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 aliases all the short names to the long
17 names in the current package. Some even have medium names, generally
18 borrowed from B<awk>. In general, it's best to use the
20 use English '-no_match_vars';
22 invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids
23 a certain performance hit with the use of regular expressions. See
26 Variables that depend on the currently selected filehandle may be set by
27 calling an appropriate object method on the IO::Handle object, although
28 this is less efficient than using the regular built-in variables. (Summary
29 lines below for this contain the word HANDLE.) First you must say
33 after which you may use either
41 Each method returns the old value of the IO::Handle attribute.
42 The methods each take an optional EXPR, which, if supplied, specifies the
43 new value for the IO::Handle attribute in question. If not supplied,
44 most methods do nothing to the current value--except for
45 autoflush(), which will assume a 1 for you, just to be different.
47 Because loading in the IO::Handle class is an expensive operation, you should
48 learn how to use the regular built-in variables.
50 A few of these variables are considered "read-only". This means that if
51 you try to assign to this variable, either directly or indirectly through
52 a reference, you'll raise a run-time exception.
54 You should be very careful when modifying the default values of most
55 special variables described in this document. In most cases you want
56 to localize these variables before changing them, since if you don't,
57 the change may affect other modules which rely on the default values
58 of the special variables that you have changed. This is one of the
59 correct ways to read the whole file at once:
61 open my $fh, "foo" or die $!;
62 local $/; # enable localized slurp mode
66 But the following code is quite bad:
68 open my $fh, "foo" or die $!;
69 undef $/; # enable slurp mode
73 since some other module, may want to read data from some file in the
74 default "line mode", so if the code we have just presented has been
75 executed, the global value of C<$/> is now changed for any other code
76 running inside the same Perl interpreter.
78 Usually when a variable is localized you want to make sure that this
79 change affects the shortest scope possible. So unless you are already
80 inside some short C<{}> block, you should create one yourself. For
84 open my $fh, "foo" or die $!;
91 Here is an example of how your own code can go broken:
99 # do something with $_
102 You probably expect this code to print:
110 Why? Because nasty_break() modifies C<$_> without localizing it
111 first. The fix is to add local():
115 It's easy to notice the problem in such a short example, but in more
116 complicated code you are looking for trouble if you don't localize
117 changes to the special variables.
119 The following list is ordered by scalar variables first, then the
120 arrays, then the hashes.
128 The default input and pattern-searching space. The following pairs are
131 while (<>) {...} # equivalent only in while!
132 while (defined($_ = <>)) {...}
143 Here are the places where Perl will assume $_ even if you
150 Various unary functions, including functions like ord() and int(), as well
151 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
156 Various list functions like print() and unlink().
160 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
161 without an C<=~> operator.
165 The default iterator variable in a C<foreach> loop if no other
166 variable is supplied.
170 The implicit iterator variable in the grep() and map() functions.
174 The default place to put an input record when a C<< <FH> >>
175 operation's result is tested by itself as the sole criterion of a C<while>
176 test. Outside a C<while> test, this will not happen.
180 (Mnemonic: underline is understood in certain operations.)
190 Special package variables when using sort(), see L<perlfunc/sort>.
191 Because of this specialness $a and $b don't need to be declared
192 (using use vars, or our()) even when using the C<strict 'vars'> pragma.
193 Don't lexicalize them with C<my $a> or C<my $b> if you want to be
194 able to use them in the sort() comparison block or function.
202 Contains the subpattern from the corresponding set of capturing
203 parentheses from the last pattern match, not counting patterns
204 matched in nested blocks that have been exited already. (Mnemonic:
205 like \digits.) These variables are all read-only and dynamically
206 scoped to the current BLOCK.
212 The string matched by the last successful pattern match (not counting
213 any matches hidden within a BLOCK or eval() enclosed by the current
214 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
215 and dynamically scoped to the current BLOCK.
217 The use of this variable anywhere in a program imposes a considerable
218 performance penalty on all regular expression matches. See L</BUGS>.
224 The string preceding whatever was matched by the last successful
225 pattern match (not counting any matches hidden within a BLOCK or eval
226 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
227 string.) This variable is read-only.
229 The use of this variable anywhere in a program imposes a considerable
230 performance penalty on all regular expression matches. See L</BUGS>.
236 The string following whatever was matched by the last successful
237 pattern match (not counting any matches hidden within a BLOCK or eval()
238 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
241 local $_ = 'abcdefghi';
243 print "$`:$&:$'\n"; # prints abc:def:ghi
245 This variable is read-only and dynamically scoped to the current BLOCK.
247 The use of this variable anywhere in a program imposes a considerable
248 performance penalty on all regular expression matches. See L</BUGS>.
250 =item $LAST_PAREN_MATCH
254 The text matched by the last bracket of the last successful search pattern.
255 This is useful if you don't know which one of a set of alternative patterns
256 matched. For example:
258 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
260 (Mnemonic: be positive and forward looking.)
261 This variable is read-only and dynamically scoped to the current BLOCK.
265 The text matched by the used group most-recently closed (i.e. the group
266 with the rightmost closing parenthesis) of the last successful search
267 pattern. (Mnemonic: the (possibly) Nested parenthesis that most
270 This is primarily used inside C<(?{...})> blocks for examining text
271 recently matched. For example, to effectively capture text to a variable
272 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
274 (?:(...)(?{ $var = $^N }))
276 By setting and then using C<$var> in this way relieves you from having to
277 worry about exactly which numbered set of parentheses they are.
