1=head1 NAME 2 3perlfunc - Perl builtin functions 4 5=head1 DESCRIPTION 6 7The functions in this section can serve as terms in an expression. 8They fall into two major categories: list operators and named unary 9operators. These differ in their precedence relationship with a 10following comma. (See the precedence table in L<perlop>.) List 11operators take more than one argument, while unary operators can never 12take more than one argument. Thus, a comma terminates the argument of 13a unary operator, but merely separates the arguments of a list 14operator. A unary operator generally provides a scalar context to its 15argument, while a list operator may provide either scalar and list 16contexts for its arguments. If it does both, the scalar arguments will 17be first, and the list argument will follow. (Note that there can only 18ever be one list argument.) For instance, splice() has three scalar 19arguments followed by a list. 20 21In the syntax descriptions that follow, list operators that expect a 22list (and provide list context for the elements of the list) are shown 23with LIST as an argument. Such a list may consist of any combination 24of scalar arguments or list values; the list values will be included 25in the list as if each individual element were interpolated at that 26point in the list, forming a longer single-dimensional list value. 27Elements of the LIST should be separated by commas. 28 29Any function in the list below may be used either with or without 30parentheses around its arguments. (The syntax descriptions omit the 31parens.) If you use the parens, the simple (but occasionally 32surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a 33function, and precedence doesn't matter. Otherwise it's a list 34operator or unary operator, and precedence does matter. And whitespace 35between the function and left parenthesis doesn't count--so you need to 36be careful sometimes: 37 38 print 1+2+3; # Prints 6. 39 print(1+2) + 3; # Prints 3. 40 print (1+2)+3; # Also prints 3! 41 print +(1+2)+3; # Prints 6. 42 print ((1+2)+3); # Prints 6. 43 44If you run Perl with the B<-w> switch it can warn you about this. For 45example, the third line above produces: 46 47 print (...) interpreted as function at - line 1. 48 Useless use of integer addition in void context at - line 1. 49 50For functions that can be used in either a scalar or list context, 51non-abortive failure is generally indicated in a scalar context by 52returning the undefined value, and in a list context by returning the 53null list. 54 55Remember the following rule: 56 57=over 8 58 59=item 60 61I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!> 62 63=back 64 65Each operator and function decides which sort of value it would be most 66appropriate to return in a scalar context. Some operators return the 67length of the list that would have been returned in a list context. Some 68operators return the first value in the list. Some operators return the 69last value in the list. Some operators return a count of successful 70operations. In general, they do what you want, unless you want 71consistency. 72 73=head2 Perl Functions by Category 74 75Here are Perl's functions (including things that look like 76functions, like some of the keywords and named operators) 77arranged by category. Some functions appear in more 78than one place. 79 80=over 81 82=item Functions for SCALARs or strings 83 84chomp, chop, chr, crypt, hex, index, lc, lcfirst, length, 85oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex, 86sprintf, substr, tr///, uc, ucfirst, y/// 87 88=item Regular expressions and pattern matching 89 90m//, pos, quotemeta, s///, split, study 91 92=item Numeric functions 93 94abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt, 95srand 96 97=item Functions for real @ARRAYs 98 99pop, push, shift, splice, unshift 100 101=item Functions for list data 102 103grep, join, map, qw/STRING/, reverse, sort, unpack 104 105=item Functions for real %HASHes 106 107delete, each, exists, keys, values 108 109=item Input and output functions 110 111binmode, close, closedir, dbmclose, dbmopen, die, eof, 112fileno, flock, format, getc, print, printf, read, readdir, 113rewinddir, seek, seekdir, select, syscall, sysread, 114syswrite, tell, telldir, truncate, warn, write 115 116=item Functions for fixed length data or records 117 118pack, read, syscall, sysread, syswrite, unpack, vec 119 120=item Functions for filehandles, files, or directories 121 122-X, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link, 123lstat, mkdir, open, opendir, readlink, rename, rmdir, 124stat, symlink, umask, unlink, utime 125 126=item Keywords related to the control flow of your perl program 127 128caller, continue, die, do, dump, eval, exit, goto, last, 129next, redo, return, sub, wantarray 130 131=item Keywords related to scoping 132 133caller, import, local, my, package, use 134 135=item Miscellaneous functions 136 137defined, dump, eval, formline, local, my, reset, scalar, 138undef, wantarray 139 140=item Functions for processes and process groups 141 142alarm, exec, fork, getpgrp, getppid, getpriority, kill, 143pipe, qx/STRING/, setpgrp, setpriority, sleep, system, 144times, wait, waitpid 145 146=item Keywords related to perl modules 147 148do, import, no, package, require, use 149 150=item Keywords related to classes and object-orientedness 151 152bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use 153 154=item Low-level socket functions 155 156accept, bind, connect, getpeername, getsockname, 157getsockopt, listen, recv, send, setsockopt, shutdown, 158socket, socketpair 159 160=item System V interprocess communication functions 161 162msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop, 163shmctl, shmget, shmread, shmwrite 164 165=item Fetching user and group info 166 167endgrent, endhostent, endnetent, endpwent, getgrent, 168getgrgid, getgrnam, getlogin, getpwent, getpwnam, 169getpwuid, setgrent, setpwent 170 171=item Fetching network info 172 173endprotoent, endservent, gethostbyaddr, gethostbyname, 174gethostent, getnetbyaddr, getnetbyname, getnetent, 175getprotobyname, getprotobynumber, getprotoent, 176getservbyname, getservbyport, getservent, sethostent, 177setnetent, setprotoent, setservent 178 179=item Time-related functions 180 181gmtime, localtime, time, times 182 183=item Functions new in perl5 184 185abs, bless, chomp, chr, exists, formline, glob, import, lc, 186lcfirst, map, my, no, qx, qw, ref, sub*, sysopen, tie, tied, uc, 187ucfirst, untie, use 188 189* - C<sub> was a keyword in perl4, but in perl5 it is an 190operator which can be used in expressions. 191 192=item Functions obsoleted in perl5 193 194dbmclose, dbmopen 195 196 197=back 198 199=head2 Alphabetical Listing of Perl Functions 200 201 202=over 8 203 204=item -X FILEHANDLE 205 206=item -X EXPR 207 208=item -X 209 210A file test, where X is one of the letters listed below. This unary 211operator takes one argument, either a filename or a filehandle, and 212tests the associated file to see if something is true about it. If the 213argument is omitted, tests $_, except for C<-t>, which tests STDIN. 214Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or 215the undefined value if the file doesn't exist. Despite the funny 216names, precedence is the same as any other named unary operator, and 217the argument may be parenthesized like any other unary operator. The 218operator may be any of: 219 220 -r File is readable by effective uid/gid. 221 -w File is writable by effective uid/gid. 222 -x File is executable by effective uid/gid. 223 -o File is owned by effective uid. 224 225 -R File is readable by real uid/gid. 226 -W File is writable by real uid/gid. 227 -X File is executable by real uid/gid. 228 -O File is owned by real uid. 229 230 -e File exists. 231 -z File has zero size. 232 -s File has non-zero size (returns size). 233 234 -f File is a plain file. 235 -d File is a directory. 236 -l File is a symbolic link. 237 -p File is a named pipe (FIFO). 238 -S File is a socket. 239 -b File is a block special file. 240 -c File is a character special file. 241 -t Filehandle is opened to a tty. 242 243 -u File has setuid bit set. 244 -g File has setgid bit set. 245 -k File has sticky bit set. 246 247 -T File is a text file. 248 -B File is a binary file (opposite of -T). 249 250 -M Age of file in days when script started. 251 -A Same for access time. 252 -C Same for inode change time. 253 254The interpretation of the file permission operators C<-r>, C<-R>, C<-w>, 255C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the 256uids and gids of the user. There may be other reasons you can't actually 257read, write or execute the file. Also note that, for the superuser, 258C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return 2591 if any execute bit is set in the mode. Scripts run by the superuser may 260thus need to do a stat() in order to determine the actual mode of the 261file, or temporarily set the uid to something else. 262 263Example: 264 265 while (<>) { 266 chop; 267 next unless -f $_; # ignore specials 268 ... 269 } 270 271Note that C<-s/a/b/> does not do a negated substitution. Saying 272C<-exp($foo)> still works as expected, however--only single letters 273following a minus are interpreted as file tests. 274 275The C<-T> and C<-B> switches work as follows. The first block or so of the 276file is examined for odd characters such as strange control codes or 277characters with the high bit set. If too many odd characters (>30%) 278are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file 279containing null in the first block is considered a binary file. If C<-T> 280or C<-B> is used on a filehandle, the current stdio buffer is examined 281rather than the first block. Both C<-T> and C<-B> return TRUE on a null 282file, or a file at EOF when testing a filehandle. Because you have to 283read a file to do the C<-T> test, on most occasions you want to use a C<-f> 284against the file first, as in C<next unless -f $file && -T $file>. 285 286If any of the file tests (or either the stat() or lstat() operators) are given the 287special filehandle consisting of a solitary underline, then the stat 288structure of the previous file test (or stat operator) is used, saving 289a system call. (This doesn't work with C<-t>, and you need to remember 290that lstat() and C<-l> will leave values in the stat structure for the 291symbolic link, not the real file.) Example: 292 293 print "Can do.\n" if -r $a || -w _ || -x _; 294 295 stat($filename); 296 print "Readable\n" if -r _; 297 print "Writable\n" if -w _; 298 print "Executable\n" if -x _; 299 print "Setuid\n" if -u _; 300 print "Setgid\n" if -g _; 301 print "Sticky\n" if -k _; 302 print "Text\n" if -T _; 303 print "Binary\n" if -B _; 304 305=item abs VALUE 306 307Returns the absolute value of its argument. 308 309=item accept NEWSOCKET,GENERICSOCKET 310 311Accepts an incoming socket connect, just as the accept(2) system call 312does. Returns the packed address if it succeeded, FALSE otherwise. 313See example in L<perlipc/"Sockets: Client/Server Communication">. 314 315=item alarm SECONDS 316 317Arranges to have a SIGALRM delivered to this process after the 318specified number of seconds have elapsed. (On some machines, 319unfortunately, the elapsed time may be up to one second less than you 320specified because of how seconds are counted.) Only one timer may be 321counting at once. Each call disables the previous timer, and an 322argument of 0 may be supplied to cancel the previous timer without 323starting a new one. The returned value is the amount of time remaining 324on the previous timer. 325 326For delays of finer granularity than one second, you may use Perl's 327syscall() interface to access setitimer(2) if your system supports it, 328or else see L</select()> below. It is not advised to intermix alarm() 329and sleep() calls. 330 331=item atan2 Y,X 332 333Returns the arctangent of Y/X in the range -PI to PI. 334 335=item bind SOCKET,NAME 336 337Binds a network address to a socket, just as the bind system call 338does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a 339packed address of the appropriate type for the socket. See the examples in 340L<perlipc/"Sockets: Client/Server Communication">. 341 342=item binmode FILEHANDLE 343 344Arranges for the file to be read or written in "binary" mode in operating 345systems that distinguish between binary and text files. Files that are 346not in binary mode have CR LF sequences translated to LF on input and LF 347translated to CR LF on output. Binmode has no effect under Unix; in DOS 348and similarly archaic systems, it may be imperative--otherwise your 349DOS-damaged C library may mangle your file. The key distinction between 350systems that need binmode and those that don't is their text file 351formats. Systems like Unix and Plan9 that delimit lines with a single 352character, and that encode that character in C as '\n', do not need 353C<binmode>. The rest need it. If FILEHANDLE is an expression, the value 354is taken as the name of the filehandle. 355 356=item bless REF,CLASSNAME 357 358=item bless REF 359 360This function tells the referenced object (passed as REF) that it is now 361an object in the CLASSNAME package--or the current package if no CLASSNAME 362is specified, which is often the case. It returns the reference for 363convenience, since a bless() is often the last thing in a constructor. 364Always use the two-argument version if the function doing the blessing 365might be inherited by a derived class. See L<perlobj> for more about the 366blessing (and blessings) of objects. 367 368=item caller EXPR 369 370=item caller 371 372Returns the context of the current subroutine call. In a scalar context, 373returns TRUE if there is a caller, that is, if we're in a subroutine or 374eval() or require(), and FALSE otherwise. In a list context, returns 375 376 ($package, $filename, $line) = caller; 377 378With EXPR, it returns some extra information that the debugger uses to 379print a stack trace. The value of EXPR indicates how many call frames 380to go back before the current one. 381 382 ($package, $filename, $line, 383 $subroutine, $hasargs, $wantargs) = caller($i); 384 385Furthermore, when called from within the DB package, caller returns more 386detailed information: it sets the list variable @DB::args to be the 387arguments with which that subroutine was invoked. 388 389=item chdir EXPR 390 391Changes the working directory to EXPR, if possible. If EXPR is 392omitted, changes to home directory. Returns TRUE upon success, FALSE 393otherwise. See example under die(). 394 395=item chmod LIST 396 397Changes the permissions of a list of files. The first element of the 398list must be the numerical mode, which should probably be an octal 399number. Returns the number of files successfully changed. 400 401 $cnt = chmod 0755, 'foo', 'bar'; 402 chmod 0755, @executables; 403 404=item chomp VARIABLE 405 406=item chomp LIST 407 408=item chomp 409 410This is a slightly safer version of chop (see below). It removes any 411line ending that corresponds to the current value of C<$/> (also known as 412$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number 413of characters removed. It's often used to remove the newline from the 414end of an input record when you're worried that the final record may be 415missing its newline. When in paragraph mode (C<$/ = "">), it removes all 416trailing newlines from the string. If VARIABLE is omitted, it chomps 417$_. Example: 418 419 while (<>) { 420 chomp; # avoid \n on last field 421 @array = split(/:/); 422 ... 