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1 =head1 NAME 2 3 perlvar - Perl predefined variables 4 5 =head1 DESCRIPTION 6 7 =head2 Predefined Names 8 9 The following names have special meaning to Perl. Most 10 punctuation names have reasonable mnemonics, or analogs in the 11 shells. Nevertheless, if you wish to use long variable names, 12 you need only say 13 14 use English; 15 16 at the top of your program. This aliases all the short names to the long 17 names in the current package. Some even have medium names, generally 18 borrowed from B<awk>. In general, it's best to use the 19 20 use English '-no_match_vars'; 21 22 invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids 23 a certain performance hit with the use of regular expressions. See 24 L<English>. 25 26 Variables that depend on the currently selected filehandle may be set by 27 calling an appropriate object method on the IO::Handle object, although 28 this is less efficient than using the regular built-in variables. (Summary 29 lines below for this contain the word HANDLE.) First you must say 30 31 use IO::Handle; 32 33 after which you may use either 34 35 method HANDLE EXPR 36 37 or more safely, 38 39 HANDLE->method(EXPR) 40 41 Each method returns the old value of the IO::Handle attribute. 42 The methods each take an optional EXPR, which, if supplied, specifies the 43 new value for the IO::Handle attribute in question. If not supplied, 44 most methods do nothing to the current value--except for 45 autoflush(), which will assume a 1 for you, just to be different. 46 47 Because loading in the IO::Handle class is an expensive operation, you should 48 learn how to use the regular built-in variables. 49 50 A few of these variables are considered "read-only". This means that if 51 you try to assign to this variable, either directly or indirectly through 52 a reference, you'll raise a run-time exception. 53 54 You should be very careful when modifying the default values of most 55 special variables described in this document. In most cases you want 56 to localize these variables before changing them, since if you don't, 57 the change may affect other modules which rely on the default values 58 of the special variables that you have changed. This is one of the 59 correct ways to read the whole file at once: 60 61 open my $fh, "foo" or die $!; 62 local $/; # enable localized slurp mode 63 my $content = <$fh>; 64 close $fh; 65 66 But the following code is quite bad: 67 68 open my $fh, "foo" or die $!; 69 undef $/; # enable slurp mode 70 my $content = <$fh>; 71 close $fh; 72 73 since some other module, may want to read data from some file in the 74 default "line mode", so if the code we have just presented has been 75 executed, the global value of C<$/> is now changed for any other code 76 running inside the same Perl interpreter. 77 78 Usually when a variable is localized you want to make sure that this 79 change affects the shortest scope possible. So unless you are already 80 inside some short C<{}> block, you should create one yourself. For 81 example: 82 83 my $content = ''; 84 open my $fh, "foo" or die $!; 85 { 86 local $/; 87 $content = <$fh>; 88 } 89 close $fh; 90 91 Here is an example of how your own code can go broken: 92 93 for (1..5){ 94 nasty_break(); 95 print "$_ "; 96 } 97 sub nasty_break { 98 $_ = 5; 99 # do something with $_ 100 } 101 102 You probably expect this code to print: 103 104 1 2 3 4 5 105 106 but instead you get: 107 108 5 5 5 5 5 109 110 Why? Because nasty_break() modifies C<$_> without localizing it 111 first. The fix is to add local(): 112 113 local $_ = 5; 114 115 It's easy to notice the problem in such a short example, but in more 116 complicated code you are looking for trouble if you don't localize 117 changes to the special variables. 118 119 The following list is ordered by scalar variables first, then the 120 arrays, then the hashes. 121 122 =over 8 123 124 =item $ARG 125 126 =item $_ 127 X<$_> X<$ARG> 128 129 The default input and pattern-searching space. The following pairs are 130 equivalent: 131 132 while (<>) {...} # equivalent only in while! 133 while (defined($_ = <>)) {...} 134 135 /^Subject:/ 136 $_ =~ /^Subject:/ 137 138 tr/a-z/A-Z/ 139 $_ =~ tr/a-z/A-Z/ 140 141 chomp 142 chomp($_) 143 144 Here are the places where Perl will assume $_ even if you 145 don't use it: 146 147 =over 3 148 149 =item * 150 151 Various unary functions, including functions like ord() and int(), as well 152 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to 153 STDIN. 154 155 =item * 156 157 Various list functions like print() and unlink(). 158 159 =item * 160 161 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used 162 without an C<=~> operator. 163 164 =item * 165 166 The default iterator variable in a C<foreach> loop if no other 167 variable is supplied. 168 169 =item * 170 171 The implicit iterator variable in the grep() and map() functions. 172 173 =item * 174 175 The default place to put an input record when a C<< <FH> >> 176 operation's result is tested by itself as the sole criterion of a C<while> 177 test. Outside a C<while> test, this will not happen. 178 179 =back 180 181 As C<$_> is a global variable, this may lead in some cases to unwanted 182 side-effects. As of perl 5.9.1, you can now use a lexical version of 183 C<$_> by declaring it in a file or in a block with C<my>. Moreover, 184 declaring C<our $_> restores the global C<$_> in the current scope. 185 186 (Mnemonic: underline is understood in certain operations.) 187 188 =back 189 190 =over 8 191 192 =item $a 193 194 =item $b 195 X<$a> X<$b> 196 197 Special package variables when using sort(), see L<perlfunc/sort>. 198 Because of this specialness $a and $b don't need to be declared 199 (using use vars, or our()) even when using the C<strict 'vars'> pragma. 200 Don't lexicalize them with C<my $a> or C<my $b> if you want to be 201 able to use them in the sort() comparison block or function. 202 203 =back 204 205 =over 8 206 207 =item $<I<digits>> 208 X<$1> X<$2> X<$3> 209 210 Contains the subpattern from the corresponding set of capturing 211 parentheses from the last pattern match, not counting patterns 212 matched in nested blocks that have been exited already. (Mnemonic: 213 like \digits.) These variables are all read-only and dynamically 214 scoped to the current BLOCK. 215 216 =item $MATCH 217 218 =item $& 219 X<$&> X<$MATCH> 220 221 The string matched by the last successful pattern match (not counting 222 any matches hidden within a BLOCK or eval() enclosed by the current 223 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only 224 and dynamically scoped to the current BLOCK. 225 226 The use of this variable anywhere in a program imposes a considerable 227 performance penalty on all regular expression matches. See L</BUGS>. 228 229 See L</@-> for a replacement. 230 231 =item ${^MATCH} 232 X<${^MATCH}> 233 234 This is similar to C<$&> (C<$POSTMATCH>) except that it does not incur the 235 performance penalty associated with that variable, and is only guaranteed 236 to return a defined value when the pattern was compiled or executed with 237 the C</p> modifier. 238 239 =item $PREMATCH 240 241 =item $` 242 X<$`> X<$PREMATCH> 243 244 The string preceding whatever was matched by the last successful 245 pattern match (not counting any matches hidden within a BLOCK or eval 246 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted 247 string.) This variable is read-only. 248 249 The use of this variable anywhere in a program imposes a considerable 250 performance penalty on all regular expression matches. See L</BUGS>. 251 252 See L</@-> for a replacement. 253 254 =item ${^PREMATCH} 255 X<${^PREMATCH}> 256 257 This is similar to C<$`> ($PREMATCH) except that it does not incur the 258 performance penalty associated with that variable, and is only guaranteed 259 to return a defined value when the pattern was compiled or executed with 260 the C</p> modifier. 