:
768
769 local $SIG{__WARN__} = sub { die $_[0] };
770 eval $proggie;
771
a8f8344d 772The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
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773is about to be thrown. The error message is passed as the first
774argument. When a __DIE__ hook routine returns, the exception
775processing continues as it would have in the absence of the hook,
cb1a09d0 776unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
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777The C<__DIE__> handler is explicitly disabled during the call, so that you
778can die from a C<__DIE__> handler. Similarly for C<__WARN__>. See
779L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval>.
a0d0e21e 780
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781=item $^M
782
783By default, running out of memory it is not trappable. However, if
784compiled for this, Perl may use the contents of C<$^M> as an emergency
785pool after die()ing with this message. Suppose that your Perl were
84902520 786compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. Then
54310121 787
68dc0745 788 $^M = 'a' x (1<<16);
54310121 789
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790would allocate a 64K buffer for use when in emergency. See the F<INSTALL>
791file for information on how to enable this option. As a disincentive to
792casual use of this advanced feature, there is no L<English> long name for
793this variable.
794
a0d0e21e 795=back