423 } 424 425You can actually chomp anything that's an lvalue, including an assignment: 426 427 chomp($cwd = `pwd`); 428 chomp($answer = <STDIN>); 429 430If you chomp a list, each element is chomped, and the total number of 431characters removed is returned. 432 433=item chop VARIABLE 434 435=item chop LIST 436 437=item chop 438 439Chops off the last character of a string and returns the character 440chopped. It's used primarily to remove the newline from the end of an 441input record, but is much more efficient than C<s/\n//> because it neither 442scans nor copies the string. If VARIABLE is omitted, chops $_. 443Example: 444 445 while (<>) { 446 chop; # avoid \n on last field 447 @array = split(/:/); 448 ... 449 } 450 451You can actually chop anything that's an lvalue, including an assignment: 452 453 chop($cwd = `pwd`); 454 chop($answer = <STDIN>); 455 456If you chop a list, each element is chopped. Only the value of the 457last chop is returned. 458 459Note that chop returns the last character. To return all but the last 460character, use C<substr($string, 0, -1)>. 461 462=item chown LIST 463 464Changes the owner (and group) of a list of files. The first two 465elements of the list must be the I<NUMERICAL> uid and gid, in that order. 466Returns the number of files successfully changed. 467 468 $cnt = chown $uid, $gid, 'foo', 'bar'; 469 chown $uid, $gid, @filenames; 470 471Here's an example that looks up non-numeric uids in the passwd file: 472 473 print "User: "; 474 chop($user = <STDIN>); 475 print "Files: " 476 chop($pattern = <STDIN>); 477 478 ($login,$pass,$uid,$gid) = getpwnam($user) 479 or die "$user not in passwd file"; 480 481 @ary = <${pattern}>; # expand filenames 482 chown $uid, $gid, @ary; 483 484On most systems, you are not allowed to change the ownership of the 485file unless you're the superuser, although you should be able to change 486the group to any of your secondary groups. On insecure systems, these 487restrictions may be relaxed, but this is not a portable assumption. 488 489=item chr NUMBER 490 491Returns the character represented by that NUMBER in the character set. 492For example, C<chr(65)> is "A" in ASCII. 493 494=item chroot FILENAME 495 496This function works as the system call by the same name: it makes the 497named directory the new root directory for all further pathnames that 498begin with a "/" by your process and all of its children. (It doesn't 499change your current working directory is unaffected.) For security 500reasons, this call is restricted to the superuser. If FILENAME is 501omitted, does chroot to $_. 502 503=item close FILEHANDLE 504 505Closes the file or pipe associated with the file handle, returning TRUE 506only if stdio successfully flushes buffers and closes the system file 507descriptor. You don't have to close FILEHANDLE if you are immediately 508going to do another open() on it, since open() will close it for you. (See 509open().) However, an explicit close on an input file resets the line 510counter ($.), while the implicit close done by open() does not. Also, 511closing a pipe will wait for the process executing on the pipe to 512complete, in case you want to look at the output of the pipe 513afterwards. Closing a pipe explicitly also puts the status value of 514the command into C<$?>. Example: 515 516 open(OUTPUT, '|sort >foo'); # pipe to sort 517 ... # print stuff to output 518 close OUTPUT; # wait for sort to finish 519 open(INPUT, 'foo'); # get sort's results 520 521FILEHANDLE may be an expression whose value gives the real filehandle name. 522 523=item closedir DIRHANDLE 524 525Closes a directory opened by opendir(). 526 527=item connect SOCKET,NAME 528 529Attempts to connect to a remote socket, just as the connect system call 530does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a 531packed address of the appropriate type for the socket. See the examples in 532L<perlipc/"Sockets: Client/Server Communication">. 533 534=item continue BLOCK 535 536Actually a flow control statement rather than a function. If there is a 537C<continue> BLOCK attached to a BLOCK (typically in a C<while> or 538C<foreach>), it is always executed just before the conditional is about to 539be evaluated again, just like the third part of a C<for> loop in C. Thus 540it can be used to increment a loop variable, even when the loop has been 541continued via the C<next> statement (which is similar to the C C<continue> 542statement). 543 544=item cos EXPR 545 546Returns the cosine of EXPR (expressed in radians). If EXPR is omitted 547takes cosine of $_. 548 549=item crypt PLAINTEXT,SALT 550 551Encrypts a string exactly like the crypt(3) function in the C library 552(assuming that you actually have a version there that has not been 553extirpated as a potential munition). This can prove useful for checking 554the password file for lousy passwords, amongst other things. Only the 555guys wearing white hats should do this. 556 557Here's an example that makes sure that whoever runs this program knows 558their own password: 559 560 $pwd = (getpwuid($<))[1]; 561 $salt = substr($pwd, 0, 2); 562 563 system "stty -echo"; 564 print "Password: "; 565 chop($word = <STDIN>); 566 print "\n"; 567 system "stty echo"; 568 569 if (crypt($word, $salt) ne $pwd) { 570 die "Sorry...\n"; 571 } else { 572 print "ok\n"; 573 } 574 575Of course, typing in your own password to whoever asks you 576for it is unwise. 577 578=item dbmclose ASSOC_ARRAY 579 580[This function has been superseded by the untie() function.] 581 582Breaks the binding between a DBM file and an associative array. 583 584=item dbmopen ASSOC,DBNAME,MODE 585 586[This function has been superseded by the tie() function.] 587 588This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an 589associative array. ASSOC is the name of the associative array. (Unlike 590normal open, the first argument is I<NOT> a filehandle, even though it 591looks like one). DBNAME is the name of the database (without the F<.dir> 592or F<.pag> extension if any). If the database does not exist, it is 593created with protection specified by MODE (as modified by the umask()). 594If your system only supports the older DBM functions, you may perform only 595one dbmopen() in your program. In older versions of Perl, if your system 596had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now 597falls back to sdbm(3). 598 599If you don't have write access to the DBM file, you can only read 600associative array variables, not set them. If you want to test whether 601you can write, either use file tests or try setting a dummy array entry 602inside an eval(), which will trap the error. 603 604Note that functions such as keys() and values() may return huge array 605values when used on large DBM files. You may prefer to use the each() 606function to iterate over large DBM files. Example: 607 608 # print out history file offsets 609 dbmopen(%HIST,'/usr/lib/news/history',0666); 610 while (($key,$val) = each %HIST) { 611 print $key, ' = ', unpack('L',$val), "\n"; 612 } 613 dbmclose(%HIST); 614 615See also L<AnyDBM_File> for a more general description of the pros and 616cons of the various dbm apparoches, as well as L<DB_File> for a particularly 617rich implementation. 618 619=item defined EXPR 620 621Returns a boolean value saying whether EXPR has a real value 622or not. Many operations return the undefined value under exceptional 623conditions, such as end of file, uninitialized variable, system error 624and such. This function allows you to distinguish between an undefined 625null scalar and a defined null scalar with operations that might return 626a real null string, such as referencing elements of an array. You may 627also check to see if arrays or subroutines exist. Use of defined on 628predefined variables is not guaranteed to produce intuitive results. 629 630When used on a hash array element, it tells you whether the value 631is defined, not whether the key exists in the hash. Use exists() for that. 632 633Examples: 634 635 print if defined $switch{'D'}; 636 print "$val\n" while defined($val = pop(@ary)); 637 die "Can't readlink $sym: $!" 638 unless defined($value = readlink $sym); 639 eval '@foo = ()' if defined(@foo); 640 die "No XYZ package defined" unless defined %_XYZ; 641 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } 642 643See also undef(). 644 645Note: many folks tend to overuse defined(), and then are surprised to 646discover that the number 0 and the null string are, in fact, defined 647concepts. For example, if you say 648 649 "ab" =~ /a(.*)b/; 650 651the pattern match succeeds, and $1 is defined, despite the fact that it 652matched "nothing". But it didn't really match nothing--rather, it 653matched something that happened to be 0 characters long. This is all 654very above-board and honest. When a function returns an undefined value, 655it's an admission that it couldn't give you an honest answer. So 656you should only use defined() when you're questioning the integrity 657of what you're trying to do. At other times, a simple comparison to 6580 or "" is what you want. 659 660=item delete EXPR 661 662Deletes the specified value from its hash array. Returns the deleted 663value, or the undefined value if nothing was deleted. Deleting from 664C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM 665file deletes the entry from the DBM file. (But deleting from a tie()d 666hash doesn't necessarily return anything.) 667 668The following deletes all the values of an associative array: 669 670 foreach $key (keys %ARRAY) { 671 delete $ARRAY{$key}; 672 } 673 674(But it would be faster to use the undef() command.) Note that the 675EXPR can be arbitrarily complicated as long as the final operation is 676a hash key lookup: 677 678 delete $ref->[$x][$y]{$key}; 679 680=item die LIST 681 682Outside of an eval(), prints the value of LIST to C<STDERR> and exits with 683the current value of $! (errno). If $! is 0, exits with the value of 684C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> is 0, 685exits with 255. Inside an eval(), the error message is stuffed into C<$@>, 686and the eval() is terminated with the undefined value; this makes die() 687the way to raise an exception. 688 689Equivalent examples: 690 691 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; 692 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" 693 694If the value of EXPR does not end in a newline, the current script line 695number and input line number (if any) are also printed, and a newline 696is supplied. Hint: sometimes appending ", stopped" to your message 697will cause it to make better sense when the string "at foo line 123" is 698appended. Suppose you are running script "canasta". 699 700 die "/etc/games is no good"; 701 die "/etc/games is no good, stopped"; 702 703produce, respectively 704 705 /etc/games is no good at canasta line 123. 706 /etc/games is no good, stopped at canasta line 123. 707 708See also exit() and warn(). 709 710=item do BLOCK 711 712Not really a function. Returns the value of the last command in the 713sequence of commands indicated by BLOCK. When modified by a loop 714modifier, executes the BLOCK once before testing the loop condition. 715(On other statements the loop modifiers test the conditional first.) 716 717=item do SUBROUTINE(LIST) 718 719A deprecated form of subroutine call. See L<perlsub>. 720 721=item do EXPR 722 723Uses the value of EXPR as a filename and executes the contents of the 724file as a Perl script. Its primary use is to include subroutines 725from a Perl subroutine library. 726 727 do 'stat.pl'; 728 729is just like 730 731 eval `cat stat.pl`; 732 733except that it's more efficient, more concise, keeps track of the 734current filename for error messages, and searches all the B<-I> 735libraries if the file isn't in the current directory (see also the @INC 736array in L<perlvar/Predefined Names>). It's the same, however, in that it does 737reparse the file every time you call it, so you probably don't want to 738do this inside a loop. 739 740Note that inclusion of library modules is better done with the 741use() and require() operators, which also do error checking 742and raise an exception if there's a problem. 743 744=item dump LABEL 745 746This causes an immediate core dump. Primarily this is so that you can 747use the B<undump> program to turn your core dump into an executable binary 748after having initialized all your variables at the beginning of the 749program. When the new binary is executed it will begin by executing a 750C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of 751it as a goto with an intervening core dump and reincarnation. If LABEL 752is omitted, restarts the program from the top. WARNING: any files 753opened at the time of the dump will NOT be open any more when the 754program is reincarnated, with possible resulting confusion on the part 755of Perl. See also B<-u> option in L<perlrun>. 756 757Example: 758 759 #!/usr/bin/perl 760 require 'getopt.pl'; 761 require 'stat.pl'; 762 %days = ( 763 'Sun' => 1, 764 'Mon' => 2, 765 'Tue' => 3, 766 'Wed' => 4, 767 'Thu' => 5, 768 'Fri' => 6, 769 'Sat' => 7, 770 ); 771 772 dump QUICKSTART if $ARGV[0] eq '-d'; 773 774 QUICKSTART: 775 Getopt('f'); 776 777=item each ASSOC_ARRAY 778 779Returns a 2-element array consisting of the key and value for the next 780value of an associative array, so that you can iterate over it. 781Entries are returned in an apparently random order. When the array is 782entirely read, a null array is returned (which when assigned produces a 783FALSE (0) value). The next call to each() after that will start 784iterating again. The iterator can be reset only by reading all the 785elements from the array. You should not add elements to an array while 786you're iterating over it. There is a single iterator for each 787associative array, shared by all each(), keys() and values() function 788calls in the program. The following prints out your environment like 789the printenv(1) program, only in a different order: 790 791 while (($key,$value) = each %ENV) { 792 print "$key=$value\n"; 793 } 794 795See also keys() and values(). 796 797=item eof FILEHANDLE 798 799=item eof () 800 801=item eof 802 803Returns 1 if the next read on FILEHANDLE will return end of file, or if 804FILEHANDLE is not open. FILEHANDLE may be an expression whose value 805gives the real filehandle name. (Note that this function actually 806reads a character and then ungetc()s it, so it is not very useful in an 807interactive context.) Do not read from a terminal file (or call 808C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such 809as terminals may lose the end-of-file condition if you do. 810 811An C<eof> without an argument uses the last file read as argument. 812Empty parentheses () may be used to indicate 813the pseudofile formed of the files listed on the command line, i.e. 814C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end 815of only the last file. Use C<eof(ARGV)> or eof without the parentheses to 816test I<EACH> file in a while (E<lt>E<gt>) loop. Examples: 817 818 # reset line numbering on each input file 819 while (<>) { 820 print "$.\t$_"; 821 close(ARGV) if (eof); # Not eof(). 822 } 823 824 # insert dashes just before last line of last file 825 while (<>) { 826 if (eof()) { 827 print "--------------\n"; 828 close(ARGV); # close or break; is needed if we 829 # are reading from the terminal 830 } 831 print; 832 } 833 834Practical hint: you almost never need to use C<eof> in Perl, because the 835input operators return undef when they run out of data. 836 837=item eval EXPR 838 839=item eval BLOCK 840 841EXPR is parsed and executed as if it were a little Perl program. It 842is executed in the context of the current Perl program, so that any 843variable settings, subroutine or format definitions remain afterwards. 