3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar and list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one list argument.) For instance, splice() has three scalar
19 arguments followed by a list.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parentheses.) If you use the parentheses, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 nonabortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following rule:
59 =item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
63 Each operator and function decides which sort of value it would be most
64 appropriate to return in a scalar context. Some operators return the
65 length of the list that would have been returned in a list context. Some
66 operators return the first value in the list. Some operators return the
67 last value in the list. Some operators return a count of successful
68 operations. In general, they do what you want, unless you want
71 =head2 Perl Functions by Category
73 Here are Perl's functions (including things that look like
74 functions, like some of the keywords and named operators)
75 arranged by category. Some functions appear in more
80 =item Functions for SCALARs or strings
82 chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
83 oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
84 sprintf, substr, tr///, uc, ucfirst, y///
86 =item Regular expressions and pattern matching
88 m//, pos, quotemeta, s///, split, study
90 =item Numeric functions
92 abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
95 =item Functions for real @ARRAYs
97 pop, push, shift, splice, unshift
99 =item Functions for list data
101 grep, join, map, qw/STRING/, reverse, sort, unpack
103 =item Functions for real %HASHes
105 delete, each, exists, keys, values
107 =item Input and output functions
109 binmode, close, closedir, dbmclose, dbmopen, die, eof,
110 fileno, flock, format, getc, print, printf, read, readdir,
111 rewinddir, seek, seekdir, select, syscall, sysread, sysseek,
112 syswrite, tell, telldir, truncate, warn, write
114 =item Functions for fixed length data or records
116 pack, read, syscall, sysread, syswrite, unpack, vec
118 =item Functions for filehandles, files, or directories
120 I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
121 lstat, mkdir, open, opendir, readlink, rename, rmdir,
122 stat, symlink, umask, unlink, utime
124 =item Keywords related to the control flow of your perl program
126 caller, continue, die, do, dump, eval, exit, goto, last,
127 next, redo, return, sub, wantarray
129 =item Keywords related to scoping
131 caller, import, local, my, package, use
133 =item Miscellaneous functions
135 defined, dump, eval, formline, local, my, reset, scalar,
138 =item Functions for processes and process groups
140 alarm, exec, fork, getpgrp, getppid, getpriority, kill,
141 pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
144 =item Keywords related to perl modules
146 do, import, no, package, require, use
148 =item Keywords related to classes and object-orientedness
150 bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
152 =item Low-level socket functions
154 accept, bind, connect, getpeername, getsockname,
155 getsockopt, listen, recv, send, setsockopt, shutdown,
158 =item System V interprocess communication functions
160 msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
161 shmctl, shmget, shmread, shmwrite
163 =item Fetching user and group info
165 endgrent, endhostent, endnetent, endpwent, getgrent,
166 getgrgid, getgrnam, getlogin, getpwent, getpwnam,
167 getpwuid, setgrent, setpwent
169 =item Fetching network info
171 endprotoent, endservent, gethostbyaddr, gethostbyname,
172 gethostent, getnetbyaddr, getnetbyname, getnetent,
173 getprotobyname, getprotobynumber, getprotoent,
174 getservbyname, getservbyport, getservent, sethostent,
175 setnetent, setprotoent, setservent
177 =item Time-related functions
179 gmtime, localtime, time, times
181 =item Functions new in perl5
183 abs, bless, chomp, chr, exists, formline, glob, import, lc,
184 lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
185 ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
187 * - C<sub> was a keyword in perl4, but in perl5 it is an
188 operator which can be used in expressions.
190 =item Functions obsoleted in perl5
196 =head2 Alphabetical Listing of Perl Functions
206 A file test, where X is one of the letters listed below. This unary
207 operator takes one argument, either a filename or a filehandle, and
208 tests the associated file to see if something is true about it. If the
209 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
210 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
211 the undefined value if the file doesn't exist. Despite the funny
212 names, precedence is the same as any other named unary operator, and
213 the argument may be parenthesized like any other unary operator. The
214 operator may be any of:
216 -r File is readable by effective uid/gid.
217 -w File is writable by effective uid/gid.
218 -x File is executable by effective uid/gid.
219 -o File is owned by effective uid.
221 -R File is readable by real uid/gid.
222 -W File is writable by real uid/gid.
223 -X File is executable by real uid/gid.
224 -O File is owned by real uid.
227 -z File has zero size.
228 -s File has nonzero size (returns size).
230 -f File is a plain file.
231 -d File is a directory.
232 -l File is a symbolic link.
233 -p File is a named pipe (FIFO).
235 -b File is a block special file.
236 -c File is a character special file.
237 -t Filehandle is opened to a tty.
239 -u File has setuid bit set.
240 -g File has setgid bit set.
241 -k File has sticky bit set.
243 -T File is a text file.
244 -B File is a binary file (opposite of -T).
246 -M Age of file in days when script started.
247 -A Same for access time.
248 -C Same for inode change time.
250 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
251 C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
252 uids and gids of the user. There may be other reasons you can't actually
253 read, write or execute the file. Also note that, for the superuser,
254 C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
255 1 if any execute bit is set in the mode. Scripts run by the superuser may
256 thus need to do a stat() to determine the actual mode of the
257 file, or temporarily set the uid to something else.
263 next unless -f $_; # ignore specials
267 Note that C<-s/a/b/> does not do a negated substitution. Saying
268 C<-exp($foo)> still works as expected, however--only single letters
269 following a minus are interpreted as file tests.
271 The C<-T> and C<-B> switches work as follows. The first block or so of the
272 file is examined for odd characters such as strange control codes or
273 characters with the high bit set. If too many odd characters (E<gt>30%)
274 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
275 containing null in the first block is considered a binary file. If C<-T>
276 or C<-B> is used on a filehandle, the current stdio buffer is examined
277 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
278 file, or a file at EOF when testing a filehandle. Because you have to
279 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
280 against the file first, as in C<next unless -f $file && -T $file>.
282 If any of the file tests (or either the stat() or lstat() operators) are given
283 the special filehandle consisting of a solitary underline, then the stat
284 structure of the previous file test (or stat operator) is used, saving
285 a system call. (This doesn't work with C<-t>, and you need to remember
286 that lstat() and C<-l> will leave values in the stat structure for the
287 symbolic link, not the real file.) Example:
289 print "Can do.\n" if -r $a || -w _ || -x _;
292 print "Readable\n" if -r _;
293 print "Writable\n" if -w _;
294 print "Executable\n" if -x _;
295 print "Setuid\n" if -u _;
296 print "Setgid\n" if -g _;
297 print "Sticky\n" if -k _;
298 print "Text\n" if -T _;
299 print "Binary\n" if -B _;
305 Returns the absolute value of its argument.
306 If VALUE is omitted, uses $_.
308 =item accept NEWSOCKET,GENERICSOCKET
310 Accepts an incoming socket connect, just as the accept(2) system call
311 does. Returns the packed address if it succeeded, FALSE otherwise.
312 See example in L<perlipc/"Sockets: Client/Server Communication">.
318 Arranges to have a SIGALRM delivered to this process after the
319 specified number of seconds have elapsed. If SECONDS is not specified,
320 the value stored in $_ is used. (On some machines,
321 unfortunately, the elapsed time may be up to one second less than you
322 specified because of how seconds are counted.) Only one timer may be
323 counting at once. Each call disables the previous timer, and an
324 argument of 0 may be supplied to cancel the previous timer without
325 starting a new one. The returned value is the amount of time remaining
326 on the previous timer.
328 For delays of finer granularity than one second, you may use Perl's
329 syscall() interface to access setitimer(2) if your system supports it,
330 or else see L</select()>. It is usually a mistake to intermix alarm()
333 If you want to use alarm() to time out a system call you need to use an
334 eval/die pair. You can't rely on the alarm causing the system call to
335 fail with $! set to EINTR because Perl sets up signal handlers to
336 restart system calls on some systems. Using eval/die always works.
339 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
341 $nread = sysread SOCKET, $buffer, $size;
344 die if $@ && $@ ne "alarm\n"; # propagate errors
354 Returns the arctangent of Y/X in the range -PI to PI.
356 For the tangent operation, you may use the POSIX::tan()
357 function, or use the familiar relation:
359 sub tan { sin($_[0]) / cos($_[0]) }
361 =item bind SOCKET,NAME
363 Binds a network address to a socket, just as the bind system call
364 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
365 packed address of the appropriate type for the socket. See the examples in
366 L<perlipc/"Sockets: Client/Server Communication">.
368 =item binmode FILEHANDLE
370 Arranges for the file to be read or written in "binary" mode in operating
371 systems that distinguish between binary and text files. Files that are
372 not in binary mode have CR LF sequences translated to LF on input and LF
373 translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
374 and similarly archaic systems, it may be imperative--otherwise your
375 MS-DOS-damaged C library may mangle your file. The key distinction between
376 systems that need binmode and those that don't is their text file
377 formats. Systems like Unix and Plan9 that delimit lines with a single
378 character, and that encode that character in C as '\n', do not need
379 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
380 is taken as the name of the filehandle.
382 =item bless REF,CLASSNAME
386 This function tells the thingy referenced by REF that it is now
387 an object in the CLASSNAME package--or the current package if no CLASSNAME
388 is specified, which is often the case. It returns the reference for
389 convenience, because a bless() is often the last thing in a constructor.
390 Always use the two-argument version if the function doing the blessing
391 might be inherited by a derived class. See L<perlobj> for more about the
392 blessing (and blessings) of objects.
398 Returns the context of the current subroutine call. In a scalar context,
399 returns the caller's package name if there is a caller, that is, if
400 we're in a subroutine or eval() or require(), and the undefined value
401 otherwise. In a list context, returns
403 ($package, $filename, $line) = caller;
405 With EXPR, it returns some extra information that the debugger uses to
406 print a stack trace. The value of EXPR indicates how many call frames
407 to go back before the current one.
409 ($package, $filename, $line, $subroutine,
410 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
412 Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
413 call, but an C<eval>. In such a case additional elements $evaltext and
414 $is_require are set: $is_require is true if the frame is created by a
415 C<require> or C<use> statement, $evaltext contains the text of the
416 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
417 $filename is C<"(eval)">, but $evaltext is undefined. (Note also that
418 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
421 Furthermore, when called from within the DB package, caller returns more
422 detailed information: it sets the list variable @DB::args to be the
423 arguments with which the subroutine was invoked.
427 Changes the working directory to EXPR, if possible. If EXPR is
428 omitted, changes to home directory. Returns TRUE upon success, FALSE
429 otherwise. See example under die().
433 Changes the permissions of a list of files. The first element of the
434 list must be the numerical mode, which should probably be an octal
435 number, and which definitely should I<not> a string of octal digits:
436 C<0644> is okay, C<'0644'> is not. Returns the number of files
437 successfully changed. See also L</oct>, if all you have is a string.
439 $cnt = chmod 0755, 'foo', 'bar';
440 chmod 0755, @executables;
441 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T
442 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
443 $mode = 0644; chmod $mode, 'foo'; # this is best
451 This is a slightly safer version of L</chop>. It removes any
452 line ending that corresponds to the current value of C<$/> (also known as
453 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
454 number of characters removed from all its arguments. It's often used to
455 remove the newline from the end of an input record when you're worried
456 that the final record may be missing its newline. When in paragraph mode
457 (C<$/ = "">), it removes all trailing newlines from the string. If
458 VARIABLE is omitted, it chomps $_. Example:
461 chomp; # avoid \n on last field
466 You can actually chomp anything that's an lvalue, including an assignment:
469 chomp($answer = <STDIN>);
471 If you chomp a list, each element is chomped, and the total number of
472 characters removed is returned.
480 Chops off the last character of a string and returns the character
481 chopped. It's used primarily to remove the newline from the end of an
482 input record, but is much more efficient than C<s/\n//> because it neither
483 scans nor copies the string. If VARIABLE is omitted, chops $_.
487 chop; # avoid \n on last field
492 You can actually chop anything that's an lvalue, including an assignment:
495 chop($answer = <STDIN>);
497 If you chop a list, each element is chopped. Only the value of the
498 last chop is returned.
