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 local(), use vars, or our()) even when using the strict
193 vars pragma. Don't lexicalize them with C<my $a> or C<my $b>
194 if you want to be able to use them in the sort() comparison block
203 Contains the subpattern from the corresponding set of capturing
204 parentheses from the last pattern match, not counting patterns
205 matched in nested blocks that have been exited already. (Mnemonic:
206 like \digits.) These variables are all read-only and dynamically
207 scoped to the current BLOCK.
213 The string matched by the last successful pattern match (not counting
214 any matches hidden within a BLOCK or eval() enclosed by the current
215 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
216 and dynamically scoped to the current BLOCK.
218 The use of this variable anywhere in a program imposes a considerable
219 performance penalty on all regular expression matches. See L</BUGS>.
225 The string preceding whatever was matched by the last successful
226 pattern match (not counting any matches hidden within a BLOCK or eval
227 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
228 string.) This variable is read-only.
230 The use of this variable anywhere in a program imposes a considerable
231 performance penalty on all regular expression matches. See L</BUGS>.
237 The string following whatever was matched by the last successful
238 pattern match (not counting any matches hidden within a BLOCK or eval()
239 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
242 local $_ = 'abcdefghi';
244 print "$`:$&:$'\n"; # prints abc:def:ghi
246 This variable is read-only and dynamically scoped to the current BLOCK.
248 The use of this variable anywhere in a program imposes a considerable
249 performance penalty on all regular expression matches. See L</BUGS>.
251 =item $LAST_PAREN_MATCH
255 The text matched by the last bracket of the last successful search pattern.
256 This is useful if you don't know which one of a set of alternative patterns
257 matched. For example:
259 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
261 (Mnemonic: be positive and forward looking.)
262 This variable is read-only and dynamically scoped to the current BLOCK.
266 The text matched by the used group most-recently closed (i.e. the group
267 with the rightmost closing parenthesis) of the last successful search
268 pattern. (Mnemonic: the (possibly) Nested parenthesis that most
271 This is primarily used inside C<(?{...})> blocks for examining text
272 recently matched. For example, to effectively capture text to a variable
273 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
275 (?:(...)(?{ $var = $^N }))
277 By setting and then using C<$var> in this way relieves you from having to
278 worry about exactly which numbered set of parentheses they are.
280 This variable is dynamically scoped to the current BLOCK.
282 =item @LAST_MATCH_END
286 This array holds the offsets of the ends of the last successful
287 submatches in the currently active dynamic scope. C<$+[0]> is
288 the offset into the string of the end of the entire match. This
289 is the same value as what the C<pos> function returns when called
290 on the variable that was matched against. The I<n>th element
291 of this array holds the offset of the I<n>th submatch, so
292 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
293 past where $2 ends, and so on. You can use C<$#+> to determine
294 how many subgroups were in the last successful match. See the
295 examples given for the C<@-> variable.
297 =item $MULTILINE_MATCHING
301 Set to a non-zero integer value to do multi-line matching within a
302 string, 0 (or undefined) to tell Perl that it can assume that strings
303 contain a single line, for the purpose of optimizing pattern matches.
304 Pattern matches on strings containing multiple newlines can produce
305 confusing results when C<$*> is 0 or undefined. Default is undefined.
306 (Mnemonic: * matches multiple things.) This variable influences the
307 interpretation of only C<^> and C<$>. A literal newline can be searched
308 for even when C<$* == 0>.
310 Use of C<$*> is deprecated in modern Perl, supplanted by
311 the C</s> and C</m> modifiers on pattern matching.
313 Assigning a non-numerical value to C<$*> triggers a warning (and makes
314 C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
315 makes that an implicit C<int> is applied on the value.
317 =item HANDLE->input_line_number(EXPR)
319 =item $INPUT_LINE_NUMBER
325 Current line number for the last filehandle accessed.
327 Each filehandle in Perl counts the number of lines that have been read
328 from it. (Depending on the value of C<$/>, Perl's idea of what
329 constitutes a line may not match yours.) When a line is read from a
330 filehandle (via readline() or C<< <> >>), or when tell() or seek() is
331 called on it, C<$.> becomes an alias to the line counter for that
334 You can adjust the counter by assigning to C<$.>, but this will not
335 actually move the seek pointer. I<Localizing C<$.> will not localize
336 the filehandle's line count>. Instead, it will localize perl's notion
337 of which filehandle C<$.> is currently aliased to.
339 C<$.> is reset when the filehandle is closed, but B<not> when an open
340 filehandle is reopened without an intervening close(). For more
341 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
342 an explicit close, line numbers increase across ARGV files (but see
343 examples in L<perlfunc/eof>).
345 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
346 line counter for a given filehandle without having to worry about
347 which handle you last accessed.
349 (Mnemonic: many programs use "." to mean the current line number.)
