3 perlvar - Perl predefined variables
7 =head2 Predefined Names
9 The following names have special meaning to Perl. Most of the
10 punctuation names have reasonable mnemonics, or analogues in one of
11 the shells. Nevertheless, if you wish to use the long variable names,
16 at the top of your program. This will alias all the short names to the
17 long names in the current package. Some of them even have medium names,
18 generally borrowed from B<awk>.
20 To go a step further, those variables that depend on the currently
21 selected filehandle may instead (and preferably) be set by calling an
22 object method on the FileHandle object. (Summary lines below for this
23 contain the word HANDLE.) First you must say
27 after which you may use either
35 Each of the methods returns the old value of the FileHandle attribute.
36 The methods each take an optional EXPR, which if supplied specifies the
37 new value for the FileHandle attribute in question. If not supplied,
38 most of the methods do nothing to the current value, except for
39 autoflush(), which will assume a 1 for you, just to be different.
41 A few of these variables are considered "read-only". This means that if
42 you try to assign to this variable, either directly or indirectly through
43 a reference, you'll raise a run-time exception.
45 The following list is ordered by scalar variables first, then the
46 arrays, then the hashes (except $^M was added in the wrong place).
47 This is somewhat obscured by the fact that %ENV and %SIG are listed as
48 $ENV{expr} and $SIG{expr}.
57 The default input and pattern-searching space. The following pairs are
60 while (<>) {...} # equivalent in only while!
61 while (defined($_ = <>)) {...}
72 Here are the places where Perl will assume $_ even if you
79 Various unary functions, including functions like ord() and int(), as well
80 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
85 Various list functions like print() and unlink().
89 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
90 without an C<=~> operator.
94 The default iterator variable in a C<foreach> loop if no other
99 The implicit iterator variable in the grep() and map() functions.
103 The default place to put an input record when a C<E<lt>FHE<gt>>
104 operation's result is tested by itself as the sole criterion of a C<while>
105 test. Note that outside of a C<while> test, this will not happen.
109 (Mnemonic: underline is understood in certain operations.)
115 =item $E<lt>I<digit>E<gt>
117 Contains the subpattern from the corresponding set of parentheses in
118 the last pattern matched, not counting patterns matched in nested
119 blocks that have been exited already. (Mnemonic: like \digit.)
120 These variables are all read-only.
126 The string matched by the last successful pattern match (not counting
127 any matches hidden within a BLOCK or eval() enclosed by the current
128 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
134 The string preceding whatever was matched by the last successful
135 pattern match (not counting any matches hidden within a BLOCK or eval
136 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
137 string.) This variable is read-only.
143 The string following whatever was matched by the last successful
144 pattern match (not counting any matches hidden within a BLOCK or eval()
145 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
150 print "$`:$&:$'\n"; # prints abc:def:ghi
152 This variable is read-only.
154 =item $LAST_PAREN_MATCH
158 The last bracket matched by the last search pattern. This is useful if
159 you don't know which of a set of alternative patterns matched. For
162 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
164 (Mnemonic: be positive and forward looking.)
165 This variable is read-only.
167 =item $MULTILINE_MATCHING
171 Set to 1 to do multi-line matching within a string, 0 to tell Perl
172 that it can assume that strings contain a single line, for the purpose
173 of optimizing pattern matches. Pattern matches on strings containing
174 multiple newlines can produce confusing results when "C<$*>" is 0. Default
175 is 0. (Mnemonic: * matches multiple things.) Note that this variable
176 influences the interpretation of only "C<^>" and "C<$>". A literal newline can
177 be searched for even when C<$* == 0>.
179 Use of "C<$*>" is deprecated in modern perls.
181 =item input_line_number HANDLE EXPR
183 =item $INPUT_LINE_NUMBER
189 The current input line number for the last file handle from
190 which you read (or performed a C<seek> or C<tell> on). An
191 explicit close on a filehandle resets the line number. Because
192 "C<E<lt>E<gt>>" never does an explicit close, line numbers increase
193 across ARGV files (but see examples under eof()). Localizing C<$.> has
194 the effect of also localizing Perl's notion of "the last read
195 filehandle". (Mnemonic: many programs use "." to mean the current line
198 =item input_record_separator HANDLE EXPR
200 =item $INPUT_RECORD_SEPARATOR
206 The input record separator, newline by default. Works like B<awk>'s RS
207 variable, including treating empty lines as delimiters if set to the
208 null string. (Note: An empty line cannot contain any spaces or tabs.)