500 Note that chop returns the last character. To return all but the last
501 character, use C<substr($string, 0, -1)>.
505 Changes the owner (and group) of a list of files. The first two
506 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
507 Returns the number of files successfully changed.
509 $cnt = chown $uid, $gid, 'foo', 'bar';
510 chown $uid, $gid, @filenames;
512 Here's an example that looks up nonnumeric uids in the passwd file:
515 chop($user = <STDIN>);
517 chop($pattern = <STDIN>);
519 ($login,$pass,$uid,$gid) = getpwnam($user)
520 or die "$user not in passwd file";
522 @ary = <${pattern}>; # expand filenames
523 chown $uid, $gid, @ary;
525 On most systems, you are not allowed to change the ownership of the
526 file unless you're the superuser, although you should be able to change
527 the group to any of your secondary groups. On insecure systems, these
528 restrictions may be relaxed, but this is not a portable assumption.
534 Returns the character represented by that NUMBER in the character set.
535 For example, C<chr(65)> is "A" in ASCII. For the reverse, use L</ord>.
537 If NUMBER is omitted, uses $_.
539 =item chroot FILENAME
543 This function works as the system call by the same name: it makes the
544 named directory the new root directory for all further pathnames that
545 begin with a "/" by your process and all of its children. (It doesn't
546 change your current working directory, which is unaffected.) For security
547 reasons, this call is restricted to the superuser. If FILENAME is
548 omitted, does chroot to $_.
550 =item close FILEHANDLE
552 Closes the file or pipe associated with the file handle, returning TRUE
553 only if stdio successfully flushes buffers and closes the system file
556 You don't have to close FILEHANDLE if you are immediately going to do
557 another open() on it, because open() will close it for you. (See
558 open().) However, an explicit close on an input file resets the line
559 counter ($.), while the implicit close done by open() does not.
561 If the file handle came from a piped open C<close> will additionally
562 return FALSE if one of the other system calls involved fails or if the
563 program exits with non-zero status. (If the only problem was that the
564 program exited non-zero $! will be set to 0.) Also, closing a pipe will
565 wait for the process executing on the pipe to complete, in case you
566 want to look at the output of the pipe afterwards. Closing a pipe
567 explicitly also puts the exit status value of the command into C<$?>.
570 open(OUTPUT, '|sort >foo') # pipe to sort
571 or die "Can't start sort: $!";
572 ... # print stuff to output
573 close OUTPUT # wait for sort to finish
574 or warn $! ? "Error closing sort pipe: $!"
575 : "Exit status $? from sort";
576 open(INPUT, 'foo') # get sort's results
577 or die "Can't open 'foo' for input: $!";
579 FILEHANDLE may be an expression whose value gives the real filehandle name.
581 =item closedir DIRHANDLE
583 Closes a directory opened by opendir().
585 =item connect SOCKET,NAME
587 Attempts to connect to a remote socket, just as the connect system call
588 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
589 packed address of the appropriate type for the socket. See the examples in
590 L<perlipc/"Sockets: Client/Server Communication">.
594 Actually a flow control statement rather than a function. If there is a
595 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
596 C<foreach>), it is always executed just before the conditional is about to
597 be evaluated again, just like the third part of a C<for> loop in C. Thus
598 it can be used to increment a loop variable, even when the loop has been
599 continued via the C<next> statement (which is similar to the C C<continue>
604 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
607 For the inverse cosine operation, you may use the POSIX::acos()
608 function, or use this relation:
610 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
612 =item crypt PLAINTEXT,SALT
614 Encrypts a string exactly like the crypt(3) function in the C library
615 (assuming that you actually have a version there that has not been
616 extirpated as a potential munition). This can prove useful for checking
617 the password file for lousy passwords, amongst other things. Only the
618 guys wearing white hats should do this.
620 Note that crypt is intended to be a one-way function, much like breaking
621 eggs to make an omelette. There is no (known) corresponding decrypt
622 function. As a result, this function isn't all that useful for
623 cryptography. (For that, see your nearby CPAN mirror.)
625 Here's an example that makes sure that whoever runs this program knows
628 $pwd = (getpwuid($<))[1];
629 $salt = substr($pwd, 0, 2);
633 chop($word = <STDIN>);
637 if (crypt($word, $salt) ne $pwd) {
643 Of course, typing in your own password to whoever asks you
648 [This function has been superseded by the untie() function.]
650 Breaks the binding between a DBM file and a hash.
652 =item dbmopen HASH,DBNAME,MODE
654 [This function has been superseded by the tie() function.]
656 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a
657 hash. HASH is the name of the hash. (Unlike normal open, the first
658 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
659 is the name of the database (without the F<.dir> or F<.pag> extension if
660 any). If the database does not exist, it is created with protection
661 specified by MODE (as modified by the umask()). If your system supports
662 only the older DBM functions, you may perform only one dbmopen() in your
663 program. In older versions of Perl, if your system had neither DBM nor
664 ndbm, calling dbmopen() produced a fatal error; it now falls back to
667 If you don't have write access to the DBM file, you can only read hash
668 variables, not set them. If you want to test whether you can write,
669 either use file tests or try setting a dummy hash entry inside an eval(),
670 which will trap the error.
672 Note that functions such as keys() and values() may return huge array
673 values when used on large DBM files. You may prefer to use the each()
674 function to iterate over large DBM files. Example:
676 # print out history file offsets
677 dbmopen(%HIST,'/usr/lib/news/history',0666);
678 while (($key,$val) = each %HIST) {
679 print $key, ' = ', unpack('L',$val), "\n";
683 See also L<AnyDBM_File> for a more general description of the pros and
684 cons of the various dbm approaches, as well as L<DB_File> for a particularly
691 Returns a Boolean value telling whether EXPR has a value other than
692 the undefined value C<undef>. If EXPR is not present, C<$_> will be
695 Many operations return C<undef> to indicate failure, end of file,
696 system error, uninitialized variable, and other exceptional
697 conditions. This function allows you to distinguish C<undef> from
698 other values. (A simple Boolean test will not distinguish among
699 C<undef>, zero, the empty string, and "0", which are all equally
700 false.) Note that since C<undef> is a valid scalar, its presence
701 doesn't I<necessarily> indicate an exceptional condition: pop()
702 returns C<undef> when its argument is an empty array, I<or> when the
703 element to return happens to be C<undef>.
705 You may also use defined() to check whether a subroutine exists. On
706 the other hand, use of defined() upon aggregates (hashes and arrays)
707 is not guaranteed to produce intuitive results, and should probably be
710 When used on a hash element, it tells you whether the value is defined,
711 not whether the key exists in the hash. Use L</exists> for the latter
716 print if defined $switch{'D'};
717 print "$val\n" while defined($val = pop(@ary));
718 die "Can't readlink $sym: $!"
719 unless defined($value = readlink $sym);
720 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
721 $debugging = 0 unless defined $debugging;
723 Note: Many folks tend to overuse defined(), and then are surprised to
724 discover that the number 0 and "" (the zero-length string) are, in fact,
725 defined values. For example, if you say
729 the pattern match succeeds, and $1 is defined, despite the fact that it
730 matched "nothing". But it didn't really match nothing--rather, it
731 matched something that happened to be 0 characters long. This is all
732 very above-board and honest. When a function returns an undefined value,
733 it's an admission that it couldn't give you an honest answer. So you
734 should use defined() only when you're questioning the integrity of what
735 you're trying to do. At other times, a simple comparison to 0 or "" is
738 Currently, using defined() on an entire array or hash reports whether
739 memory for that aggregate has ever been allocated. So an array you set
740 to the empty list appears undefined initially, and one that once was full
741 and that you then set to the empty list still appears defined. You
742 should instead use a simple test for size:
744 if (@an_array) { print "has array elements\n" }
745 if (%a_hash) { print "has hash members\n" }
747 Using undef() on these, however, does clear their memory and then report
748 them as not defined anymore, but you shoudln't do that unless you don't
749 plan to use them again, because it saves time when you load them up
750 again to have memory already ready to be filled.
752 This counterintuitive behaviour of defined() on aggregates may be
753 changed, fixed, or broken in a future release of Perl.
755 See also L</undef>, L</exists>, L</ref>.
759 Deletes the specified key(s) and their associated values from a hash.
760 For each key, returns the deleted value associated with that key, or
761 the undefined value if there was no such key. Deleting from C<$ENV{}>
762 modifies the environment. Deleting from a hash tied to a DBM file
763 deletes the entry from the DBM file. (But deleting from a tie()d hash
764 doesn't necessarily return anything.)
766 The following deletes all the values of a hash:
768 foreach $key (keys %HASH) {
774 delete @HASH{keys %HASH}
776 (But both of these are slower than the undef() command.) Note that the
777 EXPR can be arbitrarily complicated as long as the final operation is a
778 hash element lookup or hash slice:
780 delete $ref->[$x][$y]{$key};
781 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
785 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
786 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
787 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
788 is 0, exits with 255. Inside an eval(), the error message is stuffed into
789 C<$@>, and the eval() is terminated with the undefined value; this makes
790 die() the way to raise an exception.
794 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
795 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
797 If the value of EXPR does not end in a newline, the current script line
798 number and input line number (if any) are also printed, and a newline
799 is supplied. Hint: sometimes appending ", stopped" to your message
800 will cause it to make better sense when the string "at foo line 123" is
801 appended. Suppose you are running script "canasta".
803 die "/etc/games is no good";
804 die "/etc/games is no good, stopped";
806 produce, respectively
808 /etc/games is no good at canasta line 123.
809 /etc/games is no good, stopped at canasta line 123.
811 See also exit() and warn().
813 If LIST is empty and $@ already contains a value (typically from a
814 previous eval) that value is reused after appending "\t...propagated".
815 This is useful for propagating exceptions:
818 die unless $@ =~ /Expected exception/;
820 If $@ is empty then the string "Died" is used.
822 You can arrange for a callback to be called just before the die() does
823 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
824 will be called with the error text and can change the error message, if
825 it sees fit, by calling die() again. See L<perlvar/$SIG{expr}> for details on
826 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
828 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
829 blocks/strings. If one wants the hook to do nothing in such
834 as the first line of the handler (see L<perlvar/$^S>).
838 Not really a function. Returns the value of the last command in the
839 sequence of commands indicated by BLOCK. When modified by a loop
840 modifier, executes the BLOCK once before testing the loop condition.
841 (On other statements the loop modifiers test the conditional first.)
843 =item do SUBROUTINE(LIST)
845 A deprecated form of subroutine call. See L<perlsub>.
849 Uses the value of EXPR as a filename and executes the contents of the
850 file as a Perl script. Its primary use is to include subroutines
851 from a Perl subroutine library.
857 scalar eval `cat stat.pl`;
859 except that it's more efficient, more concise, keeps track of the
860 current filename for error messages, and searches all the B<-I>
861 libraries if the file isn't in the current directory (see also the @INC
862 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
863 reparse the file every time you call it, so you probably don't want to
864 do this inside a loop.
866 Note that inclusion of library modules is better done with the
867 use() and require() operators, which also do error checking
868 and raise an exception if there's a problem.
872 This causes an immediate core dump. Primarily this is so that you can
873 use the B<undump> program to turn your core dump into an executable binary
874 after having initialized all your variables at the beginning of the
875 program. When the new binary is executed it will begin by executing a
876 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
877 it as a goto with an intervening core dump and reincarnation. If LABEL
878 is omitted, restarts the program from the top. WARNING: any files
879 opened at the time of the dump will NOT be open any more when the
880 program is reincarnated, with possible resulting confusion on the part
881 of Perl. See also B<-u> option in L<perlrun>.
898 dump QUICKSTART if $ARGV[0] eq '-d';
905 When called in a list context, returns a 2-element array consisting of the
906 key and value for the next element of a hash, so that you can iterate over
907 it. When called in a scalar context, returns the key for only the next
908 element in the hash. (Note: Keys may be "0" or "", which are logically
909 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
912 Entries are returned in an apparently random order. When the hash is
913 entirely read, a null array is returned in list context (which when
914 assigned produces a FALSE (0) value), and C<undef> is returned in a
915 scalar context. The next call to each() after that will start iterating
916 again. There is a single iterator for each hash, shared by all each(),
917 keys(), and values() function calls in the program; it can be reset by
918 reading all the elements from the hash, or by evaluating C<keys HASH> or
919 C<values HASH>. If you add or delete elements of a hash while you're
920 iterating over it, you may get entries skipped or duplicated, so don't.
