3 perlvar - Perl predefined variables
7 =head2 Predefined Names
9 The following names have special meaning to Perl. Most of the
10 punctuational 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 (<>) {...} # only equivalent in while!
66 Here are the places where Perl will assume $_ even if you
73 Various unary functions, including functions like ord() and int(), as well
74 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
79 Various list functions like print() and unlink().
83 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
84 without an C<=~> operator.
88 The default iterator variable in a C<foreach> loop if no other
93 The implicit iterator variable in the grep() and map() functions.
97 The default place to put an input record when a C<E<lt>FHE<gt>>
98 operation's result is tested by itself as the sole criterion of a C<while>
99 test. Note that outside of a C<while> test, this will not happen.
103 (Mnemonic: underline is understood in certain operations.)
111 Contains the subpattern from the corresponding set of parentheses in
112 the last pattern matched, not counting patterns matched in nested
113 blocks that have been exited already. (Mnemonic: like \digit.)
114 These variables are all read-only.
120 The string matched by the last successful pattern match (not counting
121 any matches hidden within a BLOCK or eval() enclosed by the current
122 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
128 The string preceding whatever was matched by the last successful
129 pattern match (not counting any matches hidden within a BLOCK or eval
130 enclosed by the current BLOCK). (Mnemonic: ` often precedes a quoted
131 string.) This variable is read-only.
137 The string following whatever was matched by the last successful
138 pattern match (not counting any matches hidden within a BLOCK or eval()
139 enclosed by the current BLOCK). (Mnemonic: ' often follows a quoted
144 print "$`:$&:$'\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 $MULTILINE_MATCHING
165 Set to 1 to do multiline matching within a string, 0 to tell Perl
166 that it can assume that strings contain a single line, for the purpose
167 of optimizing pattern matches. Pattern matches on strings containing
168 multiple newlines can produce confusing results when "C<$*>" is 0. Default
169 is 0. (Mnemonic: * matches multiple things.) Note that this variable
170 only influences the interpretation of "C<^>" and "C<$>". A literal newline can
171 be searched for even when C<$* == 0>.
173 Use of "C<$*>" is deprecated in Perl 5.
175 =item input_line_number HANDLE EXPR
177 =item $INPUT_LINE_NUMBER
183 The current input line number for the last file handle from
184 which you read (or performed a C<seek> or <tell> on). An
185 explicit close on a filehandle resets the line number. Since
186 "C<E<lt>E<gt>>" never does an explicit close, line numbers increase
187 across ARGV files (but see examples under eof()). Localizing C<$.> 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 blank lines as delimiters if set to the
202 null string. You may set it to a multicharacter string to match a
203 multi-character delimiter. Note that setting it to C<"\n\n"> means
204 something slightly different than setting it to C<"">, if the file
205 contains consecutive blank lines. Setting it to C<""> will treat two or
206 more consecutive blank lines as a single blank line. Setting it to
207 C<"\n\n"> will blindly assume that the next input character belongs to the
208 next paragraph, even if it's a newline. (Mnemonic: / is used to
209 delimit line boundaries when quoting poetry.)
212 $_ = <FH>; # whole file now here
215 =item autoflush HANDLE EXPR
217 =item $OUTPUT_AUTOFLUSH
221 If set to nonzero, forces a flush after every write or print on the
222 currently selected output channel. Default is 0 (regardless of whether
223 the channel is actually buffered by the system or not; C<$|> only tells
224 you whether you've asked Perl to explicitly flush after each write).
225 Note that STDOUT will typically be line buffered if output is to the
226 terminal and block buffered otherwise. Setting this variable is useful
227 primarily when you are outputting to a pipe, such as when you are running
228 a Perl script under rsh and want to see the output as it's happening. This
229 has no effect on input buffering.
230 (Mnemonic: when you want your pipes to be piping hot.)
