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 $_ even if you
73 Various unary functions, including functions like ord() and int(), as well
74 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
79 Various list functions like print() and unlink().
83 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
84 without an C<=~> operator.
88 The default iterator variable in a C<foreach> loop if no other
93 The implicit iterator variable in the grep() and map() functions.
97 The default place to put an input record when a C<E<lt>FHE<gt>>
98 operation's result is tested by itself as the sole criterion of a C<while>
99 test. Note that outside of a C<while> test, this will not happen.
103 (Mnemonic: underline is understood in certain operations.)
107 Contains the subpattern from the corresponding set of parentheses in
108 the last pattern matched, not counting patterns matched in nested
109 blocks that have been exited already. (Mnemonic: like \digit.)
110 These variables are all read-only.
116 The string matched by the last successful pattern match (not counting
117 any matches hidden within a BLOCK or eval() enclosed by the current
118 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
124 The string preceding whatever was matched by the last successful
125 pattern match (not counting any matches hidden within a BLOCK or eval
126 enclosed by the current BLOCK). (Mnemonic: ` often precedes a quoted
127 string.) This variable is read-only.
133 The string following whatever was matched by the last successful
134 pattern match (not counting any matches hidden within a BLOCK or eval()
135 enclosed by the current BLOCK). (Mnemonic: ' often follows a quoted
140 print "$`:$&:$'\n"; # prints abc:def:ghi
142 This variable is read-only.
144 =item $LAST_PAREN_MATCH
148 The last bracket matched by the last search pattern. This is useful if
149 you don't know which of a set of alternative patterns matched. For
152 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
154 (Mnemonic: be positive and forward looking.)
155 This variable is read-only.
157 =item $MULTILINE_MATCHING
161 Set to 1 to do multiline matching within a string, 0 to tell Perl
162 that it can assume that strings contain a single line, for the purpose
163 of optimizing pattern matches. Pattern matches on strings containing
164 multiple newlines can produce confusing results when "C<$*>" is 0. Default
165 is 0. (Mnemonic: * matches multiple things.) Note that this variable
166 only influences the interpretation of "C<^>" and "C<$>". A literal newline can
167 be searched for even when C<$* == 0>.
169 Use of "C<$*>" is deprecated in Perl 5.
171 =item input_line_number HANDLE EXPR
173 =item $INPUT_LINE_NUMBER
179 The current input line number of the last filehandle that was read. An
180 explicit close on the filehandle resets the line number. Since
181 "C<E<lt>E<gt>>" never does an explicit close, line numbers increase
182 across ARGV files (but see examples under eof()). Localizing C<$.> has
183 the effect of also localizing Perl's notion of "the last read
184 filehandle". (Mnemonic: many programs use "." to mean the current line
187 =item input_record_separator HANDLE EXPR
189 =item $INPUT_RECORD_SEPARATOR
195 The input record separator, newline by default. Works like B<awk>'s RS
196 variable, including treating blank lines as delimiters if set to the
197 null string. You may set it to a multicharacter string to match a
198 multi-character delimiter. Note that setting it to C<"\n\n"> means
199 something slightly different than setting it to C<"">, if the file
200 contains consecutive blank lines. Setting it to C<""> will treat two or
201 more consecutive blank lines as a single blank line. Setting it to
202 C<"\n\n"> will blindly assume that the next input character belongs to the
203 next paragraph, even if it's a newline. (Mnemonic: / is used to
204 delimit line boundaries when quoting poetry.)
207 $_ = <FH>; # whole file now here
210 =item autoflush HANDLE EXPR
212 =item $OUTPUT_AUTOFLUSH
216 If set to nonzero, forces a flush after every write or print on the
217 currently selected output channel. Default is 0. Note that STDOUT
218 will typically be line buffered if output is to the terminal and block
219 buffered otherwise. Setting this variable is useful primarily when you
220 are outputting to a pipe, such as when you are running a Perl script
221 under rsh and want to see the output as it's happening. This has no
222 effect on input buffering.
223 (Mnemonic: when you want your pipes to be piping hot.)
