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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 |
14218588 GS |
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 | |
a0d0e21e LW |
13 | |
14 | use English; | |
15 | ||
a1ce9542 JF |
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 | |
a0d0e21e | 19 | |
a1ce9542 JF |
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 | |
a0d0e21e | 30 | |
19799a22 | 31 | use IO::Handle; |
a0d0e21e LW |
32 | |
33 | after which you may use either | |
34 | ||
35 | method HANDLE EXPR | |
36 | ||
5a964f20 | 37 | or more safely, |
a0d0e21e LW |
38 | |
39 | HANDLE->method(EXPR) | |
40 | ||
14218588 | 41 | Each method returns the old value of the IO::Handle attribute. |
a1ce9542 | 42 | The methods each take an optional EXPR, which, if supplied, specifies the |
19799a22 | 43 | new value for the IO::Handle attribute in question. If not supplied, |
14218588 | 44 | most methods do nothing to the current value--except for |
a0d0e21e | 45 | autoflush(), which will assume a 1 for you, just to be different. |
a1ce9542 | 46 | |
14218588 | 47 | Because loading in the IO::Handle class is an expensive operation, you should |
19799a22 | 48 | learn how to use the regular built-in variables. |
a0d0e21e | 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 | |
22d0716c SB |
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 | ||
b0169937 | 61 | open my $fh, "<", "foo" or die $!; |
22d0716c SB |
62 | local $/; # enable localized slurp mode |
63 | my $content = <$fh>; | |
64 | close $fh; | |
65 | ||
66 | But the following code is quite bad: | |
67 | ||
b0169937 | 68 | open my $fh, "<", "foo" or die $!; |
22d0716c SB |
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 = ''; | |
b0169937 | 84 | open my $fh, "<", "foo" or die $!; |
22d0716c SB |
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 | ||
fb73857a | 119 | The following list is ordered by scalar variables first, then the |
87275199 | 120 | arrays, then the hashes. |
fb73857a | 121 | |
a0d0e21e LW |
122 | =over 8 |
123 | ||
124 | =item $ARG | |
125 | ||
126 | =item $_ | |
a054c801 | 127 | X<$_> X<$ARG> |
a0d0e21e LW |
128 | |
129 | The default input and pattern-searching space. The following pairs are | |
130 | equivalent: | |
131 | ||
19799a22 | 132 | while (<>) {...} # equivalent only in while! |
54310121 | 133 | while (defined($_ = <>)) {...} |
a0d0e21e LW |
134 | |
135 | /^Subject:/ | |
136 | $_ =~ /^Subject:/ | |
137 | ||
138 | tr/a-z/A-Z/ | |
139 | $_ =~ tr/a-z/A-Z/ | |
140 | ||
19799a22 GS |
141 | chomp |
142 | chomp($_) | |
a0d0e21e | 143 | |
54310121 | 144 | Here are the places where Perl will assume $_ even if you |
cb1a09d0 AD |
145 | don't use it: |
146 | ||
147 | =over 3 | |
148 | ||
149 | =item * | |
150 | ||
db1511c8 GS |
151 | The following functions: |
152 | ||
b0169937 GS |
153 | abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, exp, glob, |
154 | hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, | |
155 | quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only), | |
156 | rmdir, sin, split (on its second argument), sqrt, stat, study, uc, ucfirst, | |
157 | unlink, unpack. | |
cb1a09d0 AD |
158 | |
159 | =item * | |
160 | ||
db1511c8 GS |
161 | All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN. |
162 | See L<perlfunc/-X> | |
163 | ||
cb1a09d0 AD |
164 | |
165 | =item * | |
166 | ||
b0169937 GS |
167 | The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>) |
168 | when used without an C<=~> operator. | |
cb1a09d0 | 169 | |
54310121 | 170 | =item * |
cb1a09d0 AD |
171 | |
172 | The default iterator variable in a C<foreach> loop if no other | |
173 | variable is supplied. | |
174 | ||
54310121 | 175 | =item * |
cb1a09d0 AD |
176 | |
177 | The implicit iterator variable in the grep() and map() functions. | |
178 | ||
54310121 | 179 | =item * |
cb1a09d0 | 180 | |
db1511c8 GS |
181 | The implicit variable of given(). |
182 | ||
183 | =item * | |
184 | ||
c47ff5f1 | 185 | The default place to put an input record when a C<< <FH> >> |
cb1a09d0 | 186 | operation's result is tested by itself as the sole criterion of a C<while> |
14218588 | 187 | test. Outside a C<while> test, this will not happen. |
cb1a09d0 AD |
188 | |
189 | =back | |
190 | ||
59f00321 RGS |
191 | As C<$_> is a global variable, this may lead in some cases to unwanted |
192 | side-effects. As of perl 5.9.1, you can now use a lexical version of | |
193 | C<$_> by declaring it in a file or in a block with C<my>. Moreover, | |
4fd88bf8 | 194 | declaring C<our $_> restores the global C<$_> in the current scope. |
59f00321 | 195 | |
a0d0e21e LW |
196 | (Mnemonic: underline is understood in certain operations.) |
197 | ||
6e2995f4 | 198 | =back |
199 | ||
200 | =over 8 | |
201 | ||
a1db74c9 JH |
202 | =item $a |
203 | ||
204 | =item $b | |
a054c801 | 205 | X<$a> X<$b> |
a1db74c9 JH |
206 | |
207 | Special package variables when using sort(), see L<perlfunc/sort>. | |
208 | Because of this specialness $a and $b don't need to be declared | |
f83912f2 JW |
209 | (using use vars, or our()) even when using the C<strict 'vars'> pragma. |
210 | Don't lexicalize them with C<my $a> or C<my $b> if you want to be | |
211 | able to use them in the sort() comparison block or function. | |
a1db74c9 JH |
212 | |
213 | =back | |
214 | ||
215 | =over 8 | |
216 | ||
8491f48b | 217 | =item $<I<digits>> ($1, $2, ...) |
a054c801 | 218 | X<$1> X<$2> X<$3> |
a0d0e21e | 219 | |
19799a22 | 220 | Contains the subpattern from the corresponding set of capturing |
8491f48b | 221 | parentheses from the last successful pattern match, not counting patterns |
19799a22 GS |
222 | matched in nested blocks that have been exited already. (Mnemonic: |
223 | like \digits.) These variables are all read-only and dynamically | |
224 | scoped to the current BLOCK. | |
a0d0e21e LW |
225 | |
226 | =item $MATCH | |
227 | ||
228 | =item $& | |
a054c801 | 229 | X<$&> X<$MATCH> |
a0d0e21e LW |
230 | |
231 | The string matched by the last successful pattern match (not counting | |
232 | any matches hidden within a BLOCK or eval() enclosed by the current | |
19799a22 GS |
233 | BLOCK). (Mnemonic: like & in some editors.) This variable is read-only |
234 | and dynamically scoped to the current BLOCK. | |
a0d0e21e | 235 | |
19ddd453 | 236 | The use of this variable anywhere in a program imposes a considerable |
667e1aea | 237 | performance penalty on all regular expression matches. See L</BUGS>. |
19ddd453 | 238 | |
a054c801 GS |
239 | See L</@-> for a replacement. |
240 | ||
cde0cee5 YO |
241 | =item ${^MATCH} |
242 | X<${^MATCH}> | |
243 | ||
6210e03f | 244 | This is similar to C<$&> (C<$MATCH>) except that it does not incur the |
cde0cee5 YO |
245 | performance penalty associated with that variable, and is only guaranteed |
246 | to return a defined value when the pattern was compiled or executed with | |
87e95b7f | 247 | the C</p> modifier. |
cde0cee5 | 248 | |
a0d0e21e LW |
249 | =item $PREMATCH |
250 | ||
251 | =item $` | |
a054c801 | 252 | X<$`> X<$PREMATCH> |
a0d0e21e LW |
253 | |
254 | The string preceding whatever was matched by the last successful | |
255 | pattern match (not counting any matches hidden within a BLOCK or eval | |
a8f8344d | 256 | enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted |
a0d0e21e LW |
257 | string.) This variable is read-only. |
258 | ||
19ddd453 | 259 | The use of this variable anywhere in a program imposes a considerable |
667e1aea | 260 | performance penalty on all regular expression matches. See L</BUGS>. |
19ddd453 | 261 | |
a054c801 GS |
262 | See L</@-> for a replacement. |
263 | ||
cde0cee5 YO |
264 | =item ${^PREMATCH} |
265 | X<${^PREMATCH}> | |
266 | ||
267 | This is similar to C<$`> ($PREMATCH) except that it does not incur the | |
268 | performance penalty associated with that variable, and is only guaranteed | |
269 | to return a defined value when the pattern was compiled or executed with | |
87e95b7f | 270 | the C</p> modifier. |
cde0cee5 | 271 | |
a0d0e21e LW |
272 | =item $POSTMATCH |
273 | ||
274 | =item $' | |
a054c801 | 275 | X<$'> X<$POSTMATCH> |
a0d0e21e LW |
276 | |
277 | The string following whatever was matched by the last successful | |
278 | pattern match (not counting any matches hidden within a BLOCK or eval() | |
a8f8344d | 279 | enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted |
a0d0e21e LW |
280 | string.) Example: |
281 | ||
22d0716c | 282 | local $_ = 'abcdefghi'; |
a0d0e21e LW |
283 | /def/; |
284 | print "$`:$&:$'\n"; # prints abc:def:ghi | |
285 | ||
19799a22 | 286 | This variable is read-only and dynamically scoped to the current BLOCK. |
a0d0e21e | 287 | |
19ddd453 | 288 | The use of this variable anywhere in a program imposes a considerable |
667e1aea | 289 | performance penalty on all regular expression matches. See L</BUGS>. |
19ddd453 | 290 | |
a054c801 GS |
291 | See L</@-> for a replacement. |
292 | ||
cde0cee5 YO |
293 | =item ${^POSTMATCH} |
294 | X<${^POSTMATCH}> | |
295 | ||
296 | This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the | |
297 | performance penalty associated with that variable, and is only guaranteed | |
298 | to return a defined value when the pattern was compiled or executed with | |
87e95b7f | 299 | the C</p> modifier. |
cde0cee5 | 300 | |
a0d0e21e LW |
301 | =item $LAST_PAREN_MATCH |
302 | ||
303 | =item $+ | |
a054c801 | 304 | X<$+> X<$LAST_PAREN_MATCH> |
a0d0e21e | 305 | |
a01268b5 JH |
306 | The text matched by the last bracket of the last successful search pattern. |
307 | This is useful if you don't know which one of a set of alternative patterns | |
308 | matched. For example: | |
a0d0e21e LW |
309 | |
310 | /Version: (.*)|Revision: (.*)/ && ($rev = $+); | |
311 | ||
312 | (Mnemonic: be positive and forward looking.) | |
19799a22 | 313 | This variable is read-only and dynamically scoped to the current BLOCK. |
a0d0e21e | 314 | |
daaddde1 MS |
315 | =item $LAST_SUBMATCH_RESULT |
316 | ||
a01268b5 | 317 | =item $^N |
a054c801 | 318 | X<$^N> |
a01268b5 JH |
319 | |
320 | The text matched by the used group most-recently closed (i.e. the group | |
