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1 | =head1 NAME |
2 | |
3 | perlvar - Perl predefined variables |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | =head2 Predefined Names |
8 | |
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, |
12 | you just need to say |
13 | |
14 | use English; |
15 | |
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>. |
19 | |
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 |
24 | |
25 | use FileHandle; |
26 | |
27 | after which you may use either |
28 | |
29 | method HANDLE EXPR |
30 | |
31 | or |
32 | |
33 | HANDLE->method(EXPR) |
34 | |
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. |
40 | |
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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. |
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44 | |
45 | =over 8 |
46 | |
47 | =item $ARG |
48 | |
49 | =item $_ |
50 | |
51 | The default input and pattern-searching space. The following pairs are |
52 | equivalent: |
53 | |
54 | while (<>) {...} # only equivalent in while! |
55 | while ($_ = <>) {...} |
56 | |
57 | /^Subject:/ |
58 | $_ =~ /^Subject:/ |
59 | |
60 | tr/a-z/A-Z/ |
61 | $_ =~ tr/a-z/A-Z/ |
62 | |
63 | chop |
64 | chop($_) |
65 | |
66 | (Mnemonic: underline is understood in certain operations.) |
67 | |
68 | =item $<I<digit>> |
69 | |
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. |
74 | |
75 | =item $MATCH |
76 | |
77 | =item $& |
78 | |
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. |
82 | |
83 | =item $PREMATCH |
84 | |
85 | =item $` |
86 | |
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. |
91 | |
92 | =item $POSTMATCH |
93 | |
94 | =item $' |
95 | |
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 |
99 | string.) Example: |
100 | |
101 | $_ = 'abcdefghi'; |
102 | /def/; |
103 | print "$`:$&:$'\n"; # prints abc:def:ghi |
104 | |
105 | This variable is read-only. |
106 | |
107 | =item $LAST_PAREN_MATCH |
108 | |
109 | =item $+ |
110 | |
111 | The last bracket matched by the last search pattern. This is useful if |
112 | you don't know which of a set of alternative patterns matched. For |
113 | example: |
114 | |
115 | /Version: (.*)|Revision: (.*)/ && ($rev = $+); |
116 | |
117 | (Mnemonic: be positive and forward looking.) |
118 | This variable is read-only. |
119 | |
120 | =item $MULTILINE_MATCHING |
121 | |
122 | =item $* |
123 | |
124 | Set to 1 to do multiline matching within a string, 0 to tell Perl |
125 | that it can assume that strings contain a single line, for the purpose |
126 | of optimizing pattern matches. Pattern matches on strings containing |
127 | multiple newlines can produce confusing results when "C<$*>" 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>. |
131 | |
132 | Use of "C<$*>" is deprecated in Perl 5. |
133 | |
134 | =item input_line_number HANDLE EXPR |
135 | |
136 | =item $INPUT_LINE_NUMBER |
137 | |
138 | =item $NR |
139 | |
140 | =item $. |
141 | |
142 | The current input line number of the last filehandle that was read. |
143 | This variable should be considered read-only. |
144 | Remember that only an explicit close on the filehandle |
145 | resets the line number. Since "C<E<lt>E<gt>>" never does an explicit close, line |
146 | numbers increase across ARGV files (but see examples under eof()). |
147 | (Mnemonic: many programs use "." to mean the current line number.) |
148 | |
149 | =item input_record_separator HANDLE EXPR |
150 | |
151 | =item $INPUT_RECORD_SEPARATOR |
152 | |
153 | =item $RS |
154 | |
155 | =item $/ |
156 | |
157 | The input record separator, newline by default. Works like B<awk>'s RS |
158 | variable, including treating blank lines as delimiters if set to the |
159 | null string. You may set it to a multicharacter string to match a |
160 | multi-character delimiter. Note that setting it to C<"\n\n"> means |
161 | something slightly different than setting it to C<"">, if the file |
162 | contains consecutive blank lines. Setting it to C<""> will treat two or |
163 | more consecutive blank lines as a single blank line. Setting it to |
164 | C<"\n\n"> will blindly assume that the next input character belongs to the |
165 | next paragraph, even if it's a newline. (Mnemonic: / is used to |
166 | delimit line boundaries when quoting poetry.) |
167 | |
168 | undef $/; |
169 | $_ = <FH>; # whole file now here |
