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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 | |
10 | punctuation names have reasonable mnemonics, or analogs in the | |
11 | shells. Nevertheless, if you wish to use long variable names, | |
12 | you need only say | |
13 | ||
14 | use English; | |
15 | ||
16 | at the top of your program. This aliases all the short names to the long | |
17 | names in the current package. Some even have medium names, generally | |
18 | borrowed from B<awk>. In general, it's best to use the | |
19 | ||
20 | use English '-no_match_vars'; | |
21 | ||
22 | invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids | |
23 | a certain performance hit with the use of regular expressions. See | |
24 | L<English>. | |
25 | ||
26 | Variables that depend on the currently selected filehandle may be set by | |
27 | calling an appropriate object method on the IO::Handle object, although | |
28 | this is less efficient than using the regular built-in variables. (Summary | |
29 | lines below for this contain the word HANDLE.) First you must say | |
30 | ||
31 | use IO::Handle; | |
32 | ||
33 | after which you may use either | |
34 | ||
35 | method HANDLE EXPR | |
36 | ||
37 | or more safely, | |
38 | ||
39 | HANDLE->method(EXPR) | |
40 | ||
41 | Each method returns the old value of the IO::Handle attribute. | |
42 | The methods each take an optional EXPR, which, if supplied, specifies the | |
43 | new value for the IO::Handle attribute in question. If not supplied, | |
44 | most methods do nothing to the current value--except for | |
45 | autoflush(), which will assume a 1 for you, just to be different. | |
46 | ||
47 | Because loading in the IO::Handle class is an expensive operation, you should | |
48 | learn how to use the regular built-in variables. | |
49 | ||
50 | A few of these variables are considered "read-only". This means that if | |
51 | you try to assign to this variable, either directly or indirectly through | |
52 | a reference, you'll raise a run-time exception. | |
53 | ||
54 | You should be very careful when modifying the default values of most | |
55 | special variables described in this document. In most cases you want | |
56 | to localize these variables before changing them, since if you don't, | |
57 | the change may affect other modules which rely on the default values | |
58 | of the special variables that you have changed. This is one of the | |
59 | correct ways to read the whole file at once: | |
60 | ||
61 | open my $fh, "foo" or die $!; | |
62 | local $/; # enable localized slurp mode | |
63 | my $content = <$fh>; | |
64 | close $fh; | |
65 | ||
66 | But the following code is quite bad: | |
67 | ||
68 | open my $fh, "foo" or die $!; | |
69 | undef $/; # enable slurp mode | |
70 | my $content = <$fh>; | |
71 | close $fh; | |
72 | ||
73 | since some other module, may want to read data from some file in the | |
74 | default "line mode", so if the code we have just presented has been | |
75 | executed, the global value of C<$/> is now changed for any other code | |
76 | running inside the same Perl interpreter. | |
77 | ||
78 | Usually when a variable is localized you want to make sure that this | |
79 | change affects the shortest scope possible. So unless you are already | |
80 | inside some short C<{}> block, you should create one yourself. For | |
81 | example: | |
82 | ||
83 | my $content = ''; | |
84 | open my $fh, "foo" or die $!; | |
85 | { | |
86 | local $/; | |
87 | $content = <$fh>; | |
88 | } | |
89 | close $fh; | |
90 | ||
91 | Here is an example of how your own code can go broken: | |
92 | ||
93 | for (1..5){ | |
94 | nasty_break(); | |
95 | print "$_ "; | |
96 | } | |
97 | sub nasty_break { | |
98 | $_ = 5; | |
99 | # do something with $_ | |
100 | } | |
101 | ||
102 | You probably expect this code to print: | |
103 | ||
104 | 1 2 3 4 5 | |
105 | ||
106 | but instead you get: | |
107 | ||
108 | 5 5 5 5 5 | |
109 | ||
110 | Why? Because nasty_break() modifies C<$_> without localizing it | |
111 | first. The fix is to add local(): | |
112 | ||
113 | local $_ = 5; | |
114 | ||
115 | It's easy to notice the problem in such a short example, but in more | |
116 | complicated code you are looking for trouble if you don't localize | |
117 | changes to the special variables. | |
118 | ||
119 | The following list is ordered by scalar variables first, then the | |
120 | arrays, then the hashes. | |
121 | ||
122 | =over 8 | |
123 | ||
124 | =item $ARG | |
125 | ||
126 | =item $_ | |
127 | ||
128 | The default input and pattern-searching space. The following pairs are | |
129 | equivalent: | |
130 | ||
131 | while (<>) {...} # equivalent only in while! | |
132 | while (defined($_ = <>)) {...} | |
133 | ||
134 | /^Subject:/ | |
135 | $_ =~ /^Subject:/ | |
136 | ||
137 | tr/a-z/A-Z/ | |
138 | $_ =~ tr/a-z/A-Z/ | |
139 | ||
140 | chomp | |
141 | chomp($_) | |
142 | ||
143 | Here are the places where Perl will assume $_ even if you | |
144 | don't use it: | |
145 | ||
146 | =over 3 | |
147 | ||
148 | =item * | |
149 | ||
150 | Various unary functions, including functions like ord() and int(), as well | |
151 | as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to | |
152 | STDIN. | |
153 | ||
154 | =item * | |
155 | ||
156 | Various list functions like print() and unlink(). | |
157 | ||
158 | =item * | |
159 | ||
160 | The pattern matching operations C<m//>, C<s///>, and C<tr///> when used | |
161 | without an C<=~> operator. | |
162 | ||
163 | =item * | |
164 | ||
165 | The default iterator variable in a C<foreach> loop if no other | |
166 | variable is supplied. | |
167 | ||
168 | =item * | |
169 | ||
170 | The implicit iterator variable in the grep() and map() functions. | |
171 | ||
172 | =item * | |
173 | ||
174 | The default place to put an input record when a C<< <FH> >> | |
175 | operation's result is tested by itself as the sole criterion of a C<while> | |
176 | test. Outside a C<while> test, this will not happen. | |
177 | ||
178 | =back | |
179 | ||
180 | (Mnemonic: underline is understood in certain operations.) | |
181 | ||
182 | =back | |
183 | ||
184 | =over 8 | |
185 | ||
186 | =item $a | |
187 | ||
188 | =item $b | |
189 | ||
190 | Special package variables when using sort(), see L<perlfunc/sort>. | |
191 | Because of this specialness $a and $b don't need to be declared | |
192 | (using use vars, or our()) even when using the C<strict 'vars'> pragma. | |
193 | Don't lexicalize them with C<my $a> or C<my $b> if you want to be | |
194 | able to use them in the sort() comparison block or function. | |
195 | ||
196 | =back | |
197 | ||
198 | =over 8 | |
199 | ||
200 | =item $<I<digits>> | |
201 | ||
202 | Contains the subpattern from the corresponding set of capturing | |
203 | parentheses from the last pattern match, not counting patterns | |
204 | matched in nested blocks that have been exited already. (Mnemonic: | |
205 | like \digits.) These variables are all read-only and dynamically | |
206 | scoped to the current BLOCK. | |
207 | ||
208 | =item $MATCH | |
209 | ||
210 | =item $& | |
211 | ||
212 | The string matched by the last successful pattern match (not counting | |
213 | any matches hidden within a BLOCK or eval() enclosed by the current | |
214 | BLOCK). (Mnemonic: like & in some editors.) This variable is read-only | |
215 | and dynamically scoped to the current BLOCK. | |
216 | ||
217 | The use of this variable anywhere in a program imposes a considerable | |
218 | performance penalty on all regular expression matches. See L</BUGS>. | |
219 | ||
220 | =item $PREMATCH | |
221 | ||
222 | =item $` | |
223 | ||
224 | The string preceding whatever was matched by the last successful | |
225 | pattern match (not counting any matches hidden within a BLOCK or eval | |
226 | enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted | |
227 | string.) This variable is read-only. | |
228 | ||
229 | The use of this variable anywhere in a program imposes a considerable | |
230 | performance penalty on all regular expression matches. See L</BUGS>. | |
231 | ||
232 | =item $POSTMATCH | |
233 | ||
234 | =item $' | |
235 | ||
236 | The string following whatever was matched by the last successful | |
237 | pattern match (not counting any matches hidden within a BLOCK or eval() | |