351 =item IO::Handle->input_record_separator(EXPR)
353 =item $INPUT_RECORD_SEPARATOR
359 The input record separator, newline by default. This
360 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
361 variable, including treating empty lines as a terminator if set to
362 the null string. (An empty line cannot contain any spaces
363 or tabs.) You may set it to a multi-character string to match a
364 multi-character terminator, or to C<undef> to read through the end
365 of file. Setting it to C<"\n\n"> means something slightly
366 different than setting to C<"">, if the file contains consecutive
367 empty lines. Setting to C<""> will treat two or more consecutive
368 empty lines as a single empty line. Setting to C<"\n\n"> will
369 blindly assume that the next input character belongs to the next
370 paragraph, even if it's a newline. (Mnemonic: / delimits
371 line boundaries when quoting poetry.)
373 local $/; # enable "slurp" mode
374 local $_ = <FH>; # whole file now here
377 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
378 better for something. :-)
380 Setting C<$/> to a reference to an integer, scalar containing an integer, or
381 scalar that's convertible to an integer will attempt to read records
382 instead of lines, with the maximum record size being the referenced
385 local $/ = \32768; # or \"32768", or \$var_containing_32768
386 open my $fh, $myfile or die $!;
389 will read a record of no more than 32768 bytes from FILE. If you're
390 not reading from a record-oriented file (or your OS doesn't have
391 record-oriented files), then you'll likely get a full chunk of data
392 with every read. If a record is larger than the record size you've
393 set, you'll get the record back in pieces.
395 On VMS, record reads are done with the equivalent of C<sysread>,
396 so it's best not to mix record and non-record reads on the same
397 file. (This is unlikely to be a problem, because any file you'd
398 want to read in record mode is probably unusable in line mode.)
399 Non-VMS systems do normal I/O, so it's safe to mix record and
400 non-record reads of a file.
402 See also L<perlport/"Newlines">. Also see C<$.>.
404 =item HANDLE->autoflush(EXPR)
406 =item $OUTPUT_AUTOFLUSH
410 If set to nonzero, forces a flush right away and after every write
411 or print on the currently selected output channel. Default is 0
412 (regardless of whether the channel is really buffered by the
413 system or not; C<$|> tells you only whether you've asked Perl
414 explicitly to flush after each write). STDOUT will
415 typically be line buffered if output is to the terminal and block
416 buffered otherwise. Setting this variable is useful primarily when
417 you are outputting to a pipe or socket, such as when you are running
418 a Perl program under B<rsh> and want to see the output as it's
419 happening. This has no effect on input buffering. See L<perlfunc/getc>
420 for that. (Mnemonic: when you want your pipes to be piping hot.)
422 =item IO::Handle->output_field_separator EXPR
424 =item $OUTPUT_FIELD_SEPARATOR
430 The output field separator for the print operator. Ordinarily the
431 print operator simply prints out its arguments without further
432 adornment. To get behavior more like B<awk>, set this variable as
433 you would set B<awk>'s OFS variable to specify what is printed
434 between fields. (Mnemonic: what is printed when there is a "," in
435 your print statement.)
437 =item IO::Handle->output_record_separator EXPR
439 =item $OUTPUT_RECORD_SEPARATOR
445 The output record separator for the print operator. Ordinarily the
446 print operator simply prints out its arguments as is, with no
447 trailing newline or other end-of-record string added. To get
448 behavior more like B<awk>, set this variable as you would set
449 B<awk>'s ORS variable to specify what is printed at the end of the
450 print. (Mnemonic: you set C<$\> instead of adding "\n" at the
451 end of the print. Also, it's just like C<$/>, but it's what you
452 get "back" from Perl.)
454 =item $LIST_SEPARATOR
458 This is like C<$,> except that it applies to array and slice values
459 interpolated into a double-quoted string (or similar interpreted
460 string). Default is a space. (Mnemonic: obvious, I think.)
462 =item $SUBSCRIPT_SEPARATOR
468 The subscript separator for multidimensional array emulation. If you
469 refer to a hash element as
475 $foo{join($;, $a, $b, $c)}
479 @foo{$a,$b,$c} # a slice--note the @
483 ($foo{$a},$foo{$b},$foo{$c})
485 Default is "\034", the same as SUBSEP in B<awk>. If your
486 keys contain binary data there might not be any safe value for C<$;>.
487 (Mnemonic: comma (the syntactic subscript separator) is a
488 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
489 taken for something more important.)