922 The following prints out your environment like the printenv(1) program,
923 only in a different order:
925 while (($key,$value) = each %ENV) {
926 print "$key=$value\n";
929 See also keys() and values().
937 Returns 1 if the next read on FILEHANDLE will return end of file, or if
938 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
939 gives the real filehandle name. (Note that this function actually
940 reads a character and then ungetc()s it, so it is not very useful in an
941 interactive context.) Do not read from a terminal file (or call
942 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
943 as terminals may lose the end-of-file condition if you do.
945 An C<eof> without an argument uses the last file read as argument.
946 Empty parentheses () may be used to indicate the pseudo file formed of
947 the files listed on the command line, i.e., C<eof()> is reasonable to
948 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
949 last file. Use C<eof(ARGV)> or eof without the parentheses to test
950 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
952 # reset line numbering on each input file
955 close(ARGV) if (eof); # Not eof().
958 # insert dashes just before last line of last file
961 print "--------------\n";
962 close(ARGV); # close or break; is needed if we
963 # are reading from the terminal
968 Practical hint: you almost never need to use C<eof> in Perl, because the
969 input operators return undef when they run out of data.
975 In the first form, the return value of EXPR is parsed and executed as if it
976 were a little Perl program. The value of the expression (which is itself
977 determined within a scalar context) is first parsed, and if there are no
978 errors, executed in the context of the current Perl program, so that any
979 variable settings or subroutine and format definitions remain afterwards.
980 Note that the value is parsed every time the eval executes. If EXPR is
981 omitted, evaluates C<$_>. This form is typically used to delay parsing
982 and subsequent execution of the text of EXPR until run time.
984 In the second form, the code within the BLOCK is parsed only once--at the
985 same time the code surrounding the eval itself was parsed--and executed
986 within the context of the current Perl program. This form is typically
987 used to trap exceptions more efficiently than the first (see below), while
988 also providing the benefit of checking the code within BLOCK at compile
991 The final semicolon, if any, may be omitted from the value of EXPR or within
994 In both forms, the value returned is the value of the last expression
995 evaluated inside the mini-program, or a return statement may be used, just
996 as with subroutines. The expression providing the return value is evaluated
997 in void, scalar or array context, depending on the context of the eval itself.
998 See L</wantarray> for more on how the evaluation context can be determined.
1000 If there is a syntax error or runtime error, or a die() statement is
1001 executed, an undefined value is returned by eval(), and C<$@> is set to the
1002 error message. If there was no error, C<$@> is guaranteed to be a null
1003 string. Beware that using eval() neither silences perl from printing
1004 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1005 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1006 L</warn> and L<perlvar>.
1008 Note that, because eval() traps otherwise-fatal errors, it is useful for
1009 determining whether a particular feature (such as socket() or symlink())
1010 is implemented. It is also Perl's exception trapping mechanism, where
1011 the die operator is used to raise exceptions.
1013 If the code to be executed doesn't vary, you may use the eval-BLOCK
1014 form to trap run-time errors without incurring the penalty of
1015 recompiling each time. The error, if any, is still returned in C<$@>.
1018 # make divide-by-zero nonfatal
1019 eval { $answer = $a / $b; }; warn $@ if $@;
1021 # same thing, but less efficient
1022 eval '$answer = $a / $b'; warn $@ if $@;
1024 # a compile-time error
1028 eval '$answer ='; # sets $@
1030 When using the eval{} form as an exception trap in libraries, you may
1031 wish not to trigger any C<__DIE__> hooks that user code may have
1032 installed. You can use the C<local $SIG{__DIE__}> construct for this
1033 purpose, as shown in this example:
1035 # a very private exception trap for divide-by-zero
1036 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
1038 This is especially significant, given that C<__DIE__> hooks can call
1039 die() again, which has the effect of changing their error messages:
1041 # __DIE__ hooks may modify error messages
1043 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1044 eval { die "foo lives here" };
1045 print $@ if $@; # prints "bar lives here"
1048 With an eval(), you should be especially careful to remember what's
1049 being looked at when:
1055 eval { $x }; # CASE 4
1057 eval "\$$x++" # CASE 5
1060 Cases 1 and 2 above behave identically: they run the code contained in
1061 the variable $x. (Although case 2 has misleading double quotes making
1062 the reader wonder what else might be happening (nothing is).) Cases 3
1063 and 4 likewise behave in the same way: they run the code '$x', which
1064 does nothing but return the value of C<$x>. (Case 4 is preferred for
1065 purely visual reasons, but it also has the advantage of compiling at
1066 compile-time instead of at run-time.) Case 5 is a place where
1067 normally you I<WOULD> like to use double quotes, except that in this
1068 particular situation, you can just use symbolic references instead, as
1073 The exec() function executes a system command I<AND NEVER RETURNS> -
1074 use system() instead of exec() if you want it to return. It fails and
1075 returns FALSE only if the command does not exist I<and> it is executed
1076 directly instead of via your system's command shell (see below).
1078 If there is more than one argument in LIST, or if LIST is an array with
1079 more than one value, calls execvp(3) with the arguments in LIST. If
1080 there is only one scalar argument, the argument is checked for shell
1081 metacharacters, and if there are any, the entire argument is passed to
1082 the system's command shell for parsing (this is C</bin/sh -c> on Unix
1083 platforms, but varies on other platforms). If there are no shell
1084 metacharacters in the argument, it is split into words and passed
1085 directly to execvp(), which is more efficient. Note: exec() and
1086 system() do not flush your output buffer, so you may need to set C<$|>
1087 to avoid lost output. Examples:
1089 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1090 exec "sort $outfile | uniq";
1092 If you don't really want to execute the first argument, but want to lie
1093 to the program you are executing about its own name, you can specify
1094 the program you actually want to run as an "indirect object" (without a
1095 comma) in front of the LIST. (This always forces interpretation of the
1096 LIST as a multivalued list, even if there is only a single scalar in
1099 $shell = '/bin/csh';
1100 exec $shell '-sh'; # pretend it's a login shell
1104 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1106 When the arguments get executed via the system shell, results will
1107 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1112 Returns TRUE if the specified hash key exists in its hash array, even
1113 if the corresponding value is undefined.
1115 print "Exists\n" if exists $array{$key};
1116 print "Defined\n" if defined $array{$key};
1117 print "True\n" if $array{$key};
1119 A hash element can be TRUE only if it's defined, and defined if
1120 it exists, but the reverse doesn't necessarily hold true.
1122 Note that the EXPR can be arbitrarily complicated as long as the final
1123 operation is a hash key lookup:
1125 if (exists $ref->[$x][$y]{$key}) { ... }
1129 Evaluates EXPR and exits immediately with that value. (Actually, it
1130 calls any defined C<END> routines first, but the C<END> routines may not
1131 abort the exit. Likewise any object destructors that need to be called
1132 are called before exit.) Example:
1135 exit 0 if $ans =~ /^[Xx]/;
1137 See also die(). If EXPR is omitted, exits with 0 status. The only
1138 universally portable values for EXPR are 0 for success and 1 for error;
1139 all other values are subject to unpredictable interpretation depending
1140 on the environment in which the Perl program is running.
1142 You shouldn't use exit() to abort a subroutine if there's any chance that
1143 someone might want to trap whatever error happened. Use die() instead,
1144 which can be trapped by an eval().
1150 Returns I<e> (the natural logarithm base) to the power of EXPR.
1151 If EXPR is omitted, gives C<exp($_)>.
1153 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1155 Implements the fcntl(2) function. You'll probably have to say
1159 first to get the correct function definitions. Argument processing and
1160 value return works just like ioctl() below. Note that fcntl() will produce
1161 a fatal error if used on a machine that doesn't implement fcntl(2).
1165 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1167 =item fileno FILEHANDLE
1169 Returns the file descriptor for a filehandle. This is useful for
1170 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1171 value is taken as the name of the filehandle.
1173 =item flock FILEHANDLE,OPERATION
1175 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1176 success, FALSE on failure. Produces a fatal error if used on a machine
1177 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1178 is Perl's portable file locking interface, although it locks only entire
1181 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1182 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1183 you can use the symbolic names if import them from the Fcntl module,
1184 either individually, or as a group using the ':flock' tag. LOCK_SH
1185 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1186 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1187 LOCK_EX then flock() will return immediately rather than blocking
1188 waiting for the lock (check the return status to see if you got it).
1190 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1191 before (un)locking it.
1193 Note that the emulation built with lockf(3) doesn't provide shared
1194 locks, and it requires that FILEHANDLE be open with write intent. These
1195 are the semantics that lockf(3) implements. Most (all?) systems
1196 implement lockf(3) in terms of fcntl(2) locking, though, so the
1197 differing semantics shouldn't bite too many people.
1199 Note also that some versions of flock() cannot lock things over the
1200 network; you would need to use the more system-specific fcntl() for
1201 that. If you like you can force Perl to ignore your system's flock(2)
1202 function, and so provide its own fcntl(2)-based emulation, by passing
1203 the switch C<-Ud_flock> to the F<Configure> program when you configure
1206 Here's a mailbox appender for BSD systems.
1208 use Fcntl ':flock'; # import LOCK_* constants
1211 flock(MBOX,LOCK_EX);
1212 # and, in case someone appended
1213 # while we were waiting...
1218 flock(MBOX,LOCK_UN);
1221 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1222 or die "Can't open mailbox: $!";
1225 print MBOX $msg,"\n\n";
1228 See also L<DB_File> for other flock() examples.
1232 Does a fork(2) system call. Returns the child pid to the parent process
1233 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1234 Note: unflushed buffers remain unflushed in both processes, which means
1235 you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1236 method of IO::Handle to avoid duplicate output.
1238 If you fork() without ever waiting on your children, you will accumulate
1241 $SIG{CHLD} = sub { wait };
1243 There's also the double-fork trick (error checking on
1244 fork() returns omitted);
1246 unless ($pid = fork) {
1248 exec "what you really wanna do";
1251 ## (some_perl_code_here)
1258 See also L<perlipc> for more examples of forking and reaping
1261 Note that if your forked child inherits system file descriptors like
1262 STDIN and STDOUT that are actually connected by a pipe or socket, even
1263 if you exit, the remote server (such as, say, httpd or rsh) won't think
1264 you're done. You should reopen those to /dev/null if it's any issue.
1268 Declare a picture format with use by the write() function. For
1272 Test: @<<<<<<<< @||||| @>>>>>
1273 $str, $%, '$' . int($num)
1277 $num = $cost/$quantity;
1281 See L<perlform> for many details and examples.
1284 =item formline PICTURE,LIST
1286 This is an internal function used by C<format>s, though you may call it
1287 too. It formats (see L<perlform>) a list of values according to the
1288 contents of PICTURE, placing the output into the format output
1289 accumulator, C<$^A> (or $ACCUMULATOR in English).
1290 Eventually, when a write() is done, the contents of
1291 C<$^A> are written to some filehandle, but you could also read C<$^A>
1292 yourself and then set C<$^A> back to "". Note that a format typically
1293 does one formline() per line of form, but the formline() function itself
1294 doesn't care how many newlines are embedded in the PICTURE. This means
1295 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1296 You may therefore need to use multiple formlines to implement a single
1297 record format, just like the format compiler.
1299 Be careful if you put double quotes around the picture, because an "C<@>"
1300 character may be taken to mean the beginning of an array name.
1301 formline() always returns TRUE. See L<perlform> for other examples.
1303 =item getc FILEHANDLE
1307 Returns the next character from the input file attached to FILEHANDLE,
1308 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1309 This is not particularly efficient. It cannot be used to get unbuffered
1310 single-characters, however. For that, try something more like:
1313 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1316 system "stty", '-icanon', 'eol', "\001";
1322 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1325 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1329 Determination of whether $BSD_STYLE should be set
1330 is left as an exercise to the reader.