238 | enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted | |
239 | string.) Example: | |
240 | ||
241 | local $_ = 'abcdefghi'; | |
242 | /def/; | |
243 | print "$`:$&:$'\n"; # prints abc:def:ghi | |
244 | ||
245 | This variable is read-only and dynamically scoped to the current BLOCK. | |
246 | ||
247 | The use of this variable anywhere in a program imposes a considerable | |
248 | performance penalty on all regular expression matches. See L</BUGS>. | |
249 | ||
250 | =item $LAST_PAREN_MATCH | |
251 | ||
252 | =item $+ | |
253 | ||
254 | The text matched by the last bracket of the last successful search pattern. | |
255 | This is useful if you don't know which one of a set of alternative patterns | |
256 | matched. For example: | |
257 | ||
258 | /Version: (.*)|Revision: (.*)/ && ($rev = $+); | |
259 | ||
260 | (Mnemonic: be positive and forward looking.) | |
261 | This variable is read-only and dynamically scoped to the current BLOCK. | |
262 | ||
263 | =item $^N | |
264 | ||
265 | The text matched by the used group most-recently closed (i.e. the group | |
266 | with the rightmost closing parenthesis) of the last successful search | |
267 | pattern. (Mnemonic: the (possibly) Nested parenthesis that most | |
268 | recently closed.) | |
269 | ||
270 | This is primarily used inside C<(?{...})> blocks for examining text | |
271 | recently matched. For example, to effectively capture text to a variable | |
272 | (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with | |
273 | ||
274 | (?:(...)(?{ $var = $^N })) | |
275 | ||
276 | By setting and then using C<$var> in this way relieves you from having to | |
277 | worry about exactly which numbered set of parentheses they are. | |
278 | ||
279 | This variable is dynamically scoped to the current BLOCK. | |
280 | ||
281 | =item @LAST_MATCH_END | |
282 | ||
283 | =item @+ | |
284 | ||
285 | This array holds the offsets of the ends of the last successful | |
286 | submatches in the currently active dynamic scope. C<$+[0]> is | |
287 | the offset into the string of the end of the entire match. This | |
288 | is the same value as what the C<pos> function returns when called | |
289 | on the variable that was matched against. The I<n>th element | |
290 | of this array holds the offset of the I<n>th submatch, so | |
291 | C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset | |
292 | past where $2 ends, and so on. You can use C<$#+> to determine | |
293 | how many subgroups were in the last successful match. See the | |
294 | examples given for the C<@-> variable. | |
295 | ||
296 | =item $* | |
297 | ||
298 | Set to a non-zero integer value to do multi-line matching within a | |
299 | string, 0 (or undefined) to tell Perl that it can assume that strings | |
300 | contain a single line, for the purpose of optimizing pattern matches. | |
301 | Pattern matches on strings containing multiple newlines can produce | |
302 | confusing results when C<$*> is 0 or undefined. Default is undefined. | |
303 | (Mnemonic: * matches multiple things.) This variable influences the | |
304 | interpretation of only C<^> and C<$>. A literal newline can be searched | |
305 | for even when C<$* == 0>. | |
306 | ||
307 | Use of C<$*> is deprecated in modern Perl, supplanted by | |
308 | the C</s> and C</m> modifiers on pattern matching. | |
309 | ||
310 | Assigning a non-numerical value to C<$*> triggers a warning (and makes | |
311 | C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*> | |
312 | makes that an implicit C<int> is applied on the value. | |
313 | ||
314 | =item HANDLE->input_line_number(EXPR) | |
315 | ||
316 | =item $INPUT_LINE_NUMBER | |
317 | ||
318 | =item $NR | |
319 | ||
320 | =item $. | |
321 | ||
322 | Current line number for the last filehandle accessed. | |
323 | ||
324 | Each filehandle in Perl counts the number of lines that have been read | |
325 | from it. (Depending on the value of C<$/>, Perl's idea of what | |
326 | constitutes a line may not match yours.) When a line is read from a | |
327 | filehandle (via readline() or C<< <> >>), or when tell() or seek() is | |
328 | called on it, C<$.> becomes an alias to the line counter for that | |
329 | filehandle. | |
330 | ||
331 | You can adjust the counter by assigning to C<$.>, but this will not | |
332 | actually move the seek pointer. I<Localizing C<$.> will not localize | |
333 | the filehandle's line count>. Instead, it will localize perl's notion | |
334 | of which filehandle C<$.> is currently aliased to. | |
335 | ||
336 | C<$.> is reset when the filehandle is closed, but B<not> when an open | |
337 | filehandle is reopened without an intervening close(). For more | |
338 | details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does | |
339 | an explicit close, line numbers increase across ARGV files (but see | |
340 | examples in L<perlfunc/eof>). | |
341 | ||
342 | You can also use C<< HANDLE->input_line_number(EXPR) >> to access the | |
343 | line counter for a given filehandle without having to worry about | |
344 | which handle you last accessed. | |
345 | ||
346 | (Mnemonic: many programs use "." to mean the current line number.) | |
347 | ||
348 | =item IO::Handle->input_record_separator(EXPR) | |
349 | ||
350 | =item $INPUT_RECORD_SEPARATOR | |
351 | ||
352 | =item $RS | |
353 | ||
354 | =item $/ | |
355 | ||
356 | The input record separator, newline by default. This | |
357 | influences Perl's idea of what a "line" is. Works like B<awk>'s RS | |
358 | variable, including treating empty lines as a terminator if set to | |
359 | the null string. (An empty line cannot contain any spaces | |
360 | or tabs.) You may set it to a multi-character string to match a | |
361 | multi-character terminator, or to C<undef> to read through the end | |
362 | of file. Setting it to C<"\n\n"> means something slightly | |
363 | different than setting to C<"">, if the file contains consecutive | |
364 | empty lines. Setting to C<""> will treat two or more consecutive | |
365 | empty lines as a single empty line. Setting to C<"\n\n"> will | |
366 | blindly assume that the next input character belongs to the next | |
367 | paragraph, even if it's a newline. (Mnemonic: / delimits | |
368 | line boundaries when quoting poetry.) | |
369 | ||
370 | local $/; # enable "slurp" mode | |
371 | local $_ = <FH>; # whole file now here | |
372 | s/\n[ \t]+/ /g; | |
373 | ||
374 | Remember: the value of C<$/> is a string, not a regex. B<awk> has to be | |
375 | better for something. :-) | |
376 | ||
377 | Setting C<$/> to a reference to an integer, scalar containing an integer, or | |
378 | scalar that's convertible to an integer will attempt to read records | |
379 | instead of lines, with the maximum record size being the referenced | |
380 | integer. So this: | |
381 | ||
382 | local $/ = \32768; # or \"32768", or \$var_containing_32768 | |
383 | open my $fh, $myfile or die $!; | |
384 | local $_ = <$fh>; | |
385 | ||
386 | will read a record of no more than 32768 bytes from FILE. If you're | |
387 | not reading from a record-oriented file (or your OS doesn't have | |
388 | record-oriented files), then you'll likely get a full chunk of data | |
389 | with every read. If a record is larger than the record size you've | |
390 | set, you'll get the record back in pieces. | |
391 | ||
392 | On VMS, record reads are done with the equivalent of C<sysread>, | |
393 | so it's best not to mix record and non-record reads on the same | |
394 | file. (This is unlikely to be a problem, because any file you'd | |
395 | want to read in record mode is probably unusable in line mode.) | |
396 | Non-VMS systems do normal I/O, so it's safe to mix record and | |
397 | non-record reads of a file. | |
398 | ||
399 | See also L<perlport/"Newlines">. Also see C<$.>. | |
400 | ||
401 | =item HANDLE->autoflush(EXPR) | |
402 | ||
403 | =item $OUTPUT_AUTOFLUSH | |
404 | ||
405 | =item $| | |
406 | ||
407 | If set to nonzero, forces a flush right away and after every write | |
408 | or print on the currently selected output channel. Default is 0 | |
409 | (regardless of whether the channel is really buffered by the | |
410 | system or not; C<$|> tells you only whether you've asked Perl | |
411 | explicitly to flush after each write). STDOUT will | |
412 | typically be line buffered if output is to the terminal and block | |
