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 be set by calling an object method on
22 the FileHandle object. (Summary lines below for this contain the word
23 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.
51 The default input and pattern-searching space. The following pairs are
54 while (<>) {...} # equivalent in only while!
55 while (defined($_ = <>)) {...}
66 Here are the places where Perl will assume $_ even if you
73 Various unary functions, including functions like ord() and int(), as well
74 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
79 Various list functions like print() and unlink().
83 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
84 without an C<=~> operator.
88 The default iterator variable in a C<foreach> loop if no other
93 The implicit iterator variable in the grep() and map() functions.
97 The default place to put an input record when a C<E<lt>FHE<gt>>
98 operation's result is tested by itself as the sole criterion of a C<while>
99 test. Note that outside of a C<while> test, this will not happen.
103 (Mnemonic: underline is understood in certain operations.)
109 =item $E<lt>I<digit>E<gt>
111 Contains the sub-pattern from the corresponding set of parentheses in
112 the last pattern matched, not counting patterns matched in nested
113 blocks that have been exited already. (Mnemonic: like \digit.)
114 These variables are all read-only.
120 The string matched by the last successful pattern match (not counting
121 any matches hidden within a BLOCK or eval() enclosed by the current
122 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
128 The string preceding whatever was matched by the last successful
129 pattern match (not counting any matches hidden within a BLOCK or eval
130 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
131 string.) This variable is read-only.
137 The string following whatever was matched by the last successful
138 pattern match (not counting any matches hidden within a BLOCK or eval()
139 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
144 print "$`:$&:$'\n"; # prints abc:def:ghi
146 This variable is read-only.
148 =item $LAST_PAREN_MATCH
152 The last bracket matched by the last search pattern. This is useful if
153 you don't know which of a set of alternative patterns matched. For
156 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
158 (Mnemonic: be positive and forward looking.)
159 This variable is read-only.
161 =item $MULTILINE_MATCHING
165 Set to 1 to do multi-line matching within a string, 0 to tell Perl
166 that it can assume that strings contain a single line, for the purpose
167 of optimizing pattern matches. Pattern matches on strings containing
168 multiple newlines can produce confusing results when "C<$*>" is 0. Default
169 is 0. (Mnemonic: * matches multiple things.) Note that this variable
170 influences the interpretation of only "C<^>" and "C<$>". A literal newline can
171 be searched for even when C<$* == 0>.
173 Use of "C<$*>" is deprecated in modern perls.
175 =item input_line_number HANDLE EXPR
177 =item $INPUT_LINE_NUMBER
183 The current input line number for the last file handle from
184 which you read (or performed a C<seek> or C<tell> on). An
185 explicit close on a filehandle resets the line number. Because
186 "C<E<lt>E<gt>>" never does an explicit close, line numbers increase
187 across ARGV files (but see examples under eof()). Localizing C<$.> has
188 the effect of also localizing Perl's notion of "the last read
189 filehandle". (Mnemonic: many programs use "." to mean the current line
192 =item input_record_separator HANDLE EXPR
194 =item $INPUT_RECORD_SEPARATOR
200 The input record separator, newline by default. Works like B<awk>'s RS
201 variable, including treating empty lines as delimiters if set to the
202 null string. (Note: An empty line cannot contain any spaces or
203 tabs.) You may set it to C<undef> if you want to treat the whole file
205 You may set it to a multicharacter string to match a
206 multi-character delimiter. Note that setting it to C<"\n\n"> means
207 something slightly different than setting it to C<"">, if the file
208 contains consecutive empty lines. Setting it to C<""> will treat two
209 or more consecutive empty lines as a single empty line. Setting it to
210 C<"\n\n"> will blindly assume that the next input character belongs to
211 the next paragraph, even if it's a newline. (Mnemonic: / is used to
212 delimit line boundaries when quoting poetry.)
215 $_ = <FH>; # whole file now here
218 Remember: the value of $/ is a string, not a regexp. AWK has to be
219 better for something :-)
221 =item autoflush HANDLE EXPR
223 =item $OUTPUT_AUTOFLUSH
227 If set to nonzero, forces a flush right away and after every write or print on the
228 currently selected output channel. Default is 0 (regardless of whether
229 the channel is actually buffered by the system or not; C<$|> tells you
230 only whether you've asked Perl explicitly to flush after each write).
231 Note that STDOUT will typically be line buffered if output is to the
232 terminal and block buffered otherwise. Setting this variable is useful
233 primarily when you are outputting to a pipe, such as when you are running
234 a Perl script under rsh and want to see the output as it's happening. This
235 has no effect on input buffering.
