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 $_ even if you
150 Various unary functions, including functions like ord() and int(), as well
151 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
156 Various list functions like print() and unlink().
160 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
161 without an C<=~> operator.
165 The default iterator variable in a C<foreach> loop if no other
166 variable is supplied.
170 The implicit iterator variable in the grep() and map() functions.
174 The default place to put an input record when a C<< <FH> >>
175 operation's result is tested by itself as the sole criterion of a C<while>
176 test. Outside a C<while> test, this will not happen.
180 As C<$_> is a global variable, this may lead in some cases to unwanted
181 side-effects. As of perl 5.9.1, you can now use a lexical version of
182 C<$_> by declaring it in a file or in a block with C<my>. Moreover,
183 declaring C<our $> restores the global C<$_> in the current scope.
185 (Mnemonic: underline is understood in certain operations.)
195 Special package variables when using sort(), see L<perlfunc/sort>.
196 Because of this specialness $a and $b don't need to be declared
197 (using use vars, or our()) even when using the C<strict 'vars'> pragma.
198 Don't lexicalize them with C<my $a> or C<my $b> if you want to be
199 able to use them in the sort() comparison block or function.
207 Contains the subpattern from the corresponding set of capturing
208 parentheses from the last pattern match, not counting patterns
209 matched in nested blocks that have been exited already. (Mnemonic:
210 like \digits.) These variables are all read-only and dynamically
211 scoped to the current BLOCK.
217 The string matched by the last successful pattern match (not counting
218 any matches hidden within a BLOCK or eval() enclosed by the current
219 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
220 and dynamically scoped to the current BLOCK.
222 The use of this variable anywhere in a program imposes a considerable
223 performance penalty on all regular expression matches. See L</BUGS>.
229 The string preceding whatever was matched by the last successful
230 pattern match (not counting any matches hidden within a BLOCK or eval
231 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
232 string.) This variable is read-only.
234 The use of this variable anywhere in a program imposes a considerable
235 performance penalty on all regular expression matches. See L</BUGS>.
241 The string following whatever was matched by the last successful
242 pattern match (not counting any matches hidden within a BLOCK or eval()
243 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
246 local $_ = 'abcdefghi';
248 print "$`:$&:$'\n"; # prints abc:def:ghi
250 This variable is read-only and dynamically scoped to the current BLOCK.
252 The use of this variable anywhere in a program imposes a considerable
253 performance penalty on all regular expression matches. See L</BUGS>.
255 =item $LAST_PAREN_MATCH
259 The text matched by the last bracket of the last successful search pattern.
260 This is useful if you don't know which one of a set of alternative patterns
261 matched. For example:
263 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
265 (Mnemonic: be positive and forward looking.)
266 This variable is read-only and dynamically scoped to the current BLOCK.
270 The text matched by the used group most-recently closed (i.e. the group
271 with the rightmost closing parenthesis) of the last successful search
272 pattern. (Mnemonic: the (possibly) Nested parenthesis that most
275 This is primarily used inside C<(?{...})> blocks for examining text
276 recently matched. For example, to effectively capture text to a variable
277 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
279 (?:(...)(?{ $var = $^N }))
281 By setting and then using C<$var> in this way relieves you from having to
282 worry about exactly which numbered set of parentheses they are.
284 This variable is dynamically scoped to the current BLOCK.
286 =item @LAST_MATCH_END
290 This array holds the offsets of the ends of the last successful
291 submatches in the currently active dynamic scope. C<$+[0]> is
292 the offset into the string of the end of the entire match. This
293 is the same value as what the C<pos> function returns when called
294 on the variable that was matched against. The I<n>th element
295 of this array holds the offset of the I<n>th submatch, so
296 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
297 past where $2 ends, and so on. You can use C<$#+> to determine
298 how many subgroups were in the last successful match. See the
299 examples given for the C<@-> variable.
301 =item HANDLE->input_line_number(EXPR)
303 =item $INPUT_LINE_NUMBER
309 Current line number for the last filehandle accessed.
311 Each filehandle in Perl counts the number of lines that have been read
312 from it. (Depending on the value of C<$/>, Perl's idea of what
313 constitutes a line may not match yours.) When a line is read from a
314 filehandle (via readline() or C<< <> >>), or when tell() or seek() is
315 called on it, C<$.> becomes an alias to the line counter for that
318 You can adjust the counter by assigning to C<$.>, but this will not
319 actually move the seek pointer. I<Localizing C<$.> will not localize
320 the filehandle's line count>. Instead, it will localize perl's notion
321 of which filehandle C<$.> is currently aliased to.
323 C<$.> is reset when the filehandle is closed, but B<not> when an open
324 filehandle is reopened without an intervening close(). For more
325 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
326 an explicit close, line numbers increase across ARGV files (but see
327 examples in L<perlfunc/eof>).
329 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
330 line counter for a given filehandle without having to worry about
331 which handle you last accessed.
333 (Mnemonic: many programs use "." to mean the current line number.)
