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1 | =head1 NAME |
2 | |
3 | perlvar - Perl predefined variables |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | =head2 Predefined Names |
8 | |
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9 | The following names have special meaning to Perl. Most |
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10 | punctuation names have reasonable mnemonics, or analogues in one of |
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11 | the shells. Nevertheless, if you wish to use long variable names, |
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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 |
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17 | long names in the current package. Some even have medium names, |
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18 | generally borrowed from B<awk>. |
19 | |
20 | To go a step further, those variables that depend on the currently |
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21 | selected filehandle may instead (and preferably) be set by calling an |
22 | object method on the FileHandle object. (Summary lines below for this |
23 | contain the word HANDLE.) First you must say |
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24 | |
25 | use FileHandle; |
26 | |
27 | after which you may use either |
28 | |
29 | method HANDLE EXPR |
30 | |
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31 | or more safely, |
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32 | |
33 | HANDLE->method(EXPR) |
34 | |
35 | Each of the methods returns the old value of the FileHandle attribute. |
36 | The methods each take an optional EXPR, which if supplied specifies the |
37 | new value for the FileHandle attribute in question. If not supplied, |
38 | most of the methods do nothing to the current value, except for |
39 | autoflush(), which will assume a 1 for you, just to be different. |
40 | |
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41 | A few of these variables are considered "read-only". This means that if |
42 | you try to assign to this variable, either directly or indirectly through |
43 | a reference, you'll raise a run-time exception. |
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44 | |
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45 | The following list is ordered by scalar variables first, then the |
46 | arrays, then the hashes (except $^M was added in the wrong place). |
47 | This is somewhat obscured by the fact that %ENV and %SIG are listed as |
48 | $ENV{expr} and $SIG{expr}. |
49 | |
50 | |
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51 | =over 8 |
52 | |
53 | =item $ARG |
54 | |
55 | =item $_ |
56 | |
57 | The default input and pattern-searching space. The following pairs are |
58 | equivalent: |
59 | |
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60 | while (<>) {...} # equivalent in only while! |
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61 | while (defined($_ = <>)) {...} |
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62 | |
63 | /^Subject:/ |
64 | $_ =~ /^Subject:/ |
65 | |
66 | tr/a-z/A-Z/ |
67 | $_ =~ tr/a-z/A-Z/ |
68 | |
69 | chop |
70 | chop($_) |
71 | |
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72 | Here are the places where Perl will assume $_ even if you |
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73 | don't use it: |
74 | |
75 | =over 3 |
76 | |
77 | =item * |
78 | |
79 | Various unary functions, including functions like ord() and int(), as well |
80 | as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to |
81 | STDIN. |
82 | |
83 | =item * |
84 | |
85 | Various list functions like print() and unlink(). |
86 | |
87 | =item * |
88 | |
89 | The pattern matching operations C<m//>, C<s///>, and C<tr///> when used |
90 | without an C<=~> operator. |
91 | |
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92 | =item * |
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93 | |
94 | The default iterator variable in a C<foreach> loop if no other |
95 | variable is supplied. |
96 | |
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97 | =item * |
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98 | |
99 | The implicit iterator variable in the grep() and map() functions. |
100 | |
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101 | =item * |
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102 | |
103 | The default place to put an input record when a C<E<lt>FHE<gt>> |
104 | operation's result is tested by itself as the sole criterion of a C<while> |
105 | test. Note that outside of a C<while> test, this will not happen. |
106 | |
107 | =back |
108 | |
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109 | (Mnemonic: underline is understood in certain operations.) |
110 | |
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111 | =back |
112 | |
113 | =over 8 |
114 | |
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115 | =item $E<lt>I<digits>E<gt> |
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116 | |
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117 | Contains the subpattern from the corresponding set of parentheses in |
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118 | the last pattern matched, not counting patterns matched in nested |
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119 | blocks that have been exited already. (Mnemonic: like \digits.) |
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120 | These variables are all read-only. |
121 | |
122 | =item $MATCH |
123 | |
124 | =item $& |
125 | |