261 262 =item $POSTMATCH 263 264 =item $' 265 X<$'> X<$POSTMATCH> 266 267 The string following whatever was matched by the last successful 268 pattern match (not counting any matches hidden within a BLOCK or eval() 269 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted 270 string.) Example: 271 272 local $_ = 'abcdefghi'; 273 /def/; 274 print "$`:$&:$'\n"; # prints abc:def:ghi 275 276 This variable is read-only and dynamically scoped to the current BLOCK. 277 278 The use of this variable anywhere in a program imposes a considerable 279 performance penalty on all regular expression matches. See L</BUGS>. 280 281 See L</@-> for a replacement. 282 283 =item ${^POSTMATCH} 284 X<${^POSTMATCH}> 285 286 This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the 287 performance penalty associated with that variable, and is only guaranteed 288 to return a defined value when the pattern was compiled or executed with 289 the C</p> modifier. 290 291 =item $LAST_PAREN_MATCH 292 293 =item $+ 294 X<$+> X<$LAST_PAREN_MATCH> 295 296 The text matched by the last bracket of the last successful search pattern. 297 This is useful if you don't know which one of a set of alternative patterns 298 matched. For example: 299 300 /Version: (.*)|Revision: (.*)/ && ($rev = $+); 301 302 (Mnemonic: be positive and forward looking.) 303 This variable is read-only and dynamically scoped to the current BLOCK. 304 305 =item $LAST_SUBMATCH_RESULT 306 307 =item $^N 308 X<$^N> 309 310 The text matched by the used group most-recently closed (i.e. the group 311 with the rightmost closing parenthesis) of the last successful search 312 pattern. (Mnemonic: the (possibly) Nested parenthesis that most 313 recently closed.) 314 315 This is primarily used inside C<(?{...})> blocks for examining text 316 recently matched. For example, to effectively capture text to a variable 317 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with 318 319 (?:(...)(?{ $var = $^N })) 320 321 By setting and then using C<$var> in this way relieves you from having to 322 worry about exactly which numbered set of parentheses they are. 323 324 This variable is dynamically scoped to the current BLOCK. 325 326 =item @LAST_MATCH_END 327 328 =item @+ 329 X<@+> X<@LAST_MATCH_END> 330 331 This array holds the offsets of the ends of the last successful 332 submatches in the currently active dynamic scope. C<$+[0]> is 333 the offset into the string of the end of the entire match. This 334 is the same value as what the C<pos> function returns when called 335 on the variable that was matched against. The I<n>th element 336 of this array holds the offset of the I<n>th submatch, so 337 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset 338 past where $2 ends, and so on. You can use C<$#+> to determine 339 how many subgroups were in the last successful match. See the 340 examples given for the C<@-> variable. 341 342 =item %+ 343 X<%+> 344 345 Similar to C<@+>, the C<%+> hash allows access to the named capture 346 buffers, should they exist, in the last successful match in the 347 currently active dynamic scope. 348 349 For example, C<$+{foo}> is equivalent to C<$1> after the following match: 350 351 'foo' =~ /(?<foo>foo)/; 352 353 The keys of the C<%+> hash list only the names of buffers that have 354 captured (and that are thus associated to defined values). 355 356 The underlying behaviour of C<%+> is provided by the 357 L<Tie::Hash::NamedCapture> module. 358 359 B<Note:> C<%-> and C<%+> are tied views into a common internal hash 360 associated with the last successful regular expression. Therefore mixing 361 iterative access to them via C<each> may have unpredictable results. 362 Likewise, if the last successful match changes, then the results may be 363 surprising. 364 365 =item HANDLE->input_line_number(EXPR) 366 367 =item $INPUT_LINE_NUMBER 368 369 =item $NR 370 371 =item $. 372 X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number> 373 374 Current line number for the last filehandle accessed. 375 376 Each filehandle in Perl counts the number of lines that have been read 377 from it. (Depending on the value of C<$/>, Perl's idea of what 378 constitutes a line may not match yours.) When a line is read from a 379 filehandle (via readline() or C<< <> >>), or when tell() or seek() is 380 called on it, C<$.> becomes an alias to the line counter for that 381 filehandle. 382 383 You can adjust the counter by assigning to C<$.>, but this will not 384 actually move the seek pointer. I<Localizing C<$.> will not localize 385 the filehandle's line count>. Instead, it will localize perl's notion 386 of which filehandle C<$.> is currently aliased to. 387 388 C<$.> is reset when the filehandle is closed, but B<not> when an open 389 filehandle is reopened without an intervening close(). For more 390 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does 391 an explicit close, line numbers increase across ARGV files (but see 392 examples in L<perlfunc/eof>). 393 394 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the 395 line counter for a given filehandle without having to worry about 396 which handle you last accessed. 397 398 (Mnemonic: many programs use "." to mean the current line number.) 399 400 =item IO::Handle->input_record_separator(EXPR) 401 402 =item $INPUT_RECORD_SEPARATOR 403 404 =item $RS 405 406 =item $/ 407 X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR> 408 409 The input record separator, newline by default. This 410 influences Perl's idea of what a "line" is. Works like B<awk>'s RS 411 variable, including treating empty lines as a terminator if set to 412 the null string. (An empty line cannot contain any spaces 413 or tabs.) You may set it to a multi-character string to match a 414 multi-character terminator, or to C<undef> to read through the end 415 of file. Setting it to C<"\n\n"> means something slightly 416 different than setting to C<"">, if the file contains consecutive 417 empty lines. Setting to C<""> will treat two or more consecutive 418 empty lines as a single empty line. Setting to C<"\n\n"> will 419 blindly assume that the next input character belongs to the next 420 paragraph, even if it's a newline. (Mnemonic: / delimits 421 line boundaries when quoting poetry.) 422 423 local $/; # enable "slurp" mode 424 local $_ = <FH>; # whole file now here 425 s/\n[ \t]+/ /g; 426 427 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be 428 better for something. :-) 429 430 Setting C<$/> to a reference to an integer, scalar containing an integer, or 431 scalar that's convertible to an integer will attempt to read records 432 instead of lines, with the maximum record size being the referenced 433 integer. So this: 434 435 local $/ = \32768; # or \"32768", or \$var_containing_32768 436 open my $fh, $myfile or die $!; 437 local $_ = <$fh>; 438 439 will read a record of no more than 32768 bytes from FILE. If you're 440 not reading from a record-oriented file (or your OS doesn't have 441 record-oriented files), then you'll likely get a full chunk of data 442 with every read. If a record is larger than the record size you've 443 set, you'll get the record back in pieces. Trying to set the record 444 size to zero or less will cause reading in the (rest of the) whole file. 445 446 On VMS, record reads are done with the equivalent of C<sysread>, 447 so it's best not to mix record and non-record reads on the same 448 file. (This is unlikely to be a problem, because any file you'd 449 want to read in record mode is probably unusable in line mode.) 450 Non-VMS systems do normal I/O, so it's safe to mix record and 451 non-record reads of a file. 452 453 See also L<perlport/"Newlines">. Also see C<$.>. 