335 =item IO::Handle->input_record_separator(EXPR)
337 =item $INPUT_RECORD_SEPARATOR
343 The input record separator, newline by default. This
344 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
345 variable, including treating empty lines as a terminator if set to
346 the null string. (An empty line cannot contain any spaces
347 or tabs.) You may set it to a multi-character string to match a
348 multi-character terminator, or to C<undef> to read through the end
349 of file. Setting it to C<"\n\n"> means something slightly
350 different than setting to C<"">, if the file contains consecutive
351 empty lines. Setting to C<""> will treat two or more consecutive
352 empty lines as a single empty line. Setting to C<"\n\n"> will
353 blindly assume that the next input character belongs to the next
354 paragraph, even if it's a newline. (Mnemonic: / delimits
355 line boundaries when quoting poetry.)
357 local $/; # enable "slurp" mode
358 local $_ = <FH>; # whole file now here
361 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
362 better for something. :-)
364 Setting C<$/> to a reference to an integer, scalar containing an integer, or
365 scalar that's convertible to an integer will attempt to read records
366 instead of lines, with the maximum record size being the referenced
369 local $/ = \32768; # or \"32768", or \$var_containing_32768
370 open my $fh, $myfile or die $!;
373 will read a record of no more than 32768 bytes from FILE. If you're
374 not reading from a record-oriented file (or your OS doesn't have
375 record-oriented files), then you'll likely get a full chunk of data
376 with every read. If a record is larger than the record size you've
377 set, you'll get the record back in pieces.
379 On VMS, record reads are done with the equivalent of C<sysread>,
380 so it's best not to mix record and non-record reads on the same
381 file. (This is unlikely to be a problem, because any file you'd
382 want to read in record mode is probably unusable in line mode.)
383 Non-VMS systems do normal I/O, so it's safe to mix record and
384 non-record reads of a file.
386 See also L<perlport/"Newlines">. Also see C<$.>.
388 =item HANDLE->autoflush(EXPR)
390 =item $OUTPUT_AUTOFLUSH
394 If set to nonzero, forces a flush right away and after every write
395 or print on the currently selected output channel. Default is 0
396 (regardless of whether the channel is really buffered by the
397 system or not; C<$|> tells you only whether you've asked Perl
398 explicitly to flush after each write). STDOUT will
399 typically be line buffered if output is to the terminal and block
400 buffered otherwise. Setting this variable is useful primarily when
401 you are outputting to a pipe or socket, such as when you are running
402 a Perl program under B<rsh> and want to see the output as it's
403 happening. This has no effect on input buffering. See L<perlfunc/getc>
404 for that. (Mnemonic: when you want your pipes to be piping hot.)
406 =item IO::Handle->output_field_separator EXPR
408 =item $OUTPUT_FIELD_SEPARATOR
414 The output field separator for the print operator. Ordinarily the
415 print operator simply prints out its arguments without further
416 adornment. To get behavior more like B<awk>, set this variable as
417 you would set B<awk>'s OFS variable to specify what is printed
418 between fields. (Mnemonic: what is printed when there is a "," in
419 your print statement.)
421 =item IO::Handle->output_record_separator EXPR
423 =item $OUTPUT_RECORD_SEPARATOR
429 The output record separator for the print operator. Ordinarily the
430 print operator simply prints out its arguments as is, with no
431 trailing newline or other end-of-record string added. To get
432 behavior more like B<awk>, set this variable as you would set
433 B<awk>'s ORS variable to specify what is printed at the end of the
434 print. (Mnemonic: you set C<$\> instead of adding "\n" at the
435 end of the print. Also, it's just like C<$/>, but it's what you
436 get "back" from Perl.)
438 =item $LIST_SEPARATOR
442 This is like C<$,> except that it applies to array and slice values
443 interpolated into a double-quoted string (or similar interpreted
444 string). Default is a space. (Mnemonic: obvious, I think.)
446 =item $SUBSCRIPT_SEPARATOR
452 The subscript separator for multidimensional array emulation. If you
453 refer to a hash element as
459 $foo{join($;, $a, $b, $c)}
463 @foo{$a,$b,$c} # a slice--note the @
467 ($foo{$a},$foo{$b},$foo{$c})
469 Default is "\034", the same as SUBSEP in B<awk>. If your
470 keys contain binary data there might not be any safe value for C<$;>.
471 (Mnemonic: comma (the syntactic subscript separator) is a
472 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
473 taken for something more important.)
475 Consider using "real" multidimensional arrays as described
480 The output format for printed numbers. This variable is a half-hearted
481 attempt to emulate B<awk>'s OFMT variable. There are times, however,
482 when B<awk> and Perl have differing notions of what counts as
483 numeric. The initial value is "%.I<n>g", where I<n> is the value
484 of the macro DBL_DIG from your system's F<float.h>. This is different from
485 B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
486 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
488 Use of C<$#> is deprecated.
490 =item HANDLE->format_page_number(EXPR)
492 =item $FORMAT_PAGE_NUMBER
496 The current page number of the currently selected output channel.
498 (Mnemonic: % is page number in B<nroff>.)