279 This variable is dynamically scoped to the current BLOCK.
281 =item @LAST_MATCH_END
285 This array holds the offsets of the ends of the last successful
286 submatches in the currently active dynamic scope. C<$+[0]> is
287 the offset into the string of the end of the entire match. This
288 is the same value as what the C<pos> function returns when called
289 on the variable that was matched against. The I<n>th element
290 of this array holds the offset of the I<n>th submatch, so
291 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
292 past where $2 ends, and so on. You can use C<$#+> to determine
293 how many subgroups were in the last successful match. See the
294 examples given for the C<@-> variable.
298 Set to a non-zero integer value to do multi-line matching within a
299 string, 0 (or undefined) to tell Perl that it can assume that strings
300 contain a single line, for the purpose of optimizing pattern matches.
301 Pattern matches on strings containing multiple newlines can produce
302 confusing results when C<$*> is 0 or undefined. Default is undefined.
303 (Mnemonic: * matches multiple things.) This variable influences the
304 interpretation of only C<^> and C<$>. A literal newline can be searched
305 for even when C<$* == 0>.
307 Use of C<$*> is deprecated in modern Perl, supplanted by
308 the C</s> and C</m> modifiers on pattern matching.
310 Assigning a non-numerical value to C<$*> triggers a warning (and makes
311 C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
312 makes that an implicit C<int> is applied on the value.
314 =item HANDLE->input_line_number(EXPR)
316 =item $INPUT_LINE_NUMBER
322 Current line number for the last filehandle accessed.
324 Each filehandle in Perl counts the number of lines that have been read
325 from it. (Depending on the value of C<$/>, Perl's idea of what
326 constitutes a line may not match yours.) When a line is read from a
327 filehandle (via readline() or C<< <> >>), or when tell() or seek() is
328 called on it, C<$.> becomes an alias to the line counter for that
331 You can adjust the counter by assigning to C<$.>, but this will not
332 actually move the seek pointer. I<Localizing C<$.> will not localize
333 the filehandle's line count>. Instead, it will localize perl's notion
334 of which filehandle C<$.> is currently aliased to.
336 C<$.> is reset when the filehandle is closed, but B<not> when an open
337 filehandle is reopened without an intervening close(). For more
338 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
339 an explicit close, line numbers increase across ARGV files (but see
340 examples in L<perlfunc/eof>).
342 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
343 line counter for a given filehandle without having to worry about
344 which handle you last accessed.
346 (Mnemonic: many programs use "." to mean the current line number.)
348 =item IO::Handle->input_record_separator(EXPR)
350 =item $INPUT_RECORD_SEPARATOR
356 The input record separator, newline by default. This
357 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
358 variable, including treating empty lines as a terminator if set to
359 the null string. (An empty line cannot contain any spaces
360 or tabs.) You may set it to a multi-character string to match a
361 multi-character terminator, or to C<undef> to read through the end
362 of file. Setting it to C<"\n\n"> means something slightly
363 different than setting to C<"">, if the file contains consecutive
364 empty lines. Setting to C<""> will treat two or more consecutive
365 empty lines as a single empty line. Setting to C<"\n\n"> will
366 blindly assume that the next input character belongs to the next
367 paragraph, even if it's a newline. (Mnemonic: / delimits
368 line boundaries when quoting poetry.)
370 local $/; # enable "slurp" mode
371 local $_ = <FH>; # whole file now here
374 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
375 better for something. :-)
377 Setting C<$/> to a reference to an integer, scalar containing an integer, or
378 scalar that's convertible to an integer will attempt to read records
379 instead of lines, with the maximum record size being the referenced
382 local $/ = \32768; # or \"32768", or \$var_containing_32768
383 open my $fh, $myfile or die $!;
386 will read a record of no more than 32768 bytes from FILE. If you're
387 not reading from a record-oriented file (or your OS doesn't have
388 record-oriented files), then you'll likely get a full chunk of data
389 with every read. If a record is larger than the record size you've
390 set, you'll get the record back in pieces.
392 On VMS, record reads are done with the equivalent of C<sysread>,
393 so it's best not to mix record and non-record reads on the same
394 file. (This is unlikely to be a problem, because any file you'd
395 want to read in record mode is probably unusable in line mode.)
396 Non-VMS systems do normal I/O, so it's safe to mix record and
397 non-record reads of a file.
399 See also L<perlport/"Newlines">. Also see C<$.>.
401 =item HANDLE->autoflush(EXPR)
403 =item $OUTPUT_AUTOFLUSH
407 If set to nonzero, forces a flush right away and after every write
408 or print on the currently selected output channel. Default is 0
409 (regardless of whether the channel is really buffered by the
410 system or not; C<$|> tells you only whether you've asked Perl
411 explicitly to flush after each write). STDOUT will
412 typically be line buffered if output is to the terminal and block
413 buffered otherwise. Setting this variable is useful primarily when
414 you are outputting to a pipe or socket, such as when you are running
415 a Perl program under B<rsh> and want to see the output as it's
416 happening. This has no effect on input buffering. See L<perlfunc/getc>
417 for that. (Mnemonic: when you want your pipes to be piping hot.)
