3 perlvar - Perl predefined variables
7 =head2 Predefined Names
9 The following names have special meaning to Perl. Most
10 punctuation names have reasonable mnemonics, or analogs in the
11 shells. Nevertheless, if you wish to use long variable names,
16 at the top of your program. This will alias all the short names to the
17 long names in the current package. Some even have medium names,
18 generally borrowed from B<awk>.
20 If you don't mind the performance hit, variables that depend on the
21 currently selected filehandle may instead be set by calling an
22 appropriate object method on the IO::Handle object. (Summary lines
23 below for this contain the word HANDLE.) First you must say
27 after which you may use either
35 Each method returns the old value of the IO::Handle attribute.
36 The methods each take an optional EXPR, which if supplied specifies the
37 new value for the IO::Handle attribute in question. If not supplied,
38 most methods do nothing to the current value--except for
39 autoflush(), which will assume a 1 for you, just to be different.
40 Because loading in the IO::Handle class is an expensive operation, you should
41 learn how to use the regular built-in variables.
43 A few of these variables are considered "read-only". This means that if
44 you try to assign to this variable, either directly or indirectly through
45 a reference, you'll raise a run-time exception.
47 The following list is ordered by scalar variables first, then the
48 arrays, then the hashes.
56 The default input and pattern-searching space. The following pairs are
59 while (<>) {...} # equivalent only in while!
60 while (defined($_ = <>)) {...}
71 Here are the places where Perl will assume $_ even if you
78 Various unary functions, including functions like ord() and int(), as well
79 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
84 Various list functions like print() and unlink().
88 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
89 without an C<=~> operator.
93 The default iterator variable in a C<foreach> loop if no other
98 The implicit iterator variable in the grep() and map() functions.
102 The default place to put an input record when a C<< <FH> >>
103 operation's result is tested by itself as the sole criterion of a C<while>
104 test. Outside a C<while> test, this will not happen.
108 (Mnemonic: underline is understood in certain operations.)
118 Special package variables when using sort(), see L<perlfunc/sort>.
119 Because of this specialness $a and $b don't need to be declared
120 (using local(), use vars, or our()) even when using the strict
129 Contains the subpattern from the corresponding set of capturing
130 parentheses from the last pattern match, not counting patterns
131 matched in nested blocks that have been exited already. (Mnemonic:
132 like \digits.) These variables are all read-only and dynamically
133 scoped to the current BLOCK.
139 The string matched by the last successful pattern match (not counting
140 any matches hidden within a BLOCK or eval() enclosed by the current
141 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
142 and dynamically scoped to the current BLOCK.
144 The use of this variable anywhere in a program imposes a considerable
145 performance penalty on all regular expression matches. See L<BUGS>.
151 The string preceding whatever was matched by the last successful
152 pattern match (not counting any matches hidden within a BLOCK or eval
153 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
154 string.) This variable is read-only.
156 The use of this variable anywhere in a program imposes a considerable
157 performance penalty on all regular expression matches. See L<BUGS>.
163 The string following whatever was matched by the last successful
164 pattern match (not counting any matches hidden within a BLOCK or eval()
165 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
170 print "$`:$&:$'\n"; # prints abc:def:ghi
172 This variable is read-only and dynamically scoped to the current BLOCK.
174 The use of this variable anywhere in a program imposes a considerable
175 performance penalty on all regular expression matches. See L<BUGS>.
177 =item $LAST_PAREN_MATCH
181 The last bracket matched by the last search pattern. This is useful if
182 you don't know which one of a set of alternative patterns matched. For
185 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
187 (Mnemonic: be positive and forward looking.)
188 This variable is read-only and dynamically scoped to the current BLOCK.
190 =item @LAST_MATCH_END
194 This array holds the offsets of the ends of the last successful
195 submatches in the currently active dynamic scope. C<$+[0]> is
196 the offset into the string of the end of the entire match. This
197 is the same value as what the C<pos> function returns when called
198 on the variable that was matched against. The I<n>th element
199 of this array holds the offset of the I<n>th submatch, so
200 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
201 past where $2 ends, and so on. You can use C<$#+> to determine
202 how many subgroups were in the last successful match. See the
203 examples given for the C<@-> variable.