491 Consider using "real" multidimensional arrays as described
498 The output format for printed numbers. This variable is a half-hearted
499 attempt to emulate B<awk>'s OFMT variable. There are times, however,
500 when B<awk> and Perl have differing notions of what counts as
501 numeric. The initial value is "%.I<n>g", where I<n> is the value
502 of the macro DBL_DIG from your system's F<float.h>. This is different from
503 B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
504 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
506 Use of C<$#> is deprecated.
508 =item HANDLE->format_page_number(EXPR)
510 =item $FORMAT_PAGE_NUMBER
514 The current page number of the currently selected output channel.
516 (Mnemonic: % is page number in B<nroff>.)
518 =item HANDLE->format_lines_per_page(EXPR)
520 =item $FORMAT_LINES_PER_PAGE
524 The current page length (printable lines) of the currently selected
525 output channel. Default is 60.
527 (Mnemonic: = has horizontal lines.)
529 =item HANDLE->format_lines_left(EXPR)
531 =item $FORMAT_LINES_LEFT
535 The number of lines left on the page of the currently selected output
538 (Mnemonic: lines_on_page - lines_printed.)
540 =item @LAST_MATCH_START
544 $-[0] is the offset of the start of the last successful match.
545 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
546 I<n>-th subpattern, or undef if the subpattern did not match.
548 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
549 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
550 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
551 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
552 matched subgroup in the last successful match. Contrast with
553 C<$#+>, the number of subgroups in the regular expression. Compare
556 This array holds the offsets of the beginnings of the last
557 successful submatches in the currently active dynamic scope.
558 C<$-[0]> is the offset into the string of the beginning of the
559 entire match. The I<n>th element of this array holds the offset
560 of the I<n>th submatch, so C<$-[1]> is the offset where $1
561 begins, C<$-[2]> the offset where $2 begins, and so on.
563 After a match against some variable $var:
567 =item C<$`> is the same as C<substr($var, 0, $-[0])>
569 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
571 =item C<$'> is the same as C<substr($var, $+[0])>
573 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
575 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
577 =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
581 =item HANDLE->format_name(EXPR)
587 The name of the current report format for the currently selected output
588 channel. Default is the name of the filehandle. (Mnemonic: brother to
591 =item HANDLE->format_top_name(EXPR)
593 =item $FORMAT_TOP_NAME
597 The name of the current top-of-page format for the currently selected
598 output channel. Default is the name of the filehandle with _TOP
599 appended. (Mnemonic: points to top of page.)
601 =item IO::Handle->format_line_break_characters EXPR
603 =item $FORMAT_LINE_BREAK_CHARACTERS
607 The current set of characters after which a string may be broken to
608 fill continuation fields (starting with ^) in a format. Default is
609 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
610 poetry is a part of a line.)
612 =item IO::Handle->format_formfeed EXPR
614 =item $FORMAT_FORMFEED
618 What formats output as a form feed. Default is \f.
624 The current value of the write() accumulator for format() lines. A format
625 contains formline() calls that put their result into C<$^A>. After
626 calling its format, write() prints out the contents of C<$^A> and empties.
627 So you never really see the contents of C<$^A> unless you call
628 formline() yourself and then look at it. See L<perlform> and
629 L<perlfunc/formline()>.
635 The status returned by the last pipe close, backtick (C<``>) command,
636 successful call to wait() or waitpid(), or from the system()
637 operator. This is just the 16-bit status word returned by the
638 wait() system call (or else is made up to look like it). Thus, the
639 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
640 C<$? & 127> gives which signal, if any, the process died from, and
641 C<$? & 128> reports whether there was a core dump. (Mnemonic:
642 similar to B<sh> and B<ksh>.)
644 Additionally, if the C<h_errno> variable is supported in C, its value
645 is returned via $? if any C<gethost*()> function fails.
647 If you have installed a signal handler for C<SIGCHLD>, the
648 value of C<$?> will usually be wrong outside that handler.
650 Inside an C<END> subroutine C<$?> contains the value that is going to be
651 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
652 change the exit status of your program. For example:
655 $? = 1 if $? == 255; # die would make it 255
658 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
659 actual VMS exit status, instead of the default emulation of POSIX
660 status; see L<perlvms/$?> for details.
662 Also see L<Error Indicators>.
666 The I<object reference> to the Encode object that is used to convert
667 the source code to Unicode. Thanks to this variable your perl script
668 does not have to be written in UTF-8. Default is I<undef>. The direct
669 manipulation of this variable is highly discouraged. See L<encoding>
678 If used numerically, yields the current value of the C C<errno>
679 variable, or in other words, if a system or library call fails, it
680 sets this variable. This means that the value of C<$!> is meaningful
681 only I<immediately> after a B<failure>:
683 if (open(FH, $filename)) {
684 # Here $! is meaningless.
687 # ONLY here is $! meaningful.
689 # Already here $! might be meaningless.
691 # Since here we might have either success or failure,
692 # here $! is meaningless.
694 In the above I<meaningless> stands for anything: zero, non-zero,
695 C<undef>. A successful system or library call does B<not> set
696 the variable to zero.