844The value returned is the value of the last expression evaluated, or a 845return statement may be used, just as with subroutines. 846 847If there is a syntax error or runtime error, or a die() statement is 848executed, an undefined value is returned by eval(), and C<$@> is set to the 849error message. If there was no error, C<$@> is guaranteed to be a null 850string. If EXPR is omitted, evaluates $_. The final semicolon, if 851any, may be omitted from the expression. 852 853Note that, since eval() traps otherwise-fatal errors, it is useful for 854determining whether a particular feature (such as socket() or symlink()) 855is implemented. It is also Perl's exception trapping mechanism, where 856the die operator is used to raise exceptions. 857 858If the code to be executed doesn't vary, you may use the eval-BLOCK 859form to trap run-time errors without incurring the penalty of 860recompiling each time. The error, if any, is still returned in C<$@>. 861Examples: 862 863 # make divide-by-zero non-fatal 864 eval { $answer = $a / $b; }; warn $@ if $@; 865 866 # same thing, but less efficient 867 eval '$answer = $a / $b'; warn $@ if $@; 868 869 # a compile-time error 870 eval { $answer = }; 871 872 # a run-time error 873 eval '$answer ='; # sets $@ 874 875With an eval(), you should be especially careful to remember what's 876being looked at when: 877 878 eval $x; # CASE 1 879 eval "$x"; # CASE 2 880 881 eval '$x'; # CASE 3 882 eval { $x }; # CASE 4 883 884 eval "\$$x++" # CASE 5 885 $$x++; # CASE 6 886 887Cases 1 and 2 above behave identically: they run the code contained in the 888variable $x. (Although case 2 has misleading double quotes making the 889reader wonder what else might be happening (nothing is).) Cases 3 and 4 890likewise behave in the same way: they run the code <$x>, which does 891nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5 892is a place where normally you I<WOULD> like to use double quotes, except 893that in that particular situation, you can just use symbolic references 894instead, as in case 6. 895 896=item exec LIST 897 898The exec() function executes a system command I<AND NEVER RETURNS>. Use 899the system() function if you want it to return. 900 901If there is more than one argument in LIST, or if LIST is an array with 902more than one value, calls execvp(3) with the arguments in LIST. If 903there is only one scalar argument, the argument is checked for shell 904metacharacters. If there are any, the entire argument is passed to 905C</bin/sh -c> for parsing. If there are none, the argument is split 906into words and passed directly to execvp(), which is more efficient. 907Note: exec() and system() do not flush your output buffer, so you may 908need to set C<$|> to avoid lost output. Examples: 909 910 exec '/bin/echo', 'Your arguments are: ', @ARGV; 911 exec "sort $outfile | uniq"; 912 913If you don't really want to execute the first argument, but want to lie 914to the program you are executing about its own name, you can specify 915the program you actually want to run as an "indirect object" (without a 916comma) in front of the LIST. (This always forces interpretation of the 917LIST as a multi-valued list, even if there is only a single scalar in 918the list.) Example: 919 920 $shell = '/bin/csh'; 921 exec $shell '-sh'; # pretend it's a login shell 922 923or, more directly, 924 925 exec {'/bin/csh'} '-sh'; # pretend it's a login shell 926 927=item exists EXPR 928 929Returns TRUE if the specified hash key exists in its hash array, even 930if the corresponding value is undefined. 931 932 print "Exists\n" if exists $array{$key}; 933 print "Defined\n" if defined $array{$key}; 934 print "True\n" if $array{$key}; 935 936A hash element can only be TRUE if it's defined, and defined if 937it exists, but the reverse doesn't necessarily hold true. 938 939Note that the EXPR can be arbitrarily complicated as long as the final 940operation is a hash key lookup: 941 942 if (exists $ref->[$x][$y]{$key}) { ... } 943 944=item exit EXPR 945 946Evaluates EXPR and exits immediately with that value. (Actually, it 947calls any defined C<END> routines first, but the C<END> routines may not 948abort the exit. Likewise any object destructors that need to be called 949are called before exit.) Example: 950 951 $ans = <STDIN>; 952 exit 0 if $ans =~ /^[Xx]/; 953 954See also die(). If EXPR is omitted, exits with 0 status. 955 956=item exp EXPR 957 958Returns I<e> (the natural logarithm base) to the power of EXPR. 959If EXPR is omitted, gives C<exp($_)>. 960 961=item fcntl FILEHANDLE,FUNCTION,SCALAR 962 963Implements the fcntl(2) function. You'll probably have to say 964 965 use Fcntl; 966 967first to get the correct function definitions. Argument processing and 968value return works just like ioctl() below. Note that fcntl() will produce 969a fatal error if used on a machine that doesn't implement fcntl(2). 970For example: 971 972 use Fcntl; 973 fcntl($filehandle, F_GETLK, $packed_return_buffer); 974 975=item fileno FILEHANDLE 976 977Returns the file descriptor for a filehandle. This is useful for 978constructing bitmaps for select(). If FILEHANDLE is an expression, the 979value is taken as the name of the filehandle. 980 981=item flock FILEHANDLE,OPERATION 982 983Calls flock(2) on FILEHANDLE. See L<flock(2)> for definition of 984OPERATION. Returns TRUE for success, FALSE on failure. Will produce a 985fatal error if used on a machine that doesn't implement either flock(2) or 986fcntl(2). The fcntl(2) system call will be automatically used if flock(2) 987is missing from your system. This makes flock() the portable file locking 988strategy, although it will only lock entire files, not records. Note also 989that some versions of flock() cannot lock things over the network; you 990would need to use the more system-specific fcntl() for that. 991 992Here's a mailbox appender for BSD systems. 993 994 $LOCK_SH = 1; 995 $LOCK_EX = 2; 996 $LOCK_NB = 4; 997 $LOCK_UN = 8; 998 999 sub lock { 1000 flock(MBOX,$LOCK_EX); 1001 # and, in case someone appended 1002 # while we were waiting... 1003 seek(MBOX, 0, 2); 1004 } 1005 1006 sub unlock { 1007 flock(MBOX,$LOCK_UN); 1008 } 1009 1010 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") 1011 or die "Can't open mailbox: $!"; 1012 1013 lock(); 1014 print MBOX $msg,"\n\n"; 1015 unlock(); 1016 1017See also L<DB_File> for other flock() examples. 1018 1019=item fork 1020 1021Does a fork(2) system call. Returns the child pid to the parent process 1022and 0 to the child process, or C<undef> if the fork is unsuccessful. 1023Note: unflushed buffers remain unflushed in both processes, which means 1024you may need to set C<$|> ($AUTOFLUSH in English) or call the 1025autoflush() FileHandle method to avoid duplicate output. 1026 1027If you fork() without ever waiting on your children, you will accumulate 1028zombies: 1029 1030 $SIG{CHLD} = sub { wait }; 1031 1032There's also the double-fork trick (error checking on 1033fork() returns omitted); 1034 1035 unless ($pid = fork) { 1036 unless (fork) { 1037 exec "what you really wanna do"; 1038 die "no exec"; 1039 # ... or ... 1040 ## (some_perl_code_here) 1041 exit 0; 1042 } 1043 exit 0; 1044 } 1045 waitpid($pid,0); 1046 1047See also L<perlipc> for more examples of forking and reaping 1048moribund children. 1049 1050=item format 1051 1052Declare a picture format with use by the write() function. For 1053example: 1054 1055 format Something = 1056 Test: @<<<<<<<< @||||| @>>>>> 1057 $str, $%, '$' . int($num) 1058 . 1059 1060 $str = "widget"; 1061 $num = $cost/$quantiy; 1062 $~ = 'Something'; 1063 write; 1064 1065See L<perlform> for many details and examples. 1066 1067 1068=item formline PICTURE, LIST 1069 1070This is an internal function used by C<format>s, though you may call it 1071too. It formats (see L<perlform>) a list of values according to the 1072contents of PICTURE, placing the output into the format output 1073accumulator, C<$^A> (or $ACCUMULATOR in English). 1074Eventually, when a write() is done, the contents of 1075C<$^A> are written to some filehandle, but you could also read C<$^A> 1076yourself and then set C<$^A> back to "". Note that a format typically 1077does one formline() per line of form, but the formline() function itself 1078doesn't care how many newlines are embedded in the PICTURE. This means 1079that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. 1080You may therefore need to use multiple formlines to implement a single 1081record format, just like the format compiler. 1082 1083Be careful if you put double quotes around the picture, since an "C<@>" 1084character may be taken to mean the beginning of an array name. 1085formline() always returns TRUE. See L<perlform> for other examples. 1086 1087=item getc FILEHANDLE 1088 1089=item getc 1090 1091Returns the next character from the input file attached to FILEHANDLE, 1092or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN. 1093This is not particularly efficient. It cannot be used to get unbuffered 1094single-characters, however. For that, try something more like: 1095 1096 if ($BSD_STYLE) { 1097 system "stty cbreak </dev/tty >/dev/tty 2>&1"; 1098 } 1099 else { 1100 system "stty", '-icanon', 'eol', "\001"; 1101 } 1102 1103 $key = getc(STDIN); 1104 1105 if ($BSD_STYLE) { 1106 system "stty -cbreak </dev/tty >/dev/tty 2>&1"; 1107 } 1108 else { 1109 system "stty", 'icanon', 'eol', '^@'; # ascii null 1110 } 1111 print "\n"; 1112 1113Determination of whether to whether $BSD_STYLE should be set 1114is left as an exercise to the reader. 1115 1116See also the C<Term::ReadKey> module from your nearest CPAN site; 1117details on CPAN can be found on L<perlmod/CPAN> 1118 1119=item getlogin 1120 1121Returns the current login from F</etc/utmp>, if any. If null, use 1122getpwuid(). 1123 1124 $login = getlogin || (getpwuid($<))[0] || "Kilroy"; 1125 1126Do not consider getlogin() for authorentication: it is not as 1127secure as getpwuid(). 1128 1129=item getpeername SOCKET 1130 1131Returns the packed sockaddr address of other end of the SOCKET connection. 1132 1133 use Socket; 1134 $hersockaddr = getpeername(SOCK); 1135 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr); 1136 $herhostname = gethostbyaddr($iaddr, AF_INET); 1137 $herstraddr = inet_ntoa($iaddr); 1138 1139=item getpgrp PID 1140 1141Returns the current process group for the specified PID, 0 for the 1142current process. Will raise an exception if used on a machine that 1143doesn't implement getpgrp(2). If PID is omitted, returns process 1144group of current process. 1145 1146=item getppid 1147 1148Returns the process id of the parent process. 1149 1150=item getpriority WHICH,WHO 1151 1152Returns the current priority for a process, a process group, or a user. 1153(See L<getpriority(2)>.) Will raise a fatal exception if used on a 1154machine that doesn't implement getpriority(2). 1155 1156=item getpwnam NAME 1157 1158=item getgrnam NAME 1159 1160=item gethostbyname NAME 1161 1162=item getnetbyname NAME 1163 1164=item getprotobyname NAME 1165 1166=item getpwuid UID 1167 1168=item getgrgid GID 1169 1170=item getservbyname NAME,PROTO 1171 1172=item gethostbyaddr ADDR,ADDRTYPE 1173 1174=item getnetbyaddr ADDR,ADDRTYPE 1175 1176=item getprotobynumber NUMBER 1177 1178=item getservbyport PORT,PROTO 1179 1180=item getpwent 1181 1182=item getgrent 1183 1184=item gethostent 1185 1186=item getnetent 1187 1188=item getprotoent 1189 1190=item getservent 1191 1192=item setpwent 1193 1194=item setgrent 1195 1196=item sethostent STAYOPEN 1197 1198=item setnetent STAYOPEN 1199 1200=item setprotoent STAYOPEN 1201 1202=item setservent STAYOPEN 1203 1204=item endpwent 1205 1206=item endgrent 1207 1208=item endhostent 1209 1210=item endnetent 1211 1212=item endprotoent 1213 1214=item endservent 1215 1216These routines perform the same functions as their counterparts in the 1217system library. Within a list context, the return values from the 1218various get routines are as follows: 1219 1220 ($name,$passwd,$uid,$gid, 1221 $quota,$comment,$gcos,$dir,$shell) = getpw* 1222 ($name,$passwd,$gid,$members) = getgr* 1223 ($name,$aliases,$addrtype,$length,@addrs) = gethost* 1224 ($name,$aliases,$addrtype,$net) = getnet* 1225 ($name,$aliases,$proto) = getproto* 1226 ($name,$aliases,$port,$proto) = getserv* 1227 1228(If the entry doesn't exist you get a null list.) 1229 1230Within a scalar context, you get the name, unless the function was a 1231lookup by name, in which case you get the other thing, whatever it is. 1232(If the entry doesn't exist you get the undefined value.) For example: 1233 1234 $uid = getpwnam 1235 $name = getpwuid 1236 $name = getpwent 1237 $gid = getgrnam 1238 $name = getgrgid 1239 $name = getgrent 1240 etc. 1241 1242The $members value returned by I<getgr*()> is a space separated list of 1243the login names of the members of the group. 1244 1245For the I<gethost*()> functions, if the C<h_errno> variable is supported in 1246C, it will be returned to you via C<$?> if the function call fails. The 1247@addrs value returned by a successful call is a list of the raw 1248addresses returned by the corresponding system library call. In the 1249Internet domain, each address is four bytes long and you can unpack it 1250by saying something like: 1251 1252 ($a,$b,$c,$d) = unpack('C4',$addr[0]); 1253 1254=item getsockname SOCKET 1255 1256Returns the packed sockaddr address of this end of the SOCKET connection. 1257 1258 use Socket; 1259 $mysockaddr = getsockname(SOCK); 1260 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr); 1261 1262=item getsockopt SOCKET,LEVEL,OPTNAME 1263 1264Returns the socket option requested, or undefined if there is an error. 1265 1266=item glob EXPR 1267 1268Returns the value of EXPR with filename expansions such as a shell 1269would do. This is the internal function implementing the <*.*> 1270operator, except it's easier to use. 1271 1272=item gmtime EXPR 1273 1274Converts a time as returned by the time function to a 9-element array 1275with the time localized for the standard Greenwich timezone. 1276Typically used as follows: 1277 1278 1279 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = 1280 gmtime(time); 1281 1282All array elements are numeric, and come straight out of a struct tm. 1283In particular this means that $mon has the range 0..11 and $wday has 1284the range 0..6. If EXPR is omitted, does C<gmtime(time())>. 1285 1286=item goto LABEL 1287 1288=item goto EXPR 1289 1290=item goto &NAME 1291 1292The goto-LABEL form finds the statement labeled with LABEL and resumes 1293execution there. It may not be used to go into any construct that 1294requires initialization, such as a subroutine or a foreach loop. It 1295also can't be used to go into a construct that is optimized away. It 1296can be used to go almost anywhere else within the dynamic scope, 1297including out of subroutines, but it's usually better to use some other 1298construct such as last or die. The author of Perl has never felt the 1299need to use this form of goto (in Perl, that is--C is another matter). 