126 | The string matched by the last successful pattern match (not counting |
127 | any matches hidden within a BLOCK or eval() enclosed by the current |
128 | BLOCK). (Mnemonic: like & in some editors.) This variable is read-only. |
129 | |
130 | =item $PREMATCH |
131 | |
132 | =item $` |
133 | |
134 | The string preceding whatever was matched by the last successful |
135 | pattern match (not counting any matches hidden within a BLOCK or eval |
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136 | enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted |
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137 | string.) This variable is read-only. |
138 | |
139 | =item $POSTMATCH |
140 | |
141 | =item $' |
142 | |
143 | The string following whatever was matched by the last successful |
144 | pattern match (not counting any matches hidden within a BLOCK or eval() |
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145 | enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted |
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146 | string.) Example: |
147 | |
148 | $_ = 'abcdefghi'; |
149 | /def/; |
150 | print "$`:$&:$'\n"; # prints abc:def:ghi |
151 | |
152 | This variable is read-only. |
153 | |
154 | =item $LAST_PAREN_MATCH |
155 | |
156 | =item $+ |
157 | |
158 | The last bracket matched by the last search pattern. This is useful if |
159 | you don't know which of a set of alternative patterns matched. For |
160 | example: |
161 | |
162 | /Version: (.*)|Revision: (.*)/ && ($rev = $+); |
163 | |
164 | (Mnemonic: be positive and forward looking.) |
165 | This variable is read-only. |
166 | |
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167 | =item @+ |
168 | |
169 | $+[0] is the offset of the end of the last successfull match. |
170 | C<$+[>I<n>C<]> is the offset of the end of the substring matched by |
171 | I<n>-th subpattern. |
172 | |
173 | Thus after a match against $_, $& coincides with C<substr $_, $-[0], |
174 | $+[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<], |
175 | $+[>I<0>C<]> if C<$-[>I<n>C<]> is defined, and $+ conincides with |
176 | C<substr $_, $-[-1], $+[-1]>. One can use C<$#+> to find the last |
177 | matched subgroup in the last successful match. Compare with L<"@-">. |
178 | |
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179 | =item $MULTILINE_MATCHING |
180 | |
181 | =item $* |
182 | |
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183 | Set to 1 to do multi-line matching within a string, 0 to tell Perl |
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184 | that it can assume that strings contain a single line, for the purpose |
185 | of optimizing pattern matches. Pattern matches on strings containing |
186 | multiple newlines can produce confusing results when "C<$*>" is 0. Default |
187 | is 0. (Mnemonic: * matches multiple things.) Note that this variable |
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188 | influences the interpretation of only "C<^>" and "C<$>". A literal newline can |
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189 | be searched for even when C<$* == 0>. |
190 | |
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191 | Use of "C<$*>" is deprecated in modern Perls, supplanted by |
192 | the C</s> and C</m> modifiers on pattern matching. |
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193 | |
194 | =item input_line_number HANDLE EXPR |
195 | |
196 | =item $INPUT_LINE_NUMBER |
197 | |
198 | =item $NR |
199 | |
200 | =item $. |
201 | |
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202 | The current input line number for the last file handle from |
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203 | which you read (or performed a C<seek> or C<tell> on). An |
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204 | explicit close on a filehandle resets the line number. Because |
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205 | "C<E<lt>E<gt>>" never does an explicit close, line numbers increase |
206 | across ARGV files (but see examples under eof()). Localizing C<$.> has |
207 | the effect of also localizing Perl's notion of "the last read |
208 | filehandle". (Mnemonic: many programs use "." to mean the current line |
209 | number.) |
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210 | |
211 | =item input_record_separator HANDLE EXPR |
212 | |
213 | =item $INPUT_RECORD_SEPARATOR |
214 | |
215 | =item $RS |
216 | |
217 | =item $/ |
218 | |
219 | The input record separator, newline by default. Works like B<awk>'s RS |
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220 | variable, including treating empty lines as delimiters if set to the |
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221 | null string. (Note: An empty line cannot contain any spaces or tabs.) |
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222 | You may set it to a multi-character string to match a multi-character |
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223 | delimiter, or to C<undef> to read to end of file. Note that setting it |
224 | to C<"\n\n"> means something slightly different than setting it to |
225 | C<"">, if the file contains consecutive empty lines. Setting it to |
226 | C<""> will treat two or more consecutive empty lines as a single empty |
227 | line. Setting it to C<"\n\n"> will blindly assume that the next input |
228 | character belongs to the next paragraph, even if it's a newline. |
229 | (Mnemonic: / is used to delimit line boundaries when quoting poetry.) |
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230 | |
231 | undef $/; |