698 If used an a string, yields the corresponding system error string.
699 You can assign a number to C<$!> to set I<errno> if, for instance,
700 you want C<"$!"> to return the string for error I<n>, or you want
701 to set the exit value for the die() operator. (Mnemonic: What just
704 Also see L<Error Indicators>.
708 Each element of C<%!> has a true value only if C<$!> is set to that
709 value. For example, C<$!{ENOENT}> is true if and only if the current
710 value of C<$!> is C<ENOENT>; that is, if the most recent error was
711 "No such file or directory" (or its moral equivalent: not all operating
712 systems give that exact error, and certainly not all languages).
713 To check if a particular key is meaningful on your system, use
714 C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
715 See L<Errno> for more information, and also see above for the
718 =item $EXTENDED_OS_ERROR
722 Error information specific to the current operating system. At
723 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
724 (and for MacPerl). On all other platforms, C<$^E> is always just
727 Under VMS, C<$^E> provides the VMS status value from the last
728 system error. This is more specific information about the last
729 system error than that provided by C<$!>. This is particularly
730 important when C<$!> is set to B<EVMSERR>.
732 Under OS/2, C<$^E> is set to the error code of the last call to
733 OS/2 API either via CRT, or directly from perl.
735 Under Win32, C<$^E> always returns the last error information
736 reported by the Win32 call C<GetLastError()> which describes
737 the last error from within the Win32 API. Most Win32-specific
738 code will report errors via C<$^E>. ANSI C and Unix-like calls
739 set C<errno> and so most portable Perl code will report errors
742 Caveats mentioned in the description of C<$!> generally apply to
743 C<$^E>, also. (Mnemonic: Extra error explanation.)
745 Also see L<Error Indicators>.
751 The Perl syntax error message from the last eval() operator.
752 If $@ is the null string, the last eval() parsed and executed
753 correctly (although the operations you invoked may have failed in the
754 normal fashion). (Mnemonic: Where was the syntax error "at"?)
756 Warning messages are not collected in this variable. You can,
757 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
760 Also see L<Error Indicators>.
768 The process number of the Perl running this script. You should
769 consider this variable read-only, although it will be altered
770 across fork() calls. (Mnemonic: same as shells.)
772 Note for Linux users: on Linux, the C functions C<getpid()> and
773 C<getppid()> return different values from different threads. In order to
774 be portable, this behavior is not reflected by C<$$>, whose value remains
775 consistent across threads. If you want to call the underlying C<getpid()>,
776 you may use the CPAN module C<Linux::Pid>.
784 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
785 if you're running setuid.) You can change both the real uid and
786 the effective uid at the same time by using POSIX::setuid().
788 =item $EFFECTIVE_USER_ID
794 The effective uid of this process. Example:
796 $< = $>; # set real to effective uid
797 ($<,$>) = ($>,$<); # swap real and effective uid
799 You can change both the effective uid and the real uid at the same
800 time by using POSIX::setuid().
802 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
803 C<< $< >> and C<< $> >> can be swapped only on machines
804 supporting setreuid().
812 The real gid of this process. If you are on a machine that supports
813 membership in multiple groups simultaneously, gives a space separated
814 list of groups you are in. The first number is the one returned by
815 getgid(), and the subsequent ones by getgroups(), one of which may be
816 the same as the first number.
818 However, a value assigned to C<$(> must be a single number used to
819 set the real gid. So the value given by C<$(> should I<not> be assigned
820 back to C<$(> without being forced numeric, such as by adding zero.
822 You can change both the real gid and the effective gid at the same
823 time by using POSIX::setgid().
825 (Mnemonic: parentheses are used to I<group> things. The real gid is the
826 group you I<left>, if you're running setgid.)
828 =item $EFFECTIVE_GROUP_ID
834 The effective gid of this process. If you are on a machine that
835 supports membership in multiple groups simultaneously, gives a space
836 separated list of groups you are in. The first number is the one
837 returned by getegid(), and the subsequent ones by getgroups(), one of
838 which may be the same as the first number.
840 Similarly, a value assigned to C<$)> must also be a space-separated
841 list of numbers. The first number sets the effective gid, and
842 the rest (if any) are passed to setgroups(). To get the effect of an
843 empty list for setgroups(), just repeat the new effective gid; that is,
844 to force an effective gid of 5 and an effectively empty setgroups()
845 list, say C< $) = "5 5" >.
847 You can change both the effective gid and the real gid at the same
848 time by using POSIX::setgid() (use only a single numeric argument).
850 (Mnemonic: parentheses are used to I<group> things. The effective gid
851 is the group that's I<right> for you, if you're running setgid.)
853 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
854 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
855 and C<$)> can be swapped only on machines supporting setregid().