170 | s/\n[ \t]+/ /g; |
171 | |
172 | =item autoflush HANDLE EXPR |
173 | |
174 | =item $OUTPUT_AUTOFLUSH |
175 | |
176 | =item $| |
177 | |
178 | If set to nonzero, forces a flush after every write or print on the |
179 | currently selected output channel. Default is 0. Note that STDOUT |
180 | will typically be line buffered if output is to the terminal and block |
181 | buffered otherwise. Setting this variable is useful primarily when you |
182 | are outputting to a pipe, such as when you are running a Perl script |
183 | under rsh and want to see the output as it's happening. (Mnemonic: |
184 | when you want your pipes to be piping hot.) |
185 | |
186 | =item output_field_separator HANDLE EXPR |
187 | |
188 | =item $OUTPUT_FIELD_SEPARATOR |
189 | |
190 | =item $OFS |
191 | |
192 | =item $, |
193 | |
194 | The output field separator for the print operator. Ordinarily the |
195 | print operator simply prints out the comma separated fields you |
196 | specify. In order to get behavior more like B<awk>, set this variable |
197 | as you would set B<awk>'s OFS variable to specify what is printed |
198 | between fields. (Mnemonic: what is printed when there is a , in your |
199 | print statement.) |
200 | |
201 | =item output_record_separator HANDLE EXPR |
202 | |
203 | =item $OUTPUT_RECORD_SEPARATOR |
204 | |
205 | =item $ORS |
206 | |
207 | =item $\ |
208 | |
209 | The output record separator for the print operator. Ordinarily the |
210 | print operator simply prints out the comma separated fields you |
211 | specify, with no trailing newline or record separator assumed. In |
212 | order to get behavior more like B<awk>, set this variable as you would |
213 | set B<awk>'s ORS variable to specify what is printed at the end of the |
214 | print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the |
215 | print. Also, it's just like /, but it's what you get "back" from |
216 | Perl.) |
217 | |
218 | =item $LIST_SEPARATOR |
219 | |
220 | =item $" |
221 | |
222 | This is like "C<$,>" except that it applies to array values interpolated |
223 | into a double-quoted string (or similar interpreted string). Default |
224 | is a space. (Mnemonic: obvious, I think.) |
225 | |
226 | =item $SUBSCRIPT_SEPARATOR |
227 | |
228 | =item $SUBSEP |
229 | |
230 | =item $; |
231 | |
232 | The subscript separator for multi-dimensional array emulation. If you |
233 | refer to a hash element as |
234 | |
235 | $foo{$a,$b,$c} |
236 | |
237 | it really means |
238 | |
239 | $foo{join($;, $a, $b, $c)} |
240 | |
241 | But don't put |
242 | |
243 | @foo{$a,$b,$c} # a slice--note the @ |
244 | |
245 | which means |
246 | |
247 | ($foo{$a},$foo{$b},$foo{$c}) |
248 | |
249 | Default is "\034", the same as SUBSEP in B<awk>. Note that if your |
250 | keys contain binary data there might not be any safe value for "C<$;>". |
251 | (Mnemonic: comma (the syntactic subscript separator) is a |
252 | semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already |
253 | taken for something more important.) |
254 | |
255 | Consider using "real" multi-dimensional arrays in Perl 5. |
256 | |
257 | =item $OFMT |
258 | |
259 | =item $# |
260 | |
261 | The output format for printed numbers. This variable is a half-hearted |
262 | attempt to emulate B<awk>'s OFMT variable. There are times, however, |
263 | when B<awk> and Perl have differing notions of what is in fact |
264 | numeric. Also, the initial value is %.20g rather than %.6g, so you |
265 | need to set "C<$#>" explicitly to get B<awk>'s value. (Mnemonic: # is the |
266 | number sign.) |
267 | |
268 | Use of "C<$#>" is deprecated in Perl 5. |
269 | |
270 | =item format_page_number HANDLE EXPR |
271 | |
272 | =item $FORMAT_PAGE_NUMBER |
273 | |
274 | =item $% |
275 | |
276 | The current page number of the currently selected output channel. |
277 | (Mnemonic: % is page number in B<nroff>.) |
278 | |
279 | =item format_lines_per_page HANDLE EXPR |
280 | |
281 | =item $FORMAT_LINES_PER_PAGE |
282 | |
283 | =item $= |
284 | |
285 | The current page length (printable lines) of the currently selected |
286 | output channel. Default is 60. (Mnemonic: = has horizontal lines.) |
287 | |
288 | =item format_lines_left HANDLE EXPR |
289 | |
290 | =item $FORMAT_LINES_LEFT |
291 | |
292 | =item $- |
293 | |
294 | The number of lines left on the page of the currently selected output |