1300 1301The goto-EXPR form expects a label name, whose scope will be resolved 1302dynamically. This allows for computed gotos per FORTRAN, but isn't 1303necessarily recommended if you're optimizing for maintainability: 1304 1305 goto ("FOO", "BAR", "GLARCH")[$i]; 1306 1307The goto-&NAME form is highly magical, and substitutes a call to the 1308named subroutine for the currently running subroutine. This is used by 1309AUTOLOAD subroutines that wish to load another subroutine and then 1310pretend that the other subroutine had been called in the first place 1311(except that any modifications to @_ in the current subroutine are 1312propagated to the other subroutine.) After the goto, not even caller() 1313will be able to tell that this routine was called first. 1314 1315=item grep BLOCK LIST 1316 1317=item grep EXPR,LIST 1318 1319Evaluates the BLOCK or EXPR for each element of LIST (locally setting 1320$_ to each element) and returns the list value consisting of those 1321elements for which the expression evaluated to TRUE. In a scalar 1322context, returns the number of times the expression was TRUE. 1323 1324 @foo = grep(!/^#/, @bar); # weed out comments 1325 1326or equivalently, 1327 1328 @foo = grep {!/^#/} @bar; # weed out comments 1329 1330Note that, since $_ is a reference into the list value, it can be used 1331to modify the elements of the array. While this is useful and 1332supported, it can cause bizarre results if the LIST is not a named 1333array. 1334 1335=item hex EXPR 1336 1337Interprets EXPR as a hex string and returns the corresponding decimal 1338value. (To convert strings that might start with 0 or 0x see 1339oct().) If EXPR is omitted, uses $_. 1340 1341=item import 1342 1343There is no built-in import() function. It is merely an ordinary 1344method (subroutine) defined (or inherited) by modules that wish to export 1345names to another module. The use() function calls the import() method 1346for the package used. See also L</use>, L<perlmod>, and L<Exporter>. 1347 1348=item index STR,SUBSTR,POSITION 1349 1350=item index STR,SUBSTR 1351 1352Returns the position of the first occurrence of SUBSTR in STR at or after 1353POSITION. If POSITION is omitted, starts searching from the beginning of 1354the string. The return value is based at 0 (or whatever you've set the $[ 1355variable to--but don't do that). If the substring is not found, returns 1356one less than the base, ordinarily -1. 1357 1358=item int EXPR 1359 1360Returns the integer portion of EXPR. If EXPR is omitted, uses $_. 1361 1362=item ioctl FILEHANDLE,FUNCTION,SCALAR 1363 1364Implements the ioctl(2) function. You'll probably have to say 1365 1366 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph 1367 1368first to get the correct function definitions. If F<ioctl.ph> doesn't 1369exist or doesn't have the correct definitions you'll have to roll your 1370own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>. 1371(There is a Perl script called B<h2ph> that comes with the Perl kit which 1372may help you in this, but it's non-trivial.) SCALAR will be read and/or 1373written depending on the FUNCTION--a pointer to the string value of SCALAR 1374will be passed as the third argument of the actual ioctl call. (If SCALAR 1375has no string value but does have a numeric value, that value will be 1376passed rather than a pointer to the string value. To guarantee this to be 1377TRUE, add a 0 to the scalar before using it.) The pack() and unpack() 1378functions are useful for manipulating the values of structures used by 1379ioctl(). The following example sets the erase character to DEL. 1380 1381 require 'ioctl.ph'; 1382 $getp = &TIOCGETP; 1383 die "NO TIOCGETP" if $@ || !$getp; 1384 $sgttyb_t = "ccccs"; # 4 chars and a short 1385 if (ioctl(STDIN,$getp,$sgttyb)) { 1386 @ary = unpack($sgttyb_t,$sgttyb); 1387 $ary[2] = 127; 1388 $sgttyb = pack($sgttyb_t,@ary); 1389 ioctl(STDIN,&TIOCSETP,$sgttyb) 1390 || die "Can't ioctl: $!"; 1391 } 1392 1393The return value of ioctl (and fcntl) is as follows: 1394 1395 if OS returns: then Perl returns: 1396 -1 undefined value 1397 0 string "0 but true" 1398 anything else that number 1399 1400Thus Perl returns TRUE on success and FALSE on failure, yet you can 1401still easily determine the actual value returned by the operating 1402system: 1403 1404 ($retval = ioctl(...)) || ($retval = -1); 1405 printf "System returned %d\n", $retval; 1406 1407=item join EXPR,LIST 1408 1409Joins the separate strings of LIST or ARRAY into a single string with 1410fields separated by the value of EXPR, and returns the string. 1411Example: 1412 1413 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); 1414 1415See L<perlfunc/split>. 1416 1417=item keys ASSOC_ARRAY 1418 1419Returns a normal array consisting of all the keys of the named 1420associative array. (In a scalar context, returns the number of keys.) 1421The keys are returned in an apparently random order, but it is the same 1422order as either the values() or each() function produces (given that 1423the associative array has not been modified). Here is yet another way 1424to print your environment: 1425 1426 @keys = keys %ENV; 1427 @values = values %ENV; 1428 while ($#keys >= 0) { 1429 print pop(@keys), '=', pop(@values), "\n"; 1430 } 1431 1432or how about sorted by key: 1433 1434 foreach $key (sort(keys %ENV)) { 1435 print $key, '=', $ENV{$key}, "\n"; 1436 } 1437 1438To sort an array by value, you'll need to use a C<sort{}> 1439function. Here's a descending numeric sort of a hash by its values: 1440 1441 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) { 1442 printf "%4d %s\n", $hash{$key}, $key; 1443 } 1444 1445=item kill LIST 1446 1447Sends a signal to a list of processes. The first element of 1448the list must be the signal to send. Returns the number of 1449processes successfully signaled. 1450 1451 $cnt = kill 1, $child1, $child2; 1452 kill 9, @goners; 1453 1454Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills 1455process groups instead of processes. (On System V, a negative I<PROCESS> 1456number will also kill process groups, but that's not portable.) That 1457means you usually want to use positive not negative signals. You may also 1458use a signal name in quotes. See the L<perlipc/"Signals"> man page for details. 1459 1460=item last LABEL 1461 1462=item last 1463 1464The C<last> command is like the C<break> statement in C (as used in 1465loops); it immediately exits the loop in question. If the LABEL is 1466omitted, the command refers to the innermost enclosing loop. The 1467C<continue> block, if any, is not executed: 1468 1469 LINE: while (<STDIN>) { 1470 last LINE if /^$/; # exit when done with header 1471 ... 1472 } 1473 1474=item lc EXPR 1475 1476Returns an lowercased version of EXPR. This is the internal function 1477implementing the \L escape in double-quoted strings. 1478Should respect any POSIX setlocale() settings. 1479 1480=item lcfirst EXPR 1481 1482Returns the value of EXPR with the first character lowercased. This is 1483the internal function implementing the \l escape in double-quoted strings. 1484Should respect any POSIX setlocale() settings. 1485 1486=item length EXPR 1487 1488Returns the length in characters of the value of EXPR. If EXPR is 1489omitted, returns length of $_. 1490 1491=item link OLDFILE,NEWFILE 1492 1493Creates a new filename linked to the old filename. Returns 1 for 1494success, 0 otherwise. 1495 1496=item listen SOCKET,QUEUESIZE 1497 1498Does the same thing that the listen system call does. Returns TRUE if 1499it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">. 1500 1501=item local EXPR 1502 1503A local modifies the listed variables to be local to the enclosing block, 1504subroutine, C<eval{}> or C<do>. If more than one value is listed, the 1505list must be placed in parens. See L<perlsub/"Temporary Values via 1506local()"> for details. 1507 1508But you really probably want to be using my() instead, because local() isn't 1509what most people think of as "local"). See L<perlsub/"Private Variables 1510via my()"> for details. 1511 1512=item localtime EXPR 1513 1514Converts a time as returned by the time function to a 9-element array 1515with the time analyzed for the local timezone. Typically used as 1516follows: 1517 1518 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = 1519 localtime(time); 1520 1521All array elements are numeric, and come straight out of a struct tm. 1522In particular this means that $mon has the range 0..11 and $wday has 1523the range 0..6. If EXPR is omitted, does localtime(time). 1524 1525In a scalar context, prints out the ctime(3) value: 1526 1527 $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994" 1528 1529Also see the F<timelocal.pl> library, and the strftime(3) function available 1530via the POSIX modulie. 1531 1532=item log EXPR 1533 1534Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log 1535of $_. 1536 1537=item lstat FILEHANDLE 1538 1539=item lstat EXPR 1540 1541Does the same thing as the stat() function, but stats a symbolic link 1542instead of the file the symbolic link points to. If symbolic links are 1543unimplemented on your system, a normal stat() is done. 1544 1545=item m// 1546 1547The match operator. See L<perlop>. 1548 1549=item map BLOCK LIST 1550 1551=item map EXPR,LIST 1552 1553Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each 1554element) and returns the list value composed of the results of each such 1555evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST 1556may produce zero, one, or more elements in the returned value. 1557 1558 @chars = map(chr, @nums); 1559 1560translates a list of numbers to the corresponding characters. And 1561 1562 %hash = map { getkey($_) => $_ } @array; 1563 1564is just a funny way to write 1565 1566 %hash = (); 1567 foreach $_ (@array) { 1568 $hash{getkey($_)} = $_; 1569 } 1570 1571=item mkdir FILENAME,MODE 1572 1573Creates the directory specified by FILENAME, with permissions specified 1574by MODE (as modified by umask). If it succeeds it returns 1, otherwise 1575it returns 0 and sets $! (errno). 1576 1577=item msgctl ID,CMD,ARG 1578 1579Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG 1580must be a variable which will hold the returned msqid_ds structure. 1581Returns like ioctl: the undefined value for error, "0 but true" for 1582zero, or the actual return value otherwise. 1583 1584=item msgget KEY,FLAGS 1585 1586Calls the System V IPC function msgget(2). Returns the message queue id, 1587or the undefined value if there is an error. 1588 1589=item msgsnd ID,MSG,FLAGS 1590 1591Calls the System V IPC function msgsnd to send the message MSG to the 1592message queue ID. MSG must begin with the long integer message type, 1593which may be created with C<pack("l", $type)>. Returns TRUE if 1594successful, or FALSE if there is an error. 1595 1596=item msgrcv ID,VAR,SIZE,TYPE,FLAGS 1597 1598Calls the System V IPC function msgrcv to receive a message from 1599message queue ID into variable VAR with a maximum message size of 1600SIZE. Note that if a message is received, the message type will be the 1601first thing in VAR, and the maximum length of VAR is SIZE plus the size 1602of the message type. Returns TRUE if successful, or FALSE if there is 1603an error. 1604 1605=item my EXPR 1606 1607A "my" declares the listed variables to be local (lexically) to the 1608enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If 1609more than one value is listed, the list must be placed in parens. See 1610L<perlsub/"Private Variables via my()"> for details. 1611 1612=item next LABEL 1613 1614=item next 1615 1616The C<next> command is like the C<continue> statement in C; it starts 1617the next iteration of the loop: 1618 1619 LINE: while (<STDIN>) { 1620 next LINE if /^#/; # discard comments 1621 ... 1622 } 1623 1624Note that if there were a C<continue> block on the above, it would get 1625executed even on discarded lines. If the LABEL is omitted, the command 1626refers to the innermost enclosing loop. 1627 1628=item no Module LIST 1629 1630See the "use" function, which "no" is the opposite of. 1631 1632=item oct EXPR 1633 1634Interprets EXPR as an octal string and returns the corresponding 1635decimal value. (If EXPR happens to start off with 0x, interprets it as 1636a hex string instead.) The following will handle decimal, octal, and 1637hex in the standard Perl or C notation: 1638 1639 $val = oct($val) if $val =~ /^0/; 1640 1641If EXPR is omitted, uses $_. 1642 1643=item open FILEHANDLE,EXPR 1644 1645=item open FILEHANDLE 1646 1647Opens the file whose filename is given by EXPR, and associates it with 1648FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name 1649of the real filehandle wanted. If EXPR is omitted, the scalar variable of 1650the same name as the FILEHANDLE contains the filename. If the filename 1651begins with "<" or nothing, the file is opened for input. If the filename 1652begins with ">", the file is opened for output. If the filename begins 1653with ">>", the file is opened for appending. You can put a '+' in front 1654of the '>' or '<' to indicate that you want both read and write access to 1655the file; thus '+<' is usually preferred for read/write updates--the '+>' 1656mode would clobber the file first. These correspond to the fopen(3) modes 1657of 'r', 'r+', 'w', 'w+', 'a', and 'a+'. 1658 1659If the filename begins with "|", the filename is interpreted 1660as a command to which output is to be piped, and if the filename ends with 1661a "|", the filename is interpreted See L<perlipc/"Using open() for IPC"> 1662for more examples of this. as command which pipes input to us. (You may 1663not have a raw open() to a command that pipes both in I<and> out, but see See L<open2>, 1664L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.) 1665 1666Opening '-' opens STDIN and opening '>-' opens STDOUT. Open returns 1667non-zero upon success, the undefined value otherwise. If the open 1668involved a pipe, the return value happens to be the pid of the 1669subprocess. 1670 1671If you're unfortunate enough to be running Perl on a system that 1672distinguishes between text files and binary files (modern operating 1673systems don't care), then you should check out L</binmode> for tips for 1674dealing with this. The key distinction between systems that need binmode 1675and those that don't is their text file formats. Systems like Unix and 1676Plan9 that delimit lines with a single character, and that encode that 1677character in C as '\n', do not need C<binmode>. The rest need it. 1678 1679Examples: 1680 1681 $ARTICLE = 100; 1682 open ARTICLE or die "Can't find article $ARTICLE: $!\n"; 1683 while (<ARTICLE>) {... 1684 1685 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) 1686 1687 open(DBASE, '+<dbase.mine'); # open for update 1688 1689 open(ARTICLE, "caesar <$article |"); # decrypt article 1690 1691 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id 1692 1693 # process argument list of files along with any includes 1694 1695 foreach $file (@ARGV) { 1696 process($file, 'fh00'); 1697 } 1698 1699 sub process { 1700 local($filename, $input) = @_; 1701 $input++; # this is a string increment 1702 unless (open($input, $filename)) { 1703 print STDERR "Can't open $filename: $!\n"; 1704 return; 1705 } 1706 1707 while (<$input>) { # note use of indirection 1708 if (/^#include "(.*)"/) { 1709 process($1, $input); 1710 next; 1711 } 1712 ... # whatever 1713 } 1714 } 1715 1716You may also, in the Bourne shell tradition, specify an EXPR beginning 1717with ">&", in which case the rest of the string is interpreted as the 1718name of a filehandle (or file descriptor, if numeric) which is to be 1719duped and opened. You may use & after >, >>, <, +>, +>> and +<. The 1720mode you specify should match the mode of the original filehandle. 1721(Duping a filehandle does not take into acount any existing contents of 1722stdio buffers.) 