232 =item output_field_separator HANDLE EXPR
234 =item $OUTPUT_FIELD_SEPARATOR
240 The output field separator for the print operator. Ordinarily the
241 print operator simply prints out the comma separated fields you
242 specify. In order to get behavior more like B<awk>, set this variable
243 as you would set B<awk>'s OFS variable to specify what is printed
244 between fields. (Mnemonic: what is printed when there is a , in your
247 =item output_record_separator HANDLE EXPR
249 =item $OUTPUT_RECORD_SEPARATOR
255 The output record separator for the print operator. Ordinarily the
256 print operator simply prints out the comma separated fields you
257 specify, with no trailing newline or record separator assumed. In
258 order to get behavior more like B<awk>, set this variable as you would
259 set B<awk>'s ORS variable to specify what is printed at the end of the
260 print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
261 print. Also, it's just like /, but it's what you get "back" from
264 =item $LIST_SEPARATOR
268 This is like "C<$,>" except that it applies to array values interpolated
269 into a double-quoted string (or similar interpreted string). Default
270 is a space. (Mnemonic: obvious, I think.)
272 =item $SUBSCRIPT_SEPARATOR
278 The subscript separator for multi-dimensional array emulation. If you
279 refer to a hash element as
285 $foo{join($;, $a, $b, $c)}
289 @foo{$a,$b,$c} # a slice--note the @
293 ($foo{$a},$foo{$b},$foo{$c})
295 Default is "\034", the same as SUBSEP in B<awk>. Note that if your
296 keys contain binary data there might not be any safe value for "C<$;>".
297 (Mnemonic: comma (the syntactic subscript separator) is a
298 semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
299 taken for something more important.)
301 Consider using "real" multi-dimensional arrays in Perl 5.
307 The output format for printed numbers. This variable is a half-hearted
308 attempt to emulate B<awk>'s OFMT variable. There are times, however,
309 when B<awk> and Perl have differing notions of what is in fact
310 numeric. The initial value is %.I<n>g, where I<n> is the value
311 of the macro DBL_DIG from your system's F<float.h>. This is different from
312 B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
313 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
315 Use of "C<$#>" is deprecated in Perl 5.
317 =item format_page_number HANDLE EXPR
319 =item $FORMAT_PAGE_NUMBER
323 The current page number of the currently selected output channel.
324 (Mnemonic: % is page number in B<nroff>.)
326 =item format_lines_per_page HANDLE EXPR
328 =item $FORMAT_LINES_PER_PAGE
332 The current page length (printable lines) of the currently selected
333 output channel. Default is 60. (Mnemonic: = has horizontal lines.)
335 =item format_lines_left HANDLE EXPR
337 =item $FORMAT_LINES_LEFT
341 The number of lines left on the page of the currently selected output
342 channel. (Mnemonic: lines_on_page - lines_printed.)
344 =item format_name HANDLE EXPR
350 The name of the current report format for the currently selected output
351 channel. Default is name of the filehandle. (Mnemonic: brother to
354 =item format_top_name HANDLE EXPR
356 =item $FORMAT_TOP_NAME
360 The name of the current top-of-page format for the currently selected
361 output channel. Default is name of the filehandle with _TOP
362 appended. (Mnemonic: points to top of page.)
364 =item format_line_break_characters HANDLE EXPR
366 =item $FORMAT_LINE_BREAK_CHARACTERS
370 The current set of characters after which a string may be broken to
371 fill continuation fields (starting with ^) in a format. Default is
372 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
373 poetry is a part of a line.)
375 =item format_formfeed HANDLE EXPR
377 =item $FORMAT_FORMFEED
381 What formats output to perform a formfeed. Default is \f.
387 The current value of the write() accumulator for format() lines. A format
388 contains formline() commands that put their result into C<$^A>. After
389 calling its format, write() prints out the contents of C<$^A> and empties.
390 So you never actually see the contents of C<$^A> unless you call
391 formline() yourself and then look at it. See L<perlform> and
392 L<perlfunc/formline()>.
398 The status returned by the last pipe close, backtick (C<``>) command,
399 or system() operator. Note that this is the status word returned by
400 the wait() system call, so the exit value of the subprocess is actually
401 (C<$? E<gt>E<gt> 8>). Thus on many systems, C<$? & 255> gives which signal,
402 if any, the process died from, and whether there was a core dump.
