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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
10punctuational names have reasonable mnemonics, or analogues in one of
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
41A few of these variables are considered "read-only". This means that if you
42try to assign to this variable, either directly or indirectly through
43a reference. If you attempt to do so, you'll raise a run-time exception.
44
45=over 8
46
47=item $ARG
48
49=item $_
50
51The default input and pattern-searching space. The following pairs are
52equivalent:
53
54 while (<>) {...} # only equivalent in while!
55 while ($_ = <>) {...}
56
57 /^Subject:/
58 $_ =~ /^Subject:/
59
60 tr/a-z/A-Z/
61 $_ =~ tr/a-z/A-Z/
62
63 chop
64 chop($_)
65
66(Mnemonic: underline is understood in certain operations.)
67
68=item $<I<digit>>
69
70Contains the subpattern from the corresponding set of parentheses in
71the last pattern matched, not counting patterns matched in nested
72blocks that have been exited already. (Mnemonic: like \digit.)
73These variables are all read-only.
74
75=item $MATCH
76
77=item $&
78
79The string matched by the last successful pattern match (not counting
80any matches hidden within a BLOCK or eval() enclosed by the current
81BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
82
83=item $PREMATCH
84
85=item $`
86
87The string preceding whatever was matched by the last successful
88pattern match (not counting any matches hidden within a BLOCK or eval
89enclosed by the current BLOCK). (Mnemonic: ` often precedes a quoted
90string.) This variable is read-only.
91
92=item $POSTMATCH
93
94=item $'
95
96The string following whatever was matched by the last successful
97pattern match (not counting any matches hidden within a BLOCK or eval()
98enclosed by the current BLOCK). (Mnemonic: ' often follows a quoted
99string.) Example:
100
101 $_ = 'abcdefghi';
102 /def/;
103 print "$`:$&:$'\n"; # prints abc:def:ghi
104
105This variable is read-only.
106
107=item $LAST_PAREN_MATCH
108
109=item $+
110
111The last bracket matched by the last search pattern. This is useful if
112you don't know which of a set of alternative patterns matched. For
113example:
114
115 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
116
117(Mnemonic: be positive and forward looking.)
118This variable is read-only.
119
120=item $MULTILINE_MATCHING
121
122=item $*
123
124Set to 1 to do multiline matching within a string, 0 to tell Perl
125that it can assume that strings contain a single line, for the purpose
126of optimizing pattern matches. Pattern matches on strings containing
127multiple newlines can produce confusing results when "C<$*>" is 0. Default
128is 0. (Mnemonic: * matches multiple things.) Note that this variable
129only influences the interpretation of "C<^>" and "C<$>". A literal newline can
130be searched for even when C<$* == 0>.
131
132Use of "C<$*>" is deprecated in Perl 5.
133
134=item input_line_number HANDLE EXPR
135
136=item $INPUT_LINE_NUMBER
137
138=item $NR
139
140=item $.
141
142The current input line number of the last filehandle that was read.
143This variable should be considered read-only.
144Remember that only an explicit close on the filehandle
145resets the line number. Since "C<E<lt>E<gt>>" never does an explicit close, line
146numbers increase across ARGV files (but see examples under eof()).
147(Mnemonic: many programs use "." to mean the current line number.)
148
149=item input_record_separator HANDLE EXPR
150
151=item $INPUT_RECORD_SEPARATOR
152
153=item $RS
154
155=item $/
156
157The input record separator, newline by default. Works like B<awk>'s RS
158variable, including treating blank lines as delimiters if set to the
159null string. You may set it to a multicharacter string to match a
160multi-character delimiter. Note that setting it to C<"\n\n"> means
161something slightly different than setting it to C<"">, if the file
162contains consecutive blank lines. Setting it to C<""> will treat two or
163more consecutive blank lines as a single blank line. Setting it to
164C<"\n\n"> will blindly assume that the next input character belongs to the
165next paragraph, even if it's a newline. (Mnemonic: / is used to
166delimit line boundaries when quoting poetry.)
167
168 undef $/;
169 $_ = <FH>; # whole file now here
170 s/\n[ \t]+/ /g;
171
172=item autoflush HANDLE EXPR
173
174=item $OUTPUT_AUTOFLUSH
175
176=item $|
177
178If set to nonzero, forces a flush after every write or print on the
179currently selected output channel. Default is 0. Note that STDOUT
180will typically be line buffered if output is to the terminal and block
181buffered otherwise. Setting this variable is useful primarily when you
182are outputting to a pipe, such as when you are running a Perl script
183under rsh and want to see the output as it's happening. (Mnemonic:
184when you want your pipes to be piping hot.)