500 =item HANDLE->format_lines_per_page(EXPR)
502 =item $FORMAT_LINES_PER_PAGE
506 The current page length (printable lines) of the currently selected
507 output channel. Default is 60.
509 (Mnemonic: = has horizontal lines.)
511 =item HANDLE->format_lines_left(EXPR)
513 =item $FORMAT_LINES_LEFT
517 The number of lines left on the page of the currently selected output
520 (Mnemonic: lines_on_page - lines_printed.)
522 =item @LAST_MATCH_START
526 $-[0] is the offset of the start of the last successful match.
527 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
528 I<n>-th subpattern, or undef if the subpattern did not match.
530 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
531 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
532 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
533 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
534 matched subgroup in the last successful match. Contrast with
535 C<$#+>, the number of subgroups in the regular expression. Compare
538 This array holds the offsets of the beginnings of the last
539 successful submatches in the currently active dynamic scope.
540 C<$-[0]> is the offset into the string of the beginning of the
541 entire match. The I<n>th element of this array holds the offset
542 of the I<n>th submatch, so C<$-[1]> is the offset where $1
543 begins, C<$-[2]> the offset where $2 begins, and so on.
545 After a match against some variable $var:
549 =item C<$`> is the same as C<substr($var, 0, $-[0])>
551 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
553 =item C<$'> is the same as C<substr($var, $+[0])>
555 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
557 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
559 =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
563 =item HANDLE->format_name(EXPR)
569 The name of the current report format for the currently selected output
570 channel. Default is the name of the filehandle. (Mnemonic: brother to
573 =item HANDLE->format_top_name(EXPR)
575 =item $FORMAT_TOP_NAME
579 The name of the current top-of-page format for the currently selected
580 output channel. Default is the name of the filehandle with _TOP
581 appended. (Mnemonic: points to top of page.)
583 =item IO::Handle->format_line_break_characters EXPR
585 =item $FORMAT_LINE_BREAK_CHARACTERS
589 The current set of characters after which a string may be broken to
590 fill continuation fields (starting with ^) in a format. Default is
591 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
592 poetry is a part of a line.)
594 =item IO::Handle->format_formfeed EXPR
596 =item $FORMAT_FORMFEED
600 What formats output as a form feed. Default is \f.
606 The current value of the write() accumulator for format() lines. A format
607 contains formline() calls that put their result into C<$^A>. After
608 calling its format, write() prints out the contents of C<$^A> and empties.
609 So you never really see the contents of C<$^A> unless you call
610 formline() yourself and then look at it. See L<perlform> and
611 L<perlfunc/formline()>.
617 The status returned by the last pipe close, backtick (C<``>) command,
618 successful call to wait() or waitpid(), or from the system()
619 operator. This is just the 16-bit status word returned by the
620 wait() system call (or else is made up to look like it). Thus, the
621 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
622 C<$? & 127> gives which signal, if any, the process died from, and
623 C<$? & 128> reports whether there was a core dump. (Mnemonic:
624 similar to B<sh> and B<ksh>.)
626 Additionally, if the C<h_errno> variable is supported in C, its value
627 is returned via $? if any C<gethost*()> function fails.
629 If you have installed a signal handler for C<SIGCHLD>, the
630 value of C<$?> will usually be wrong outside that handler.
632 Inside an C<END> subroutine C<$?> contains the value that is going to be
633 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
634 change the exit status of your program. For example:
637 $? = 1 if $? == 255; # die would make it 255
640 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
641 actual VMS exit status, instead of the default emulation of POSIX
642 status; see L<perlvms/$?> for details.
644 Also see L<Error Indicators>.
648 The I<object reference> to the Encode object that is used to convert
649 the source code to Unicode. Thanks to this variable your perl script
650 does not have to be written in UTF-8. Default is I<undef>. The direct
651 manipulation of this variable is highly discouraged. See L<encoding>
660 If used numerically, yields the current value of the C C<errno>
661 variable, or in other words, if a system or library call fails, it
662 sets this variable. This means that the value of C<$!> is meaningful
663 only I<immediately> after a B<failure>:
665 if (open(FH, $filename)) {
666 # Here $! is meaningless.
669 # ONLY here is $! meaningful.
671 # Already here $! might be meaningless.
673 # Since here we might have either success or failure,
674 # here $! is meaningless.
676 In the above I<meaningless> stands for anything: zero, non-zero,
677 C<undef>. A successful system or library call does B<not> set
678 the variable to zero.
680 If used as a string, yields the corresponding system error string.
681 You can assign a number to C<$!> to set I<errno> if, for instance,
682 you want C<"$!"> to return the string for error I<n>, or you want
683 to set the exit value for the die() operator. (Mnemonic: What just
686 Also see L<Error Indicators>.
690 Each element of C<%!> has a true value only if C<$!> is set to that
691 value. For example, C<$!{ENOENT}> is true if and only if the current
692 value of C<$!> is C<ENOENT>; that is, if the most recent error was
693 "No such file or directory" (or its moral equivalent: not all operating
694 systems give that exact error, and certainly not all languages).
