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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perlvar - Perl predefined variables | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | =head2 Predefined Names | |
8 | ||
5a964f20 | 9 | The following names have special meaning to Perl. Most |
5f05dabc | 10 | punctuation names have reasonable mnemonics, or analogues in one of |
5a964f20 | 11 | the shells. Nevertheless, if you wish to use long variable names, |
a0d0e21e LW |
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 | |
5a964f20 | 17 | long names in the current package. Some even have medium names, |
a0d0e21e LW |
18 | generally borrowed from B<awk>. |
19 | ||
19ddd453 JT |
20 | Due to an unfortunate accident of Perl's implementation, "C<use English>" |
21 | imposes a considerable performance penalty on all regular expression | |
22 | matches in a program, regardless of whether they occur in the scope of | |
23 | "C<use English>". For that reason, saying "C<use English>" in | |
24 | libraries is strongly discouraged. See the Devel::SawAmpersand module | |
25 | documentation from CPAN | |
26 | (http://www.perl.com/CPAN/modules/by-module/Devel/Devel-SawAmpersand-0.10.readme) | |
27 | for more information. | |
28 | ||
a0d0e21e | 29 | To go a step further, those variables that depend on the currently |
fb73857a | 30 | selected filehandle may instead (and preferably) be set by calling an |
31 | object method on the FileHandle object. (Summary lines below for this | |
32 | contain the word HANDLE.) First you must say | |
a0d0e21e LW |
33 | |
34 | use FileHandle; | |
35 | ||
36 | after which you may use either | |
37 | ||
38 | method HANDLE EXPR | |
39 | ||
5a964f20 | 40 | or more safely, |
a0d0e21e LW |
41 | |
42 | HANDLE->method(EXPR) | |
43 | ||
44 | Each of the methods returns the old value of the FileHandle attribute. | |
45 | The methods each take an optional EXPR, which if supplied specifies the | |
46 | new value for the FileHandle attribute in question. If not supplied, | |
47 | most of the methods do nothing to the current value, except for | |
48 | autoflush(), which will assume a 1 for you, just to be different. | |
49 | ||
748a9306 LW |
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. | |
a0d0e21e | 53 | |
fb73857a | 54 | The following list is ordered by scalar variables first, then the |
55 | arrays, then the hashes (except $^M was added in the wrong place). | |
56 | This is somewhat obscured by the fact that %ENV and %SIG are listed as | |
57 | $ENV{expr} and $SIG{expr}. | |
58 | ||
59 | ||
a0d0e21e LW |
60 | =over 8 |
61 | ||
62 | =item $ARG | |
63 | ||
64 | =item $_ | |
65 | ||
66 | The default input and pattern-searching space. The following pairs are | |
67 | equivalent: | |
68 | ||
5f05dabc | 69 | while (<>) {...} # equivalent in only while! |
54310121 | 70 | while (defined($_ = <>)) {...} |
a0d0e21e LW |
71 | |
72 | /^Subject:/ | |
73 | $_ =~ /^Subject:/ | |
74 | ||
75 | tr/a-z/A-Z/ | |
76 | $_ =~ tr/a-z/A-Z/ | |
77 | ||
78 | chop | |
79 | chop($_) | |
80 | ||
54310121 | 81 | Here are the places where Perl will assume $_ even if you |
cb1a09d0 AD |
82 | don't use it: |
83 | ||
84 | =over 3 | |
85 | ||
86 | =item * | |
87 | ||
88 | Various unary functions, including functions like ord() and int(), as well | |
89 | as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to | |
90 | STDIN. | |
91 | ||
92 | =item * | |
93 | ||
94 | Various list functions like print() and unlink(). | |
95 | ||
96 | =item * | |
97 | ||
98 | The pattern matching operations C<m//>, C<s///>, and C<tr///> when used | |
99 | without an C<=~> operator. | |
100 | ||
54310121 | 101 | =item * |
cb1a09d0 AD |
102 | |
103 | The default iterator variable in a C<foreach> loop if no other | |
104 | variable is supplied. | |
105 | ||
54310121 | 106 | =item * |
cb1a09d0 AD |
107 | |
108 | The implicit iterator variable in the grep() and map() functions. | |
109 | ||
54310121 | 110 | =item * |
cb1a09d0 AD |
111 | |
112 | The default place to put an input record when a C<E<lt>FHE<gt>> | |
113 | operation's result is tested by itself as the sole criterion of a C<while> | |
114 | test. Note that outside of a C<while> test, this will not happen. | |
115 | ||
116 | =back | |
117 | ||
a0d0e21e LW |
118 | (Mnemonic: underline is understood in certain operations.) |
119 | ||
6e2995f4 | 120 | =back |
121 | ||
122 | =over 8 | |
123 | ||
5a964f20 | 124 | =item $E<lt>I<digits>E<gt> |
a0d0e21e | 125 | |
54310121 | 126 | Contains the subpattern from the corresponding set of parentheses in |
a0d0e21e | 127 | the last pattern matched, not counting patterns matched in nested |
5a964f20 | 128 | blocks that have been exited already. (Mnemonic: like \digits.) |
a0d0e21e LW |
129 | These variables are all read-only. |
130 | ||
131 | =item $MATCH | |
132 | ||
133 | =item $& | |
134 | ||
135 | The string matched by the last successful pattern match (not counting | |
136 | any matches hidden within a BLOCK or eval() enclosed by the current | |
137 | BLOCK). (Mnemonic: like & in some editors.) This variable is read-only. | |
138 | ||
19ddd453 JT |
139 | The use of this variable anywhere in a program imposes a considerable |
140 | performance penalty on all regular expression matches. See the | |
141 | Devel::SawAmpersand module from CPAN for more information. | |
142 | ||
a0d0e21e LW |
143 | =item $PREMATCH |
144 | ||
145 | =item $` | |
146 | ||
147 | The string preceding whatever was matched by the last successful | |