419 =item IO::Handle->output_field_separator EXPR
421 =item $OUTPUT_FIELD_SEPARATOR
427 The output field separator for the print operator. If defined, this
428 value is printed between each of print's arguments. Default is C<undef>.
429 (Mnemonic: what is printed when there is a "," in your print statement.)
431 =item IO::Handle->output_record_separator EXPR
433 =item $OUTPUT_RECORD_SEPARATOR
439 The output record separator for the print operator. If defined, this
440 value is printed after the last of print's arguments. Default is C<undef>.
441 (Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
442 Also, it's just like C<$/>, but it's what you get "back" from Perl.)
444 =item $LIST_SEPARATOR
448 This is like C<$,> except that it applies to array and slice values
449 interpolated into a double-quoted string (or similar interpreted
450 string). Default is a space. (Mnemonic: obvious, I think.)
452 =item $SUBSCRIPT_SEPARATOR
458 The subscript separator for multidimensional array emulation. If you
459 refer to a hash element as
465 $foo{join($;, $a, $b, $c)}
469 @foo{$a,$b,$c} # a slice--note the @
473 ($foo{$a},$foo{$b},$foo{$c})
475 Default is "\034", the same as SUBSEP in B<awk>. If your
476 keys contain binary data there might not be any safe value for C<$;>.
477 (Mnemonic: comma (the syntactic subscript separator) is a
478 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
479 taken for something more important.)
481 Consider using "real" multidimensional arrays as described
486 The output format for printed numbers. This variable is a half-hearted
487 attempt to emulate B<awk>'s OFMT variable. There are times, however,
488 when B<awk> and Perl have differing notions of what counts as
489 numeric. The initial value is "%.I<n>g", where I<n> is the value
490 of the macro DBL_DIG from your system's F<float.h>. This is different from
491 B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
492 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
494 Use of C<$#> is deprecated.
496 =item HANDLE->format_page_number(EXPR)
498 =item $FORMAT_PAGE_NUMBER
502 The current page number of the currently selected output channel.
504 (Mnemonic: % is page number in B<nroff>.)
506 =item HANDLE->format_lines_per_page(EXPR)
508 =item $FORMAT_LINES_PER_PAGE
512 The current page length (printable lines) of the currently selected
513 output channel. Default is 60.
515 (Mnemonic: = has horizontal lines.)
517 =item HANDLE->format_lines_left(EXPR)
519 =item $FORMAT_LINES_LEFT
523 The number of lines left on the page of the currently selected output
526 (Mnemonic: lines_on_page - lines_printed.)
528 =item @LAST_MATCH_START
532 $-[0] is the offset of the start of the last successful match.
533 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
534 I<n>-th subpattern, or undef if the subpattern did not match.
536 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
537 $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
538 $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
539 C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
540 matched subgroup in the last successful match. Contrast with
541 C<$#+>, the number of subgroups in the regular expression. Compare
544 This array holds the offsets of the beginnings of the last
545 successful submatches in the currently active dynamic scope.
546 C<$-[0]> is the offset into the string of the beginning of the
547 entire match. The I<n>th element of this array holds the offset
548 of the I<n>th submatch, so C<$-[1]> is the offset where $1
549 begins, C<$-[2]> the offset where $2 begins, and so on.
551 After a match against some variable $var:
555 =item C<$`> is the same as C<substr($var, 0, $-[0])>
557 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
559 =item C<$'> is the same as C<substr($var, $+[0])>
561 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
563 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
565 =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
569 =item HANDLE->format_name(EXPR)
575 The name of the current report format for the currently selected output
576 channel. Default is the name of the filehandle. (Mnemonic: brother to
579 =item HANDLE->format_top_name(EXPR)
581 =item $FORMAT_TOP_NAME
585 The name of the current top-of-page format for the currently selected
586 output channel. Default is the name of the filehandle with _TOP
587 appended. (Mnemonic: points to top of page.)
589 =item IO::Handle->format_line_break_characters EXPR
591 =item $FORMAT_LINE_BREAK_CHARACTERS
595 The current set of characters after which a string may be broken to
596 fill continuation fields (starting with ^) in a format. Default is
597 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
598 poetry is a part of a line.)
600 =item IO::Handle->format_formfeed EXPR
602 =item $FORMAT_FORMFEED
606 What formats output as a form feed. Default is \f.
612 The current value of the write() accumulator for format() lines. A format
613 contains formline() calls that put their result into C<$^A>. After
614 calling its format, write() prints out the contents of C<$^A> and empties.
615 So you never really see the contents of C<$^A> unless you call
616 formline() yourself and then look at it. See L<perlform> and
617 L<perlfunc/formline()>.
623 The status returned by the last pipe close, backtick (C<``>) command,
624 successful call to wait() or waitpid(), or from the system()
625 operator. This is just the 16-bit status word returned by the
626 wait() system call (or else is made up to look like it). Thus, the
627 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
628 C<$? & 127> gives which signal, if any, the process died from, and
629 C<$? & 128> reports whether there was a core dump. (Mnemonic:
630 similar to B<sh> and B<ksh>.)
632 Additionally, if the C<h_errno> variable is supported in C, its value
633 is returned via $? if any C<gethost*()> function fails.
635 If you have installed a signal handler for C<SIGCHLD>, the
636 value of C<$?> will usually be wrong outside that handler.