232 | $_ = <FH>; # whole file now here |
233 | s/\n[ \t]+/ /g; |
234 | |
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235 | Remember: the value of $/ is a string, not a regexp. AWK has to be |
236 | better for something :-) |
237 | |
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238 | Setting $/ to a reference to an integer, scalar containing an integer, or |
239 | scalar that's convertable to an integer will attempt to read records |
240 | instead of lines, with the maximum record size being the referenced |
241 | integer. So this: |
242 | |
243 | $/ = \32768; # or \"32768", or \$var_containing_32768 |
244 | open(FILE, $myfile); |
245 | $_ = <FILE>; |
246 | |
247 | will read a record of no more than 32768 bytes from FILE. If you're not |
248 | reading from a record-oriented file (or your OS doesn't have |
249 | record-oriented files), then you'll likely get a full chunk of data with |
250 | every read. If a record is larger than the record size you've set, you'll |
251 | get the record back in pieces. |
252 | |
253 | On VMS, record reads are done with the equivalent of C<sysread>, so it's |
254 | best not to mix record and non-record reads on the same file. (This is |
255 | likely not a problem, as any file you'd want to read in record mode is |
256 | proably usable in line mode) Non-VMS systems perform normal I/O, so |
257 | it's safe to mix record and non-record reads of a file. |
258 | |
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259 | =item autoflush HANDLE EXPR |
260 | |
261 | =item $OUTPUT_AUTOFLUSH |
262 | |
263 | =item $| |
264 | |
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265 | If set to nonzero, forces a flush right away and after every write or print on the |
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266 | currently selected output channel. Default is 0 (regardless of whether |
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267 | the channel is actually buffered by the system or not; C<$|> tells you |
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268 | only whether you've asked Perl explicitly to flush after each write). |
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269 | Note that STDOUT will typically be line buffered if output is to the |
270 | terminal and block buffered otherwise. Setting this variable is useful |
271 | primarily when you are outputting to a pipe, such as when you are running |
272 | a Perl script under rsh and want to see the output as it's happening. This |
273 | has no effect on input buffering. |
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274 | (Mnemonic: when you want your pipes to be piping hot.) |
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275 | |
276 | =item output_field_separator HANDLE EXPR |
277 | |
278 | =item $OUTPUT_FIELD_SEPARATOR |
279 | |
280 | =item $OFS |
281 | |
282 | =item $, |
283 | |
284 | The output field separator for the print operator. Ordinarily the |
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285 | print operator simply prints out the comma-separated fields you |
286 | specify. To get behavior more like B<awk>, set this variable |
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287 | as you would set B<awk>'s OFS variable to specify what is printed |
288 | between fields. (Mnemonic: what is printed when there is a , in your |
289 | print statement.) |
290 | |
291 | =item output_record_separator HANDLE EXPR |
292 | |
293 | =item $OUTPUT_RECORD_SEPARATOR |
294 | |
295 | =item $ORS |
296 | |
297 | =item $\ |
298 | |
299 | The output record separator for the print operator. Ordinarily the |
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300 | print operator simply prints out the comma-separated fields you |
301 | specify, with no trailing newline or record separator assumed. |
302 | To get behavior more like B<awk>, set this variable as you would |
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303 | set B<awk>'s ORS variable to specify what is printed at the end of the |
304 | print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the |
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305 | print. Also, it's just like C<$/>, but it's what you get "back" from |
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306 | Perl.) |
307 | |
308 | =item $LIST_SEPARATOR |
309 | |
310 | =item $" |
311 | |
312 | This is like "C<$,>" except that it applies to array values interpolated |
313 | into a double-quoted string (or similar interpreted string). Default |
314 | is a space. (Mnemonic: obvious, I think.) |
315 | |
316 | =item $SUBSCRIPT_SEPARATOR |
317 | |
318 | =item $SUBSEP |
319 | |
320 | =item $; |
321 | |
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322 | The subscript separator for multidimensional array emulation. If you |
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323 | refer to a hash element as |
324 | |
325 | $foo{$a,$b,$c} |
326 | |
327 | it really means |
328 | |
329 | $foo{join($;, $a, $b, $c)} |
330 | |
331 | But don't put |
332 | |
333 | @foo{$a,$b,$c} # a slice--note the @ |
334 | |
335 | which means |
336 | |
337 | ($foo{$a},$foo{$b},$foo{$c}) |
338 | |
339 | Default is "\034", the same as SUBSEP in B<awk>. Note that if your |
340 | keys contain binary data there might not be any safe value for "C<$;>". |
341 | (Mnemonic: comma (the syntactic subscript separator) is a |
342 | semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already |
343 | taken for something more important.) |
344 | |
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345 | Consider using "real" multidimensional arrays. |