205 =item $MULTILINE_MATCHING
209 Set to a non-zero integer value to do multi-line matching within a
210 string, 0 (or undefined) to tell Perl that it can assume that strings
211 contain a single line, for the purpose of optimizing pattern matches.
212 Pattern matches on strings containing multiple newlines can produce
213 confusing results when C<$*> is 0 or undefined. Default is undefined.
214 (Mnemonic: * matches multiple things.) This variable influences the
215 interpretation of only C<^> and C<$>. A literal newline can be searched
216 for even when C<$* == 0>.
218 Use of C<$*> is deprecated in modern Perl, supplanted by
219 the C</s> and C</m> modifiers on pattern matching.
221 Assigning a non-numerical value to C<$*> triggers a warning (and makes
222 C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
223 makes that an implicit C<int> is applied on the value.
225 =item input_line_number HANDLE EXPR
227 =item $INPUT_LINE_NUMBER
233 The current input record number for the last file handle from which
234 you just read() (or called a C<seek> or C<tell> on). The value
235 may be different from the actual physical line number in the file,
236 depending on what notion of "line" is in effect--see C<$/> on how
237 to change that. An explicit close on a filehandle resets the line
238 number. Because C<< <> >> never does an explicit close, line
239 numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
240 Consider this variable read-only: setting it does not reposition
241 the seek pointer; you'll have to do that on your own. Localizing C<$.>
242 has the effect of also localizing Perl's notion of "the last read
243 filehandle". (Mnemonic: many programs use "." to mean the current line
246 =item input_record_separator HANDLE EXPR
248 =item $INPUT_RECORD_SEPARATOR
254 The input record separator, newline by default. This
255 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
256 variable, including treating empty lines as a terminator if set to
257 the null string. (An empty line cannot contain any spaces
258 or tabs.) You may set it to a multi-character string to match a
259 multi-character terminator, or to C<undef> to read through the end
260 of file. Setting it to C<"\n\n"> means something slightly
261 different than setting to C<"">, if the file contains consecutive
262 empty lines. Setting to C<""> will treat two or more consecutive
263 empty lines as a single empty line. Setting to C<"\n\n"> will
264 blindly assume that the next input character belongs to the next
265 paragraph, even if it's a newline. (Mnemonic: / delimits
266 line boundaries when quoting poetry.)
268 undef $/; # enable "slurp" mode
269 $_ = <FH>; # whole file now here
272 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
273 better for something. :-)
275 Setting C<$/> to a reference to an integer, scalar containing an integer, or
276 scalar that's convertible to an integer will attempt to read records
277 instead of lines, with the maximum record size being the referenced
280 $/ = \32768; # or \"32768", or \$var_containing_32768
284 will read a record of no more than 32768 bytes from FILE. If you're
285 not reading from a record-oriented file (or your OS doesn't have
286 record-oriented files), then you'll likely get a full chunk of data
287 with every read. If a record is larger than the record size you've
288 set, you'll get the record back in pieces.
290 On VMS, record reads are done with the equivalent of C<sysread>,
291 so it's best not to mix record and non-record reads on the same
292 file. (This is unlikely to be a problem, because any file you'd
293 want to read in record mode is probably unusable in line mode.)
294 Non-VMS systems do normal I/O, so it's safe to mix record and
295 non-record reads of a file.
297 See also L<perlport/"Newlines">. Also see C<$.>.
299 =item autoflush HANDLE EXPR
301 =item $OUTPUT_AUTOFLUSH
305 If set to nonzero, forces a flush right away and after every write
306 or print on the currently selected output channel. Default is 0
307 (regardless of whether the channel is really buffered by the
308 system or not; C<$|> tells you only whether you've asked Perl
309 explicitly to flush after each write). STDOUT will
310 typically be line buffered if output is to the terminal and block
311 buffered otherwise. Setting this variable is useful primarily when
312 you are outputting to a pipe or socket, such as when you are running
313 a Perl program under B<rsh> and want to see the output as it's
314 happening. This has no effect on input buffering. See L<perlfunc/getc>
315 for that. (Mnemonic: when you want your pipes to be piping hot.)
317 =item output_field_separator HANDLE EXPR
319 =item $OUTPUT_FIELD_SEPARATOR
325 The output field separator for the print operator. Ordinarily the
326 print operator simply prints out its arguments without further
327 adornment. To get behavior more like B<awk>, set this variable as
328 you would set B<awk>'s OFS variable to specify what is printed
329 between fields. (Mnemonic: what is printed when there is a "," in
330 your print statement.)