861 Contains the name of the program being executed. On some operating
862 systems assigning to C<$0> modifies the argument area that the B<ps>
863 program sees. This is more useful as a way of indicating the current
864 program state than it is for hiding the program you're running.
865 (Mnemonic: same as B<sh> and B<ksh>.)
867 Note for BSD users: setting C<$0> does not completely remove "perl"
868 from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
869 result in C<"perl: foobar (perl)">. This is an operating system
874 The index of the first element in an array, and of the first character
875 in a substring. Default is 0, but you could theoretically set it
876 to 1 to make Perl behave more like B<awk> (or Fortran) when
877 subscripting and when evaluating the index() and substr() functions.
878 (Mnemonic: [ begins subscripts.)
880 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
881 directive, and cannot influence the behavior of any other file.
882 Its use is highly discouraged.
886 The version + patchlevel / 1000 of the Perl interpreter. This variable
887 can be used to determine whether the Perl interpreter executing a
888 script is in the right range of versions. (Mnemonic: Is this version
889 of perl in the right bracket?) Example:
891 warn "No checksumming!\n" if $] < 3.019;
893 See also the documentation of C<use VERSION> and C<require VERSION>
894 for a convenient way to fail if the running Perl interpreter is too old.
896 The floating point representation can sometimes lead to inaccurate
897 numeric comparisons. See C<$^V> for a more modern representation of
898 the Perl version that allows accurate string comparisons.
904 The current value of the flag associated with the B<-c> switch.
905 Mainly of use with B<-MO=...> to allow code to alter its behavior
906 when being compiled, such as for example to AUTOLOAD at compile
907 time rather than normal, deferred loading. See L<perlcc>. Setting
908 C<$^C = 1> is similar to calling C<B::minus_c>.
914 The current value of the debugging flags. (Mnemonic: value of B<-D>
921 The maximum system file descriptor, ordinarily 2. System file
922 descriptors are passed to exec()ed processes, while higher file
923 descriptors are not. Also, during an open(), system file descriptors are
924 preserved even if the open() fails. (Ordinary file descriptors are
925 closed before the open() is attempted.) The close-on-exec
926 status of a file descriptor will be decided according to the value of
927 C<$^F> when the corresponding file, pipe, or socket was opened, not the
932 WARNING: This variable is strictly for internal use only. Its availability,
933 behavior, and contents are subject to change without notice.
935 This variable contains compile-time hints for the Perl interpreter. At the
936 end of compilation of a BLOCK the value of this variable is restored to the
937 value when the interpreter started to compile the BLOCK.
939 When perl begins to parse any block construct that provides a lexical scope
940 (e.g., eval body, required file, subroutine body, loop body, or conditional
941 block), the existing value of $^H is saved, but its value is left unchanged.
942 When the compilation of the block is completed, it regains the saved value.
943 Between the points where its value is saved and restored, code that
944 executes within BEGIN blocks is free to change the value of $^H.
946 This behavior provides the semantic of lexical scoping, and is used in,
947 for instance, the C<use strict> pragma.
949 The contents should be an integer; different bits of it are used for
950 different pragmatic flags. Here's an example:
952 sub add_100 { $^H |= 0x100 }
959 Consider what happens during execution of the BEGIN block. At this point
960 the BEGIN block has already been compiled, but the body of foo() is still
961 being compiled. The new value of $^H will therefore be visible only while
962 the body of foo() is being compiled.
964 Substitution of the above BEGIN block with:
966 BEGIN { require strict; strict->import('vars') }
968 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
969 version of the same lexical pragma:
971 BEGIN { require strict; strict->import('vars') if $condition }
975 WARNING: This variable is strictly for internal use only. Its availability,
976 behavior, and contents are subject to change without notice.
978 The %^H hash provides the same scoping semantic as $^H. This makes it
979 useful for implementation of lexically scoped pragmas.
985 The current value of the inplace-edit extension. Use C<undef> to disable
986 inplace editing. (Mnemonic: value of B<-i> switch.)
990 By default, running out of memory is an untrappable, fatal error.
991 However, if suitably built, Perl can use the contents of C<$^M>
992 as an emergency memory pool after die()ing. Suppose that your Perl
993 were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
996 $^M = 'a' x (1 << 16);
998 would allocate a 64K buffer for use in an emergency. See the
999 F<INSTALL> file in the Perl distribution for information on how to
1000 enable this option. To discourage casual use of this advanced
1001 feature, there is no L<English|English> long name for this variable.
1007 The name of the operating system under which this copy of Perl was
1008 built, as determined during the configuration process. The value
1009 is identical to C<$Config{'osname'}>. See also L<Config> and the
1010 B<-V> command-line switch documented in L<perlrun>.
1012 In Windows platforms, $^O is not very helpful: since it is always
1013 C<MSWin32>, it doesn't tell the difference between
1014 95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1015 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1016 between the variants.