321 | with the rightmost closing parenthesis) of the last successful search | |
ad83b128 PN |
322 | pattern. (Mnemonic: the (possibly) Nested parenthesis that most |
323 | recently closed.) | |
324 | ||
210b36aa | 325 | This is primarily used inside C<(?{...})> blocks for examining text |
a01268b5 JH |
326 | recently matched. For example, to effectively capture text to a variable |
327 | (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with | |
328 | ||
329 | (?:(...)(?{ $var = $^N })) | |
330 | ||
331 | By setting and then using C<$var> in this way relieves you from having to | |
332 | worry about exactly which numbered set of parentheses they are. | |
333 | ||
334 | This variable is dynamically scoped to the current BLOCK. | |
335 | ||
fe307981 GS |
336 | =item @LAST_MATCH_END |
337 | ||
6cef1e77 | 338 | =item @+ |
a054c801 | 339 | X<@+> X<@LAST_MATCH_END> |
6cef1e77 | 340 | |
4ba05bdc GS |
341 | This array holds the offsets of the ends of the last successful |
342 | submatches in the currently active dynamic scope. C<$+[0]> is | |
343 | the offset into the string of the end of the entire match. This | |
344 | is the same value as what the C<pos> function returns when called | |
345 | on the variable that was matched against. The I<n>th element | |
346 | of this array holds the offset of the I<n>th submatch, so | |
347 | C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset | |
348 | past where $2 ends, and so on. You can use C<$#+> to determine | |
349 | how many subgroups were in the last successful match. See the | |
350 | examples given for the C<@-> variable. | |
6cef1e77 | 351 | |
43239f90 | 352 | =item %LAST_PAREN_MATCH |
8e08999f | 353 | |
81714fb9 YO |
354 | =item %+ |
355 | X<%+> | |
356 | ||
357 | Similar to C<@+>, the C<%+> hash allows access to the named capture | |
358 | buffers, should they exist, in the last successful match in the | |
359 | currently active dynamic scope. | |
360 | ||
3195cf34 | 361 | For example, C<$+{foo}> is equivalent to C<$1> after the following match: |
81714fb9 | 362 | |
3195cf34 | 363 | 'foo' =~ /(?<foo>foo)/; |
81714fb9 | 364 | |
3195cf34 RGS |
365 | The keys of the C<%+> hash list only the names of buffers that have |
366 | captured (and that are thus associated to defined values). | |
44a2ac75 | 367 | |
3195cf34 | 368 | The underlying behaviour of C<%+> is provided by the |
80305961 | 369 | L<Tie::Hash::NamedCapture> module. |
3195cf34 RGS |
370 | |
371 | B<Note:> C<%-> and C<%+> are tied views into a common internal hash | |
44a2ac75 YO |
372 | associated with the last successful regular expression. Therefore mixing |
373 | iterative access to them via C<each> may have unpredictable results. | |
3195cf34 | 374 | Likewise, if the last successful match changes, then the results may be |
44a2ac75 YO |
375 | surprising. |
376 | ||
fcc7d916 | 377 | =item HANDLE->input_line_number(EXPR) |
a0d0e21e LW |
378 | |
379 | =item $INPUT_LINE_NUMBER | |
380 | ||
381 | =item $NR | |
382 | ||
383 | =item $. | |
a054c801 | 384 | X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number> |
a0d0e21e | 385 | |
81714fb9 | 386 | Current line number for the last filehandle accessed. |
fcc7d916 IK |
387 | |
388 | Each filehandle in Perl counts the number of lines that have been read | |
389 | from it. (Depending on the value of C<$/>, Perl's idea of what | |
390 | constitutes a line may not match yours.) When a line is read from a | |
391 | filehandle (via readline() or C<< <> >>), or when tell() or seek() is | |
392 | called on it, C<$.> becomes an alias to the line counter for that | |
393 | filehandle. | |
394 | ||
395 | You can adjust the counter by assigning to C<$.>, but this will not | |
396 | actually move the seek pointer. I<Localizing C<$.> will not localize | |
397 | the filehandle's line count>. Instead, it will localize perl's notion | |
398 | of which filehandle C<$.> is currently aliased to. | |
399 | ||
400 | C<$.> is reset when the filehandle is closed, but B<not> when an open | |
401 | filehandle is reopened without an intervening close(). For more | |
e48df184 | 402 | details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does |
fcc7d916 IK |
403 | an explicit close, line numbers increase across ARGV files (but see |
404 | examples in L<perlfunc/eof>). | |
405 | ||
406 | You can also use C<< HANDLE->input_line_number(EXPR) >> to access the | |
407 | line counter for a given filehandle without having to worry about | |
408 | which handle you last accessed. | |
409 | ||
410 | (Mnemonic: many programs use "." to mean the current line number.) | |
411 | ||
412 | =item IO::Handle->input_record_separator(EXPR) | |
a0d0e21e LW |
413 | |
414 | =item $INPUT_RECORD_SEPARATOR | |
415 | ||
416 | =item $RS | |
417 | ||
418 | =item $/ | |
a054c801 | 419 | X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR> |
a0d0e21e | 420 | |
14218588 GS |
421 | The input record separator, newline by default. This |
422 | influences Perl's idea of what a "line" is. Works like B<awk>'s RS | |
19799a22 | 423 | variable, including treating empty lines as a terminator if set to |
14218588 GS |
424 | the null string. (An empty line cannot contain any spaces |
425 | or tabs.) You may set it to a multi-character string to match a | |
19799a22 GS |
426 | multi-character terminator, or to C<undef> to read through the end |
427 | of file. Setting it to C<"\n\n"> means something slightly | |
428 | different than setting to C<"">, if the file contains consecutive | |
429 | empty lines. Setting to C<""> will treat two or more consecutive | |
430 | empty lines as a single empty line. Setting to C<"\n\n"> will | |
431 | blindly assume that the next input character belongs to the next | |
14218588 | 432 | paragraph, even if it's a newline. (Mnemonic: / delimits |
19799a22 | 433 | line boundaries when quoting poetry.) |
a0d0e21e | 434 | |
22d0716c SB |
435 | local $/; # enable "slurp" mode |
436 | local $_ = <FH>; # whole file now here | |
a0d0e21e LW |
437 | s/\n[ \t]+/ /g; |
438 | ||
19799a22 GS |
439 | Remember: the value of C<$/> is a string, not a regex. B<awk> has to be |
440 | better for something. :-) | |
68dc0745 | 441 | |
19799a22 GS |
442 | Setting C<$/> to a reference to an integer, scalar containing an integer, or |
443 | scalar that's convertible to an integer will attempt to read records | |
5b2b9c68 | 444 | instead of lines, with the maximum record size being the referenced |
19799a22 | 445 | integer. So this: |
5b2b9c68 | 446 | |
22d0716c | 447 | local $/ = \32768; # or \"32768", or \$var_containing_32768 |
b0169937 | 448 | open my $fh, "<", $myfile or die $!; |
22d0716c | 449 | local $_ = <$fh>; |
5b2b9c68 | 450 | |
19799a22 GS |
451 | will read a record of no more than 32768 bytes from FILE. If you're |
452 | not reading from a record-oriented file (or your OS doesn't have | |
453 | record-oriented files), then you'll likely get a full chunk of data | |
454 | with every read. If a record is larger than the record size you've | |
acbd132f JH |
455 | set, you'll get the record back in pieces. Trying to set the record |
456 | size to zero or less will cause reading in the (rest of the) whole file. | |
5b2b9c68 | 457 | |
19799a22 GS |
458 | On VMS, record reads are done with the equivalent of C<sysread>, |
459 | so it's best not to mix record and non-record reads on the same | |
460 | file. (This is unlikely to be a problem, because any file you'd | |
83763826 | 461 | want to read in record mode is probably unusable in line mode.) |
14218588 | 462 | Non-VMS systems do normal I/O, so it's safe to mix record and |
19799a22 | 463 | non-record reads of a file. |
5b2b9c68 | 464 | |
14218588 | 465 | See also L<perlport/"Newlines">. Also see C<$.>. |
883faa13 | 466 | |
fcc7d916 | 467 | =item HANDLE->autoflush(EXPR) |
a0d0e21e LW |
468 | |
469 | =item $OUTPUT_AUTOFLUSH | |
470 | ||
471 | =item $| | |
a054c801 | 472 | X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH> |
a0d0e21e | 473 | |
19799a22 GS |
474 | If set to nonzero, forces a flush right away and after every write |
475 | or print on the currently selected output channel. Default is 0 | |
14218588 | 476 | (regardless of whether the channel is really buffered by the |
19799a22 GS |
477 | system or not; C<$|> tells you only whether you've asked Perl |
478 | explicitly to flush after each write). STDOUT will | |
479 | typically be line buffered if output is to the terminal and block | |
480 | buffered otherwise. Setting this variable is useful primarily when | |
481 | you are outputting to a pipe or socket, such as when you are running | |
482 | a Perl program under B<rsh> and want to see the output as it's | |
483 | happening. This has no effect on input buffering. See L<perlfunc/getc> | |
b0169937 GS |
484 | for that. See L<perldoc/select> on how to select the output channel. |
485 | See also L<IO::Handle>. (Mnemonic: when you want your pipes to be piping hot.) | |
a0d0e21e | 486 | |
46550894 | 487 | =item IO::Handle->output_field_separator EXPR |
a0d0e21e LW |
488 | |
489 | =item $OUTPUT_FIELD_SEPARATOR | |
490 | ||
491 | =item $OFS | |
492 | ||
493 | =item $, | |
a054c801 | 494 | X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR> |
a0d0e21e | 495 | |
d6584ed8 XN |
496 | The output field separator for the print operator. If defined, this |
497 | value is printed between each of print's arguments. Default is C<undef>. | |
498 | (Mnemonic: what is printed when there is a "," in your print statement.) | |
a0d0e21e | 499 | |
46550894 | 500 | =item IO::Handle->output_record_separator EXPR |
a0d0e21e LW |
501 | |
502 | =item $OUTPUT_RECORD_SEPARATOR | |
503 | ||
504 | =item $ORS | |
505 | ||
506 | =item $\ | |
a054c801 | 507 | X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR> |
a0d0e21e | 508 | |
d6584ed8 XN |
509 | The output record separator for the print operator. If defined, this |
510 | value is printed after the last of print's arguments. Default is C<undef>. | |
511 | (Mnemonic: you set C<$\> instead of adding "\n" at the end of the print. | |
512 | Also, it's just like C<$/>, but it's what you get "back" from Perl.) | |