695 To check if a particular key is meaningful on your system, use
696 C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
697 See L<Errno> for more information, and also see above for the
700 =item $EXTENDED_OS_ERROR
704 Error information specific to the current operating system. At
705 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
706 (and for MacPerl). On all other platforms, C<$^E> is always just
709 Under VMS, C<$^E> provides the VMS status value from the last
710 system error. This is more specific information about the last
711 system error than that provided by C<$!>. This is particularly
712 important when C<$!> is set to B<EVMSERR>.
714 Under OS/2, C<$^E> is set to the error code of the last call to
715 OS/2 API either via CRT, or directly from perl.
717 Under Win32, C<$^E> always returns the last error information
718 reported by the Win32 call C<GetLastError()> which describes
719 the last error from within the Win32 API. Most Win32-specific
720 code will report errors via C<$^E>. ANSI C and Unix-like calls
721 set C<errno> and so most portable Perl code will report errors
724 Caveats mentioned in the description of C<$!> generally apply to
725 C<$^E>, also. (Mnemonic: Extra error explanation.)
727 Also see L<Error Indicators>.
733 The Perl syntax error message from the last eval() operator.
734 If $@ is the null string, the last eval() parsed and executed
735 correctly (although the operations you invoked may have failed in the
736 normal fashion). (Mnemonic: Where was the syntax error "at"?)
738 Warning messages are not collected in this variable. You can,
739 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
742 Also see L<Error Indicators>.
750 The process number of the Perl running this script. You should
751 consider this variable read-only, although it will be altered
752 across fork() calls. (Mnemonic: same as shells.)
754 Note for Linux users: on Linux, the C functions C<getpid()> and
755 C<getppid()> return different values from different threads. In order to
756 be portable, this behavior is not reflected by C<$$>, whose value remains
757 consistent across threads. If you want to call the underlying C<getpid()>,
758 you may use the CPAN module C<Linux::Pid>.
766 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
767 if you're running setuid.) You can change both the real uid and
768 the effective uid at the same time by using POSIX::setuid(). Since
769 changes to $< require a system call, check $! after a change attempt to
770 detect any possible errors.
772 =item $EFFECTIVE_USER_ID
778 The effective uid of this process. Example:
780 $< = $>; # set real to effective uid
781 ($<,$>) = ($>,$<); # swap real and effective uid
783 You can change both the effective uid and the real uid at the same
784 time by using POSIX::setuid(). Changes to $> require a check to $!
785 to detect any possible errors after an attempted change.
787 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
788 C<< $< >> and C<< $> >> can be swapped only on machines
789 supporting setreuid().
797 The real gid of this process. If you are on a machine that supports
798 membership in multiple groups simultaneously, gives a space separated
799 list of groups you are in. The first number is the one returned by
800 getgid(), and the subsequent ones by getgroups(), one of which may be
801 the same as the first number.
803 However, a value assigned to C<$(> must be a single number used to
804 set the real gid. So the value given by C<$(> should I<not> be assigned
805 back to C<$(> without being forced numeric, such as by adding zero.
807 You can change both the real gid and the effective gid at the same
808 time by using POSIX::setgid(). Changes to $( require a check to $!
809 to detect any possible errors after an attempted change.
811 (Mnemonic: parentheses are used to I<group> things. The real gid is the
812 group you I<left>, if you're running setgid.)
814 =item $EFFECTIVE_GROUP_ID
820 The effective gid of this process. If you are on a machine that
821 supports membership in multiple groups simultaneously, gives a space
822 separated list of groups you are in. The first number is the one
823 returned by getegid(), and the subsequent ones by getgroups(), one of
824 which may be the same as the first number.
826 Similarly, a value assigned to C<$)> must also be a space-separated
827 list of numbers. The first number sets the effective gid, and
828 the rest (if any) are passed to setgroups(). To get the effect of an
829 empty list for setgroups(), just repeat the new effective gid; that is,
830 to force an effective gid of 5 and an effectively empty setgroups()
831 list, say C< $) = "5 5" >.
833 You can change both the effective gid and the real gid at the same
834 time by using POSIX::setgid() (use only a single numeric argument).
835 Changes to $) require a check to $! to detect any possible errors
836 after an attempted change.
838 (Mnemonic: parentheses are used to I<group> things. The effective gid
839 is the group that's I<right> for you, if you're running setgid.)
841 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
842 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
843 and C<$)> can be swapped only on machines supporting setregid().
849 Contains the name of the program being executed.
851 On some (read: not all) operating systems assigning to C<$0> modifies
852 the argument area that the C<ps> program sees. On some platforms you
853 may have to use special C<ps> options or a different C<ps> to see the
854 changes. Modifying the $0 is more useful as a way of indicating the
855 current program state than it is for hiding the program you're
856 running. (Mnemonic: same as B<sh> and B<ksh>.)
858 Note that there are platform specific limitations on the the maximum
859 length of C<$0>. In the most extreme case it may be limited to the
860 space occupied by the original C<$0>.