1020 An internal variable used by PerlIO. A string in two parts, separated
1021 by a C<\0> byte, the first part describes the input layers, the second
1022 part describes the output layers.
1028 The internal variable for debugging support. The meanings of the
1029 various bits are subject to change, but currently indicate:
1035 Debug subroutine enter/exit.
1039 Line-by-line debugging.
1043 Switch off optimizations.
1047 Preserve more data for future interactive inspections.
1051 Keep info about source lines on which a subroutine is defined.
1055 Start with single-step on.
1059 Use subroutine address instead of name when reporting.
1063 Report C<goto &subroutine> as well.
1067 Provide informative "file" names for evals based on the place they were compiled.
1071 Provide informative names to anonymous subroutines based on the place they
1076 Some bits may be relevant at compile-time only, some at
1077 run-time only. This is a new mechanism and the details may change.
1079 =item $LAST_REGEXP_CODE_RESULT
1083 The result of evaluation of the last successful C<(?{ code })>
1084 regular expression assertion (see L<perlre>). May be written to.
1086 =item $EXCEPTIONS_BEING_CAUGHT
1090 Current state of the interpreter. Undefined if parsing of the current
1091 module/eval is not finished (may happen in $SIG{__DIE__} and
1092 $SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
1098 The time at which the program began running, in seconds since the
1099 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
1100 and B<-C> filetests are based on this value.
1104 Reflects if taint mode is on or off (i.e. if the program was run with
1105 B<-T> or not). True for on, false for off.
1111 The revision, version, and subversion of the Perl interpreter, represented
1112 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
1113 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1114 C<$^V eq v5.6.0>. Note that the characters in this string value can
1115 potentially be in Unicode range.
1117 This can be used to determine whether the Perl interpreter executing a
1118 script is in the right range of versions. (Mnemonic: use ^V for Version
1121 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
1123 To convert C<$^V> into its string representation use sprintf()'s
1124 C<"%vd"> conversion:
1126 printf "version is v%vd\n", $^V; # Perl's version
1128 See the documentation of C<use VERSION> and C<require VERSION>
1129 for a convenient way to fail if the running Perl interpreter is too old.
1131 See also C<$]> for an older representation of the Perl version.
1137 The current value of the warning switch, initially true if B<-w>
1138 was used, false otherwise, but directly modifiable. (Mnemonic:
1139 related to the B<-w> switch.) See also L<warnings>.
1141 =item ${^WARNING_BITS}
1143 The current set of warning checks enabled by the C<use warnings> pragma.
1144 See the documentation of C<warnings> for more details.
1146 =item ${^WIDE_SYSTEM_CALLS}
1148 Global flag that enables system calls made by Perl to use wide character
1149 APIs native to the system, if available. This is currently only implemented
1150 on the Windows platform.
1152 This can also be enabled from the command line using the C<-C> switch.
1154 The initial value is typically C<0> for compatibility with Perl versions
1155 earlier than 5.6, but may be automatically set to C<1> by Perl if the system
1156 provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
1158 The C<bytes> pragma always overrides the effect of this flag in the current
1159 lexical scope. See L<bytes>.
1161 =item $EXECUTABLE_NAME
1165 The name used to execute the current copy of Perl, from C's
1168 Depending on the host operating system, the value of $^X may be
1169 a relative or absolute pathname of the perl program file, or may
1170 be the string used to invoke perl but not the pathname of the
1171 perl program file. Also, most operating systems permit invoking
1172 programs that are not in the PATH environment variable, so there
1173 is no guarantee that the value of $^X is in PATH. For VMS, the
1174 value may or may not include a version number.
1176 You usually can use the value of $^X to re-invoke an independent
1177 copy of the same perl that is currently running, e.g.,
1179 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1181 But recall that not all operating systems support forking or
1182 capturing of the output of commands, so this complex statement
1183 may not be portable.
1185 It is not safe to use the value of $^X as a path name of a file,
1186 as some operating systems that have a mandatory suffix on
1187 executable files do not require use of the suffix when invoking
1188 a command. To convert the value of $^X to a path name, use the
1189 following statements:
1191 # Build up a set of file names (not command names).
1195 {$this_perl .= $Config{_exe}
1196 unless $this_perl =~ m/$Config{_exe}$/i;}
1198 Because many operating systems permit anyone with read access to
1199 the Perl program file to make a copy of it, patch the copy, and
1200 then execute the copy, the security-conscious Perl programmer
1201 should take care to invoke the installed copy of perl, not the
1202 copy referenced by $^X. The following statements accomplish
1203 this goal, and produce a pathname that can be invoked as a
1204 command or referenced as a file.
