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!
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 a multicharacter string to match a
204 multi-character delimiter. Note that setting it to C<"\n\n"> means
205 something slightly different than setting it to C<"">, if the file
206 contains consecutive empty lines. Setting it to C<""> will treat two
207 or more consecutive empty lines as a single empty line. Setting it to
208 C<"\n\n"> will blindly assume that the next input character belongs to
209 the next paragraph, even if it's a newline. (Mnemonic: / is used to
210 delimit line boundaries when quoting poetry.)
213 $_ = <FH>; # whole file now here
216 =item autoflush HANDLE EXPR
218 =item $OUTPUT_AUTOFLUSH
222 If set to nonzero, forces a flush after every write or print on the
223 currently selected output channel. Default is 0 (regardless of whether
224 the channel is actually buffered by the system or not; C<$|> tells you
225 only whether you've asked Perl explicitly to flush after each write).
226 Note that STDOUT will typically be line buffered if output is to the
227 terminal and block buffered otherwise. Setting this variable is useful
228 primarily when you are outputting to a pipe, such as when you are running
229 a Perl script under rsh and want to see the output as it's happening. This
230 has no effect on input buffering.
231 (Mnemonic: when you want your pipes to be piping hot.)
233 =item output_field_separator HANDLE EXPR
235 =item $OUTPUT_FIELD_SEPARATOR
241 The output field separator for the print operator. Ordinarily the
242 print operator simply prints out the comma-separated fields you
243 specify. To get behavior more like B<awk>, set this variable
244 as you would set B<awk>'s OFS variable to specify what is printed
245 between fields. (Mnemonic: what is printed when there is a , in your
248 =item output_record_separator HANDLE EXPR
250 =item $OUTPUT_RECORD_SEPARATOR
256 The output record separator for the print operator. Ordinarily the
257 print operator simply prints out the comma-separated fields you
258 specify, with no trailing newline or record separator assumed.
259 To get behavior more like B<awk>, set this variable as you would
260 set B<awk>'s ORS variable to specify what is printed at the end of the
261 print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
262 print. Also, it's just like C<$/>, but it's what you get "back" from
265 =item $LIST_SEPARATOR
269 This is like "C<$,>" except that it applies to array values interpolated
270 into a double-quoted string (or similar interpreted string). Default
271 is a space. (Mnemonic: obvious, I think.)
273 =item $SUBSCRIPT_SEPARATOR
279 The subscript separator for multi-dimensional array emulation. If you
280 refer to a hash element as
286 $foo{join($;, $a, $b, $c)}
290 @foo{$a,$b,$c} # a slice--note the @
294 ($foo{$a},$foo{$b},$foo{$c})
296 Default is "\034", the same as SUBSEP in B<awk>. Note that if your
297 keys contain binary data there might not be any safe value for "C<$;>".
298 (Mnemonic: comma (the syntactic subscript separator) is a
299 semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
300 taken for something more important.)
302 Consider using "real" multi-dimensional arrays.
308 The output format for printed numbers. This variable is a half-hearted
309 attempt to emulate B<awk>'s OFMT variable. There are times, however,
310 when B<awk> and Perl have differing notions of what is in fact
311 numeric. The initial value is %.I<n>g, where I<n> is the value
312 of the macro DBL_DIG from your system's F<float.h>. This is different from
313 B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
314 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
316 Use of "C<$#>" is deprecated.
318 =item format_page_number HANDLE EXPR
320 =item $FORMAT_PAGE_NUMBER
324 The current page number of the currently selected output channel.
325 (Mnemonic: % is page number in B<nroff>.)
327 =item format_lines_per_page HANDLE EXPR
329 =item $FORMAT_LINES_PER_PAGE
333 The current page length (printable lines) of the currently selected
334 output channel. Default is 60. (Mnemonic: = has horizontal lines.)
336 =item format_lines_left HANDLE EXPR
338 =item $FORMAT_LINES_LEFT
342 The number of lines left on the page of the currently selected output
343 channel. (Mnemonic: lines_on_page - lines_printed.)
345 =item format_name HANDLE EXPR
351 The name of the current report format for the currently selected output
352 channel. Default is name of the filehandle. (Mnemonic: brother to
355 =item format_top_name HANDLE EXPR
357 =item $FORMAT_TOP_NAME
361 The name of the current top-of-page format for the currently selected
362 output channel. Default is name of the filehandle with _TOP
363 appended. (Mnemonic: points to top of page.)
365 =item format_line_break_characters HANDLE EXPR
367 =item $FORMAT_LINE_BREAK_CHARACTERS
371 The current set of characters after which a string may be broken to
372 fill continuation fields (starting with ^) in a format. Default is
373 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
374 poetry is a part of a line.)