148 | pattern match (not counting any matches hidden within a BLOCK or eval | |
a8f8344d | 149 | enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted |
a0d0e21e LW |
150 | string.) This variable is read-only. |
151 | ||
19ddd453 JT |
152 | The use of this variable anywhere in a program imposes a considerable |
153 | performance penalty on all regular expression matches. See the | |
154 | Devel::SawAmpersand module from CPAN for more information. | |
155 | ||
a0d0e21e LW |
156 | =item $POSTMATCH |
157 | ||
158 | =item $' | |
159 | ||
160 | The string following whatever was matched by the last successful | |
161 | pattern match (not counting any matches hidden within a BLOCK or eval() | |
a8f8344d | 162 | enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted |
a0d0e21e LW |
163 | string.) Example: |
164 | ||
165 | $_ = 'abcdefghi'; | |
166 | /def/; | |
167 | print "$`:$&:$'\n"; # prints abc:def:ghi | |
168 | ||
169 | This variable is read-only. | |
170 | ||
19ddd453 JT |
171 | The use of this variable anywhere in a program imposes a considerable |
172 | performance penalty on all regular expression matches. See the | |
173 | Devel::SawAmpersand module from CPAN for more information. | |
174 | ||
a0d0e21e LW |
175 | =item $LAST_PAREN_MATCH |
176 | ||
177 | =item $+ | |
178 | ||
179 | The last bracket matched by the last search pattern. This is useful if | |
180 | you don't know which of a set of alternative patterns matched. For | |
181 | example: | |
182 | ||
183 | /Version: (.*)|Revision: (.*)/ && ($rev = $+); | |
184 | ||
185 | (Mnemonic: be positive and forward looking.) | |
186 | This variable is read-only. | |
187 | ||
6cef1e77 IZ |
188 | =item @+ |
189 | ||
190 | $+[0] is the offset of the end of the last successfull match. | |
191 | C<$+[>I<n>C<]> is the offset of the end of the substring matched by | |
8f580fb8 | 192 | I<n>-th subpattern, or undef if the subpattern did not match. |
6cef1e77 IZ |
193 | |
194 | Thus after a match against $_, $& coincides with C<substr $_, $-[0], | |
8f580fb8 IZ |
195 | $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<], |
196 | $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with | |
197 | C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#+> to find the number | |
198 | of subgroups in the last successful match. Note the difference with | |
199 | C<$#->, which is the last I<matched> subgroup. Compare with L<"@-">. | |
6cef1e77 | 200 | |
a0d0e21e LW |
201 | =item $MULTILINE_MATCHING |
202 | ||
203 | =item $* | |
204 | ||
4a6725af | 205 | Set to 1 to do multi-line matching within a string, 0 to tell Perl |
a0d0e21e LW |
206 | that it can assume that strings contain a single line, for the purpose |
207 | of optimizing pattern matches. Pattern matches on strings containing | |
208 | multiple newlines can produce confusing results when "C<$*>" is 0. Default | |
209 | is 0. (Mnemonic: * matches multiple things.) Note that this variable | |
5f05dabc | 210 | influences the interpretation of only "C<^>" and "C<$>". A literal newline can |
a0d0e21e LW |
211 | be searched for even when C<$* == 0>. |
212 | ||
5a964f20 TC |
213 | Use of "C<$*>" is deprecated in modern Perls, supplanted by |
214 | the C</s> and C</m> modifiers on pattern matching. | |
a0d0e21e LW |
215 | |
216 | =item input_line_number HANDLE EXPR | |
217 | ||
218 | =item $INPUT_LINE_NUMBER | |
219 | ||
220 | =item $NR | |
221 | ||
222 | =item $. | |
223 | ||
6e2995f4 | 224 | The current input line number for the last file handle from |
883faa13 GS |
225 | which you read (or performed a C<seek> or C<tell> on). The value |
226 | may be different from the actual physical line number in the file, | |
227 | depending on what notion of "line" is in effect--see L<$/> on how | |
228 | to affect that. An | |
5f05dabc | 229 | explicit close on a filehandle resets the line number. Because |
4633a7c4 LW |
230 | "C<E<lt>E<gt>>" never does an explicit close, line numbers increase |
231 | across ARGV files (but see examples under eof()). Localizing C<$.> has | |
232 | the effect of also localizing Perl's notion of "the last read | |
233 | filehandle". (Mnemonic: many programs use "." to mean the current line | |
234 | number.) | |
a0d0e21e LW |
235 | |
236 | =item input_record_separator HANDLE EXPR | |
237 | ||
238 | =item $INPUT_RECORD_SEPARATOR | |
239 | ||
240 | =item $RS | |
241 | ||
242 | =item $/ | |
243 | ||
883faa13 GS |
244 | The input record separator, newline by default. This is used to |
245 | influence Perl's idea of what a "line" is. Works like B<awk>'s RS | |
303f2f76 | 246 | variable, including treating empty lines as delimiters if set to the |
54310121 | 247 | null string. (Note: An empty line cannot contain any spaces or tabs.) |
4a6725af | 248 | You may set it to a multi-character string to match a multi-character |
54310121 | 249 | delimiter, or to C<undef> to read to end of file. Note that setting it |
250 | to C<"\n\n"> means something slightly different than setting it to | |
251 | C<"">, if the file contains consecutive empty lines. Setting it to | |
252 | C<""> will treat two or more consecutive empty lines as a single empty | |
253 | line. Setting it to C<"\n\n"> will blindly assume that the next input | |
254 | character belongs to the next paragraph, even if it's a newline. | |
255 | (Mnemonic: / is used to delimit line boundaries when quoting poetry.) | |
a0d0e21e | 256 | |
fbad3eb5 GS |
257 | undef $/; # enable "slurp" mode |
258 | $_ = <FH>; # whole file now here | |
a0d0e21e LW |
259 | s/\n[ \t]+/ /g; |