332 =item output_record_separator HANDLE EXPR
334 =item $OUTPUT_RECORD_SEPARATOR
340 The output record separator for the print operator. Ordinarily the
341 print operator simply prints out its arguments as is, with no
342 trailing newline or other end-of-record string added. To get
343 behavior more like B<awk>, set this variable as you would set
344 B<awk>'s ORS variable to specify what is printed at the end of the
345 print. (Mnemonic: you set C<$\> instead of adding "\n" at the
346 end of the print. Also, it's just like C<$/>, but it's what you
347 get "back" from Perl.)
349 =item $LIST_SEPARATOR
353 This is like C<$,> except that it applies to array and slice values
354 interpolated into a double-quoted string (or similar interpreted
355 string). Default is a space. (Mnemonic: obvious, I think.)
357 =item $SUBSCRIPT_SEPARATOR
363 The subscript separator for multidimensional array emulation. If you
364 refer to a hash element as
370 $foo{join($;, $a, $b, $c)}
374 @foo{$a,$b,$c} # a slice--note the @
378 ($foo{$a},$foo{$b},$foo{$c})
380 Default is "\034", the same as SUBSEP in B<awk>. If your
381 keys contain binary data there might not be any safe value for C<$;>.
382 (Mnemonic: comma (the syntactic subscript separator) is a
383 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
384 taken for something more important.)
386 Consider using "real" multidimensional arrays as described
393 The output format for printed numbers. This variable is a half-hearted
394 attempt to emulate B<awk>'s OFMT variable. There are times, however,
395 when B<awk> and Perl have differing notions of what counts as
396 numeric. The initial value is "%.I<n>g", where I<n> is the value
397 of the macro DBL_DIG from your system's F<float.h>. This is different from
398 B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
399 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
401 Use of C<$#> is deprecated.
403 =item format_page_number HANDLE EXPR
405 =item $FORMAT_PAGE_NUMBER
409 The current page number of the currently selected output channel.
411 (Mnemonic: % is page number in B<nroff>.)
413 =item format_lines_per_page HANDLE EXPR
415 =item $FORMAT_LINES_PER_PAGE
419 The current page length (printable lines) of the currently selected
420 output channel. Default is 60.
422 (Mnemonic: = has horizontal lines.)
424 =item format_lines_left HANDLE EXPR
426 =item $FORMAT_LINES_LEFT
430 The number of lines left on the page of the currently selected output
433 (Mnemonic: lines_on_page - lines_printed.)
435 =item @LAST_MATCH_START
439 $-[0] is the offset of the start of the last successful match.
440 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
441 I<n>-th subpattern, or undef if the subpattern did not match.
443 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
444 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
445 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
446 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
447 matched subgroup in the last successful match. Contrast with
448 C<$#+>, the number of subgroups in the regular expression. Compare
451 This array holds the offsets of the beginnings of the last
452 successful submatches in the currently active dynamic scope.
453 C<$-[0]> is the offset into the string of the beginning of the
454 entire match. The I<n>th element of this array holds the offset
455 of the I<n>th submatch, so C<$+[1]> is the offset where $1
456 begins, C<$+[2]> the offset where $2 begins, and so on.
457 You can use C<$#-> to determine how many subgroups were in the
458 last successful match. Compare with the C<@+> variable.
460 After a match against some variable $var:
464 =item C<$`> is the same as C<substr($var, 0, $-[0])>
466 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
468 =item C<$'> is the same as C<substr($var, $+[0])>
470 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
472 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
474 =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
478 =item format_name HANDLE EXPR
484 The name of the current report format for the currently selected output
485 channel. Default is the name of the filehandle. (Mnemonic: brother to
488 =item format_top_name HANDLE EXPR
490 =item $FORMAT_TOP_NAME
494 The name of the current top-of-page format for the currently selected
495 output channel. Default is the name of the filehandle with _TOP
496 appended. (Mnemonic: points to top of page.)
498 =item format_line_break_characters HANDLE EXPR
500 =item $FORMAT_LINE_BREAK_CHARACTERS
504 The current set of characters after which a string may be broken to
505 fill continuation fields (starting with ^) in a format. Default is
506 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
507 poetry is a part of a line.)