454 455 =item HANDLE->autoflush(EXPR) 456 457 =item $OUTPUT_AUTOFLUSH 458 459 =item $| 460 X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH> 461 462 If set to nonzero, forces a flush right away and after every write 463 or print on the currently selected output channel. Default is 0 464 (regardless of whether the channel is really buffered by the 465 system or not; C<$|> tells you only whether you've asked Perl 466 explicitly to flush after each write). STDOUT will 467 typically be line buffered if output is to the terminal and block 468 buffered otherwise. Setting this variable is useful primarily when 469 you are outputting to a pipe or socket, such as when you are running 470 a Perl program under B<rsh> and want to see the output as it's 471 happening. This has no effect on input buffering. See L<perlfunc/getc> 472 for that. (Mnemonic: when you want your pipes to be piping hot.) 473 474 =item IO::Handle->output_field_separator EXPR 475 476 =item $OUTPUT_FIELD_SEPARATOR 477 478 =item $OFS 479 480 =item $, 481 X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR> 482 483 The output field separator for the print operator. If defined, this 484 value is printed between each of print's arguments. Default is C<undef>. 485 (Mnemonic: what is printed when there is a "," in your print statement.) 486 487 =item IO::Handle->output_record_separator EXPR 488 489 =item $OUTPUT_RECORD_SEPARATOR 490 491 =item $ORS 492 493 =item $\ 494 X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR> 495 496 The output record separator for the print operator. If defined, this 497 value is printed after the last of print's arguments. Default is C<undef>. 498 (Mnemonic: you set C<$\> instead of adding "\n" at the end of the print. 499 Also, it's just like C<$/>, but it's what you get "back" from Perl.) 500 501 =item $LIST_SEPARATOR 502 503 =item $" 504 X<$"> X<$LIST_SEPARATOR> 505 506 This is like C<$,> except that it applies to array and slice values 507 interpolated into a double-quoted string (or similar interpreted 508 string). Default is a space. (Mnemonic: obvious, I think.) 509 510 =item $SUBSCRIPT_SEPARATOR 511 512 =item $SUBSEP 513 514 =item $; 515 X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR> 516 517 The subscript separator for multidimensional array emulation. If you 518 refer to a hash element as 519 520 $foo{$a,$b,$c} 521 522 it really means 523 524 $foo{join($;, $a, $b, $c)} 525 526 But don't put 527 528 @foo{$a,$b,$c} # a slice--note the @ 529 530 which means 531 532 ($foo{$a},$foo{$b},$foo{$c}) 533 534 Default is "\034", the same as SUBSEP in B<awk>. If your 535 keys contain binary data there might not be any safe value for C<$;>. 536 (Mnemonic: comma (the syntactic subscript separator) is a 537 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already 538 taken for something more important.) 539 540 Consider using "real" multidimensional arrays as described 541 in L<perllol>. 542 543 =item HANDLE->format_page_number(EXPR) 544 545 =item $FORMAT_PAGE_NUMBER 546 547 =item $% 548 X<$%> X<$FORMAT_PAGE_NUMBER> 549 550 The current page number of the currently selected output channel. 551 Used with formats. 552 (Mnemonic: % is page number in B<nroff>.) 553 554 =item HANDLE->format_lines_per_page(EXPR) 555 556 =item $FORMAT_LINES_PER_PAGE 557 558 =item $= 559 X<$=> X<$FORMAT_LINES_PER_PAGE> 560 561 The current page length (printable lines) of the currently selected 562 output channel. Default is 60. 563 Used with formats. 564 (Mnemonic: = has horizontal lines.) 565 566 =item HANDLE->format_lines_left(EXPR) 567 568 =item $FORMAT_LINES_LEFT 569 570 =item $- 571 X<$-> X<$FORMAT_LINES_LEFT> 572 573 The number of lines left on the page of the currently selected output 574 channel. 575 Used with formats. 576 (Mnemonic: lines_on_page - lines_printed.) 577 578 =item @LAST_MATCH_START 579 580 =item @- 581 X<@-> X<@LAST_MATCH_START> 582 583 $-[0] is the offset of the start of the last successful match. 584 C<$-[>I<n>C<]> is the offset of the start of the substring matched by 585 I<n>-th subpattern, or undef if the subpattern did not match. 586 587 Thus after a match against $_, $& coincides with C<substr $_, $-[0], 588 $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n], 589 $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with 590 C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last 591 matched subgroup in the last successful match. Contrast with 592 C<$#+>, the number of subgroups in the regular expression. Compare 593 with C<@+>. 594 595 This array holds the offsets of the beginnings of the last 596 successful submatches in the currently active dynamic scope. 597 C<$-[0]> is the offset into the string of the beginning of the 598 entire match. The I<n>th element of this array holds the offset 599 of the I<n>th submatch, so C<$-[1]> is the offset where $1 600 begins, C<$-[2]> the offset where $2 begins, and so on. 601 602 After a match against some variable $var: 603 604 =over 5 605 606 =item C<$`> is the same as C<substr($var, 0, $-[0])> 607 608 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])> 609 610 =item C<$'> is the same as C<substr($var, $+[0])> 611 612 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])> 613 614 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])> 615 616 =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])> 617 618 =back 619 620 =item %- 621 X<%-> 622 623 Similar to C<%+>, this variable allows access to the named capture buffers 624 in the last successful match in the currently active dynamic scope. To 625 each capture buffer name found in the regular expression, it associates a 626 reference to an array containing the list of values captured by all 627 buffers with that name (should there be several of them), in the order 628 where they appear. 629 630 Here's an example: 631 632 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) { 633 foreach my $bufname (sort keys %-) { 634 my $ary = $-{$bufname}; 635 foreach my $idx (0..$#$ary) { 636 print "\$-{$bufname}[$idx] : ", 637 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"), 638 "\n"; 639 } 640 } 641 } 642 643 would print out: 644 645 $-{A}[0] : '1' 646 $-{A}[1] : '3' 647 $-{B}[0] : '2' 648 $-{B}[1] : '4' 649 650 The keys of the C<%-> hash correspond to all buffer names found in 651 the regular expression. 652 653 The behaviour of C<%-> is implemented via the 654 L<Tie::Hash::NamedCapture> module. 655 656 B<Note:> C<%-> and C<%+> are tied views into a common internal hash 657 associated with the last successful regular expression. Therefore mixing 658 iterative access to them via C<each> may have unpredictable results. 659 Likewise, if the last successful match changes, then the results may be 660 surprising. 661 662 =item HANDLE->format_name(EXPR) 663 664 =item $FORMAT_NAME 665 666 =item $~ 667 X<$~> X<$FORMAT_NAME> 668 669 The name of the current report format for the currently selected output 670 channel. Default is the name of the filehandle. (Mnemonic: brother to 671 C<$^>.) 672 673 =item HANDLE->format_top_name(EXPR) 674 675 =item $FORMAT_TOP_NAME 676 677 =item $^ 678 X<$^> X<$FORMAT_TOP_NAME> 679 680 The name of the current top-of-page format for the currently selected 681 output channel. Default is the name of the filehandle with _TOP 682 appended. (Mnemonic: points to top of page.) 683 684 =item IO::Handle->format_line_break_characters EXPR 685 686 =item $FORMAT_LINE_BREAK_CHARACTERS 687 688 =item $: 689 X<$:> X<FORMAT_LINE_BREAK_CHARACTERS> 690 691 The current set of characters after which a string may be broken to 692 fill continuation fields (starting with ^) in a format. Default is 693 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in 694 poetry is a part of a line.) 695 696 =item IO::Handle->format_formfeed EXPR 697 698 =item $FORMAT_FORMFEED 699 700 =item $^L 701 X<$^L> X<$FORMAT_FORMFEED> 702 703 What formats output as a form feed. Default is \f. 704 705 =item $ACCUMULATOR 706 707 =item $^A 708 X<$^A> X<$ACCUMULATOR> 709 710 The current value of the write() accumulator for format() lines. A format 711 contains formline() calls that put their result into C<$^A>. After 712 calling its format, write() prints out the contents of C<$^A> and empties. 713 So you never really see the contents of C<$^A> unless you call 714 formline() yourself and then look at it. See L<perlform> and 715 L<perlfunc/formline()>. 716 717 =item $CHILD_ERROR 718 719 =item $? 