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 (Mnemonic: underline is understood in certain operations.)
70 Contains the subpattern from the corresponding set of parentheses in
71 the last pattern matched, not counting patterns matched in nested
72 blocks that have been exited already. (Mnemonic: like \digit.)
73 These variables are all read-only.
79 The string matched by the last successful pattern match (not counting
80 any matches hidden within a BLOCK or eval() enclosed by the current
81 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
87 The string preceding whatever was matched by the last successful
88 pattern match (not counting any matches hidden within a BLOCK or eval
89 enclosed by the current BLOCK). (Mnemonic: ` often precedes a quoted
90 string.) This variable is read-only.
96 The string following whatever was matched by the last successful
97 pattern match (not counting any matches hidden within a BLOCK or eval()
98 enclosed by the current BLOCK). (Mnemonic: ' often follows a quoted
103 print "$`:$&:$'\n"; # prints abc:def:ghi
105 This variable is read-only.
107 =item $LAST_PAREN_MATCH
111 The last bracket matched by the last search pattern. This is useful if
112 you don't know which of a set of alternative patterns matched. For
115 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
117 (Mnemonic: be positive and forward looking.)
118 This variable is read-only.
120 =item $MULTILINE_MATCHING
124 Set to 1 to do multiline matching within a string, 0 to tell Perl
125 that it can assume that strings contain a single line, for the purpose
126 of optimizing pattern matches. Pattern matches on strings containing
127 multiple newlines can produce confusing results when "C<$*>" is 0. Default
128 is 0. (Mnemonic: * matches multiple things.) Note that this variable
129 only influences the interpretation of "C<^>" and "C<$>". A literal newline can
130 be searched for even when C<$* == 0>.
132 Use of "C<$*>" is deprecated in Perl 5.
134 =item input_line_number HANDLE EXPR
136 =item $INPUT_LINE_NUMBER
142 The current input line number of the last filehandle that was read. An
143 explicit close on the filehandle resets the line number. Since
144 "C<E<lt>E<gt>>" never does an explicit close, line numbers increase
145 across ARGV files (but see examples under eof()). Localizing C<$.> has
146 the effect of also localizing Perl's notion of "the last read
147 filehandle". (Mnemonic: many programs use "." to mean the current line
150 =item input_record_separator HANDLE EXPR
152 =item $INPUT_RECORD_SEPARATOR
158 The input record separator, newline by default. Works like B<awk>'s RS
159 variable, including treating blank lines as delimiters if set to the
160 null string. You may set it to a multicharacter string to match a
161 multi-character delimiter. Note that setting it to C<"\n\n"> means
162 something slightly different than setting it to C<"">, if the file
163 contains consecutive blank lines. Setting it to C<""> will treat two or
164 more consecutive blank lines as a single blank line. Setting it to
165 C<"\n\n"> will blindly assume that the next input character belongs to the
166 next paragraph, even if it's a newline. (Mnemonic: / is used to
167 delimit line boundaries when quoting poetry.)
170 $_ = <FH>; # whole file now here
173 =item autoflush HANDLE EXPR
175 =item $OUTPUT_AUTOFLUSH
179 If set to nonzero, forces a flush after every write or print on the
180 currently selected output channel. Default is 0. Note that STDOUT
181 will typically be line buffered if output is to the terminal and block
182 buffered otherwise. Setting this variable is useful primarily when you
183 are outputting to a pipe, such as when you are running a Perl script
184 under rsh and want to see the output as it's happening. (Mnemonic:
185 when you want your pipes to be piping hot.)
187 =item output_field_separator HANDLE EXPR
189 =item $OUTPUT_FIELD_SEPARATOR
195 The output field separator for the print operator. Ordinarily the
196 print operator simply prints out the comma separated fields you
197 specify. In order to get behavior more like B<awk>, set this variable
198 as you would set B<awk>'s OFS variable to specify what is printed
199 between fields. (Mnemonic: what is printed when there is a , in your
202 =item output_record_separator HANDLE EXPR
204 =item $OUTPUT_RECORD_SEPARATOR
210 The output record separator for the print operator. Ordinarily the
211 print operator simply prints out the comma separated fields you
212 specify, with no trailing newline or record separator assumed. In
213 order to get behavior more like B<awk>, set this variable as you would
214 set B<awk>'s ORS variable to specify what is printed at the end of the
215 print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
216 print. Also, it's just like /, but it's what you get "back" from
219 =item $LIST_SEPARATOR
223 This is like "C<$,>" except that it applies to array values interpolated
224 into a double-quoted string (or similar interpreted string). Default
225 is a space. (Mnemonic: obvious, I think.)
227 =item $SUBSCRIPT_SEPARATOR
233 The subscript separator for multi-dimensional array emulation. If you
234 refer to a hash element as
240 $foo{join($;, $a, $b, $c)}
244 @foo{$a,$b,$c} # a slice--note the @
248 ($foo{$a},$foo{$b},$foo{$c})
250 Default is "\034", the same as SUBSEP in B<awk>. Note that if your
251 keys contain binary data there might not be any safe value for "C<$;>".