209 You may set it to a multi-character string to match a multi-character
210 delimiter, or to C<undef> to read to end of file. Note that setting it
211 to C<"\n\n"> means something slightly different than setting it to
212 C<"">, if the file contains consecutive empty lines. Setting it to
213 C<""> will treat two or more consecutive empty lines as a single empty
214 line. Setting it to C<"\n\n"> will blindly assume that the next input
215 character belongs to the next paragraph, even if it's a newline.
216 (Mnemonic: / is used to delimit line boundaries when quoting poetry.)
219 $_ = <FH>; # whole file now here
222 Remember: the value of $/ is a string, not a regexp. AWK has to be
223 better for something :-)
225 =item autoflush HANDLE EXPR
227 =item $OUTPUT_AUTOFLUSH
231 If set to nonzero, forces a flush right away and after every write or print on the
232 currently selected output channel. Default is 0 (regardless of whether
233 the channel is actually buffered by the system or not; C<$|> tells you
234 only whether you've asked Perl explicitly to flush after each write).
235 Note that STDOUT will typically be line buffered if output is to the
236 terminal and block buffered otherwise. Setting this variable is useful
237 primarily when you are outputting to a pipe, such as when you are running
238 a Perl script under rsh and want to see the output as it's happening. This
239 has no effect on input buffering.
240 (Mnemonic: when you want your pipes to be piping hot.)
242 =item output_field_separator HANDLE EXPR
244 =item $OUTPUT_FIELD_SEPARATOR
250 The output field separator for the print operator. Ordinarily the
251 print operator simply prints out the comma-separated fields you
252 specify. To get behavior more like B<awk>, set this variable
253 as you would set B<awk>'s OFS variable to specify what is printed
254 between fields. (Mnemonic: what is printed when there is a , in your
257 =item output_record_separator HANDLE EXPR
259 =item $OUTPUT_RECORD_SEPARATOR
265 The output record separator for the print operator. Ordinarily the
266 print operator simply prints out the comma-separated fields you
267 specify, with no trailing newline or record separator assumed.
268 To get behavior more like B<awk>, set this variable as you would
269 set B<awk>'s ORS variable to specify what is printed at the end of the
270 print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
271 print. Also, it's just like C<$/>, but it's what you get "back" from
274 =item $LIST_SEPARATOR
278 This is like "C<$,>" except that it applies to array values interpolated
279 into a double-quoted string (or similar interpreted string). Default
280 is a space. (Mnemonic: obvious, I think.)
282 =item $SUBSCRIPT_SEPARATOR
288 The subscript separator for multidimensional array emulation. If you
289 refer to a hash element as
295 $foo{join($;, $a, $b, $c)}
299 @foo{$a,$b,$c} # a slice--note the @
303 ($foo{$a},$foo{$b},$foo{$c})
305 Default is "\034", the same as SUBSEP in B<awk>. Note that if your
306 keys contain binary data there might not be any safe value for "C<$;>".
307 (Mnemonic: comma (the syntactic subscript separator) is a
308 semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
309 taken for something more important.)
311 Consider using "real" multidimensional arrays.
317 The output format for printed numbers. This variable is a half-hearted
318 attempt to emulate B<awk>'s OFMT variable. There are times, however,
319 when B<awk> and Perl have differing notions of what is in fact
320 numeric. The initial value is %.I<n>g, where I<n> is the value
321 of the macro DBL_DIG from your system's F<float.h>. This is different from
322 B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
323 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
325 Use of "C<$#>" is deprecated.
327 =item format_page_number HANDLE EXPR
329 =item $FORMAT_PAGE_NUMBER
333 The current page number of the currently selected output channel.
334 (Mnemonic: % is page number in B<nroff>.)
336 =item format_lines_per_page HANDLE EXPR
338 =item $FORMAT_LINES_PER_PAGE
342 The current page length (printable lines) of the currently selected
343 output channel. Default is 60. (Mnemonic: = has horizontal lines.)
345 =item format_lines_left HANDLE EXPR
347 =item $FORMAT_LINES_LEFT
351 The number of lines left on the page of the currently selected output
352 channel. (Mnemonic: lines_on_page - lines_printed.)