1723Here is a script that saves, redirects, and restores STDOUT and 1724STDERR: 1725 1726 #!/usr/bin/perl 1727 open(SAVEOUT, ">&STDOUT"); 1728 open(SAVEERR, ">&STDERR"); 1729 1730 open(STDOUT, ">foo.out") || die "Can't redirect stdout"; 1731 open(STDERR, ">&STDOUT") || die "Can't dup stdout"; 1732 1733 select(STDERR); $| = 1; # make unbuffered 1734 select(STDOUT); $| = 1; # make unbuffered 1735 1736 print STDOUT "stdout 1\n"; # this works for 1737 print STDERR "stderr 1\n"; # subprocesses too 1738 1739 close(STDOUT); 1740 close(STDERR); 1741 1742 open(STDOUT, ">&SAVEOUT"); 1743 open(STDERR, ">&SAVEERR"); 1744 1745 print STDOUT "stdout 2\n"; 1746 print STDERR "stderr 2\n"; 1747 1748 1749If you specify "<&=N", where N is a number, then Perl will do an 1750equivalent of C's fdopen() of that file descriptor; this is more 1751parsimonious of file descriptors. For example: 1752 1753 open(FILEHANDLE, "<&=$fd") 1754 1755If you open a pipe on the command "-", i.e. either "|-" or "-|", then 1756there is an implicit fork done, and the return value of open is the pid 1757of the child within the parent process, and 0 within the child 1758process. (Use defined($pid) to determine whether the open was successful.) 1759The filehandle behaves normally for the parent, but i/o to that 1760filehandle is piped from/to the STDOUT/STDIN of the child process. 1761In the child process the filehandle isn't opened--i/o happens from/to 1762the new STDOUT or STDIN. Typically this is used like the normal 1763piped open when you want to exercise more control over just how the 1764pipe command gets executed, such as when you are running setuid, and 1765don't want to have to scan shell commands for metacharacters. 1766The following pairs are more or less equivalent: 1767 1768 open(FOO, "|tr '[a-z]' '[A-Z]'"); 1769 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]'; 1770 1771 open(FOO, "cat -n '$file'|"); 1772 open(FOO, "-|") || exec 'cat', '-n', $file; 1773 1774See L<perlipc/"Safe Pipe Opens"> for more examples of this. 1775 1776Explicitly closing any piped filehandle causes the parent process to 1777wait for the child to finish, and returns the status value in $?. 1778Note: on any operation which may do a fork, unflushed buffers remain 1779unflushed in both processes, which means you may need to set $| to 1780avoid duplicate output. 1781 1782Using the FileHandle constructor from the FileHandle package, 1783you can generate anonymous filehandles which have the scope of whatever 1784variables hold references to them, and automatically close whenever 1785and however you leave that scope: 1786 1787 use FileHandle; 1788 ... 1789 sub read_myfile_munged { 1790 my $ALL = shift; 1791 my $handle = new FileHandle; 1792 open($handle, "myfile") or die "myfile: $!"; 1793 $first = <$handle> 1794 or return (); # Automatically closed here. 1795 mung $first or die "mung failed"; # Or here. 1796 return $first, <$handle> if $ALL; # Or here. 1797 $first; # Or here. 1798 } 1799 1800The filename that is passed to open will have leading and trailing 1801whitespace deleted. In order to open a file with arbitrary weird 1802characters in it, it's necessary to protect any leading and trailing 1803whitespace thusly: 1804 1805 $file =~ s#^(\s)#./$1#; 1806 open(FOO, "< $file\0"); 1807 1808If you want a "real" C open() (see L<open(2)> on your system), then 1809you should use the sysopen() function. This is another way to 1810protect your filenames from interpretation. For example: 1811 1812 use FileHandle; 1813 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700) 1814 or die "sysopen $path: $!"; 1815 HANDLE->autoflush(1); 1816 HANDLE->print("stuff $$\n"); 1817 seek(HANDLE, 0, 0); 1818 print "File contains: ", <HANDLE>; 1819 1820See L</seek()> for some details about mixing reading and writing. 1821 1822=item opendir DIRHANDLE,EXPR 1823 1824Opens a directory named EXPR for processing by readdir(), telldir(), 1825seekdir(), rewinddir() and closedir(). Returns TRUE if successful. 1826DIRHANDLEs have their own namespace separate from FILEHANDLEs. 1827 1828=item ord EXPR 1829 1830Returns the numeric ascii value of the first character of EXPR. If 1831EXPR is omitted, uses $_. 1832 1833=item pack TEMPLATE,LIST 1834 1835Takes an array or list of values and packs it into a binary structure, 1836returning the string containing the structure. The TEMPLATE is a 1837sequence of characters that give the order and type of values, as 1838follows: 1839 1840 A An ascii string, will be space padded. 1841 a An ascii string, will be null padded. 1842 b A bit string (ascending bit order, like vec()). 1843 B A bit string (descending bit order). 1844 h A hex string (low nybble first). 1845 H A hex string (high nybble first). 1846 1847 c A signed char value. 1848 C An unsigned char value. 1849 s A signed short value. 1850 S An unsigned short value. 1851 i A signed integer value. 1852 I An unsigned integer value. 1853 l A signed long value. 1854 L An unsigned long value. 1855 1856 n A short in "network" order. 1857 N A long in "network" order. 1858 v A short in "VAX" (little-endian) order. 1859 V A long in "VAX" (little-endian) order. 1860 1861 f A single-precision float in the native format. 1862 d A double-precision float in the native format. 1863 1864 p A pointer to a null-terminated string. 1865 P A pointer to a structure (fixed-length string). 1866 1867 u A uuencoded string. 1868 1869 x A null byte. 1870 X Back up a byte. 1871 @ Null fill to absolute position. 1872 1873Each letter may optionally be followed by a number which gives a repeat 1874count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the 1875pack function will gobble up that many values from the LIST. A * for the 1876repeat count means to use however many items are left. The "a" and "A" 1877types gobble just one value, but pack it as a string of length count, 1878padding with nulls or spaces as necessary. (When unpacking, "A" strips 1879trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B" 1880fields pack a string that many bits long. The "h" and "H" fields pack a 1881string that many nybbles long. The "P" packs a pointer to a structure of 1882the size indicated by the length. Real numbers (floats and doubles) are 1883in the native machine format only; due to the multiplicity of floating 1884formats around, and the lack of a standard "network" representation, no 1885facility for interchange has been made. This means that packed floating 1886point data written on one machine may not be readable on another - even if 1887both use IEEE floating point arithmetic (as the endian-ness of the memory 1888representation is not part of the IEEE spec). Note that Perl uses doubles 1889internally for all numeric calculation, and converting from double into 1890float and thence back to double again will lose precision (i.e. 1891C<unpack("f", pack("f", $foo)>) will not in general equal $foo). 1892 1893Examples: 1894 1895 $foo = pack("cccc",65,66,67,68); 1896 # foo eq "ABCD" 1897 $foo = pack("c4",65,66,67,68); 1898 # same thing 1899 1900 $foo = pack("ccxxcc",65,66,67,68); 1901 # foo eq "AB\0\0CD" 1902 1903 $foo = pack("s2",1,2); 1904 # "\1\0\2\0" on little-endian 1905 # "\0\1\0\2" on big-endian 1906 1907 $foo = pack("a4","abcd","x","y","z"); 1908 # "abcd" 1909 1910 $foo = pack("aaaa","abcd","x","y","z"); 1911 # "axyz" 1912 1913 $foo = pack("a14","abcdefg"); 1914 # "abcdefg\0\0\0\0\0\0\0" 1915 1916 $foo = pack("i9pl", gmtime); 1917 # a real struct tm (on my system anyway) 1918 1919 sub bintodec { 1920 unpack("N", pack("B32", substr("0" x 32 . shift, -32))); 1921 } 1922 1923The same template may generally also be used in the unpack function. 1924 1925=item package NAMESPACE 1926 1927Declares the compilation unit as being in the given namespace. The scope 1928of the package declaration is from the declaration itself through the end of 1929the enclosing block (the same scope as the local() operator). All further 1930unqualified dynamic identifiers will be in this namespace. A package 1931statement only affects dynamic variables--including those you've used 1932local() on--but I<not> lexical variables created with my(). Typically it 1933would be the first declaration in a file to be included by the C<require> 1934or C<use> operator. You can switch into a package in more than one place; 1935it merely influences which symbol table is used by the compiler for the 1936rest of that block. You can refer to variables and filehandles in other 1937packages by prefixing the identifier with the package name and a double 1938colon: C<$Package::Variable>. If the package name is null, the C<main> 1939package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>. 1940 1941See L<perlmod/"Packages"> for more information about packages, modules, 1942and classes. See L<perlsub> for other scoping issues. 1943 1944=item pipe READHANDLE,WRITEHANDLE 1945 1946Opens a pair of connected pipes like the corresponding system call. 1947Note that if you set up a loop of piped processes, deadlock can occur 1948unless you are very careful. In addition, note that Perl's pipes use 1949stdio buffering, so you may need to set $| to flush your WRITEHANDLE 1950after each command, depending on the application. 1951 1952See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication"> 1953for examples of such things. 1954 1955=item pop ARRAY 1956 1957Pops and returns the last value of the array, shortening the array by 19581. Has a similar effect to 1959 1960 $tmp = $ARRAY[$#ARRAY--]; 1961 1962If there are no elements in the array, returns the undefined value. 1963If ARRAY is omitted, pops the 1964@ARGV array in the main program, and the @_ array in subroutines, just 1965like shift(). 1966 1967=item pos SCALAR 1968 1969Returns the offset of where the last C<m//g> search left off for the variable 1970in question. May be modified to change that offset. 1971 1972=item print FILEHANDLE LIST 1973 1974=item print LIST 1975 1976=item print 1977 1978Prints a string or a comma-separated list of strings. Returns TRUE 1979if successful. FILEHANDLE may be a scalar variable name, in which case 1980the variable contains the name of or a reference to the filehandle, thus introducing one 1981level of indirection. (NOTE: If FILEHANDLE is a variable and the next 1982token is a term, it may be misinterpreted as an operator unless you 1983interpose a + or put parens around the arguments.) If FILEHANDLE is 1984omitted, prints by default to standard output (or to the last selected 1985output channel--see select()). If LIST is also omitted, prints $_ to 1986STDOUT. To set the default output channel to something other than 1987STDOUT use the select operation. Note that, because print takes a 1988LIST, anything in the LIST is evaluated in a list context, and any 1989subroutine that you call will have one or more of its expressions 1990evaluated in a list context. Also be careful not to follow the print 1991keyword with a left parenthesis unless you want the corresponding right 1992parenthesis to terminate the arguments to the print--interpose a + or 1993put parens around all the arguments. 1994 1995Note that if you're storing FILEHANDLES in an array or other expression, 1996you will have to use a block returning its value instead 1997 1998 print { $files[$i] } "stuff\n"; 1999 print { $OK ? STDOUT : STDERR } "stuff\n"; 2000 2001=item printf FILEHANDLE LIST 2002 2003=item printf LIST 2004 2005Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument 2006of the list will be interpreted as the printf format. 2007 2008=item push ARRAY,LIST 2009 2010Treats ARRAY as a stack, and pushes the values of LIST 2011onto the end of ARRAY. The length of ARRAY increases by the length of 2012LIST. Has the same effect as 2013 2014 for $value (LIST) { 2015 $ARRAY[++$#ARRAY] = $value; 2016 } 2017 2018but is more efficient. Returns the new number of elements in the array. 2019 2020=item q/STRING/ 2021 2022=item qq/STRING/ 2023 2024=item qx/STRING/ 2025 2026=item qw/STRING/ 2027 2028Generalized quotes. See L<perlop>. 2029 2030=item quotemeta EXPR 2031 2032Returns the value of EXPR with with all regular expression 2033metacharacters backslashed. This is the internal function implementing 2034the \Q escape in double-quoted strings. 2035 2036=item rand EXPR 2037 2038=item rand 2039 2040Returns a random fractional number between 0 and the value of EXPR. 2041(EXPR should be positive.) If EXPR is omitted, returns a value between 20420 and 1. This function produces repeatable sequences unless srand() 2043is invoked. See also srand(). 2044 2045(Note: if your rand function consistently returns numbers that are too 2046large or too small, then your version of Perl was probably compiled 2047with the wrong number of RANDBITS. As a workaround, you can usually 2048multiply EXPR by the correct power of 2 to get the range you want. 2049This will make your script unportable, however. It's better to recompile 2050if you can.) 2051 2052=item read FILEHANDLE,SCALAR,LENGTH,OFFSET 2053 2054=item read FILEHANDLE,SCALAR,LENGTH 2055 2056Attempts to read LENGTH bytes of data into variable SCALAR from the 2057specified FILEHANDLE. Returns the number of bytes actually read, or 2058undef if there was an error. SCALAR will be grown or shrunk to the 2059length actually read. An OFFSET may be specified to place the read 2060data at some other place than the beginning of the string. This call 2061is actually implemented in terms of stdio's fread call. To get a true 2062read system call, see sysread(). 2063 2064=item readdir DIRHANDLE 2065 2066Returns the next directory entry for a directory opened by opendir(). 2067If used in a list context, returns all the rest of the entries in the 2068directory. If there are no more entries, returns an undefined value in 2069a scalar context or a null list in a list context. 2070 2071If you're planning to filetest the return values out of a readdir(), you'd 2072better prepend the directory in question. Otherwise, since we didn't 2073chdir() there, it would have been testing the wrong file. 2074 2075 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; 2076 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); 2077 closedir DIR; 2078 2079=item readlink EXPR 2080 2081Returns the value of a symbolic link, if symbolic links are 2082implemented. If not, gives a fatal error. If there is some system 2083error, returns the undefined value and sets $! (errno). If EXPR is 2084omitted, uses $_. 2085 2086=item recv SOCKET,SCALAR,LEN,FLAGS 2087 2088Receives a message on a socket. Attempts to receive LENGTH bytes of 2089data into variable SCALAR from the specified SOCKET filehandle. 2090Actually does a C recvfrom(), so that it can returns the address of the 2091sender. Returns the undefined value if there's an error. SCALAR will 2092be grown or shrunk to the length actually read. Takes the same flags 2093as the system call of the same name. 2094See L<perlipc/"UDP: Message Passing"> for examples. 2095 2096=item redo LABEL 2097 2098=item redo 2099 2100The C<redo> command restarts the loop block without evaluating the 2101conditional again. The C<continue> block, if any, is not executed. If 2102the LABEL is omitted, the command refers to the innermost enclosing 2103loop. This command is normally used by programs that want to lie to 2104themselves about what was just input: 2105 2106 # a simpleminded Pascal comment stripper 2107 # (warning: assumes no { or } in strings) 2108 LINE: while (<STDIN>) { 2109 while (s|({.*}.*){.*}|$1 |) {} 2110 s|{.*}| |; 2111 if (s|{.*| |) { 2112 $front = $_; 2113 while (<STDIN>) { 2114 if (/}/) { # end of comment? 