376 =item format_formfeed HANDLE EXPR
378 =item $FORMAT_FORMFEED
382 What formats output to perform a form feed. Default is \f.
388 The current value of the write() accumulator for format() lines. A format
389 contains formline() commands that put their result into C<$^A>. After
390 calling its format, write() prints out the contents of C<$^A> and empties.
391 So you never actually see the contents of C<$^A> unless you call
392 formline() yourself and then look at it. See L<perlform> and
393 L<perlfunc/formline()>.
399 The status returned by the last pipe close, back-tick (C<``>) command,
400 or system() operator. Note that this is the status word returned by
401 the wait() system call (or else is made up to look like it). Thus,
402 the exit value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and
403 C<$? & 255> gives which signal, if any, the process died from, and
404 whether there was a core dump. (Mnemonic: similar to B<sh> and
407 Inside an C<END> subroutine C<$?> contains the value that is going to be
408 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
409 change the exit status of the script.
411 Under VMS, the pragma C<use vmsish 'status'> make C<$?> reflect the
412 actual VMS exit status, instead of the default emulation of POSIX
421 If used in a numeric context, yields the current value of errno, with
422 all the usual caveats. (This means that you shouldn't depend on the
423 value of "C<$!>" to be anything in particular unless you've gotten a
424 specific error return indicating a system error.) If used in a string
425 context, yields the corresponding system error string. You can assign
426 to "C<$!>" to set I<errno> if, for instance, you want "C<$!>" to return the
427 string for error I<n>, or you want to set the exit value for the die()
428 operator. (Mnemonic: What just went bang?)
430 =item $EXTENDED_OS_ERROR
434 More specific information about the last system error than that provided by
435 C<$!>, if available. (If not, it's just C<$!> again, except under OS/2.)
436 At the moment, this differs from C<$!> under only VMS and OS/2, where it
437 provides the VMS status value from the last system error, and OS/2 error
438 code of the last call to OS/2 API which was not directed via CRT. The
439 caveats mentioned in the description of C<$!> apply here, too.
440 (Mnemonic: Extra error explanation.)
442 Note that under OS/2 C<$!> and C<$^E> do not track each other, so if an
443 OS/2-specific call is performed, you may need to check both.
449 The Perl syntax error message from the last eval() command. If null, the
450 last eval() parsed and executed correctly (although the operations you
451 invoked may have failed in the normal fashion). (Mnemonic: Where was
452 the syntax error "at"?)
454 Note that warning messages are not collected in this variable. You can,
455 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
464 The process number of the Perl running this script. (Mnemonic: same
473 The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
474 if you're running setuid.)
476 =item $EFFECTIVE_USER_ID
482 The effective uid of this process. Example:
484 $< = $>; # set real to effective uid
485 ($<,$>) = ($>,$<); # swap real and effective uid
487 (Mnemonic: it's the uid you went I<TO>, if you're running setuid.) Note:
488 "C<$E<lt>>" and "C<$E<gt>>" can be swapped on only machines supporting setreuid().
496 The real gid of this process. If you are on a machine that supports
497 membership in multiple groups simultaneously, gives a space separated
498 list of groups you are in. The first number is the one returned by
499 getgid(), and the subsequent ones by getgroups(), one of which may be
500 the same as the first number. (Mnemonic: parentheses are used to I<GROUP>
501 things. The real gid is the group you I<LEFT>, if you're running setgid.)
503 =item $EFFECTIVE_GROUP_ID
509 The effective gid of this process. If you are on a machine that
510 supports membership in multiple groups simultaneously, gives a space
511 separated list of groups you are in. The first number is the one
512 returned by getegid(), and the subsequent ones by getgroups(), one of
513 which may be the same as the first number. (Mnemonic: parentheses are
514 used to I<GROUP> things. The effective gid is the group that's I<RIGHT> for
515 you, if you're running setgid.)
517 Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
518 machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
519 and "C<$)>" can be swapped on only machines supporting setregid(). Because
520 Perl doesn't currently use initgroups(), you can't set your group vector to
527 Contains the name of the file containing the Perl script being
528 executed. Assigning to "C<$0>" modifies the argument area that the ps(1)
529 program sees. This is more useful as a way of indicating the
530 current program state than it is for hiding the program you're running.
531 (Mnemonic: same as B<sh> and B<ksh>.)
535 The index of the first element in an array, and of the first character
536 in a substring. Default is 0, but you could set it to 1 to make
537 Perl behave more like B<awk> (or Fortran) when subscripting and when
538 evaluating the index() and substr() functions. (Mnemonic: [ begins
541 As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
542 and cannot influence the behavior of any other file. Its use is
549 The string printed out when you say C<perl -v>.
550 (This is currently I<BROKEN>).