260 | ||
68dc0745 | 261 | Remember: the value of $/ is a string, not a regexp. AWK has to be |
262 | better for something :-) | |
263 | ||
5b2b9c68 HM |
264 | Setting $/ to a reference to an integer, scalar containing an integer, or |
265 | scalar that's convertable to an integer will attempt to read records | |
266 | instead of lines, with the maximum record size being the referenced | |
267 | integer. So this: | |
268 | ||
269 | $/ = \32768; # or \"32768", or \$var_containing_32768 | |
270 | open(FILE, $myfile); | |
271 | $_ = <FILE>; | |
272 | ||
273 | will read a record of no more than 32768 bytes from FILE. If you're not | |
274 | reading from a record-oriented file (or your OS doesn't have | |
275 | record-oriented files), then you'll likely get a full chunk of data with | |
276 | every read. If a record is larger than the record size you've set, you'll | |
277 | get the record back in pieces. | |
278 | ||
279 | On VMS, record reads are done with the equivalent of C<sysread>, so it's | |
280 | best not to mix record and non-record reads on the same file. (This is | |
281 | likely not a problem, as any file you'd want to read in record mode is | |
c2611fb3 | 282 | probably usable in line mode) Non-VMS systems perform normal I/O, so |
5b2b9c68 HM |
283 | it's safe to mix record and non-record reads of a file. |
284 | ||
883faa13 GS |
285 | Also see L<$.>. |
286 | ||
a0d0e21e LW |
287 | =item autoflush HANDLE EXPR |
288 | ||
289 | =item $OUTPUT_AUTOFLUSH | |
290 | ||
291 | =item $| | |
292 | ||
54310121 | 293 | If set to nonzero, forces a flush right away and after every write or print on the |
6e2995f4 | 294 | currently selected output channel. Default is 0 (regardless of whether |
5f05dabc | 295 | the channel is actually buffered by the system or not; C<$|> tells you |
54310121 | 296 | only whether you've asked Perl explicitly to flush after each write). |
6e2995f4 | 297 | Note that STDOUT will typically be line buffered if output is to the |
298 | terminal and block buffered otherwise. Setting this variable is useful | |
299 | primarily when you are outputting to a pipe, such as when you are running | |
300 | a Perl script under rsh and want to see the output as it's happening. This | |
301 | has no effect on input buffering. | |
cb1a09d0 | 302 | (Mnemonic: when you want your pipes to be piping hot.) |
a0d0e21e LW |
303 | |
304 | =item output_field_separator HANDLE EXPR | |
305 | ||
306 | =item $OUTPUT_FIELD_SEPARATOR | |
307 | ||
308 | =item $OFS | |
309 | ||
310 | =item $, | |
311 | ||
312 | The output field separator for the print operator. Ordinarily the | |
5f05dabc | 313 | print operator simply prints out the comma-separated fields you |
314 | specify. To get behavior more like B<awk>, set this variable | |
a0d0e21e LW |
315 | as you would set B<awk>'s OFS variable to specify what is printed |
316 | between fields. (Mnemonic: what is printed when there is a , in your | |
317 | print statement.) | |
318 | ||
319 | =item output_record_separator HANDLE EXPR | |
320 | ||
321 | =item $OUTPUT_RECORD_SEPARATOR | |
322 | ||
323 | =item $ORS | |
324 | ||
325 | =item $\ | |
326 | ||
327 | The output record separator for the print operator. Ordinarily the | |
5f05dabc | 328 | print operator simply prints out the comma-separated fields you |
329 | specify, with no trailing newline or record separator assumed. | |
330 | To get behavior more like B<awk>, set this variable as you would | |
a0d0e21e LW |
331 | set B<awk>'s ORS variable to specify what is printed at the end of the |
332 | print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the | |
a8f8344d | 333 | print. Also, it's just like C<$/>, but it's what you get "back" from |
a0d0e21e LW |
334 | Perl.) |
335 | ||
336 | =item $LIST_SEPARATOR | |
337 | ||
338 | =item $" | |
339 | ||
340 | This is like "C<$,>" except that it applies to array values interpolated | |
341 | into a double-quoted string (or similar interpreted string). Default | |
342 | is a space. (Mnemonic: obvious, I think.) | |
343 | ||
344 | =item $SUBSCRIPT_SEPARATOR | |
345 | ||
346 | =item $SUBSEP | |
347 | ||
348 | =item $; | |
349 | ||
54310121 | 350 | The subscript separator for multidimensional array emulation. If you |
a0d0e21e LW |
351 | refer to a hash element as |
352 | ||
353 | $foo{$a,$b,$c} | |
354 | ||
355 | it really means | |
356 | ||
357 | $foo{join($;, $a, $b, $c)} | |
358 | ||
359 | But don't put | |
360 | ||
361 | @foo{$a,$b,$c} # a slice--note the @ | |
362 | ||
363 | which means | |
364 | ||
365 | ($foo{$a},$foo{$b},$foo{$c}) | |
366 | ||
367 | Default is "\034", the same as SUBSEP in B<awk>. Note that if your | |
368 | keys contain binary data there might not be any safe value for "C<$;>". | |
369 | (Mnemonic: comma (the syntactic subscript separator) is a | |
370 | semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already | |
371 | taken for something more important.) | |
372 | ||
54310121 | 373 | Consider using "real" multidimensional arrays. |
a0d0e21e LW |
374 | |
375 | =item $OFMT | |
376 | ||
377 | =item $# | |
378 | ||
379 | The output format for printed numbers. This variable is a half-hearted | |
380 | attempt to emulate B<awk>'s OFMT variable. There are times, however, | |
381 | when B<awk> and Perl have differing notions of what is in fact | |
6e2995f4 | 382 | numeric. The initial value is %.I<n>g, where I<n> is the value |
383 | of the macro DBL_DIG from your system's F<float.h>. This is different from | |
384 | B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>" | |