413 | buffered otherwise. Setting this variable is useful primarily when | |
414 | you are outputting to a pipe or socket, such as when you are running | |
415 | a Perl program under B<rsh> and want to see the output as it's | |
416 | happening. This has no effect on input buffering. See L<perlfunc/getc> | |
417 | for that. (Mnemonic: when you want your pipes to be piping hot.) | |
418 | ||
419 | =item IO::Handle->output_field_separator EXPR | |
420 | ||
421 | =item $OUTPUT_FIELD_SEPARATOR | |
422 | ||
423 | =item $OFS | |
424 | ||
425 | =item $, | |
426 | ||
427 | The output field separator for the print operator. If defined, this | |
428 | value is printed between each of print's arguments. Default is C<undef>. | |
429 | (Mnemonic: what is printed when there is a "," in your print statement.) | |
430 | ||
431 | =item IO::Handle->output_record_separator EXPR | |
432 | ||
433 | =item $OUTPUT_RECORD_SEPARATOR | |
434 | ||
435 | =item $ORS | |
436 | ||
437 | =item $\ | |
438 | ||
439 | The output record separator for the print operator. If defined, this | |
440 | value is printed after the last of print's arguments. Default is C<undef>. | |
441 | (Mnemonic: you set C<$\> instead of adding "\n" at the end of the print. | |
442 | Also, it's just like C<$/>, but it's what you get "back" from Perl.) | |
443 | ||
444 | =item $LIST_SEPARATOR | |
445 | ||
446 | =item $" | |
447 | ||
448 | This is like C<$,> except that it applies to array and slice values | |
449 | interpolated into a double-quoted string (or similar interpreted | |
450 | string). Default is a space. (Mnemonic: obvious, I think.) | |
451 | ||
452 | =item $SUBSCRIPT_SEPARATOR | |
453 | ||
454 | =item $SUBSEP | |
455 | ||
456 | =item $; | |
457 | ||
458 | The subscript separator for multidimensional array emulation. If you | |
459 | refer to a hash element as | |
460 | ||
461 | $foo{$a,$b,$c} | |
462 | ||
463 | it really means | |
464 | ||
465 | $foo{join($;, $a, $b, $c)} | |
466 | ||
467 | But don't put | |
468 | ||
469 | @foo{$a,$b,$c} # a slice--note the @ | |
470 | ||
471 | which means | |
472 | ||
473 | ($foo{$a},$foo{$b},$foo{$c}) | |
474 | ||
475 | Default is "\034", the same as SUBSEP in B<awk>. If your | |
476 | keys contain binary data there might not be any safe value for C<$;>. | |
477 | (Mnemonic: comma (the syntactic subscript separator) is a | |
478 | semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already | |
479 | taken for something more important.) | |
480 | ||
481 | Consider using "real" multidimensional arrays as described | |
482 | in L<perllol>. | |
483 | ||
484 | =item $# | |
485 | ||
486 | The output format for printed numbers. This variable is a half-hearted | |
487 | attempt to emulate B<awk>'s OFMT variable. There are times, however, | |
488 | when B<awk> and Perl have differing notions of what counts as | |
489 | numeric. The initial value is "%.I<n>g", where I<n> is the value | |
490 | of the macro DBL_DIG from your system's F<float.h>. This is different from | |
491 | B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#> | |
492 | explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.) | |
493 | ||
494 | Use of C<$#> is deprecated. | |
495 | ||
496 | =item HANDLE->format_page_number(EXPR) | |
497 | ||
498 | =item $FORMAT_PAGE_NUMBER | |
499 | ||
500 | =item $% | |
501 | ||
502 | The current page number of the currently selected output channel. | |
503 | Used with formats. | |
504 | (Mnemonic: % is page number in B<nroff>.) | |
505 | ||
506 | =item HANDLE->format_lines_per_page(EXPR) | |
507 | ||
508 | =item $FORMAT_LINES_PER_PAGE | |
509 | ||
510 | =item $= | |
511 | ||
512 | The current page length (printable lines) of the currently selected | |
513 | output channel. Default is 60. | |
514 | Used with formats. | |
515 | (Mnemonic: = has horizontal lines.) | |
516 | ||
517 | =item HANDLE->format_lines_left(EXPR) | |
518 | ||
519 | =item $FORMAT_LINES_LEFT | |
520 | ||
521 | =item $- | |
522 | ||
523 | The number of lines left on the page of the currently selected output | |
524 | channel. | |
525 | Used with formats. | |
526 | (Mnemonic: lines_on_page - lines_printed.) | |
527 | ||
528 | =item @LAST_MATCH_START | |
529 | ||
530 | =item @- | |
531 | ||
532 | $-[0] is the offset of the start of the last successful match. | |
533 | C<$-[>I<n>C<]> is the offset of the start of the substring matched by | |
534 | I<n>-th subpattern, or undef if the subpattern did not match. | |
535 | ||
536 | Thus after a match against $_, $& coincides with C<substr $_, $-[0], | |
537 | $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n], | |
538 | $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with | |
539 | C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last | |
540 | matched subgroup in the last successful match. Contrast with | |
541 | C<$#+>, the number of subgroups in the regular expression. Compare | |
542 | with C<@+>. | |
543 | ||
544 | This array holds the offsets of the beginnings of the last | |
545 | successful submatches in the currently active dynamic scope. | |
546 | C<$-[0]> is the offset into the string of the beginning of the | |
547 | entire match. The I<n>th element of this array holds the offset | |
548 | of the I<n>th submatch, so C<$-[1]> is the offset where $1 | |
549 | begins, C<$-[2]> the offset where $2 begins, and so on. | |
550 | ||
551 | After a match against some variable $var: | |
552 | ||
553 | =over 5 | |
554 | ||
555 | =item C<$`> is the same as C<substr($var, 0, $-[0])> | |
556 | ||
557 | =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])> | |
558 | ||
559 | =item C<$'> is the same as C<substr($var, $+[0])> | |
560 | ||
561 | =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])> | |
562 | ||
563 | =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])> | |
564 | ||
565 | =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])> | |
566 | ||
567 | =back | |
568 | ||
569 | =item HANDLE->format_name(EXPR) | |
570 | ||
571 | =item $FORMAT_NAME | |
572 | ||
573 | =item $~ | |
574 | ||
575 | The name of the current report format for the currently selected output | |
576 | channel. Default is the name of the filehandle. (Mnemonic: brother to | |
577 | C<$^>.) | |
578 | ||
579 | =item HANDLE->format_top_name(EXPR) | |
580 | ||
581 | =item $FORMAT_TOP_NAME | |
582 | ||
583 | =item $^ | |
584 | ||
585 | The name of the current top-of-page format for the currently selected | |
586 | output channel. Default is the name of the filehandle with _TOP | |
587 | appended. (Mnemonic: points to top of page.) | |
588 | ||
589 | =item IO::Handle->format_line_break_characters EXPR | |
590 | ||
591 | =item $FORMAT_LINE_BREAK_CHARACTERS | |
592 | ||
593 | =item $: | |
594 | ||
595 | The current set of characters after which a string may be broken to | |
596 | fill continuation fields (starting with ^) in a format. Default is | |
597 | S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in | |
598 | poetry is a part of a line.) | |
599 | ||
600 | =item IO::Handle->format_formfeed EXPR | |
601 | ||
602 | =item $FORMAT_FORMFEED | |
603 | ||
604 | =item $^L | |
605 | ||
606 | What formats output as a form feed. Default is \f. | |
607 | ||
608 | =item $ACCUMULATOR | |
609 | ||
610 | =item $^A | |
611 | ||
612 | The current value of the write() accumulator for format() lines. A format | |
613 | contains formline() calls that put their result into C<$^A>. After | |
614 | calling its format, write() prints out the contents of C<$^A> and empties. | |
615 | So you never really see the contents of C<$^A> unless you call | |
616 | formline() yourself and then look at it. See L<perlform> and | |
617 | L<perlfunc/formline()>. | |
618 | ||
619 | =item $CHILD_ERROR | |
620 | ||
621 | =item $? | |
622 | ||
623 | The status returned by the last pipe close, backtick (C<``>) command, | |
624 | successful call to wait() or waitpid(), or from the system() | |
625 | operator. This is just the 16-bit status word returned by the | |
626 | wait() system call (or else is made up to look like it). Thus, the | |
627 | exit value of the subprocess is really (C<<< $? >> 8 >>>), and | |
628 | C<$? & 127> gives which signal, if any, the process died from, and | |
629 | C<$? & 128> reports whether there was a core dump. (Mnemonic: | |