236 (Mnemonic: when you want your pipes to be piping hot.)
238 =item output_field_separator HANDLE EXPR
240 =item $OUTPUT_FIELD_SEPARATOR
246 The output field separator for the print operator. Ordinarily the
247 print operator simply prints out the comma-separated fields you
248 specify. To get behavior more like B<awk>, set this variable
249 as you would set B<awk>'s OFS variable to specify what is printed
250 between fields. (Mnemonic: what is printed when there is a , in your
253 =item output_record_separator HANDLE EXPR
255 =item $OUTPUT_RECORD_SEPARATOR
261 The output record separator for the print operator. Ordinarily the
262 print operator simply prints out the comma-separated fields you
263 specify, with no trailing newline or record separator assumed.
264 To get behavior more like B<awk>, set this variable as you would
265 set B<awk>'s ORS variable to specify what is printed at the end of the
266 print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
267 print. Also, it's just like C<$/>, but it's what you get "back" from
270 =item $LIST_SEPARATOR
274 This is like "C<$,>" except that it applies to array values interpolated
275 into a double-quoted string (or similar interpreted string). Default
276 is a space. (Mnemonic: obvious, I think.)
278 =item $SUBSCRIPT_SEPARATOR
284 The subscript separator for multi-dimensional array emulation. If you
285 refer to a hash element as
291 $foo{join($;, $a, $b, $c)}
295 @foo{$a,$b,$c} # a slice--note the @
299 ($foo{$a},$foo{$b},$foo{$c})
301 Default is "\034", the same as SUBSEP in B<awk>. Note that if your
302 keys contain binary data there might not be any safe value for "C<$;>".
303 (Mnemonic: comma (the syntactic subscript separator) is a
304 semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
305 taken for something more important.)
307 Consider using "real" multi-dimensional arrays.
313 The output format for printed numbers. This variable is a half-hearted
314 attempt to emulate B<awk>'s OFMT variable. There are times, however,
315 when B<awk> and Perl have differing notions of what is in fact
316 numeric. The initial value is %.I<n>g, where I<n> is the value
317 of the macro DBL_DIG from your system's F<float.h>. This is different from
318 B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
319 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
321 Use of "C<$#>" is deprecated.
323 =item format_page_number HANDLE EXPR
325 =item $FORMAT_PAGE_NUMBER
329 The current page number of the currently selected output channel.
330 (Mnemonic: % is page number in B<nroff>.)
332 =item format_lines_per_page HANDLE EXPR
334 =item $FORMAT_LINES_PER_PAGE
338 The current page length (printable lines) of the currently selected
339 output channel. Default is 60. (Mnemonic: = has horizontal lines.)
341 =item format_lines_left HANDLE EXPR
343 =item $FORMAT_LINES_LEFT
347 The number of lines left on the page of the currently selected output
348 channel. (Mnemonic: lines_on_page - lines_printed.)
350 =item format_name HANDLE EXPR
356 The name of the current report format for the currently selected output
357 channel. Default is name of the filehandle. (Mnemonic: brother to
360 =item format_top_name HANDLE EXPR
362 =item $FORMAT_TOP_NAME
366 The name of the current top-of-page format for the currently selected
367 output channel. Default is name of the filehandle with _TOP
368 appended. (Mnemonic: points to top of page.)
370 =item format_line_break_characters HANDLE EXPR
372 =item $FORMAT_LINE_BREAK_CHARACTERS
376 The current set of characters after which a string may be broken to
377 fill continuation fields (starting with ^) in a format. Default is
378 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
379 poetry is a part of a line.)
381 =item format_formfeed HANDLE EXPR
383 =item $FORMAT_FORMFEED
387 What formats output to perform a form feed. Default is \f.
393 The current value of the write() accumulator for format() lines. A format
394 contains formline() commands that put their result into C<$^A>. After
395 calling its format, write() prints out the contents of C<$^A> and empties.
396 So you never actually see the contents of C<$^A> unless you call
397 formline() yourself and then look at it. See L<perlform> and
398 L<perlfunc/formline()>.
404 The status returned by the last pipe close, back-tick (C<``>) command,
405 or system() operator. Note that this is the status word returned by
406 the wait() system call (or else is made up to look like it). Thus,
407 the exit value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and
408 C<$? & 255> gives which signal, if any, the process died from, and
409 whether there was a core dump. (Mnemonic: similar to B<sh> and
412 Note that if you have installed a signal handler for C<SIGCHLD>, the
413 value of C<$?> will usually be wrong outside that handler.