403 (Mnemonic: similar to B<sh> and B<ksh>.)
411 If used in a numeric context, yields the current value of errno, with
412 all the usual caveats. (This means that you shouldn't depend on the
413 value of "C<$!>" to be anything in particular unless you've gotten a
414 specific error return indicating a system error.) If used in a string
415 context, yields the corresponding system error string. You can assign
416 to "C<$!>" in order to set I<errno> if, for instance, you want "C<$!>" to return the
417 string for error I<n>, or you want to set the exit value for the die()
418 operator. (Mnemonic: What just went bang?)
420 =item $EXTENDED_OS_ERROR
424 More specific information about the last system error than that
425 provided by C<$!>, if available. (If not, it's just C<$!> again.)
426 At the moment, this differs from C<$!> only under VMS, where it
427 provides the VMS status value from the last system error. The
428 caveats mentioned in the description of C<$!> apply here, too.
429 (Mnemonic: Extra error explanation.)
436 The Perl syntax error message from the last eval() command. If null, the
437 last eval() parsed and executed correctly (although the operations you
438 invoked may have failed in the normal fashion). (Mnemonic: Where was
439 the syntax error "at"?)
441 Note that warning messages are not collected in this variable. You can,
442 however, set up a routine to process warnings by setting $SIG{__WARN__} below.
450 The process number of the Perl running this script. (Mnemonic: same
459 The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
460 if you're running setuid.)
462 =item $EFFECTIVE_USER_ID
468 The effective uid of this process. Example:
470 $< = $>; # set real to effective uid
471 ($<,$>) = ($>,$<); # swap real and effective uid
473 (Mnemonic: it's the uid you went I<TO>, if you're running setuid.) Note:
474 "C<$E<lt>>" and "C<$E<gt>>" can only be swapped on machines supporting setreuid().
482 The real gid of this process. If you are on a machine that supports
483 membership in multiple groups simultaneously, gives a space separated
484 list of groups you are in. The first number is the one returned by
485 getgid(), and the subsequent ones by getgroups(), one of which may be
486 the same as the first number. (Mnemonic: parentheses are used to I<GROUP>
487 things. The real gid is the group you I<LEFT>, if you're running setgid.)
489 =item $EFFECTIVE_GROUP_ID
495 The effective gid of this process. If you are on a machine that
496 supports membership in multiple groups simultaneously, gives a space
497 separated list of groups you are in. The first number is the one
498 returned by getegid(), and the subsequent ones by getgroups(), one of
499 which may be the same as the first number. (Mnemonic: parentheses are
500 used to I<GROUP> things. The effective gid is the group that's I<RIGHT> for
501 you, if you're running setgid.)
503 Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can only be set on machines
504 that support the corresponding I<set[re][ug]id()> routine. "C<$(>" and "C<$)>"
505 can only be swapped on machines supporting setregid(). Because Perl doesn't
506 currently use initgroups(), you can't set your group vector to multiple groups.
512 Contains the name of the file containing the Perl script being
513 executed. Assigning to "C<$0>" modifies the argument area that the ps(1)
514 program sees. This is more useful as a way of indicating the
515 current program state than it is for hiding the program you're running.
516 (Mnemonic: same as B<sh> and B<ksh>.)
520 The index of the first element in an array, and of the first character
521 in a substring. Default is 0, but you could set it to 1 to make
522 Perl behave more like B<awk> (or Fortran) when subscripting and when
523 evaluating the index() and substr() functions. (Mnemonic: [ begins
526 As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
527 and cannot influence the behavior of any other file. Its use is
534 The string printed out when you say C<perl -v>.
535 (This is currently I<BROKEN>).
537 determine at the beginning of a script whether the perl interpreter
538 executing the script is in the right range of versions. If used in a
539 numeric context, returns the version + patchlevel / 1000. Example:
541 # see if getc is available
542 ($version,$patchlevel) =
543 $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
544 print STDERR "(No filename completion available.)\n"
545 if $version * 1000 + $patchlevel < 2016;
547 or, used numerically,
549 warn "No checksumming!\n" if $] < 3.019;
551 (Mnemonic: Is this version of perl in the right bracket?)