354 =item format_name HANDLE EXPR
360 The name of the current report format for the currently selected output
361 channel. Default is name of the filehandle. (Mnemonic: brother to
364 =item format_top_name HANDLE EXPR
366 =item $FORMAT_TOP_NAME
370 The name of the current top-of-page format for the currently selected
371 output channel. Default is name of the filehandle with _TOP
372 appended. (Mnemonic: points to top of page.)
374 =item format_line_break_characters HANDLE EXPR
376 =item $FORMAT_LINE_BREAK_CHARACTERS
380 The current set of characters after which a string may be broken to
381 fill continuation fields (starting with ^) in a format. Default is
382 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
383 poetry is a part of a line.)
385 =item format_formfeed HANDLE EXPR
387 =item $FORMAT_FORMFEED
391 What formats output to perform a form feed. Default is \f.
397 The current value of the write() accumulator for format() lines. A format
398 contains formline() commands that put their result into C<$^A>. After
399 calling its format, write() prints out the contents of C<$^A> and empties.
400 So you never actually see the contents of C<$^A> unless you call
401 formline() yourself and then look at it. See L<perlform> and
402 L<perlfunc/formline()>.
408 The status returned by the last pipe close, backtick (C<``>) command,
409 or system() operator. Note that this is the status word returned by
410 the wait() system call (or else is made up to look like it). Thus,
411 the exit value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and
412 C<$? & 255> gives which signal, if any, the process died from, and
413 whether there was a core dump. (Mnemonic: similar to B<sh> and
416 Note that if you have installed a signal handler for C<SIGCHLD>, the
417 value of C<$?> will usually be wrong outside that handler.
419 Inside an C<END> subroutine C<$?> contains the value that is going to be
420 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
421 change the exit status of the script.
423 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
424 actual VMS exit status, instead of the default emulation of POSIX
433 If used in a numeric context, yields the current value of errno, with
434 all the usual caveats. (This means that you shouldn't depend on the
435 value of C<$!> to be anything in particular unless you've gotten a
436 specific error return indicating a system error.) If used in a string
437 context, yields the corresponding system error string. You can assign
438 to C<$!> to set I<errno> if, for instance, you want C<"$!"> to return the
439 string for error I<n>, or you want to set the exit value for the die()
440 operator. (Mnemonic: What just went bang?)
442 =item $EXTENDED_OS_ERROR
446 Error information specific to the current operating system. At
447 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
448 (and for MacPerl). On all other platforms, C<$^E> is always just
451 Under VMS, C<$^E> provides the VMS status value from the last
452 system error. This is more specific information about the last
453 system error than that provided by C<$!>. This is particularly
454 important when C<$!> is set to B<EVMSERR>.
456 Under OS/2, C<$^E> is set to the error code of the last call to
457 OS/2 API either via CRT, or directly from perl.
459 Under Win32, C<$^E> always returns the last error information
460 reported by the Win32 call C<GetLastError()> which describes
461 the last error from within the Win32 API. Most Win32-specific
462 code will report errors via C<$^E>. ANSI C and UNIX-like calls
463 set C<errno> and so most portable Perl code will report errors
466 Caveats mentioned in the description of C<$!> generally apply to
467 C<$^E>, also. (Mnemonic: Extra error explanation.)
473 The Perl syntax error message from the last eval() command. If null, the
474 last eval() parsed and executed correctly (although the operations you
475 invoked may have failed in the normal fashion). (Mnemonic: Where was
476 the syntax error "at"?)
478 Note that warning messages are not collected in this variable. You can,
479 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
488 The process number of the Perl running this script. (Mnemonic: same
497 The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
498 if you're running setuid.)
500 =item $EFFECTIVE_USER_ID
506 The effective uid of this process. Example:
508 $< = $>; # set real to effective uid
509 ($<,$>) = ($>,$<); # swap real and effective uid
511 (Mnemonic: it's the uid you went I<TO>, if you're running setuid.)
512 Note: "C<$E<lt>>" and "C<$E<gt>>" can be swapped only on machines
513 supporting setreuid().