638 Inside an C<END> subroutine C<$?> contains the value that is going to be
639 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
640 change the exit status of your program. For example:
643 $? = 1 if $? == 255; # die would make it 255
646 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
647 actual VMS exit status, instead of the default emulation of POSIX
648 status; see L<perlvms/$?> for details.
650 Also see L<Error Indicators>.
654 The I<object reference> to the Encode object that is used to convert
655 the source code to Unicode. Thanks to this variable your perl script
656 does not have to be written in UTF-8. Default is I<undef>. The direct
657 manipulation of this variable is highly discouraged. See L<encoding>
666 If used numerically, yields the current value of the C C<errno>
667 variable, or in other words, if a system or library call fails, it
668 sets this variable. This means that the value of C<$!> is meaningful
669 only I<immediately> after a B<failure>:
671 if (open(FH, $filename)) {
672 # Here $! is meaningless.
675 # ONLY here is $! meaningful.
677 # Already here $! might be meaningless.
679 # Since here we might have either success or failure,
680 # here $! is meaningless.
682 In the above I<meaningless> stands for anything: zero, non-zero,
683 C<undef>. A successful system or library call does B<not> set
684 the variable to zero.
686 If used as a string, yields the corresponding system error string.
687 You can assign a number to C<$!> to set I<errno> if, for instance,
688 you want C<"$!"> to return the string for error I<n>, or you want
689 to set the exit value for the die() operator. (Mnemonic: What just
692 Also see L<Error Indicators>.
696 Each element of C<%!> has a true value only if C<$!> is set to that
697 value. For example, C<$!{ENOENT}> is true if and only if the current
698 value of C<$!> is C<ENOENT>; that is, if the most recent error was
699 "No such file or directory" (or its moral equivalent: not all operating
700 systems give that exact error, and certainly not all languages).
701 To check if a particular key is meaningful on your system, use
702 C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
703 See L<Errno> for more information, and also see above for the
706 =item $EXTENDED_OS_ERROR
710 Error information specific to the current operating system. At
711 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
712 (and for MacPerl). On all other platforms, C<$^E> is always just
715 Under VMS, C<$^E> provides the VMS status value from the last
716 system error. This is more specific information about the last
717 system error than that provided by C<$!>. This is particularly
718 important when C<$!> is set to B<EVMSERR>.
720 Under OS/2, C<$^E> is set to the error code of the last call to
721 OS/2 API either via CRT, or directly from perl.
723 Under Win32, C<$^E> always returns the last error information
724 reported by the Win32 call C<GetLastError()> which describes
725 the last error from within the Win32 API. Most Win32-specific
726 code will report errors via C<$^E>. ANSI C and Unix-like calls
727 set C<errno> and so most portable Perl code will report errors
730 Caveats mentioned in the description of C<$!> generally apply to
731 C<$^E>, also. (Mnemonic: Extra error explanation.)
733 Also see L<Error Indicators>.
739 The Perl syntax error message from the last eval() operator.
740 If $@ is the null string, the last eval() parsed and executed
741 correctly (although the operations you invoked may have failed in the
742 normal fashion). (Mnemonic: Where was the syntax error "at"?)
744 Warning messages are not collected in this variable. You can,
745 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
748 Also see L<Error Indicators>.
756 The process number of the Perl running this script. You should
757 consider this variable read-only, although it will be altered
758 across fork() calls. (Mnemonic: same as shells.)
760 Note for Linux users: on Linux, the C functions C<getpid()> and
761 C<getppid()> return different values from different threads. In order to
762 be portable, this behavior is not reflected by C<$$>, whose value remains
763 consistent across threads. If you want to call the underlying C<getpid()>,
764 you may use the CPAN module C<Linux::Pid>.
772 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
773 if you're running setuid.) You can change both the real uid and
774 the effective uid at the same time by using POSIX::setuid(). Since
775 changes to $< require a system call, check $! after a change attempt to
776 detect any possible errors.
778 =item $EFFECTIVE_USER_ID
784 The effective uid of this process. Example:
786 $< = $>; # set real to effective uid
787 ($<,$>) = ($>,$<); # swap real and effective uid
789 You can change both the effective uid and the real uid at the same
790 time by using POSIX::setuid(). Changes to $> require a check to $!
791 to detect any possible errors after an attempted change.
793 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
794 C<< $< >> and C<< $> >> can be swapped only on machines
795 supporting setreuid().
803 The real gid of this process. If you are on a machine that supports
804 membership in multiple groups simultaneously, gives a space separated
805 list of groups you are in. The first number is the one returned by
806 getgid(), and the subsequent ones by getgroups(), one of which may be
807 the same as the first number.
809 However, a value assigned to C<$(> must be a single number used to
810 set the real gid. So the value given by C<$(> should I<not> be assigned
811 back to C<$(> without being forced numeric, such as by adding zero.
813 You can change both the real gid and the effective gid at the same
814 time by using POSIX::setgid(). Changes to $( require a check to $!
815 to detect any possible errors after an attempted change.
817 (Mnemonic: parentheses are used to I<group> things. The real gid is the
818 group you I<left>, if you're running setgid.)
820 =item $EFFECTIVE_GROUP_ID
826 The effective gid of this process. If you are on a machine that
827 supports membership in multiple groups simultaneously, gives a space
828 separated list of groups you are in. The first number is the one
829 returned by getegid(), and the subsequent ones by getgroups(), one of
830 which may be the same as the first number.