225 =item output_field_separator HANDLE EXPR
227 =item $OUTPUT_FIELD_SEPARATOR
233 The output field separator for the print operator. Ordinarily the
234 print operator simply prints out the comma separated fields you
235 specify. In order to get behavior more like B<awk>, set this variable
236 as you would set B<awk>'s OFS variable to specify what is printed
237 between fields. (Mnemonic: what is printed when there is a , in your
240 =item output_record_separator HANDLE EXPR
242 =item $OUTPUT_RECORD_SEPARATOR
248 The output record separator for the print operator. Ordinarily the
249 print operator simply prints out the comma separated fields you
250 specify, with no trailing newline or record separator assumed. In
251 order to get behavior more like B<awk>, set this variable as you would
252 set B<awk>'s ORS variable to specify what is printed at the end of the
253 print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
254 print. Also, it's just like /, but it's what you get "back" from
257 =item $LIST_SEPARATOR
261 This is like "C<$,>" except that it applies to array values interpolated
262 into a double-quoted string (or similar interpreted string). Default
263 is a space. (Mnemonic: obvious, I think.)
265 =item $SUBSCRIPT_SEPARATOR
271 The subscript separator for multi-dimensional array emulation. If you
272 refer to a hash element as
278 $foo{join($;, $a, $b, $c)}
282 @foo{$a,$b,$c} # a slice--note the @
286 ($foo{$a},$foo{$b},$foo{$c})
288 Default is "\034", the same as SUBSEP in B<awk>. Note that if your
289 keys contain binary data there might not be any safe value for "C<$;>".
290 (Mnemonic: comma (the syntactic subscript separator) is a
291 semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
292 taken for something more important.)
294 Consider using "real" multi-dimensional arrays in Perl 5.
300 The output format for printed numbers. This variable is a half-hearted
301 attempt to emulate B<awk>'s OFMT variable. There are times, however,
302 when B<awk> and Perl have differing notions of what is in fact
303 numeric. Also, the initial value is %.20g rather than %.6g, so you
304 need to set "C<$#>" explicitly to get B<awk>'s value. (Mnemonic: # is the
307 Use of "C<$#>" is deprecated in Perl 5.
309 =item format_page_number HANDLE EXPR
311 =item $FORMAT_PAGE_NUMBER
315 The current page number of the currently selected output channel.
316 (Mnemonic: % is page number in B<nroff>.)
318 =item format_lines_per_page HANDLE EXPR
320 =item $FORMAT_LINES_PER_PAGE
324 The current page length (printable lines) of the currently selected
325 output channel. Default is 60. (Mnemonic: = has horizontal lines.)
327 =item format_lines_left HANDLE EXPR
329 =item $FORMAT_LINES_LEFT
333 The number of lines left on the page of the currently selected output
334 channel. (Mnemonic: lines_on_page - lines_printed.)
336 =item format_name HANDLE EXPR
342 The name of the current report format for the currently selected output
343 channel. Default is name of the filehandle. (Mnemonic: brother to
346 =item format_top_name HANDLE EXPR
348 =item $FORMAT_TOP_NAME
352 The name of the current top-of-page format for the currently selected
353 output channel. Default is name of the filehandle with _TOP
354 appended. (Mnemonic: points to top of page.)
356 =item format_line_break_characters HANDLE EXPR
358 =item $FORMAT_LINE_BREAK_CHARACTERS
362 The current set of characters after which a string may be broken to
363 fill continuation fields (starting with ^) in a format. Default is
364 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
365 poetry is a part of a line.)
367 =item format_formfeed HANDLE EXPR
369 =item $FORMAT_FORMFEED
373 What formats output to perform a formfeed. Default is \f.
379 The current value of the write() accumulator for format() lines. A format
380 contains formline() commands that put their result into C<$^A>. After
381 calling its format, write() prints out the contents of C<$^A> and empties.
382 So you never actually see the contents of C<$^A> unless you call
383 formline() yourself and then look at it. See L<perlform> and
384 L<perlfunc/formline()>.
390 The status returned by the last pipe close, backtick (C<``>) command,
391 or system() operator. Note that this is the status word returned by
392 the wait() system call, so the exit value of the subprocess is actually
393 (C<$? E<gt>E<gt> 8>). Thus on many systems, C<$? & 255> gives which signal,
394 if any, the process died from, and whether there was a core dump.