509 =item format_formfeed HANDLE EXPR
511 =item $FORMAT_FORMFEED
515 What formats output as a form feed. Default is \f.
521 The current value of the write() accumulator for format() lines. A format
522 contains formline() calls that put their result into C<$^A>. After
523 calling its format, write() prints out the contents of C<$^A> and empties.
524 So you never really see the contents of C<$^A> unless you call
525 formline() yourself and then look at it. See L<perlform> and
526 L<perlfunc/formline()>.
532 The status returned by the last pipe close, backtick (C<``>) command,
533 successful call to wait() or waitpid(), or from the system()
534 operator. This is just the 16-bit status word returned by the
535 wait() system call (or else is made up to look like it). Thus, the
536 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
537 C<$? & 127> gives which signal, if any, the process died from, and
538 C<$? & 128> reports whether there was a core dump. (Mnemonic:
539 similar to B<sh> and B<ksh>.)
541 Additionally, if the C<h_errno> variable is supported in C, its value
542 is returned via $? if any C<gethost*()> function fails.
544 If you have installed a signal handler for C<SIGCHLD>, the
545 value of C<$?> will usually be wrong outside that handler.
547 Inside an C<END> subroutine C<$?> contains the value that is going to be
548 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
549 change the exit status of your program. For example:
552 $? = 1 if $? == 255; # die would make it 255
555 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
556 actual VMS exit status, instead of the default emulation of POSIX
559 Also see L<Error Indicators>.
567 If used numerically, yields the current value of the C C<errno>
568 variable, with all the usual caveats. (This means that you shouldn't
569 depend on the value of C<$!> to be anything in particular unless
570 you've gotten a specific error return indicating a system error.)
571 If used an a string, yields the corresponding system error string.
572 You can assign a number to C<$!> to set I<errno> if, for instance,
573 you want C<"$!"> to return the string for error I<n>, or you want
574 to set the exit value for the die() operator. (Mnemonic: What just
577 Also see L<Error Indicators>.
579 =item $EXTENDED_OS_ERROR
583 Error information specific to the current operating system. At
584 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
585 (and for MacPerl). On all other platforms, C<$^E> is always just
588 Under VMS, C<$^E> provides the VMS status value from the last
589 system error. This is more specific information about the last
590 system error than that provided by C<$!>. This is particularly
591 important when C<$!> is set to B<EVMSERR>.
593 Under OS/2, C<$^E> is set to the error code of the last call to
594 OS/2 API either via CRT, or directly from perl.
596 Under Win32, C<$^E> always returns the last error information
597 reported by the Win32 call C<GetLastError()> which describes
598 the last error from within the Win32 API. Most Win32-specific
599 code will report errors via C<$^E>. ANSI C and Unix-like calls
600 set C<errno> and so most portable Perl code will report errors
603 Caveats mentioned in the description of C<$!> generally apply to
604 C<$^E>, also. (Mnemonic: Extra error explanation.)
606 Also see L<Error Indicators>.
612 The Perl syntax error message from the last eval() operator. If null, the
613 last eval() parsed and executed correctly (although the operations you
614 invoked may have failed in the normal fashion). (Mnemonic: Where was
615 the syntax error "at"?)
617 Warning messages are not collected in this variable. You can,
618 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
621 Also see L<Error Indicators>.
629 The process number of the Perl running this script. You should
630 consider this variable read-only, although it will be altered
631 across fork() calls. (Mnemonic: same as shells.)
639 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
640 if you're running setuid.) You can change both the real uid and
641 the effective uid at the same time by using POSIX::setuid().
643 =item $EFFECTIVE_USER_ID
649 The effective uid of this process. Example:
651 $< = $>; # set real to effective uid
652 ($<,$>) = ($>,$<); # swap real and effective uid
654 You can change both the effective uid and the real uid at the same
655 time by using POSIX::setuid().
657 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
658 C<< $< >> and C<< $> >> can be swapped only on machines
659 supporting setreuid().
667 The real gid of this process. If you are on a machine that supports
668 membership in multiple groups simultaneously, gives a space separated
669 list of groups you are in. The first number is the one returned by
670 getgid(), and the subsequent ones by getgroups(), one of which may be
671 the same as the first number.