295 | channel. (Mnemonic: lines_on_page - lines_printed.) |
296 | |
297 | =item format_name HANDLE EXPR |
298 | |
299 | =item $FORMAT_NAME |
300 | |
301 | =item $~ |
302 | |
303 | The name of the current report format for the currently selected output |
304 | channel. Default is name of the filehandle. (Mnemonic: brother to |
305 | "C<$^>".) |
306 | |
307 | =item format_top_name HANDLE EXPR |
308 | |
309 | =item $FORMAT_TOP_NAME |
310 | |
311 | =item $^ |
312 | |
313 | The name of the current top-of-page format for the currently selected |
314 | output channel. Default is name of the filehandle with _TOP |
315 | appended. (Mnemonic: points to top of page.) |
316 | |
317 | =item format_line_break_characters HANDLE EXPR |
318 | |
319 | =item $FORMAT_LINE_BREAK_CHARACTERS |
320 | |
321 | =item $: |
322 | |
323 | The current set of characters after which a string may be broken to |
324 | fill continuation fields (starting with ^) in a format. Default is |
325 | S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in |
326 | poetry is a part of a line.) |
327 | |
328 | =item format_formfeed HANDLE EXPR |
329 | |
330 | =item $FORMAT_FORMFEED |
331 | |
332 | =item $^L |
333 | |
334 | What formats output to perform a formfeed. Default is \f. |
335 | |
336 | =item $ACCUMULATOR |
337 | |
338 | =item $^A |
339 | |
340 | The current value of the write() accumulator for format() lines. A format |
341 | contains formline() commands that put their result into C<$^A>. After |
342 | calling its format, write() prints out the contents of C<$^A> and empties. |
343 | So you never actually see the contents of C<$^A> unless you call |
344 | formline() yourself and then look at it. See L<perlform> and |
345 | L<perlfunc/formline()>. |
346 | |
347 | =item $CHILD_ERROR |
348 | |
349 | =item $? |
350 | |
351 | The status returned by the last pipe close, backtick (C<``>) command, |
352 | or system() operator. Note that this is the status word returned by |
353 | the wait() system call, so the exit value of the subprocess is actually |
354 | (C<$? E<gt>E<gt> 8>). Thus on many systems, C<$? & 255> gives which signal, |
355 | if any, the process died from, and whether there was a core dump. |
356 | (Mnemonic: similar to B<sh> and B<ksh>.) |
357 | |
358 | =item $OS_ERROR |
359 | |
360 | =item $ERRNO |
361 | |
362 | =item $! |
363 | |
364 | If used in a numeric context, yields the current value of errno, with |
365 | all the usual caveats. (This means that you shouldn't depend on the |
366 | value of "C<$!>" to be anything in particular unless you've gotten a |
367 | specific error return indicating a system error.) If used in a string |
368 | context, yields the corresponding system error string. You can assign |
369 | to "C<$!>" in order to set I<errno> if, for instance, you want "C<$!>" to return the |
370 | string for error I<n>, or you want to set the exit value for the die() |
371 | operator. (Mnemonic: What just went bang?) |
372 | |
373 | =item $EVAL_ERROR |
374 | |
375 | =item $@ |
376 | |
377 | The Perl syntax error message from the last eval() command. If null, the |
378 | last eval() parsed and executed correctly (although the operations you |
379 | invoked may have failed in the normal fashion). (Mnemonic: Where was |
380 | the syntax error "at"?) |
381 | |
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382 | Note that warning messages are not collected in this variable. You can, |
383 | however, set up a routine to process warnings by setting $SIG{__WARN__} below. |
384 | |
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385 | =item $PROCESS_ID |
386 | |
387 | =item $PID |
388 | |
389 | =item $$ |
390 | |
391 | The process number of the Perl running this script. (Mnemonic: same |
392 | as shells.) |
393 | |
394 | =item $REAL_USER_ID |
395 | |
396 | =item $UID |
397 | |
398 | =item $< |
399 | |
400 | The real uid of this process. (Mnemonic: it's the uid you came I<FROM>, |
401 | if you're running setuid.) |
402 | |
403 | =item $EFFECTIVE_USER_ID |
404 | |
405 | =item $EUID |
406 | |
407 | =item $> |
408 | |
409 | The effective uid of this process. Example: |
410 | |
411 | $< = $>; # set real to effective uid |
412 | ($<,$>) = ($>,$<); # swap real and effective uid |
413 | |
414 | (Mnemonic: it's the uid you went I<TO>, if you're running setuid.) Note: |