415 Inside an C<END> subroutine C<$?> contains the value that is going to be
416 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
417 change the exit status of the script.
419 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
420 actual VMS exit status, instead of the default emulation of POSIX
429 If used in a numeric context, yields the current value of errno, with
430 all the usual caveats. (This means that you shouldn't depend on the
431 value of "C<$!>" to be anything in particular unless you've gotten a
432 specific error return indicating a system error.) If used in a string
433 context, yields the corresponding system error string. You can assign
434 to "C<$!>" to set I<errno> if, for instance, you want "C<$!>" to return the
435 string for error I<n>, or you want to set the exit value for the die()
436 operator. (Mnemonic: What just went bang?)
438 =item $EXTENDED_OS_ERROR
442 More specific information about the last system error than that provided by
443 C<$!>, if available. (If not, it's just C<$!> again, except under OS/2.)
444 At the moment, this differs from C<$!> under only VMS and OS/2, where it
445 provides the VMS status value from the last system error, and OS/2 error
446 code of the last call to OS/2 API which was not directed via CRT. The
447 caveats mentioned in the description of C<$!> apply here, too.
448 (Mnemonic: Extra error explanation.)
450 Note that under OS/2 C<$!> and C<$^E> do not track each other, so if an
451 OS/2-specific call is performed, you may need to check both.
457 The Perl syntax error message from the last eval() command. If null, the
458 last eval() parsed and executed correctly (although the operations you
459 invoked may have failed in the normal fashion). (Mnemonic: Where was
460 the syntax error "at"?)
462 Note that warning messages are not collected in this variable. You can,
463 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
472 The process number of the Perl running this script. (Mnemonic: same
481 The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
482 if you're running setuid.)
484 =item $EFFECTIVE_USER_ID
490 The effective uid of this process. Example:
492 $< = $>; # set real to effective uid
493 ($<,$>) = ($>,$<); # swap real and effective uid
495 (Mnemonic: it's the uid you went I<TO>, if you're running setuid.) Note:
496 "C<$E<lt>>" and "C<$E<gt>>" can be swapped on only machines supporting setreuid().
504 The real gid of this process. If you are on a machine that supports
505 membership in multiple groups simultaneously, gives a space separated
506 list of groups you are in. The first number is the one returned by
507 getgid(), and the subsequent ones by getgroups(), one of which may be
508 the same as the first number. (Mnemonic: parentheses are used to I<GROUP>
509 things. The real gid is the group you I<LEFT>, if you're running setgid.)
511 =item $EFFECTIVE_GROUP_ID
517 The effective gid of this process. If you are on a machine that
518 supports membership in multiple groups simultaneously, gives a space
519 separated list of groups you are in. The first number is the one
520 returned by getegid(), and the subsequent ones by getgroups(), one of
521 which may be the same as the first number. (Mnemonic: parentheses are
522 used to I<GROUP> things. The effective gid is the group that's I<RIGHT> for
523 you, if you're running setgid.)
525 Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
526 machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
527 and "C<$)>" can be swapped on only machines supporting setregid(). Because
528 Perl doesn't currently use initgroups(), you can't set your group vector to
535 Contains the name of the file containing the Perl script being
536 executed. On some operating systems
537 assigning to "C<$0>" modifies the argument area that the ps(1)
538 program sees. This is more useful as a way of indicating the
539 current program state than it is for hiding the program you're running.
540 (Mnemonic: same as B<sh> and B<ksh>.)
544 The index of the first element in an array, and of the first character
545 in a substring. Default is 0, but you could set it to 1 to make
546 Perl behave more like B<awk> (or Fortran) when subscripting and when
547 evaluating the index() and substr() functions. (Mnemonic: [ begins
550 As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
551 and cannot influence the behavior of any other file. Its use is
558 The string printed out when you say C<perl -v>.
559 (This is currently I<BROKEN>).
561 determine at the beginning of a script whether the perl interpreter
562 executing the script is in the right range of versions. If used in a
563 numeric context, returns the version + patchlevel / 1000. Example:
565 # see if getc is available
566 ($version,$patchlevel) =
567 $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
568 print STDERR "(No filename completion available.)\n"
569 if $version * 1000 + $patchlevel < 2016;
571 or, used numerically,
573 warn "No checksumming!\n" if $] < 3.019;
575 (Mnemonic: Is this version of perl in the right bracket?)