252 (Mnemonic: comma (the syntactic subscript separator) is a
253 semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
254 taken for something more important.)
256 Consider using "real" multi-dimensional arrays in Perl 5.
262 The output format for printed numbers. This variable is a half-hearted
263 attempt to emulate B<awk>'s OFMT variable. There are times, however,
264 when B<awk> and Perl have differing notions of what is in fact
265 numeric. Also, the initial value is %.20g rather than %.6g, so you
266 need to set "C<$#>" explicitly to get B<awk>'s value. (Mnemonic: # is the
269 Use of "C<$#>" is deprecated in Perl 5.
271 =item format_page_number HANDLE EXPR
273 =item $FORMAT_PAGE_NUMBER
277 The current page number of the currently selected output channel.
278 (Mnemonic: % is page number in B<nroff>.)
280 =item format_lines_per_page HANDLE EXPR
282 =item $FORMAT_LINES_PER_PAGE
286 The current page length (printable lines) of the currently selected
287 output channel. Default is 60. (Mnemonic: = has horizontal lines.)
289 =item format_lines_left HANDLE EXPR
291 =item $FORMAT_LINES_LEFT
295 The number of lines left on the page of the currently selected output
296 channel. (Mnemonic: lines_on_page - lines_printed.)
298 =item format_name HANDLE EXPR
304 The name of the current report format for the currently selected output
305 channel. Default is name of the filehandle. (Mnemonic: brother to
308 =item format_top_name HANDLE EXPR
310 =item $FORMAT_TOP_NAME
314 The name of the current top-of-page format for the currently selected
315 output channel. Default is name of the filehandle with _TOP
316 appended. (Mnemonic: points to top of page.)
318 =item format_line_break_characters HANDLE EXPR
320 =item $FORMAT_LINE_BREAK_CHARACTERS
324 The current set of characters after which a string may be broken to
325 fill continuation fields (starting with ^) in a format. Default is
326 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
327 poetry is a part of a line.)
329 =item format_formfeed HANDLE EXPR
331 =item $FORMAT_FORMFEED
335 What formats output to perform a formfeed. Default is \f.
341 The current value of the write() accumulator for format() lines. A format
342 contains formline() commands that put their result into C<$^A>. After
343 calling its format, write() prints out the contents of C<$^A> and empties.
344 So you never actually see the contents of C<$^A> unless you call
345 formline() yourself and then look at it. See L<perlform> and
346 L<perlfunc/formline()>.
352 The status returned by the last pipe close, backtick (C<``>) command,
353 or system() operator. Note that this is the status word returned by
354 the wait() system call, so the exit value of the subprocess is actually
355 (C<$? E<gt>E<gt> 8>). Thus on many systems, C<$? & 255> gives which signal,
356 if any, the process died from, and whether there was a core dump.
357 (Mnemonic: similar to B<sh> and B<ksh>.)
365 If used in a numeric context, yields the current value of errno, with
366 all the usual caveats. (This means that you shouldn't depend on the
367 value of "C<$!>" to be anything in particular unless you've gotten a
368 specific error return indicating a system error.) If used in a string
369 context, yields the corresponding system error string. You can assign
370 to "C<$!>" in order to set I<errno> if, for instance, you want "C<$!>" to return the
371 string for error I<n>, or you want to set the exit value for the die()
372 operator. (Mnemonic: What just went bang?)
378 The Perl syntax error message from the last eval() command. If null, the
379 last eval() parsed and executed correctly (although the operations you
380 invoked may have failed in the normal fashion). (Mnemonic: Where was
381 the syntax error "at"?)
383 Note that warning messages are not collected in this variable. You can,
384 however, set up a routine to process warnings by setting $SIG{__WARN__} below.
392 The process number of the Perl running this script. (Mnemonic: same
401 The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
402 if you're running setuid.)
404 =item $EFFECTIVE_USER_ID
410 The effective uid of this process. Example:
412 $< = $>; # set real to effective uid
413 ($<,$>) = ($>,$<); # swap real and effective uid
415 (Mnemonic: it's the uid you went I<TO>, if you're running setuid.) Note:
416 "C<$E<lt>>" and "C<$E<gt>>" can only be swapped on machines supporting setreuid().
424 The real gid of this process. If you are on a machine that supports
425 membership in multiple groups simultaneously, gives a space separated
426 list of groups you are in. The first number is the one returned by
427 getgid(), and the subsequent ones by getgroups(), one of which may be
428 the same as the first number. (Mnemonic: parentheses are used to I<GROUP>
429 things. The real gid is the group you I<LEFT>, if you're running setgid.)
431 =item $EFFECTIVE_GROUP_ID
437 The effective gid of this process. If you are on a machine that
438 supports membership in multiple groups simultaneously, gives a space
439 separated list of groups you are in. The first number is the one
440 returned by getegid(), and the subsequent ones by getgroups(), one of
441 which may be the same as the first number. (Mnemonic: parentheses are
442 used to I<GROUP> things. The effective gid is the group that's I<RIGHT> for
443 you, if you're running setgid.)