673 However, a value assigned to C<$(> must be a single number used to
674 set the real gid. So the value given by C<$(> should I<not> be assigned
675 back to C<$(> without being forced numeric, such as by adding zero.
677 You can change both the real gid and the effective gid at the same
678 time by using POSIX::setgid().
680 (Mnemonic: parentheses are used to I<group> things. The real gid is the
681 group you I<left>, if you're running setgid.)
683 =item $EFFECTIVE_GROUP_ID
689 The effective gid of this process. If you are on a machine that
690 supports membership in multiple groups simultaneously, gives a space
691 separated list of groups you are in. The first number is the one
692 returned by getegid(), and the subsequent ones by getgroups(), one of
693 which may be the same as the first number.
695 Similarly, a value assigned to C<$)> must also be a space-separated
696 list of numbers. The first number sets the effective gid, and
697 the rest (if any) are passed to setgroups(). To get the effect of an
698 empty list for setgroups(), just repeat the new effective gid; that is,
699 to force an effective gid of 5 and an effectively empty setgroups()
700 list, say C< $) = "5 5" >.
702 You can change both the effective gid and the real gid at the same
703 time by using POSIX::setgid() (use only a single numeric argument).
705 (Mnemonic: parentheses are used to I<group> things. The effective gid
706 is the group that's I<right> for you, if you're running setgid.)
708 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
709 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
710 and C<$)> can be swapped only on machines supporting setregid().
716 Contains the name of the program being executed. On some operating
717 systems assigning to C<$0> modifies the argument area that the B<ps>
718 program sees. This is more useful as a way of indicating the current
719 program state than it is for hiding the program you're running.
720 (Mnemonic: same as B<sh> and B<ksh>.)
722 Note for BSD users: setting C<$0> does not completely remove "perl"
723 from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
724 result in C<"perl: foobar (perl)">. This is an operating system
729 The index of the first element in an array, and of the first character
730 in a substring. Default is 0, but you could theoretically set it
731 to 1 to make Perl behave more like B<awk> (or Fortran) when
732 subscripting and when evaluating the index() and substr() functions.
733 (Mnemonic: [ begins subscripts.)
735 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
736 directive, and cannot influence the behavior of any other file.
737 Its use is highly discouraged.
741 The version + patchlevel / 1000 of the Perl interpreter. This variable
742 can be used to determine whether the Perl interpreter executing a
743 script is in the right range of versions. (Mnemonic: Is this version
744 of perl in the right bracket?) Example:
746 warn "No checksumming!\n" if $] < 3.019;
748 See also the documentation of C<use VERSION> and C<require VERSION>
749 for a convenient way to fail if the running Perl interpreter is too old.
751 The use of this variable is deprecated. The floating point representation
752 can sometimes lead to inaccurate numeric comparisons. See C<$^V> for a
753 more modern representation of the Perl version that allows accurate string
760 The current value of the flag associated with the B<-c> switch.
761 Mainly of use with B<-MO=...> to allow code to alter its behavior
762 when being compiled, such as for example to AUTOLOAD at compile
763 time rather than normal, deferred loading. See L<perlcc>. Setting
764 C<$^C = 1> is similar to calling C<B::minus_c>.
770 The current value of the debugging flags. (Mnemonic: value of B<-D>
777 The maximum system file descriptor, ordinarily 2. System file
778 descriptors are passed to exec()ed processes, while higher file
779 descriptors are not. Also, during an open(), system file descriptors are
780 preserved even if the open() fails. (Ordinary file descriptors are
781 closed before the open() is attempted.) The close-on-exec
782 status of a file descriptor will be decided according to the value of
783 C<$^F> when the corresponding file, pipe, or socket was opened, not the
788 WARNING: This variable is strictly for internal use only. Its availability,
789 behavior, and contents are subject to change without notice.
791 This variable contains compile-time hints for the Perl interpreter. At the
792 end of compilation of a BLOCK the value of this variable is restored to the
793 value when the interpreter started to compile the BLOCK.
795 When perl begins to parse any block construct that provides a lexical scope
796 (e.g., eval body, required file, subroutine body, loop body, or conditional
797 block), the existing value of $^H is saved, but its value is left unchanged.
798 When the compilation of the block is completed, it regains the saved value.
799 Between the points where its value is saved and restored, code that
800 executes within BEGIN blocks is free to change the value of $^H.