581 The current value of the debugging flags. (Mnemonic: value of B<-D>
588 The maximum system file descriptor, ordinarily 2. System file
589 descriptors are passed to exec()ed processes, while higher file
590 descriptors are not. Also, during an open(), system file descriptors are
591 preserved even if the open() fails. (Ordinary file descriptors are
592 closed before the open() is attempted.) Note that the close-on-exec
593 status of a file descriptor will be decided according to the value of
594 C<$^F> at the time of the open, not the time of the exec.
598 The current set of syntax checks enabled by C<use strict>. See the
599 documentation of C<strict> for more details.
605 The current value of the inplace-edit extension. Use C<undef> to disable
606 inplace editing. (Mnemonic: value of B<-i> switch.)
612 The name of the operating system under which this copy of Perl was
613 built, as determined during the configuration process. The value
614 is identical to C<$Config{'osname'}>.
620 The internal flag that the debugger clears so that it doesn't debug
621 itself. You could conceivably disable debugging yourself by clearing
628 The time at which the script began running, in seconds since the
629 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
630 and B<-C> filetests are
637 The current value of the warning switch, either TRUE or FALSE.
638 (Mnemonic: related to the B<-w> switch.)
640 =item $EXECUTABLE_NAME
644 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
648 contains the name of the current file when reading from E<lt>E<gt>.
652 The array @ARGV contains the command line arguments intended for the
653 script. Note that C<$#ARGV> is the generally number of arguments minus
654 one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
655 "C<$0>" for the command name.
659 The array @INC contains the list of places to look for Perl scripts to
660 be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
661 initially consists of the arguments to any B<-I> command line switches,
662 followed by the default Perl library, probably F</usr/local/lib/perl>,
663 followed by ".", to represent the current directory. If you need to
664 modify this at runtime, you should use the C<use lib> pragma
665 to get the machine-dependent library properly loaded also:
667 use lib '/mypath/libdir/';
672 The hash %INC contains entries for each filename that has
673 been included via C<do> or C<require>. The key is the filename you
674 specified, and the value is the location of the file actually found.
675 The C<require> command uses this array to determine whether a given file
676 has already been included.
680 The hash %ENV contains your current environment. Setting a
681 value in C<ENV> changes the environment for child processes.
685 The hash %SIG is used to set signal handlers for various
688 sub handler { # 1st argument is signal name
690 print "Caught a SIG$sig--shutting down\n";
695 $SIG{'INT'} = 'handler';
696 $SIG{'QUIT'} = 'handler';
698 $SIG{'INT'} = 'DEFAULT'; # restore default action
699 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
701 The %SIG array contains values for only the signals actually set within
702 the Perl script. Here are some other examples:
704 $SIG{PIPE} = Plumber; # SCARY!!
705 $SIG{"PIPE"} = "Plumber"; # just fine, assumes main::Plumber
706 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
707 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
709 The one marked scary is problematic because it's a bareword, which means
710 sometimes it's a string representing the function, and sometimes it's
711 going to call the subroutine call right then and there! Best to be sure
712 and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
714 If your system has the sigaction() function then signal handlers are
715 installed using it. This means you get reliable signal handling. If
716 your system has the SA_RESTART flag it is used when signals handlers are
717 installed. This means that system calls for which it is supported
718 continue rather than returning when a signal arrives. If you want your
719 system calls to be interrupted by signal delivery then do something like
722 use POSIX ':signal_h';
725 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
726 or die "Error setting SIGALRM handler: $!\n";
730 Certain internal hooks can be also set using the %SIG hash. The
731 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
732 about to be printed. The warning message is passed as the first
733 argument. The presence of a __WARN__ hook causes the ordinary printing
734 of warnings to STDERR to be suppressed. You can use this to save warnings
735 in a variable, or turn warnings into fatal errors, like this:
737 local $SIG{__WARN__} = sub { die $_[0] };
740 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
741 is about to be thrown. The error message is passed as the first
742 argument. When a __DIE__ hook routine returns, the exception
743 processing continues as it would have in the absence of the hook,
744 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
745 The C<__DIE__> handler is explicitly disabled during the call, so that you
746 can die from a C<__DIE__> handler. Similarly for C<__WARN__>. See
747 L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval>.
751 By default, running out of memory it is not trappable. However, if
752 compiled for this, Perl may use the contents of C<$^M> as an emergency
753 pool after die()ing with this message. Suppose that your Perl were
754 compiled with -DEMERGENCY_SBRK and used Perl's malloc. Then
758 would allocate a 64K buffer for use when in emergency. See the F<INSTALL>
759 file for information on how to enable this option. As a disincentive to
760 casual use of this advanced feature, there is no L<English> long name for