a0d0e21e LW |
513 | |
514 | =item $LIST_SEPARATOR | |
515 | ||
516 | =item $" | |
a054c801 | 517 | X<$"> X<$LIST_SEPARATOR> |
a0d0e21e | 518 | |
19799a22 GS |
519 | This is like C<$,> except that it applies to array and slice values |
520 | interpolated into a double-quoted string (or similar interpreted | |
521 | string). Default is a space. (Mnemonic: obvious, I think.) | |
a0d0e21e LW |
522 | |
523 | =item $SUBSCRIPT_SEPARATOR | |
524 | ||
525 | =item $SUBSEP | |
526 | ||
527 | =item $; | |
a054c801 | 528 | X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR> |
a0d0e21e | 529 | |
54310121 | 530 | The subscript separator for multidimensional array emulation. If you |
a0d0e21e LW |
531 | refer to a hash element as |
532 | ||
533 | $foo{$a,$b,$c} | |
534 | ||
535 | it really means | |
536 | ||
537 | $foo{join($;, $a, $b, $c)} | |
538 | ||
539 | But don't put | |
540 | ||
541 | @foo{$a,$b,$c} # a slice--note the @ | |
542 | ||
543 | which means | |
544 | ||
545 | ($foo{$a},$foo{$b},$foo{$c}) | |
546 | ||
19799a22 GS |
547 | Default is "\034", the same as SUBSEP in B<awk>. If your |
548 | keys contain binary data there might not be any safe value for C<$;>. | |
a0d0e21e | 549 | (Mnemonic: comma (the syntactic subscript separator) is a |
19799a22 | 550 | semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already |
a0d0e21e LW |
551 | taken for something more important.) |
552 | ||
19799a22 GS |
553 | Consider using "real" multidimensional arrays as described |
554 | in L<perllol>. | |
a0d0e21e | 555 | |
fcc7d916 | 556 | =item HANDLE->format_page_number(EXPR) |
a0d0e21e LW |
557 | |
558 | =item $FORMAT_PAGE_NUMBER | |
559 | ||
560 | =item $% | |
a054c801 | 561 | X<$%> X<$FORMAT_PAGE_NUMBER> |
a0d0e21e LW |
562 | |
563 | The current page number of the currently selected output channel. | |
19799a22 | 564 | Used with formats. |
a0d0e21e LW |
565 | (Mnemonic: % is page number in B<nroff>.) |
566 | ||
fcc7d916 | 567 | =item HANDLE->format_lines_per_page(EXPR) |
a0d0e21e LW |
568 | |
569 | =item $FORMAT_LINES_PER_PAGE | |
570 | ||
571 | =item $= | |
a054c801 | 572 | X<$=> X<$FORMAT_LINES_PER_PAGE> |
a0d0e21e LW |
573 | |
574 | The current page length (printable lines) of the currently selected | |
19799a22 GS |
575 | output channel. Default is 60. |
576 | Used with formats. | |
577 | (Mnemonic: = has horizontal lines.) | |
a0d0e21e | 578 | |
fcc7d916 | 579 | =item HANDLE->format_lines_left(EXPR) |
a0d0e21e LW |
580 | |
581 | =item $FORMAT_LINES_LEFT | |
582 | ||
583 | =item $- | |
a054c801 | 584 | X<$-> X<$FORMAT_LINES_LEFT> |
a0d0e21e LW |
585 | |
586 | The number of lines left on the page of the currently selected output | |
19799a22 GS |
587 | channel. |
588 | Used with formats. | |
589 | (Mnemonic: lines_on_page - lines_printed.) | |
a0d0e21e | 590 | |
fe307981 GS |
591 | =item @LAST_MATCH_START |
592 | ||
6cef1e77 | 593 | =item @- |
a054c801 | 594 | X<@-> X<@LAST_MATCH_START> |
6cef1e77 | 595 | |
19799a22 | 596 | $-[0] is the offset of the start of the last successful match. |
6cef1e77 | 597 | C<$-[>I<n>C<]> is the offset of the start of the substring matched by |
8f580fb8 | 598 | I<n>-th subpattern, or undef if the subpattern did not match. |
6cef1e77 IZ |
599 | |
600 | Thus after a match against $_, $& coincides with C<substr $_, $-[0], | |
5060ef7b RGS |
601 | $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n], |
602 | $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with | |
603 | C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last | |
14218588 GS |
604 | matched subgroup in the last successful match. Contrast with |
605 | C<$#+>, the number of subgroups in the regular expression. Compare | |
19799a22 | 606 | with C<@+>. |
6cef1e77 | 607 | |
4ba05bdc GS |
608 | This array holds the offsets of the beginnings of the last |
609 | successful submatches in the currently active dynamic scope. | |
610 | C<$-[0]> is the offset into the string of the beginning of the | |
611 | entire match. The I<n>th element of this array holds the offset | |
0926d669 JP |
612 | of the I<n>th submatch, so C<$-[1]> is the offset where $1 |
613 | begins, C<$-[2]> the offset where $2 begins, and so on. | |
4ba05bdc GS |
614 | |
615 | After a match against some variable $var: | |
616 | ||
617 | =over 5 | |
618 | ||
4375e838 | 619 | =item C<$`> is the same as C<substr($var, 0, $-[0])> |
4ba05bdc | 620 | |
4375e838 | 621 | =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])> |
4ba05bdc | 622 | |
4375e838 | 623 | =item C<$'> is the same as C<substr($var, $+[0])> |
4ba05bdc GS |
624 | |
625 | =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])> | |
626 | ||
627 | =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])> | |
628 | ||
80dc6883 | 629 | =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])> |
4ba05bdc GS |
630 | |
631 | =back | |
632 | ||
44a2ac75 YO |
633 | =item %- |
634 | X<%-> | |
635 | ||
3195cf34 RGS |
636 | Similar to C<%+>, this variable allows access to the named capture buffers |
637 | in the last successful match in the currently active dynamic scope. To | |
638 | each capture buffer name found in the regular expression, it associates a | |
639 | reference to an array containing the list of values captured by all | |
640 | buffers with that name (should there be several of them), in the order | |
641 | where they appear. | |
642 | ||
643 | Here's an example: | |
44a2ac75 | 644 | |
3195cf34 RGS |
645 | if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) { |
646 | foreach my $bufname (sort keys %-) { | |
647 | my $ary = $-{$bufname}; | |
44a2ac75 | 648 | foreach my $idx (0..$#$ary) { |
3195cf34 | 649 | print "\$-{$bufname}[$idx] : ", |
44a2ac75 YO |
650 | (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"), |
651 | "\n"; | |
652 | } | |
653 | } | |
654 | } | |
655 | ||
656 | would print out: | |
657 | ||
658 | $-{A}[0] : '1' | |
659 | $-{A}[1] : '3' | |
660 | $-{B}[0] : '2' | |
661 | $-{B}[1] : '4' | |
662 | ||
3195cf34 RGS |
663 | The keys of the C<%-> hash correspond to all buffer names found in |
664 | the regular expression. | |
665 | ||
666 | The behaviour of C<%-> is implemented via the | |
80305961 | 667 | L<Tie::Hash::NamedCapture> module. |
44a2ac75 | 668 | |
3195cf34 | 669 | B<Note:> C<%-> and C<%+> are tied views into a common internal hash |
44a2ac75 YO |
670 | associated with the last successful regular expression. Therefore mixing |
671 | iterative access to them via C<each> may have unpredictable results. | |
3195cf34 | 672 | Likewise, if the last successful match changes, then the results may be |
44a2ac75 YO |
673 | surprising. |
674 | ||
fcc7d916 | 675 | =item HANDLE->format_name(EXPR) |
a0d0e21e LW |
676 | |
677 | =item $FORMAT_NAME | |
678 | ||
679 | =item $~ | |
a054c801 | 680 | X<$~> X<$FORMAT_NAME> |
a0d0e21e LW |
681 | |
682 | The name of the current report format for the currently selected output | |
14218588 | 683 | channel. Default is the name of the filehandle. (Mnemonic: brother to |
19799a22 | 684 | C<$^>.) |
a0d0e21e | 685 | |
fcc7d916 | 686 | =item HANDLE->format_top_name(EXPR) |
a0d0e21e LW |
687 | |
688 | =item $FORMAT_TOP_NAME | |
689 | ||
690 | =item $^ | |
a054c801 | 691 | X<$^> X<$FORMAT_TOP_NAME> |
a0d0e21e LW |
692 | |
693 | The name of the current top-of-page format for the currently selected | |
14218588 | 694 | output channel. Default is the name of the filehandle with _TOP |
a0d0e21e LW |
695 | appended. (Mnemonic: points to top of page.) |
696 | ||
46550894 | 697 | =item IO::Handle->format_line_break_characters EXPR |
a0d0e21e LW |
698 | |
699 | =item $FORMAT_LINE_BREAK_CHARACTERS | |
700 | ||
701 | =item $: | |
a054c801 | 702 | X<$:> X<FORMAT_LINE_BREAK_CHARACTERS> |
a0d0e21e LW |
703 | |
704 | The current set of characters after which a string may be broken to | |
54310121 | 705 | fill continuation fields (starting with ^) in a format. Default is |
a0d0e21e LW |
706 | S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in |
707 | poetry is a part of a line.) | |
708 | ||
46550894 | 709 | =item IO::Handle->format_formfeed EXPR |
a0d0e21e LW |
710 | |
711 | =item $FORMAT_FORMFEED | |
712 | ||
713 | =item $^L | |
a054c801 | 714 | X<$^L> X<$FORMAT_FORMFEED> |
a0d0e21e | 715 | |
14218588 | 716 | What formats output as a form feed. Default is \f. |
a0d0e21e LW |
717 | |
718 | =item $ACCUMULATOR | |
719 | ||
720 | =item $^A | |
a054c801 | 721 | X<$^A> X<$ACCUMULATOR> |
a0d0e21e LW |
722 | |
723 | The current value of the write() accumulator for format() lines. A format | |
19799a22 | 724 | contains formline() calls that put their result into C<$^A>. After |
a0d0e21e | 725 | calling its format, write() prints out the contents of C<$^A> and empties. |
14218588 | 726 | So you never really see the contents of C<$^A> unless you call |
a0d0e21e LW |
727 | formline() yourself and then look at it. See L<perlform> and |
728 | L<perlfunc/formline()>. | |
729 | ||
730 | =item $CHILD_ERROR | |
731 | ||
732 | =item $? | |
a054c801 | 733 | X<$?> X<$CHILD_ERROR> |
a0d0e21e | 734 | |
54310121 | 735 | The status returned by the last pipe close, backtick (C<``>) command, |
19799a22 GS |
736 | successful call to wait() or waitpid(), or from the system() |
737 | operator. This is just the 16-bit status word returned by the | |
e5218da5 | 738 | traditional Unix wait() system call (or else is made up to look like it). Thus, the |
c47ff5f1 | 739 | exit value of the subprocess is really (C<<< $? >> 8 >>>), and |
19799a22 GS |
740 | C<$? & 127> gives which signal, if any, the process died from, and |
741 | C<$? & 128> reports whether there was a core dump. (Mnemonic: | |
742 | similar to B<sh> and B<ksh>.) | |
a0d0e21e | 743 | |
7b8d334a | 744 | Additionally, if the C<h_errno> variable is supported in C, its value |
14218588 | 745 | is returned via $? if any C<gethost*()> function fails. |
7b8d334a | 746 | |
19799a22 | 747 | If you have installed a signal handler for C<SIGCHLD>, the |
aa689395 | 748 | value of C<$?> will usually be wrong outside that handler. |
749 | ||
a8f8344d | 750 | Inside an C<END> subroutine C<$?> contains the value that is going to be |