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346 | |
347 | =item $OFMT |
348 | |
349 | =item $# |
350 | |
351 | The output format for printed numbers. This variable is a half-hearted |
352 | attempt to emulate B<awk>'s OFMT variable. There are times, however, |
353 | when B<awk> and Perl have differing notions of what is in fact |
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354 | numeric. The initial value is %.I<n>g, where I<n> is the value |
355 | of the macro DBL_DIG from your system's F<float.h>. This is different from |
356 | B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>" |
357 | explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.) |
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358 | |
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359 | Use of "C<$#>" is deprecated. |
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360 | |
361 | =item format_page_number HANDLE EXPR |
362 | |
363 | =item $FORMAT_PAGE_NUMBER |
364 | |
365 | =item $% |
366 | |
367 | The current page number of the currently selected output channel. |
368 | (Mnemonic: % is page number in B<nroff>.) |
369 | |
370 | =item format_lines_per_page HANDLE EXPR |
371 | |
372 | =item $FORMAT_LINES_PER_PAGE |
373 | |
374 | =item $= |
375 | |
376 | The current page length (printable lines) of the currently selected |
377 | output channel. Default is 60. (Mnemonic: = has horizontal lines.) |
378 | |
379 | =item format_lines_left HANDLE EXPR |
380 | |
381 | =item $FORMAT_LINES_LEFT |
382 | |
383 | =item $- |
384 | |
385 | The number of lines left on the page of the currently selected output |
386 | channel. (Mnemonic: lines_on_page - lines_printed.) |
387 | |
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388 | =item @- |
389 | |
390 | $-[0] is the offset of the start of the last successfull match. |
391 | C<$-[>I<n>C<]> is the offset of the start of the substring matched by |
392 | I<n>-th subpattern. |
393 | |
394 | Thus after a match against $_, $& coincides with C<substr $_, $-[0], |
395 | $+[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<], |
396 | $+[>I<0>C<]> if C<$-[>I<n>C<]> is defined, and $+ conincides with |
397 | C<substr $_, $-[-1], $+[-1]>. One can use C<$#-> to find the last |
398 | matched subgroup in the last successful match. Compare with L<"@+">. |
399 | |
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400 | =item format_name HANDLE EXPR |
401 | |
402 | =item $FORMAT_NAME |
403 | |
404 | =item $~ |
405 | |
406 | The name of the current report format for the currently selected output |
407 | channel. Default is name of the filehandle. (Mnemonic: brother to |
408 | "C<$^>".) |
409 | |
410 | =item format_top_name HANDLE EXPR |
411 | |
412 | =item $FORMAT_TOP_NAME |
413 | |
414 | =item $^ |
415 | |
416 | The name of the current top-of-page format for the currently selected |
417 | output channel. Default is name of the filehandle with _TOP |
418 | appended. (Mnemonic: points to top of page.) |
419 | |
420 | =item format_line_break_characters HANDLE EXPR |
421 | |
422 | =item $FORMAT_LINE_BREAK_CHARACTERS |
423 | |
424 | =item $: |
425 | |
426 | The current set of characters after which a string may be broken to |
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427 | fill continuation fields (starting with ^) in a format. Default is |
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428 | S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in |
429 | poetry is a part of a line.) |
430 | |
431 | =item format_formfeed HANDLE EXPR |
432 | |
433 | =item $FORMAT_FORMFEED |
434 | |
435 | =item $^L |
436 | |
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437 | What formats output to perform a form feed. Default is \f. |
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438 | |
439 | =item $ACCUMULATOR |
440 | |
441 | =item $^A |
442 | |
443 | The current value of the write() accumulator for format() lines. A format |
444 | contains formline() commands that put their result into C<$^A>. After |
445 | calling its format, write() prints out the contents of C<$^A> and empties. |
446 | So you never actually see the contents of C<$^A> unless you call |
447 | formline() yourself and then look at it. See L<perlform> and |
448 | L<perlfunc/formline()>. |
449 | |
450 | =item $CHILD_ERROR |
451 | |
452 | =item $? |
453 | |
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454 | The status returned by the last pipe close, backtick (C<``>) command, |
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455 | or system() operator. Note that this is the status word returned by the |
456 | wait() system call (or else is made up to look like it). Thus, the exit |
457 | value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and C<$? & 127> |
458 | gives which signal, if any, the process died from, and C<$? & 128> reports |
459 | whether there was a core dump. (Mnemonic: similar to B<sh> and B<ksh>.) |
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460 | |
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461 | Additionally, if the C<h_errno> variable is supported in C, its value |
462 | is returned via $? if any of the C<gethost*()> functions fail. |
463 | |
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464 | Note that if you have installed a signal handler for C<SIGCHLD>, the |
465 | value of C<$?> will usually be wrong outside that handler. |
466 | |
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467 | Inside an C<END> subroutine C<$?> contains the value that is going to be |
468 | given to C<exit()>. You can modify C<$?> in an C<END> subroutine to |
469 | change the exit status of the script. |