2115 s|^|$front{|; 2116 redo LINE; 2117 } 2118 } 2119 } 2120 print; 2121 } 2122 2123=item ref EXPR 2124 2125Returns a TRUE value if EXPR is a reference, FALSE otherwise. The value 2126returned depends on the type of thing the reference is a reference to. 2127Builtin types include: 2128 2129 REF 2130 SCALAR 2131 ARRAY 2132 HASH 2133 CODE 2134 GLOB 2135 2136If the referenced object has been blessed into a package, then that package 2137name is returned instead. You can think of ref() as a typeof() operator. 2138 2139 if (ref($r) eq "HASH") { 2140 print "r is a reference to an associative array.\n"; 2141 } 2142 if (!ref ($r) { 2143 print "r is not a reference at all.\n"; 2144 } 2145 2146See also L<perlref>. 2147 2148=item rename OLDNAME,NEWNAME 2149 2150Changes the name of a file. Returns 1 for success, 0 otherwise. Will 2151not work across filesystem boundaries. 2152 2153=item require EXPR 2154 2155=item require 2156 2157Demands some semantics specified by EXPR, or by $_ if EXPR is not 2158supplied. If EXPR is numeric, demands that the current version of Perl 2159($] or $PERL_VERSION) be equal or greater than EXPR. 2160 2161Otherwise, demands that a library file be included if it hasn't already 2162been included. The file is included via the do-FILE mechanism, which is 2163essentially just a variety of eval(). Has semantics similar to the following 2164subroutine: 2165 2166 sub require { 2167 local($filename) = @_; 2168 return 1 if $INC{$filename}; 2169 local($realfilename,$result); 2170 ITER: { 2171 foreach $prefix (@INC) { 2172 $realfilename = "$prefix/$filename"; 2173 if (-f $realfilename) { 2174 $result = do $realfilename; 2175 last ITER; 2176 } 2177 } 2178 die "Can't find $filename in \@INC"; 2179 } 2180 die $@ if $@; 2181 die "$filename did not return true value" unless $result; 2182 $INC{$filename} = $realfilename; 2183 $result; 2184 } 2185 2186Note that the file will not be included twice under the same specified 2187name. The file must return TRUE as the last statement to indicate 2188successful execution of any initialization code, so it's customary to 2189end such a file with "1;" unless you're sure it'll return TRUE 2190otherwise. But it's better just to put the "C<1;>", in case you add more 2191statements. 2192 2193If EXPR is a bare word, the require assumes a "F<.pm>" extension for you, 2194to make it easy to load standard modules. This form of loading of 2195modules does not risk altering your namespace. 2196 2197For a yet-more-powerful import facility, see the L</use()> and 2198L<perlmod>. 2199 2200=item reset EXPR 2201 2202=item reset 2203 2204Generally used in a C<continue> block at the end of a loop to clear 2205variables and reset ?? searches so that they work again. The 2206expression is interpreted as a list of single characters (hyphens 2207allowed for ranges). All variables and arrays beginning with one of 2208those letters are reset to their pristine state. If the expression is 2209omitted, one-match searches (?pattern?) are reset to match again. Only 2210resets variables or searches in the current package. Always returns 22111. Examples: 2212 2213 reset 'X'; # reset all X variables 2214 reset 'a-z'; # reset lower case variables 2215 reset; # just reset ?? searches 2216 2217Resetting "A-Z" is not recommended since you'll wipe out your 2218ARGV and ENV arrays. Only resets package variables--lexical variables 2219are unaffected, but they clean themselves up on scope exit anyway, 2220so anymore you probably want to use them instead. See L</my>. 2221 2222=item return LIST 2223 2224Returns from a subroutine or eval with the value specified. (Note that 2225in the absence of a return a subroutine or eval() will automatically 2226return the value of the last expression evaluated.) 2227 2228=item reverse LIST 2229 2230In a list context, returns a list value consisting of the elements 2231of LIST in the opposite order. In a scalar context, returns a string 2232value consisting of the bytes of the first element of LIST in the 2233opposite order. 2234 2235 print reverse <>; # line tac 2236 2237 undef $/; 2238 print scalar reverse scalar <>; # byte tac 2239 2240=item rewinddir DIRHANDLE 2241 2242Sets the current position to the beginning of the directory for the 2243readdir() routine on DIRHANDLE. 2244 2245=item rindex STR,SUBSTR,POSITION 2246 2247=item rindex STR,SUBSTR 2248 2249Works just like index except that it returns the position of the LAST 2250occurrence of SUBSTR in STR. If POSITION is specified, returns the 2251last occurrence at or before that position. 2252 2253=item rmdir FILENAME 2254 2255Deletes the directory specified by FILENAME if it is empty. If it 2256succeeds it returns 1, otherwise it returns 0 and sets $! (errno). If 2257FILENAME is omitted, uses $_. 2258 2259=item s/// 2260 2261The substitution operator. See L<perlop>. 2262 2263=item scalar EXPR 2264 2265Forces EXPR to be interpreted in a scalar context and returns the value 2266of EXPR. 2267 2268 @counts = ( scalar @a, scalar @b, scalar @c ); 2269 2270There is no equivalent operator to force an expression to 2271be interpolated in a list context because it's in practice never 2272needed. If you really wanted to do so, however, you could use 2273the construction C<@{[ (some expression) ]}>, but usually a simple 2274C<(some expression)> suffices. 2275 2276=item seek FILEHANDLE,POSITION,WHENCE 2277 2278Randomly positions the file pointer for FILEHANDLE, just like the fseek() 2279call of stdio. FILEHANDLE may be an expression whose value gives the name 2280of the filehandle. The values for WHENCE are 0 to set the file pointer to 2281POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF 2282plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for 2283this from POSIX module. Returns 1 upon success, 0 otherwise. 2284 2285On some systems you have to do a seek whenever you switch between reading 2286and writing. Amongst other things, this may have the effect of calling 2287stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving 2288the file pointer: 2289 2290 seek(TEST,0,1); 2291 2292This is also useful for applications emulating C<tail -f>. Once you hit 2293EOF on your read, and then sleep for a while, you might have to stick in a 2294seek() to reset things. First the simple trick listed above to clear the 2295filepointer. The seek() doesn't change the current position, but it 2296I<does> clear the end-of-file condition on the handle, so that the next 2297C<E<lt>FILEE<gt>> makes Perl try again to read something. Hopefully. 2298 2299If that doesn't work (some stdios are particularly cantankerous), then 2300you may need something more like this: 2301 2302 for (;;) { 2303 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) { 2304 # search for some stuff and put it into files 2305 } 2306 sleep($for_a_while); 2307 seek(FILE, $curpos, 0); 2308 } 2309 2310=item seekdir DIRHANDLE,POS 2311 2312Sets the current position for the readdir() routine on DIRHANDLE. POS 2313must be a value returned by telldir(). Has the same caveats about 2314possible directory compaction as the corresponding system library 2315routine. 2316 2317=item select FILEHANDLE 2318 2319=item select 2320 2321Returns the currently selected filehandle. Sets the current default 2322filehandle for output, if FILEHANDLE is supplied. This has two 2323effects: first, a C<write> or a C<print> without a filehandle will 2324default to this FILEHANDLE. Second, references to variables related to 2325output will refer to this output channel. For example, if you have to 2326set the top of form format for more than one output channel, you might 2327do the following: 2328 2329 select(REPORT1); 2330 $^ = 'report1_top'; 2331 select(REPORT2); 2332 $^ = 'report2_top'; 2333 2334FILEHANDLE may be an expression whose value gives the name of the 2335actual filehandle. Thus: 2336 2337 $oldfh = select(STDERR); $| = 1; select($oldfh); 2338 2339Some programmers may prefer to think of filehandles as objects with 2340methods, preferring to write the last example as: 2341 2342 use FileHandle; 2343 STDERR->autoflush(1); 2344 2345=item select RBITS,WBITS,EBITS,TIMEOUT 2346 2347This calls the select(2) system call with the bitmasks specified, which 2348can be constructed using fileno() and vec(), along these lines: 2349 2350 $rin = $win = $ein = ''; 2351 vec($rin,fileno(STDIN),1) = 1; 2352 vec($win,fileno(STDOUT),1) = 1; 2353 $ein = $rin | $win; 2354 2355If you want to select on many filehandles you might wish to write a 2356subroutine: 2357 2358 sub fhbits { 2359 local(@fhlist) = split(' ',$_[0]); 2360 local($bits); 2361 for (@fhlist) { 2362 vec($bits,fileno($_),1) = 1; 2363 } 2364 $bits; 2365 } 2366 $rin = fhbits('STDIN TTY SOCK'); 2367 2368The usual idiom is: 2369 2370 ($nfound,$timeleft) = 2371 select($rout=$rin, $wout=$win, $eout=$ein, $timeout); 2372 2373or to block until something becomes ready just do this 2374 2375 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); 2376 2377Most systems do not both to return anything useful in $timeleft, so 2378calling select() in a scalar context just returns $nfound. 2379 2380Any of the bitmasks can also be undef. The timeout, if specified, is 2381in seconds, which may be fractional. Note: not all implementations are 2382capable of returning the $timeleft. If not, they always return 2383$timeleft equal to the supplied $timeout. 2384 2385You can effect a 250-microsecond sleep this way: 2386 2387 select(undef, undef, undef, 0.25); 2388 2389B<WARNING>: Do not attempt to mix buffered I/O (like read() or <FH>) 2390with select(). You have to use sysread() instead. 2391 2392=item semctl ID,SEMNUM,CMD,ARG 2393 2394Calls the System V IPC function semctl. If CMD is &IPC_STAT or 2395&GETALL, then ARG must be a variable which will hold the returned 2396semid_ds structure or semaphore value array. Returns like ioctl: the 2397undefined value for error, "0 but true" for zero, or the actual return 2398value otherwise. 2399 2400=item semget KEY,NSEMS,FLAGS 2401 2402Calls the System V IPC function semget. Returns the semaphore id, or 2403the undefined value if there is an error. 2404 2405=item semop KEY,OPSTRING 2406 2407Calls the System V IPC function semop to perform semaphore operations 2408such as signaling and waiting. OPSTRING must be a packed array of 2409semop structures. Each semop structure can be generated with 2410C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore 2411operations is implied by the length of OPSTRING. Returns TRUE if 2412successful, or FALSE if there is an error. As an example, the 2413following code waits on semaphore $semnum of semaphore id $semid: 2414 2415 $semop = pack("sss", $semnum, -1, 0); 2416 die "Semaphore trouble: $!\n" unless semop($semid, $semop); 2417 2418To signal the semaphore, replace "-1" with "1". 2419 2420=item send SOCKET,MSG,FLAGS,TO 2421 2422=item send SOCKET,MSG,FLAGS 2423 2424Sends a message on a socket. Takes the same flags as the system call 2425of the same name. On unconnected sockets you must specify a 2426destination to send TO, in which case it does a C sendto(). Returns 2427the number of characters sent, or the undefined value if there is an 2428error. 2429See L<perlipc/"UDP: Message Passing"> for examples. 2430 2431=item setpgrp PID,PGRP 2432 2433Sets the current process group for the specified PID, 0 for the current 2434process. Will produce a fatal error if used on a machine that doesn't 2435implement setpgrp(2). 2436 2437=item setpriority WHICH,WHO,PRIORITY 2438 2439Sets the current priority for a process, a process group, or a user. 2440(See setpriority(2).) Will produce a fatal error if used on a machine 2441that doesn't implement setpriority(2). 2442 2443=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL 2444 2445Sets the socket option requested. Returns undefined if there is an 2446error. OPTVAL may be specified as undef if you don't want to pass an 2447argument. 2448 2449=item shift ARRAY 2450 2451=item shift 2452 2453Shifts the first value of the array off and returns it, shortening the 2454array by 1 and moving everything down. If there are no elements in the 2455array, returns the undefined value. If ARRAY is omitted, shifts the 2456@ARGV array in the main program, and the @_ array in subroutines. 2457(This is determined lexically.) See also unshift(), push(), and pop(). 2458Shift() and unshift() do the same thing to the left end of an array 2459that push() and pop() do to the right end. 2460 2461=item shmctl ID,CMD,ARG 2462 2463Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG 2464must be a variable which will hold the returned shmid_ds structure. 2465Returns like ioctl: the undefined value for error, "0 but true" for 2466zero, or the actual return value otherwise. 2467 2468=item shmget KEY,SIZE,FLAGS 2469 2470Calls the System V IPC function shmget. Returns the shared memory 2471segment id, or the undefined value if there is an error. 2472 2473=item shmread ID,VAR,POS,SIZE 2474 2475=item shmwrite ID,STRING,POS,SIZE 2476 2477Reads or writes the System V shared memory segment ID starting at 2478position POS for size SIZE by attaching to it, copying in/out, and 2479detaching from it. When reading, VAR must be a variable which will 2480hold the data read. When writing, if STRING is too long, only SIZE 2481bytes are used; if STRING is too short, nulls are written to fill out 2482SIZE bytes. Return TRUE if successful, or FALSE if there is an error. 2483 2484=item shutdown SOCKET,HOW 2485 2486Shuts down a socket connection in the manner indicated by HOW, which 2487has the same interpretation as in the system call of the same name. 2488 2489=item sin EXPR 2490 2491Returns the sine of EXPR (expressed in radians). If EXPR is omitted, 2492returns sine of $_. 2493 2494=item sleep EXPR 2495 2496=item sleep 2497 2498Causes the script to sleep for EXPR seconds, or forever if no EXPR. 2499May be interrupted by sending the process a SIGALRM. Returns the 2500number of seconds actually slept. You probably cannot mix alarm() and 2501sleep() calls, since sleep() is often implemented using alarm(). 2502 2503On some older systems, it may sleep up to a full second less than what 2504you requested, depending on how it counts seconds. Most modern systems 2505always sleep the full amount. 2506 2507For delays of finer granularity than one second, you may use Perl's 2508syscall() interface to access setitimer(2) if your system supports it, 2509or else see L</select()> below. 2510 2511=item socket SOCKET,DOMAIN,TYPE,PROTOCOL 2512 2513Opens a socket of the specified kind and attaches it to filehandle 2514SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the 2515system call of the same name. You should "use Socket;" first to get 2516the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">. 2517 2518=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL 2519 2520Creates an unnamed pair of sockets in the specified domain, of the 2521specified type. DOMAIN, TYPE and PROTOCOL are specified the same as 2522for the system call of the same name. If unimplemented, yields a fatal 2523error. Returns TRUE if successful. 2524 2525=item sort SUBNAME LIST 2526 2527=item sort BLOCK LIST 2528 2529=item sort LIST 2530 2531Sorts the LIST and returns the sorted list value. Nonexistent values 2532of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts 2533in standard string comparison order. If SUBNAME is specified, it 2534gives the name of a subroutine that returns an integer less than, equal 2535to, or greater than 0, depending on how the elements of the array are 2536to be ordered. (The <=> and cmp operators are extremely useful in such 2537routines.) SUBNAME may be a scalar variable name, in which case the 2538value provides the name of the subroutine to use. In place of a 2539SUBNAME, you can provide a BLOCK as an anonymous, in-line sort 2540subroutine. 