751 | given to C<exit()>. You can modify C<$?> in an C<END> subroutine to | |
19799a22 GS |
752 | change the exit status of your program. For example: |
753 | ||
754 | END { | |
755 | $? = 1 if $? == 255; # die would make it 255 | |
756 | } | |
a8f8344d | 757 | |
aa689395 | 758 | Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the |
ff0cee69 | 759 | actual VMS exit status, instead of the default emulation of POSIX |
9bc98430 | 760 | status; see L<perlvms/$?> for details. |
f86702cc | 761 | |
55602bd2 IZ |
762 | Also see L<Error Indicators>. |
763 | ||
e5218da5 | 764 | =item ${^CHILD_ERROR_NATIVE} |
a054c801 | 765 | X<$^CHILD_ERROR_NATIVE> |
e5218da5 GA |
766 | |
767 | The native status returned by the last pipe close, backtick (C<``>) | |
768 | command, successful call to wait() or waitpid(), or from the system() | |
769 | operator. On POSIX-like systems this value can be decoded with the | |
770 | WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, WSTOPSIG | |
771 | and WIFCONTINUED functions provided by the L<POSIX> module. | |
772 | ||
773 | Under VMS this reflects the actual VMS exit status; i.e. it is the same | |
774 | as $? when the pragma C<use vmsish 'status'> is in effect. | |
775 | ||
0a378802 | 776 | =item ${^ENCODING} |
a054c801 | 777 | X<$^ENCODING> |
0a378802 | 778 | |
740bd165 PN |
779 | The I<object reference> to the Encode object that is used to convert |
780 | the source code to Unicode. Thanks to this variable your perl script | |
781 | does not have to be written in UTF-8. Default is I<undef>. The direct | |
749e08de | 782 | manipulation of this variable is highly discouraged. |
0a378802 | 783 | |
a0d0e21e LW |
784 | =item $OS_ERROR |
785 | ||
786 | =item $ERRNO | |
787 | ||
788 | =item $! | |
a054c801 | 789 | X<$!> X<$ERRNO> X<$OS_ERROR> |
a0d0e21e | 790 | |
19799a22 | 791 | If used numerically, yields the current value of the C C<errno> |
6ab308ee JH |
792 | variable, or in other words, if a system or library call fails, it |
793 | sets this variable. This means that the value of C<$!> is meaningful | |
794 | only I<immediately> after a B<failure>: | |
795 | ||
b0169937 | 796 | if (open my $fh, "<", $filename) { |
6ab308ee JH |
797 | # Here $! is meaningless. |
798 | ... | |
799 | } else { | |
800 | # ONLY here is $! meaningful. | |
801 | ... | |
802 | # Already here $! might be meaningless. | |
803 | } | |
804 | # Since here we might have either success or failure, | |
805 | # here $! is meaningless. | |
806 | ||
807 | In the above I<meaningless> stands for anything: zero, non-zero, | |
808 | C<undef>. A successful system or library call does B<not> set | |
809 | the variable to zero. | |
810 | ||
271df126 | 811 | If used as a string, yields the corresponding system error string. |
19799a22 GS |
812 | You can assign a number to C<$!> to set I<errno> if, for instance, |
813 | you want C<"$!"> to return the string for error I<n>, or you want | |
814 | to set the exit value for the die() operator. (Mnemonic: What just | |
815 | went bang?) | |
a0d0e21e | 816 | |
55602bd2 IZ |
817 | Also see L<Error Indicators>. |
818 | ||
daaddde1 MS |
819 | =item %OS_ERROR |
820 | ||
821 | =item %ERRNO | |
822 | ||
4c5cef9b | 823 | =item %! |
a054c801 | 824 | X<%!> |
4c5cef9b MJD |
825 | |
826 | Each element of C<%!> has a true value only if C<$!> is set to that | |
827 | value. For example, C<$!{ENOENT}> is true if and only if the current | |
3be065a1 JH |
828 | value of C<$!> is C<ENOENT>; that is, if the most recent error was |
829 | "No such file or directory" (or its moral equivalent: not all operating | |
830 | systems give that exact error, and certainly not all languages). | |
831 | To check if a particular key is meaningful on your system, use | |
832 | C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>. | |
833 | See L<Errno> for more information, and also see above for the | |
834 | validity of C<$!>. | |
4c5cef9b | 835 | |
5c055ba3 | 836 | =item $EXTENDED_OS_ERROR |
837 | ||
838 | =item $^E | |
a054c801 | 839 | X<$^E> X<$EXTENDED_OS_ERROR> |
5c055ba3 | 840 | |
22fae026 TM |
841 | Error information specific to the current operating system. At |
842 | the moment, this differs from C<$!> under only VMS, OS/2, and Win32 | |
843 | (and for MacPerl). On all other platforms, C<$^E> is always just | |
844 | the same as C<$!>. | |
845 | ||
846 | Under VMS, C<$^E> provides the VMS status value from the last | |
847 | system error. This is more specific information about the last | |
848 | system error than that provided by C<$!>. This is particularly | |
d516a115 | 849 | important when C<$!> is set to B<EVMSERR>. |
22fae026 | 850 | |
1c1c7f20 GS |
851 | Under OS/2, C<$^E> is set to the error code of the last call to |
852 | OS/2 API either via CRT, or directly from perl. | |
22fae026 TM |
853 | |
854 | Under Win32, C<$^E> always returns the last error information | |
855 | reported by the Win32 call C<GetLastError()> which describes | |
856 | the last error from within the Win32 API. Most Win32-specific | |
19799a22 | 857 | code will report errors via C<$^E>. ANSI C and Unix-like calls |
22fae026 TM |
858 | set C<errno> and so most portable Perl code will report errors |
859 | via C<$!>. | |
860 | ||
861 | Caveats mentioned in the description of C<$!> generally apply to | |
862 | C<$^E>, also. (Mnemonic: Extra error explanation.) | |
5c055ba3 | 863 | |
55602bd2 IZ |
864 | Also see L<Error Indicators>. |
865 | ||
a0d0e21e LW |
866 | =item $EVAL_ERROR |
867 | ||
868 | =item $@ | |
a054c801 | 869 | X<$@> X<$EVAL_ERROR> |
a0d0e21e | 870 | |
4a280ebe JG |
871 | The Perl syntax error message from the last eval() operator. |
872 | If $@ is the null string, the last eval() parsed and executed | |
873 | correctly (although the operations you invoked may have failed in the | |
874 | normal fashion). (Mnemonic: Where was the syntax error "at"?) | |
a0d0e21e | 875 | |
19799a22 | 876 | Warning messages are not collected in this variable. You can, |
a8f8344d | 877 | however, set up a routine to process warnings by setting C<$SIG{__WARN__}> |
54310121 | 878 | as described below. |
748a9306 | 879 | |
55602bd2 IZ |
880 | Also see L<Error Indicators>. |
881 | ||
a0d0e21e LW |
882 | =item $PROCESS_ID |
883 | ||
884 | =item $PID | |
885 | ||
886 | =item $$ | |
a054c801 | 887 | X<$$> X<$PID> X<$PROCESS_ID> |
a0d0e21e | 888 | |
19799a22 GS |
889 | The process number of the Perl running this script. You should |
890 | consider this variable read-only, although it will be altered | |
891 | across fork() calls. (Mnemonic: same as shells.) | |
a0d0e21e | 892 | |
4d76a344 RGS |
893 | Note for Linux users: on Linux, the C functions C<getpid()> and |
894 | C<getppid()> return different values from different threads. In order to | |
895 | be portable, this behavior is not reflected by C<$$>, whose value remains | |
896 | consistent across threads. If you want to call the underlying C<getpid()>, | |
e3256f86 | 897 | you may use the CPAN module C<Linux::Pid>. |
4d76a344 | 898 | |
a0d0e21e LW |
899 | =item $REAL_USER_ID |
900 | ||
901 | =item $UID | |
902 | ||
903 | =item $< | |
a054c801 | 904 | X<< $< >> X<$UID> X<$REAL_USER_ID> |
a0d0e21e | 905 | |
19799a22 | 906 | The real uid of this process. (Mnemonic: it's the uid you came I<from>, |
a043a685 | 907 | if you're running setuid.) You can change both the real uid and |
a537debe SP |
908 | the effective uid at the same time by using POSIX::setuid(). Since |
909 | changes to $< require a system call, check $! after a change attempt to | |
910 | detect any possible errors. | |
a0d0e21e LW |
911 | |
912 | =item $EFFECTIVE_USER_ID | |
913 | ||
914 | =item $EUID | |
915 | ||
916 | =item $> | |
a054c801 | 917 | X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID> |
a0d0e21e LW |
918 | |
919 | The effective uid of this process. Example: | |
920 | ||
921 | $< = $>; # set real to effective uid | |
922 | ($<,$>) = ($>,$<); # swap real and effective uid | |
923 | ||
a043a685 | 924 | You can change both the effective uid and the real uid at the same |
a537debe SP |
925 | time by using POSIX::setuid(). Changes to $> require a check to $! |
926 | to detect any possible errors after an attempted change. | |
a043a685 | 927 | |
19799a22 | 928 | (Mnemonic: it's the uid you went I<to>, if you're running setuid.) |
c47ff5f1 | 929 | C<< $< >> and C<< $> >> can be swapped only on machines |
8cc95fdb | 930 | supporting setreuid(). |
a0d0e21e LW |
931 | |
932 | =item $REAL_GROUP_ID | |
933 | ||
934 | =item $GID | |
935 | ||
936 | =item $( | |
a054c801 | 937 | X<$(> X<$GID> X<$REAL_GROUP_ID> |
a0d0e21e LW |
938 | |
939 | The real gid of this process. If you are on a machine that supports | |
940 | membership in multiple groups simultaneously, gives a space separated | |
941 | list of groups you are in. The first number is the one returned by | |
942 | getgid(), and the subsequent ones by getgroups(), one of which may be | |
8cc95fdb | 943 | the same as the first number. |
944 | ||
19799a22 GS |
945 | However, a value assigned to C<$(> must be a single number used to |
946 | set the real gid. So the value given by C<$(> should I<not> be assigned | |
2bc0d022 DL |
947 | back to C<$(> without being forced numeric, such as by adding zero. Note |
948 | that this is different to the effective gid (C<$)>) which does take a | |
949 | list. | |
8cc95fdb | 950 | |
a043a685 | 951 | You can change both the real gid and the effective gid at the same |
a537debe SP |
952 | time by using POSIX::setgid(). Changes to $( require a check to $! |
953 | to detect any possible errors after an attempted change. | |
a043a685 | 954 | |
19799a22 GS |
955 | (Mnemonic: parentheses are used to I<group> things. The real gid is the |
956 | group you I<left>, if you're running setgid.) | |
a0d0e21e LW |
957 | |
958 | =item $EFFECTIVE_GROUP_ID | |
959 | ||
960 | =item $EGID | |
961 | ||
962 | =item $) | |
a054c801 | 963 | X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID> |
a0d0e21e LW |
964 | |
965 | The effective gid of this process. If you are on a machine that | |