1332 The POSIX::getattr() function can do this more portably on systems
1333 alleging POSIX compliance.
1334 See also the C<Term::ReadKey> module from your nearest CPAN site;
1335 details on CPAN can be found on L<perlmod/CPAN>.
1339 Returns the current login from F</etc/utmp>, if any. If null, use
1342 $login = getlogin || getpwuid($<) || "Kilroy";
1344 Do not consider getlogin() for authentication: it is not as
1345 secure as getpwuid().
1347 =item getpeername SOCKET
1349 Returns the packed sockaddr address of other end of the SOCKET connection.
1352 $hersockaddr = getpeername(SOCK);
1353 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1354 $herhostname = gethostbyaddr($iaddr, AF_INET);
1355 $herstraddr = inet_ntoa($iaddr);
1359 Returns the current process group for the specified PID. Use
1360 a PID of 0 to get the current process group for the
1361 current process. Will raise an exception if used on a machine that
1362 doesn't implement getpgrp(2). If PID is omitted, returns process
1363 group of current process. Note that the POSIX version of getpgrp()
1364 does not accept a PID argument, so only PID==0 is truly portable.
1368 Returns the process id of the parent process.
1370 =item getpriority WHICH,WHO
1372 Returns the current priority for a process, a process group, or a user.
1373 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1374 machine that doesn't implement getpriority(2).
1380 =item gethostbyname NAME
1382 =item getnetbyname NAME
1384 =item getprotobyname NAME
1390 =item getservbyname NAME,PROTO
1392 =item gethostbyaddr ADDR,ADDRTYPE
1394 =item getnetbyaddr ADDR,ADDRTYPE
1396 =item getprotobynumber NUMBER
1398 =item getservbyport PORT,PROTO
1416 =item sethostent STAYOPEN
1418 =item setnetent STAYOPEN
1420 =item setprotoent STAYOPEN
1422 =item setservent STAYOPEN
1436 These routines perform the same functions as their counterparts in the
1437 system library. Within a list context, the return values from the
1438 various get routines are as follows:
1440 ($name,$passwd,$uid,$gid,
1441 $quota,$comment,$gcos,$dir,$shell) = getpw*
1442 ($name,$passwd,$gid,$members) = getgr*
1443 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1444 ($name,$aliases,$addrtype,$net) = getnet*
1445 ($name,$aliases,$proto) = getproto*
1446 ($name,$aliases,$port,$proto) = getserv*
1448 (If the entry doesn't exist you get a null list.)
1450 Within a scalar context, you get the name, unless the function was a
1451 lookup by name, in which case you get the other thing, whatever it is.
1452 (If the entry doesn't exist you get the undefined value.) For example:
1462 The $members value returned by I<getgr*()> is a space separated list of
1463 the login names of the members of the group.
1465 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1466 C, it will be returned to you via C<$?> if the function call fails. The
1467 @addrs value returned by a successful call is a list of the raw
1468 addresses returned by the corresponding system library call. In the
1469 Internet domain, each address is four bytes long and you can unpack it
1470 by saying something like:
1472 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1474 =item getsockname SOCKET
1476 Returns the packed sockaddr address of this end of the SOCKET connection.
1479 $mysockaddr = getsockname(SOCK);
1480 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1482 =item getsockopt SOCKET,LEVEL,OPTNAME
1484 Returns the socket option requested, or undefined if there is an error.
1490 Returns the value of EXPR with filename expansions such as a shell would
1491 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1492 operator, but you can use it directly. If EXPR is omitted, $_ is used.
1493 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1494 L<perlop/"I/O Operators">.
1498 Converts a time as returned by the time function to a 9-element array
1499 with the time localized for the standard Greenwich time zone.
1500 Typically used as follows:
1503 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1506 All array elements are numeric, and come straight out of a struct tm.
1507 In particular this means that $mon has the range 0..11 and $wday has
1508 the range 0..6 with sunday as day 0. Also, $year is the number of
1509 years since 1900, I<not> simply the last two digits of the year.
1511 If EXPR is omitted, does C<gmtime(time())>.
1513 In a scalar context, returns the ctime(3) value:
1515 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1517 Also see the timegm() function provided by the Time::Local module,
1518 and the strftime(3) function available via the POSIX module.
1526 The goto-LABEL form finds the statement labeled with LABEL and resumes
1527 execution there. It may not be used to go into any construct that
1528 requires initialization, such as a subroutine or a foreach loop. It
1529 also can't be used to go into a construct that is optimized away,
1530 or to get out of a block or subroutine given to sort().
1531 It can be used to go almost anywhere else within the dynamic scope,
1532 including out of subroutines, but it's usually better to use some other
1533 construct such as last or die. The author of Perl has never felt the
1534 need to use this form of goto (in Perl, that is--C is another matter).
1536 The goto-EXPR form expects a label name, whose scope will be resolved
1537 dynamically. This allows for computed gotos per FORTRAN, but isn't
1538 necessarily recommended if you're optimizing for maintainability:
1540 goto ("FOO", "BAR", "GLARCH")[$i];
1542 The goto-&NAME form is highly magical, and substitutes a call to the
1543 named subroutine for the currently running subroutine. This is used by
1544 AUTOLOAD subroutines that wish to load another subroutine and then
1545 pretend that the other subroutine had been called in the first place
1546 (except that any modifications to @_ in the current subroutine are
1547 propagated to the other subroutine.) After the goto, not even caller()
1548 will be able to tell that this routine was called first.
1550 =item grep BLOCK LIST
1552 =item grep EXPR,LIST
1554 This is similar in spirit to, but not the same as, grep(1)
1555 and its relatives. In particular, it is not limited to using
1556 regular expressions.
1558 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1559 $_ to each element) and returns the list value consisting of those
1560 elements for which the expression evaluated to TRUE. In a scalar
1561 context, returns the number of times the expression was TRUE.
1563 @foo = grep(!/^#/, @bar); # weed out comments
1567 @foo = grep {!/^#/} @bar; # weed out comments
1569 Note that, because $_ is a reference into the list value, it can be used
1570 to modify the elements of the array. While this is useful and
1571 supported, it can cause bizarre results if the LIST is not a named
1572 array. Similarly, grep returns aliases into the original list,
1573 much like the way that L<Foreach Loops>'s index variable aliases the list
1574 elements. That is, modifying an element of a list returned by grep
1575 (for example, in a C<foreach>, C<map> or another C<grep>)
1576 actually modifies the element in the original list.
1578 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1583 Interprets EXPR as a hex string and returns the corresponding
1584 value. (To convert strings that might start with either 0 or 0x
1585 see L</oct>.) If EXPR is omitted, uses $_.
1587 print hex '0xAf'; # prints '175'
1588 print hex 'aF'; # same
1592 There is no builtin import() function. It is merely an ordinary
1593 method (subroutine) defined (or inherited) by modules that wish to export
1594 names to another module. The use() function calls the import() method
1595 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1597 =item index STR,SUBSTR,POSITION
1599 =item index STR,SUBSTR
1601 Returns the position of the first occurrence of SUBSTR in STR at or after
1602 POSITION. If POSITION is omitted, starts searching from the beginning of
1603 the string. The return value is based at 0 (or whatever you've set the C<$[>
1604 variable to--but don't do that). If the substring is not found, returns
1605 one less than the base, ordinarily -1.
1611 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1613 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1615 Implements the ioctl(2) function. You'll probably have to say
1617 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1619 first to get the correct function definitions. If F<ioctl.ph> doesn't
1620 exist or doesn't have the correct definitions you'll have to roll your
1621 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1622 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1623 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1624 written depending on the FUNCTION--a pointer to the string value of SCALAR
1625 will be passed as the third argument of the actual ioctl call. (If SCALAR
1626 has no string value but does have a numeric value, that value will be
1627 passed rather than a pointer to the string value. To guarantee this to be
1628 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1629 functions are useful for manipulating the values of structures used by
1630 ioctl(). The following example sets the erase character to DEL.
1634 die "NO TIOCGETP" if $@ || !$getp;
1635 $sgttyb_t = "ccccs"; # 4 chars and a short
1636 if (ioctl(STDIN,$getp,$sgttyb)) {
1637 @ary = unpack($sgttyb_t,$sgttyb);
1639 $sgttyb = pack($sgttyb_t,@ary);
1640 ioctl(STDIN,&TIOCSETP,$sgttyb)
1641 || die "Can't ioctl: $!";
1644 The return value of ioctl (and fcntl) is as follows:
1646 if OS returns: then Perl returns:
1648 0 string "0 but true"
1649 anything else that number
1651 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1652 still easily determine the actual value returned by the operating
1655 ($retval = ioctl(...)) || ($retval = -1);
1656 printf "System returned %d\n", $retval;
1658 =item join EXPR,LIST
1660 Joins the separate strings of LIST into a single string with
1661 fields separated by the value of EXPR, and returns the string.
1664 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1666 See L<perlfunc/split>.
1670 Returns a normal array consisting of all the keys of the named hash. (In
1671 a scalar context, returns the number of keys.) The keys are returned in
1672 an apparently random order, but it is the same order as either the
1673 values() or each() function produces (given that the hash has not been
1674 modified). As a side effect, it resets HASH's iterator.
1676 Here is yet another way to print your environment:
1679 @values = values %ENV;
1680 while ($#keys >= 0) {
1681 print pop(@keys), '=', pop(@values), "\n";
1684 or how about sorted by key:
1686 foreach $key (sort(keys %ENV)) {
1687 print $key, '=', $ENV{$key}, "\n";
1690 To sort an array by value, you'll need to use a C<sort> function.
1691 Here's a descending numeric sort of a hash by its values:
1693 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1694 printf "%4d %s\n", $hash{$key}, $key;
1697 As an lvalue C<keys> allows you to increase the number of hash buckets
1698 allocated for the given hash. This can gain you a measure of efficiency if
1699 you know the hash is going to get big. (This is similar to pre-extending
1700 an array by assigning a larger number to $#array.) If you say
1704 then C<%hash> will have at least 200 buckets allocated for it. These
1705 buckets will be retained even if you do C<%hash = ()>, use C<undef
1706 %hash> if you want to free the storage while C<%hash> is still in scope.
1707 You can't shrink the number of buckets allocated for the hash using
1708 C<keys> in this way (but you needn't worry about doing this by accident,
1709 as trying has no effect).
1713 Sends a signal to a list of processes. The first element of
1714 the list must be the signal to send. Returns the number of
1715 processes successfully signaled.
1717 $cnt = kill 1, $child1, $child2;
1720 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1721 process groups instead of processes. (On System V, a negative I<PROCESS>
1722 number will also kill process groups, but that's not portable.) That
1723 means you usually want to use positive not negative signals. You may also
1724 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1730 The C<last> command is like the C<break> statement in C (as used in
1731 loops); it immediately exits the loop in question. If the LABEL is
1732 omitted, the command refers to the innermost enclosing loop. The
1733 C<continue> block, if any, is not executed:
1735 LINE: while (<STDIN>) {
1736 last LINE if /^$/; # exit when done with header
1744 Returns an lowercased version of EXPR. This is the internal function
1745 implementing the \L escape in double-quoted strings.
1746 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1748 If EXPR is omitted, uses $_.
1754 Returns the value of EXPR with the first character lowercased. This is
1755 the internal function implementing the \l escape in double-quoted strings.
1756 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1758 If EXPR is omitted, uses $_.
1764 Returns the length in characters of the value of EXPR. If EXPR is
1765 omitted, returns length of $_.
1767 =item link OLDFILE,NEWFILE
1769 Creates a new filename linked to the old filename. Returns 1 for
1770 success, 0 otherwise.
1772 =item listen SOCKET,QUEUESIZE
1774 Does the same thing that the listen system call does. Returns TRUE if
1775 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1779 A local modifies the listed variables to be local to the enclosing block,
1780 subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1781 list must be placed in parentheses. See L<perlsub/"Temporary Values via
1782 local()"> for details, including issues with tied arrays and hashes.