415 | "C<$E<lt>>" and "C<$E<gt>>" can only be swapped on machines supporting setreuid(). |
416 | |
417 | =item $REAL_GROUP_ID |
418 | |
419 | =item $GID |
420 | |
421 | =item $( |
422 | |
423 | The real gid of this process. If you are on a machine that supports |
424 | membership in multiple groups simultaneously, gives a space separated |
425 | list of groups you are in. The first number is the one returned by |
426 | getgid(), and the subsequent ones by getgroups(), one of which may be |
427 | the same as the first number. (Mnemonic: parentheses are used to I<GROUP> |
428 | things. The real gid is the group you I<LEFT>, if you're running setgid.) |
429 | |
430 | =item $EFFECTIVE_GROUP_ID |
431 | |
432 | =item $EGID |
433 | |
434 | =item $) |
435 | |
436 | The effective gid of this process. If you are on a machine that |
437 | supports membership in multiple groups simultaneously, gives a space |
438 | separated list of groups you are in. The first number is the one |
439 | returned by getegid(), and the subsequent ones by getgroups(), one of |
440 | which may be the same as the first number. (Mnemonic: parentheses are |
441 | used to I<GROUP> things. The effective gid is the group that's I<RIGHT> for |
442 | you, if you're running setgid.) |
443 | |
444 | Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can only be set on machines |
445 | that support the corresponding I<set[re][ug]id()> routine. "C<$(>" and "C<$)>" |
446 | can only be swapped on machines supporting setregid(). |
447 | |
448 | =item $PROGRAM_NAME |
449 | |
450 | =item $0 |
451 | |
452 | Contains the name of the file containing the Perl script being |
453 | executed. Assigning to "C<$0>" modifies the argument area that the ps(1) |
454 | program sees. This is more useful as a way of indicating the |
455 | current program state than it is for hiding the program you're running. |
456 | (Mnemonic: same as B<sh> and B<ksh>.) |
457 | |
458 | =item $[ |
459 | |
460 | The index of the first element in an array, and of the first character |
461 | in a substring. Default is 0, but you could set it to 1 to make |
462 | Perl behave more like B<awk> (or Fortran) when subscripting and when |
463 | evaluating the index() and substr() functions. (Mnemonic: [ begins |
464 | subscripts.) |
465 | |
466 | As of Perl 5, assignment to "C<$[>" is treated as a compiler directive, |
467 | and cannot influence the behavior of any other file. Its use is |
468 | discouraged. |
469 | |
470 | =item $PERL_VERSION |
471 | |
472 | =item $] |
473 | |
474 | The string printed out when you say C<perl -v>. It can be used to |
475 | determine at the beginning of a script whether the perl interpreter |
476 | executing the script is in the right range of versions. If used in a |
477 | numeric context, returns the version + patchlevel / 1000. Example: |
478 | |
479 | # see if getc is available |
480 | ($version,$patchlevel) = |
481 | $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/; |
482 | print STDERR "(No filename completion available.)\n" |
483 | if $version * 1000 + $patchlevel < 2016; |
484 | |
485 | or, used numerically, |
486 | |
487 | warn "No checksumming!\n" if $] < 3.019; |
488 | |
489 | (Mnemonic: Is this version of perl in the right bracket?) |
490 | |
491 | =item $DEBUGGING |
492 | |
493 | =item $^D |
494 | |
495 | The current value of the debugging flags. (Mnemonic: value of B<-D> |
496 | switch.) |
497 | |
498 | =item $SYSTEM_FD_MAX |
499 | |
500 | =item $^F |
501 | |
502 | The maximum system file descriptor, ordinarily 2. System file |
503 | descriptors are passed to exec()ed processes, while higher file |
504 | descriptors are not. Also, during an open(), system file descriptors are |
505 | preserved even if the open() fails. (Ordinary file descriptors are |
506 | closed before the open() is attempted.) Note that the close-on-exec |
507 | status of a file descriptor will be decided according to the value of |
508 | C<$^F> at the time of the open, not the time of the exec. |
509 | |
510 | =item $INPLACE_EDIT |
511 | |
512 | =item $^I |
513 | |
514 | The current value of the inplace-edit extension. Use C<undef> to disable |
515 | inplace editing. (Mnemonic: value of B<-i> switch.) |
516 | |
517 | =item $PERLDB |
518 | |
519 | =item $^P |
520 | |
521 | The internal flag that the debugger clears so that it doesn't debug |