966 | supports membership in multiple groups simultaneously, gives a space | |
967 | separated list of groups you are in. The first number is the one | |
968 | returned by getegid(), and the subsequent ones by getgroups(), one of | |
8cc95fdb | 969 | which may be the same as the first number. |
970 | ||
19799a22 | 971 | Similarly, a value assigned to C<$)> must also be a space-separated |
14218588 | 972 | list of numbers. The first number sets the effective gid, and |
8cc95fdb | 973 | the rest (if any) are passed to setgroups(). To get the effect of an |
974 | empty list for setgroups(), just repeat the new effective gid; that is, | |
975 | to force an effective gid of 5 and an effectively empty setgroups() | |
976 | list, say C< $) = "5 5" >. | |
977 | ||
a043a685 GW |
978 | You can change both the effective gid and the real gid at the same |
979 | time by using POSIX::setgid() (use only a single numeric argument). | |
a537debe SP |
980 | Changes to $) require a check to $! to detect any possible errors |
981 | after an attempted change. | |
a043a685 | 982 | |
19799a22 GS |
983 | (Mnemonic: parentheses are used to I<group> things. The effective gid |
984 | is the group that's I<right> for you, if you're running setgid.) | |
a0d0e21e | 985 | |
c47ff5f1 | 986 | C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on |
19799a22 GS |
987 | machines that support the corresponding I<set[re][ug]id()> routine. C<$(> |
988 | and C<$)> can be swapped only on machines supporting setregid(). | |
a0d0e21e LW |
989 | |
990 | =item $PROGRAM_NAME | |
991 | ||
992 | =item $0 | |
a054c801 | 993 | X<$0> X<$PROGRAM_NAME> |
a0d0e21e | 994 | |
80bca1b4 JH |
995 | Contains the name of the program being executed. |
996 | ||
997 | On some (read: not all) operating systems assigning to C<$0> modifies | |
998 | the argument area that the C<ps> program sees. On some platforms you | |
999 | may have to use special C<ps> options or a different C<ps> to see the | |
1000 | changes. Modifying the $0 is more useful as a way of indicating the | |
1001 | current program state than it is for hiding the program you're | |
1002 | running. (Mnemonic: same as B<sh> and B<ksh>.) | |
f9cbb277 | 1003 | |
cf525c36 | 1004 | Note that there are platform specific limitations on the maximum |
f9cbb277 JH |
1005 | length of C<$0>. In the most extreme case it may be limited to the |
1006 | space occupied by the original C<$0>. | |
a0d0e21e | 1007 | |
80bca1b4 JH |
1008 | In some platforms there may be arbitrary amount of padding, for |
1009 | example space characters, after the modified name as shown by C<ps>. | |
dda345b7 | 1010 | In some platforms this padding may extend all the way to the original |
c80e2480 JH |
1011 | length of the argument area, no matter what you do (this is the case |
1012 | for example with Linux 2.2). | |
80bca1b4 | 1013 | |
4bc88a62 | 1014 | Note for BSD users: setting C<$0> does not completely remove "perl" |
6a4647a3 JH |
1015 | from the ps(1) output. For example, setting C<$0> to C<"foobar"> may |
1016 | result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix | |
1017 | and the " (perl)" suffix are shown depends on your exact BSD variant | |
1018 | and version). This is an operating system feature, Perl cannot help it. | |
4bc88a62 | 1019 | |
e2975953 JH |
1020 | In multithreaded scripts Perl coordinates the threads so that any |
1021 | thread may modify its copy of the C<$0> and the change becomes visible | |
cf525c36 | 1022 | to ps(1) (assuming the operating system plays along). Note that |
80bca1b4 JH |
1023 | the view of C<$0> the other threads have will not change since they |
1024 | have their own copies of it. | |
e2975953 | 1025 | |
52c447a8 RGS |
1026 | If the program has been given to perl via the switches C<-e> or C<-E>, |
1027 | C<$0> will contain the string C<"-e">. | |
1028 | ||
7636ea95 AB |
1029 | On Linux as of perl 5.14 the legacy process name will be set with |
1030 | L<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as | |
1031 | perl has done since version 4.000. Now system utilities that read the | |
1032 | legacy process name such as ps, top and killall will recognize the | |
1033 | name you set when assigning to C<$0>. The string you supply will be | |
1034 | cut off at 16 bytes, this is a limitation imposed by Linux. | |
1035 | ||
a0d0e21e | 1036 | =item $[ |
a054c801 | 1037 | X<$[> |
a0d0e21e LW |
1038 | |
1039 | The index of the first element in an array, and of the first character | |
19799a22 GS |
1040 | in a substring. Default is 0, but you could theoretically set it |
1041 | to 1 to make Perl behave more like B<awk> (or Fortran) when | |
1042 | subscripting and when evaluating the index() and substr() functions. | |
1043 | (Mnemonic: [ begins subscripts.) | |
a0d0e21e | 1044 | |
19799a22 GS |
1045 | As of release 5 of Perl, assignment to C<$[> is treated as a compiler |
1046 | directive, and cannot influence the behavior of any other file. | |
55b67815 | 1047 | (That's why you can only assign compile-time constants to it.) Its |
d1d15184 | 1048 | use is deprecated, and by default will trigger a warning. |
a0d0e21e | 1049 | |
f83ed198 | 1050 | Note that, unlike other compile-time directives (such as L<strict>), |
af7a0647 RGS |
1051 | assignment to C<$[> can be seen from outer lexical scopes in the same file. |
1052 | However, you can use local() on it to strictly bind its value to a | |
f83ed198 RGS |
1053 | lexical block. |
1054 | ||
a0d0e21e | 1055 | =item $] |
a054c801 | 1056 | X<$]> |
a0d0e21e | 1057 | |
54310121 | 1058 | The version + patchlevel / 1000 of the Perl interpreter. This variable |
1059 | can be used to determine whether the Perl interpreter executing a | |
1060 | script is in the right range of versions. (Mnemonic: Is this version | |
1061 | of perl in the right bracket?) Example: | |
a0d0e21e LW |
1062 | |
1063 | warn "No checksumming!\n" if $] < 3.019; | |
1064 | ||
54310121 | 1065 | See also the documentation of C<use VERSION> and C<require VERSION> |
19799a22 | 1066 | for a convenient way to fail if the running Perl interpreter is too old. |
a0d0e21e | 1067 | |
0c8d858b MS |
1068 | The floating point representation can sometimes lead to inaccurate |
1069 | numeric comparisons. See C<$^V> for a more modern representation of | |
1070 | the Perl version that allows accurate string comparisons. | |
16070b82 | 1071 | |
305aace0 NIS |
1072 | =item $COMPILING |
1073 | ||
1074 | =item $^C | |
a054c801 | 1075 | X<$^C> X<$COMPILING> |
305aace0 | 1076 | |
19799a22 GS |
1077 | The current value of the flag associated with the B<-c> switch. |
1078 | Mainly of use with B<-MO=...> to allow code to alter its behavior | |
1079 | when being compiled, such as for example to AUTOLOAD at compile | |
59f521f4 | 1080 | time rather than normal, deferred loading. Setting |
19799a22 | 1081 | C<$^C = 1> is similar to calling C<B::minus_c>. |
305aace0 | 1082 | |
a0d0e21e LW |
1083 | =item $DEBUGGING |
1084 | ||
1085 | =item $^D | |
a054c801 | 1086 | X<$^D> X<$DEBUGGING> |
a0d0e21e LW |
1087 | |
1088 | The current value of the debugging flags. (Mnemonic: value of B<-D> | |
b4ab917c DM |
1089 | switch.) May be read or set. Like its command-line equivalent, you can use |
1090 | numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">. | |
a0d0e21e | 1091 | |
a3621e74 YO |
1092 | =item ${^RE_DEBUG_FLAGS} |
1093 | ||
1094 | The current value of the regex debugging flags. Set to 0 for no debug output | |
1095 | even when the re 'debug' module is loaded. See L<re> for details. | |
1096 | ||
0111c4fd | 1097 | =item ${^RE_TRIE_MAXBUF} |
a3621e74 YO |
1098 | |
1099 | Controls how certain regex optimisations are applied and how much memory they | |
1100 | utilize. This value by default is 65536 which corresponds to a 512kB temporary | |
1101 | cache. Set this to a higher value to trade memory for speed when matching | |
1102 | large alternations. Set it to a lower value if you want the optimisations to | |
1103 | be as conservative of memory as possible but still occur, and set it to a | |
1104 | negative value to prevent the optimisation and conserve the most memory. | |
1105 | Under normal situations this variable should be of no interest to you. | |
1106 | ||
a0d0e21e LW |
1107 | =item $SYSTEM_FD_MAX |
1108 | ||
1109 | =item $^F | |
a054c801 | 1110 | X<$^F> X<$SYSTEM_FD_MAX> |
a0d0e21e LW |
1111 | |
1112 | The maximum system file descriptor, ordinarily 2. System file | |
1113 | descriptors are passed to exec()ed processes, while higher file | |
1114 | descriptors are not. Also, during an open(), system file descriptors are | |
1115 | preserved even if the open() fails. (Ordinary file descriptors are | |
19799a22 | 1116 | closed before the open() is attempted.) The close-on-exec |
a0d0e21e | 1117 | status of a file descriptor will be decided according to the value of |
8d2a6795 GS |
1118 | C<$^F> when the corresponding file, pipe, or socket was opened, not the |
1119 | time of the exec(). | |
a0d0e21e | 1120 | |
6e2995f4 | 1121 | =item $^H |
1122 | ||
0462a1ab GS |
1123 | WARNING: This variable is strictly for internal use only. Its availability, |
1124 | behavior, and contents are subject to change without notice. | |
1125 | ||
1126 | This variable contains compile-time hints for the Perl interpreter. At the | |
1127 | end of compilation of a BLOCK the value of this variable is restored to the | |
1128 | value when the interpreter started to compile the BLOCK. | |
1129 | ||
1130 | When perl begins to parse any block construct that provides a lexical scope | |
1131 | (e.g., eval body, required file, subroutine body, loop body, or conditional | |
1132 | block), the existing value of $^H is saved, but its value is left unchanged. | |
1133 | When the compilation of the block is completed, it regains the saved value. | |
1134 | Between the points where its value is saved and restored, code that | |
1135 | executes within BEGIN blocks is free to change the value of $^H. | |
1136 | ||
1137 | This behavior provides the semantic of lexical scoping, and is used in, | |
1138 | for instance, the C<use strict> pragma. | |