185
186=item output_field_separator HANDLE EXPR
187
188=item $OUTPUT_FIELD_SEPARATOR
189
190=item $OFS
191
192=item $,
193
194The output field separator for the print operator. Ordinarily the
195print operator simply prints out the comma separated fields you
196specify. In order to get behavior more like B<awk>, set this variable
197as you would set B<awk>'s OFS variable to specify what is printed
198between fields. (Mnemonic: what is printed when there is a , in your
199print statement.)
200
201=item output_record_separator HANDLE EXPR
202
203=item $OUTPUT_RECORD_SEPARATOR
204
205=item $ORS
206
207=item $\
208
209The output record separator for the print operator. Ordinarily the
210print operator simply prints out the comma separated fields you
211specify, with no trailing newline or record separator assumed. In
212order to get behavior more like B<awk>, set this variable as you would
213set B<awk>'s ORS variable to specify what is printed at the end of the
214print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
215print. Also, it's just like /, but it's what you get "back" from
216Perl.)
217
218=item $LIST_SEPARATOR
219
220=item $"
221
222This is like "C<$,>" except that it applies to array values interpolated
223into a double-quoted string (or similar interpreted string). Default
224is a space. (Mnemonic: obvious, I think.)
225
226=item $SUBSCRIPT_SEPARATOR
227
228=item $SUBSEP
229
230=item $;
231
232The subscript separator for multi-dimensional array emulation. If you
233refer to a hash element as
234
235 $foo{$a,$b,$c}
236
237it really means
238
239 $foo{join($;, $a, $b, $c)}
240
241But don't put
242
243 @foo{$a,$b,$c} # a slice--note the @
244
245which means
246
247 ($foo{$a},$foo{$b},$foo{$c})
248
249Default is "\034", the same as SUBSEP in B<awk>. Note that if your
250keys contain binary data there might not be any safe value for "C<$;>".
251(Mnemonic: comma (the syntactic subscript separator) is a
252semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
253taken for something more important.)
254
255Consider using "real" multi-dimensional arrays in Perl 5.
256
257=item $OFMT
258
259=item $#
260
261The output format for printed numbers. This variable is a half-hearted
262attempt to emulate B<awk>'s OFMT variable. There are times, however,
263when B<awk> and Perl have differing notions of what is in fact
264numeric. Also, the initial value is %.20g rather than %.6g, so you
265need to set "C<$#>" explicitly to get B<awk>'s value. (Mnemonic: # is the
266number sign.)
267
268Use of "C<$#>" is deprecated in Perl 5.
269
270=item format_page_number HANDLE EXPR
271
272=item $FORMAT_PAGE_NUMBER
273
274=item $%
275
276The current page number of the currently selected output channel.
277(Mnemonic: % is page number in B<nroff>.)
278
279=item format_lines_per_page HANDLE EXPR
280
281=item $FORMAT_LINES_PER_PAGE
282
283=item $=
284
285The current page length (printable lines) of the currently selected
286output channel. Default is 60. (Mnemonic: = has horizontal lines.)
287
288=item format_lines_left HANDLE EXPR
289
290=item $FORMAT_LINES_LEFT
291
292=item $-
293
294The number of lines left on the page of the currently selected output
295channel. (Mnemonic: lines_on_page - lines_printed.)
296
297=item format_name HANDLE EXPR
298
299=item $FORMAT_NAME
300
301=item $~
302
303The name of the current report format for the currently selected output
304channel. Default is name of the filehandle. (Mnemonic: brother to
305"C<$^>".)
306
307=item format_top_name HANDLE EXPR
308
309=item $FORMAT_TOP_NAME
310
311=item $^
312
313The name of the current top-of-page format for the currently selected
314output channel. Default is name of the filehandle with _TOP
315appended. (Mnemonic: points to top of page.)
316
317=item format_line_break_characters HANDLE EXPR
318
319=item $FORMAT_LINE_BREAK_CHARACTERS
320
321=item $:
322
323The current set of characters after which a string may be broken to
324fill continuation fields (starting with ^) in a format. Default is
325S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
326poetry is a part of a line.)