1207 $secure_perl_path = $Config{perlpath};
1209 {$secure_perl_path .= $Config{_exe}
1210 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
1214 The special filehandle that iterates over command-line filenames in
1215 C<@ARGV>. Usually written as the null filehandle in the angle operator
1216 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1217 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1218 corresponding to the last file opened by C<< <> >>. In particular,
1219 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1220 may not cause your function to automatically read the contents of all the
1225 contains the name of the current file when reading from <>.
1229 The array @ARGV contains the command-line arguments intended for
1230 the script. C<$#ARGV> is generally the number of arguments minus
1231 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1232 command name itself. See C<$0> for the command name.
1236 The special filehandle that points to the currently open output file
1237 when doing edit-in-place processing with B<-i>. Useful when you have
1238 to do a lot of inserting and don't want to keep modifying $_. See
1239 L<perlrun> for the B<-i> switch.
1243 The array @F contains the fields of each line read in when autosplit
1244 mode is turned on. See L<perlrun> for the B<-a> switch. This array
1245 is package-specific, and must be declared or given a full package name
1246 if not in package main when running under C<strict 'vars'>.
1250 The array @INC contains the list of places that the C<do EXPR>,
1251 C<require>, or C<use> constructs look for their library files. It
1252 initially consists of the arguments to any B<-I> command-line
1253 switches, followed by the default Perl library, probably
1254 F</usr/local/lib/perl>, followed by ".", to represent the current
1255 directory. ("." will not be appended if taint checks are enabled, either by
1256 C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
1257 the C<use lib> pragma to get the machine-dependent library properly
1260 use lib '/mypath/libdir/';
1263 You can also insert hooks into the file inclusion system by putting Perl
1264 code directly into @INC. Those hooks may be subroutine references, array
1265 references or blessed objects. See L<perlfunc/require> for details.
1269 Within a subroutine the array @_ contains the parameters passed to that
1270 subroutine. See L<perlsub>.
1274 The hash %INC contains entries for each filename included via the
1275 C<do>, C<require>, or C<use> operators. The key is the filename
1276 you specified (with module names converted to pathnames), and the
1277 value is the location of the file found. The C<require>
1278 operator uses this hash to determine whether a particular file has
1279 already been included.
1281 If the file was loaded via a hook (e.g. a subroutine reference, see
1282 L<perlfunc/require> for a description of these hooks), this hook is
1283 by default inserted into %INC in place of a filename. Note, however,
1284 that the hook may have set the %INC entry by itself to provide some more
1291 The hash %ENV contains your current environment. Setting a
1292 value in C<ENV> changes the environment for any child processes
1293 you subsequently fork() off.
1299 The hash %SIG contains signal handlers for signals. For example:
1301 sub handler { # 1st argument is signal name
1303 print "Caught a SIG$sig--shutting down\n";
1308 $SIG{'INT'} = \&handler;
1309 $SIG{'QUIT'} = \&handler;
1311 $SIG{'INT'} = 'DEFAULT'; # restore default action
1312 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1314 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1315 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1318 Here are some other examples:
1320 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1321 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1322 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1323 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1325 Be sure not to use a bareword as the name of a signal handler,
1326 lest you inadvertently call it.
1328 If your system has the sigaction() function then signal handlers are
1329 installed using it. This means you get reliable signal handling. If
1330 your system has the SA_RESTART flag it is used when signals handlers are
1331 installed. This means that system calls for which restarting is supported
1332 continue rather than returning when a signal arrives. If you want your
1333 system calls to be interrupted by signal delivery then do something like
1336 use POSIX ':signal_h';
1339 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1340 or die "Error setting SIGALRM handler: $!\n";
1344 Certain internal hooks can be also set using the %SIG hash. The
1345 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1346 about to be printed. The warning message is passed as the first
1347 argument. The presence of a __WARN__ hook causes the ordinary printing
1348 of warnings to STDERR to be suppressed. You can use this to save warnings
1349 in a variable, or turn warnings into fatal errors, like this:
1351 local $SIG{__WARN__} = sub { die $_[0] };
1354 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1355 is about to be thrown. The error message is passed as the first
1356 argument. When a __DIE__ hook routine returns, the exception
1357 processing continues as it would have in the absence of the hook,
1358 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
1359 The C<__DIE__> handler is explicitly disabled during the call, so that you
1360 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1362 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1363 even inside an eval(). Do not use this to rewrite a pending exception
1364 in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1365 This strange action at a distance may be fixed in a future release
1366 so that C<$SIG{__DIE__}> is only called if your program is about
1367 to exit, as was the original intent. Any other use is deprecated.
1369 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1370 they may be called to report (probable) errors found by the parser.
1371 In such a case the parser may be in inconsistent state, so any
1372 attempt to evaluate Perl code from such a handler will probably
1373 result in a segfault. This means that warnings or errors that
1374 result from parsing Perl should be used with extreme caution, like
1377 require Carp if defined $^S;
1378 Carp::confess("Something wrong") if defined &Carp::confess;
1379 die "Something wrong, but could not load Carp to give backtrace...