630 | similar to B<sh> and B<ksh>.) | |
631 | ||
632 | Additionally, if the C<h_errno> variable is supported in C, its value | |
633 | is returned via $? if any C<gethost*()> function fails. | |
634 | ||
635 | If you have installed a signal handler for C<SIGCHLD>, the | |
636 | value of C<$?> will usually be wrong outside that handler. | |
637 | ||
638 | Inside an C<END> subroutine C<$?> contains the value that is going to be | |
639 | given to C<exit()>. You can modify C<$?> in an C<END> subroutine to | |
640 | change the exit status of your program. For example: | |
641 | ||
642 | END { | |
643 | $? = 1 if $? == 255; # die would make it 255 | |
644 | } | |
645 | ||
646 | Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the | |
647 | actual VMS exit status, instead of the default emulation of POSIX | |
648 | status; see L<perlvms/$?> for details. | |
649 | ||
650 | Also see L<Error Indicators>. | |
651 | ||
652 | =item ${^ENCODING} | |
653 | ||
654 | The I<object reference> to the Encode object that is used to convert | |
655 | the source code to Unicode. Thanks to this variable your perl script | |
656 | does not have to be written in UTF-8. Default is I<undef>. The direct | |
657 | manipulation of this variable is highly discouraged. See L<encoding> | |
658 | for more details. | |
659 | ||
660 | =item $OS_ERROR | |
661 | ||
662 | =item $ERRNO | |
663 | ||
664 | =item $! | |
665 | ||
666 | If used numerically, yields the current value of the C C<errno> | |
667 | variable, or in other words, if a system or library call fails, it | |
668 | sets this variable. This means that the value of C<$!> is meaningful | |
669 | only I<immediately> after a B<failure>: | |
670 | ||
671 | if (open(FH, $filename)) { | |
672 | # Here $! is meaningless. | |
673 | ... | |
674 | } else { | |
675 | # ONLY here is $! meaningful. | |
676 | ... | |
677 | # Already here $! might be meaningless. | |
678 | } | |
679 | # Since here we might have either success or failure, | |
680 | # here $! is meaningless. | |
681 | ||
682 | In the above I<meaningless> stands for anything: zero, non-zero, | |
683 | C<undef>. A successful system or library call does B<not> set | |
684 | the variable to zero. | |
685 | ||
686 | If used as a string, yields the corresponding system error string. | |
687 | You can assign a number to C<$!> to set I<errno> if, for instance, | |
688 | you want C<"$!"> to return the string for error I<n>, or you want | |
689 | to set the exit value for the die() operator. (Mnemonic: What just | |
690 | went bang?) | |
691 | ||
692 | Also see L<Error Indicators>. | |
693 | ||
694 | =item %! | |
695 | ||
696 | Each element of C<%!> has a true value only if C<$!> is set to that | |
697 | value. For example, C<$!{ENOENT}> is true if and only if the current | |
698 | value of C<$!> is C<ENOENT>; that is, if the most recent error was | |
699 | "No such file or directory" (or its moral equivalent: not all operating | |
700 | systems give that exact error, and certainly not all languages). | |
701 | To check if a particular key is meaningful on your system, use | |
702 | C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>. | |
703 | See L<Errno> for more information, and also see above for the | |
704 | validity of C<$!>. | |
705 | ||
706 | =item $EXTENDED_OS_ERROR | |
707 | ||
708 | =item $^E | |
709 | ||
710 | Error information specific to the current operating system. At | |
711 | the moment, this differs from C<$!> under only VMS, OS/2, and Win32 | |
712 | (and for MacPerl). On all other platforms, C<$^E> is always just | |
713 | the same as C<$!>. | |
714 | ||
715 | Under VMS, C<$^E> provides the VMS status value from the last | |
716 | system error. This is more specific information about the last | |
717 | system error than that provided by C<$!>. This is particularly | |
718 | important when C<$!> is set to B<EVMSERR>. | |
719 | ||
720 | Under OS/2, C<$^E> is set to the error code of the last call to | |
721 | OS/2 API either via CRT, or directly from perl. | |
722 | ||
723 | Under Win32, C<$^E> always returns the last error information | |
724 | reported by the Win32 call C<GetLastError()> which describes | |
725 | the last error from within the Win32 API. Most Win32-specific | |
726 | code will report errors via C<$^E>. ANSI C and Unix-like calls | |
727 | set C<errno> and so most portable Perl code will report errors | |
728 | via C<$!>. | |
729 | ||
730 | Caveats mentioned in the description of C<$!> generally apply to | |
731 | C<$^E>, also. (Mnemonic: Extra error explanation.) | |
732 | ||
733 | Also see L<Error Indicators>. | |
734 | ||
735 | =item $EVAL_ERROR | |
736 | ||
737 | =item $@ | |
738 | ||
739 | The Perl syntax error message from the last eval() operator. | |
740 | If $@ is the null string, the last eval() parsed and executed | |
741 | correctly (although the operations you invoked may have failed in the | |
742 | normal fashion). (Mnemonic: Where was the syntax error "at"?) | |
743 | ||
744 | Warning messages are not collected in this variable. You can, | |
745 | however, set up a routine to process warnings by setting C<$SIG{__WARN__}> | |
746 | as described below. | |
747 | ||
748 | Also see L<Error Indicators>. | |
749 | ||
750 | =item $PROCESS_ID | |
751 | ||
752 | =item $PID | |
753 | ||
754 | =item $$ | |
755 | ||
756 | The process number of the Perl running this script. You should | |
757 | consider this variable read-only, although it will be altered | |
758 | across fork() calls. (Mnemonic: same as shells.) | |
759 | ||
760 | Note for Linux users: on Linux, the C functions C<getpid()> and | |
761 | C<getppid()> return different values from different threads. In order to | |
762 | be portable, this behavior is not reflected by C<$$>, whose value remains | |
763 | consistent across threads. If you want to call the underlying C<getpid()>, | |
764 | you may use the CPAN module C<Linux::Pid>. | |
765 | ||
766 | =item $REAL_USER_ID | |
767 | ||
768 | =item $UID | |
769 | ||
770 | =item $< | |
771 | ||
772 | The real uid of this process. (Mnemonic: it's the uid you came I<from>, | |
773 | if you're running setuid.) You can change both the real uid and | |
774 | the effective uid at the same time by using POSIX::setuid(). Since | |
775 | changes to $< require a system call, check $! after a change attempt to | |
776 | detect any possible errors. | |
777 | ||
778 | =item $EFFECTIVE_USER_ID | |
779 | ||
780 | =item $EUID | |
781 | ||
782 | =item $> | |
783 | ||
784 | The effective uid of this process. Example: | |
785 | ||
786 | $< = $>; # set real to effective uid | |
787 | ($<,$>) = ($>,$<); # swap real and effective uid | |
788 | ||
789 | You can change both the effective uid and the real uid at the same | |
790 | time by using POSIX::setuid(). Changes to $> require a check to $! | |
791 | to detect any possible errors after an attempted change. | |
792 | ||
793 | (Mnemonic: it's the uid you went I<to>, if you're running setuid.) | |
794 | C<< $< >> and C<< $> >> can be swapped only on machines | |
795 | supporting setreuid(). | |
796 | ||
797 | =item $REAL_GROUP_ID | |
798 | ||
799 | =item $GID | |
800 | ||
801 | =item $( | |
802 | ||
803 | The real gid of this process. If you are on a machine that supports | |
804 | membership in multiple groups simultaneously, gives a space separated | |
805 | list of groups you are in. The first number is the one returned by | |
806 | getgid(), and the subsequent ones by getgroups(), one of which may be | |
807 | the same as the first number. | |
808 | ||
809 | However, a value assigned to C<$(> must be a single number used to | |
810 | set the real gid. So the value given by C<$(> should I<not> be assigned | |
811 | back to C<$(> without being forced numeric, such as by adding zero. | |
812 | ||
813 | You can change both the real gid and the effective gid at the same | |
814 | time by using POSIX::setgid(). Changes to $( require a check to $! | |
815 | to detect any possible errors after an attempted change. | |
816 | ||
817 | (Mnemonic: parentheses are used to I<group> things. The real gid is the | |
818 | group you I<left>, if you're running setgid.) | |
819 | ||
820 | =item $EFFECTIVE_GROUP_ID | |
821 | ||
822 | =item $EGID | |
823 | ||
824 | =item $) | |
825 | ||
826 | The effective gid of this process. If you are on a machine that | |