832 Similarly, a value assigned to C<$)> must also be a space-separated
833 list of numbers. The first number sets the effective gid, and
834 the rest (if any) are passed to setgroups(). To get the effect of an
835 empty list for setgroups(), just repeat the new effective gid; that is,
836 to force an effective gid of 5 and an effectively empty setgroups()
837 list, say C< $) = "5 5" >.
839 You can change both the effective gid and the real gid at the same
840 time by using POSIX::setgid() (use only a single numeric argument).
841 Changes to $) require a check to $! to detect any possible errors
842 after an attempted change.
844 (Mnemonic: parentheses are used to I<group> things. The effective gid
845 is the group that's I<right> for you, if you're running setgid.)
847 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
848 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
849 and C<$)> can be swapped only on machines supporting setregid().
855 Contains the name of the program being executed.
857 On some (read: not all) operating systems assigning to C<$0> modifies
858 the argument area that the C<ps> program sees. On some platforms you
859 may have to use special C<ps> options or a different C<ps> to see the
860 changes. Modifying the $0 is more useful as a way of indicating the
861 current program state than it is for hiding the program you're
862 running. (Mnemonic: same as B<sh> and B<ksh>.)
864 Note that there are platform specific limitations on the maximum
865 length of C<$0>. In the most extreme case it may be limited to the
866 space occupied by the original C<$0>.
868 In some platforms there may be arbitrary amount of padding, for
869 example space characters, after the modified name as shown by C<ps>.
870 In some platforms this padding may extend all the way to the original
871 length of the argument area, no matter what you do (this is the case
872 for example with Linux 2.2).
874 Note for BSD users: setting C<$0> does not completely remove "perl"
875 from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
876 result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
877 and the " (perl)" suffix are shown depends on your exact BSD variant
878 and version). This is an operating system feature, Perl cannot help it.
880 In multithreaded scripts Perl coordinates the threads so that any
881 thread may modify its copy of the C<$0> and the change becomes visible
882 to ps(1) (assuming the operating system plays along). Note that
883 the view of C<$0> the other threads have will not change since they
884 have their own copies of it.
888 The index of the first element in an array, and of the first character
889 in a substring. Default is 0, but you could theoretically set it
890 to 1 to make Perl behave more like B<awk> (or Fortran) when
891 subscripting and when evaluating the index() and substr() functions.
892 (Mnemonic: [ begins subscripts.)
894 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
895 directive, and cannot influence the behavior of any other file.
896 (That's why you can only assign compile-time constants to it.)
897 Its use is highly discouraged.
899 Note that, unlike other compile-time directives (such as L<strict>),
900 assignment to C<$[> can be seen from outer lexical scopes in the same file.
901 However, you can use local() on it to strictly bind its value to a
906 The version + patchlevel / 1000 of the Perl interpreter. This variable
907 can be used to determine whether the Perl interpreter executing a
908 script is in the right range of versions. (Mnemonic: Is this version
909 of perl in the right bracket?) Example:
911 warn "No checksumming!\n" if $] < 3.019;
913 See also the documentation of C<use VERSION> and C<require VERSION>
914 for a convenient way to fail if the running Perl interpreter is too old.
916 When testing the variable, to steer clear of floating point
917 inaccuracies you might want to prefer the inequality tests C<< < >>
918 and C<< > >> to the tests containing equivalence: C<< <= >>, C<< == >>,
921 The floating point representation can sometimes lead to inaccurate
922 numeric comparisons. See C<$^V> for a more modern representation of
923 the Perl version that allows accurate string comparisons.
929 The current value of the flag associated with the B<-c> switch.
930 Mainly of use with B<-MO=...> to allow code to alter its behavior
931 when being compiled, such as for example to AUTOLOAD at compile
932 time rather than normal, deferred loading. See L<perlcc>. Setting
933 C<$^C = 1> is similar to calling C<B::minus_c>.
939 The current value of the debugging flags. (Mnemonic: value of B<-D>
940 switch.) May be read or set. Like its command-line equivalent, you can use
941 numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
947 The maximum system file descriptor, ordinarily 2. System file
948 descriptors are passed to exec()ed processes, while higher file
949 descriptors are not. Also, during an open(), system file descriptors are
950 preserved even if the open() fails. (Ordinary file descriptors are
951 closed before the open() is attempted.) The close-on-exec
952 status of a file descriptor will be decided according to the value of
953 C<$^F> when the corresponding file, pipe, or socket was opened, not the
958 WARNING: This variable is strictly for internal use only. Its availability,
959 behavior, and contents are subject to change without notice.
961 This variable contains compile-time hints for the Perl interpreter. At the
962 end of compilation of a BLOCK the value of this variable is restored to the
963 value when the interpreter started to compile the BLOCK.
965 When perl begins to parse any block construct that provides a lexical scope
966 (e.g., eval body, required file, subroutine body, loop body, or conditional
967 block), the existing value of $^H is saved, but its value is left unchanged.
968 When the compilation of the block is completed, it regains the saved value.
969 Between the points where its value is saved and restored, code that
970 executes within BEGIN blocks is free to change the value of $^H.
972 This behavior provides the semantic of lexical scoping, and is used in,
973 for instance, the C<use strict> pragma.