395 (Mnemonic: similar to B<sh> and B<ksh>.)
403 If used in a numeric context, yields the current value of errno, with
404 all the usual caveats. (This means that you shouldn't depend on the
405 value of "C<$!>" to be anything in particular unless you've gotten a
406 specific error return indicating a system error.) If used in a string
407 context, yields the corresponding system error string. You can assign
408 to "C<$!>" in order to set I<errno> if, for instance, you want "C<$!>" to return the
409 string for error I<n>, or you want to set the exit value for the die()
410 operator. (Mnemonic: What just went bang?)
412 =item $EXTENDED_OS_ERROR
416 More specific information about the last system error than that
417 provided by C<$!>, if available. (If not, it's just C<$!> again.)
418 At the moment, this differs from C<$!> only under VMS, where it
419 provides the VMS status value from the last system error. The
420 caveats mentioned in the description of C<$!> apply here, too.
421 (Mnemonic: Extra error explanation.)
428 The Perl syntax error message from the last eval() command. If null, the
429 last eval() parsed and executed correctly (although the operations you
430 invoked may have failed in the normal fashion). (Mnemonic: Where was
431 the syntax error "at"?)
433 Note that warning messages are not collected in this variable. You can,
434 however, set up a routine to process warnings by setting $SIG{__WARN__} below.
442 The process number of the Perl running this script. (Mnemonic: same
451 The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
452 if you're running setuid.)
454 =item $EFFECTIVE_USER_ID
460 The effective uid of this process. Example:
462 $< = $>; # set real to effective uid
463 ($<,$>) = ($>,$<); # swap real and effective uid
465 (Mnemonic: it's the uid you went I<TO>, if you're running setuid.) Note:
466 "C<$E<lt>>" and "C<$E<gt>>" can only be swapped on machines supporting setreuid().
474 The real gid of this process. If you are on a machine that supports
475 membership in multiple groups simultaneously, gives a space separated
476 list of groups you are in. The first number is the one returned by
477 getgid(), and the subsequent ones by getgroups(), one of which may be
478 the same as the first number. (Mnemonic: parentheses are used to I<GROUP>
479 things. The real gid is the group you I<LEFT>, if you're running setgid.)
481 =item $EFFECTIVE_GROUP_ID
487 The effective gid of this process. If you are on a machine that
488 supports membership in multiple groups simultaneously, gives a space
489 separated list of groups you are in. The first number is the one
490 returned by getegid(), and the subsequent ones by getgroups(), one of
491 which may be the same as the first number. (Mnemonic: parentheses are
492 used to I<GROUP> things. The effective gid is the group that's I<RIGHT> for
493 you, if you're running setgid.)
495 Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can only be set on machines
496 that support the corresponding I<set[re][ug]id()> routine. "C<$(>" and "C<$)>"
497 can only be swapped on machines supporting setregid(). Because Perl doesn't
498 currently use initgroups(), you can't set your group vector to multiple groups.
504 Contains the name of the file containing the Perl script being
505 executed. Assigning to "C<$0>" modifies the argument area that the ps(1)
506 program sees. This is more useful as a way of indicating the
507 current program state than it is for hiding the program you're running.
508 (Mnemonic: same as B<sh> and B<ksh>.)
512 The index of the first element in an array, and of the first character
513 in a substring. Default is 0, but you could set it to 1 to make
514 Perl behave more like B<awk> (or Fortran) when subscripting and when
515 evaluating the index() and substr() functions. (Mnemonic: [ begins
518 As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
519 and cannot influence the behavior of any other file. Its use is
526 The string printed out when you say C<perl -v>.
527 (This is currently I<BROKEN>).
529 determine at the beginning of a script whether the perl interpreter
530 executing the script is in the right range of versions. If used in a
531 numeric context, returns the version + patchlevel / 1000. Example:
533 # see if getc is available
534 ($version,$patchlevel) =
535 $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
536 print STDERR "(No filename completion available.)\n"
537 if $version * 1000 + $patchlevel < 2016;
539 or, used numerically,
541 warn "No checksumming!\n" if $] < 3.019;
543 (Mnemonic: Is this version of perl in the right bracket?)