385 | explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.) | |
a0d0e21e | 386 | |
5f05dabc | 387 | Use of "C<$#>" is deprecated. |
a0d0e21e LW |
388 | |
389 | =item format_page_number HANDLE EXPR | |
390 | ||
391 | =item $FORMAT_PAGE_NUMBER | |
392 | ||
393 | =item $% | |
394 | ||
395 | The current page number of the currently selected output channel. | |
396 | (Mnemonic: % is page number in B<nroff>.) | |
397 | ||
398 | =item format_lines_per_page HANDLE EXPR | |
399 | ||
400 | =item $FORMAT_LINES_PER_PAGE | |
401 | ||
402 | =item $= | |
403 | ||
404 | The current page length (printable lines) of the currently selected | |
405 | output channel. Default is 60. (Mnemonic: = has horizontal lines.) | |
406 | ||
407 | =item format_lines_left HANDLE EXPR | |
408 | ||
409 | =item $FORMAT_LINES_LEFT | |
410 | ||
411 | =item $- | |
412 | ||
413 | The number of lines left on the page of the currently selected output | |
414 | channel. (Mnemonic: lines_on_page - lines_printed.) | |
415 | ||
6cef1e77 IZ |
416 | =item @- |
417 | ||
418 | $-[0] is the offset of the start of the last successfull match. | |
419 | C<$-[>I<n>C<]> is the offset of the start of the substring matched by | |
8f580fb8 | 420 | I<n>-th subpattern, or undef if the subpattern did not match. |
6cef1e77 IZ |
421 | |
422 | Thus after a match against $_, $& coincides with C<substr $_, $-[0], | |
8f580fb8 IZ |
423 | $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<], |
424 | $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with | |
425 | C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last | |
426 | matched subgroup in the last successful match. Note the difference with | |
427 | C<$#+>, which is the number of subgroups in the regular expression. Compare | |
428 | with L<"@+">. | |
6cef1e77 | 429 | |
a0d0e21e LW |
430 | =item format_name HANDLE EXPR |
431 | ||
432 | =item $FORMAT_NAME | |
433 | ||
434 | =item $~ | |
435 | ||
436 | The name of the current report format for the currently selected output | |
437 | channel. Default is name of the filehandle. (Mnemonic: brother to | |
438 | "C<$^>".) | |
439 | ||
440 | =item format_top_name HANDLE EXPR | |
441 | ||
442 | =item $FORMAT_TOP_NAME | |
443 | ||
444 | =item $^ | |
445 | ||
446 | The name of the current top-of-page format for the currently selected | |
447 | output channel. Default is name of the filehandle with _TOP | |
448 | appended. (Mnemonic: points to top of page.) | |
449 | ||
450 | =item format_line_break_characters HANDLE EXPR | |
451 | ||
452 | =item $FORMAT_LINE_BREAK_CHARACTERS | |
453 | ||
454 | =item $: | |
455 | ||
456 | The current set of characters after which a string may be broken to | |
54310121 | 457 | fill continuation fields (starting with ^) in a format. Default is |
a0d0e21e LW |
458 | S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in |
459 | poetry is a part of a line.) | |
460 | ||
461 | =item format_formfeed HANDLE EXPR | |
462 | ||
463 | =item $FORMAT_FORMFEED | |
464 | ||
465 | =item $^L | |
466 | ||
5f05dabc | 467 | What formats output to perform a form feed. Default is \f. |
a0d0e21e LW |
468 | |
469 | =item $ACCUMULATOR | |
470 | ||
471 | =item $^A | |
472 | ||
473 | The current value of the write() accumulator for format() lines. A format | |
474 | contains formline() commands that put their result into C<$^A>. After | |
475 | calling its format, write() prints out the contents of C<$^A> and empties. | |
476 | So you never actually see the contents of C<$^A> unless you call | |
477 | formline() yourself and then look at it. See L<perlform> and | |
478 | L<perlfunc/formline()>. | |
479 | ||
480 | =item $CHILD_ERROR | |
481 | ||
482 | =item $? | |
483 | ||
54310121 | 484 | The status returned by the last pipe close, backtick (C<``>) command, |
5a964f20 TC |
485 | or system() operator. Note that this is the status word returned by the |
486 | wait() system call (or else is made up to look like it). Thus, the exit | |
487 | value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and C<$? & 127> | |
488 | gives which signal, if any, the process died from, and C<$? & 128> reports | |
489 | whether there was a core dump. (Mnemonic: similar to B<sh> and B<ksh>.) | |
a0d0e21e | 490 | |
7b8d334a GS |
491 | Additionally, if the C<h_errno> variable is supported in C, its value |
492 | is returned via $? if any of the C<gethost*()> functions fail. | |
493 | ||
aa689395 | 494 | Note that if you have installed a signal handler for C<SIGCHLD>, the |
495 | value of C<$?> will usually be wrong outside that handler. | |
496 | ||
a8f8344d | 497 | Inside an C<END> subroutine C<$?> contains the value that is going to be |
498 | given to C<exit()>. You can modify C<$?> in an C<END> subroutine to | |
499 | change the exit status of the script. | |
500 | ||
aa689395 | 501 | Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the |
ff0cee69 | 502 | actual VMS exit status, instead of the default emulation of POSIX |
503 | status. | |
f86702cc | 504 | |
55602bd2 IZ |
505 | Also see L<Error Indicators>. |
506 | ||
a0d0e21e LW |
507 | =item $OS_ERROR |
508 | ||
509 | =item $ERRNO | |
510 | ||
511 | =item $! | |
512 | ||
513 | If used in a numeric context, yields the current value of errno, with | |
514 | all the usual caveats. (This means that you shouldn't depend on the | |
22fae026 | 515 | value of C<$!> to be anything in particular unless you've gotten a |
a0d0e21e LW |
516 | specific error return indicating a system error.) If used in a string |