2541 2542In the interests of efficiency the normal calling code for subroutines is 2543bypassed, with the following effects: the subroutine may not be a 2544recursive subroutine, and the two elements to be compared are passed into 2545the subroutine not via @_ but as the package global variables $a and 2546$b (see example below). They are passed by reference, so don't 2547modify $a and $b. And don't try to declare them as lexicals either. 2548 2549Examples: 2550 2551 # sort lexically 2552 @articles = sort @files; 2553 2554 # same thing, but with explicit sort routine 2555 @articles = sort {$a cmp $b} @files; 2556 2557 # now case-insensitively 2558 @articles = sort { uc($a) cmp uc($b)} @files; 2559 2560 # same thing in reversed order 2561 @articles = sort {$b cmp $a} @files; 2562 2563 # sort numerically ascending 2564 @articles = sort {$a <=> $b} @files; 2565 2566 # sort numerically descending 2567 @articles = sort {$b <=> $a} @files; 2568 2569 # sort using explicit subroutine name 2570 sub byage { 2571 $age{$a} <=> $age{$b}; # presuming integers 2572 } 2573 @sortedclass = sort byage @class; 2574 2575 # this sorts the %age associative arrays by value 2576 # instead of key using an inline function 2577 @eldest = sort { $age{$b} <=> $age{$a} } keys %age; 2578 2579 sub backwards { $b cmp $a; } 2580 @harry = ('dog','cat','x','Cain','Abel'); 2581 @george = ('gone','chased','yz','Punished','Axed'); 2582 print sort @harry; 2583 # prints AbelCaincatdogx 2584 print sort backwards @harry; 2585 # prints xdogcatCainAbel 2586 print sort @george, 'to', @harry; 2587 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz 2588 2589 # inefficiently sort by descending numeric compare using 2590 # the first integer after the first = sign, or the 2591 # whole record case-insensitively otherwise 2592 2593 @new = sort { 2594 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] 2595 || 2596 uc($a) cmp uc($b) 2597 } @old; 2598 2599 # same thing, but much more efficiently; 2600 # we'll build auxiliary indices instead 2601 # for speed 2602 @nums = @caps = (); 2603 for (@old) { 2604 push @nums, /=(\d+)/; 2605 push @caps, uc($_); 2606 } 2607 2608 @new = @old[ sort { 2609 $nums[$b] <=> $nums[$a] 2610 || 2611 $caps[$a] cmp $caps[$b] 2612 } 0..$#old 2613 ]; 2614 2615 # same thing using a Schwartzian Transform (no temps) 2616 @new = map { $_->[0] } 2617 sort { $b->[1] <=> $a->[1] 2618 || 2619 $a->[2] cmp $b->[2] 2620 } map { [$_, /=(\d+)/, uc($_)] } @old; 2621 2622If you're and using strict, you I<MUST NOT> declare $a 2623and $b as lexicals. They are package globals. That means 2624if you're in the C<main> package, it's 2625 2626 @articles = sort {$main::b <=> $main::a} @files; 2627 2628or just 2629 2630 @articles = sort {$::b <=> $::a} @files; 2631 2632but if you're in the C<FooPack> package, it's 2633 2634 @articles = sort {$FooPack::b <=> $FooPack::a} @files; 2635 2636=item splice ARRAY,OFFSET,LENGTH,LIST 2637 2638=item splice ARRAY,OFFSET,LENGTH 2639 2640=item splice ARRAY,OFFSET 2641 2642Removes the elements designated by OFFSET and LENGTH from an array, and 2643replaces them with the elements of LIST, if any. Returns the elements 2644removed from the array. The array grows or shrinks as necessary. If 2645LENGTH is omitted, removes everything from OFFSET onward. The 2646following equivalencies hold (assuming $[ == 0): 2647 2648 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y) 2649 pop(@a) splice(@a,-1) 2650 shift(@a) splice(@a,0,1) 2651 unshift(@a,$x,$y) splice(@a,0,0,$x,$y) 2652 $a[$x] = $y splice(@a,$x,1,$y); 2653 2654Example, assuming array lengths are passed before arrays: 2655 2656 sub aeq { # compare two list values 2657 local(@a) = splice(@_,0,shift); 2658 local(@b) = splice(@_,0,shift); 2659 return 0 unless @a == @b; # same len? 2660 while (@a) { 2661 return 0 if pop(@a) ne pop(@b); 2662 } 2663 return 1; 2664 } 2665 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } 2666 2667=item split /PATTERN/,EXPR,LIMIT 2668 2669=item split /PATTERN/,EXPR 2670 2671=item split /PATTERN/ 2672 2673=item split 2674 2675Splits a string into an array of strings, and returns it. 2676 2677If not in a list context, returns the number of fields found and splits into 2678the @_ array. (In a list context, you can force the split into @_ by 2679using C<??> as the pattern delimiters, but it still returns the array 2680value.) The use of implicit split to @_ is deprecated, however. 2681 2682If EXPR is omitted, splits the $_ string. If PATTERN is also omitted, 2683splits on whitespace (after skipping any leading whitespace). Anything 2684matching PATTERN is taken to be a delimiter separating the fields. (Note 2685that the delimiter may be longer than one character.) If LIMIT is 2686specified and is not negative, splits into no more than that many fields 2687(though it may split into fewer). If LIMIT is unspecified, trailing null 2688fields are stripped (which potential users of pop() would do well to 2689remember). If LIMIT is negative, it is treated as if an arbitrarily large 2690LIMIT had been specified. 2691 2692A pattern matching the null string (not to be confused with 2693a null pattern C<//>, which is just one member of the set of patterns 2694matching a null string) will split the value of EXPR into separate 2695characters at each point it matches that way. For example: 2696 2697 print join(':', split(/ */, 'hi there')); 2698 2699produces the output 'h:i:t:h:e:r:e'. 2700 2701The LIMIT parameter can be used to partially split a line 2702 2703 ($login, $passwd, $remainder) = split(/:/, $_, 3); 2704 2705When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT 2706one larger than the number of variables in the list, to avoid 2707unnecessary work. For the list above LIMIT would have been 4 by 2708default. In time critical applications it behooves you not to split 2709into more fields than you really need. 2710 2711If the PATTERN contains parentheses, additional array elements are 2712created from each matching substring in the delimiter. 2713 2714 split(/([,-])/, "1-10,20"); 2715 2716produces the list value 2717 2718 (1, '-', 10, ',', 20) 2719 2720If you had the entire header of a normal Unix email message in $header, 2721you could split it up into fields and their values this way: 2722 2723 $header =~ s/\n\s+/ /g; # fix continuation lines 2724 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header); 2725 2726The pattern C</PATTERN/> may be replaced with an expression to specify 2727patterns that vary at runtime. (To do runtime compilation only once, 2728use C</$variable/o>.) 2729 2730As a special case, specifying a PATTERN of space (C<' '>) will split on 2731white space just as split with no arguments does. Thus, split(' ') can 2732be used to emulate B<awk>'s default behavior, whereas C<split(/ /)> 2733will give you as many null initial fields as there are leading spaces. 2734A split on /\s+/ is like a split(' ') except that any leading 2735whitespace produces a null first field. A split with no arguments 2736really does a C<split(' ', $_)> internally. 2737 2738Example: 2739 2740 open(passwd, '/etc/passwd'); 2741 while (<passwd>) { 2742 ($login, $passwd, $uid, $gid, $gcos, 2743 $home, $shell) = split(/:/); 2744 ... 2745 } 2746 2747(Note that $shell above will still have a newline on it. See L</chop>, 2748L</chomp>, and L</join>.) 2749 2750=item sprintf FORMAT,LIST 2751 2752Returns a string formatted by the usual printf conventions of the C 2753language. See L<sprintf(3)> or L<printf(3)> on your system for details. 2754(The * character for an indirectly specified length is not 2755supported, but you can get the same effect by interpolating a variable 2756into the pattern.) Some C libraries' implementations of sprintf() can 2757dump core when fed ludicrous arguments. 2758 2759=item sqrt EXPR 2760 2761Return the square root of EXPR. If EXPR is omitted, returns square 2762root of $_. 2763 2764=item srand EXPR 2765 2766Sets the random number seed for the C<rand> operator. If EXPR is omitted, 2767does C<srand(time)>. Many folks use an explicit C<srand(time ^ $$)> 2768instead. Of course, you'd need something much more random than that for 2769cryptographic purposes, since it's easy to guess the current time. 2770Checksumming the compressed output of rapidly changing operating system 2771status programs is the usual method. Examples are posted regularly to 2772the comp.security.unix newsgroup. 2773 2774=item stat FILEHANDLE 2775 2776=item stat EXPR 2777 2778Returns a 13-element array giving the status info for a file, either the 2779file opened via FILEHANDLE, or named by EXPR. Returns a null list if 2780the stat fails. Typically used as follows: 2781 2782 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, 2783 $atime,$mtime,$ctime,$blksize,$blocks) 2784 = stat($filename); 2785 2786Not all fields are supported on all filesystem types. Here are the 2787meaning of the fields: 2788 2789 dev device number of filesystem 2790 ino inode number 2791 mode file mode (type and permissions) 2792 nlink number of (hard) links to the file 2793 uid numeric user ID of file's owner 2794 gid numer group ID of file's owner 2795 rdev the device identifier (special files only) 2796 size total size of file, in bytes 2797 atime last access time since the epoch 2798 mtime last modify time since the epoch 2799 ctime inode change time (NOT creation type!) since the epoch 2800 blksize preferred blocksize for file system I/O 2801 blocks actual number of blocks allocated 2802 2803(The epoch was at 00:00 January 1, 1970 GMT.) 2804 2805If stat is passed the special filehandle consisting of an underline, no 2806stat is done, but the current contents of the stat structure from the 2807last stat or filetest are returned. Example: 2808 2809 if (-x $file && (($d) = stat(_)) && $d < 0) { 2810 print "$file is executable NFS file\n"; 2811 } 2812 2813(This only works on machines for which the device number is negative under NFS.) 2814 2815=item study SCALAR 2816 2817=item study 2818 2819Takes extra time to study SCALAR ($_ if unspecified) in anticipation of 2820doing many pattern matches on the string before it is next modified. 2821This may or may not save time, depending on the nature and number of 2822patterns you are searching on, and on the distribution of character 2823frequencies in the string to be searched--you probably want to compare 2824runtimes with and without it to see which runs faster. Those loops 2825which scan for many short constant strings (including the constant 2826parts of more complex patterns) will benefit most. You may have only 2827one study active at a time--if you study a different scalar the first 2828is "unstudied". (The way study works is this: a linked list of every 2829character in the string to be searched is made, so we know, for 2830example, where all the 'k' characters are. From each search string, 2831the rarest character is selected, based on some static frequency tables 2832constructed from some C programs and English text. Only those places 2833that contain this "rarest" character are examined.) 2834 2835For example, here is a loop which inserts index producing entries 2836before any line containing a certain pattern: 2837 2838 while (<>) { 2839 study; 2840 print ".IX foo\n" if /\bfoo\b/; 2841 print ".IX bar\n" if /\bbar\b/; 2842 print ".IX blurfl\n" if /\bblurfl\b/; 2843 ... 2844 print; 2845 } 2846 2847In searching for /\bfoo\b/, only those locations in $_ that contain "f" 2848will be looked at, because "f" is rarer than "o". In general, this is 2849a big win except in pathological cases. The only question is whether 2850it saves you more time than it took to build the linked list in the 2851first place. 2852 2853Note that if you have to look for strings that you don't know till 2854runtime, you can build an entire loop as a string and eval that to 2855avoid recompiling all your patterns all the time. Together with 2856undefining $/ to input entire files as one record, this can be very 2857fast, often faster than specialized programs like fgrep(1). The following 2858scans a list of files (@files) for a list of words (@words), and prints 2859out the names of those files that contain a match: 2860 2861 $search = 'while (<>) { study;'; 2862 foreach $word (@words) { 2863 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; 2864 } 2865 $search .= "}"; 2866 @ARGV = @files; 2867 undef $/; 2868 eval $search; # this screams 2869 $/ = "\n"; # put back to normal input delim 2870 foreach $file (sort keys(%seen)) { 2871 print $file, "\n"; 2872 } 2873 2874=item sub BLOCK 2875 2876=item sub NAME 2877 2878=item sub NAME BLOCK 2879 2880This is subroutine definition, not a real function I<per se>. With just a 2881NAME (and possibly prototypes), it's just a forward declaration. Without 2882a NAME, it's an anonymous function declaration, and does actually return a 2883value: the CODE ref of the closure you just created. See L<perlsub> and 2884L<perlref> for details. 2885 2886=item substr EXPR,OFFSET,LEN 2887 2888=item substr EXPR,OFFSET 2889 2890Extracts a substring out of EXPR and returns it. First character is at 2891offset 0, or whatever you've set $[ to. If OFFSET is negative, starts 2892that far from the end of the string. If LEN is omitted, returns 2893everything to the end of the string. If LEN is negative, leaves that 2894many characters off the end of the string. 2895 2896You can use the substr() function 2897as an lvalue, in which case EXPR must be an lvalue. If you assign 2898something shorter than LEN, the string will shrink, and if you assign 2899something longer than LEN, the string will grow to accommodate it. To 2900keep the string the same length you may need to pad or chop your value 2901using sprintf(). 2902 2903=item symlink OLDFILE,NEWFILE 2904 2905Creates a new filename symbolically linked to the old filename. 2906Returns 1 for success, 0 otherwise. On systems that don't support 2907symbolic links, produces a fatal error at run time. To check for that, 2908use eval: 2909 2910 $symlink_exists = (eval 'symlink("","");', $@ eq ''); 2911 2912=item syscall LIST 2913 2914Calls the system call specified as the first element of the list, 2915passing the remaining elements as arguments to the system call. If 2916unimplemented, produces a fatal error. The arguments are interpreted 2917as follows: if a given argument is numeric, the argument is passed as 2918an int. If not, the pointer to the string value is passed. You are 2919responsible to make sure a string is pre-extended long enough to 2920receive any result that might be written into a string. If your 2921integer arguments are not literals and have never been interpreted in a 2922numeric context, you may need to add 0 to them to force them to look 2923like numbers. 2924 2925 require 'syscall.ph'; # may need to run h2ph 2926 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9); 2927 2928Note that Perl only supports passing of up to 14 arguments to your system call, 2929which in practice should usually suffice. 2930 2931=item sysopen FILEHANDLE,FILENAME,MODE 2932 2933=item sysopen FILEHANDLE,FILENAME,MODE,PERMS 2934 2935Opens the file whose filename is given by FILENAME, and associates it 2936with FILEHANDLE. If FILEHANDLE is an expression, its value is used as 2937the name of the real filehandle wanted. This function calls the 2938underlying operating system's C<open> function with the parameters 2939FILENAME, MODE, PERMS. 