720 X<$?> X<$CHILD_ERROR> 721 722 The status returned by the last pipe close, backtick (C<``>) command, 723 successful call to wait() or waitpid(), or from the system() 724 operator. This is just the 16-bit status word returned by the 725 traditional Unix wait() system call (or else is made up to look like it). Thus, the 726 exit value of the subprocess is really (C<<< $? >> 8 >>>), and 727 C<$? & 127> gives which signal, if any, the process died from, and 728 C<$? & 128> reports whether there was a core dump. (Mnemonic: 729 similar to B<sh> and B<ksh>.) 730 731 Additionally, if the C<h_errno> variable is supported in C, its value 732 is returned via $? if any C<gethost*()> function fails. 733 734 If you have installed a signal handler for C<SIGCHLD>, the 735 value of C<$?> will usually be wrong outside that handler. 736 737 Inside an C<END> subroutine C<$?> contains the value that is going to be 738 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to 739 change the exit status of your program. For example: 740 741 END { 742 $? = 1 if $? == 255; # die would make it 255 743 } 744 745 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the 746 actual VMS exit status, instead of the default emulation of POSIX 747 status; see L<perlvms/$?> for details. 748 749 Also see L<Error Indicators>. 750 751 =item ${^CHILD_ERROR_NATIVE} 752 X<$^CHILD_ERROR_NATIVE> 753 754 The native status returned by the last pipe close, backtick (C<``>) 755 command, successful call to wait() or waitpid(), or from the system() 756 operator. On POSIX-like systems this value can be decoded with the 757 WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, WSTOPSIG 758 and WIFCONTINUED functions provided by the L<POSIX> module. 759 760 Under VMS this reflects the actual VMS exit status; i.e. it is the same 761 as $? when the pragma C<use vmsish 'status'> is in effect. 762 763 =item ${^ENCODING} 764 X<$^ENCODING> 765 766 The I<object reference> to the Encode object that is used to convert 767 the source code to Unicode. Thanks to this variable your perl script 768 does not have to be written in UTF-8. Default is I<undef>. The direct 769 manipulation of this variable is highly discouraged. 770 771 =item $OS_ERROR 772 773 =item $ERRNO 774 775 =item $! 776 X<$!> X<$ERRNO> X<$OS_ERROR> 777 778 If used numerically, yields the current value of the C C<errno> 779 variable, or in other words, if a system or library call fails, it 780 sets this variable. This means that the value of C<$!> is meaningful 781 only I<immediately> after a B<failure>: 782 783 if (open(FH, $filename)) { 784 # Here $! is meaningless. 785 ... 786 } else { 787 # ONLY here is $! meaningful. 788 ... 789 # Already here $! might be meaningless. 790 } 791 # Since here we might have either success or failure, 792 # here $! is meaningless. 793 794 In the above I<meaningless> stands for anything: zero, non-zero, 795 C<undef>. A successful system or library call does B<not> set 796 the variable to zero. 797 798 If used as a string, yields the corresponding system error string. 799 You can assign a number to C<$!> to set I<errno> if, for instance, 800 you want C<"$!"> to return the string for error I<n>, or you want 801 to set the exit value for the die() operator. (Mnemonic: What just 802 went bang?) 803 804 Also see L<Error Indicators>. 805 806 =item %OS_ERROR 807 808 =item %ERRNO 809 810 =item %! 811 X<%!> 812 813 Each element of C<%!> has a true value only if C<$!> is set to that 814 value. For example, C<$!{ENOENT}> is true if and only if the current 815 value of C<$!> is C<ENOENT>; that is, if the most recent error was 816 "No such file or directory" (or its moral equivalent: not all operating 817 systems give that exact error, and certainly not all languages). 818 To check if a particular key is meaningful on your system, use 819 C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>. 820 See L<Errno> for more information, and also see above for the 821 validity of C<$!>. 822 823 =item $EXTENDED_OS_ERROR 824 825 =item $^E 826 X<$^E> X<$EXTENDED_OS_ERROR> 827 828 Error information specific to the current operating system. At 829 the moment, this differs from C<$!> under only VMS, OS/2, and Win32 830 (and for MacPerl). On all other platforms, C<$^E> is always just 831 the same as C<$!>. 832 833 Under VMS, C<$^E> provides the VMS status value from the last 834 system error. This is more specific information about the last 835 system error than that provided by C<$!>. This is particularly 836 important when C<$!> is set to B<EVMSERR>. 837 838 Under OS/2, C<$^E> is set to the error code of the last call to 839 OS/2 API either via CRT, or directly from perl. 840 841 Under Win32, C<$^E> always returns the last error information 842 reported by the Win32 call C<GetLastError()> which describes 843 the last error from within the Win32 API. Most Win32-specific 844 code will report errors via C<$^E>. ANSI C and Unix-like calls 845 set C<errno> and so most portable Perl code will report errors 846 via C<$!>. 847 848 Caveats mentioned in the description of C<$!> generally apply to 849 C<$^E>, also. (Mnemonic: Extra error explanation.) 850 851 Also see L<Error Indicators>. 852 853 =item $EVAL_ERROR 854 855 =item $@ 856 X<$@> X<$EVAL_ERROR> 857 858 The Perl syntax error message from the last eval() operator. 859 If $@ is the null string, the last eval() parsed and executed 860 correctly (although the operations you invoked may have failed in the 861 normal fashion). (Mnemonic: Where was the syntax error "at"?) 862 863 Warning messages are not collected in this variable. You can, 864 however, set up a routine to process warnings by setting C<$SIG{__WARN__}> 865 as described below. 866 867 Also see L<Error Indicators>. 868 869 =item $PROCESS_ID 870 871 =item $PID 872 873 =item $$ 874 X<$$> X<$PID> X<$PROCESS_ID> 875 876 The process number of the Perl running this script. You should 877 consider this variable read-only, although it will be altered 878 across fork() calls. (Mnemonic: same as shells.) 879 880 Note for Linux users: on Linux, the C functions C<getpid()> and 881 C<getppid()> return different values from different threads. In order to 882 be portable, this behavior is not reflected by C<$$>, whose value remains 883 consistent across threads. If you want to call the underlying C<getpid()>, 884 you may use the CPAN module C<Linux::Pid>. 885 886 =item $REAL_USER_ID 887 888 =item $UID 889 890 =item $< 891 X<< $< >> X<$UID> X<$REAL_USER_ID> 892 893 The real uid of this process. (Mnemonic: it's the uid you came I<from>, 894 if you're running setuid.) You can change both the real uid and 895 the effective uid at the same time by using POSIX::setuid(). Since 896 changes to $< require a system call, check $! after a change attempt to 897 detect any possible errors. 898 899 =item $EFFECTIVE_USER_ID 900 901 =item $EUID 902 903 =item $> 904 X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID> 905 906 The effective uid of this process. Example: 907 908 $< = $>; # set real to effective uid 909 ($<,$>) = ($>,$<); # swap real and effective uid 910 911 You can change both the effective uid and the real uid at the same 912 time by using POSIX::setuid(). Changes to $> require a check to $! 913 to detect any possible errors after an attempted change. 914 915 (Mnemonic: it's the uid you went I<to>, if you're running setuid.) 916 C<< $< >> and C<< $> >> can be swapped only on machines 917 supporting setreuid(). 918 919 =item $REAL_GROUP_ID 920 921 =item $GID 922 923 =item $( 924 X<$(> X<$GID> X<$REAL_GROUP_ID> 925 926 The real gid of this process. If you are on a machine that supports 927 membership in multiple groups simultaneously, gives a space separated 928 list of groups you are in. The first number is the one returned by 929 getgid(), and the subsequent ones by getgroups(), one of which may be 930 the same as the first number. 