517 | context, yields the corresponding system error string. You can assign | |
22fae026 | 518 | to C<$!> to set I<errno> if, for instance, you want C<"$!"> to return the |
a0d0e21e LW |
519 | string for error I<n>, or you want to set the exit value for the die() |
520 | operator. (Mnemonic: What just went bang?) | |
521 | ||
55602bd2 IZ |
522 | Also see L<Error Indicators>. |
523 | ||
5c055ba3 | 524 | =item $EXTENDED_OS_ERROR |
525 | ||
526 | =item $^E | |
527 | ||
22fae026 TM |
528 | Error information specific to the current operating system. At |
529 | the moment, this differs from C<$!> under only VMS, OS/2, and Win32 | |
530 | (and for MacPerl). On all other platforms, C<$^E> is always just | |
531 | the same as C<$!>. | |
532 | ||
533 | Under VMS, C<$^E> provides the VMS status value from the last | |
534 | system error. This is more specific information about the last | |
535 | system error than that provided by C<$!>. This is particularly | |
d516a115 | 536 | important when C<$!> is set to B<EVMSERR>. |
22fae026 | 537 | |
1c1c7f20 GS |
538 | Under OS/2, C<$^E> is set to the error code of the last call to |
539 | OS/2 API either via CRT, or directly from perl. | |
22fae026 TM |
540 | |
541 | Under Win32, C<$^E> always returns the last error information | |
542 | reported by the Win32 call C<GetLastError()> which describes | |
543 | the last error from within the Win32 API. Most Win32-specific | |
544 | code will report errors via C<$^E>. ANSI C and UNIX-like calls | |
545 | set C<errno> and so most portable Perl code will report errors | |
546 | via C<$!>. | |
547 | ||
548 | Caveats mentioned in the description of C<$!> generally apply to | |
549 | C<$^E>, also. (Mnemonic: Extra error explanation.) | |
5c055ba3 | 550 | |
55602bd2 IZ |
551 | Also see L<Error Indicators>. |
552 | ||
a0d0e21e LW |
553 | =item $EVAL_ERROR |
554 | ||
555 | =item $@ | |
556 | ||
557 | The Perl syntax error message from the last eval() command. If null, the | |
558 | last eval() parsed and executed correctly (although the operations you | |
559 | invoked may have failed in the normal fashion). (Mnemonic: Where was | |
560 | the syntax error "at"?) | |
561 | ||
748a9306 | 562 | Note that warning messages are not collected in this variable. You can, |
a8f8344d | 563 | however, set up a routine to process warnings by setting C<$SIG{__WARN__}> |
54310121 | 564 | as described below. |
748a9306 | 565 | |
55602bd2 IZ |
566 | Also see L<Error Indicators>. |
567 | ||
a0d0e21e LW |
568 | =item $PROCESS_ID |
569 | ||
570 | =item $PID | |
571 | ||
572 | =item $$ | |
573 | ||
574 | The process number of the Perl running this script. (Mnemonic: same | |
575 | as shells.) | |
576 | ||
577 | =item $REAL_USER_ID | |
578 | ||
579 | =item $UID | |
580 | ||
581 | =item $< | |
582 | ||
583 | The real uid of this process. (Mnemonic: it's the uid you came I<FROM>, | |
584 | if you're running setuid.) | |
585 | ||
586 | =item $EFFECTIVE_USER_ID | |
587 | ||
588 | =item $EUID | |
589 | ||
590 | =item $> | |
591 | ||
592 | The effective uid of this process. Example: | |
593 | ||
594 | $< = $>; # set real to effective uid | |
595 | ($<,$>) = ($>,$<); # swap real and effective uid | |
596 | ||
8cc95fdb | 597 | (Mnemonic: it's the uid you went I<TO>, if you're running setuid.) |
598 | Note: "C<$E<lt>>" and "C<$E<gt>>" can be swapped only on machines | |
599 | supporting setreuid(). | |
a0d0e21e LW |
600 | |
601 | =item $REAL_GROUP_ID | |
602 | ||
603 | =item $GID | |
604 | ||
605 | =item $( | |
606 | ||
607 | The real gid of this process. If you are on a machine that supports | |
608 | membership in multiple groups simultaneously, gives a space separated | |
609 | list of groups you are in. The first number is the one returned by | |
610 | getgid(), and the subsequent ones by getgroups(), one of which may be | |
8cc95fdb | 611 | the same as the first number. |
612 | ||
613 | However, a value assigned to "C<$(>" must be a single number used to | |
614 | set the real gid. So the value given by "C<$(>" should I<not> be assigned | |
615 | back to "C<$(>" without being forced numeric, such as by adding zero. | |
616 | ||
617 | (Mnemonic: parentheses are used to I<GROUP> things. The real gid is the | |
618 | group you I<LEFT>, if you're running setgid.) | |
a0d0e21e LW |
619 | |
620 | =item $EFFECTIVE_GROUP_ID | |
621 | ||
622 | =item $EGID | |
623 | ||
624 | =item $) | |
625 | ||
626 | The effective gid of this process. If you are on a machine that | |
627 | supports membership in multiple groups simultaneously, gives a space | |
628 | separated list of groups you are in. The first number is the one | |
629 | returned by getegid(), and the subsequent ones by getgroups(), one of | |
8cc95fdb | 630 | which may be the same as the first number. |
631 | ||
632 | Similarly, a value assigned to "C<$)>" must also be a space-separated | |
633 | list of numbers. The first number is used to set the effective gid, and | |
634 | the rest (if any) are passed to setgroups(). To get the effect of an | |
635 | empty list for setgroups(), just repeat the new effective gid; that is, | |
636 | to force an effective gid of 5 and an effectively empty setgroups() | |
637 | list, say C< $) = "5 5" >. | |
638 | ||
639 | (Mnemonic: parentheses are used to I<GROUP> things. The effective gid | |
640 | is the group that's I<RIGHT> for you, if you're running setgid.) | |
a0d0e21e | 641 | |
5f05dabc | 642 | Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on |