1784 But you really probably want to be using my() instead, because local() isn't
1785 what most people think of as "local"). See L<perlsub/"Private Variables
1786 via my()"> for details.
1788 =item localtime EXPR
1790 Converts a time as returned by the time function to a 9-element array
1791 with the time analyzed for the local time zone. Typically used as
1795 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1798 All array elements are numeric, and come straight out of a struct tm.
1799 In particular this means that $mon has the range 0..11 and $wday has
1800 the range 0..6 with sunday as day 0. Also, $year is the number of
1801 years since 1900, that is, $year is 123 in year 2023.
1803 If EXPR is omitted, uses the current time (C<localtime(time)>).
1805 In a scalar context, returns the ctime(3) value:
1807 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
1809 This scalar value is B<not> locale dependent, see L<perllocale>,
1810 but instead a Perl builtin.
1811 Also see the Time::Local module, and the strftime(3) and mktime(3)
1812 function available via the POSIX module.
1818 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1821 =item lstat FILEHANDLE
1827 Does the same thing as the stat() function, but stats a symbolic link
1828 instead of the file the symbolic link points to. If symbolic links are
1829 unimplemented on your system, a normal stat() is done.
1831 If EXPR is omitted, stats $_.
1835 The match operator. See L<perlop>.
1837 =item map BLOCK LIST
1841 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1842 element) and returns the list value composed of the results of each such
1843 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1844 may produce zero, one, or more elements in the returned value.
1846 @chars = map(chr, @nums);
1848 translates a list of numbers to the corresponding characters. And
1850 %hash = map { getkey($_) => $_ } @array;
1852 is just a funny way to write
1855 foreach $_ (@array) {
1856 $hash{getkey($_)} = $_;
1859 Note that, because $_ is a reference into the list value, it can be used
1860 to modify the elements of the array. While this is useful and
1861 supported, it can cause bizarre results if the LIST is not a named
1862 array. See also L</grep> for an array composed of those items of the
1863 original list for which the BLOCK or EXPR evaluates to true.
1865 =item mkdir FILENAME,MODE
1867 Creates the directory specified by FILENAME, with permissions specified
1868 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1869 it returns 0 and sets C<$!> (errno).
1871 =item msgctl ID,CMD,ARG
1873 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1874 must be a variable which will hold the returned msqid_ds structure.
1875 Returns like ioctl: the undefined value for error, "0 but true" for
1876 zero, or the actual return value otherwise.
1878 =item msgget KEY,FLAGS
1880 Calls the System V IPC function msgget(2). Returns the message queue id,
1881 or the undefined value if there is an error.
1883 =item msgsnd ID,MSG,FLAGS
1885 Calls the System V IPC function msgsnd to send the message MSG to the
1886 message queue ID. MSG must begin with the long integer message type,
1887 which may be created with C<pack("l", $type)>. Returns TRUE if
1888 successful, or FALSE if there is an error.
1890 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1892 Calls the System V IPC function msgrcv to receive a message from
1893 message queue ID into variable VAR with a maximum message size of
1894 SIZE. Note that if a message is received, the message type will be the
1895 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1896 of the message type. Returns TRUE if successful, or FALSE if there is
1901 A "my" declares the listed variables to be local (lexically) to the
1902 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1903 more than one value is listed, the list must be placed in parentheses. See
1904 L<perlsub/"Private Variables via my()"> for details.
1910 The C<next> command is like the C<continue> statement in C; it starts
1911 the next iteration of the loop:
1913 LINE: while (<STDIN>) {
1914 next LINE if /^#/; # discard comments
1918 Note that if there were a C<continue> block on the above, it would get
1919 executed even on discarded lines. If the LABEL is omitted, the command
1920 refers to the innermost enclosing loop.
1922 =item no Module LIST
1924 See the "use" function, which "no" is the opposite of.
1930 Interprets EXPR as an octal string and returns the corresponding
1931 value. (If EXPR happens to start off with 0x, interprets it as
1932 a hex string instead.) The following will handle decimal, octal, and
1933 hex in the standard Perl or C notation:
1935 $val = oct($val) if $val =~ /^0/;
1937 If EXPR is omitted, uses $_. This function is commonly used when
1938 a string such as "644" needs to be converted into a file mode, for
1939 example. (Although perl will automatically convert strings into
1940 numbers as needed, this automatic conversion assumes base 10.)
1942 =item open FILEHANDLE,EXPR
1944 =item open FILEHANDLE
1946 Opens the file whose filename is given by EXPR, and associates it with
1947 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1948 name of the real filehandle wanted. If EXPR is omitted, the scalar
1949 variable of the same name as the FILEHANDLE contains the filename.
1950 (Note that lexical variables--those declared with C<my>--will not work
1951 for this purpose; so if you're using C<my>, specify EXPR in your call
1954 If the filename begins with '<' or nothing, the file is opened for input.
1955 If the filename begins with '>', the file is truncated and opened for
1956 output. If the filename begins with '>>', the file is opened for
1957 appending. You can put a '+' in front of the '>' or '<' to indicate that
1958 you want both read and write access to the file; thus '+<' is almost
1959 always preferred for read/write updates--the '+>' mode would clobber the
1960 file first. The prefix and the filename may be separated with spaces.
1961 These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1962 'w+', 'a', and 'a+'.
1964 If the filename begins with "|", the filename is interpreted as a command
1965 to which output is to be piped, and if the filename ends with a "|", the
1966 filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1967 examples of this. as command which pipes input to us. (You may not have
1968 a raw open() to a command that pipes both in I<and> out, but see
1969 L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1972 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1973 nonzero upon success, the undefined value otherwise. If the open
1974 involved a pipe, the return value happens to be the pid of the
1977 If you're unfortunate enough to be running Perl on a system that
1978 distinguishes between text files and binary files (modern operating
1979 systems don't care), then you should check out L</binmode> for tips for
1980 dealing with this. The key distinction between systems that need binmode
1981 and those that don't is their text file formats. Systems like Unix and
1982 Plan9 that delimit lines with a single character, and that encode that
1983 character in C as '\n', do not need C<binmode>. The rest need it.
1985 When opening a file, it's usually a bad idea to continue normal execution
1986 if the request failed, so C<open> is frequently used in connection with
1987 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
1988 where you want to make a nicely formatted error message (but there are
1989 modules which can help with that problem)) you should always check
1990 the return value from opening a file. The infrequent exception is when
1991 working with an unopened filehandle is actually what you want to do.
1996 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1997 while (<ARTICLE>) {...
1999 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2000 # if the open fails, output is discarded
2002 open(DBASE, '+<dbase.mine') # open for update
2003 or die "Can't open 'dbase.mine' for update: $!";
2005 open(ARTICLE, "caesar <$article |") # decrypt article
2006 or die "Can't start caesar: $!";
2008 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2009 or die "Can't start sort: $!";
2011 # process argument list of files along with any includes
2013 foreach $file (@ARGV) {
2014 process($file, 'fh00');
2018 local($filename, $input) = @_;
2019 $input++; # this is a string increment
2020 unless (open($input, $filename)) {
2021 print STDERR "Can't open $filename: $!\n";
2025 while (<$input>) { # note use of indirection
2026 if (/^#include "(.*)"/) {
2027 process($1, $input);
2034 You may also, in the Bourne shell tradition, specify an EXPR beginning
2035 with "E<gt>&", in which case the rest of the string is interpreted as the
2036 name of a filehandle (or file descriptor, if numeric) which is to be
2037 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
2038 +E<gt>E<gt>, and +E<lt>. The
2039 mode you specify should match the mode of the original filehandle.
2040 (Duping a filehandle does not take into account any existing contents of
2042 Here is a script that saves, redirects, and restores STDOUT and
2046 open(SAVEOUT, ">&STDOUT");
2047 open(SAVEERR, ">&STDERR");
2049 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2050 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2052 select(STDERR); $| = 1; # make unbuffered
2053 select(STDOUT); $| = 1; # make unbuffered
2055 print STDOUT "stdout 1\n"; # this works for
2056 print STDERR "stderr 1\n"; # subprocesses too
2061 open(STDOUT, ">&SAVEOUT");
2062 open(STDERR, ">&SAVEERR");
2064 print STDOUT "stdout 2\n";
2065 print STDERR "stderr 2\n";
2068 If you specify "E<lt>&=N", where N is a number, then Perl will do an
2069 equivalent of C's fdopen() of that file descriptor; this is more
2070 parsimonious of file descriptors. For example:
2072 open(FILEHANDLE, "<&=$fd")
2074 If you open a pipe on the command "-", i.e., either "|-" or "-|", then
2075 there is an implicit fork done, and the return value of open is the pid
2076 of the child within the parent process, and 0 within the child
2077 process. (Use C<defined($pid)> to determine whether the open was successful.)
2078 The filehandle behaves normally for the parent, but i/o to that
2079 filehandle is piped from/to the STDOUT/STDIN of the child process.
2080 In the child process the filehandle isn't opened--i/o happens from/to
2081 the new STDOUT or STDIN. Typically this is used like the normal
2082 piped open when you want to exercise more control over just how the
2083 pipe command gets executed, such as when you are running setuid, and
2084 don't want to have to scan shell commands for metacharacters.
2085 The following pairs are more or less equivalent:
2087 open(FOO, "|tr '[a-z]' '[A-Z]'");
2088 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2090 open(FOO, "cat -n '$file'|");
2091 open(FOO, "-|") || exec 'cat', '-n', $file;
2093 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2095 NOTE: On any operation which may do a fork, unflushed buffers remain
2096 unflushed in both processes, which means you may need to set C<$|> to
2097 avoid duplicate output.
2099 Closing any piped filehandle causes the parent process to wait for the
2100 child to finish, and returns the status value in C<$?>.
2102 Using the constructor from the IO::Handle package (or one of its
2103 subclasses, such as IO::File or IO::Socket),
2104 you can generate anonymous filehandles which have the scope of whatever
2105 variables hold references to them, and automatically close whenever
2106 and however you leave that scope:
2110 sub read_myfile_munged {
2112 my $handle = new IO::File;
2113 open($handle, "myfile") or die "myfile: $!";
2115 or return (); # Automatically closed here.
2116 mung $first or die "mung failed"; # Or here.
2117 return $first, <$handle> if $ALL; # Or here.
2121 The filename that is passed to open will have leading and trailing
2122 whitespace deleted. To open a file with arbitrary weird
2123 characters in it, it's necessary to protect any leading and trailing
2126 $file =~ s#^(\s)#./$1#;
2127 open(FOO, "< $file\0");
2129 If you want a "real" C open() (see L<open(2)> on your system), then
2130 you should use the sysopen() function. This is another way to
2131 protect your filenames from interpretation. For example:
2134 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2135 or die "sysopen $path: $!";
2136 HANDLE->autoflush(1);
2137 HANDLE->print("stuff $$\n");
2139 print "File contains: ", <HANDLE>;
2141 See L</seek()> for some details about mixing reading and writing.
2143 =item opendir DIRHANDLE,EXPR
2145 Opens a directory named EXPR for processing by readdir(), telldir(),
2146 seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
2147 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2153 Returns the numeric ascii value of the first character of EXPR. If
2154 EXPR is omitted, uses $_. For the reverse, see L</chr>.
2156 =item pack TEMPLATE,LIST
2158 Takes an array or list of values and packs it into a binary structure,
2159 returning the string containing the structure. The TEMPLATE is a
2160 sequence of characters that give the order and type of values, as
2163 A An ascii string, will be space padded.
2164 a An ascii string, will be null padded.
2165 b A bit string (ascending bit order, like vec()).
2166 B A bit string (descending bit order).
2167 h A hex string (low nybble first).
2168 H A hex string (high nybble first).
2170 c A signed char value.
2171 C An unsigned char value.
2173 s A signed short value.
2174 S An unsigned short value.
2175 (This 'short' is _exactly_ 16 bits, which may differ from
2176 what a local C compiler calls 'short'.)
2178 i A signed integer value.
2179 I An unsigned integer value.
2180 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2181 depends on what a local C compiler calls 'int', and may
2182 even be larger than the 'long' described in the next item.)
2184 l A signed long value.