470 | |
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471 | Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the |
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472 | actual VMS exit status, instead of the default emulation of POSIX |
473 | status. |
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474 | |
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475 | Also see L<Error Indicators>. |
476 | |
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477 | =item $OS_ERROR |
478 | |
479 | =item $ERRNO |
480 | |
481 | =item $! |
482 | |
483 | If used in a numeric context, yields the current value of errno, with |
484 | all the usual caveats. (This means that you shouldn't depend on the |
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485 | value of C<$!> to be anything in particular unless you've gotten a |
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486 | specific error return indicating a system error.) If used in a string |
487 | context, yields the corresponding system error string. You can assign |
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488 | to C<$!> to set I<errno> if, for instance, you want C<"$!"> to return the |
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489 | string for error I<n>, or you want to set the exit value for the die() |
490 | operator. (Mnemonic: What just went bang?) |
491 | |
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492 | Also see L<Error Indicators>. |
493 | |
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494 | =item $EXTENDED_OS_ERROR |
495 | |
496 | =item $^E |
497 | |
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498 | Error information specific to the current operating system. At |
499 | the moment, this differs from C<$!> under only VMS, OS/2, and Win32 |
500 | (and for MacPerl). On all other platforms, C<$^E> is always just |
501 | the same as C<$!>. |
502 | |
503 | Under VMS, C<$^E> provides the VMS status value from the last |
504 | system error. This is more specific information about the last |
505 | system error than that provided by C<$!>. This is particularly |
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506 | important when C<$!> is set to B<EVMSERR>. |
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507 | |
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508 | Under OS/2, C<$^E> is set to the error code of the last call to |
509 | OS/2 API either via CRT, or directly from perl. |
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510 | |
511 | Under Win32, C<$^E> always returns the last error information |
512 | reported by the Win32 call C<GetLastError()> which describes |
513 | the last error from within the Win32 API. Most Win32-specific |
514 | code will report errors via C<$^E>. ANSI C and UNIX-like calls |
515 | set C<errno> and so most portable Perl code will report errors |
516 | via C<$!>. |
517 | |
518 | Caveats mentioned in the description of C<$!> generally apply to |
519 | C<$^E>, also. (Mnemonic: Extra error explanation.) |
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520 | |
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521 | Also see L<Error Indicators>. |
522 | |
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523 | =item $EVAL_ERROR |
524 | |
525 | =item $@ |
526 | |
527 | The Perl syntax error message from the last eval() command. If null, the |
528 | last eval() parsed and executed correctly (although the operations you |
529 | invoked may have failed in the normal fashion). (Mnemonic: Where was |
530 | the syntax error "at"?) |
531 | |
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532 | Note that warning messages are not collected in this variable. You can, |
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533 | however, set up a routine to process warnings by setting C<$SIG{__WARN__}> |
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534 | as described below. |
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535 | |
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536 | Also see L<Error Indicators>. |
537 | |
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538 | =item $PROCESS_ID |
539 | |
540 | =item $PID |
541 | |
542 | =item $$ |
543 | |
544 | The process number of the Perl running this script. (Mnemonic: same |
545 | as shells.) |
546 | |
547 | =item $REAL_USER_ID |
548 | |
549 | =item $UID |
550 | |
551 | =item $< |
552 | |
553 | The real uid of this process. (Mnemonic: it's the uid you came I<FROM>, |
554 | if you're running setuid.) |
555 | |
556 | =item $EFFECTIVE_USER_ID |
557 | |
558 | =item $EUID |
559 | |
560 | =item $> |
561 | |
562 | The effective uid of this process. Example: |
563 | |
564 | $< = $>; # set real to effective uid |
565 | ($<,$>) = ($>,$<); # swap real and effective uid |
566 | |
8cc95fdb |
567 | (Mnemonic: it's the uid you went I<TO>, if you're running setuid.) |
568 | Note: "C<$E<lt>>" and "C<$E<gt>>" can be swapped only on machines |
569 | supporting setreuid(). |
a0d0e21e |
570 | |
571 | =item $REAL_GROUP_ID |
572 | |
573 | =item $GID |
574 | |
575 | =item $( |
576 | |
577 | The real gid of this process. If you are on a machine that supports |
578 | membership in multiple groups simultaneously, gives a space separated |
579 | list of groups you are in. The first number is the one returned by |
580 | getgid(), and the subsequent ones by getgroups(), one of which may be |
8cc95fdb |
581 | the same as the first number. |
582 | |
583 | However, a value assigned to "C<$(>" must be a single number used to |
584 | set the real gid. So the value given by "C<$(>" should I<not> be assigned |