643 | machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>" | |
8cc95fdb | 644 | and "C<$)>" can be swapped only on machines supporting setregid(). |
a0d0e21e LW |
645 | |
646 | =item $PROGRAM_NAME | |
647 | ||
648 | =item $0 | |
649 | ||
650 | Contains the name of the file containing the Perl script being | |
54310121 | 651 | executed. On some operating systems |
652 | assigning to "C<$0>" modifies the argument area that the ps(1) | |
a0d0e21e LW |
653 | program sees. This is more useful as a way of indicating the |
654 | current program state than it is for hiding the program you're running. | |
655 | (Mnemonic: same as B<sh> and B<ksh>.) | |
656 | ||
657 | =item $[ | |
658 | ||
659 | The index of the first element in an array, and of the first character | |
660 | in a substring. Default is 0, but you could set it to 1 to make | |
661 | Perl behave more like B<awk> (or Fortran) when subscripting and when | |
662 | evaluating the index() and substr() functions. (Mnemonic: [ begins | |
663 | subscripts.) | |
664 | ||
665 | As of Perl 5, assignment to "C<$[>" is treated as a compiler directive, | |
666 | and cannot influence the behavior of any other file. Its use is | |
667 | discouraged. | |
668 | ||
669 | =item $PERL_VERSION | |
670 | ||
671 | =item $] | |
672 | ||
54310121 | 673 | The version + patchlevel / 1000 of the Perl interpreter. This variable |
674 | can be used to determine whether the Perl interpreter executing a | |
675 | script is in the right range of versions. (Mnemonic: Is this version | |
676 | of perl in the right bracket?) Example: | |
a0d0e21e LW |
677 | |
678 | warn "No checksumming!\n" if $] < 3.019; | |
679 | ||
54310121 | 680 | See also the documentation of C<use VERSION> and C<require VERSION> |
681 | for a convenient way to fail if the Perl interpreter is too old. | |
a0d0e21e | 682 | |
305aace0 NIS |
683 | =item $COMPILING |
684 | ||
685 | =item $^C | |
686 | ||
687 | The current value of the flag associated with the B<-c> switch. Mainly | |
688 | of use with B<-MO=...> to allow code to alter its behaviour when being compiled. | |
689 | (For example to automatically AUTOLOADing at compile time rather than normal | |
690 | deferred loading.) Setting C<$^C = 1> is similar to calling C<B::minus_c>. | |
691 | ||
a0d0e21e LW |
692 | =item $DEBUGGING |
693 | ||
694 | =item $^D | |
695 | ||
696 | The current value of the debugging flags. (Mnemonic: value of B<-D> | |
697 | switch.) | |
698 | ||
699 | =item $SYSTEM_FD_MAX | |
700 | ||
701 | =item $^F | |
702 | ||
703 | The maximum system file descriptor, ordinarily 2. System file | |
704 | descriptors are passed to exec()ed processes, while higher file | |
705 | descriptors are not. Also, during an open(), system file descriptors are | |
706 | preserved even if the open() fails. (Ordinary file descriptors are | |
707 | closed before the open() is attempted.) Note that the close-on-exec | |
708 | status of a file descriptor will be decided according to the value of | |
4771b018 | 709 | C<$^F> when the open() or pipe() was called, not the time of the exec(). |
a0d0e21e | 710 | |
6e2995f4 | 711 | =item $^H |
712 | ||
fb73857a | 713 | The current set of syntax checks enabled by C<use strict> and other block |
714 | scoped compiler hints. See the documentation of C<strict> for more details. | |
6e2995f4 | 715 | |
a0d0e21e LW |
716 | =item $INPLACE_EDIT |
717 | ||
718 | =item $^I | |
719 | ||
720 | The current value of the inplace-edit extension. Use C<undef> to disable | |
721 | inplace editing. (Mnemonic: value of B<-i> switch.) | |
722 | ||
fb73857a | 723 | =item $^M |
724 | ||
725 | By default, running out of memory it is not trappable. However, if | |
726 | compiled for this, Perl may use the contents of C<$^M> as an emergency | |
727 | pool after die()ing with this message. Suppose that your Perl were | |
728 | compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. Then | |
729 | ||
730 | $^M = 'a' x (1<<16); | |
731 | ||
732 | would allocate a 64K buffer for use when in emergency. See the F<INSTALL> | |
733 | file for information on how to enable this option. As a disincentive to | |
734 | casual use of this advanced feature, there is no L<English> long name for | |
735 | this variable. | |
736 | ||
5c055ba3 | 737 | =item $OSNAME |
6e2995f4 | 738 | |
5c055ba3 | 739 | =item $^O |
740 | ||
741 | The name of the operating system under which this copy of Perl was | |
742 | built, as determined during the configuration process. The value | |
743 | is identical to C<$Config{'osname'}>. | |
744 | ||
a0d0e21e LW |
745 | =item $PERLDB |
746 | ||
747 | =item $^P | |
748 | ||
84902520 TB |
749 | The internal variable for debugging support. Different bits mean the |
750 | following (subject to change): | |
751 | ||
752 | =over 6 | |
753 | ||
754 | =item 0x01 | |
755 | ||
756 | Debug subroutine enter/exit. | |
757 | ||
758 | =item 0x02 | |
759 | ||
760 | Line-by-line debugging. | |
761 | ||
762 | =item 0x04 | |
763 | ||
764 | Switch off optimizations. | |
765 | ||
766 | =item 0x08 | |
767 | ||
768 | Preserve more data for future interactive inspections. | |
769 | ||
770 | =item 0x10 | |
771 | ||
772 | Keep info about source lines on which a subroutine is defined. | |
773 | ||
774 | =item 0x20 | |
775 | ||
776 | Start with single-step on. | |
777 | ||
778 | =back | |
779 | ||
c2611fb3 | 780 | Note that some bits may be relevant at compile-time only, some at |
84902520 | 781 | run-time only. This is a new mechanism and the details may change. |
a0d0e21e | 782 | |
b9ac3b5b GS |