931 932 However, a value assigned to C<$(> must be a single number used to 933 set the real gid. So the value given by C<$(> should I<not> be assigned 934 back to C<$(> without being forced numeric, such as by adding zero. Note 935 that this is different to the effective gid (C<$)>) which does take a 936 list. 937 938 You can change both the real gid and the effective gid at the same 939 time by using POSIX::setgid(). Changes to $( require a check to $! 940 to detect any possible errors after an attempted change. 941 942 (Mnemonic: parentheses are used to I<group> things. The real gid is the 943 group you I<left>, if you're running setgid.) 944 945 =item $EFFECTIVE_GROUP_ID 946 947 =item $EGID 948 949 =item $) 950 X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID> 951 952 The effective gid of this process. If you are on a machine that 953 supports membership in multiple groups simultaneously, gives a space 954 separated list of groups you are in. The first number is the one 955 returned by getegid(), and the subsequent ones by getgroups(), one of 956 which may be the same as the first number. 957 958 Similarly, a value assigned to C<$)> must also be a space-separated 959 list of numbers. The first number sets the effective gid, and 960 the rest (if any) are passed to setgroups(). To get the effect of an 961 empty list for setgroups(), just repeat the new effective gid; that is, 962 to force an effective gid of 5 and an effectively empty setgroups() 963 list, say C< $) = "5 5" >. 964 965 You can change both the effective gid and the real gid at the same 966 time by using POSIX::setgid() (use only a single numeric argument). 967 Changes to $) require a check to $! to detect any possible errors 968 after an attempted change. 969 970 (Mnemonic: parentheses are used to I<group> things. The effective gid 971 is the group that's I<right> for you, if you're running setgid.) 972 973 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on 974 machines that support the corresponding I<set[re][ug]id()> routine. C<$(> 975 and C<$)> can be swapped only on machines supporting setregid(). 976 977 =item $PROGRAM_NAME 978 979 =item $0 980 X<$0> X<$PROGRAM_NAME> 981 982 Contains the name of the program being executed. 983 984 On some (read: not all) operating systems assigning to C<$0> modifies 985 the argument area that the C<ps> program sees. On some platforms you 986 may have to use special C<ps> options or a different C<ps> to see the 987 changes. Modifying the $0 is more useful as a way of indicating the 988 current program state than it is for hiding the program you're 989 running. (Mnemonic: same as B<sh> and B<ksh>.) 990 991 Note that there are platform specific limitations on the maximum 992 length of C<$0>. In the most extreme case it may be limited to the 993 space occupied by the original C<$0>. 994 995 In some platforms there may be arbitrary amount of padding, for 996 example space characters, after the modified name as shown by C<ps>. 997 In some platforms this padding may extend all the way to the original 998 length of the argument area, no matter what you do (this is the case 999 for example with Linux 2.2). 1000 1001 Note for BSD users: setting C<$0> does not completely remove "perl" 1002 from the ps(1) output. For example, setting C<$0> to C<"foobar"> may 1003 result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix 1004 and the " (perl)" suffix are shown depends on your exact BSD variant 1005 and version). This is an operating system feature, Perl cannot help it. 1006 1007 In multithreaded scripts Perl coordinates the threads so that any 1008 thread may modify its copy of the C<$0> and the change becomes visible 1009 to ps(1) (assuming the operating system plays along). Note that 1010 the view of C<$0> the other threads have will not change since they 1011 have their own copies of it. 1012 1013 =item $[ 1014 X<$[> 1015 1016 The index of the first element in an array, and of the first character 1017 in a substring. Default is 0, but you could theoretically set it 1018 to 1 to make Perl behave more like B<awk> (or Fortran) when 1019 subscripting and when evaluating the index() and substr() functions. 1020 (Mnemonic: [ begins subscripts.) 1021 1022 As of release 5 of Perl, assignment to C<$[> is treated as a compiler 1023 directive, and cannot influence the behavior of any other file. 1024 (That's why you can only assign compile-time constants to it.) 1025 Its use is highly discouraged. 1026 1027 Note that, unlike other compile-time directives (such as L<strict>), 1028 assignment to C<$[> can be seen from outer lexical scopes in the same file. 1029 However, you can use local() on it to strictly bind its value to a 1030 lexical block. 1031 1032 =item $] 1033 X<$]> 1034 1035 The version + patchlevel / 1000 of the Perl interpreter. This variable 1036 can be used to determine whether the Perl interpreter executing a 1037 script is in the right range of versions. (Mnemonic: Is this version 1038 of perl in the right bracket?) Example: 1039 1040 warn "No checksumming!\n" if $] < 3.019; 1041 1042 See also the documentation of C<use VERSION> and C<require VERSION> 1043 for a convenient way to fail if the running Perl interpreter is too old. 1044 1045 The floating point representation can sometimes lead to inaccurate 1046 numeric comparisons. See C<$^V> for a more modern representation of 1047 the Perl version that allows accurate string comparisons. 1048 1049 =item $COMPILING 1050 1051 =item $^C 1052 X<$^C> X<$COMPILING> 1053 1054 The current value of the flag associated with the B<-c> switch. 1055 Mainly of use with B<-MO=...> to allow code to alter its behavior 1056 when being compiled, such as for example to AUTOLOAD at compile 1057 time rather than normal, deferred loading. Setting 1058 C<$^C = 1> is similar to calling C<B::minus_c>. 1059 1060 =item $DEBUGGING 1061 1062 =item $^D 1063 X<$^D> X<$DEBUGGING> 1064 1065 The current value of the debugging flags. (Mnemonic: value of B<-D> 1066 switch.) May be read or set. Like its command-line equivalent, you can use 1067 numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">. 1068 1069 =item ${^RE_DEBUG_FLAGS} 1070 1071 The current value of the regex debugging flags. Set to 0 for no debug output 1072 even when the re 'debug' module is loaded. See L<re> for details. 1073 1074 =item ${^RE_TRIE_MAXBUF} 1075 1076 Controls how certain regex optimisations are applied and how much memory they 1077 utilize. This value by default is 65536 which corresponds to a 512kB temporary 1078 cache. Set this to a higher value to trade memory for speed when matching 1079 large alternations. Set it to a lower value if you want the optimisations to 1080 be as conservative of memory as possible but still occur, and set it to a 1081 negative value to prevent the optimisation and conserve the most memory. 1082 Under normal situations this variable should be of no interest to you. 1083 1084 =item $SYSTEM_FD_MAX 1085 1086 =item $^F 1087 X<$^F> X<$SYSTEM_FD_MAX> 1088 1089 The maximum system file descriptor, ordinarily 2. System file 1090 descriptors are passed to exec()ed processes, while higher file 1091 descriptors are not. Also, during an open(), system file descriptors are 1092 preserved even if the open() fails. (Ordinary file descriptors are 1093 closed before the open() is attempted.) The close-on-exec 1094 status of a file descriptor will be decided according to the value of 1095 C<$^F> when the corresponding file, pipe, or socket was opened, not the 1096 time of the exec(). 1097 1098 =item $^H 1099 1100 WARNING: This variable is strictly for internal use only. Its availability, 1101 behavior, and contents are subject to change without notice. 1102 1103 This variable contains compile-time hints for the Perl interpreter. At the 1104 end of compilation of a BLOCK the value of this variable is restored to the 1105 value when the interpreter started to compile the BLOCK. 1106 1107 When perl begins to parse any block construct that provides a lexical scope 1108 (e.g., eval body, required file, subroutine body, loop body, or conditional 1109 block), the existing value of $^H is saved, but its value is left unchanged. 