327
328=item format_formfeed HANDLE EXPR
329
330=item $FORMAT_FORMFEED
331
332=item $^L
333
334What formats output to perform a formfeed. Default is \f.
335
336=item $ACCUMULATOR
337
338=item $^A
339
340The current value of the write() accumulator for format() lines. A format
341contains formline() commands that put their result into C<$^A>. After
342calling its format, write() prints out the contents of C<$^A> and empties.
343So you never actually see the contents of C<$^A> unless you call
344formline() yourself and then look at it. See L<perlform> and
345L<perlfunc/formline()>.
346
347=item $CHILD_ERROR
348
349=item $?
350
351The status returned by the last pipe close, backtick (C<``>) command,
352or system() operator. Note that this is the status word returned by
353the wait() system call, so the exit value of the subprocess is actually
354(C<$? E<gt>E<gt> 8>). Thus on many systems, C<$? & 255> gives which signal,
355if any, the process died from, and whether there was a core dump.
356(Mnemonic: similar to B<sh> and B<ksh>.)
357
358=item $OS_ERROR
359
360=item $ERRNO
361
362=item $!
363
364If used in a numeric context, yields the current value of errno, with
365all the usual caveats. (This means that you shouldn't depend on the
366value of "C<$!>" to be anything in particular unless you've gotten a
367specific error return indicating a system error.) If used in a string
368context, yields the corresponding system error string. You can assign
369to "C<$!>" in order to set I<errno> if, for instance, you want "C<$!>" to return the
370string for error I<n>, or you want to set the exit value for the die()
371operator. (Mnemonic: What just went bang?)
372
373=item $EVAL_ERROR
374
375=item $@
376
377The Perl syntax error message from the last eval() command. If null, the
378last eval() parsed and executed correctly (although the operations you
379invoked may have failed in the normal fashion). (Mnemonic: Where was
380the syntax error "at"?)
381
382=item $PROCESS_ID
383
384=item $PID
385
386=item $$
387
388The process number of the Perl running this script. (Mnemonic: same
389as shells.)
390
391=item $REAL_USER_ID
392
393=item $UID
394
395=item $<
396
397The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
398if you're running setuid.)
399
400=item $EFFECTIVE_USER_ID
401
402=item $EUID
403
404=item $>
405
406The effective uid of this process. Example:
407
408 $< = $>; # set real to effective uid
409 ($<,$>) = ($>,$<); # swap real and effective uid
410
411(Mnemonic: it's the uid you went I<TO>, if you're running setuid.) Note:
412"C<$E<lt>>" and "C<$E<gt>>" can only be swapped on machines supporting setreuid().
413
414=item $REAL_GROUP_ID
415
416=item $GID
417
418=item $(
419
420The real gid of this process. If you are on a machine that supports
421membership in multiple groups simultaneously, gives a space separated
422list of groups you are in. The first number is the one returned by
423getgid(), and the subsequent ones by getgroups(), one of which may be
424the same as the first number. (Mnemonic: parentheses are used to I<GROUP>
425things. The real gid is the group you I<LEFT>, if you're running setgid.)
426
427=item $EFFECTIVE_GROUP_ID
428
429=item $EGID
430
431=item $)
432
433The effective gid of this process. If you are on a machine that
434supports membership in multiple groups simultaneously, gives a space
435separated list of groups you are in. The first number is the one
436returned by getegid(), and the subsequent ones by getgroups(), one of
437which may be the same as the first number. (Mnemonic: parentheses are
438used to I<GROUP> things. The effective gid is the group that's I<RIGHT> for
439you, if you're running setgid.)
440
441Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can only be set on machines
442that support the corresponding I<set[re][ug]id()> routine. "C<$(>" and "C<$)>"
443can only be swapped on machines supporting setregid().
444
445=item $PROGRAM_NAME
446
447=item $0
448
449Contains the name of the file containing the Perl script being
450executed. Assigning to "C<$0>" modifies the argument area that the ps(1)
451program sees. This is more useful as a way of indicating the
452current program state than it is for hiding the program you're running.
453(Mnemonic: same as B<sh> and B<ksh>.)
454
455=item $[
456
457The index of the first element in an array, and of the first character
458in a substring. Default is 0, but you could set it to 1 to make
459Perl behave more like B<awk> (or Fortran) when subscripting and when
460evaluating the index() and substr() functions. (Mnemonic: [ begins
461subscripts.)