975 The contents should be an integer; different bits of it are used for
976 different pragmatic flags. Here's an example:
978 sub add_100 { $^H |= 0x100 }
985 Consider what happens during execution of the BEGIN block. At this point
986 the BEGIN block has already been compiled, but the body of foo() is still
987 being compiled. The new value of $^H will therefore be visible only while
988 the body of foo() is being compiled.
990 Substitution of the above BEGIN block with:
992 BEGIN { require strict; strict->import('vars') }
994 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
995 version of the same lexical pragma:
997 BEGIN { require strict; strict->import('vars') if $condition }
1001 WARNING: This variable is strictly for internal use only. Its availability,
1002 behavior, and contents are subject to change without notice.
1004 The %^H hash provides the same scoping semantic as $^H. This makes it
1005 useful for implementation of lexically scoped pragmas.
1011 The current value of the inplace-edit extension. Use C<undef> to disable
1012 inplace editing. (Mnemonic: value of B<-i> switch.)
1016 By default, running out of memory is an untrappable, fatal error.
1017 However, if suitably built, Perl can use the contents of C<$^M>
1018 as an emergency memory pool after die()ing. Suppose that your Perl
1019 were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
1022 $^M = 'a' x (1 << 16);
1024 would allocate a 64K buffer for use in an emergency. See the
1025 F<INSTALL> file in the Perl distribution for information on how to
1026 add custom C compilation flags when compiling perl. To discourage casual
1027 use of this advanced feature, there is no L<English|English> long name for
1034 The name of the operating system under which this copy of Perl was
1035 built, as determined during the configuration process. The value
1036 is identical to C<$Config{'osname'}>. See also L<Config> and the
1037 B<-V> command-line switch documented in L<perlrun>.
1039 In Windows platforms, $^O is not very helpful: since it is always
1040 C<MSWin32>, it doesn't tell the difference between
1041 95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1042 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1043 between the variants.
1047 An internal variable used by PerlIO. A string in two parts, separated
1048 by a C<\0> byte, the first part describes the input layers, the second
1049 part describes the output layers.
1055 The internal variable for debugging support. The meanings of the
1056 various bits are subject to change, but currently indicate:
1062 Debug subroutine enter/exit.
1066 Line-by-line debugging.
1070 Switch off optimizations.
1074 Preserve more data for future interactive inspections.
1078 Keep info about source lines on which a subroutine is defined.
1082 Start with single-step on.
1086 Use subroutine address instead of name when reporting.
1090 Report C<goto &subroutine> as well.
1094 Provide informative "file" names for evals based on the place they were compiled.
1098 Provide informative names to anonymous subroutines based on the place they
1103 Debug assertion subroutines enter/exit.
1107 Some bits may be relevant at compile-time only, some at
1108 run-time only. This is a new mechanism and the details may change.
1110 =item $LAST_REGEXP_CODE_RESULT
1114 The result of evaluation of the last successful C<(?{ code })>
1115 regular expression assertion (see L<perlre>). May be written to.
1117 =item $EXCEPTIONS_BEING_CAUGHT
1121 Current state of the interpreter.
1124 --------- -------------------
1125 undef Parsing module/eval
1126 true (1) Executing an eval
1129 The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
1135 The time at which the program began running, in seconds since the
1136 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
1137 and B<-C> filetests are based on this value.
1141 Reflects if taint mode is on or off. 1 for on (the program was run with
1142 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
1147 Reflects certain Unicode settings of Perl. See L<perlrun>
1148 documentation for the C<-C> switch for more information about
1149 the possible values. This variable is set during Perl startup
1150 and is thereafter read-only.
1152 =item ${^UTF8LOCALE}
1154 This variable indicates whether an UTF-8 locale was detected by perl at
1155 startup. This information is used by perl when it's in
1156 adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
1157 switch); see L<perlrun> for more info on this.
1163 The revision, version, and subversion of the Perl interpreter, represented
1164 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
1165 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1166 C<$^V eq v5.6.0>. Note that the characters in this string value can
1167 potentially be in Unicode range.
1169 This can be used to determine whether the Perl interpreter executing a
1170 script is in the right range of versions. (Mnemonic: use ^V for Version
1173 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
1175 To convert C<$^V> into its string representation use sprintf()'s
1176 C<"%vd"> conversion:
1178 printf "version is v%vd\n", $^V; # Perl's version
1180 See the documentation of C<use VERSION> and C<require VERSION>
1181 for a convenient way to fail if the running Perl interpreter is too old.
1183 See also C<$]> for an older representation of the Perl version.
1189 The current value of the warning switch, initially true if B<-w>
1190 was used, false otherwise, but directly modifiable. (Mnemonic:
1191 related to the B<-w> switch.) See also L<warnings>.
1193 =item ${^WARNING_BITS}
1195 The current set of warning checks enabled by the C<use warnings> pragma.
1196 See the documentation of C<warnings> for more details.
1198 =item $EXECUTABLE_NAME
1202 The name used to execute the current copy of Perl, from C's
1203 C<argv[0]> or (where supported) F</proc/self/exe>.
1205 Depending on the host operating system, the value of $^X may be
1206 a relative or absolute pathname of the perl program file, or may
1207 be the string used to invoke perl but not the pathname of the
1208 perl program file. Also, most operating systems permit invoking
1209 programs that are not in the PATH environment variable, so there
1210 is no guarantee that the value of $^X is in PATH. For VMS, the
1211 value may or may not include a version number.