1110 When the compilation of the block is completed, it regains the saved value. 1111 Between the points where its value is saved and restored, code that 1112 executes within BEGIN blocks is free to change the value of $^H. 1113 1114 This behavior provides the semantic of lexical scoping, and is used in, 1115 for instance, the C<use strict> pragma. 1116 1117 The contents should be an integer; different bits of it are used for 1118 different pragmatic flags. Here's an example: 1119 1120 sub add_100 { $^H |= 0x100 } 1121 1122 sub foo { 1123 BEGIN { add_100() } 1124 bar->baz($boon); 1125 } 1126 1127 Consider what happens during execution of the BEGIN block. At this point 1128 the BEGIN block has already been compiled, but the body of foo() is still 1129 being compiled. The new value of $^H will therefore be visible only while 1130 the body of foo() is being compiled. 1131 1132 Substitution of the above BEGIN block with: 1133 1134 BEGIN { require strict; strict->import('vars') } 1135 1136 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional 1137 version of the same lexical pragma: 1138 1139 BEGIN { require strict; strict->import('vars') if $condition } 1140 1141 =item %^H 1142 1143 The %^H hash provides the same scoping semantic as $^H. This makes it 1144 useful for implementation of lexically scoped pragmas. See L<perlpragma>. 1145 1146 =item $INPLACE_EDIT 1147 1148 =item $^I 1149 X<$^I> X<$INPLACE_EDIT> 1150 1151 The current value of the inplace-edit extension. Use C<undef> to disable 1152 inplace editing. (Mnemonic: value of B<-i> switch.) 1153 1154 =item $^M 1155 X<$^M> 1156 1157 By default, running out of memory is an untrappable, fatal error. 1158 However, if suitably built, Perl can use the contents of C<$^M> 1159 as an emergency memory pool after die()ing. Suppose that your Perl 1160 were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc. 1161 Then 1162 1163 $^M = 'a' x (1 << 16); 1164 1165 would allocate a 64K buffer for use in an emergency. See the 1166 F<INSTALL> file in the Perl distribution for information on how to 1167 add custom C compilation flags when compiling perl. To discourage casual 1168 use of this advanced feature, there is no L<English|English> long name for 1169 this variable. 1170 1171 =item $OSNAME 1172 1173 =item $^O 1174 X<$^O> X<$OSNAME> 1175 1176 The name of the operating system under which this copy of Perl was 1177 built, as determined during the configuration process. The value 1178 is identical to C<$Config{'osname'}>. See also L<Config> and the 1179 B<-V> command-line switch documented in L<perlrun>. 1180 1181 In Windows platforms, $^O is not very helpful: since it is always 1182 C<MSWin32>, it doesn't tell the difference between 1183 95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or 1184 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish 1185 between the variants. 1186 1187 =item ${^OPEN} 1188 1189 An internal variable used by PerlIO. A string in two parts, separated 1190 by a C<\0> byte, the first part describes the input layers, the second 1191 part describes the output layers. 1192 1193 =item $PERLDB 1194 1195 =item $^P 1196 X<$^P> X<$PERLDB> 1197 1198 The internal variable for debugging support. The meanings of the 1199 various bits are subject to change, but currently indicate: 1200 1201 =over 6 1202 1203 =item 0x01 1204 1205 Debug subroutine enter/exit. 1206 1207 =item 0x02 1208 1209 Line-by-line debugging. 1210 1211 =item 0x04 1212 1213 Switch off optimizations. 1214 1215 =item 0x08 1216 1217 Preserve more data for future interactive inspections. 1218 1219 =item 0x10 1220 1221 Keep info about source lines on which a subroutine is defined. 1222 1223 =item 0x20 1224 1225 Start with single-step on. 1226 1227 =item 0x40 1228 1229 Use subroutine address instead of name when reporting. 1230 1231 =item 0x80 1232 1233 Report C<goto &subroutine> as well. 1234 1235 =item 0x100 1236 1237 Provide informative "file" names for evals based on the place they were compiled. 1238 1239 =item 0x200 1240 1241 Provide informative names to anonymous subroutines based on the place they 1242 were compiled. 1243 1244 =item 0x400 1245 1246 Debug assertion subroutines enter/exit. 1247 1248 =back 1249 1250 Some bits may be relevant at compile-time only, some at 1251 run-time only. This is a new mechanism and the details may change. 1252 1253 =item $LAST_REGEXP_CODE_RESULT 1254 1255 =item $^R 1256 X<$^R> X<$LAST_REGEXP_CODE_RESULT> 1257 1258 The result of evaluation of the last successful C<(?{ code })> 1259 regular expression assertion (see L<perlre>). May be written to. 1260 1261 =item $EXCEPTIONS_BEING_CAUGHT 1262 1263 =item $^S 1264 X<$^S> X<$EXCEPTIONS_BEING_CAUGHT> 1265 1266 Current state of the interpreter. 1267 1268 $^S State 1269 --------- ------------------- 1270 undef Parsing module/eval 1271 true (1) Executing an eval 1272 false (0) Otherwise 1273 1274 The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers. 1275 1276 =item $BASETIME 1277 1278 =item $^T 1279 X<$^T> X<$BASETIME> 1280 1281 The time at which the program began running, in seconds since the 1282 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>, 1283 and B<-C> filetests are based on this value. 1284 1285 =item ${^TAINT} 1286 1287 Reflects if taint mode is on or off. 1 for on (the program was run with 1288 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with 1289 B<-t> or B<-TU>). This variable is read-only. 1290 1291 =item ${^UNICODE} 1292 1293 Reflects certain Unicode settings of Perl. See L<perlrun> 1294 documentation for the C<-C> switch for more information about 1295 the possible values. This variable is set during Perl startup 1296 and is thereafter read-only. 1297 1298 =item ${^UTF8CACHE} 1299 1300 This variable controls the state of the internal UTF-8 offset caching code. 1301 1 for on (the default), 0 for off, -1 to debug the caching code by checking 1302 all its results against linear scans, and panicking on any discrepancy. 1303 1304 =item ${^UTF8LOCALE} 1305 1306 This variable indicates whether an UTF-8 locale was detected by perl at 1307 startup. This information is used by perl when it's in 1308 adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line 1309 switch); see L<perlrun> for more info on this. 1310 1311 =item $PERL_VERSION 1312 1313 =item $^V 1314 X<$^V> X<$PERL_VERSION> 1315 1316 The revision, version, and subversion of the Perl interpreter, represented 1317 as a C<version> object. 1318 1319 This variable first appeared in perl 5.6.0; earlier versions of perl will 1320 see an undefined value. Before perl 5.10.0 $^V was represented as a v-string. 1321 1322 $^V can be used to determine whether the Perl interpreter executing a 1323 script is in the right range of versions. (Mnemonic: use ^V for Version 1324 Control.) Example: 1325 1326 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1 1327 1328 To convert C<$^V> into its string representation use sprintf()'s 1329 C<"%vd"> conversion: 1330 1331 printf "version is v%vd\n", $^V; # Perl's version 1332 1333 See the documentation of C<use VERSION> and C<require VERSION> 1334 for a convenient way to fail if the running Perl interpreter is too old. 1335 1336 See also C<$]> for an older representation of the Perl version. 1337 1338 =item $WARNING 1339 1340 =item $^W 1341 X<$^W> X<$WARNING> 1342 1343 The current value of the warning switch, initially true if B<-w> 1344 was used, false otherwise, but directly modifiable. (Mnemonic: 1345 related to the B<-w> switch.) See also L<warnings>. 1346 1347 =item ${^WARNING_BITS} 1348 1349 The current set of warning checks enabled by the C<use warnings> pragma. 1350 See the documentation of C<warnings> for more details. 1351 1352 =item ${^WIN32_SLOPPY_STAT} 1353 1354 If this variable is set to a true value, then stat() on Windows will 1355 not try to open the file. This means that the link count cannot be 1356 determined and file attributes may be out of date if additional 1357 hardlinks to the file exist. On the other hand, not opening the file 1358 is considerably faster, especially for files on network drives. 