521 The real gid of this process. If you are on a machine that supports
522 membership in multiple groups simultaneously, gives a space separated
523 list of groups you are in. The first number is the one returned by
524 getgid(), and the subsequent ones by getgroups(), one of which may be
525 the same as the first number.
527 However, a value assigned to "C<$(>" must be a single number used to
528 set the real gid. So the value given by "C<$(>" should I<not> be assigned
529 back to "C<$(>" without being forced numeric, such as by adding zero.
531 (Mnemonic: parentheses are used to I<GROUP> things. The real gid is the
532 group you I<LEFT>, if you're running setgid.)
534 =item $EFFECTIVE_GROUP_ID
540 The effective gid of this process. If you are on a machine that
541 supports membership in multiple groups simultaneously, gives a space
542 separated list of groups you are in. The first number is the one
543 returned by getegid(), and the subsequent ones by getgroups(), one of
544 which may be the same as the first number.
546 Similarly, a value assigned to "C<$)>" must also be a space-separated
547 list of numbers. The first number is used to set the effective gid, and
548 the rest (if any) are passed to setgroups(). To get the effect of an
549 empty list for setgroups(), just repeat the new effective gid; that is,
550 to force an effective gid of 5 and an effectively empty setgroups()
551 list, say C< $) = "5 5" >.
553 (Mnemonic: parentheses are used to I<GROUP> things. The effective gid
554 is the group that's I<RIGHT> for you, if you're running setgid.)
556 Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
557 machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
558 and "C<$)>" can be swapped only on machines supporting setregid().
564 Contains the name of the file containing the Perl script being
565 executed. On some operating systems
566 assigning to "C<$0>" modifies the argument area that the ps(1)
567 program sees. This is more useful as a way of indicating the
568 current program state than it is for hiding the program you're running.
569 (Mnemonic: same as B<sh> and B<ksh>.)
573 The index of the first element in an array, and of the first character
574 in a substring. Default is 0, but you could set it to 1 to make
575 Perl behave more like B<awk> (or Fortran) when subscripting and when
576 evaluating the index() and substr() functions. (Mnemonic: [ begins
579 As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
580 and cannot influence the behavior of any other file. Its use is
587 The version + patchlevel / 1000 of the Perl interpreter. This variable
588 can be used to determine whether the Perl interpreter executing a
589 script is in the right range of versions. (Mnemonic: Is this version
590 of perl in the right bracket?) Example:
592 warn "No checksumming!\n" if $] < 3.019;
594 See also the documentation of C<use VERSION> and C<require VERSION>
595 for a convenient way to fail if the Perl interpreter is too old.
601 The current value of the debugging flags. (Mnemonic: value of B<-D>
608 The maximum system file descriptor, ordinarily 2. System file
609 descriptors are passed to exec()ed processes, while higher file
610 descriptors are not. Also, during an open(), system file descriptors are
611 preserved even if the open() fails. (Ordinary file descriptors are
612 closed before the open() is attempted.) Note that the close-on-exec
613 status of a file descriptor will be decided according to the value of
614 C<$^F> at the time of the open, not the time of the exec.
618 The current set of syntax checks enabled by C<use strict> and other block
619 scoped compiler hints. See the documentation of C<strict> for more details.
625 The current value of the inplace-edit extension. Use C<undef> to disable
626 inplace editing. (Mnemonic: value of B<-i> switch.)
630 By default, running out of memory it is not trappable. However, if
631 compiled for this, Perl may use the contents of C<$^M> as an emergency
632 pool after die()ing with this message. Suppose that your Perl were
633 compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. Then
637 would allocate a 64K buffer for use when in emergency. See the F<INSTALL>
638 file for information on how to enable this option. As a disincentive to
639 casual use of this advanced feature, there is no L<English> long name for
646 The name of the operating system under which this copy of Perl was
647 built, as determined during the configuration process. The value
648 is identical to C<$Config{'osname'}>.
654 The internal variable for debugging support. Different bits mean the
655 following (subject to change):
661 Debug subroutine enter/exit.
665 Line-by-line debugging.
669 Switch off optimizations.
673 Preserve more data for future interactive inspections.
677 Keep info about source lines on which a subroutine is defined.
681 Start with single-step on.
685 Note that some bits may be relevent at compile-time only, some at
686 run-time only. This is a new mechanism and the details may change.