802 This behavior provides the semantic of lexical scoping, and is used in,
803 for instance, the C<use strict> pragma.
805 The contents should be an integer; different bits of it are used for
806 different pragmatic flags. Here's an example:
808 sub add_100 { $^H |= 0x100 }
815 Consider what happens during execution of the BEGIN block. At this point
816 the BEGIN block has already been compiled, but the body of foo() is still
817 being compiled. The new value of $^H will therefore be visible only while
818 the body of foo() is being compiled.
820 Substitution of the above BEGIN block with:
822 BEGIN { require strict; strict->import('vars') }
824 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
825 version of the same lexical pragma:
827 BEGIN { require strict; strict->import('vars') if $condition }
831 WARNING: This variable is strictly for internal use only. Its availability,
832 behavior, and contents are subject to change without notice.
834 The %^H hash provides the same scoping semantic as $^H. This makes it
835 useful for implementation of lexically scoped pragmas.
841 The current value of the inplace-edit extension. Use C<undef> to disable
842 inplace editing. (Mnemonic: value of B<-i> switch.)
846 By default, running out of memory is an untrappable, fatal error.
847 However, if suitably built, Perl can use the contents of C<$^M>
848 as an emergency memory pool after die()ing. Suppose that your Perl
849 were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
852 $^M = 'a' x (1 << 16);
854 would allocate a 64K buffer for use in an emergency. See the
855 F<INSTALL> file in the Perl distribution for information on how to
856 enable this option. To discourage casual use of this advanced
857 feature, there is no L<English|English> long name for this variable.
863 The name of the operating system under which this copy of Perl was
864 built, as determined during the configuration process. The value
865 is identical to C<$Config{'osname'}>. See also L<Config> and the
866 B<-V> command-line switch documented in L<perlrun>.
872 The internal variable for debugging support. The meanings of the
873 various bits are subject to change, but currently indicate:
879 Debug subroutine enter/exit.
883 Line-by-line debugging.
887 Switch off optimizations.
891 Preserve more data for future interactive inspections.
895 Keep info about source lines on which a subroutine is defined.
899 Start with single-step on.
903 Use subroutine address instead of name when reporting.
907 Report C<goto &subroutine> as well.
911 Provide informative "file" names for evals based on the place they were compiled.
915 Provide informative names to anonymous subroutines based on the place they
920 Some bits may be relevant at compile-time only, some at
921 run-time only. This is a new mechanism and the details may change.
923 =item $LAST_REGEXP_CODE_RESULT
927 The result of evaluation of the last successful C<(?{ code })>
928 regular expression assertion (see L<perlre>). May be written to.
930 =item $EXCEPTIONS_BEING_CAUGHT
934 Current state of the interpreter. Undefined if parsing of the current
935 module/eval is not finished (may happen in $SIG{__DIE__} and
936 $SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
942 The time at which the program began running, in seconds since the
943 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
944 and B<-C> filetests are based on this value.
950 The revision, version, and subversion of the Perl interpreter, represented
951 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
952 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
953 C<$^V eq v5.6.0>. Note that the characters in this string value can
954 potentially be in Unicode range.
956 This can be used to determine whether the Perl interpreter executing a
957 script is in the right range of versions. (Mnemonic: use ^V for Version
960 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
962 See the documentation of C<use VERSION> and C<require VERSION>
963 for a convenient way to fail if the running Perl interpreter is too old.
965 See also C<$]> for an older representation of the Perl version.
971 The current value of the warning switch, initially true if B<-w>
972 was used, false otherwise, but directly modifiable. (Mnemonic:
973 related to the B<-w> switch.) See also L<warnings>.
975 =item ${^WARNING_BITS}
977 The current set of warning checks enabled by the C<use warnings> pragma.
978 See the documentation of C<warnings> for more details.
980 =item ${^WIDE_SYSTEM_CALLS}
982 Global flag that enables system calls made by Perl to use wide character
983 APIs native to the system, if available. This is currently only implemented
984 on the Windows platform.
986 This can also be enabled from the command line using the C<-C> switch.
988 The initial value is typically C<0> for compatibility with Perl versions
989 earlier than 5.6, but may be automatically set to C<1> by Perl if the system
990 provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
992 The C<bytes> pragma always overrides the effect of this flag in the current
993 lexical scope. See L<bytes>.