1359 1360 This variable could be set in the F<sitecustomize.pl> file to 1361 configure the local Perl installation to use "sloppy" stat() by 1362 default. See L<perlrun> for more information about site 1363 customization. 1364 1365 =item $EXECUTABLE_NAME 1366 1367 =item $^X 1368 X<$^X> X<$EXECUTABLE_NAME> 1369 1370 The name used to execute the current copy of Perl, from C's 1371 C<argv[0]> or (where supported) F</proc/self/exe>. 1372 1373 Depending on the host operating system, the value of $^X may be 1374 a relative or absolute pathname of the perl program file, or may 1375 be the string used to invoke perl but not the pathname of the 1376 perl program file. Also, most operating systems permit invoking 1377 programs that are not in the PATH environment variable, so there 1378 is no guarantee that the value of $^X is in PATH. For VMS, the 1379 value may or may not include a version number. 1380 1381 You usually can use the value of $^X to re-invoke an independent 1382 copy of the same perl that is currently running, e.g., 1383 1384 @first_run = `$^X -le "print int rand 100 for 1..100"`; 1385 1386 But recall that not all operating systems support forking or 1387 capturing of the output of commands, so this complex statement 1388 may not be portable. 1389 1390 It is not safe to use the value of $^X as a path name of a file, 1391 as some operating systems that have a mandatory suffix on 1392 executable files do not require use of the suffix when invoking 1393 a command. To convert the value of $^X to a path name, use the 1394 following statements: 1395 1396 # Build up a set of file names (not command names). 1397 use Config; 1398 $this_perl = $^X; 1399 if ($^O ne 'VMS') 1400 {$this_perl .= $Config{_exe} 1401 unless $this_perl =~ m/$Config{_exe}$/i;} 1402 1403 Because many operating systems permit anyone with read access to 1404 the Perl program file to make a copy of it, patch the copy, and 1405 then execute the copy, the security-conscious Perl programmer 1406 should take care to invoke the installed copy of perl, not the 1407 copy referenced by $^X. The following statements accomplish 1408 this goal, and produce a pathname that can be invoked as a 1409 command or referenced as a file. 1410 1411 use Config; 1412 $secure_perl_path = $Config{perlpath}; 1413 if ($^O ne 'VMS') 1414 {$secure_perl_path .= $Config{_exe} 1415 unless $secure_perl_path =~ m/$Config{_exe}$/i;} 1416 1417 =item ARGV 1418 X<ARGV> 1419 1420 The special filehandle that iterates over command-line filenames in 1421 C<@ARGV>. Usually written as the null filehandle in the angle operator 1422 C<< <> >>. Note that currently C<ARGV> only has its magical effect 1423 within the C<< <> >> operator; elsewhere it is just a plain filehandle 1424 corresponding to the last file opened by C<< <> >>. In particular, 1425 passing C<\*ARGV> as a parameter to a function that expects a filehandle 1426 may not cause your function to automatically read the contents of all the 1427 files in C<@ARGV>. 1428 1429 =item $ARGV 1430 X<$ARGV> 1431 1432 contains the name of the current file when reading from <>. 1433 1434 =item @ARGV 1435 X<@ARGV> 1436 1437 The array @ARGV contains the command-line arguments intended for 1438 the script. C<$#ARGV> is generally the number of arguments minus 1439 one, because C<$ARGV[0]> is the first argument, I<not> the program's 1440 command name itself. See C<$0> for the command name. 1441 1442 =item ARGVOUT 1443 X<ARGVOUT> 1444 1445 The special filehandle that points to the currently open output file 1446 when doing edit-in-place processing with B<-i>. Useful when you have 1447 to do a lot of inserting and don't want to keep modifying $_. See 1448 L<perlrun> for the B<-i> switch. 1449 1450 =item @F 1451 X<@F> 1452 1453 The array @F contains the fields of each line read in when autosplit 1454 mode is turned on. See L<perlrun> for the B<-a> switch. This array 1455 is package-specific, and must be declared or given a full package name 1456 if not in package main when running under C<strict 'vars'>. 1457 1458 =item @INC 1459 X<@INC> 1460 1461 The array @INC contains the list of places that the C<do EXPR>, 1462 C<require>, or C<use> constructs look for their library files. It 1463 initially consists of the arguments to any B<-I> command-line 1464 switches, followed by the default Perl library, probably 1465 F</usr/local/lib/perl>, followed by ".", to represent the current 1466 directory. ("." will not be appended if taint checks are enabled, either by 1467 C<-T> or by C<-t>.) If you need to modify this at runtime, you should use 1468 the C<use lib> pragma to get the machine-dependent library properly 1469 loaded also: 1470 1471 use lib '/mypath/libdir/'; 1472 use SomeMod; 1473 1474 You can also insert hooks into the file inclusion system by putting Perl 1475 code directly into @INC. Those hooks may be subroutine references, array 1476 references or blessed objects. See L<perlfunc/require> for details. 1477 1478 =item @ARG 1479 1480 =item @_ 1481 X<@_> X<@ARG> 1482 1483 Within a subroutine the array @_ contains the parameters passed to that 1484 subroutine. See L<perlsub>. 1485 1486 =item %INC 1487 X<%INC> 1488 1489 The hash %INC contains entries for each filename included via the 1490 C<do>, C<require>, or C<use> operators. The key is the filename 1491 you specified (with module names converted to pathnames), and the 1492 value is the location of the file found. The C<require> 1493 operator uses this hash to determine whether a particular file has 1494 already been included. 1495 1496 If the file was loaded via a hook (e.g. a subroutine reference, see 1497 L<perlfunc/require> for a description of these hooks), this hook is 1498 by default inserted into %INC in place of a filename. Note, however, 1499 that the hook may have set the %INC entry by itself to provide some more 1500 specific info. 1501 1502 =item %ENV 1503 1504 =item $ENV{expr} 1505 X<%ENV> 1506 1507 The hash %ENV contains your current environment. Setting a 1508 value in C<ENV> changes the environment for any child processes 1509 you subsequently fork() off. 1510 1511 =item %SIG 1512 1513 =item $SIG{expr} 1514 X<%SIG> 1515 1516 The hash C<%SIG> contains signal handlers for signals. For example: 1517 1518 sub handler { # 1st argument is signal name 1519 my($sig) = @_; 1520 print "Caught a SIG$sig--shutting down\n"; 1521 close(LOG); 1522 exit(0); 1523 } 1524 1525 $SIG{'INT'} = \&handler; 1526 $SIG{'QUIT'} = \&handler; 1527 ... 1528 $SIG{'INT'} = 'DEFAULT'; # restore default action 1529 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT 1530 1531 Using a value of C<'IGNORE'> usually has the effect of ignoring the 1532 signal, except for the C<CHLD> signal. See L<perlipc> for more about 1533 this special case. 1534 1535 Here are some other examples: 1536 1537 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended) 1538 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber 1539 $SIG{"PIPE"} = *Plumber; # somewhat esoteric 1540 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return?? 1541 1542 Be sure not to use a bareword as the name of a signal handler, 1543 lest you inadvertently call it. 1544 1545 If your system has the sigaction() function then signal handlers are 1546 installed using it. This means you get reliable signal handling. 1547 1548 The default delivery policy of signals changed in Perl 5.8.0 from 1549 immediate (also known as "unsafe") to deferred, also known as 1550 "safe signals". See L<perlipc> for more information. 