690 Current state of the interpreter. Undefined if parsing of the current
691 module/eval is not finished (may happen in $SIG{__DIE__} and
692 $SIG{__WARN__} handlers). True if inside an eval, othewise false.
698 The time at which the script began running, in seconds since the
699 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
700 and B<-C> filetests are
707 The current value of the warning switch, either TRUE or FALSE.
708 (Mnemonic: related to the B<-w> switch.)
710 =item $EXECUTABLE_NAME
714 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
718 contains the name of the current file when reading from E<lt>E<gt>.
722 The array @ARGV contains the command line arguments intended for the
723 script. Note that C<$#ARGV> is the generally number of arguments minus
724 one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
725 "C<$0>" for the command name.
729 The array @INC contains the list of places to look for Perl scripts to
730 be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
731 initially consists of the arguments to any B<-I> command line switches,
732 followed by the default Perl library, probably F</usr/local/lib/perl>,
733 followed by ".", to represent the current directory. If you need to
734 modify this at runtime, you should use the C<use lib> pragma
735 to get the machine-dependent library properly loaded also:
737 use lib '/mypath/libdir/';
742 Within a subroutine the array @_ contains the parameters passed to that
743 subroutine. See L<perlsub>.
747 The hash %INC contains entries for each filename that has
748 been included via C<do> or C<require>. The key is the filename you
749 specified, and the value is the location of the file actually found.
750 The C<require> command uses this array to determine whether a given file
751 has already been included.
753 =item %ENV $ENV{expr}
755 The hash %ENV contains your current environment. Setting a
756 value in C<ENV> changes the environment for child processes.
758 =item %SIG $SIG{expr}
760 The hash %SIG is used to set signal handlers for various
763 sub handler { # 1st argument is signal name
765 print "Caught a SIG$sig--shutting down\n";
770 $SIG{'INT'} = \&handler;
771 $SIG{'QUIT'} = \&handler;
773 $SIG{'INT'} = 'DEFAULT'; # restore default action
774 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
776 The %SIG array contains values for only the signals actually set within
777 the Perl script. Here are some other examples:
779 $SIG{"PIPE"} = Plumber; # SCARY!!
780 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
781 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
782 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
784 The one marked scary is problematic because it's a bareword, which means
785 sometimes it's a string representing the function, and sometimes it's
786 going to call the subroutine call right then and there! Best to be sure
787 and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
789 If your system has the sigaction() function then signal handlers are
790 installed using it. This means you get reliable signal handling. If
791 your system has the SA_RESTART flag it is used when signals handlers are
792 installed. This means that system calls for which it is supported
793 continue rather than returning when a signal arrives. If you want your
794 system calls to be interrupted by signal delivery then do something like
797 use POSIX ':signal_h';
800 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
801 or die "Error setting SIGALRM handler: $!\n";
805 Certain internal hooks can be also set using the %SIG hash. The
806 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
807 about to be printed. The warning message is passed as the first
808 argument. The presence of a __WARN__ hook causes the ordinary printing
809 of warnings to STDERR to be suppressed. You can use this to save warnings
810 in a variable, or turn warnings into fatal errors, like this:
812 local $SIG{__WARN__} = sub { die $_[0] };
815 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
816 is about to be thrown. The error message is passed as the first
817 argument. When a __DIE__ hook routine returns, the exception
818 processing continues as it would have in the absence of the hook,
819 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
820 The C<__DIE__> handler is explicitly disabled during the call, so that you
821 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
823 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
824 blocks/strings. See L<perlfunc/die>, L<perlvar/$^S> for how to
827 Note that C<__DIE__>/C<__WARN__> handlers are very special in one
828 respect: they may be called to report (probable) errors found by the
829 parser. In such a case the parser may be in inconsistent state, so
830 any attempt to evaluate Perl code from such a handler will probably
831 result in a segfault. This means that calls which result/may-result
832 in parsing Perl should be used with extreme causion, like this:
834 require Carp if defined $^S;
835 Carp::confess("Something wrong") if defined &Carp::confess;
836 die "Something wrong, but could not load Carp to give backtrace...
837 To see backtrace try starting Perl with -MCarp switch";
839 Here the first line will load Carp I<unless> it is the parser who
840 called the handler. The second line will print backtrace and die if
841 Carp was available. The third line will be executed only if Carp was
844 See L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval> for