522 | itself. You could conceivable disable debugging yourself by clearing |
523 | it. |
524 | |
525 | =item $BASETIME |
526 | |
527 | =item $^T |
528 | |
529 | The time at which the script began running, in seconds since the |
530 | epoch (beginning of 1970). The values returned by the B<-M>, B<-A> |
531 | and B<-C> filetests are |
532 | based on this value. |
533 | |
534 | =item $WARNING |
535 | |
536 | =item $^W |
537 | |
538 | The current value of the warning switch, either TRUE or FALSE. (Mnemonic: related to the |
539 | B<-w> switch.) |
540 | |
541 | =item $EXECUTABLE_NAME |
542 | |
543 | =item $^X |
544 | |
545 | The name that the Perl binary itself was executed as, from C's C<argv[0]>. |
546 | |
547 | =item $ARGV |
548 | |
549 | contains the name of the current file when reading from <>. |
550 | |
551 | =item @ARGV |
552 | |
553 | The array @ARGV contains the command line arguments intended for the |
554 | script. Note that C<$#ARGV> is the generally number of arguments minus |
555 | one, since C<$ARGV[0]> is the first argument, I<NOT> the command name. See |
556 | "C<$0>" for the command name. |
557 | |
558 | =item @INC |
559 | |
560 | The array @INC contains the list of places to look for Perl scripts to |
561 | be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It |
562 | initially consists of the arguments to any B<-I> command line switches, |
563 | followed by the default Perl library, probably "/usr/local/lib/perl", |
564 | followed by ".", to represent the current directory. |
565 | |
566 | =item %INC |
567 | |
568 | The hash %INC contains entries for each filename that has |
569 | been included via C<do> or C<require>. The key is the filename you |
570 | specified, and the value is the location of the file actually found. |
571 | The C<require> command uses this array to determine whether a given file |
572 | has already been included. |
573 | |
574 | =item $ENV{expr} |
575 | |
576 | The hash %ENV contains your current environment. Setting a |
577 | value in C<ENV> changes the environment for child processes. |
578 | |
579 | =item $SIG{expr} |
580 | |
581 | The hash %SIG is used to set signal handlers for various |
582 | signals. Example: |
583 | |
584 | sub handler { # 1st argument is signal name |
585 | local($sig) = @_; |
586 | print "Caught a SIG$sig--shutting down\n"; |
587 | close(LOG); |
588 | exit(0); |
589 | } |
590 | |
591 | $SIG{'INT'} = 'handler'; |
592 | $SIG{'QUIT'} = 'handler'; |
593 | ... |
594 | $SIG{'INT'} = 'DEFAULT'; # restore default action |
595 | $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT |
596 | |
597 | The %SIG array only contains values for the signals actually set within |
598 | the Perl script. Here are some other examples: |
599 | |
600 | $SIG{PIPE} = Plumber; # SCARY!! |
601 | $SIG{"PIPE"} = "Plumber"; # just fine, assumes main::Plumber |
602 | $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber |
603 | $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return?? |
604 | |
605 | The one marked scary is problematic because it's a bareword, which means |
606 | sometimes it's a string representing the function, and sometimes it's |
607 | going to call the subroutine call right then and there! Best to be sure |
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608 | and quote it or take a reference to it. *Plumber works too. See L<perlsubs>. |
609 | |
610 | Certain internal hooks can be also set using the %SIG hash. The |
611 | routine indicated by $SIG{__WARN__} is called when a warning message is |
612 | about to be printed. The warning message is passed as the first |
613 | argument. The presence of a __WARN__ hook causes the ordinary printing |
614 | of warnings to STDERR to be suppressed. You can use this to save warnings |
615 | in a variable, or turn warnings into fatal errors, like this: |
616 | |
617 | local $SIG{__WARN__} = sub { die $_[0] }; |
618 | eval $proggie; |
619 | |
620 | The routine indicated by $SIG{__DIE__} is called when a fatal exception |
621 | is about to be thrown. The error message is passed as the first |
622 | argument. When a __DIE__ hook routine returns, the exception |
623 | processing continues as it would have in the absence of the hook, |
624 | unless the hook routine itself exits via a goto, a loop exit, or a die. |
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625 | |
626 | =back |
627 | |