1213 You usually can use the value of $^X to re-invoke an independent
1214 copy of the same perl that is currently running, e.g.,
1216 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1218 But recall that not all operating systems support forking or
1219 capturing of the output of commands, so this complex statement
1220 may not be portable.
1222 It is not safe to use the value of $^X as a path name of a file,
1223 as some operating systems that have a mandatory suffix on
1224 executable files do not require use of the suffix when invoking
1225 a command. To convert the value of $^X to a path name, use the
1226 following statements:
1228 # Build up a set of file names (not command names).
1232 {$this_perl .= $Config{_exe}
1233 unless $this_perl =~ m/$Config{_exe}$/i;}
1235 Because many operating systems permit anyone with read access to
1236 the Perl program file to make a copy of it, patch the copy, and
1237 then execute the copy, the security-conscious Perl programmer
1238 should take care to invoke the installed copy of perl, not the
1239 copy referenced by $^X. The following statements accomplish
1240 this goal, and produce a pathname that can be invoked as a
1241 command or referenced as a file.
1244 $secure_perl_path = $Config{perlpath};
1246 {$secure_perl_path .= $Config{_exe}
1247 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
1251 The special filehandle that iterates over command-line filenames in
1252 C<@ARGV>. Usually written as the null filehandle in the angle operator
1253 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1254 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1255 corresponding to the last file opened by C<< <> >>. In particular,
1256 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1257 may not cause your function to automatically read the contents of all the
1262 contains the name of the current file when reading from <>.
1266 The array @ARGV contains the command-line arguments intended for
1267 the script. C<$#ARGV> is generally the number of arguments minus
1268 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1269 command name itself. See C<$0> for the command name.
1273 The special filehandle that points to the currently open output file
1274 when doing edit-in-place processing with B<-i>. Useful when you have
1275 to do a lot of inserting and don't want to keep modifying $_. See
1276 L<perlrun> for the B<-i> switch.
1280 The array @F contains the fields of each line read in when autosplit
1281 mode is turned on. See L<perlrun> for the B<-a> switch. This array
1282 is package-specific, and must be declared or given a full package name
1283 if not in package main when running under C<strict 'vars'>.
1287 The array @INC contains the list of places that the C<do EXPR>,
1288 C<require>, or C<use> constructs look for their library files. It
1289 initially consists of the arguments to any B<-I> command-line
1290 switches, followed by the default Perl library, probably
1291 F</usr/local/lib/perl>, followed by ".", to represent the current
1292 directory. ("." will not be appended if taint checks are enabled, either by
1293 C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
1294 the C<use lib> pragma to get the machine-dependent library properly
1297 use lib '/mypath/libdir/';
1300 You can also insert hooks into the file inclusion system by putting Perl
1301 code directly into @INC. Those hooks may be subroutine references, array
1302 references or blessed objects. See L<perlfunc/require> for details.
1306 Within a subroutine the array @_ contains the parameters passed to that
1307 subroutine. See L<perlsub>.
1311 The hash %INC contains entries for each filename included via the
1312 C<do>, C<require>, or C<use> operators. The key is the filename
1313 you specified (with module names converted to pathnames), and the
1314 value is the location of the file found. The C<require>
1315 operator uses this hash to determine whether a particular file has
1316 already been included.
1318 If the file was loaded via a hook (e.g. a subroutine reference, see
1319 L<perlfunc/require> for a description of these hooks), this hook is
1320 by default inserted into %INC in place of a filename. Note, however,
1321 that the hook may have set the %INC entry by itself to provide some more
1328 The hash %ENV contains your current environment. Setting a
1329 value in C<ENV> changes the environment for any child processes
1330 you subsequently fork() off.
1336 The hash %SIG contains signal handlers for signals. For example:
1338 sub handler { # 1st argument is signal name
1340 print "Caught a SIG$sig--shutting down\n";
1345 $SIG{'INT'} = \&handler;
1346 $SIG{'QUIT'} = \&handler;
1348 $SIG{'INT'} = 'DEFAULT'; # restore default action
1349 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1351 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1352 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1355 Here are some other examples:
1357 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1358 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1359 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1360 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1362 Be sure not to use a bareword as the name of a signal handler,
1363 lest you inadvertently call it.
1365 If your system has the sigaction() function then signal handlers are
1366 installed using it. This means you get reliable signal handling.
1368 The default delivery policy of signals changed in Perl 5.8.0 from
1369 immediate (also known as "unsafe") to deferred, also known as
1370 "safe signals". See L<perlipc> for more information.
1372 Certain internal hooks can be also set using the %SIG hash. The
1373 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1374 about to be printed. The warning message is passed as the first
1375 argument. The presence of a __WARN__ hook causes the ordinary printing
1376 of warnings to STDERR to be suppressed. You can use this to save warnings
1377 in a variable, or turn warnings into fatal errors, like this:
1379 local $SIG{__WARN__} = sub { die $_[0] };
1382 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1383 is about to be thrown. The error message is passed as the first
1384 argument. When a __DIE__ hook routine returns, the exception
1385 processing continues as it would have in the absence of the hook,
1386 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
1387 The C<__DIE__> handler is explicitly disabled during the call, so that you
1388 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1390 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1391 even inside an eval(). Do not use this to rewrite a pending exception
1392 in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1393 This strange action at a distance may be fixed in a future release
1394 so that C<$SIG{__DIE__}> is only called if your program is about
1395 to exit, as was the original intent. Any other use is deprecated.