1380 To see backtrace try starting Perl with -MCarp switch";
1382 Here the first line will load Carp I<unless> it is the parser who
1383 called the handler. The second line will print backtrace and die if
1384 Carp was available. The third line will be executed only if Carp was
1387 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1388 L<warnings> for additional information.
1392 =head2 Error Indicators
1394 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1395 about different types of error conditions that may appear during
1396 execution of a Perl program. The variables are shown ordered by
1397 the "distance" between the subsystem which reported the error and
1398 the Perl process. They correspond to errors detected by the Perl
1399 interpreter, C library, operating system, or an external program,
1402 To illustrate the differences between these variables, consider the
1403 following Perl expression, which uses a single-quoted string:
1406 open my $pipe, "/cdrom/install |" or die $!;
1408 close $pipe or die "bad pipe: $?, $!";
1411 After execution of this statement all 4 variables may have been set.
1413 C<$@> is set if the string to be C<eval>-ed did not compile (this
1414 may happen if C<open> or C<close> were imported with bad prototypes),
1415 or if Perl code executed during evaluation die()d . In these cases
1416 the value of $@ is the compile error, or the argument to C<die>
1417 (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1420 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1421 and C<close> are translated to calls in the C run-time library and
1422 thence to the operating system kernel. C<$!> is set to the C library's
1423 C<errno> if one of these calls fails.
1425 Under a few operating systems, C<$^E> may contain a more verbose
1426 error indicator, such as in this case, "CDROM tray not closed."
1427 Systems that do not support extended error messages leave C<$^E>
1430 Finally, C<$?> may be set to non-0 value if the external program
1431 F</cdrom/install> fails. The upper eight bits reflect specific
1432 error conditions encountered by the program (the program's exit()
1433 value). The lower eight bits reflect mode of failure, like signal
1434 death and core dump information See wait(2) for details. In
1435 contrast to C<$!> and C<$^E>, which are set only if error condition
1436 is detected, the variable C<$?> is set on each C<wait> or pipe
1437 C<close>, overwriting the old value. This is more like C<$@>, which
1438 on every eval() is always set on failure and cleared on success.
1440 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1443 =head2 Technical Note on the Syntax of Variable Names
1445 Variable names in Perl can have several formats. Usually, they
1446 must begin with a letter or underscore, in which case they can be
1447 arbitrarily long (up to an internal limit of 251 characters) and
1448 may contain letters, digits, underscores, or the special sequence
1449 C<::> or C<'>. In this case, the part before the last C<::> or
1450 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1452 Perl variable names may also be a sequence of digits or a single
1453 punctuation or control character. These names are all reserved for
1454 special uses by Perl; for example, the all-digits names are used
1455 to hold data captured by backreferences after a regular expression
1456 match. Perl has a special syntax for the single-control-character
1457 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1458 character. For example, the notation C<$^W> (dollar-sign caret
1459 C<W>) is the scalar variable whose name is the single character
1460 control-C<W>. This is better than typing a literal control-C<W>
1463 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1464 strings that begin with control characters (or better yet, a caret).
1465 These variables must be written in the form C<${^Foo}>; the braces
1466 are not optional. C<${^Foo}> denotes the scalar variable whose
1467 name is a control-C<F> followed by two C<o>'s. These variables are
1468 reserved for future special uses by Perl, except for the ones that
1469 begin with C<^_> (control-underscore or caret-underscore). No
1470 control-character name that begins with C<^_> will acquire a special
1471 meaning in any future version of Perl; such names may therefore be
1472 used safely in programs. C<$^_> itself, however, I<is> reserved.
1474 Perl identifiers that begin with digits, control characters, or
1475 punctuation characters are exempt from the effects of the C<package>
1476 declaration and are always forced to be in package C<main>; they are
1477 also exempt from C<strict 'vars'> errors. A few other names are also
1478 exempt in these ways:
1486 In particular, the new special C<${^_XYZ}> variables are always taken
1487 to be in package C<main>, regardless of any C<package> declarations
1492 Due to an unfortunate accident of Perl's implementation, C<use
1493 English> imposes a considerable performance penalty on all regular
1494 expression matches in a program, regardless of whether they occur
1495 in the scope of C<use English>. For that reason, saying C<use
1496 English> in libraries is strongly discouraged. See the
1497 Devel::SawAmpersand module documentation from CPAN
1498 ( http://www.cpan.org/modules/by-module/Devel/ )
1499 for more information.
1501 Having to even think about the C<$^S> variable in your exception
1502 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1503 invites grievous and difficult to track down errors. Avoid it
1504 and use an C<END{}> or CORE::GLOBAL::die override instead.