783 | =item $^R |
784 | ||
785 | The result of evaluation of the last successful L<perlre/C<(?{ code })>> | |
786 | regular expression assertion. (Excluding those used as switches.) May | |
787 | be written to. | |
788 | ||
fb73857a | 789 | =item $^S |
790 | ||
791 | Current state of the interpreter. Undefined if parsing of the current | |
792 | module/eval is not finished (may happen in $SIG{__DIE__} and | |
a3cb178b | 793 | $SIG{__WARN__} handlers). True if inside an eval, otherwise false. |
fb73857a | 794 | |
a0d0e21e LW |
795 | =item $BASETIME |
796 | ||
797 | =item $^T | |
798 | ||
799 | The time at which the script began running, in seconds since the | |
5f05dabc | 800 | epoch (beginning of 1970). The values returned by the B<-M>, B<-A>, |
a0d0e21e LW |
801 | and B<-C> filetests are |
802 | based on this value. | |
803 | ||
804 | =item $WARNING | |
805 | ||
806 | =item $^W | |
807 | ||
303f2f76 | 808 | The current value of the warning switch, either TRUE or FALSE. |
809 | (Mnemonic: related to the B<-w> switch.) | |
a0d0e21e LW |
810 | |
811 | =item $EXECUTABLE_NAME | |
812 | ||
813 | =item $^X | |
814 | ||
815 | The name that the Perl binary itself was executed as, from C's C<argv[0]>. | |
816 | ||
817 | =item $ARGV | |
818 | ||
a8f8344d | 819 | contains the name of the current file when reading from E<lt>E<gt>. |
a0d0e21e LW |
820 | |
821 | =item @ARGV | |
822 | ||
823 | The array @ARGV contains the command line arguments intended for the | |
824 | script. Note that C<$#ARGV> is the generally number of arguments minus | |
5f05dabc | 825 | one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See |
a0d0e21e LW |
826 | "C<$0>" for the command name. |
827 | ||
828 | =item @INC | |
829 | ||
830 | The array @INC contains the list of places to look for Perl scripts to | |
831 | be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It | |
832 | initially consists of the arguments to any B<-I> command line switches, | |
6e2995f4 | 833 | followed by the default Perl library, probably F</usr/local/lib/perl>, |
cb1a09d0 | 834 | followed by ".", to represent the current directory. If you need to |
5f05dabc | 835 | modify this at runtime, you should use the C<use lib> pragma |
836 | to get the machine-dependent library properly loaded also: | |
a0d0e21e | 837 | |
cb1a09d0 AD |
838 | use lib '/mypath/libdir/'; |
839 | use SomeMod; | |
303f2f76 | 840 | |
fb73857a | 841 | =item @_ |
842 | ||
843 | Within a subroutine the array @_ contains the parameters passed to that | |
844 | subroutine. See L<perlsub>. | |
845 | ||
a0d0e21e LW |
846 | =item %INC |
847 | ||
848 | The hash %INC contains entries for each filename that has | |
849 | been included via C<do> or C<require>. The key is the filename you | |
850 | specified, and the value is the location of the file actually found. | |
851 | The C<require> command uses this array to determine whether a given file | |
852 | has already been included. | |
853 | ||
b687b08b TC |
854 | =item %ENV |
855 | ||
856 | =item $ENV{expr} | |
a0d0e21e LW |
857 | |
858 | The hash %ENV contains your current environment. Setting a | |
859 | value in C<ENV> changes the environment for child processes. | |
860 | ||
b687b08b TC |
861 | =item %SIG |
862 | ||
863 | =item $SIG{expr} | |
a0d0e21e LW |
864 | |
865 | The hash %SIG is used to set signal handlers for various | |
866 | signals. Example: | |
867 | ||
868 | sub handler { # 1st argument is signal name | |
fb73857a | 869 | my($sig) = @_; |
a0d0e21e LW |
870 | print "Caught a SIG$sig--shutting down\n"; |
871 | close(LOG); | |
872 | exit(0); | |
873 | } | |
874 | ||
fb73857a | 875 | $SIG{'INT'} = \&handler; |
876 | $SIG{'QUIT'} = \&handler; | |
a0d0e21e LW |
877 | ... |
878 | $SIG{'INT'} = 'DEFAULT'; # restore default action | |
879 | $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT | |
880 | ||
f648820c GS |
881 | Using a value of C<'IGNORE'> usually has the effect of ignoring the |
882 | signal, except for the C<CHLD> signal. See L<perlipc> for more about | |
883 | this special case. | |
884 | ||
5f05dabc | 885 | The %SIG array contains values for only the signals actually set within |
a0d0e21e LW |
886 | the Perl script. Here are some other examples: |
887 | ||
fb73857a | 888 | $SIG{"PIPE"} = Plumber; # SCARY!! |
889 | $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended) | |
a0d0e21e LW |
890 | $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber |
891 | $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return?? | |
892 | ||
893 | The one marked scary is problematic because it's a bareword, which means | |
54310121 | 894 | sometimes it's a string representing the function, and sometimes it's |
a0d0e21e | 895 | going to call the subroutine call right then and there! Best to be sure |
a8f8344d | 896 | and quote it or take a reference to it. *Plumber works too. See L<perlsub>. |
748a9306 | 897 | |
44a8e56a | 898 | If your system has the sigaction() function then signal handlers are |
899 | installed using it. This means you get reliable signal handling. If | |
900 | your system has the SA_RESTART flag it is used when signals handlers are | |
901 | installed. This means that system calls for which it is supported | |
902 | continue rather than returning when a signal arrives. If you want your | |
903 | system calls to be interrupted by signal delivery then do something like | |
904 | this: | |
905 | ||
906 | use POSIX ':signal_h'; | |
907 | ||
908 | my $alarm = 0; | |
909 | sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 } | |