585 | back to "C<$(>" without being forced numeric, such as by adding zero. |
586 | |
587 | (Mnemonic: parentheses are used to I<GROUP> things. The real gid is the |
588 | group you I<LEFT>, if you're running setgid.) |
a0d0e21e |
589 | |
590 | =item $EFFECTIVE_GROUP_ID |
591 | |
592 | =item $EGID |
593 | |
594 | =item $) |
595 | |
596 | The effective gid of this process. If you are on a machine that |
597 | supports membership in multiple groups simultaneously, gives a space |
598 | separated list of groups you are in. The first number is the one |
599 | returned by getegid(), and the subsequent ones by getgroups(), one of |
8cc95fdb |
600 | which may be the same as the first number. |
601 | |
602 | Similarly, a value assigned to "C<$)>" must also be a space-separated |
603 | list of numbers. The first number is used to set the effective gid, and |
604 | the rest (if any) are passed to setgroups(). To get the effect of an |
605 | empty list for setgroups(), just repeat the new effective gid; that is, |
606 | to force an effective gid of 5 and an effectively empty setgroups() |
607 | list, say C< $) = "5 5" >. |
608 | |
609 | (Mnemonic: parentheses are used to I<GROUP> things. The effective gid |
610 | is the group that's I<RIGHT> for you, if you're running setgid.) |
a0d0e21e |
611 | |
5f05dabc |
612 | Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on |
613 | machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>" |
8cc95fdb |
614 | and "C<$)>" can be swapped only on machines supporting setregid(). |
a0d0e21e |
615 | |
616 | =item $PROGRAM_NAME |
617 | |
618 | =item $0 |
619 | |
620 | Contains the name of the file containing the Perl script being |
54310121 |
621 | executed. On some operating systems |
622 | assigning to "C<$0>" modifies the argument area that the ps(1) |
a0d0e21e |
623 | program sees. This is more useful as a way of indicating the |
624 | current program state than it is for hiding the program you're running. |
625 | (Mnemonic: same as B<sh> and B<ksh>.) |
626 | |
627 | =item $[ |
628 | |
629 | The index of the first element in an array, and of the first character |
630 | in a substring. Default is 0, but you could set it to 1 to make |
631 | Perl behave more like B<awk> (or Fortran) when subscripting and when |
632 | evaluating the index() and substr() functions. (Mnemonic: [ begins |
633 | subscripts.) |
634 | |
635 | As of Perl 5, assignment to "C<$[>" is treated as a compiler directive, |
636 | and cannot influence the behavior of any other file. Its use is |
637 | discouraged. |
638 | |
639 | =item $PERL_VERSION |
640 | |
641 | =item $] |
642 | |
54310121 |
643 | The version + patchlevel / 1000 of the Perl interpreter. This variable |
644 | can be used to determine whether the Perl interpreter executing a |
645 | script is in the right range of versions. (Mnemonic: Is this version |
646 | of perl in the right bracket?) Example: |
a0d0e21e |
647 | |
648 | warn "No checksumming!\n" if $] < 3.019; |
649 | |
54310121 |
650 | See also the documentation of C<use VERSION> and C<require VERSION> |
651 | for a convenient way to fail if the Perl interpreter is too old. |
a0d0e21e |
652 | |
653 | =item $DEBUGGING |
654 | |
655 | =item $^D |
656 | |
657 | The current value of the debugging flags. (Mnemonic: value of B<-D> |
658 | switch.) |
659 | |
660 | =item $SYSTEM_FD_MAX |
661 | |
662 | =item $^F |
663 | |
664 | The maximum system file descriptor, ordinarily 2. System file |
665 | descriptors are passed to exec()ed processes, while higher file |
666 | descriptors are not. Also, during an open(), system file descriptors are |
667 | preserved even if the open() fails. (Ordinary file descriptors are |
668 | closed before the open() is attempted.) Note that the close-on-exec |
669 | status of a file descriptor will be decided according to the value of |
670 | C<$^F> at the time of the open, not the time of the exec. |
671 | |
6e2995f4 |
672 | =item $^H |
673 | |
fb73857a |
674 | The current set of syntax checks enabled by C<use strict> and other block |
675 | scoped compiler hints. See the documentation of C<strict> for more details. |
6e2995f4 |
676 | |
a0d0e21e |
677 | =item $INPLACE_EDIT |
678 | |
679 | =item $^I |
680 | |
681 | The current value of the inplace-edit extension. Use C<undef> to disable |
682 | inplace editing. (Mnemonic: value of B<-i> switch.) |
683 | |
fb73857a |
684 | =item $^M |
685 | |
686 | By default, running out of memory it is not trappable. However, if |
687 | compiled for this, Perl may use the contents of C<$^M> as an emergency |
688 | pool after die()ing with this message. Suppose that your Perl were |
689 | compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. Then |
690 | |
691 | $^M = 'a' x (1<<16); |
692 | |
693 | would allocate a 64K buffer for use when in emergency. See the F<INSTALL> |
694 | file for information on how to enable this option. As a disincentive to |
695 | casual use of this advanced feature, there is no L<English> long name for |
696 | this variable. |
697 | |
5c055ba3 |
698 | =item $OSNAME |
6e2995f4 |
699 | |
5c055ba3 |
700 | =item $^O |
701 | |
702 | The name of the operating system under which this copy of Perl was |
703 | built, as determined during the configuration process. The value |
704 | is identical to C<$Config{'osname'}>. |
705 | |
a0d0e21e |
706 | =item $PERLDB |
707 | |
708 | =item $^P |
709 | |
84902520 |
710 | The internal variable for debugging support. Different bits mean the |
711 | following (subject to change): |
712 | |
713 | =over 6 |
714 | |
715 | =item 0x01 |
716 | |
717 | Debug subroutine enter/exit. |