552 determine at the beginning of a script whether the perl interpreter
553 executing the script is in the right range of versions. If used in a
554 numeric context, returns the version + patchlevel / 1000. Example:
556 # see if getc is available
557 ($version,$patchlevel) =
558 $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
559 print STDERR "(No filename completion available.)\n"
560 if $version * 1000 + $patchlevel < 2016;
562 or, used numerically,
564 warn "No checksumming!\n" if $] < 3.019;
566 (Mnemonic: Is this version of perl in the right bracket?)
572 The current value of the debugging flags. (Mnemonic: value of B<-D>
579 The maximum system file descriptor, ordinarily 2. System file
580 descriptors are passed to exec()ed processes, while higher file
581 descriptors are not. Also, during an open(), system file descriptors are
582 preserved even if the open() fails. (Ordinary file descriptors are
583 closed before the open() is attempted.) Note that the close-on-exec
584 status of a file descriptor will be decided according to the value of
585 C<$^F> at the time of the open, not the time of the exec.
589 The current set of syntax checks enabled by C<use strict>. See the
590 documentation of C<strict> for more details.
596 The current value of the inplace-edit extension. Use C<undef> to disable
597 inplace editing. (Mnemonic: value of B<-i> switch.)
603 The name of the operating system under which this copy of Perl was
604 built, as determined during the configuration process. The value
605 is identical to C<$Config{'osname'}>.
611 The internal flag that the debugger clears so that it doesn't debug
612 itself. You could conceivably disable debugging yourself by clearing
619 The time at which the script began running, in seconds since the
620 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
621 and B<-C> filetests are
628 The current value of the warning switch, either TRUE or FALSE.
629 (Mnemonic: related to the B<-w> switch.)
631 =item $EXECUTABLE_NAME
635 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
639 contains the name of the current file when reading from E<lt>E<gt>.
643 The array @ARGV contains the command line arguments intended for the
644 script. Note that C<$#ARGV> is the generally number of arguments minus
645 one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
646 "C<$0>" for the command name.
650 The array @INC contains the list of places to look for Perl scripts to
651 be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
652 initially consists of the arguments to any B<-I> command line switches,
653 followed by the default Perl library, probably F</usr/local/lib/perl>,
654 followed by ".", to represent the current directory. If you need to
655 modify this at runtime, you should use the C<use lib> pragma
656 to get the machine-dependent library properly loaded also:
658 use lib '/mypath/libdir/';
663 The hash %INC contains entries for each filename that has
664 been included via C<do> or C<require>. The key is the filename you
665 specified, and the value is the location of the file actually found.
666 The C<require> command uses this array to determine whether a given file
667 has already been included.
671 The hash %ENV contains your current environment. Setting a
672 value in C<ENV> changes the environment for child processes.
676 The hash %SIG is used to set signal handlers for various
679 sub handler { # 1st argument is signal name
681 print "Caught a SIG$sig--shutting down\n";
686 $SIG{'INT'} = 'handler';
687 $SIG{'QUIT'} = 'handler';
689 $SIG{'INT'} = 'DEFAULT'; # restore default action
690 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
692 The %SIG array contains values for only the signals actually set within
693 the Perl script. Here are some other examples:
695 $SIG{PIPE} = Plumber; # SCARY!!
696 $SIG{"PIPE"} = "Plumber"; # just fine, assumes main::Plumber
697 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
698 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
700 The one marked scary is problematic because it's a bareword, which means
701 sometimes it's a string representing the function, and sometimes it's
702 going to call the subroutine call right then and there! Best to be sure
703 and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
705 If your system has the sigaction() function then signal handlers are
706 installed using it. This means you get reliable signal handling. If
707 your system has the SA_RESTART flag it is used when signals handlers are
708 installed. This means that system calls for which it is supported
709 continue rather than returning when a signal arrives. If you want your
710 system calls to be interrupted by signal delivery then do something like
713 use POSIX ':signal_h';
716 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
717 or die "Error setting SIGALRM handler: $!\n";
721 Certain internal hooks can be also set using the %SIG hash. The
722 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
723 about to be printed. The warning message is passed as the first
724 argument. The presence of a __WARN__ hook causes the ordinary printing
725 of warnings to STDERR to be suppressed. You can use this to save warnings
726 in a variable, or turn warnings into fatal errors, like this:
728 local $SIG{__WARN__} = sub { die $_[0] };
731 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
732 is about to be thrown. The error message is passed as the first
733 argument. When a __DIE__ hook routine returns, the exception
734 processing continues as it would have in the absence of the hook,
735 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
736 The C<__DIE__> handler is explicitly disabled during the call, so that you
737 can die from a C<__DIE__> handler. Similarly for C<__WARN__>. See
738 L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval>.