2940 2941The possible values and flag bits of the MODE parameter are 2942system-dependent; they are available via the standard module C<Fcntl>. 2943However, for historical reasons, some values are universal: zero means 2944read-only, one means write-only, and two means read/write. 2945 2946If the file named by FILENAME does not exist and the C<open> call 2947creates it (typically because MODE includes the O_CREAT flag), then 2948the value of PERMS specifies the permissions of the newly created 2949file. If PERMS is omitted, the default value is 0666, which allows 2950read and write for all. This default is reasonable: see C<umask>. 2951 2952=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET 2953 2954=item sysread FILEHANDLE,SCALAR,LENGTH 2955 2956Attempts to read LENGTH bytes of data into variable SCALAR from the 2957specified FILEHANDLE, using the system call read(2). It bypasses 2958stdio, so mixing this with other kinds of reads may cause confusion. 2959Returns the number of bytes actually read, or undef if there was an 2960error. SCALAR will be grown or shrunk to the length actually read. An 2961OFFSET may be specified to place the read data at some other place than 2962the beginning of the string. 2963 2964=item system LIST 2965 2966Does exactly the same thing as "exec LIST" except that a fork is done 2967first, and the parent process waits for the child process to complete. 2968Note that argument processing varies depending on the number of 2969arguments. The return value is the exit status of the program as 2970returned by the wait() call. To get the actual exit value divide by 2971256. See also L</exec>. This is I<NOT> what you want to use to capture 2972the output from a command, for that you should merely use backticks, as 2973described in L<perlop/"`STRING`">. 2974 2975=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET 2976 2977=item syswrite FILEHANDLE,SCALAR,LENGTH 2978 2979Attempts to write LENGTH bytes of data from variable SCALAR to the 2980specified FILEHANDLE, using the system call write(2). It bypasses 2981stdio, so mixing this with prints may cause confusion. Returns the 2982number of bytes actually written, or undef if there was an error. An 2983OFFSET may be specified to get the write data from some other place than 2984the beginning of the string. 2985 2986=item tell FILEHANDLE 2987 2988=item tell 2989 2990Returns the current file position for FILEHANDLE. FILEHANDLE may be an 2991expression whose value gives the name of the actual filehandle. If 2992FILEHANDLE is omitted, assumes the file last read. 2993 2994=item telldir DIRHANDLE 2995 2996Returns the current position of the readdir() routines on DIRHANDLE. 2997Value may be given to seekdir() to access a particular location in a 2998directory. Has the same caveats about possible directory compaction as 2999the corresponding system library routine. 3000 3001=item tie VARIABLE,CLASSNAME,LIST 3002 3003This function binds a variable to a package class that will provide the 3004implementation for the variable. VARIABLE is the name of the variable 3005to be enchanted. CLASSNAME is the name of a class implementing objects 3006of correct type. Any additional arguments are passed to the "new" 3007method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH). 3008Typically these are arguments such as might be passed to the dbm_open() 3009function of C. The object returned by the "new" method is also 3010returned by the tie() function, which would be useful if you want to 3011access other methods in CLASSNAME. 3012 3013Note that functions such as keys() and values() may return huge array 3014values when used on large objects, like DBM files. You may prefer to 3015use the each() function to iterate over such. Example: 3016 3017 # print out history file offsets 3018 use NDBM_File; 3019 tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0); 3020 while (($key,$val) = each %HIST) { 3021 print $key, ' = ', unpack('L',$val), "\n"; 3022 } 3023 untie(%HIST); 3024 3025A class implementing an associative array should have the following 3026methods: 3027 3028 TIEHASH classname, LIST 3029 DESTROY this 3030 FETCH this, key 3031 STORE this, key, value 3032 DELETE this, key 3033 EXISTS this, key 3034 FIRSTKEY this 3035 NEXTKEY this, lastkey 3036 3037A class implementing an ordinary array should have the following methods: 3038 3039 TIEARRAY classname, LIST 3040 DESTROY this 3041 FETCH this, key 3042 STORE this, key, value 3043 [others TBD] 3044 3045A class implementing a scalar should have the following methods: 3046 3047 TIESCALAR classname, LIST 3048 DESTROY this 3049 FETCH this, 3050 STORE this, value 3051 3052Unlike dbmopen(), the tie() function will not use or require a module 3053for you--you need to do that explicitly yourself. See L<DB_File> 3054or the F<Config> module for interesting tie() implementations. 3055 3056=item tied VARIABLE 3057 3058Returns a reference to the object underlying VARIABLE (the same value 3059that was originally returned by the tie() call which bound the variable 3060to a package.) Returns the undefined value if VARIABLE isn't tied to a 3061package. 3062 3063=item time 3064 3065Returns the number of non-leap seconds since 00:00:00 UTC, January 1, 30661970. Suitable for feeding to gmtime() and localtime(). 3067 3068=item times 3069 3070Returns a four-element array giving the user and system times, in 3071seconds, for this process and the children of this process. 3072 3073 ($user,$system,$cuser,$csystem) = times; 3074 3075=item tr/// 3076 3077The translation operator. See L<perlop>. 3078 3079=item truncate FILEHANDLE,LENGTH 3080 3081=item truncate EXPR,LENGTH 3082 3083Truncates the file opened on FILEHANDLE, or named by EXPR, to the 3084specified length. Produces a fatal error if truncate isn't implemented 3085on your system. 3086 3087=item uc EXPR 3088 3089Returns an uppercased version of EXPR. This is the internal function 3090implementing the \U escape in double-quoted strings. 3091Should respect any POSIX setlocale() settings. 3092 3093=item ucfirst EXPR 3094 3095Returns the value of EXPR with the first character uppercased. This is 3096the internal function implementing the \u escape in double-quoted strings. 3097Should respect any POSIX setlocale() settings. 3098 3099=item umask EXPR 3100 3101=item umask 3102 3103Sets the umask for the process and returns the old one. If EXPR is 3104omitted, merely returns current umask. 3105 3106=item undef EXPR 3107 3108=item undef 3109 3110Undefines the value of EXPR, which must be an lvalue. Use only on a 3111scalar value, an entire array, or a subroutine name (using "&"). (Using undef() 3112will probably not do what you expect on most predefined variables or 3113DBM list values, so don't do that.) Always returns the undefined value. You can omit 3114the EXPR, in which case nothing is undefined, but you still get an 3115undefined value that you could, for instance, return from a 3116subroutine. Examples: 3117 3118 undef $foo; 3119 undef $bar{'blurfl'}; 3120 undef @ary; 3121 undef %assoc; 3122 undef &mysub; 3123 return (wantarray ? () : undef) if $they_blew_it; 3124 3125=item unlink LIST 3126 3127Deletes a list of files. Returns the number of files successfully 3128deleted. 3129 3130 $cnt = unlink 'a', 'b', 'c'; 3131 unlink @goners; 3132 unlink <*.bak>; 3133 3134Note: unlink will not delete directories unless you are superuser and 3135the B<-U> flag is supplied to Perl. Even if these conditions are 3136met, be warned that unlinking a directory can inflict damage on your 3137filesystem. Use rmdir instead. 3138 3139=item unpack TEMPLATE,EXPR 3140 3141Unpack does the reverse of pack: it takes a string representing a 3142structure and expands it out into a list value, returning the array 3143value. (In a scalar context, it merely returns the first value 3144produced.) The TEMPLATE has the same format as in the pack function. 3145Here's a subroutine that does substring: 3146 3147 sub substr { 3148 local($what,$where,$howmuch) = @_; 3149 unpack("x$where a$howmuch", $what); 3150 } 3151 3152and then there's 3153 3154 sub ordinal { unpack("c",$_[0]); } # same as ord() 3155 3156In addition, you may prefix a field with a %<number> to indicate that 3157you want a <number>-bit checksum of the items instead of the items 3158themselves. Default is a 16-bit checksum. For example, the following 3159computes the same number as the System V sum program: 3160 3161 while (<>) { 3162 $checksum += unpack("%16C*", $_); 3163 } 3164 $checksum %= 65536; 3165 3166The following efficiently counts the number of set bits in a bit vector: 3167 3168 $setbits = unpack("%32b*", $selectmask); 3169 3170=item untie VARIABLE 3171 3172Breaks the binding between a variable and a package. (See tie().) 3173 3174=item unshift ARRAY,LIST 3175 3176Does the opposite of a C<shift>. Or the opposite of a C<push>, 3177depending on how you look at it. Prepends list to the front of the 3178array, and returns the new number of elements in the array. 3179 3180 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/; 3181 3182Note the LIST is prepended whole, not one element at a time, so the 3183prepended elements stay in the same order. Use reverse to do the 3184reverse. 3185 3186=item use Module LIST 3187 3188=item use Module 3189 3190Imports some semantics into the current package from the named module, 3191generally by aliasing certain subroutine or variable names into your 3192package. It is exactly equivalent to 3193 3194 BEGIN { require Module; import Module LIST; } 3195 3196The BEGIN forces the require and import to happen at compile time. The 3197require makes sure the module is loaded into memory if it hasn't been 3198yet. The import is not a builtin--it's just an ordinary static method 3199call into the "Module" package to tell the module to import the list of 3200features back into the current package. The module can implement its 3201import method any way it likes, though most modules just choose to 3202derive their import method via inheritance from the Exporter class that 3203is defined in the Exporter module. See L<Exporter>. 3204 3205If you don't want your namespace altered, explicitly supply an empty list: 3206 3207 use Module (); 3208 3209That is exactly equivalent to 3210 3211 BEGIN { require Module; } 3212 3213Because this is a wide-open interface, pragmas (compiler directives) 3214are also implemented this way. Currently implemented pragmas are: 3215 3216 use integer; 3217 use diagnostics; 3218 use sigtrap qw(SEGV BUS); 3219 use strict qw(subs vars refs); 3220 use subs qw(afunc blurfl); 3221 3222These pseudomodules import semantics into the current block scope, unlike 3223ordinary modules, which import symbols into the current package (which are 3224effective through the end of the file). 3225 3226There's a corresponding "no" command that unimports meanings imported 3227by use. 3228 3229 no integer; 3230 no strict 'refs'; 3231 3232See L<perlmod> for a list of standard modules and pragmas. 3233 3234=item utime LIST 3235 3236Changes the access and modification times on each file of a list of 3237files. The first two elements of the list must be the NUMERICAL access 3238and modification times, in that order. Returns the number of files 3239successfully changed. The inode modification time of each file is set 3240to the current time. Example of a "touch" command: 3241 3242 #!/usr/bin/perl 3243 $now = time; 3244 utime $now, $now, @ARGV; 3245 3246=item values ASSOC_ARRAY 3247 3248Returns a normal array consisting of all the values of the named 3249associative array. (In a scalar context, returns the number of 3250values.) The values are returned in an apparently random order, but it 3251is the same order as either the keys() or each() function would produce 3252on the same array. See also keys(), each(), and sort(). 3253 3254=item vec EXPR,OFFSET,BITS 3255 3256Treats the string in EXPR as a vector of unsigned integers, and 3257returns the value of the bitfield specified by OFFSET. BITS specifies 3258the number of bits that are reserved for each entry in the bit 3259vector. This must be a power of two from 1 to 32. vec() may also be 3260assigned to, in which case parens are needed to give the expression 3261the correct precedence as in 3262 3263 vec($image, $max_x * $x + $y, 8) = 3; 3264 3265Vectors created with vec() can also be manipulated with the logical 3266operators |, & and ^, which will assume a bit vector operation is 3267desired when both operands are strings. 3268 3269To transform a bit vector into a string or array of 0's and 1's, use these: 3270 3271 $bits = unpack("b*", $vector); 3272 @bits = split(//, unpack("b*", $vector)); 3273 3274If you know the exact length in bits, it can be used in place of the *. 3275 3276=item wait 3277 3278Waits for a child process to terminate and returns the pid of the 3279deceased process, or -1 if there are no child processes. The status is 3280returned in $?. 3281 3282=item waitpid PID,FLAGS 3283 3284Waits for a particular child process to terminate and returns the pid 3285of the deceased process, or -1 if there is no such child process. The 3286status is returned in $?. If you say 3287 3288 use POSIX "wait_h"; 3289 ... 3290 waitpid(-1,&WNOHANG); 3291 3292then you can do a non-blocking wait for any process. Non-blocking wait 3293is only available on machines supporting either the waitpid(2) or 3294wait4(2) system calls. However, waiting for a particular pid with 3295FLAGS of 0 is implemented everywhere. (Perl emulates the system call 3296by remembering the status values of processes that have exited but have 3297not been harvested by the Perl script yet.) 3298 3299=item wantarray 3300 3301Returns TRUE if the context of the currently executing subroutine is 3302looking for a list value. Returns FALSE if the context is looking 3303for a scalar. 3304 3305 return wantarray ? () : undef; 3306 3307=item warn LIST 3308 3309Produces a message on STDERR just like die(), but doesn't exit or 3310on an exception. 3311 3312=item write FILEHANDLE 3313 3314=item write EXPR 3315 3316=item write 3317 3318Writes a formatted record (possibly multi-line) to the specified file, 3319using the format associated with that file. By default the format for 3320a file is the one having the same name is the filehandle, but the 3321format for the current output channel (see the select() function) may be set 3322explicitly by assigning the name of the format to the $~ variable. 3323 3324Top of form processing is handled automatically: if there is 3325insufficient room on the current page for the formatted record, the 3326page is advanced by writing a form feed, a special top-of-page format 3327is used to format the new page header, and then the record is written. 3328By default the top-of-page format is the name of the filehandle with 3329"_TOP" appended, but it may be dynamically set to the format of your 3330choice by assigning the name to the $^ variable while the filehandle is 3331selected. The number of lines remaining on the current page is in 3332variable $-, which can be set to 0 to force a new page. 3333 3334If FILEHANDLE is unspecified, output goes to the current default output 3335channel, which starts out as STDOUT but may be changed by the 3336C<select> operator. If the FILEHANDLE is an EXPR, then the expression 3337is evaluated and the resulting string is used to look up the name of 3338the FILEHANDLE at run time. For more on formats, see L<perlform>. 3339 3340Note that write is I<NOT> the opposite of read. Unfortunately. 3341 3342=item y/// 3343 3344The translation operator. See L<perlop>. 3345 3346=back 3347