910 | or die "Error setting SIGALRM handler: $!\n"; | |
911 | ||
912 | See L<POSIX>. | |
913 | ||
748a9306 | 914 | Certain internal hooks can be also set using the %SIG hash. The |
a8f8344d | 915 | routine indicated by C<$SIG{__WARN__}> is called when a warning message is |
748a9306 LW |
916 | about to be printed. The warning message is passed as the first |
917 | argument. The presence of a __WARN__ hook causes the ordinary printing | |
918 | of warnings to STDERR to be suppressed. You can use this to save warnings | |
919 | in a variable, or turn warnings into fatal errors, like this: | |
920 | ||
921 | local $SIG{__WARN__} = sub { die $_[0] }; | |
922 | eval $proggie; | |
923 | ||
a8f8344d | 924 | The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception |
748a9306 LW |
925 | is about to be thrown. The error message is passed as the first |
926 | argument. When a __DIE__ hook routine returns, the exception | |
927 | processing continues as it would have in the absence of the hook, | |
cb1a09d0 | 928 | unless the hook routine itself exits via a C<goto>, a loop exit, or a die(). |
774d564b | 929 | The C<__DIE__> handler is explicitly disabled during the call, so that you |
fb73857a | 930 | can die from a C<__DIE__> handler. Similarly for C<__WARN__>. |
931 | ||
932 | Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed | |
7b8d334a | 933 | blocks/strings. See L<perlfunc/die> and L<perlvar/$^S> for how to |
fb73857a | 934 | circumvent this. |
935 | ||
936 | Note that C<__DIE__>/C<__WARN__> handlers are very special in one | |
937 | respect: they may be called to report (probable) errors found by the | |
938 | parser. In such a case the parser may be in inconsistent state, so | |
939 | any attempt to evaluate Perl code from such a handler will probably | |
940 | result in a segfault. This means that calls which result/may-result | |
c2611fb3 | 941 | in parsing Perl should be used with extreme caution, like this: |
fb73857a | 942 | |
943 | require Carp if defined $^S; | |
944 | Carp::confess("Something wrong") if defined &Carp::confess; | |
945 | die "Something wrong, but could not load Carp to give backtrace... | |
946 | To see backtrace try starting Perl with -MCarp switch"; | |
947 | ||
948 | Here the first line will load Carp I<unless> it is the parser who | |
949 | called the handler. The second line will print backtrace and die if | |
950 | Carp was available. The third line will be executed only if Carp was | |
951 | not available. | |
952 | ||
953 | See L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval> for | |
954 | additional info. | |
68dc0745 | 955 | |
a0d0e21e | 956 | =back |
55602bd2 IZ |
957 | |
958 | =head2 Error Indicators | |
959 | ||
960 | The variables L<$@>, L<$!>, L<$^E>, and L<$?> contain information about | |
961 | different types of error conditions that may appear during execution of | |
962 | Perl script. The variables are shown ordered by the "distance" between | |
963 | the subsystem which reported the error and the Perl process, and | |
964 | correspond to errors detected by the Perl interpreter, C library, | |
965 | operating system, or an external program, respectively. | |
966 | ||
967 | To illustrate the differences between these variables, consider the | |
968 | following Perl expression: | |
969 | ||
970 | eval ' | |
971 | open PIPE, "/cdrom/install |"; | |
972 | @res = <PIPE>; | |
973 | close PIPE or die "bad pipe: $?, $!"; | |
974 | '; | |
975 | ||
976 | After execution of this statement all 4 variables may have been set. | |
977 | ||
978 | $@ is set if the string to be C<eval>-ed did not compile (this may happen if | |
979 | C<open> or C<close> were imported with bad prototypes), or if Perl | |
980 | code executed during evaluation die()d (either implicitly, say, | |
981 | if C<open> was imported from module L<Fatal>, or the C<die> after | |
982 | C<close> was triggered). In these cases the value of $@ is the compile | |
983 | error, or C<Fatal> error (which will interpolate C<$!>!), or the argument | |
984 | to C<die> (which will interpolate C<$!> and C<$?>!). | |
985 | ||
986 | When the above expression is executed, open(), C<<PIPEE<gt>>, and C<close> | |
987 | are translated to C run-time library calls. $! is set if one of these | |
988 | calls fails. The value is a symbolic indicator chosen by the C run-time | |
989 | library, say C<No such file or directory>. | |
990 | ||
991 | On some systems the above C library calls are further translated | |
992 | to calls to the kernel. The kernel may have set more verbose error | |
993 | indicator that one of the handful of standard C errors. In such cases $^E | |
994 | contains this verbose error indicator, which may be, say, C<CDROM tray not | |
995 | closed>. On systems where C library calls are identical to system calls | |
996 | $^E is a duplicate of $!. | |
997 | ||
998 | Finally, $? may be set to non-C<0> value if the external program | |
999 | C</cdrom/install> fails. Upper bits of the particular value may reflect | |
1000 | specific error conditions encountered by this program (this is | |
1001 | program-dependent), lower-bits reflect mode of failure (segfault, completion, | |
1002 | etc.). Note that in contrast to $@, $!, and $^E, which are set only | |
1003 | if error condition is detected, the variable $? is set on each C<wait> or | |
1004 | pipe C<close>, overwriting the old value. | |
1005 | ||
1006 | For more details, see the individual descriptions at L<$@>, L<$!>, L<$^E>, | |
1007 | and L<$?>. |