462
463As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
464and cannot influence the behavior of any other file. Its use is
465discouraged.
466
467=item $PERL_VERSION
468
469=item $]
470
471The string printed out when you say C<perl -v>. It can be used to
472determine at the beginning of a script whether the perl interpreter
473executing the script is in the right range of versions. If used in a
474numeric context, returns the version + patchlevel / 1000. Example:
475
476 # see if getc is available
477 ($version,$patchlevel) =
478 $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
479 print STDERR "(No filename completion available.)\n"
480 if $version * 1000 + $patchlevel < 2016;
481
482or, used numerically,
483
484 warn "No checksumming!\n" if $] < 3.019;
485
486(Mnemonic: Is this version of perl in the right bracket?)
487
488=item $DEBUGGING
489
490=item $^D
491
492The current value of the debugging flags. (Mnemonic: value of B<-D>
493switch.)
494
495=item $SYSTEM_FD_MAX
496
497=item $^F
498
499The maximum system file descriptor, ordinarily 2. System file
500descriptors are passed to exec()ed processes, while higher file
501descriptors are not. Also, during an open(), system file descriptors are
502preserved even if the open() fails. (Ordinary file descriptors are
503closed before the open() is attempted.) Note that the close-on-exec
504status of a file descriptor will be decided according to the value of
505C<$^F> at the time of the open, not the time of the exec.
506
507=item $INPLACE_EDIT
508
509=item $^I
510
511The current value of the inplace-edit extension. Use C<undef> to disable
512inplace editing. (Mnemonic: value of B<-i> switch.)
513
514=item $PERLDB
515
516=item $^P
517
518The internal flag that the debugger clears so that it doesn't debug
519itself. You could conceivable disable debugging yourself by clearing
520it.
521
522=item $BASETIME
523
524=item $^T
525
526The time at which the script began running, in seconds since the
527epoch (beginning of 1970). The values returned by the B<-M>, B<-A>
528and B<-C> filetests are
529based on this value.
530
531=item $WARNING
532
533=item $^W
534
535The current value of the warning switch, either TRUE or FALSE. (Mnemonic: related to the
536B<-w> switch.)
537
538=item $EXECUTABLE_NAME
539
540=item $^X
541
542The name that the Perl binary itself was executed as, from C's C<argv[0]>.
543
544=item $ARGV
545
546contains the name of the current file when reading from <>.
547
548=item @ARGV
549
550The array @ARGV contains the command line arguments intended for the
551script. Note that C<$#ARGV> is the generally number of arguments minus
552one, since C<$ARGV[0]> is the first argument, I<NOT> the command name. See
553"C<$0>" for the command name.
554
555=item @INC
556
557The array @INC contains the list of places to look for Perl scripts to
558be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
559initially consists of the arguments to any B<-I> command line switches,
560followed by the default Perl library, probably "/usr/local/lib/perl",
561followed by ".", to represent the current directory.
562
563=item %INC
564
565The hash %INC contains entries for each filename that has
566been included via C<do> or C<require>. The key is the filename you
567specified, and the value is the location of the file actually found.
568The C<require> command uses this array to determine whether a given file
569has already been included.
570
571=item $ENV{expr}
572
573The hash %ENV contains your current environment. Setting a
574value in C<ENV> changes the environment for child processes.
575
576=item $SIG{expr}
577
578The hash %SIG is used to set signal handlers for various
579signals. Example:
580
581 sub handler { # 1st argument is signal name
582 local($sig) = @_;
583 print "Caught a SIG$sig--shutting down\n";
584 close(LOG);
585 exit(0);
586 }
587
588 $SIG{'INT'} = 'handler';
589 $SIG{'QUIT'} = 'handler';
590 ...
591 $SIG{'INT'} = 'DEFAULT'; # restore default action
592 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
593
594The %SIG array only contains values for the signals actually set within
595the Perl script. Here are some other examples:
596
597 $SIG{PIPE} = Plumber; # SCARY!!
598 $SIG{"PIPE"} = "Plumber"; # just fine, assumes main::Plumber
599 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
600 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
601
602The one marked scary is problematic because it's a bareword, which means
603sometimes it's a string representing the function, and sometimes it's
604going to call the subroutine call right then and there! Best to be sure
605and quote it or take a reference to it. *Plumber works too. See <perlsubs>.
606
607=back
608