557 The current value of the debugging flags. (Mnemonic: value of B<-D>
564 The maximum system file descriptor, ordinarily 2. System file
565 descriptors are passed to exec()ed processes, while higher file
566 descriptors are not. Also, during an open(), system file descriptors are
567 preserved even if the open() fails. (Ordinary file descriptors are
568 closed before the open() is attempted.) Note that the close-on-exec
569 status of a file descriptor will be decided according to the value of
570 C<$^F> at the time of the open, not the time of the exec.
574 The current set of syntax checks enabled by C<use strict>. See the
575 documentation of C<strict> for more details.
581 The current value of the inplace-edit extension. Use C<undef> to disable
582 inplace editing. (Mnemonic: value of B<-i> switch.)
588 The name of the operating system under which this copy of Perl was
589 built, as determined during the configuration process. The value
590 is identical to C<$Config{'osname'}>.
596 The internal flag that the debugger clears so that it doesn't debug
597 itself. You could conceivably disable debugging yourself by clearing
604 The time at which the script began running, in seconds since the
605 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>
606 and B<-C> filetests are
613 The current value of the warning switch, either TRUE or FALSE. (Mnemonic: related to the
616 =item $EXECUTABLE_NAME
620 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
624 contains the name of the current file when reading from <>.
628 The array @ARGV contains the command line arguments intended for the
629 script. Note that C<$#ARGV> is the generally number of arguments minus
630 one, since C<$ARGV[0]> is the first argument, I<NOT> the command name. See
631 "C<$0>" for the command name.
635 The array @INC contains the list of places to look for Perl scripts to
636 be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
637 initially consists of the arguments to any B<-I> command line switches,
638 followed by the default Perl library, probably F</usr/local/lib/perl>,
639 followed by ".", to represent the current directory. If you need to
640 modify this at runtime, you should use the C<use lib> pragma in order
641 to also get the machine-dependent library properly loaded:
643 use lib '/mypath/libdir/';
648 The hash %INC contains entries for each filename that has
649 been included via C<do> or C<require>. The key is the filename you
650 specified, and the value is the location of the file actually found.
651 The C<require> command uses this array to determine whether a given file
652 has already been included.
656 The hash %ENV contains your current environment. Setting a
657 value in C<ENV> changes the environment for child processes.
661 The hash %SIG is used to set signal handlers for various
664 sub handler { # 1st argument is signal name
666 print "Caught a SIG$sig--shutting down\n";
671 $SIG{'INT'} = 'handler';
672 $SIG{'QUIT'} = 'handler';
674 $SIG{'INT'} = 'DEFAULT'; # restore default action
675 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
677 The %SIG array only contains values for the signals actually set within
678 the Perl script. Here are some other examples:
680 $SIG{PIPE} = Plumber; # SCARY!!
681 $SIG{"PIPE"} = "Plumber"; # just fine, assumes main::Plumber
682 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
683 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
685 The one marked scary is problematic because it's a bareword, which means
686 sometimes it's a string representing the function, and sometimes it's
687 going to call the subroutine call right then and there! Best to be sure
688 and quote it or take a reference to it. *Plumber works too. See L<perlsubs>.
690 Certain internal hooks can be also set using the %SIG hash. The
691 routine indicated by $SIG{__WARN__} is called when a warning message is
692 about to be printed. The warning message is passed as the first
693 argument. The presence of a __WARN__ hook causes the ordinary printing
694 of warnings to STDERR to be suppressed. You can use this to save warnings
695 in a variable, or turn warnings into fatal errors, like this:
697 local $SIG{__WARN__} = sub { die $_[0] };
700 The routine indicated by $SIG{__DIE__} is called when a fatal exception
701 is about to be thrown. The error message is passed as the first
702 argument. When a __DIE__ hook routine returns, the exception
703 processing continues as it would have in the absence of the hook,
704 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
705 The __DIE__ handler is explicitly disabled during the call, so that you
706 can die from a __DIE__ handler. Similarly for __WARN__.