827 | supports membership in multiple groups simultaneously, gives a space | |
828 | separated list of groups you are in. The first number is the one | |
829 | returned by getegid(), and the subsequent ones by getgroups(), one of | |
830 | which may be the same as the first number. | |
831 | ||
832 | Similarly, a value assigned to C<$)> must also be a space-separated | |
833 | list of numbers. The first number sets the effective gid, and | |
834 | the rest (if any) are passed to setgroups(). To get the effect of an | |
835 | empty list for setgroups(), just repeat the new effective gid; that is, | |
836 | to force an effective gid of 5 and an effectively empty setgroups() | |
837 | list, say C< $) = "5 5" >. | |
838 | ||
839 | You can change both the effective gid and the real gid at the same | |
840 | time by using POSIX::setgid() (use only a single numeric argument). | |
841 | Changes to $) require a check to $! to detect any possible errors | |
842 | after an attempted change. | |
843 | ||
844 | (Mnemonic: parentheses are used to I<group> things. The effective gid | |
845 | is the group that's I<right> for you, if you're running setgid.) | |
846 | ||
847 | C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on | |
848 | machines that support the corresponding I<set[re][ug]id()> routine. C<$(> | |
849 | and C<$)> can be swapped only on machines supporting setregid(). | |
850 | ||
851 | =item $PROGRAM_NAME | |
852 | ||
853 | =item $0 | |
854 | ||
855 | Contains the name of the program being executed. | |
856 | ||
857 | On some (read: not all) operating systems assigning to C<$0> modifies | |
858 | the argument area that the C<ps> program sees. On some platforms you | |
859 | may have to use special C<ps> options or a different C<ps> to see the | |
860 | changes. Modifying the $0 is more useful as a way of indicating the | |
861 | current program state than it is for hiding the program you're | |
862 | running. (Mnemonic: same as B<sh> and B<ksh>.) | |
863 | ||
864 | Note that there are platform specific limitations on the maximum | |
865 | length of C<$0>. In the most extreme case it may be limited to the | |
866 | space occupied by the original C<$0>. | |
867 | ||
868 | In some platforms there may be arbitrary amount of padding, for | |
869 | example space characters, after the modified name as shown by C<ps>. | |
870 | In some platforms this padding may extend all the way to the original | |
871 | length of the argument area, no matter what you do (this is the case | |
872 | for example with Linux 2.2). | |
873 | ||
874 | Note for BSD users: setting C<$0> does not completely remove "perl" | |
875 | from the ps(1) output. For example, setting C<$0> to C<"foobar"> may | |
876 | result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix | |
877 | and the " (perl)" suffix are shown depends on your exact BSD variant | |
878 | and version). This is an operating system feature, Perl cannot help it. | |
879 | ||
880 | In multithreaded scripts Perl coordinates the threads so that any | |
881 | thread may modify its copy of the C<$0> and the change becomes visible | |
882 | to ps(1) (assuming the operating system plays along). Note that | |
883 | the view of C<$0> the other threads have will not change since they | |
884 | have their own copies of it. | |
885 | ||
886 | =item $[ | |
887 | ||
888 | The index of the first element in an array, and of the first character | |
889 | in a substring. Default is 0, but you could theoretically set it | |
890 | to 1 to make Perl behave more like B<awk> (or Fortran) when | |
891 | subscripting and when evaluating the index() and substr() functions. | |
892 | (Mnemonic: [ begins subscripts.) | |
893 | ||
894 | As of release 5 of Perl, assignment to C<$[> is treated as a compiler | |
895 | directive, and cannot influence the behavior of any other file. | |
896 | (That's why you can only assign compile-time constants to it.) | |
897 | Its use is highly discouraged. | |
898 | ||
899 | Note that, unlike other compile-time directives (such as L<strict>), | |
900 | assignment to C<$[> can be seen from outer lexical scopes in the same file. | |
901 | However, you can use local() on it to strictly bind its value to a | |
902 | lexical block. | |
903 | ||
904 | =item $] | |
905 | ||
906 | The version + patchlevel / 1000 of the Perl interpreter. This variable | |
907 | can be used to determine whether the Perl interpreter executing a | |
908 | script is in the right range of versions. (Mnemonic: Is this version | |
909 | of perl in the right bracket?) Example: | |
910 | ||
911 | warn "No checksumming!\n" if $] < 3.019; | |
912 | ||
913 | See also the documentation of C<use VERSION> and C<require VERSION> | |
914 | for a convenient way to fail if the running Perl interpreter is too old. | |
915 | ||
916 | When testing the variable, to steer clear of floating point | |
917 | inaccuracies you might want to prefer the inequality tests C<< < >> | |
918 | and C<< > >> to the tests containing equivalence: C<< <= >>, C<< == >>, | |
919 | and C<< >= >>. | |
920 | ||
921 | The floating point representation can sometimes lead to inaccurate | |
922 | numeric comparisons. See C<$^V> for a more modern representation of | |
923 | the Perl version that allows accurate string comparisons. | |
924 | ||
925 | =item $COMPILING | |
926 | ||
927 | =item $^C | |
928 | ||
929 | The current value of the flag associated with the B<-c> switch. | |
930 | Mainly of use with B<-MO=...> to allow code to alter its behavior | |
931 | when being compiled, such as for example to AUTOLOAD at compile | |
932 | time rather than normal, deferred loading. See L<perlcc>. Setting | |
933 | C<$^C = 1> is similar to calling C<B::minus_c>. | |
934 | ||
935 | =item $DEBUGGING | |
936 | ||
937 | =item $^D | |
938 | ||
939 | The current value of the debugging flags. (Mnemonic: value of B<-D> | |
940 | switch.) May be read or set. Like its command-line equivalent, you can use | |
941 | numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">. | |
942 | ||
943 | =item $SYSTEM_FD_MAX | |
944 | ||
945 | =item $^F | |
946 | ||
947 | The maximum system file descriptor, ordinarily 2. System file | |
948 | descriptors are passed to exec()ed processes, while higher file | |
949 | descriptors are not. Also, during an open(), system file descriptors are | |
950 | preserved even if the open() fails. (Ordinary file descriptors are | |
951 | closed before the open() is attempted.) The close-on-exec | |
952 | status of a file descriptor will be decided according to the value of | |
953 | C<$^F> when the corresponding file, pipe, or socket was opened, not the | |
954 | time of the exec(). | |
955 | ||
956 | =item $^H | |
957 | ||
958 | WARNING: This variable is strictly for internal use only. Its availability, | |
959 | behavior, and contents are subject to change without notice. | |
960 | ||
961 | This variable contains compile-time hints for the Perl interpreter. At the | |
962 | end of compilation of a BLOCK the value of this variable is restored to the | |
963 | value when the interpreter started to compile the BLOCK. | |
964 | ||
965 | When perl begins to parse any block construct that provides a lexical scope | |
966 | (e.g., eval body, required file, subroutine body, loop body, or conditional | |
967 | block), the existing value of $^H is saved, but its value is left unchanged. | |
968 | When the compilation of the block is completed, it regains the saved value. | |
969 | Between the points where its value is saved and restored, code that | |
970 | executes within BEGIN blocks is free to change the value of $^H. | |
971 | ||
972 | This behavior provides the semantic of lexical scoping, and is used in, | |
973 | for instance, the C<use strict> pragma. | |
974 | ||
975 | The contents should be an integer; different bits of it are used for | |
976 | different pragmatic flags. Here's an example: | |
977 | ||
978 | sub add_100 { $^H |= 0x100 } | |
979 | ||
980 | sub foo { | |
981 | BEGIN { add_100() } | |
982 | bar->baz($boon); | |
983 | } | |
984 | ||
985 | Consider what happens during execution of the BEGIN block. At this point | |
986 | the BEGIN block has already been compiled, but the body of foo() is still | |
987 | being compiled. The new value of $^H will therefore be visible only while | |