2185 L An unsigned long value.
2186 (This 'long' is _exactly_ 32 bits, which may differ from
2187 what a local C compiler calls 'long'.)
2189 n A short in "network" (big-endian) order.
2190 N A long in "network" (big-endian) order.
2191 v A short in "VAX" (little-endian) order.
2192 V A long in "VAX" (little-endian) order.
2193 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2194 _exactly_ 32 bits, respectively.)
2196 f A single-precision float in the native format.
2197 d A double-precision float in the native format.
2199 p A pointer to a null-terminated string.
2200 P A pointer to a structure (fixed-length string).
2202 u A uuencoded string.
2204 w A BER compressed integer. Its bytes represent an unsigned
2205 integer in base 128, most significant digit first, with as few
2206 digits as possible. Bit eight (the high bit) is set on each
2207 byte except the last.
2211 @ Null fill to absolute position.
2213 Each letter may optionally be followed by a number which gives a repeat
2214 count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
2215 pack function will gobble up that many values from the LIST. A * for the
2216 repeat count means to use however many items are left. The "a" and "A"
2217 types gobble just one value, but pack it as a string of length count,
2218 padding with nulls or spaces as necessary. (When unpacking, "A" strips
2219 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2220 fields pack a string that many bits long. The "h" and "H" fields pack a
2221 string that many nybbles long. The "p" type packs a pointer to a null-
2222 terminated string. You are responsible for ensuring the string is not a
2223 temporary value (which can potentially get deallocated before you get
2224 around to using the packed result). The "P" packs a pointer to a structure
2225 of the size indicated by the length. A NULL pointer is created if the
2226 corresponding value for "p" or "P" is C<undef>.
2227 Real numbers (floats and doubles) are
2228 in the native machine format only; due to the multiplicity of floating
2229 formats around, and the lack of a standard "network" representation, no
2230 facility for interchange has been made. This means that packed floating
2231 point data written on one machine may not be readable on another - even if
2232 both use IEEE floating point arithmetic (as the endian-ness of the memory
2233 representation is not part of the IEEE spec). Note that Perl uses doubles
2234 internally for all numeric calculation, and converting from double into
2235 float and thence back to double again will lose precision (i.e.,
2236 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2240 $foo = pack("cccc",65,66,67,68);
2242 $foo = pack("c4",65,66,67,68);
2245 $foo = pack("ccxxcc",65,66,67,68);
2248 $foo = pack("s2",1,2);
2249 # "\1\0\2\0" on little-endian
2250 # "\0\1\0\2" on big-endian
2252 $foo = pack("a4","abcd","x","y","z");
2255 $foo = pack("aaaa","abcd","x","y","z");
2258 $foo = pack("a14","abcdefg");
2259 # "abcdefg\0\0\0\0\0\0\0"
2261 $foo = pack("i9pl", gmtime);
2262 # a real struct tm (on my system anyway)
2265 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2268 The same template may generally also be used in the unpack function.
2270 =item package NAMESPACE
2272 Declares the compilation unit as being in the given namespace. The scope
2273 of the package declaration is from the declaration itself through the end of
2274 the enclosing block (the same scope as the local() operator). All further
2275 unqualified dynamic identifiers will be in this namespace. A package
2276 statement affects only dynamic variables--including those you've used
2277 local() on--but I<not> lexical variables created with my(). Typically it
2278 would be the first declaration in a file to be included by the C<require>
2279 or C<use> operator. You can switch into a package in more than one place;
2280 it influences merely which symbol table is used by the compiler for the
2281 rest of that block. You can refer to variables and filehandles in other
2282 packages by prefixing the identifier with the package name and a double
2283 colon: C<$Package::Variable>. If the package name is null, the C<main>
2284 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2286 See L<perlmod/"Packages"> for more information about packages, modules,
2287 and classes. See L<perlsub> for other scoping issues.
2289 =item pipe READHANDLE,WRITEHANDLE
2291 Opens a pair of connected pipes like the corresponding system call.
2292 Note that if you set up a loop of piped processes, deadlock can occur
2293 unless you are very careful. In addition, note that Perl's pipes use
2294 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2295 after each command, depending on the application.
2297 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2298 for examples of such things.
2304 Pops and returns the last value of the array, shortening the array by
2305 1. Has a similar effect to
2307 $tmp = $ARRAY[$#ARRAY--];
2309 If there are no elements in the array, returns the undefined value.
2310 If ARRAY is omitted, pops the
2311 @ARGV array in the main program, and the @_ array in subroutines, just
2318 Returns the offset of where the last C<m//g> search left off for the variable
2319 is in question ($_ is used when the variable is not specified). May be
2320 modified to change that offset. Such modification will also influence
2321 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2324 =item print FILEHANDLE LIST
2330 Prints a string or a comma-separated list of strings. Returns TRUE
2331 if successful. FILEHANDLE may be a scalar variable name, in which case
2332 the variable contains the name of or a reference to the filehandle, thus introducing one
2333 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2334 token is a term, it may be misinterpreted as an operator unless you
2335 interpose a + or put parentheses around the arguments.) If FILEHANDLE is
2336 omitted, prints by default to standard output (or to the last selected
2337 output channel--see L</select>). If LIST is also omitted, prints $_ to
2338 STDOUT. To set the default output channel to something other than
2339 STDOUT use the select operation. Note that, because print takes a
2340 LIST, anything in the LIST is evaluated in a list context, and any
2341 subroutine that you call will have one or more of its expressions
2342 evaluated in a list context. Also be careful not to follow the print
2343 keyword with a left parenthesis unless you want the corresponding right
2344 parenthesis to terminate the arguments to the print--interpose a + or
2345 put parentheses around all the arguments.
2347 Note that if you're storing FILEHANDLES in an array or other expression,
2348 you will have to use a block returning its value instead:
2350 print { $files[$i] } "stuff\n";
2351 print { $OK ? STDOUT : STDERR } "stuff\n";
2353 =item printf FILEHANDLE FORMAT, LIST
2355 =item printf FORMAT, LIST
2357 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2358 of the list will be interpreted as the printf format. If C<use locale> is
2359 in effect, the character used for the decimal point in formatted real numbers
2360 is affected by the LC_NUMERIC locale. See L<perllocale>.
2362 Don't fall into the trap of using a printf() when a simple
2363 print() would do. The print() is more efficient, and less
2366 =item prototype FUNCTION
2368 Returns the prototype of a function as a string (or C<undef> if the
2369 function has no prototype). FUNCTION is a reference to, or the name of,
2370 the function whose prototype you want to retrieve.
2372 =item push ARRAY,LIST
2374 Treats ARRAY as a stack, and pushes the values of LIST
2375 onto the end of ARRAY. The length of ARRAY increases by the length of
2376 LIST. Has the same effect as
2379 $ARRAY[++$#ARRAY] = $value;
2382 but is more efficient. Returns the new number of elements in the array.
2392 Generalized quotes. See L<perlop>.
2394 =item quotemeta EXPR
2398 Returns the value of EXPR with all non-alphanumeric
2399 characters backslashed. (That is, all characters not matching
2400 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2401 returned string, regardless of any locale settings.)
2402 This is the internal function implementing
2403 the \Q escape in double-quoted strings.
2405 If EXPR is omitted, uses $_.
2411 Returns a random fractional number greater than or equal to 0 and less
2412 than the value of EXPR. (EXPR should be positive.) If EXPR is
2413 omitted, the value 1 is used. Automatically calls srand() unless
2414 srand() has already been called. See also srand().
2416 (Note: If your rand function consistently returns numbers that are too
2417 large or too small, then your version of Perl was probably compiled
2418 with the wrong number of RANDBITS.)
2420 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2422 =item read FILEHANDLE,SCALAR,LENGTH
2424 Attempts to read LENGTH bytes of data into variable SCALAR from the
2425 specified FILEHANDLE. Returns the number of bytes actually read, or
2426 undef if there was an error. SCALAR will be grown or shrunk to the
2427 length actually read. An OFFSET may be specified to place the read
2428 data at some other place than the beginning of the string. This call
2429 is actually implemented in terms of stdio's fread call. To get a true
2430 read system call, see sysread().
2432 =item readdir DIRHANDLE
2434 Returns the next directory entry for a directory opened by opendir().
2435 If used in a list context, returns all the rest of the entries in the
2436 directory. If there are no more entries, returns an undefined value in
2437 a scalar context or a null list in a list context.
2439 If you're planning to filetest the return values out of a readdir(), you'd
2440 better prepend the directory in question. Otherwise, because we didn't
2441 chdir() there, it would have been testing the wrong file.
2443 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2444 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2449 Reads from the file handle EXPR. In scalar context, a single line
2450 is read and returned. In list context, reads until end-of-file is
2451 reached and returns a list of lines (however you've defined lines
2452 with $/ or $INPUT_RECORD_SEPARATOR).
2453 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2454 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2455 operator is discussed in more detail in L<perlop/"I/O Operators">.
2461 Returns the value of a symbolic link, if symbolic links are
2462 implemented. If not, gives a fatal error. If there is some system
2463 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2468 EXPR is interpolated and then executed as a system command.
2469 The collected standard output of the command is returned.
2470 In scalar context, it comes back as a single (potentially
2471 multi-line) string. In list context, returns a list of lines
2472 (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).
2473 This is the internal function implementing the C<qx/EXPR/>
2474 operator, but you can use it directly. The C<qx/EXPR/>
2475 operator is discussed in more detail in L<perlop/"I/O Operators">.
2477 =item recv SOCKET,SCALAR,LEN,FLAGS
2479 Receives a message on a socket. Attempts to receive LENGTH bytes of
2480 data into variable SCALAR from the specified SOCKET filehandle.
2481 Actually does a C recvfrom(), so that it can returns the address of the
2482 sender. Returns the undefined value if there's an error. SCALAR will
2483 be grown or shrunk to the length actually read. Takes the same flags
2484 as the system call of the same name.
2485 See L<perlipc/"UDP: Message Passing"> for examples.
2491 The C<redo> command restarts the loop block without evaluating the
2492 conditional again. The C<continue> block, if any, is not executed. If
2493 the LABEL is omitted, the command refers to the innermost enclosing
2494 loop. This command is normally used by programs that want to lie to
2495 themselves about what was just input:
2497 # a simpleminded Pascal comment stripper
2498 # (warning: assumes no { or } in strings)
2499 LINE: while (<STDIN>) {
2500 while (s|({.*}.*){.*}|$1 |) {}
2505 if (/}/) { # end of comment?
2518 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2519 is not specified, $_ will be used. The value returned depends on the
2520 type of thing the reference is a reference to.
2521 Builtin types include:
2530 If the referenced object has been blessed into a package, then that package
2531 name is returned instead. You can think of ref() as a typeof() operator.
2533 if (ref($r) eq "HASH") {
2534 print "r is a reference to a hash.\n";
2537 print "r is not a reference at all.\n";
2540 See also L<perlref>.
2542 =item rename OLDNAME,NEWNAME
2544 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2545 not work across file system boundaries.
2551 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2552 supplied. If EXPR is numeric, demands that the current version of Perl
2553 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2555 Otherwise, demands that a library file be included if it hasn't already
2556 been included. The file is included via the do-FILE mechanism, which is
2557 essentially just a variety of eval(). Has semantics similar to the following
2561 local($filename) = @_;
2562 return 1 if $INC{$filename};
2563 local($realfilename,$result);
2565 foreach $prefix (@INC) {
2566 $realfilename = "$prefix/$filename";
2567 if (-f $realfilename) {
2568 $result = do $realfilename;
2572 die "Can't find $filename in \@INC";
2575 die "$filename did not return true value" unless $result;
2576 $INC{$filename} = $realfilename;
2580 Note that the file will not be included twice under the same specified
2581 name. The file must return TRUE as the last statement to indicate
2582 successful execution of any initialization code, so it's customary to
2583 end such a file with "1;" unless you're sure it'll return TRUE
2584 otherwise. But it's better just to put the "C<1;>", in case you add more
2587 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
2588 replaces "F<::>" with "F</>" in the filename for you,
2589 to make it easy to load standard modules. This form of loading of
2590 modules does not risk altering your namespace.