718 | |
719 | =item 0x02 |
720 | |
721 | Line-by-line debugging. |
722 | |
723 | =item 0x04 |
724 | |
725 | Switch off optimizations. |
726 | |
727 | =item 0x08 |
728 | |
729 | Preserve more data for future interactive inspections. |
730 | |
731 | =item 0x10 |
732 | |
733 | Keep info about source lines on which a subroutine is defined. |
734 | |
735 | =item 0x20 |
736 | |
737 | Start with single-step on. |
738 | |
739 | =back |
740 | |
741 | Note that some bits may be relevent at compile-time only, some at |
742 | run-time only. This is a new mechanism and the details may change. |
a0d0e21e |
743 | |
b9ac3b5b |
744 | =item $^R |
745 | |
746 | The result of evaluation of the last successful L<perlre/C<(?{ code })>> |
747 | regular expression assertion. (Excluding those used as switches.) May |
748 | be written to. |
749 | |
fb73857a |
750 | =item $^S |
751 | |
752 | Current state of the interpreter. Undefined if parsing of the current |
753 | module/eval is not finished (may happen in $SIG{__DIE__} and |
a3cb178b |
754 | $SIG{__WARN__} handlers). True if inside an eval, otherwise false. |
fb73857a |
755 | |
a0d0e21e |
756 | =item $BASETIME |
757 | |
758 | =item $^T |
759 | |
760 | The time at which the script began running, in seconds since the |
5f05dabc |
761 | epoch (beginning of 1970). The values returned by the B<-M>, B<-A>, |
a0d0e21e |
762 | and B<-C> filetests are |
763 | based on this value. |
764 | |
765 | =item $WARNING |
766 | |
767 | =item $^W |
768 | |
303f2f76 |
769 | The current value of the warning switch, either TRUE or FALSE. |
770 | (Mnemonic: related to the B<-w> switch.) |
a0d0e21e |
771 | |
772 | =item $EXECUTABLE_NAME |
773 | |
774 | =item $^X |
775 | |
776 | The name that the Perl binary itself was executed as, from C's C<argv[0]>. |
777 | |
778 | =item $ARGV |
779 | |
a8f8344d |
780 | contains the name of the current file when reading from E<lt>E<gt>. |
a0d0e21e |
781 | |
782 | =item @ARGV |
783 | |
784 | The array @ARGV contains the command line arguments intended for the |
785 | script. Note that C<$#ARGV> is the generally number of arguments minus |
5f05dabc |
786 | one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See |
a0d0e21e |
787 | "C<$0>" for the command name. |
788 | |
789 | =item @INC |
790 | |
791 | The array @INC contains the list of places to look for Perl scripts to |
792 | be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It |
793 | initially consists of the arguments to any B<-I> command line switches, |
6e2995f4 |
794 | followed by the default Perl library, probably F</usr/local/lib/perl>, |
cb1a09d0 |
795 | followed by ".", to represent the current directory. If you need to |
5f05dabc |
796 | modify this at runtime, you should use the C<use lib> pragma |
797 | to get the machine-dependent library properly loaded also: |
a0d0e21e |
798 | |
cb1a09d0 |
799 | use lib '/mypath/libdir/'; |
800 | use SomeMod; |
303f2f76 |
801 | |
fb73857a |
802 | =item @_ |
803 | |
804 | Within a subroutine the array @_ contains the parameters passed to that |
805 | subroutine. See L<perlsub>. |
806 | |
a0d0e21e |
807 | =item %INC |
808 | |
809 | The hash %INC contains entries for each filename that has |
810 | been included via C<do> or C<require>. The key is the filename you |
811 | specified, and the value is the location of the file actually found. |
812 | The C<require> command uses this array to determine whether a given file |
813 | has already been included. |
814 | |
fb73857a |
815 | =item %ENV $ENV{expr} |
a0d0e21e |
816 | |
817 | The hash %ENV contains your current environment. Setting a |
818 | value in C<ENV> changes the environment for child processes. |
819 | |
fb73857a |
820 | =item %SIG $SIG{expr} |
a0d0e21e |
821 | |
822 | The hash %SIG is used to set signal handlers for various |
823 | signals. Example: |
824 | |
825 | sub handler { # 1st argument is signal name |
fb73857a |
826 | my($sig) = @_; |
a0d0e21e |
827 | print "Caught a SIG$sig--shutting down\n"; |
828 | close(LOG); |
829 | exit(0); |
830 | } |
831 | |
fb73857a |
832 | $SIG{'INT'} = \&handler; |
833 | $SIG{'QUIT'} = \&handler; |
a0d0e21e |
834 | ... |
835 | $SIG{'INT'} = 'DEFAULT'; # restore default action |
836 | $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT |
837 | |
f648820c |
838 | Using a value of C<'IGNORE'> usually has the effect of ignoring the |
839 | signal, except for the C<CHLD> signal. See L<perlipc> for more about |
840 | this special case. |
841 | |
5f05dabc |
842 | The %SIG array contains values for only the signals actually set within |
a0d0e21e |
843 | the Perl script. Here are some other examples: |
844 | |
fb73857a |
845 | $SIG{"PIPE"} = Plumber; # SCARY!! |
846 | $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended) |
a0d0e21e |
847 | $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber |
848 | $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return?? |
849 | |
850 | The one marked scary is problematic because it's a bareword, which means |
54310121 |
851 | sometimes it's a string representing the function, and sometimes it's |
a0d0e21e |
852 | going to call the subroutine call right then and there! Best to be sure |
a8f8344d |
853 | and quote it or take a reference to it. *Plumber works too. See L<perlsub>. |
748a9306 |
854 | |
44a8e56a |
855 | If your system has the sigaction() function then signal handlers are |
856 | installed using it. This means you get reliable signal handling. If |
857 | your system has the SA_RESTART flag it is used when signals handlers are |
858 | installed. This means that system calls for which it is supported |