1551 1552 Certain internal hooks can be also set using the %SIG hash. The 1553 routine indicated by C<$SIG{__WARN__}> is called when a warning message is 1554 about to be printed. The warning message is passed as the first 1555 argument. The presence of a C<__WARN__> hook causes the ordinary printing 1556 of warnings to C<STDERR> to be suppressed. You can use this to save warnings 1557 in a variable, or turn warnings into fatal errors, like this: 1558 1559 local $SIG{__WARN__} = sub { die $_[0] }; 1560 eval $proggie; 1561 1562 As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can 1563 disable warnings using the empty subroutine: 1564 1565 local $SIG{__WARN__} = sub {}; 1566 1567 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception 1568 is about to be thrown. The error message is passed as the first 1569 argument. When a C<__DIE__> hook routine returns, the exception 1570 processing continues as it would have in the absence of the hook, 1571 unless the hook routine itself exits via a C<goto>, a loop exit, or a C<die()>. 1572 The C<__DIE__> handler is explicitly disabled during the call, so that you 1573 can die from a C<__DIE__> handler. Similarly for C<__WARN__>. 1574 1575 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called 1576 even inside an eval(). Do not use this to rewrite a pending exception 1577 in C<$@>, or as a bizarre substitute for overriding C<CORE::GLOBAL::die()>. 1578 This strange action at a distance may be fixed in a future release 1579 so that C<$SIG{__DIE__}> is only called if your program is about 1580 to exit, as was the original intent. Any other use is deprecated. 1581 1582 C<__DIE__>/C<__WARN__> handlers are very special in one respect: 1583 they may be called to report (probable) errors found by the parser. 1584 In such a case the parser may be in inconsistent state, so any 1585 attempt to evaluate Perl code from such a handler will probably 1586 result in a segfault. This means that warnings or errors that 1587 result from parsing Perl should be used with extreme caution, like 1588 this: 1589 1590 require Carp if defined $^S; 1591 Carp::confess("Something wrong") if defined &Carp::confess; 1592 die "Something wrong, but could not load Carp to give backtrace... 1593 To see backtrace try starting Perl with -MCarp switch"; 1594 1595 Here the first line will load Carp I<unless> it is the parser who 1596 called the handler. The second line will print backtrace and die if 1597 Carp was available. The third line will be executed only if Carp was 1598 not available. 1599 1600 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and 1601 L<warnings> for additional information. 1602 1603 =back 1604 1605 =head2 Error Indicators 1606 X<error> X<exception> 1607 1608 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information 1609 about different types of error conditions that may appear during 1610 execution of a Perl program. The variables are shown ordered by 1611 the "distance" between the subsystem which reported the error and 1612 the Perl process. They correspond to errors detected by the Perl 1613 interpreter, C library, operating system, or an external program, 1614 respectively. 1615 1616 To illustrate the differences between these variables, consider the 1617 following Perl expression, which uses a single-quoted string: 1618 1619 eval q{ 1620 open my $pipe, "/cdrom/install |" or die $!; 1621 my @res = <$pipe>; 1622 close $pipe or die "bad pipe: $?, $!"; 1623 }; 1624 1625 After execution of this statement all 4 variables may have been set. 1626 1627 C<$@> is set if the string to be C<eval>-ed did not compile (this 1628 may happen if C<open> or C<close> were imported with bad prototypes), 1629 or if Perl code executed during evaluation die()d . In these cases 1630 the value of $@ is the compile error, or the argument to C<die> 1631 (which will interpolate C<$!> and C<$?>). (See also L<Fatal>, 1632 though.) 1633 1634 When the eval() expression above is executed, open(), C<< <PIPE> >>, 1635 and C<close> are translated to calls in the C run-time library and 1636 thence to the operating system kernel. C<$!> is set to the C library's 1637 C<errno> if one of these calls fails. 1638 1639 Under a few operating systems, C<$^E> may contain a more verbose 1640 error indicator, such as in this case, "CDROM tray not closed." 1641 Systems that do not support extended error messages leave C<$^E> 1642 the same as C<$!>. 1643 1644 Finally, C<$?> may be set to non-0 value if the external program 1645 F</cdrom/install> fails. The upper eight bits reflect specific 1646 error conditions encountered by the program (the program's exit() 1647 value). The lower eight bits reflect mode of failure, like signal 1648 death and core dump information See wait(2) for details. In 1649 contrast to C<$!> and C<$^E>, which are set only if error condition 1650 is detected, the variable C<$?> is set on each C<wait> or pipe 1651 C<close>, overwriting the old value. This is more like C<$@>, which 1652 on every eval() is always set on failure and cleared on success. 1653 1654 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>, 1655 and C<$?>. 1656 1657 =head2 Technical Note on the Syntax of Variable Names 1658 1659 Variable names in Perl can have several formats. Usually, they 1660 must begin with a letter or underscore, in which case they can be 1661 arbitrarily long (up to an internal limit of 251 characters) and 1662 may contain letters, digits, underscores, or the special sequence 1663 C<::> or C<'>. In this case, the part before the last C<::> or 1664 C<'> is taken to be a I<package qualifier>; see L<perlmod>. 1665 1666 Perl variable names may also be a sequence of digits or a single 1667 punctuation or control character. These names are all reserved for 1668 special uses by Perl; for example, the all-digits names are used 1669 to hold data captured by backreferences after a regular expression 1670 match. Perl has a special syntax for the single-control-character 1671 names: It understands C<^X> (caret C<X>) to mean the control-C<X> 1672 character. For example, the notation C<$^W> (dollar-sign caret 1673 C<W>) is the scalar variable whose name is the single character 1674 control-C<W>. This is better than typing a literal control-C<W> 1675 into your program. 1676 1677 Finally, new in Perl 5.6, Perl variable names may be alphanumeric 1678 strings that begin with control characters (or better yet, a caret). 1679 These variables must be written in the form C<${^Foo}>; the braces 1680 are not optional. C<${^Foo}> denotes the scalar variable whose 1681 name is a control-C<F> followed by two C<o>'s. These variables are 1682 reserved for future special uses by Perl, except for the ones that 1683 begin with C<^_> (control-underscore or caret-underscore). No 1684 control-character name that begins with C<^_> will acquire a special 1685 meaning in any future version of Perl; such names may therefore be 1686 used safely in programs. C<$^_> itself, however, I<is> reserved. 1687 1688 Perl identifiers that begin with digits, control characters, or 1689 punctuation characters are exempt from the effects of the C<package> 1690 declaration and are always forced to be in package C<main>; they are 1691 also exempt from C<strict 'vars'> errors. A few other names are also 1692 exempt in these ways: 1693 1694 ENV STDIN 1695 INC STDOUT 1696 ARGV STDERR 1697 ARGVOUT _ 1698 SIG 1699 1700 In particular, the new special C<${^_XYZ}> variables are always taken 1701 to be in package C<main>, regardless of any C<package> declarations 1702 presently in scope. 1703 1704 =head1 BUGS 1705 1706 Due to an unfortunate accident of Perl's implementation, C<use 1707 English> imposes a considerable performance penalty on all regular 1708 expression matches in a program, regardless of whether they occur 1709 in the scope of C<use English>. For that reason, saying C<use 1710 English> in libraries is strongly discouraged. See the 1711 Devel::SawAmpersand module documentation from CPAN 1712 ( http://www.cpan.org/modules/by-module/Devel/ ) 1713 for more information. Writing C<use English '-no_match_vars';> 1714 avoids the performance penalty. 1715 1716 Having to even think about the C<$^S> variable in your exception 1717 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented 1718 invites grievous and difficult to track down errors. Avoid it 1719 and use an C<END{}> or CORE::GLOBAL::die override instead.
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