988 | the body of foo() is being compiled. | |
989 | ||
990 | Substitution of the above BEGIN block with: | |
991 | ||
992 | BEGIN { require strict; strict->import('vars') } | |
993 | ||
994 | demonstrates how C<use strict 'vars'> is implemented. Here's a conditional | |
995 | version of the same lexical pragma: | |
996 | ||
997 | BEGIN { require strict; strict->import('vars') if $condition } | |
998 | ||
999 | =item %^H | |
1000 | ||
1001 | WARNING: This variable is strictly for internal use only. Its availability, | |
1002 | behavior, and contents are subject to change without notice. | |
1003 | ||
1004 | The %^H hash provides the same scoping semantic as $^H. This makes it | |
1005 | useful for implementation of lexically scoped pragmas. | |
1006 | ||
1007 | =item $INPLACE_EDIT | |
1008 | ||
1009 | =item $^I | |
1010 | ||
1011 | The current value of the inplace-edit extension. Use C<undef> to disable | |
1012 | inplace editing. (Mnemonic: value of B<-i> switch.) | |
1013 | ||
1014 | =item $^M | |
1015 | ||
1016 | By default, running out of memory is an untrappable, fatal error. | |
1017 | However, if suitably built, Perl can use the contents of C<$^M> | |
1018 | as an emergency memory pool after die()ing. Suppose that your Perl | |
1019 | were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc. | |
1020 | Then | |
1021 | ||
1022 | $^M = 'a' x (1 << 16); | |
1023 | ||
1024 | would allocate a 64K buffer for use in an emergency. See the | |
1025 | F<INSTALL> file in the Perl distribution for information on how to | |
1026 | add custom C compilation flags when compiling perl. To discourage casual | |
1027 | use of this advanced feature, there is no L<English|English> long name for | |
1028 | this variable. | |
1029 | ||
1030 | =item $OSNAME | |
1031 | ||
1032 | =item $^O | |
1033 | ||
1034 | The name of the operating system under which this copy of Perl was | |
1035 | built, as determined during the configuration process. The value | |
1036 | is identical to C<$Config{'osname'}>. See also L<Config> and the | |
1037 | B<-V> command-line switch documented in L<perlrun>. | |
1038 | ||
1039 | In Windows platforms, $^O is not very helpful: since it is always | |
1040 | C<MSWin32>, it doesn't tell the difference between | |
1041 | 95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or | |
1042 | Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish | |
1043 | between the variants. | |
1044 | ||
1045 | =item ${^OPEN} | |
1046 | ||
1047 | An internal variable used by PerlIO. A string in two parts, separated | |
1048 | by a C<\0> byte, the first part describes the input layers, the second | |
1049 | part describes the output layers. | |
1050 | ||
1051 | =item $PERLDB | |
1052 | ||
1053 | =item $^P | |
1054 | ||
1055 | The internal variable for debugging support. The meanings of the | |
1056 | various bits are subject to change, but currently indicate: | |
1057 | ||
1058 | =over 6 | |
1059 | ||
1060 | =item 0x01 | |
1061 | ||
1062 | Debug subroutine enter/exit. | |
1063 | ||
1064 | =item 0x02 | |
1065 | ||
1066 | Line-by-line debugging. | |
1067 | ||
1068 | =item 0x04 | |
1069 | ||
1070 | Switch off optimizations. | |
1071 | ||
1072 | =item 0x08 | |
1073 | ||
1074 | Preserve more data for future interactive inspections. | |
1075 | ||
1076 | =item 0x10 | |
1077 | ||
1078 | Keep info about source lines on which a subroutine is defined. | |
1079 | ||
1080 | =item 0x20 | |
1081 | ||
1082 | Start with single-step on. | |
1083 | ||
1084 | =item 0x40 | |
1085 | ||
1086 | Use subroutine address instead of name when reporting. | |
1087 | ||
1088 | =item 0x80 | |
1089 | ||
1090 | Report C<goto &subroutine> as well. | |
1091 | ||
1092 | =item 0x100 | |
1093 | ||
1094 | Provide informative "file" names for evals based on the place they were compiled. | |
1095 | ||
1096 | =item 0x200 | |
1097 | ||
1098 | Provide informative names to anonymous subroutines based on the place they | |
1099 | were compiled. | |
1100 | ||
1101 | =item 0x400 | |
1102 | ||
1103 | Debug assertion subroutines enter/exit. | |
1104 | ||
1105 | =back | |
1106 | ||
1107 | Some bits may be relevant at compile-time only, some at | |
1108 | run-time only. This is a new mechanism and the details may change. | |
1109 | ||
1110 | =item $LAST_REGEXP_CODE_RESULT | |
1111 | ||
1112 | =item $^R | |
1113 | ||
1114 | The result of evaluation of the last successful C<(?{ code })> | |
1115 | regular expression assertion (see L<perlre>). May be written to. | |
1116 | ||
1117 | =item $EXCEPTIONS_BEING_CAUGHT | |
1118 | ||
1119 | =item $^S | |
1120 | ||
1121 | Current state of the interpreter. | |
1122 | ||
1123 | $^S State | |
1124 | --------- ------------------- | |
1125 | undef Parsing module/eval | |
1126 | true (1) Executing an eval | |
1127 | false (0) Otherwise | |
1128 | ||
1129 | The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers. | |
1130 | ||
1131 | =item $BASETIME | |
1132 | ||
1133 | =item $^T | |
1134 | ||
1135 | The time at which the program began running, in seconds since the | |
1136 | epoch (beginning of 1970). The values returned by the B<-M>, B<-A>, | |
1137 | and B<-C> filetests are based on this value. | |
1138 | ||
1139 | =item ${^TAINT} | |
1140 | ||
1141 | Reflects if taint mode is on or off. 1 for on (the program was run with | |
1142 | B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with | |
1143 | B<-t> or B<-TU>). | |
1144 | ||
1145 | =item ${^UNICODE} | |
1146 | ||
1147 | Reflects certain Unicode settings of Perl. See L<perlrun> | |
1148 | documentation for the C<-C> switch for more information about | |
1149 | the possible values. This variable is set during Perl startup | |
1150 | and is thereafter read-only. | |
1151 | ||
1152 | =item ${^UTF8LOCALE} | |
1153 | ||
1154 | This variable indicates whether an UTF-8 locale was detected by perl at | |
1155 | startup. This information is used by perl when it's in | |
1156 | adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line | |
1157 | switch); see L<perlrun> for more info on this. | |
1158 | ||
1159 | =item $PERL_VERSION | |
1160 | ||
1161 | =item $^V | |
1162 | ||
1163 | The revision, version, and subversion of the Perl interpreter, represented | |
1164 | as a string composed of characters with those ordinals. Thus in Perl v5.6.0 | |
1165 | it equals C<chr(5) . chr(6) . chr(0)> and will return true for | |
1166 | C<$^V eq v5.6.0>. Note that the characters in this string value can | |
1167 | potentially be in Unicode range. | |
1168 | ||
1169 | This can be used to determine whether the Perl interpreter executing a | |
1170 | script is in the right range of versions. (Mnemonic: use ^V for Version | |
1171 | Control.) Example: | |
1172 | ||
1173 | warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0; | |
1174 | ||
1175 | To convert C<$^V> into its string representation use sprintf()'s | |
1176 | C<"%vd"> conversion: | |
1177 | ||
1178 | printf "version is v%vd\n", $^V; # Perl's version | |
1179 | ||
1180 | See the documentation of C<use VERSION> and C<require VERSION> | |
1181 | for a convenient way to fail if the running Perl interpreter is too old. | |
1182 | ||
1183 | See also C<$]> for an older representation of the Perl version. | |
1184 | ||
1185 | =item $WARNING | |
1186 | ||
1187 | =item $^W | |
1188 | ||
1189 | The current value of the warning switch, initially true if B<-w> | |
1190 | was used, false otherwise, but directly modifiable. (Mnemonic: | |
1191 | related to the B<-w> switch.) See also L<warnings>. | |
1192 | ||
1193 | =item ${^WARNING_BITS} | |
1194 | ||
1195 | The current set of warning checks enabled by the C<use warnings> pragma. | |
1196 | See the documentation of C<warnings> for more details. | |
1197 | ||
1198 | =item $EXECUTABLE_NAME | |
1199 | ||
1200 | =item $^X | |
1201 | ||
1202 | The name used to execute the current copy of Perl, from C's | |
1203 | C<argv[0]> or (where supported) F</proc/self/exe>. | |
1204 | ||
1205 | Depending on the host operating system, the value of $^X may be | |
1206 | a relative or absolute pathname of the perl program file, or may | |
1207 | be the string used to invoke perl but not the pathname of the | |
1208 | perl program file. Also, most operating systems permit invoking | |
1209 | programs that are not in the PATH environment variable, so there | |