1139 | ||
1140 | The contents should be an integer; different bits of it are used for | |
1141 | different pragmatic flags. Here's an example: | |
1142 | ||
1143 | sub add_100 { $^H |= 0x100 } | |
1144 | ||
1145 | sub foo { | |
1146 | BEGIN { add_100() } | |
1147 | bar->baz($boon); | |
1148 | } | |
1149 | ||
1150 | Consider what happens during execution of the BEGIN block. At this point | |
1151 | the BEGIN block has already been compiled, but the body of foo() is still | |
1152 | being compiled. The new value of $^H will therefore be visible only while | |
1153 | the body of foo() is being compiled. | |
1154 | ||
1155 | Substitution of the above BEGIN block with: | |
1156 | ||
1157 | BEGIN { require strict; strict->import('vars') } | |
1158 | ||
1159 | demonstrates how C<use strict 'vars'> is implemented. Here's a conditional | |
1160 | version of the same lexical pragma: | |
1161 | ||
1162 | BEGIN { require strict; strict->import('vars') if $condition } | |
1163 | ||
1164 | =item %^H | |
1165 | ||
0462a1ab | 1166 | The %^H hash provides the same scoping semantic as $^H. This makes it |
46e5f5f4 | 1167 | useful for implementation of lexically scoped pragmas. See L<perlpragma>. |
6e2995f4 | 1168 | |
a0d0e21e LW |
1169 | =item $INPLACE_EDIT |
1170 | ||
1171 | =item $^I | |
a054c801 | 1172 | X<$^I> X<$INPLACE_EDIT> |
a0d0e21e LW |
1173 | |
1174 | The current value of the inplace-edit extension. Use C<undef> to disable | |
1175 | inplace editing. (Mnemonic: value of B<-i> switch.) | |
1176 | ||
fb73857a | 1177 | =item $^M |
a054c801 | 1178 | X<$^M> |
fb73857a | 1179 | |
19799a22 GS |
1180 | By default, running out of memory is an untrappable, fatal error. |
1181 | However, if suitably built, Perl can use the contents of C<$^M> | |
1182 | as an emergency memory pool after die()ing. Suppose that your Perl | |
0acca065 | 1183 | were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc. |
19799a22 | 1184 | Then |
fb73857a | 1185 | |
19799a22 | 1186 | $^M = 'a' x (1 << 16); |
fb73857a | 1187 | |
51ee6500 | 1188 | would allocate a 64K buffer for use in an emergency. See the |
19799a22 | 1189 | F<INSTALL> file in the Perl distribution for information on how to |
0acca065 RGS |
1190 | add custom C compilation flags when compiling perl. To discourage casual |
1191 | use of this advanced feature, there is no L<English|English> long name for | |
1192 | this variable. | |
fb73857a | 1193 | |
5c055ba3 | 1194 | =item $OSNAME |
6e2995f4 | 1195 | |
5c055ba3 | 1196 | =item $^O |
a054c801 | 1197 | X<$^O> X<$OSNAME> |
5c055ba3 | 1198 | |
1199 | The name of the operating system under which this copy of Perl was | |
1200 | built, as determined during the configuration process. The value | |
19799a22 GS |
1201 | is identical to C<$Config{'osname'}>. See also L<Config> and the |
1202 | B<-V> command-line switch documented in L<perlrun>. | |
5c055ba3 | 1203 | |
443f6d01 | 1204 | In Windows platforms, $^O is not very helpful: since it is always |
7f510801 GS |
1205 | C<MSWin32>, it doesn't tell the difference between |
1206 | 95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or | |
1207 | Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish | |
1208 | between the variants. | |
916d64a3 | 1209 | |
e2e27056 JH |
1210 | =item ${^OPEN} |
1211 | ||
1212 | An internal variable used by PerlIO. A string in two parts, separated | |
fae2c0fb RGS |
1213 | by a C<\0> byte, the first part describes the input layers, the second |
1214 | part describes the output layers. | |
e2e27056 | 1215 | |
a0d0e21e LW |
1216 | =item $PERLDB |
1217 | ||
1218 | =item $^P | |
a054c801 | 1219 | X<$^P> X<$PERLDB> |
a0d0e21e | 1220 | |
19799a22 GS |
1221 | The internal variable for debugging support. The meanings of the |
1222 | various bits are subject to change, but currently indicate: | |
84902520 TB |
1223 | |
1224 | =over 6 | |
1225 | ||
1226 | =item 0x01 | |
1227 | ||
1228 | Debug subroutine enter/exit. | |
1229 | ||
1230 | =item 0x02 | |
1231 | ||
4c85b59c TB |
1232 | Line-by-line debugging. Causes DB::DB() subroutine to be called for each |
1233 | statement executed. Also causes saving source code lines (like 0x400). | |
84902520 TB |
1234 | |
1235 | =item 0x04 | |
1236 | ||
1237 | Switch off optimizations. | |
1238 | ||
1239 | =item 0x08 | |
1240 | ||
1241 | Preserve more data for future interactive inspections. | |
1242 | ||
1243 | =item 0x10 | |
1244 | ||
1245 | Keep info about source lines on which a subroutine is defined. | |
1246 | ||
1247 | =item 0x20 | |
1248 | ||
1249 | Start with single-step on. | |
1250 | ||
83ee9e09 GS |
1251 | =item 0x40 |
1252 | ||
1253 | Use subroutine address instead of name when reporting. | |
1254 | ||
1255 | =item 0x80 | |
1256 | ||
1257 | Report C<goto &subroutine> as well. | |
1258 | ||
1259 | =item 0x100 | |
1260 | ||
1261 | Provide informative "file" names for evals based on the place they were compiled. | |
1262 | ||
1263 | =item 0x200 | |
1264 | ||
1265 | Provide informative names to anonymous subroutines based on the place they | |
1266 | were compiled. | |
1267 | ||
7619c85e RG |
1268 | =item 0x400 |
1269 | ||
4c85b59c | 1270 | Save source code lines into C<@{"_<$filename"}>. |
7619c85e | 1271 | |
84902520 TB |
1272 | =back |
1273 | ||
19799a22 GS |
1274 | Some bits may be relevant at compile-time only, some at |
1275 | run-time only. This is a new mechanism and the details may change. | |
4c85b59c | 1276 | See also L<perldebguts>. |
a0d0e21e | 1277 | |
66558a10 GS |
1278 | =item $LAST_REGEXP_CODE_RESULT |
1279 | ||
b9ac3b5b | 1280 | =item $^R |
a054c801 | 1281 | X<$^R> X<$LAST_REGEXP_CODE_RESULT> |
b9ac3b5b | 1282 | |
19799a22 GS |
1283 | The result of evaluation of the last successful C<(?{ code })> |
1284 | regular expression assertion (see L<perlre>). May be written to. | |
b9ac3b5b | 1285 | |
66558a10 GS |
1286 | =item $EXCEPTIONS_BEING_CAUGHT |
1287 | ||
fb73857a | 1288 | =item $^S |
a054c801 | 1289 | X<$^S> X<$EXCEPTIONS_BEING_CAUGHT> |
fb73857a | 1290 | |
fa05a9fd IT |
1291 | Current state of the interpreter. |
1292 | ||
1293 | $^S State | |
1294 | --------- ------------------- | |
1295 | undef Parsing module/eval | |
1296 | true (1) Executing an eval | |
1297 | false (0) Otherwise | |
1298 | ||
1299 | The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers. | |
fb73857a | 1300 | |
a0d0e21e LW |
1301 | =item $BASETIME |
1302 | ||
1303 | =item $^T | |
a054c801 | 1304 | X<$^T> X<$BASETIME> |
a0d0e21e | 1305 | |
19799a22 | 1306 | The time at which the program began running, in seconds since the |
5f05dabc | 1307 | epoch (beginning of 1970). The values returned by the B<-M>, B<-A>, |
19799a22 | 1308 | and B<-C> filetests are based on this value. |
a0d0e21e | 1309 | |
7c36658b MS |
1310 | =item ${^TAINT} |
1311 | ||
9aa05f58 RGS |
1312 | Reflects if taint mode is on or off. 1 for on (the program was run with |
1313 | B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with | |
18e8c5b0 | 1314 | B<-t> or B<-TU>). This variable is read-only. |
7c36658b | 1315 | |
a05d7ebb JH |
1316 | =item ${^UNICODE} |
1317 | ||
ab9e1bb7 JH |
1318 | Reflects certain Unicode settings of Perl. See L<perlrun> |
1319 | documentation for the C<-C> switch for more information about | |
1320 | the possible values. This variable is set during Perl startup | |
1321 | and is thereafter read-only. | |
fde18df1 | 1322 | |
e07ea26a NC |
1323 | =item ${^UTF8CACHE} |
1324 | ||
1325 | This variable controls the state of the internal UTF-8 offset caching code. | |
16d9fe92 NC |
1326 | 1 for on (the default), 0 for off, -1 to debug the caching code by checking |
1327 | all its results against linear scans, and panicking on any discrepancy. | |
e07ea26a | 1328 | |
ea8eae40 RGS |
1329 | =item ${^UTF8LOCALE} |
1330 | ||
a69635b7 | 1331 | This variable indicates whether a UTF-8 locale was detected by perl at |
ea8eae40 RGS |
1332 | startup. This information is used by perl when it's in |
1333 | adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line | |
1334 | switch); see L<perlrun> for more info on this. | |
1335 | ||
44dcb63b | 1336 | =item $PERL_VERSION |
b459063d | 1337 | |
16070b82 | 1338 | =item $^V |
a054c801 | 1339 | X<$^V> X<$PERL_VERSION> |
16070b82 GS |
1340 | |
1341 | The revision, version, and subversion of the Perl interpreter, represented | |
a32521b7 | 1342 | as a C<version> object. |
16070b82 | 1343 | |
7d2b1222 | 1344 | This variable first appeared in perl 5.6.0; earlier versions of perl will |
a32521b7 | 1345 | see an undefined value. Before perl 5.10.0 $^V was represented as a v-string. |
7d2b1222 | 1346 | |
a32521b7 | 1347 | $^V can be used to determine whether the Perl interpreter executing a |
16070b82 | 1348 | script is in the right range of versions. (Mnemonic: use ^V for Version |
44dcb63b | 1349 | Control.) Example: |
16070b82 | 1350 | |
7d2b1222 | 1351 | warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1 |
16070b82 | 1352 | |
aa2f2a36 AMS |
1353 | To convert C<$^V> into its string representation use sprintf()'s |
1354 | C<"%vd"> conversion: | |
1355 | ||
1356 | printf "version is v%vd\n", $^V; # Perl's version | |
1357 | ||
44dcb63b | 1358 | See the documentation of C<use VERSION> and C<require VERSION> |
16070b82 GS |
1359 | for a convenient way to fail if the running Perl interpreter is too old. |
1360 | ||
1361 | See also C<$]> for an older representation of the Perl version. | |
1362 | ||
a0d0e21e LW |
1363 | =item $WARNING |
1364 | ||
1365 | =item $^W | |
a054c801 | 1366 | X<$^W> X<$WARNING> |
a0d0e21e | 1367 | |
19799a22 GS |
1368 | The current value of the warning switch, initially true if B<-w> |
1369 | was used, false otherwise, but directly modifiable. (Mnemonic: | |
4438c4b7 JH |
1370 | related to the B<-w> switch.) See also L<warnings>. |
1371 | ||
6a818117 | 1372 | =item ${^WARNING_BITS} |
4438c4b7 JH |
1373 | |
1374 | The current set of warning checks enabled by the C<use warnings> pragma. | |
1375 | See the documentation of C<warnings> for more details. | |
a0d0e21e | 1376 | |
2a8c8378 | 1377 | =item ${^WIN32_SLOPPY_STAT} |
174299ac | 1378 | X<sitecustomize> X<sitecustomize.pl> |