1397 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1398 they may be called to report (probable) errors found by the parser.
1399 In such a case the parser may be in inconsistent state, so any
1400 attempt to evaluate Perl code from such a handler will probably
1401 result in a segfault. This means that warnings or errors that
1402 result from parsing Perl should be used with extreme caution, like
1405 require Carp if defined $^S;
1406 Carp::confess("Something wrong") if defined &Carp::confess;
1407 die "Something wrong, but could not load Carp to give backtrace...
1408 To see backtrace try starting Perl with -MCarp switch";
1410 Here the first line will load Carp I<unless> it is the parser who
1411 called the handler. The second line will print backtrace and die if
1412 Carp was available. The third line will be executed only if Carp was
1415 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1416 L<warnings> for additional information.
1420 =head2 Error Indicators
1422 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1423 about different types of error conditions that may appear during
1424 execution of a Perl program. The variables are shown ordered by
1425 the "distance" between the subsystem which reported the error and
1426 the Perl process. They correspond to errors detected by the Perl
1427 interpreter, C library, operating system, or an external program,
1430 To illustrate the differences between these variables, consider the
1431 following Perl expression, which uses a single-quoted string:
1434 open my $pipe, "/cdrom/install |" or die $!;
1436 close $pipe or die "bad pipe: $?, $!";
1439 After execution of this statement all 4 variables may have been set.
1441 C<$@> is set if the string to be C<eval>-ed did not compile (this
1442 may happen if C<open> or C<close> were imported with bad prototypes),
1443 or if Perl code executed during evaluation die()d . In these cases
1444 the value of $@ is the compile error, or the argument to C<die>
1445 (which will interpolate C<$!> and C<$?>). (See also L<Fatal>,
1448 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1449 and C<close> are translated to calls in the C run-time library and
1450 thence to the operating system kernel. C<$!> is set to the C library's
1451 C<errno> if one of these calls fails.
1453 Under a few operating systems, C<$^E> may contain a more verbose
1454 error indicator, such as in this case, "CDROM tray not closed."
1455 Systems that do not support extended error messages leave C<$^E>
1458 Finally, C<$?> may be set to non-0 value if the external program
1459 F</cdrom/install> fails. The upper eight bits reflect specific
1460 error conditions encountered by the program (the program's exit()
1461 value). The lower eight bits reflect mode of failure, like signal
1462 death and core dump information See wait(2) for details. In
1463 contrast to C<$!> and C<$^E>, which are set only if error condition
1464 is detected, the variable C<$?> is set on each C<wait> or pipe
1465 C<close>, overwriting the old value. This is more like C<$@>, which
1466 on every eval() is always set on failure and cleared on success.
1468 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1471 =head2 Technical Note on the Syntax of Variable Names
1473 Variable names in Perl can have several formats. Usually, they
1474 must begin with a letter or underscore, in which case they can be
1475 arbitrarily long (up to an internal limit of 251 characters) and
1476 may contain letters, digits, underscores, or the special sequence
1477 C<::> or C<'>. In this case, the part before the last C<::> or
1478 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1480 Perl variable names may also be a sequence of digits or a single
1481 punctuation or control character. These names are all reserved for
1482 special uses by Perl; for example, the all-digits names are used
1483 to hold data captured by backreferences after a regular expression
1484 match. Perl has a special syntax for the single-control-character
1485 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1486 character. For example, the notation C<$^W> (dollar-sign caret
1487 C<W>) is the scalar variable whose name is the single character
1488 control-C<W>. This is better than typing a literal control-C<W>
1491 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1492 strings that begin with control characters (or better yet, a caret).
1493 These variables must be written in the form C<${^Foo}>; the braces
1494 are not optional. C<${^Foo}> denotes the scalar variable whose
1495 name is a control-C<F> followed by two C<o>'s. These variables are
1496 reserved for future special uses by Perl, except for the ones that
1497 begin with C<^_> (control-underscore or caret-underscore). No
1498 control-character name that begins with C<^_> will acquire a special
1499 meaning in any future version of Perl; such names may therefore be
1500 used safely in programs. C<$^_> itself, however, I<is> reserved.
1502 Perl identifiers that begin with digits, control characters, or
1503 punctuation characters are exempt from the effects of the C<package>
1504 declaration and are always forced to be in package C<main>; they are
1505 also exempt from C<strict 'vars'> errors. A few other names are also
1506 exempt in these ways:
1514 In particular, the new special C<${^_XYZ}> variables are always taken
1515 to be in package C<main>, regardless of any C<package> declarations
1520 Due to an unfortunate accident of Perl's implementation, C<use
1521 English> imposes a considerable performance penalty on all regular
1522 expression matches in a program, regardless of whether they occur
1523 in the scope of C<use English>. For that reason, saying C<use
1524 English> in libraries is strongly discouraged. See the
1525 Devel::SawAmpersand module documentation from CPAN
1526 ( http://www.cpan.org/modules/by-module/Devel/ )
1527 for more information.
1529 Having to even think about the C<$^S> variable in your exception
1530 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1531 invites grievous and difficult to track down errors. Avoid it
1532 and use an C<END{}> or CORE::GLOBAL::die override instead.