1210 | is no guarantee that the value of $^X is in PATH. For VMS, the | |
1211 | value may or may not include a version number. | |
1212 | ||
1213 | You usually can use the value of $^X to re-invoke an independent | |
1214 | copy of the same perl that is currently running, e.g., | |
1215 | ||
1216 | @first_run = `$^X -le "print int rand 100 for 1..100"`; | |
1217 | ||
1218 | But recall that not all operating systems support forking or | |
1219 | capturing of the output of commands, so this complex statement | |
1220 | may not be portable. | |
1221 | ||
1222 | It is not safe to use the value of $^X as a path name of a file, | |
1223 | as some operating systems that have a mandatory suffix on | |
1224 | executable files do not require use of the suffix when invoking | |
1225 | a command. To convert the value of $^X to a path name, use the | |
1226 | following statements: | |
1227 | ||
1228 | # Build up a set of file names (not command names). | |
1229 | use Config; | |
1230 | $this_perl = $^X; | |
1231 | if ($^O ne 'VMS') | |
1232 | {$this_perl .= $Config{_exe} | |
1233 | unless $this_perl =~ m/$Config{_exe}$/i;} | |
1234 | ||
1235 | Because many operating systems permit anyone with read access to | |
1236 | the Perl program file to make a copy of it, patch the copy, and | |
1237 | then execute the copy, the security-conscious Perl programmer | |
1238 | should take care to invoke the installed copy of perl, not the | |
1239 | copy referenced by $^X. The following statements accomplish | |
1240 | this goal, and produce a pathname that can be invoked as a | |
1241 | command or referenced as a file. | |
1242 | ||
1243 | use Config; | |
1244 | $secure_perl_path = $Config{perlpath}; | |
1245 | if ($^O ne 'VMS') | |
1246 | {$secure_perl_path .= $Config{_exe} | |
1247 | unless $secure_perl_path =~ m/$Config{_exe}$/i;} | |
1248 | ||
1249 | =item ARGV | |
1250 | ||
1251 | The special filehandle that iterates over command-line filenames in | |
1252 | C<@ARGV>. Usually written as the null filehandle in the angle operator | |
1253 | C<< <> >>. Note that currently C<ARGV> only has its magical effect | |
1254 | within the C<< <> >> operator; elsewhere it is just a plain filehandle | |
1255 | corresponding to the last file opened by C<< <> >>. In particular, | |
1256 | passing C<\*ARGV> as a parameter to a function that expects a filehandle | |
1257 | may not cause your function to automatically read the contents of all the | |
1258 | files in C<@ARGV>. | |
1259 | ||
1260 | =item $ARGV | |
1261 | ||
1262 | contains the name of the current file when reading from <>. | |
1263 | ||
1264 | =item @ARGV | |
1265 | ||
1266 | The array @ARGV contains the command-line arguments intended for | |
1267 | the script. C<$#ARGV> is generally the number of arguments minus | |
1268 | one, because C<$ARGV[0]> is the first argument, I<not> the program's | |
1269 | command name itself. See C<$0> for the command name. | |
1270 | ||
1271 | =item ARGVOUT | |
1272 | ||
1273 | The special filehandle that points to the currently open output file | |
1274 | when doing edit-in-place processing with B<-i>. Useful when you have | |
1275 | to do a lot of inserting and don't want to keep modifying $_. See | |
1276 | L<perlrun> for the B<-i> switch. | |
1277 | ||
1278 | =item @F | |
1279 | ||
1280 | The array @F contains the fields of each line read in when autosplit | |
1281 | mode is turned on. See L<perlrun> for the B<-a> switch. This array | |
1282 | is package-specific, and must be declared or given a full package name | |
1283 | if not in package main when running under C<strict 'vars'>. | |
1284 | ||
1285 | =item @INC | |
1286 | ||
1287 | The array @INC contains the list of places that the C<do EXPR>, | |
1288 | C<require>, or C<use> constructs look for their library files. It | |
1289 | initially consists of the arguments to any B<-I> command-line | |
1290 | switches, followed by the default Perl library, probably | |
1291 | F</usr/local/lib/perl>, followed by ".", to represent the current | |
1292 | directory. ("." will not be appended if taint checks are enabled, either by | |
1293 | C<-T> or by C<-t>.) If you need to modify this at runtime, you should use | |
1294 | the C<use lib> pragma to get the machine-dependent library properly | |
1295 | loaded also: | |
1296 | ||
1297 | use lib '/mypath/libdir/'; | |
1298 | use SomeMod; | |
1299 | ||
1300 | You can also insert hooks into the file inclusion system by putting Perl | |
1301 | code directly into @INC. Those hooks may be subroutine references, array | |
1302 | references or blessed objects. See L<perlfunc/require> for details. | |
1303 | ||
1304 | =item @_ | |
1305 | ||
1306 | Within a subroutine the array @_ contains the parameters passed to that | |
1307 | subroutine. See L<perlsub>. | |
1308 | ||
1309 | =item %INC | |
1310 | ||
1311 | The hash %INC contains entries for each filename included via the | |
1312 | C<do>, C<require>, or C<use> operators. The key is the filename | |
1313 | you specified (with module names converted to pathnames), and the | |
1314 | value is the location of the file found. The C<require> | |
1315 | operator uses this hash to determine whether a particular file has | |
1316 | already been included. | |
1317 | ||
1318 | If the file was loaded via a hook (e.g. a subroutine reference, see | |
1319 | L<perlfunc/require> for a description of these hooks), this hook is | |
1320 | by default inserted into %INC in place of a filename. Note, however, | |
1321 | that the hook may have set the %INC entry by itself to provide some more | |
1322 | specific info. | |
1323 | ||
1324 | =item %ENV | |
1325 | ||
1326 | =item $ENV{expr} | |
1327 | ||
1328 | The hash %ENV contains your current environment. Setting a | |
1329 | value in C<ENV> changes the environment for any child processes | |
1330 | you subsequently fork() off. | |
1331 | ||
1332 | =item %SIG | |
1333 | ||
1334 | =item $SIG{expr} | |
1335 | ||
1336 | The hash %SIG contains signal handlers for signals. For example: | |
1337 | ||
1338 | sub handler { # 1st argument is signal name | |
1339 | my($sig) = @_; | |
1340 | print "Caught a SIG$sig--shutting down\n"; | |
1341 | close(LOG); | |
1342 | exit(0); | |
1343 | } | |
1344 | ||
1345 | $SIG{'INT'} = \&handler; | |
1346 | $SIG{'QUIT'} = \&handler; | |
1347 | ... | |
1348 | $SIG{'INT'} = 'DEFAULT'; # restore default action | |
1349 | $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT | |
1350 | ||
1351 | Using a value of C<'IGNORE'> usually has the effect of ignoring the | |
1352 | signal, except for the C<CHLD> signal. See L<perlipc> for more about | |
1353 | this special case. | |
1354 | ||
1355 | Here are some other examples: | |
1356 | ||
1357 | $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended) | |
1358 | $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber | |
1359 | $SIG{"PIPE"} = *Plumber; # somewhat esoteric | |
1360 | $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return?? | |
1361 | ||
1362 | Be sure not to use a bareword as the name of a signal handler, | |
1363 | lest you inadvertently call it. | |
1364 | ||
1365 | If your system has the sigaction() function then signal handlers are | |
1366 | installed using it. This means you get reliable signal handling. | |
1367 | ||
1368 | The default delivery policy of signals changed in Perl 5.8.0 from | |
1369 | immediate (also known as "unsafe") to deferred, also known as | |
1370 | "safe signals". See L<perlipc> for more information. | |
1371 | ||
1372 | Certain internal hooks can be also set using the %SIG hash. The | |
1373 | routine indicated by C<$SIG{__WARN__}> is called when a warning message is | |
1374 | about to be printed. The warning message is passed as the first | |
1375 | argument. The presence of a __WARN__ hook causes the ordinary printing | |
1376 | of warnings to STDERR to be suppressed. You can use this to save warnings | |
1377 | in a variable, or turn warnings into fatal errors, like this: | |
1378 | ||
1379 | local $SIG{__WARN__} = sub { die $_[0] }; | |
1380 | eval $proggie; | |
1381 | ||
1382 | The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception | |
1383 | is about to be thrown. The error message is passed as the first | |
1384 | argument. When a __DIE__ hook routine returns, the exception | |