859 | continue rather than returning when a signal arrives. If you want your |
860 | system calls to be interrupted by signal delivery then do something like |
861 | this: |
862 | |
863 | use POSIX ':signal_h'; |
864 | |
865 | my $alarm = 0; |
866 | sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 } |
867 | or die "Error setting SIGALRM handler: $!\n"; |
868 | |
869 | See L<POSIX>. |
870 | |
748a9306 |
871 | Certain internal hooks can be also set using the %SIG hash. The |
a8f8344d |
872 | routine indicated by C<$SIG{__WARN__}> is called when a warning message is |
748a9306 |
873 | about to be printed. The warning message is passed as the first |
874 | argument. The presence of a __WARN__ hook causes the ordinary printing |
875 | of warnings to STDERR to be suppressed. You can use this to save warnings |
876 | in a variable, or turn warnings into fatal errors, like this: |
877 | |
878 | local $SIG{__WARN__} = sub { die $_[0] }; |
879 | eval $proggie; |
880 | |
a8f8344d |
881 | The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception |
748a9306 |
882 | is about to be thrown. The error message is passed as the first |
883 | argument. When a __DIE__ hook routine returns, the exception |
884 | processing continues as it would have in the absence of the hook, |
cb1a09d0 |
885 | unless the hook routine itself exits via a C<goto>, a loop exit, or a die(). |
774d564b |
886 | The C<__DIE__> handler is explicitly disabled during the call, so that you |
fb73857a |
887 | can die from a C<__DIE__> handler. Similarly for C<__WARN__>. |
888 | |
889 | Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed |
7b8d334a |
890 | blocks/strings. See L<perlfunc/die> and L<perlvar/$^S> for how to |
fb73857a |
891 | circumvent this. |
892 | |
893 | Note that C<__DIE__>/C<__WARN__> handlers are very special in one |
894 | respect: they may be called to report (probable) errors found by the |
895 | parser. In such a case the parser may be in inconsistent state, so |
896 | any attempt to evaluate Perl code from such a handler will probably |
897 | result in a segfault. This means that calls which result/may-result |
898 | in parsing Perl should be used with extreme causion, like this: |
899 | |
900 | require Carp if defined $^S; |
901 | Carp::confess("Something wrong") if defined &Carp::confess; |
902 | die "Something wrong, but could not load Carp to give backtrace... |
903 | To see backtrace try starting Perl with -MCarp switch"; |
904 | |
905 | Here the first line will load Carp I<unless> it is the parser who |
906 | called the handler. The second line will print backtrace and die if |
907 | Carp was available. The third line will be executed only if Carp was |
908 | not available. |
909 | |
910 | See L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval> for |
911 | additional info. |
68dc0745 |
912 | |
a0d0e21e |
913 | =back |
55602bd2 |
914 | |
915 | =head2 Error Indicators |
916 | |
917 | The variables L<$@>, L<$!>, L<$^E>, and L<$?> contain information about |
918 | different types of error conditions that may appear during execution of |
919 | Perl script. The variables are shown ordered by the "distance" between |
920 | the subsystem which reported the error and the Perl process, and |
921 | correspond to errors detected by the Perl interpreter, C library, |
922 | operating system, or an external program, respectively. |
923 | |
924 | To illustrate the differences between these variables, consider the |
925 | following Perl expression: |
926 | |
927 | eval ' |
928 | open PIPE, "/cdrom/install |"; |
929 | @res = <PIPE>; |
930 | close PIPE or die "bad pipe: $?, $!"; |
931 | '; |
932 | |
933 | After execution of this statement all 4 variables may have been set. |
934 | |
935 | $@ is set if the string to be C<eval>-ed did not compile (this may happen if |
936 | C<open> or C<close> were imported with bad prototypes), or if Perl |
937 | code executed during evaluation die()d (either implicitly, say, |
938 | if C<open> was imported from module L<Fatal>, or the C<die> after |
939 | C<close> was triggered). In these cases the value of $@ is the compile |
940 | error, or C<Fatal> error (which will interpolate C<$!>!), or the argument |
941 | to C<die> (which will interpolate C<$!> and C<$?>!). |
942 | |
943 | When the above expression is executed, open(), C<<PIPEE<gt>>, and C<close> |
944 | are translated to C run-time library calls. $! is set if one of these |
945 | calls fails. The value is a symbolic indicator chosen by the C run-time |
946 | library, say C<No such file or directory>. |
947 | |
948 | On some systems the above C library calls are further translated |
949 | to calls to the kernel. The kernel may have set more verbose error |
950 | indicator that one of the handful of standard C errors. In such cases $^E |
951 | contains this verbose error indicator, which may be, say, C<CDROM tray not |
952 | closed>. On systems where C library calls are identical to system calls |
953 | $^E is a duplicate of $!. |
954 | |
955 | Finally, $? may be set to non-C<0> value if the external program |
956 | C</cdrom/install> fails. Upper bits of the particular value may reflect |
957 | specific error conditions encountered by this program (this is |
958 | program-dependent), lower-bits reflect mode of failure (segfault, completion, |
959 | etc.). Note that in contrast to $@, $!, and $^E, which are set only |
960 | if error condition is detected, the variable $? is set on each C<wait> or |
961 | pipe C<close>, overwriting the old value. |
962 | |
963 | For more details, see the individual descriptions at L<$@>, L<$!>, L<$^E>, |
964 | and L<$?>. |