2a8c8378 JD |
1379 | |
1380 | If this variable is set to a true value, then stat() on Windows will | |
1381 | not try to open the file. This means that the link count cannot be | |
1382 | determined and file attributes may be out of date if additional | |
1383 | hardlinks to the file exist. On the other hand, not opening the file | |
1384 | is considerably faster, especially for files on network drives. | |
1385 | ||
1386 | This variable could be set in the F<sitecustomize.pl> file to | |
1387 | configure the local Perl installation to use "sloppy" stat() by | |
ee811f5e AB |
1388 | default. See the documentation for B<-f> in |
1389 | L<perlrun|perlrun/"Command Switches"> for more information about site | |
2a8c8378 JD |
1390 | customization. |
1391 | ||
a0d0e21e LW |
1392 | =item $EXECUTABLE_NAME |
1393 | ||
1394 | =item $^X | |
a054c801 | 1395 | X<$^X> X<$EXECUTABLE_NAME> |
a0d0e21e | 1396 | |
e71940de | 1397 | The name used to execute the current copy of Perl, from C's |
21c1191d | 1398 | C<argv[0]> or (where supported) F</proc/self/exe>. |
38e4f4ae | 1399 | |
e71940de PG |
1400 | Depending on the host operating system, the value of $^X may be |
1401 | a relative or absolute pathname of the perl program file, or may | |
1402 | be the string used to invoke perl but not the pathname of the | |
1403 | perl program file. Also, most operating systems permit invoking | |
1404 | programs that are not in the PATH environment variable, so there | |
a10d74f3 PG |
1405 | is no guarantee that the value of $^X is in PATH. For VMS, the |
1406 | value may or may not include a version number. | |
38e4f4ae | 1407 | |
e71940de PG |
1408 | You usually can use the value of $^X to re-invoke an independent |
1409 | copy of the same perl that is currently running, e.g., | |
1410 | ||
1411 | @first_run = `$^X -le "print int rand 100 for 1..100"`; | |
1412 | ||
1413 | But recall that not all operating systems support forking or | |
1414 | capturing of the output of commands, so this complex statement | |
1415 | may not be portable. | |
38e4f4ae | 1416 | |
e71940de PG |
1417 | It is not safe to use the value of $^X as a path name of a file, |
1418 | as some operating systems that have a mandatory suffix on | |
1419 | executable files do not require use of the suffix when invoking | |
1420 | a command. To convert the value of $^X to a path name, use the | |
1421 | following statements: | |
1422 | ||
304dea91 | 1423 | # Build up a set of file names (not command names). |
e71940de | 1424 | use Config; |
68fb0eb7 PG |
1425 | $this_perl = $^X; |
1426 | if ($^O ne 'VMS') | |
1427 | {$this_perl .= $Config{_exe} | |
1428 | unless $this_perl =~ m/$Config{_exe}$/i;} | |
e71940de PG |
1429 | |
1430 | Because many operating systems permit anyone with read access to | |
1431 | the Perl program file to make a copy of it, patch the copy, and | |
1432 | then execute the copy, the security-conscious Perl programmer | |
1433 | should take care to invoke the installed copy of perl, not the | |
1434 | copy referenced by $^X. The following statements accomplish | |
1435 | this goal, and produce a pathname that can be invoked as a | |
1436 | command or referenced as a file. | |
38e4f4ae SB |
1437 | |
1438 | use Config; | |
68fb0eb7 PG |
1439 | $secure_perl_path = $Config{perlpath}; |
1440 | if ($^O ne 'VMS') | |
1441 | {$secure_perl_path .= $Config{_exe} | |
1442 | unless $secure_perl_path =~ m/$Config{_exe}$/i;} | |
a0d0e21e | 1443 | |
2d84a16a | 1444 | =item ARGV |
a054c801 | 1445 | X<ARGV> |
2d84a16a DM |
1446 | |
1447 | The special filehandle that iterates over command-line filenames in | |
1448 | C<@ARGV>. Usually written as the null filehandle in the angle operator | |
1449 | C<< <> >>. Note that currently C<ARGV> only has its magical effect | |
1450 | within the C<< <> >> operator; elsewhere it is just a plain filehandle | |
1451 | corresponding to the last file opened by C<< <> >>. In particular, | |
1452 | passing C<\*ARGV> as a parameter to a function that expects a filehandle | |
1453 | may not cause your function to automatically read the contents of all the | |
1454 | files in C<@ARGV>. | |
1455 | ||
a0d0e21e | 1456 | =item $ARGV |
a054c801 | 1457 | X<$ARGV> |
a0d0e21e | 1458 | |
c47ff5f1 | 1459 | contains the name of the current file when reading from <>. |
a0d0e21e LW |
1460 | |
1461 | =item @ARGV | |
a054c801 | 1462 | X<@ARGV> |
a0d0e21e | 1463 | |
19799a22 | 1464 | The array @ARGV contains the command-line arguments intended for |
14218588 | 1465 | the script. C<$#ARGV> is generally the number of arguments minus |
19799a22 GS |
1466 | one, because C<$ARGV[0]> is the first argument, I<not> the program's |
1467 | command name itself. See C<$0> for the command name. | |
a0d0e21e | 1468 | |
5ccee41e | 1469 | =item ARGVOUT |
a054c801 | 1470 | X<ARGVOUT> |
5ccee41e JA |
1471 | |
1472 | The special filehandle that points to the currently open output file | |
1473 | when doing edit-in-place processing with B<-i>. Useful when you have | |
1474 | to do a lot of inserting and don't want to keep modifying $_. See | |
1475 | L<perlrun> for the B<-i> switch. | |
1476 | ||
9b0e6e7a | 1477 | =item @F |
a054c801 | 1478 | X<@F> |
9b0e6e7a JP |
1479 | |
1480 | The array @F contains the fields of each line read in when autosplit | |
1481 | mode is turned on. See L<perlrun> for the B<-a> switch. This array | |
1482 | is package-specific, and must be declared or given a full package name | |
1483 | if not in package main when running under C<strict 'vars'>. | |
1484 | ||
a0d0e21e | 1485 | =item @INC |
a054c801 | 1486 | X<@INC> |
a0d0e21e | 1487 | |
19799a22 GS |
1488 | The array @INC contains the list of places that the C<do EXPR>, |
1489 | C<require>, or C<use> constructs look for their library files. It | |
1490 | initially consists of the arguments to any B<-I> command-line | |
1491 | switches, followed by the default Perl library, probably | |
1492 | F</usr/local/lib/perl>, followed by ".", to represent the current | |
e48df184 RGS |
1493 | directory. ("." will not be appended if taint checks are enabled, either by |
1494 | C<-T> or by C<-t>.) If you need to modify this at runtime, you should use | |
19799a22 GS |
1495 | the C<use lib> pragma to get the machine-dependent library properly |
1496 | loaded also: | |
a0d0e21e | 1497 | |
cb1a09d0 AD |
1498 | use lib '/mypath/libdir/'; |
1499 | use SomeMod; | |
303f2f76 | 1500 | |
d54b56d5 RGS |
1501 | You can also insert hooks into the file inclusion system by putting Perl |
1502 | code directly into @INC. Those hooks may be subroutine references, array | |
1503 | references or blessed objects. See L<perlfunc/require> for details. | |
1504 | ||
314d39ce MG |
1505 | =item @ARG |
1506 | ||
fb73857a | 1507 | =item @_ |
a054c801 | 1508 | X<@_> X<@ARG> |
fb73857a | 1509 | |
1510 | Within a subroutine the array @_ contains the parameters passed to that | |
19799a22 | 1511 | subroutine. See L<perlsub>. |
fb73857a | 1512 | |
a0d0e21e | 1513 | =item %INC |
a054c801 | 1514 | X<%INC> |
a0d0e21e | 1515 | |
19799a22 GS |
1516 | The hash %INC contains entries for each filename included via the |
1517 | C<do>, C<require>, or C<use> operators. The key is the filename | |
1518 | you specified (with module names converted to pathnames), and the | |
14218588 | 1519 | value is the location of the file found. The C<require> |
87275199 | 1520 | operator uses this hash to determine whether a particular file has |
19799a22 | 1521 | already been included. |
a0d0e21e | 1522 | |
89ccab8c RGS |
1523 | If the file was loaded via a hook (e.g. a subroutine reference, see |
1524 | L<perlfunc/require> for a description of these hooks), this hook is | |
9ae8cd5b RGS |
1525 | by default inserted into %INC in place of a filename. Note, however, |
1526 | that the hook may have set the %INC entry by itself to provide some more | |
1527 | specific info. | |
44f0be63 | 1528 | |
b687b08b TC |
1529 | =item %ENV |
1530 | ||
1531 | =item $ENV{expr} | |
a054c801 | 1532 | X<%ENV> |
a0d0e21e LW |
1533 | |
1534 | The hash %ENV contains your current environment. Setting a | |
19799a22 GS |
1535 | value in C<ENV> changes the environment for any child processes |
1536 | you subsequently fork() off. | |
a0d0e21e | 1537 | |
b687b08b TC |
1538 | =item %SIG |
1539 | ||
1540 | =item $SIG{expr} | |
a054c801 | 1541 | X<%SIG> |
a0d0e21e | 1542 | |
efbd929d | 1543 | The hash C<%SIG> contains signal handlers for signals. For example: |
a0d0e21e LW |
1544 | |
1545 | sub handler { # 1st argument is signal name | |
fb73857a | 1546 | my($sig) = @_; |
a0d0e21e LW |
1547 | print "Caught a SIG$sig--shutting down\n"; |
1548 | close(LOG); | |
1549 | exit(0); | |
1550 | } | |
1551 | ||
fb73857a | 1552 | $SIG{'INT'} = \&handler; |
1553 | $SIG{'QUIT'} = \&handler; | |
a0d0e21e | 1554 | ... |
19799a22 | 1555 | $SIG{'INT'} = 'DEFAULT'; # restore default action |
a0d0e21e LW |
1556 | $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT |
1557 | ||
f648820c GS |
1558 | Using a value of C<'IGNORE'> usually has the effect of ignoring the |
1559 | signal, except for the C<CHLD> signal. See L<perlipc> for more about | |
1560 | this special case. | |
1561 | ||
19799a22 | 1562 | Here are some other examples: |
a0d0e21e | 1563 | |
fb73857a | 1564 | $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended) |
a0d0e21e | 1565 | $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber |
19799a22 | 1566 | $SIG{"PIPE"} = *Plumber; # somewhat esoteric |
a0d0e21e LW |
1567 | $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return?? |
1568 | ||
19799a22 GS |
1569 | Be sure not to use a bareword as the name of a signal handler, |
1570 | lest you inadvertently call it. | |
748a9306 | 1571 | |
44a8e56a | 1572 | If your system has the sigaction() function then signal handlers are |
9ce5b4ad | 1573 | installed using it. This means you get reliable signal handling. |
44a8e56a | 1574 | |
9ce5b4ad SG |
1575 | The default delivery policy of signals changed in Perl 5.8.0 from |
1576 | immediate (also known as "unsafe") to deferred, also known as | |
1577 | "safe signals". See L<perlipc> for more information. | |