862 In some platforms there may be arbitrary amount of padding, for
863 example space characters, after the modified name as shown by C<ps>.
864 In some platforms this padding may extend all the way to the original
865 length of the argument area, no matter what you do (this is the case
866 for example with Linux 2.2).
868 Note for BSD users: setting C<$0> does not completely remove "perl"
869 from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
870 result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
871 and the " (perl)" suffix are shown depends on your exact BSD variant
872 and version). This is an operating system feature, Perl cannot help it.
874 In multithreaded scripts Perl coordinates the threads so that any
875 thread may modify its copy of the C<$0> and the change becomes visible
876 to ps(1) (assuming the operating system plays along). Note that the
877 the view of C<$0> the other threads have will not change since they
878 have their own copies of it.
882 The index of the first element in an array, and of the first character
883 in a substring. Default is 0, but you could theoretically set it
884 to 1 to make Perl behave more like B<awk> (or Fortran) when
885 subscripting and when evaluating the index() and substr() functions.
886 (Mnemonic: [ begins subscripts.)
888 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
889 directive, and cannot influence the behavior of any other file.
890 (That's why you can only assign compile-time constants to it.)
891 Its use is highly discouraged.
893 Note that, unlike other compile-time directives (such as L<strict>),
894 assignment to $[ can be seen from outer lexical scopes in the same file.
895 However, you can use local() on it to strictly bound its value to a
900 The version + patchlevel / 1000 of the Perl interpreter. This variable
901 can be used to determine whether the Perl interpreter executing a
902 script is in the right range of versions. (Mnemonic: Is this version
903 of perl in the right bracket?) Example:
905 warn "No checksumming!\n" if $] < 3.019;
907 See also the documentation of C<use VERSION> and C<require VERSION>
908 for a convenient way to fail if the running Perl interpreter is too old.
910 The floating point representation can sometimes lead to inaccurate
911 numeric comparisons. See C<$^V> for a more modern representation of
912 the Perl version that allows accurate string comparisons.
918 The current value of the flag associated with the B<-c> switch.
919 Mainly of use with B<-MO=...> to allow code to alter its behavior
920 when being compiled, such as for example to AUTOLOAD at compile
921 time rather than normal, deferred loading. See L<perlcc>. Setting
922 C<$^C = 1> is similar to calling C<B::minus_c>.
928 The current value of the debugging flags. (Mnemonic: value of B<-D>
929 switch.) May be read or set. Like its command-line equivalent, you can use
930 numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
932 =item ${^RE_DEBUG_FLAGS}
934 The current value of the regex debugging flags. Set to 0 for no debug output
935 even when the re 'debug' module is loaded. See L<re> for details.
937 =item ${^RE_TRIE_MAXBUFF}
939 Controls how certain regex optimisations are applied and how much memory they
940 utilize. This value by default is 65536 which corresponds to a 512kB temporary
941 cache. Set this to a higher value to trade memory for speed when matching
942 large alternations. Set it to a lower value if you want the optimisations to
943 be as conservative of memory as possible but still occur, and set it to a
944 negative value to prevent the optimisation and conserve the most memory.
945 Under normal situations this variable should be of no interest to you.
951 The maximum system file descriptor, ordinarily 2. System file
952 descriptors are passed to exec()ed processes, while higher file
953 descriptors are not. Also, during an open(), system file descriptors are
954 preserved even if the open() fails. (Ordinary file descriptors are
955 closed before the open() is attempted.) The close-on-exec
956 status of a file descriptor will be decided according to the value of
957 C<$^F> when the corresponding file, pipe, or socket was opened, not the
962 WARNING: This variable is strictly for internal use only. Its availability,
963 behavior, and contents are subject to change without notice.
965 This variable contains compile-time hints for the Perl interpreter. At the
966 end of compilation of a BLOCK the value of this variable is restored to the
967 value when the interpreter started to compile the BLOCK.
969 When perl begins to parse any block construct that provides a lexical scope
970 (e.g., eval body, required file, subroutine body, loop body, or conditional
971 block), the existing value of $^H is saved, but its value is left unchanged.
972 When the compilation of the block is completed, it regains the saved value.
973 Between the points where its value is saved and restored, code that
974 executes within BEGIN blocks is free to change the value of $^H.
976 This behavior provides the semantic of lexical scoping, and is used in,
977 for instance, the C<use strict> pragma.
979 The contents should be an integer; different bits of it are used for
980 different pragmatic flags. Here's an example:
982 sub add_100 { $^H |= 0x100 }
989 Consider what happens during execution of the BEGIN block. At this point
990 the BEGIN block has already been compiled, but the body of foo() is still
991 being compiled. The new value of $^H will therefore be visible only while
992 the body of foo() is being compiled.
994 Substitution of the above BEGIN block with:
996 BEGIN { require strict; strict->import('vars') }
998 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
999 version of the same lexical pragma:
1001 BEGIN { require strict; strict->import('vars') if $condition }
1005 WARNING: This variable is strictly for internal use only. Its availability,
1006 behavior, and contents are subject to change without notice.