995 =item $EXECUTABLE_NAME
999 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
1000 This may not be a full pathname, nor even necessarily in your path.
1004 contains the name of the current file when reading from <>.
1008 The array @ARGV contains the command-line arguments intended for
1009 the script. C<$#ARGV> is generally the number of arguments minus
1010 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1011 command name itself. See C<$0> for the command name.
1015 The array @F contains the fields of each line read in when autosplit
1016 mode is turned on. See L<perlrun> for the B<-a> switch. This array
1017 is package-specific, and must be declared or given a full package name
1018 if not in package main when running under C<strict 'vars'>.
1022 The array @INC contains the list of places that the C<do EXPR>,
1023 C<require>, or C<use> constructs look for their library files. It
1024 initially consists of the arguments to any B<-I> command-line
1025 switches, followed by the default Perl library, probably
1026 F</usr/local/lib/perl>, followed by ".", to represent the current
1027 directory. If you need to modify this at runtime, you should use
1028 the C<use lib> pragma to get the machine-dependent library properly
1031 use lib '/mypath/libdir/';
1036 Within a subroutine the array @_ contains the parameters passed to that
1037 subroutine. See L<perlsub>.
1041 The hash %INC contains entries for each filename included via the
1042 C<do>, C<require>, or C<use> operators. The key is the filename
1043 you specified (with module names converted to pathnames), and the
1044 value is the location of the file found. The C<require>
1045 operator uses this hash to determine whether a particular file has
1046 already been included.
1052 The hash %ENV contains your current environment. Setting a
1053 value in C<ENV> changes the environment for any child processes
1054 you subsequently fork() off.
1060 The hash %SIG contains signal handlers for signals. For example:
1062 sub handler { # 1st argument is signal name
1064 print "Caught a SIG$sig--shutting down\n";
1069 $SIG{'INT'} = \&handler;
1070 $SIG{'QUIT'} = \&handler;
1072 $SIG{'INT'} = 'DEFAULT'; # restore default action
1073 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1075 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1076 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1079 Here are some other examples:
1081 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1082 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1083 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1084 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1086 Be sure not to use a bareword as the name of a signal handler,
1087 lest you inadvertently call it.
1089 If your system has the sigaction() function then signal handlers are
1090 installed using it. This means you get reliable signal handling. If
1091 your system has the SA_RESTART flag it is used when signals handlers are
1092 installed. This means that system calls for which restarting is supported
1093 continue rather than returning when a signal arrives. If you want your
1094 system calls to be interrupted by signal delivery then do something like
1097 use POSIX ':signal_h';
1100 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1101 or die "Error setting SIGALRM handler: $!\n";
1105 Certain internal hooks can be also set using the %SIG hash. The
1106 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1107 about to be printed. The warning message is passed as the first
1108 argument. The presence of a __WARN__ hook causes the ordinary printing
1109 of warnings to STDERR to be suppressed. You can use this to save warnings
1110 in a variable, or turn warnings into fatal errors, like this:
1112 local $SIG{__WARN__} = sub { die $_[0] };
1115 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1116 is about to be thrown. The error message is passed as the first
1117 argument. When a __DIE__ hook routine returns, the exception
1118 processing continues as it would have in the absence of the hook,
1119 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
1120 The C<__DIE__> handler is explicitly disabled during the call, so that you
1121 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1123 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1124 even inside an eval(). Do not use this to rewrite a pending exception
1125 in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1126 This strange action at a distance may be fixed in a future release
1127 so that C<$SIG{__DIE__}> is only called if your program is about
1128 to exit, as was the original intent. Any other use is deprecated.
1130 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1131 they may be called to report (probable) errors found by the parser.
1132 In such a case the parser may be in inconsistent state, so any
1133 attempt to evaluate Perl code from such a handler will probably
1134 result in a segfault. This means that warnings or errors that
1135 result from parsing Perl should be used with extreme caution, like
1138 require Carp if defined $^S;
1139 Carp::confess("Something wrong") if defined &Carp::confess;
1140 die "Something wrong, but could not load Carp to give backtrace...
1141 To see backtrace try starting Perl with -MCarp switch";
1143 Here the first line will load Carp I<unless> it is the parser who
1144 called the handler. The second line will print backtrace and die if
1145 Carp was available. The third line will be executed only if Carp was
1148 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1149 L<warnings> for additional information.