2592 For a yet-more-powerful import facility, see L</use> and
2599 Generally used in a C<continue> block at the end of a loop to clear
2600 variables and reset ?? searches so that they work again. The
2601 expression is interpreted as a list of single characters (hyphens
2602 allowed for ranges). All variables and arrays beginning with one of
2603 those letters are reset to their pristine state. If the expression is
2604 omitted, one-match searches (?pattern?) are reset to match again. Resets
2605 only variables or searches in the current package. Always returns
2608 reset 'X'; # reset all X variables
2609 reset 'a-z'; # reset lower case variables
2610 reset; # just reset ?? searches
2612 Resetting "A-Z" is not recommended because you'll wipe out your
2613 ARGV and ENV arrays. Resets only package variables--lexical variables
2614 are unaffected, but they clean themselves up on scope exit anyway,
2615 so you'll probably want to use them instead. See L</my>.
2621 Returns from a subroutine, eval(), or do FILE with the value of the
2622 given EXPR. Evaluation of EXPR may be in a list, scalar, or void
2623 context, depending on how the return value will be used, and the context
2624 may vary from one execution to the next (see wantarray()). If no EXPR
2625 is given, returns an empty list in a list context, an undefined value in
2626 a scalar context, or nothing in a void context.
2628 (Note that in the absence of a return, a subroutine, eval, or do FILE
2629 will automatically return the value of the last expression evaluated.)
2633 In a list context, returns a list value consisting of the elements
2634 of LIST in the opposite order. In a scalar context, concatenates the
2635 elements of LIST, and returns a string value consisting of those bytes,
2636 but in the opposite order.
2638 print reverse <>; # line tac, last line first
2640 undef $/; # for efficiency of <>
2641 print scalar reverse <>; # byte tac, last line tsrif
2643 This operator is also handy for inverting a hash, although there are some
2644 caveats. If a value is duplicated in the original hash, only one of those
2645 can be represented as a key in the inverted hash. Also, this has to
2646 unwind one hash and build a whole new one, which may take some time
2649 %by_name = reverse %by_address; # Invert the hash
2651 =item rewinddir DIRHANDLE
2653 Sets the current position to the beginning of the directory for the
2654 readdir() routine on DIRHANDLE.
2656 =item rindex STR,SUBSTR,POSITION
2658 =item rindex STR,SUBSTR
2660 Works just like index except that it returns the position of the LAST
2661 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2662 last occurrence at or before that position.
2664 =item rmdir FILENAME
2668 Deletes the directory specified by FILENAME if it is empty. If it
2669 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2670 FILENAME is omitted, uses $_.
2674 The substitution operator. See L<perlop>.
2678 Forces EXPR to be interpreted in a scalar context and returns the value
2681 @counts = ( scalar @a, scalar @b, scalar @c );
2683 There is no equivalent operator to force an expression to
2684 be interpolated in a list context because it's in practice never
2685 needed. If you really wanted to do so, however, you could use
2686 the construction C<@{[ (some expression) ]}>, but usually a simple
2687 C<(some expression)> suffices.
2689 =item seek FILEHANDLE,POSITION,WHENCE
2691 Sets FILEHANDLE's position, just like the fseek() call of stdio.
2692 FILEHANDLE may be an expression whose value gives the name of the
2693 filehandle. The values for WHENCE are 0 to set the new position to
2694 POSITION, 1 to set it to the current position plus POSITION, and 2 to
2695 set it to EOF plus POSITION (typically negative). For WHENCE you may
2696 use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2697 IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2699 If you want to position file for sysread() or syswrite(), don't use
2700 seek() -- buffering makes its effect on the file's system position
2701 unpredictable and non-portable. Use sysseek() instead.
2703 On some systems you have to do a seek whenever you switch between reading
2704 and writing. Amongst other things, this may have the effect of calling
2705 stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2710 This is also useful for applications emulating C<tail -f>. Once you hit
2711 EOF on your read, and then sleep for a while, you might have to stick in a
2712 seek() to reset things. The seek() doesn't change the current position,
2713 but it I<does> clear the end-of-file condition on the handle, so that the
2714 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2716 If that doesn't work (some stdios are particularly cantankerous), then
2717 you may need something more like this:
2720 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2721 # search for some stuff and put it into files
2723 sleep($for_a_while);
2724 seek(FILE, $curpos, 0);
2727 =item seekdir DIRHANDLE,POS
2729 Sets the current position for the readdir() routine on DIRHANDLE. POS
2730 must be a value returned by telldir(). Has the same caveats about
2731 possible directory compaction as the corresponding system library
2734 =item select FILEHANDLE
2738 Returns the currently selected filehandle. Sets the current default
2739 filehandle for output, if FILEHANDLE is supplied. This has two
2740 effects: first, a C<write> or a C<print> without a filehandle will
2741 default to this FILEHANDLE. Second, references to variables related to
2742 output will refer to this output channel. For example, if you have to
2743 set the top of form format for more than one output channel, you might
2751 FILEHANDLE may be an expression whose value gives the name of the
2752 actual filehandle. Thus:
2754 $oldfh = select(STDERR); $| = 1; select($oldfh);
2756 Some programmers may prefer to think of filehandles as objects with
2757 methods, preferring to write the last example as:
2760 STDERR->autoflush(1);
2762 =item select RBITS,WBITS,EBITS,TIMEOUT
2764 This calls the select(2) system call with the bit masks specified, which
2765 can be constructed using fileno() and vec(), along these lines:
2767 $rin = $win = $ein = '';
2768 vec($rin,fileno(STDIN),1) = 1;
2769 vec($win,fileno(STDOUT),1) = 1;
2772 If you want to select on many filehandles you might wish to write a
2776 local(@fhlist) = split(' ',$_[0]);
2779 vec($bits,fileno($_),1) = 1;
2783 $rin = fhbits('STDIN TTY SOCK');
2787 ($nfound,$timeleft) =
2788 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2790 or to block until something becomes ready just do this
2792 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2794 Most systems do not bother to return anything useful in $timeleft, so
2795 calling select() in a scalar context just returns $nfound.
2797 Any of the bit masks can also be undef. The timeout, if specified, is
2798 in seconds, which may be fractional. Note: not all implementations are
2799 capable of returning the $timeleft. If not, they always return
2800 $timeleft equal to the supplied $timeout.
2802 You can effect a sleep of 250 milliseconds this way:
2804 select(undef, undef, undef, 0.25);
2806 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2807 with select(). You have to use sysread() instead.
2809 =item semctl ID,SEMNUM,CMD,ARG
2811 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2812 &GETALL, then ARG must be a variable which will hold the returned
2813 semid_ds structure or semaphore value array. Returns like ioctl: the
2814 undefined value for error, "0 but true" for zero, or the actual return
2817 =item semget KEY,NSEMS,FLAGS
2819 Calls the System V IPC function semget. Returns the semaphore id, or
2820 the undefined value if there is an error.
2822 =item semop KEY,OPSTRING
2824 Calls the System V IPC function semop to perform semaphore operations
2825 such as signaling and waiting. OPSTRING must be a packed array of
2826 semop structures. Each semop structure can be generated with
2827 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2828 operations is implied by the length of OPSTRING. Returns TRUE if
2829 successful, or FALSE if there is an error. As an example, the
2830 following code waits on semaphore $semnum of semaphore id $semid:
2832 $semop = pack("sss", $semnum, -1, 0);
2833 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2835 To signal the semaphore, replace "-1" with "1".
2837 =item send SOCKET,MSG,FLAGS,TO
2839 =item send SOCKET,MSG,FLAGS
2841 Sends a message on a socket. Takes the same flags as the system call
2842 of the same name. On unconnected sockets you must specify a
2843 destination to send TO, in which case it does a C sendto(). Returns
2844 the number of characters sent, or the undefined value if there is an
2846 See L<perlipc/"UDP: Message Passing"> for examples.
2848 =item setpgrp PID,PGRP
2850 Sets the current process group for the specified PID, 0 for the current
2851 process. Will produce a fatal error if used on a machine that doesn't
2852 implement setpgrp(2). If the arguments are omitted, it defaults to
2853 0,0. Note that the POSIX version of setpgrp() does not accept any
2854 arguments, so only setpgrp 0,0 is portable.
2856 =item setpriority WHICH,WHO,PRIORITY
2858 Sets the current priority for a process, a process group, or a user.
2859 (See setpriority(2).) Will produce a fatal error if used on a machine
2860 that doesn't implement setpriority(2).
2862 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2864 Sets the socket option requested. Returns undefined if there is an
2865 error. OPTVAL may be specified as undef if you don't want to pass an
2872 Shifts the first value of the array off and returns it, shortening the
2873 array by 1 and moving everything down. If there are no elements in the
2874 array, returns the undefined value. If ARRAY is omitted, shifts the
2875 @_ array within the lexical scope of subroutines and formats, and the
2876 @ARGV array at file scopes or within the lexical scopes established by
2877 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
2878 See also unshift(), push(), and pop(). Shift() and unshift() do the
2879 same thing to the left end of an array that pop() and push() do to the
2882 =item shmctl ID,CMD,ARG
2884 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2885 must be a variable which will hold the returned shmid_ds structure.
2886 Returns like ioctl: the undefined value for error, "0 but true" for
2887 zero, or the actual return value otherwise.
2889 =item shmget KEY,SIZE,FLAGS
2891 Calls the System V IPC function shmget. Returns the shared memory
2892 segment id, or the undefined value if there is an error.
2894 =item shmread ID,VAR,POS,SIZE
2896 =item shmwrite ID,STRING,POS,SIZE
2898 Reads or writes the System V shared memory segment ID starting at
2899 position POS for size SIZE by attaching to it, copying in/out, and
2900 detaching from it. When reading, VAR must be a variable which will
2901 hold the data read. When writing, if STRING is too long, only SIZE
2902 bytes are used; if STRING is too short, nulls are written to fill out
2903 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2905 =item shutdown SOCKET,HOW
2907 Shuts down a socket connection in the manner indicated by HOW, which
2908 has the same interpretation as in the system call of the same name.
2914 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2917 For the inverse sine operation, you may use the POSIX::asin()
2918 function, or use this relation:
2920 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2926 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2927 May be interrupted by sending the process a SIGALRM. Returns the
2928 number of seconds actually slept. You probably cannot mix alarm() and
2929 sleep() calls, because sleep() is often implemented using alarm().
2931 On some older systems, it may sleep up to a full second less than what
2932 you requested, depending on how it counts seconds. Most modern systems
2933 always sleep the full amount.
2935 For delays of finer granularity than one second, you may use Perl's
2936 syscall() interface to access setitimer(2) if your system supports it,
2937 or else see L</select()> below.
2939 See also the POSIX module's sigpause() function.
2941 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2943 Opens a socket of the specified kind and attaches it to filehandle
2944 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
2945 system call of the same name. You should "use Socket;" first to get
2946 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2948 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2950 Creates an unnamed pair of sockets in the specified domain, of the
2951 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
2952 for the system call of the same name. If unimplemented, yields a fatal
2953 error. Returns TRUE if successful.
2955 =item sort SUBNAME LIST
2957 =item sort BLOCK LIST
2961 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
2962 is omitted, sorts in standard string comparison order. If SUBNAME is
2963 specified, it gives the name of a subroutine that returns an integer
2964 less than, equal to, or greater than 0, depending on how the elements
2965 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
2966 operators are extremely useful in such routines.) SUBNAME may be a
2967 scalar variable name, in which case the value provides the name of the
2968 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
2969 an anonymous, in-line sort subroutine.
2971 In the interests of efficiency the normal calling code for subroutines is
2972 bypassed, with the following effects: the subroutine may not be a
2973 recursive subroutine, and the two elements to be compared are passed into
2974 the subroutine not via @_ but as the package global variables $a and
2975 $b (see example below). They are passed by reference, so don't
2976 modify $a and $b. And don't try to declare them as lexicals either.
2978 You also cannot exit out of the sort block or subroutine using any of the
2979 loop control operators described in L<perlsyn> or with goto().
2981 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2982 current collation locale. See L<perllocale>.