1008 The %^H hash provides the same scoping semantic as $^H. This makes it
1009 useful for implementation of lexically scoped pragmas.
1015 The current value of the inplace-edit extension. Use C<undef> to disable
1016 inplace editing. (Mnemonic: value of B<-i> switch.)
1020 By default, running out of memory is an untrappable, fatal error.
1021 However, if suitably built, Perl can use the contents of C<$^M>
1022 as an emergency memory pool after die()ing. Suppose that your Perl
1023 were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
1026 $^M = 'a' x (1 << 16);
1028 would allocate a 64K buffer for use in an emergency. See the
1029 F<INSTALL> file in the Perl distribution for information on how to
1030 enable this option. To discourage casual use of this advanced
1031 feature, there is no L<English|English> long name for this variable.
1037 The name of the operating system under which this copy of Perl was
1038 built, as determined during the configuration process. The value
1039 is identical to C<$Config{'osname'}>. See also L<Config> and the
1040 B<-V> command-line switch documented in L<perlrun>.
1042 In Windows platforms, $^O is not very helpful: since it is always
1043 C<MSWin32>, it doesn't tell the difference between
1044 95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1045 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1046 between the variants.
1050 An internal variable used by PerlIO. A string in two parts, separated
1051 by a C<\0> byte, the first part describes the input layers, the second
1052 part describes the output layers.
1058 The internal variable for debugging support. The meanings of the
1059 various bits are subject to change, but currently indicate:
1065 Debug subroutine enter/exit.
1069 Line-by-line debugging.
1073 Switch off optimizations.
1077 Preserve more data for future interactive inspections.
1081 Keep info about source lines on which a subroutine is defined.
1085 Start with single-step on.
1089 Use subroutine address instead of name when reporting.
1093 Report C<goto &subroutine> as well.
1097 Provide informative "file" names for evals based on the place they were compiled.
1101 Provide informative names to anonymous subroutines based on the place they
1106 Debug assertion subroutines enter/exit.
1110 Some bits may be relevant at compile-time only, some at
1111 run-time only. This is a new mechanism and the details may change.
1113 =item $LAST_REGEXP_CODE_RESULT
1117 The result of evaluation of the last successful C<(?{ code })>
1118 regular expression assertion (see L<perlre>). May be written to.
1120 =item $EXCEPTIONS_BEING_CAUGHT
1124 Current state of the interpreter.
1127 --------- -------------------
1128 undef Parsing module/eval
1129 true (1) Executing an eval
1132 The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
1138 The time at which the program began running, in seconds since the
1139 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
1140 and B<-C> filetests are based on this value.
1144 Reflects if taint mode is on or off. 1 for on (the program was run with
1145 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
1146 B<-t> or B<-TU>). This variable is read-only.
1150 Reflects certain Unicode settings of Perl. See L<perlrun>
1151 documentation for the C<-C> switch for more information about
1152 the possible values. This variable is set during Perl startup
1153 and is thereafter read-only.
1159 The revision, version, and subversion of the Perl interpreter, represented
1160 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
1161 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1162 C<$^V eq v5.6.0>. Note that the characters in this string value can
1163 potentially be in Unicode range.
1165 This can be used to determine whether the Perl interpreter executing a
1166 script is in the right range of versions. (Mnemonic: use ^V for Version
1169 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
1171 To convert C<$^V> into its string representation use sprintf()'s
1172 C<"%vd"> conversion:
1174 printf "version is v%vd\n", $^V; # Perl's version
1176 See the documentation of C<use VERSION> and C<require VERSION>
1177 for a convenient way to fail if the running Perl interpreter is too old.
1179 See also C<$]> for an older representation of the Perl version.
1185 The current value of the warning switch, initially true if B<-w>
1186 was used, false otherwise, but directly modifiable. (Mnemonic:
1187 related to the B<-w> switch.) See also L<warnings>.
1189 =item ${^WARNING_BITS}
1191 The current set of warning checks enabled by the C<use warnings> pragma.
1192 See the documentation of C<warnings> for more details.
1194 =item $EXECUTABLE_NAME
1198 The name used to execute the current copy of Perl, from C's
1201 Depending on the host operating system, the value of $^X may be
1202 a relative or absolute pathname of the perl program file, or may
1203 be the string used to invoke perl but not the pathname of the
1204 perl program file. Also, most operating systems permit invoking
1205 programs that are not in the PATH environment variable, so there
1206 is no guarantee that the value of $^X is in PATH. For VMS, the
1207 value may or may not include a version number.
1209 You usually can use the value of $^X to re-invoke an independent
1210 copy of the same perl that is currently running, e.g.,
1212 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1214 But recall that not all operating systems support forking or
1215 capturing of the output of commands, so this complex statement
1216 may not be portable.
1218 It is not safe to use the value of $^X as a path name of a file,
1219 as some operating systems that have a mandatory suffix on
1220 executable files do not require use of the suffix when invoking
1221 a command. To convert the value of $^X to a path name, use the
1222 following statements:
1224 # Build up a set of file names (not command names).