445 Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can only be set on machines
446 that support the corresponding I<set[re][ug]id()> routine. "C<$(>" and "C<$)>"
447 can only be swapped on machines supporting setregid().
453 Contains the name of the file containing the Perl script being
454 executed. Assigning to "C<$0>" modifies the argument area that the ps(1)
455 program sees. This is more useful as a way of indicating the
456 current program state than it is for hiding the program you're running.
457 (Mnemonic: same as B<sh> and B<ksh>.)
461 The index of the first element in an array, and of the first character
462 in a substring. Default is 0, but you could set it to 1 to make
463 Perl behave more like B<awk> (or Fortran) when subscripting and when
464 evaluating the index() and substr() functions. (Mnemonic: [ begins
467 As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
468 and cannot influence the behavior of any other file. Its use is
475 The string printed out when you say C<perl -v>. It can be used to
476 determine at the beginning of a script whether the perl interpreter
477 executing the script is in the right range of versions. If used in a
478 numeric context, returns the version + patchlevel / 1000. Example:
480 # see if getc is available
481 ($version,$patchlevel) =
482 $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
483 print STDERR "(No filename completion available.)\n"
484 if $version * 1000 + $patchlevel < 2016;
486 or, used numerically,
488 warn "No checksumming!\n" if $] < 3.019;
490 (Mnemonic: Is this version of perl in the right bracket?)
496 The current value of the debugging flags. (Mnemonic: value of B<-D>
503 The maximum system file descriptor, ordinarily 2. System file
504 descriptors are passed to exec()ed processes, while higher file
505 descriptors are not. Also, during an open(), system file descriptors are
506 preserved even if the open() fails. (Ordinary file descriptors are
507 closed before the open() is attempted.) Note that the close-on-exec
508 status of a file descriptor will be decided according to the value of
509 C<$^F> at the time of the open, not the time of the exec.
515 The current value of the inplace-edit extension. Use C<undef> to disable
516 inplace editing. (Mnemonic: value of B<-i> switch.)
522 The internal flag that the debugger clears so that it doesn't debug
523 itself. You could conceivable disable debugging yourself by clearing
530 The time at which the script began running, in seconds since the
531 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>
532 and B<-C> filetests are
539 The current value of the warning switch, either TRUE or FALSE. (Mnemonic: related to the
542 =item $EXECUTABLE_NAME
546 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
550 contains the name of the current file when reading from <>.
554 The array @ARGV contains the command line arguments intended for the
555 script. Note that C<$#ARGV> is the generally number of arguments minus
556 one, since C<$ARGV[0]> is the first argument, I<NOT> the command name. See
557 "C<$0>" for the command name.
561 The array @INC contains the list of places to look for Perl scripts to
562 be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
563 initially consists of the arguments to any B<-I> command line switches,
564 followed by the default Perl library, probably "/usr/local/lib/perl",
565 followed by ".", to represent the current directory.
569 The hash %INC contains entries for each filename that has
570 been included via C<do> or C<require>. The key is the filename you
571 specified, and the value is the location of the file actually found.
572 The C<require> command uses this array to determine whether a given file
573 has already been included.
577 The hash %ENV contains your current environment. Setting a
578 value in C<ENV> changes the environment for child processes.
582 The hash %SIG is used to set signal handlers for various
585 sub handler { # 1st argument is signal name
587 print "Caught a SIG$sig--shutting down\n";
592 $SIG{'INT'} = 'handler';
593 $SIG{'QUIT'} = 'handler';
595 $SIG{'INT'} = 'DEFAULT'; # restore default action
596 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
598 The %SIG array only contains values for the signals actually set within
599 the Perl script. Here are some other examples:
601 $SIG{PIPE} = Plumber; # SCARY!!
602 $SIG{"PIPE"} = "Plumber"; # just fine, assumes main::Plumber
603 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
604 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
606 The one marked scary is problematic because it's a bareword, which means
607 sometimes it's a string representing the function, and sometimes it's
608 going to call the subroutine call right then and there! Best to be sure
609 and quote it or take a reference to it. *Plumber works too. See L<perlsubs>.
611 Certain internal hooks can be also set using the %SIG hash. The
612 routine indicated by $SIG{__WARN__} is called when a warning message is
613 about to be printed. The warning message is passed as the first
614 argument. The presence of a __WARN__ hook causes the ordinary printing
615 of warnings to STDERR to be suppressed. You can use this to save warnings
616 in a variable, or turn warnings into fatal errors, like this:
618 local $SIG{__WARN__} = sub { die $_[0] };
621 The routine indicated by $SIG{__DIE__} is called when a fatal exception
622 is about to be thrown. The error message is passed as the first
623 argument. When a __DIE__ hook routine returns, the exception
624 processing continues as it would have in the absence of the hook,
625 unless the hook routine itself exits via a goto, a loop exit, or a die.