1153 =head2 Error Indicators
1155 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1156 about different types of error conditions that may appear during
1157 execution of a Perl program. The variables are shown ordered by
1158 the "distance" between the subsystem which reported the error and
1159 the Perl process. They correspond to errors detected by the Perl
1160 interpreter, C library, operating system, or an external program,
1163 To illustrate the differences between these variables, consider the
1164 following Perl expression, which uses a single-quoted string:
1167 open PIPE, "/cdrom/install |";
1169 close PIPE or die "bad pipe: $?, $!";
1172 After execution of this statement all 4 variables may have been set.
1174 C<$@> is set if the string to be C<eval>-ed did not compile (this
1175 may happen if C<open> or C<close> were imported with bad prototypes),
1176 or if Perl code executed during evaluation die()d . In these cases
1177 the value of $@ is the compile error, or the argument to C<die>
1178 (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1181 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1182 and C<close> are translated to calls in the C run-time library and
1183 thence to the operating system kernel. C<$!> is set to the C library's
1184 C<errno> if one of these calls fails.
1186 Under a few operating systems, C<$^E> may contain a more verbose
1187 error indicator, such as in this case, "CDROM tray not closed."
1188 Systems that do not support extended error messages leave C<$^E>
1191 Finally, C<$?> may be set to non-0 value if the external program
1192 F</cdrom/install> fails. The upper eight bits reflect specific
1193 error conditions encountered by the program (the program's exit()
1194 value). The lower eight bits reflect mode of failure, like signal
1195 death and core dump information See wait(2) for details. In
1196 contrast to C<$!> and C<$^E>, which are set only if error condition
1197 is detected, the variable C<$?> is set on each C<wait> or pipe
1198 C<close>, overwriting the old value. This is more like C<$@>, which
1199 on every eval() is always set on failure and cleared on success.
1201 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1204 =head2 Technical Note on the Syntax of Variable Names
1206 Variable names in Perl can have several formats. Usually, they
1207 must begin with a letter or underscore, in which case they can be
1208 arbitrarily long (up to an internal limit of 251 characters) and
1209 may contain letters, digits, underscores, or the special sequence
1210 C<::> or C<'>. In this case, the part before the last C<::> or
1211 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1213 Perl variable names may also be a sequence of digits or a single
1214 punctuation or control character. These names are all reserved for
1215 special uses by Perl; for example, the all-digits names are used
1216 to hold data captured by backreferences after a regular expression
1217 match. Perl has a special syntax for the single-control-character
1218 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1219 character. For example, the notation C<$^W> (dollar-sign caret
1220 C<W>) is the scalar variable whose name is the single character
1221 control-C<W>. This is better than typing a literal control-C<W>
1224 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1225 strings that begin with control characters (or better yet, a caret).
1226 These variables must be written in the form C<${^Foo}>; the braces
1227 are not optional. C<${^Foo}> denotes the scalar variable whose
1228 name is a control-C<F> followed by two C<o>'s. These variables are
1229 reserved for future special uses by Perl, except for the ones that
1230 begin with C<^_> (control-underscore or caret-underscore). No
1231 control-character name that begins with C<^_> will acquire a special
1232 meaning in any future version of Perl; such names may therefore be
1233 used safely in programs. C<$^_> itself, however, I<is> reserved.
1235 Perl identifiers that begin with digits, control characters, or
1236 punctuation characters are exempt from the effects of the C<package>
1237 declaration and are always forced to be in package C<main>. A few
1238 other names are also exempt:
1246 In particular, the new special C<${^_XYZ}> variables are always taken
1247 to be in package C<main>, regardless of any C<package> declarations
1252 Due to an unfortunate accident of Perl's implementation, C<use
1253 English> imposes a considerable performance penalty on all regular
1254 expression matches in a program, regardless of whether they occur
1255 in the scope of C<use English>. For that reason, saying C<use
1256 English> in libraries is strongly discouraged. See the
1257 Devel::SawAmpersand module documentation from CPAN
1258 (http://www.perl.com/CPAN/modules/by-module/Devel/)
1259 for more information.
1261 Having to even think about the C<$^S> variable in your exception
1262 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1263 invites grievous and difficult to track down errors. Avoid it
1264 and use an C<END{}> or CORE::GLOBAL::die override instead.