1228 {$this_perl .= $Config{_exe}
1229 unless $this_perl =~ m/$Config{_exe}$/i;}
1231 Because many operating systems permit anyone with read access to
1232 the Perl program file to make a copy of it, patch the copy, and
1233 then execute the copy, the security-conscious Perl programmer
1234 should take care to invoke the installed copy of perl, not the
1235 copy referenced by $^X. The following statements accomplish
1236 this goal, and produce a pathname that can be invoked as a
1237 command or referenced as a file.
1240 $secure_perl_path = $Config{perlpath};
1242 {$secure_perl_path .= $Config{_exe}
1243 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
1247 The special filehandle that iterates over command-line filenames in
1248 C<@ARGV>. Usually written as the null filehandle in the angle operator
1249 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1250 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1251 corresponding to the last file opened by C<< <> >>. In particular,
1252 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1253 may not cause your function to automatically read the contents of all the
1258 contains the name of the current file when reading from <>.
1262 The array @ARGV contains the command-line arguments intended for
1263 the script. C<$#ARGV> is generally the number of arguments minus
1264 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1265 command name itself. See C<$0> for the command name.
1269 The special filehandle that points to the currently open output file
1270 when doing edit-in-place processing with B<-i>. Useful when you have
1271 to do a lot of inserting and don't want to keep modifying $_. See
1272 L<perlrun> for the B<-i> switch.
1276 The array @F contains the fields of each line read in when autosplit
1277 mode is turned on. See L<perlrun> for the B<-a> switch. This array
1278 is package-specific, and must be declared or given a full package name
1279 if not in package main when running under C<strict 'vars'>.
1283 The array @INC contains the list of places that the C<do EXPR>,
1284 C<require>, or C<use> constructs look for their library files. It
1285 initially consists of the arguments to any B<-I> command-line
1286 switches, followed by the default Perl library, probably
1287 F</usr/local/lib/perl>, followed by ".", to represent the current
1288 directory. ("." will not be appended if taint checks are enabled, either by
1289 C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
1290 the C<use lib> pragma to get the machine-dependent library properly
1293 use lib '/mypath/libdir/';
1296 You can also insert hooks into the file inclusion system by putting Perl
1297 code directly into @INC. Those hooks may be subroutine references, array
1298 references or blessed objects. See L<perlfunc/require> for details.
1302 Within a subroutine the array @_ contains the parameters passed to that
1303 subroutine. See L<perlsub>.
1307 The hash %INC contains entries for each filename included via the
1308 C<do>, C<require>, or C<use> operators. The key is the filename
1309 you specified (with module names converted to pathnames), and the
1310 value is the location of the file found. The C<require>
1311 operator uses this hash to determine whether a particular file has
1312 already been included.
1314 If the file was loaded via a hook (e.g. a subroutine reference, see
1315 L<perlfunc/require> for a description of these hooks), this hook is
1316 by default inserted into %INC in place of a filename. Note, however,
1317 that the hook may have set the %INC entry by itself to provide some more
1324 The hash %ENV contains your current environment. Setting a
1325 value in C<ENV> changes the environment for any child processes
1326 you subsequently fork() off.
1332 The hash %SIG contains signal handlers for signals. For example:
1334 sub handler { # 1st argument is signal name
1336 print "Caught a SIG$sig--shutting down\n";
1341 $SIG{'INT'} = \&handler;
1342 $SIG{'QUIT'} = \&handler;
1344 $SIG{'INT'} = 'DEFAULT'; # restore default action
1345 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1347 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1348 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1351 Here are some other examples:
1353 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1354 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1355 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1356 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1358 Be sure not to use a bareword as the name of a signal handler,
1359 lest you inadvertently call it.
1361 If your system has the sigaction() function then signal handlers are
1362 installed using it. This means you get reliable signal handling.
1364 The default delivery policy of signals changed in Perl 5.8.0 from
1365 immediate (also known as "unsafe") to deferred, also known as
1366 "safe signals". See L<perlipc> for more information.
1368 Certain internal hooks can be also set using the %SIG hash. The
1369 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1370 about to be printed. The warning message is passed as the first
1371 argument. The presence of a __WARN__ hook causes the ordinary printing
1372 of warnings to STDERR to be suppressed. You can use this to save warnings
1373 in a variable, or turn warnings into fatal errors, like this:
1375 local $SIG{__WARN__} = sub { die $_[0] };
1378 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1379 is about to be thrown. The error message is passed as the first
1380 argument. When a __DIE__ hook routine returns, the exception
1381 processing continues as it would have in the absence of the hook,
1382 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
1383 The C<__DIE__> handler is explicitly disabled during the call, so that you
1384 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1386 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1387 even inside an eval(). Do not use this to rewrite a pending exception
1388 in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1389 This strange action at a distance may be fixed in a future release
1390 so that C<$SIG{__DIE__}> is only called if your program is about
1391 to exit, as was the original intent. Any other use is deprecated.
1393 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1394 they may be called to report (probable) errors found by the parser.