1385 | processing continues as it would have in the absence of the hook, | |
1386 | unless the hook routine itself exits via a C<goto>, a loop exit, or a die(). | |
1387 | The C<__DIE__> handler is explicitly disabled during the call, so that you | |
1388 | can die from a C<__DIE__> handler. Similarly for C<__WARN__>. | |
1389 | ||
1390 | Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called | |
1391 | even inside an eval(). Do not use this to rewrite a pending exception | |
1392 | in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die(). | |
1393 | This strange action at a distance may be fixed in a future release | |
1394 | so that C<$SIG{__DIE__}> is only called if your program is about | |
1395 | to exit, as was the original intent. Any other use is deprecated. | |
1396 | ||
1397 | C<__DIE__>/C<__WARN__> handlers are very special in one respect: | |
1398 | they may be called to report (probable) errors found by the parser. | |
1399 | In such a case the parser may be in inconsistent state, so any | |
1400 | attempt to evaluate Perl code from such a handler will probably | |
1401 | result in a segfault. This means that warnings or errors that | |
1402 | result from parsing Perl should be used with extreme caution, like | |
1403 | this: | |
1404 | ||
1405 | require Carp if defined $^S; | |
1406 | Carp::confess("Something wrong") if defined &Carp::confess; | |
1407 | die "Something wrong, but could not load Carp to give backtrace... | |
1408 | To see backtrace try starting Perl with -MCarp switch"; | |
1409 | ||
1410 | Here the first line will load Carp I<unless> it is the parser who | |
1411 | called the handler. The second line will print backtrace and die if | |
1412 | Carp was available. The third line will be executed only if Carp was | |
1413 | not available. | |
1414 | ||
1415 | See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and | |
1416 | L<warnings> for additional information. | |
1417 | ||
1418 | =back | |
1419 | ||
1420 | =head2 Error Indicators | |
1421 | ||
1422 | The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information | |
1423 | about different types of error conditions that may appear during | |
1424 | execution of a Perl program. The variables are shown ordered by | |
1425 | the "distance" between the subsystem which reported the error and | |
1426 | the Perl process. They correspond to errors detected by the Perl | |
1427 | interpreter, C library, operating system, or an external program, | |
1428 | respectively. | |
1429 | ||
1430 | To illustrate the differences between these variables, consider the | |
1431 | following Perl expression, which uses a single-quoted string: | |
1432 | ||
1433 | eval q{ | |
1434 | open my $pipe, "/cdrom/install |" or die $!; | |
1435 | my @res = <$pipe>; | |
1436 | close $pipe or die "bad pipe: $?, $!"; | |
1437 | }; | |
1438 | ||
1439 | After execution of this statement all 4 variables may have been set. | |
1440 | ||
1441 | C<$@> is set if the string to be C<eval>-ed did not compile (this | |
1442 | may happen if C<open> or C<close> were imported with bad prototypes), | |
1443 | or if Perl code executed during evaluation die()d . In these cases | |
1444 | the value of $@ is the compile error, or the argument to C<die> | |
1445 | (which will interpolate C<$!> and C<$?>). (See also L<Fatal>, | |
1446 | though.) | |
1447 | ||
1448 | When the eval() expression above is executed, open(), C<< <PIPE> >>, | |
1449 | and C<close> are translated to calls in the C run-time library and | |
1450 | thence to the operating system kernel. C<$!> is set to the C library's | |
1451 | C<errno> if one of these calls fails. | |
1452 | ||
1453 | Under a few operating systems, C<$^E> may contain a more verbose | |
1454 | error indicator, such as in this case, "CDROM tray not closed." | |
1455 | Systems that do not support extended error messages leave C<$^E> | |
1456 | the same as C<$!>. | |
1457 | ||
1458 | Finally, C<$?> may be set to non-0 value if the external program | |
1459 | F</cdrom/install> fails. The upper eight bits reflect specific | |
1460 | error conditions encountered by the program (the program's exit() | |
1461 | value). The lower eight bits reflect mode of failure, like signal | |
1462 | death and core dump information See wait(2) for details. In | |
1463 | contrast to C<$!> and C<$^E>, which are set only if error condition | |
1464 | is detected, the variable C<$?> is set on each C<wait> or pipe | |
1465 | C<close>, overwriting the old value. This is more like C<$@>, which | |
1466 | on every eval() is always set on failure and cleared on success. | |
1467 | ||
1468 | For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>, | |
1469 | and C<$?>. | |
1470 | ||
1471 | =head2 Technical Note on the Syntax of Variable Names | |
1472 | ||
1473 | Variable names in Perl can have several formats. Usually, they | |
1474 | must begin with a letter or underscore, in which case they can be | |
1475 | arbitrarily long (up to an internal limit of 251 characters) and | |
1476 | may contain letters, digits, underscores, or the special sequence | |
1477 | C<::> or C<'>. In this case, the part before the last C<::> or | |
1478 | C<'> is taken to be a I<package qualifier>; see L<perlmod>. | |
1479 | ||
1480 | Perl variable names may also be a sequence of digits or a single | |
1481 | punctuation or control character. These names are all reserved for | |
1482 | special uses by Perl; for example, the all-digits names are used | |
1483 | to hold data captured by backreferences after a regular expression | |
1484 | match. Perl has a special syntax for the single-control-character | |
1485 | names: It understands C<^X> (caret C<X>) to mean the control-C<X> | |
1486 | character. For example, the notation C<$^W> (dollar-sign caret | |
1487 | C<W>) is the scalar variable whose name is the single character | |
1488 | control-C<W>. This is better than typing a literal control-C<W> | |
1489 | into your program. | |
1490 | ||
1491 | Finally, new in Perl 5.6, Perl variable names may be alphanumeric | |
1492 | strings that begin with control characters (or better yet, a caret). | |
1493 | These variables must be written in the form C<${^Foo}>; the braces | |
1494 | are not optional. C<${^Foo}> denotes the scalar variable whose | |
1495 | name is a control-C<F> followed by two C<o>'s. These variables are | |
1496 | reserved for future special uses by Perl, except for the ones that | |
1497 | begin with C<^_> (control-underscore or caret-underscore). No | |
1498 | control-character name that begins with C<^_> will acquire a special | |
1499 | meaning in any future version of Perl; such names may therefore be | |
1500 | used safely in programs. C<$^_> itself, however, I<is> reserved. | |
1501 | ||
1502 | Perl identifiers that begin with digits, control characters, or | |
1503 | punctuation characters are exempt from the effects of the C<package> | |
1504 | declaration and are always forced to be in package C<main>; they are | |
1505 | also exempt from C<strict 'vars'> errors. A few other names are also | |
1506 | exempt in these ways: | |
1507 | ||
1508 | ENV STDIN | |
1509 | INC STDOUT | |
1510 | ARGV STDERR | |
1511 | ARGVOUT _ | |
1512 | SIG | |
1513 | ||
1514 | In particular, the new special C<${^_XYZ}> variables are always taken | |
1515 | to be in package C<main>, regardless of any C<package> declarations | |
1516 | presently in scope. | |
1517 | ||
1518 | =head1 BUGS | |
1519 | ||
1520 | Due to an unfortunate accident of Perl's implementation, C<use | |
1521 | English> imposes a considerable performance penalty on all regular | |
1522 | expression matches in a program, regardless of whether they occur | |
1523 | in the scope of C<use English>. For that reason, saying C<use | |
1524 | English> in libraries is strongly discouraged. See the | |
1525 | Devel::SawAmpersand module documentation from CPAN | |
1526 | ( http://www.cpan.org/modules/by-module/Devel/ ) | |
1527 | for more information. | |
1528 | ||
1529 | Having to even think about the C<$^S> variable in your exception | |
1530 | handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented | |
1531 | invites grievous and difficult to track down errors. Avoid it | |
1532 | and use an C<END{}> or CORE::GLOBAL::die override instead. |