549 The current value of the debugging flags. (Mnemonic: value of B<-D>
556 The maximum system file descriptor, ordinarily 2. System file
557 descriptors are passed to exec()ed processes, while higher file
558 descriptors are not. Also, during an open(), system file descriptors are
559 preserved even if the open() fails. (Ordinary file descriptors are
560 closed before the open() is attempted.) Note that the close-on-exec
561 status of a file descriptor will be decided according to the value of
562 C<$^F> at the time of the open, not the time of the exec.
568 The current value of the inplace-edit extension. Use C<undef> to disable
569 inplace editing. (Mnemonic: value of B<-i> switch.)
574 The name of the operating system under which this copy of Perl was
575 built, as determined during the configuration process. The value
576 is identical to C<$Config{'osname'}>.
582 The internal flag that the debugger clears so that it doesn't debug
583 itself. You could conceivably disable debugging yourself by clearing
590 The time at which the script began running, in seconds since the
591 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>
592 and B<-C> filetests are
599 The current value of the warning switch, either TRUE or FALSE. (Mnemonic: related to the
602 =item $EXECUTABLE_NAME
606 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
610 contains the name of the current file when reading from <>.
614 The array @ARGV contains the command line arguments intended for the
615 script. Note that C<$#ARGV> is the generally number of arguments minus
616 one, since C<$ARGV[0]> is the first argument, I<NOT> the command name. See
617 "C<$0>" for the command name.
621 The array @INC contains the list of places to look for Perl scripts to
622 be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
623 initially consists of the arguments to any B<-I> command line switches,
624 followed by the default Perl library, probably "/usr/local/lib/perl",
625 followed by ".", to represent the current directory. If you need to
626 modify this at runtime, you should use the C<use lib> pragma in order
627 to also get the machine-dependent library properly loaded:
629 use lib '/mypath/libdir/';
634 The hash %INC contains entries for each filename that has
635 been included via C<do> or C<require>. The key is the filename you
636 specified, and the value is the location of the file actually found.
637 The C<require> command uses this array to determine whether a given file
638 has already been included.
642 The hash %ENV contains your current environment. Setting a
643 value in C<ENV> changes the environment for child processes.
647 The hash %SIG is used to set signal handlers for various
650 sub handler { # 1st argument is signal name
652 print "Caught a SIG$sig--shutting down\n";
657 $SIG{'INT'} = 'handler';
658 $SIG{'QUIT'} = 'handler';
660 $SIG{'INT'} = 'DEFAULT'; # restore default action
661 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
663 The %SIG array only contains values for the signals actually set within
664 the Perl script. Here are some other examples:
666 $SIG{PIPE} = Plumber; # SCARY!!
667 $SIG{"PIPE"} = "Plumber"; # just fine, assumes main::Plumber
668 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
669 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
671 The one marked scary is problematic because it's a bareword, which means
672 sometimes it's a string representing the function, and sometimes it's
673 going to call the subroutine call right then and there! Best to be sure
674 and quote it or take a reference to it. *Plumber works too. See L<perlsubs>.
676 Certain internal hooks can be also set using the %SIG hash. The
677 routine indicated by $SIG{__WARN__} is called when a warning message is
678 about to be printed. The warning message is passed as the first
679 argument. The presence of a __WARN__ hook causes the ordinary printing
680 of warnings to STDERR to be suppressed. You can use this to save warnings
681 in a variable, or turn warnings into fatal errors, like this:
683 local $SIG{__WARN__} = sub { die $_[0] };
686 The routine indicated by $SIG{__DIE__} is called when a fatal exception
687 is about to be thrown. The error message is passed as the first
688 argument. When a __DIE__ hook routine returns, the exception
689 processing continues as it would have in the absence of the hook,
690 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
691 The __DIE__ handler is explicitly disabled during the call, so that you
692 can die from a __DIE__ handler. Similarly for __WARN__.