45c0772f | 1578 | |
748a9306 | 1579 | Certain internal hooks can be also set using the %SIG hash. The |
a8f8344d | 1580 | routine indicated by C<$SIG{__WARN__}> is called when a warning message is |
748a9306 | 1581 | about to be printed. The warning message is passed as the first |
efbd929d AT |
1582 | argument. The presence of a C<__WARN__> hook causes the ordinary printing |
1583 | of warnings to C<STDERR> to be suppressed. You can use this to save warnings | |
748a9306 LW |
1584 | in a variable, or turn warnings into fatal errors, like this: |
1585 | ||
1586 | local $SIG{__WARN__} = sub { die $_[0] }; | |
1587 | eval $proggie; | |
1588 | ||
efbd929d AT |
1589 | As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can |
1590 | disable warnings using the empty subroutine: | |
1591 | ||
1592 | local $SIG{__WARN__} = sub {}; | |
1593 | ||
a8f8344d | 1594 | The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception |
748a9306 | 1595 | is about to be thrown. The error message is passed as the first |
efbd929d | 1596 | argument. When a C<__DIE__> hook routine returns, the exception |
748a9306 | 1597 | processing continues as it would have in the absence of the hook, |
efbd929d | 1598 | unless the hook routine itself exits via a C<goto>, a loop exit, or a C<die()>. |
774d564b | 1599 | The C<__DIE__> handler is explicitly disabled during the call, so that you |
fb73857a | 1600 | can die from a C<__DIE__> handler. Similarly for C<__WARN__>. |
1601 | ||
19799a22 GS |
1602 | Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called |
1603 | even inside an eval(). Do not use this to rewrite a pending exception | |
efbd929d | 1604 | in C<$@>, or as a bizarre substitute for overriding C<CORE::GLOBAL::die()>. |
19799a22 GS |
1605 | This strange action at a distance may be fixed in a future release |
1606 | so that C<$SIG{__DIE__}> is only called if your program is about | |
1607 | to exit, as was the original intent. Any other use is deprecated. | |
1608 | ||
1609 | C<__DIE__>/C<__WARN__> handlers are very special in one respect: | |
1610 | they may be called to report (probable) errors found by the parser. | |
1611 | In such a case the parser may be in inconsistent state, so any | |
1612 | attempt to evaluate Perl code from such a handler will probably | |
1613 | result in a segfault. This means that warnings or errors that | |
1614 | result from parsing Perl should be used with extreme caution, like | |
1615 | this: | |
fb73857a | 1616 | |
1617 | require Carp if defined $^S; | |
1618 | Carp::confess("Something wrong") if defined &Carp::confess; | |
1619 | die "Something wrong, but could not load Carp to give backtrace... | |
1620 | To see backtrace try starting Perl with -MCarp switch"; | |
1621 | ||
1622 | Here the first line will load Carp I<unless> it is the parser who | |
1623 | called the handler. The second line will print backtrace and die if | |
1624 | Carp was available. The third line will be executed only if Carp was | |
1625 | not available. | |
1626 | ||
19799a22 | 1627 | See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and |
4438c4b7 | 1628 | L<warnings> for additional information. |
68dc0745 | 1629 | |
a0d0e21e | 1630 | =back |
55602bd2 IZ |
1631 | |
1632 | =head2 Error Indicators | |
a054c801 | 1633 | X<error> X<exception> |
55602bd2 | 1634 | |
19799a22 GS |
1635 | The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information |
1636 | about different types of error conditions that may appear during | |
1637 | execution of a Perl program. The variables are shown ordered by | |
1638 | the "distance" between the subsystem which reported the error and | |
1639 | the Perl process. They correspond to errors detected by the Perl | |
1640 | interpreter, C library, operating system, or an external program, | |
1641 | respectively. | |
55602bd2 IZ |
1642 | |
1643 | To illustrate the differences between these variables, consider the | |
19799a22 | 1644 | following Perl expression, which uses a single-quoted string: |
55602bd2 | 1645 | |
19799a22 | 1646 | eval q{ |
22d0716c SB |
1647 | open my $pipe, "/cdrom/install |" or die $!; |
1648 | my @res = <$pipe>; | |
1649 | close $pipe or die "bad pipe: $?, $!"; | |
19799a22 | 1650 | }; |
55602bd2 IZ |
1651 | |
1652 | After execution of this statement all 4 variables may have been set. | |
1653 | ||
19799a22 GS |
1654 | C<$@> is set if the string to be C<eval>-ed did not compile (this |
1655 | may happen if C<open> or C<close> were imported with bad prototypes), | |
1656 | or if Perl code executed during evaluation die()d . In these cases | |
1657 | the value of $@ is the compile error, or the argument to C<die> | |
4cb1c523 | 1658 | (which will interpolate C<$!> and C<$?>). (See also L<Fatal>, |
19799a22 GS |
1659 | though.) |
1660 | ||
c47ff5f1 | 1661 | When the eval() expression above is executed, open(), C<< <PIPE> >>, |
19799a22 GS |
1662 | and C<close> are translated to calls in the C run-time library and |
1663 | thence to the operating system kernel. C<$!> is set to the C library's | |
1664 | C<errno> if one of these calls fails. | |
1665 | ||
1666 | Under a few operating systems, C<$^E> may contain a more verbose | |
1667 | error indicator, such as in this case, "CDROM tray not closed." | |
14218588 | 1668 | Systems that do not support extended error messages leave C<$^E> |
19799a22 GS |
1669 | the same as C<$!>. |
1670 | ||
1671 | Finally, C<$?> may be set to non-0 value if the external program | |
1672 | F</cdrom/install> fails. The upper eight bits reflect specific | |
1673 | error conditions encountered by the program (the program's exit() | |
1674 | value). The lower eight bits reflect mode of failure, like signal | |
1675 | death and core dump information See wait(2) for details. In | |
1676 | contrast to C<$!> and C<$^E>, which are set only if error condition | |
1677 | is detected, the variable C<$?> is set on each C<wait> or pipe | |
1678 | C<close>, overwriting the old value. This is more like C<$@>, which | |
1679 | on every eval() is always set on failure and cleared on success. | |
2b92dfce | 1680 | |
19799a22 GS |
1681 | For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>, |
1682 | and C<$?>. | |
2b92dfce GS |
1683 | |
1684 | =head2 Technical Note on the Syntax of Variable Names | |
1685 | ||
19799a22 GS |
1686 | Variable names in Perl can have several formats. Usually, they |
1687 | must begin with a letter or underscore, in which case they can be | |
1688 | arbitrarily long (up to an internal limit of 251 characters) and | |
1689 | may contain letters, digits, underscores, or the special sequence | |
1690 | C<::> or C<'>. In this case, the part before the last C<::> or | |
1691 | C<'> is taken to be a I<package qualifier>; see L<perlmod>. | |
2b92dfce GS |
1692 | |
1693 | Perl variable names may also be a sequence of digits or a single | |
1694 | punctuation or control character. These names are all reserved for | |
19799a22 GS |
1695 | special uses by Perl; for example, the all-digits names are used |
1696 | to hold data captured by backreferences after a regular expression | |
1697 | match. Perl has a special syntax for the single-control-character | |
1698 | names: It understands C<^X> (caret C<X>) to mean the control-C<X> | |
1699 | character. For example, the notation C<$^W> (dollar-sign caret | |
1700 | C<W>) is the scalar variable whose name is the single character | |
1701 | control-C<W>. This is better than typing a literal control-C<W> | |
1702 | into your program. | |
2b92dfce | 1703 | |
87275199 | 1704 | Finally, new in Perl 5.6, Perl variable names may be alphanumeric |
19799a22 GS |
1705 | strings that begin with control characters (or better yet, a caret). |
1706 | These variables must be written in the form C<${^Foo}>; the braces | |
1707 | are not optional. C<${^Foo}> denotes the scalar variable whose | |
1708 | name is a control-C<F> followed by two C<o>'s. These variables are | |
1709 | reserved for future special uses by Perl, except for the ones that | |
1710 | begin with C<^_> (control-underscore or caret-underscore). No | |
1711 | control-character name that begins with C<^_> will acquire a special | |
1712 | meaning in any future version of Perl; such names may therefore be | |
1713 | used safely in programs. C<$^_> itself, however, I<is> reserved. | |
1714 | ||
1fcb18de RGS |
1715 | Perl identifiers that begin with digits, control characters, or |
1716 | punctuation characters are exempt from the effects of the C<package> | |
1717 | declaration and are always forced to be in package C<main>; they are | |
1718 | also exempt from C<strict 'vars'> errors. A few other names are also | |
1719 | exempt in these ways: | |
2b92dfce GS |
1720 | |
1721 | ENV STDIN | |
1722 | INC STDOUT | |
1723 | ARGV STDERR | |
5b88253b | 1724 | ARGVOUT _ |
2b92dfce GS |
1725 | SIG |
1726 | ||
1727 | In particular, the new special C<${^_XYZ}> variables are always taken | |
19799a22 | 1728 | to be in package C<main>, regardless of any C<package> declarations |
747fafda | 1729 | presently in scope. |
2b92dfce | 1730 | |
19799a22 GS |
1731 | =head1 BUGS |
1732 | ||
1733 | Due to an unfortunate accident of Perl's implementation, C<use | |
1734 | English> imposes a considerable performance penalty on all regular | |
1735 | expression matches in a program, regardless of whether they occur | |
1736 | in the scope of C<use English>. For that reason, saying C<use | |
1737 | English> in libraries is strongly discouraged. See the | |
1738 | Devel::SawAmpersand module documentation from CPAN | |
1577cd80 | 1739 | ( http://www.cpan.org/modules/by-module/Devel/ ) |
a054c801 GS |
1740 | for more information. Writing C<use English '-no_match_vars';> |
1741 | avoids the performance penalty. | |
2b92dfce | 1742 | |
19799a22 GS |
1743 | Having to even think about the C<$^S> variable in your exception |
1744 | handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented | |
1745 | invites grievous and difficult to track down errors. Avoid it | |
1746 | and use an C<END{}> or CORE::GLOBAL::die override instead. |