1395 In such a case the parser may be in inconsistent state, so any
1396 attempt to evaluate Perl code from such a handler will probably
1397 result in a segfault. This means that warnings or errors that
1398 result from parsing Perl should be used with extreme caution, like
1401 require Carp if defined $^S;
1402 Carp::confess("Something wrong") if defined &Carp::confess;
1403 die "Something wrong, but could not load Carp to give backtrace...
1404 To see backtrace try starting Perl with -MCarp switch";
1406 Here the first line will load Carp I<unless> it is the parser who
1407 called the handler. The second line will print backtrace and die if
1408 Carp was available. The third line will be executed only if Carp was
1411 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1412 L<warnings> for additional information.
1416 =head2 Error Indicators
1418 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1419 about different types of error conditions that may appear during
1420 execution of a Perl program. The variables are shown ordered by
1421 the "distance" between the subsystem which reported the error and
1422 the Perl process. They correspond to errors detected by the Perl
1423 interpreter, C library, operating system, or an external program,
1426 To illustrate the differences between these variables, consider the
1427 following Perl expression, which uses a single-quoted string:
1430 open my $pipe, "/cdrom/install |" or die $!;
1432 close $pipe or die "bad pipe: $?, $!";
1435 After execution of this statement all 4 variables may have been set.
1437 C<$@> is set if the string to be C<eval>-ed did not compile (this
1438 may happen if C<open> or C<close> were imported with bad prototypes),
1439 or if Perl code executed during evaluation die()d . In these cases
1440 the value of $@ is the compile error, or the argument to C<die>
1441 (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1444 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1445 and C<close> are translated to calls in the C run-time library and
1446 thence to the operating system kernel. C<$!> is set to the C library's
1447 C<errno> if one of these calls fails.
1449 Under a few operating systems, C<$^E> may contain a more verbose
1450 error indicator, such as in this case, "CDROM tray not closed."
1451 Systems that do not support extended error messages leave C<$^E>
1454 Finally, C<$?> may be set to non-0 value if the external program
1455 F</cdrom/install> fails. The upper eight bits reflect specific
1456 error conditions encountered by the program (the program's exit()
1457 value). The lower eight bits reflect mode of failure, like signal
1458 death and core dump information See wait(2) for details. In
1459 contrast to C<$!> and C<$^E>, which are set only if error condition
1460 is detected, the variable C<$?> is set on each C<wait> or pipe
1461 C<close>, overwriting the old value. This is more like C<$@>, which
1462 on every eval() is always set on failure and cleared on success.
1464 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1467 =head2 Technical Note on the Syntax of Variable Names
1469 Variable names in Perl can have several formats. Usually, they
1470 must begin with a letter or underscore, in which case they can be
1471 arbitrarily long (up to an internal limit of 251 characters) and
1472 may contain letters, digits, underscores, or the special sequence
1473 C<::> or C<'>. In this case, the part before the last C<::> or
1474 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1476 Perl variable names may also be a sequence of digits or a single
1477 punctuation or control character. These names are all reserved for
1478 special uses by Perl; for example, the all-digits names are used
1479 to hold data captured by backreferences after a regular expression
1480 match. Perl has a special syntax for the single-control-character
1481 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1482 character. For example, the notation C<$^W> (dollar-sign caret
1483 C<W>) is the scalar variable whose name is the single character
1484 control-C<W>. This is better than typing a literal control-C<W>
1487 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1488 strings that begin with control characters (or better yet, a caret).
1489 These variables must be written in the form C<${^Foo}>; the braces
1490 are not optional. C<${^Foo}> denotes the scalar variable whose
1491 name is a control-C<F> followed by two C<o>'s. These variables are
1492 reserved for future special uses by Perl, except for the ones that
1493 begin with C<^_> (control-underscore or caret-underscore). No
1494 control-character name that begins with C<^_> will acquire a special
1495 meaning in any future version of Perl; such names may therefore be
1496 used safely in programs. C<$^_> itself, however, I<is> reserved.
1498 Perl identifiers that begin with digits, control characters, or
1499 punctuation characters are exempt from the effects of the C<package>
1500 declaration and are always forced to be in package C<main>; they are
1501 also exempt from C<strict 'vars'> errors. A few other names are also
1502 exempt in these ways:
1510 In particular, the new special C<${^_XYZ}> variables are always taken
1511 to be in package C<main>, regardless of any C<package> declarations
1516 Due to an unfortunate accident of Perl's implementation, C<use
1517 English> imposes a considerable performance penalty on all regular
1518 expression matches in a program, regardless of whether they occur
1519 in the scope of C<use English>. For that reason, saying C<use
1520 English> in libraries is strongly discouraged. See the
1521 Devel::SawAmpersand module documentation from CPAN
1522 ( http://www.cpan.org/modules/by-module/Devel/ )
1523 for more information.
1525 Having to even think about the C<$^S> variable in your exception
1526 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1527 invites grievous and difficult to track down errors. Avoid it
1528 and use an C<END{}> or CORE::GLOBAL::die override instead.