2987 @articles = sort @files;
2989 # same thing, but with explicit sort routine
2990 @articles = sort {$a cmp $b} @files;
2992 # now case-insensitively
2993 @articles = sort {uc($a) cmp uc($b)} @files;
2995 # same thing in reversed order
2996 @articles = sort {$b cmp $a} @files;
2998 # sort numerically ascending
2999 @articles = sort {$a <=> $b} @files;
3001 # sort numerically descending
3002 @articles = sort {$b <=> $a} @files;
3004 # sort using explicit subroutine name
3006 $age{$a} <=> $age{$b}; # presuming numeric
3008 @sortedclass = sort byage @class;
3010 # this sorts the %age hash by value instead of key
3011 # using an in-line function
3012 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3014 sub backwards { $b cmp $a; }
3015 @harry = ('dog','cat','x','Cain','Abel');
3016 @george = ('gone','chased','yz','Punished','Axed');
3018 # prints AbelCaincatdogx
3019 print sort backwards @harry;
3020 # prints xdogcatCainAbel
3021 print sort @george, 'to', @harry;
3022 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3024 # inefficiently sort by descending numeric compare using
3025 # the first integer after the first = sign, or the
3026 # whole record case-insensitively otherwise
3029 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3034 # same thing, but much more efficiently;
3035 # we'll build auxiliary indices instead
3039 push @nums, /=(\d+)/;
3044 $nums[$b] <=> $nums[$a]
3046 $caps[$a] cmp $caps[$b]
3050 # same thing using a Schwartzian Transform (no temps)
3051 @new = map { $_->[0] }
3052 sort { $b->[1] <=> $a->[1]
3055 } map { [$_, /=(\d+)/, uc($_)] } @old;
3057 If you're using strict, you I<MUST NOT> declare $a
3058 and $b as lexicals. They are package globals. That means
3059 if you're in the C<main> package, it's
3061 @articles = sort {$main::b <=> $main::a} @files;
3065 @articles = sort {$::b <=> $::a} @files;
3067 but if you're in the C<FooPack> package, it's
3069 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3071 The comparison function is required to behave. If it returns
3072 inconsistent results (sometimes saying $x[1] is less than $x[2] and
3073 sometimes saying the opposite, for example) the Perl interpreter will
3074 probably crash and dump core. This is entirely due to and dependent
3075 upon your system's qsort(3) library routine; this routine often avoids
3076 sanity checks in the interest of speed.
3078 =item splice ARRAY,OFFSET,LENGTH,LIST
3080 =item splice ARRAY,OFFSET,LENGTH
3082 =item splice ARRAY,OFFSET
3084 Removes the elements designated by OFFSET and LENGTH from an array, and
3085 replaces them with the elements of LIST, if any. Returns the elements
3086 removed from the array. The array grows or shrinks as necessary. If
3087 LENGTH is omitted, removes everything from OFFSET onward. The
3088 following equivalences hold (assuming C<$[ == 0>):
3090 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
3091 pop(@a) splice(@a,-1)
3092 shift(@a) splice(@a,0,1)
3093 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3094 $a[$x] = $y splice(@a,$x,1,$y);
3096 Example, assuming array lengths are passed before arrays:
3098 sub aeq { # compare two list values
3099 local(@a) = splice(@_,0,shift);
3100 local(@b) = splice(@_,0,shift);
3101 return 0 unless @a == @b; # same len?
3103 return 0 if pop(@a) ne pop(@b);
3107 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3109 =item split /PATTERN/,EXPR,LIMIT
3111 =item split /PATTERN/,EXPR
3113 =item split /PATTERN/
3117 Splits a string into an array of strings, and returns it.
3119 If not in a list context, returns the number of fields found and splits into
3120 the @_ array. (In a list context, you can force the split into @_ by
3121 using C<??> as the pattern delimiters, but it still returns the array
3122 value.) The use of implicit split to @_ is deprecated, however.
3124 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
3125 splits on whitespace (after skipping any leading whitespace). Anything
3126 matching PATTERN is taken to be a delimiter separating the fields. (Note
3127 that the delimiter may be longer than one character.)
3129 If LIMIT is specified and is not negative, splits into no more than
3130 that many fields (though it may split into fewer). If LIMIT is
3131 unspecified, trailing null fields are stripped (which potential users
3132 of pop() would do well to remember). If LIMIT is negative, it is
3133 treated as if an arbitrarily large LIMIT had been specified.
3135 A pattern matching the null string (not to be confused with
3136 a null pattern C<//>, which is just one member of the set of patterns
3137 matching a null string) will split the value of EXPR into separate
3138 characters at each point it matches that way. For example:
3140 print join(':', split(/ */, 'hi there'));
3142 produces the output 'h:i:t:h:e:r:e'.
3144 The LIMIT parameter can be used to split a line partially
3146 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3148 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3149 one larger than the number of variables in the list, to avoid
3150 unnecessary work. For the list above LIMIT would have been 4 by
3151 default. In time critical applications it behooves you not to split
3152 into more fields than you really need.
3154 If the PATTERN contains parentheses, additional array elements are
3155 created from each matching substring in the delimiter.
3157 split(/([,-])/, "1-10,20", 3);
3159 produces the list value
3161 (1, '-', 10, ',', 20)
3163 If you had the entire header of a normal Unix email message in $header,
3164 you could split it up into fields and their values this way:
3166 $header =~ s/\n\s+/ /g; # fix continuation lines
3167 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3169 The pattern C</PATTERN/> may be replaced with an expression to specify
3170 patterns that vary at runtime. (To do runtime compilation only once,
3171 use C</$variable/o>.)
3173 As a special case, specifying a PATTERN of space (C<' '>) will split on
3174 white space just as split with no arguments does. Thus, split(' ') can
3175 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3176 will give you as many null initial fields as there are leading spaces.
3177 A split on /\s+/ is like a split(' ') except that any leading
3178 whitespace produces a null first field. A split with no arguments
3179 really does a C<split(' ', $_)> internally.
3183 open(passwd, '/etc/passwd');
3185 ($login, $passwd, $uid, $gid, $gcos,
3186 $home, $shell) = split(/:/);
3190 (Note that $shell above will still have a newline on it. See L</chop>,
3191 L</chomp>, and L</join>.)
3193 =item sprintf FORMAT, LIST
3195 Returns a string formatted by the usual printf conventions of the
3196 C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3197 on your system for an explanation of the general principles.
3199 Perl does all of its own sprintf() formatting -- it emulates the C
3200 function sprintf(), but it doesn't use it (except for floating-point
3201 numbers, and even then only the standard modifiers are allowed). As a
3202 result, any non-standard extensions in your local sprintf() are not
3203 available from Perl.
3205 Perl's sprintf() permits the following universally-known conversions:
3208 %c a character with the given number
3210 %d a signed integer, in decimal
3211 %u an unsigned integer, in decimal
3212 %o an unsigned integer, in octal
3213 %x an unsigned integer, in hexadecimal
3214 %e a floating-point number, in scientific notation
3215 %f a floating-point number, in fixed decimal notation
3216 %g a floating-point number, in %e or %f notation
3218 In addition, Perl permits the following widely-supported conversions:
3220 %X like %x, but using upper-case letters
3221 %E like %e, but using an upper-case "E"
3222 %G like %g, but with an upper-case "E" (if applicable)
3223 %p a pointer (outputs the Perl value's address in hexadecimal)
3224 %n special: *stores* the number of characters output so far
3225 into the next variable in the parameter list
3227 Finally, for backward (and we do mean "backward") compatibility, Perl
3228 permits these unnecessary but widely-supported conversions:
3231 %D a synonym for %ld
3232 %U a synonym for %lu
3233 %O a synonym for %lo
3236 Perl permits the following universally-known flags between the C<%>
3237 and the conversion letter:
3239 space prefix positive number with a space
3240 + prefix positive number with a plus sign
3241 - left-justify within the field
3242 0 use zeros, not spaces, to right-justify
3243 # prefix octal with "0", hex with "0x"
3244 number minimum field width
3245 .number "precision": digits after decimal point for floating-point,
3246 max length for string, minimum length for integer
3247 l interpret integer as C type "long" or "unsigned long"
3248 h interpret integer as C type "short" or "unsigned short"
3250 There is also one Perl-specific flag:
3252 V interpret integer as Perl's standard integer type
3254 Where a number would appear in the flags, an asterisk ("*") may be
3255 used instead, in which case Perl uses the next item in the parameter
3256 list as the given number (that is, as the field width or precision).
3257 If a field width obtained through "*" is negative, it has the same
3258 effect as the '-' flag: left-justification.
3260 If C<use locale> is in effect, the character used for the decimal
3261 point in formatted real numbers is affected by the LC_NUMERIC locale.
3268 Return the square root of EXPR. If EXPR is omitted, returns square
3275 Sets the random number seed for the C<rand> operator. If EXPR is
3276 omitted, uses a semi-random value based on the current time and process
3277 ID, among other things. In versions of Perl prior to 5.004 the default
3278 seed was just the current time(). This isn't a particularly good seed,
3279 so many old programs supply their own seed value (often C<time ^ $$> or
3280 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
3282 In fact, it's usually not necessary to call srand() at all, because if
3283 it is not called explicitly, it is called implicitly at the first use of
3284 the C<rand> operator. However, this was not the case in version of Perl
3285 before 5.004, so if your script will run under older Perl versions, it
3286 should call srand().
3288 Note that you need something much more random than the default seed for
3289 cryptographic purposes. Checksumming the compressed output of one or more
3290 rapidly changing operating system status programs is the usual method. For
3293 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3295 If you're particularly concerned with this, see the Math::TrulyRandom
3298 Do I<not> call srand() multiple times in your program unless you know
3299 exactly what you're doing and why you're doing it. The point of the
3300 function is to "seed" the rand() function so that rand() can produce
3301 a different sequence each time you run your program. Just do it once at the
3302 top of your program, or you I<won't> get random numbers out of rand()!
3304 Frequently called programs (like CGI scripts) that simply use
3308 for a seed can fall prey to the mathematical property that
3312 one-third of the time. So don't do that.
3314 =item stat FILEHANDLE
3320 Returns a 13-element array giving the status info for a file, either the
3321 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
3322 stats $_. Returns a null list if the stat fails. Typically used as
3326 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3327 $atime,$mtime,$ctime,$blksize,$blocks)
3330 Not all fields are supported on all filesystem types. Here are the
3331 meaning of the fields:
3333 0 dev device number of filesystem
3335 2 mode file mode (type and permissions)
3336 3 nlink number of (hard) links to the file
3337 4 uid numeric user ID of file's owner
3338 5 gid numeric group ID of file's owner
3339 6 rdev the device identifier (special files only)
3340 7 size total size of file, in bytes
3341 8 atime last access time since the epoch
3342 9 mtime last modify time since the epoch
3343 10 ctime inode change time (NOT creation time!) since the epoch
3344 11 blksize preferred block size for file system I/O
3345 12 blocks actual number of blocks allocated
3347 (The epoch was at 00:00 January 1, 1970 GMT.)
3349 If stat is passed the special filehandle consisting of an underline, no
3350 stat is done, but the current contents of the stat structure from the
3351 last stat or filetest are returned. Example:
3353 if (-x $file && (($d) = stat(_)) && $d < 0) {
3354 print "$file is executable NFS file\n";
3357 (This works on machines only for which the device number is negative under NFS.)
3363 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3364 doing many pattern matches on the string before it is next modified.
3365 This may or may not save time, depending on the nature and number of
3366 patterns you are searching on, and on the distribution of character
3367 frequencies in the string to be searched -- you probably want to compare
3368 run times with and without it to see which runs faster. Those loops
3369 which scan for many short constant strings (including the constant
3370 parts of more complex patterns) will benefit most. You may have only
3371 one study active at a time -- if you study a different scalar the first
3372 is "unstudied". (The way study works is this: a linked list of every
3373 character in the string to be searched is made, so we know, for
3374 example, where all the 'k' characters are. From each search string,
3375 the rarest character is selected, based on some static frequency tables
3376 constructed from some C programs and English text. Only those places
3377 that contain this "rarest" character are examined.)
3379 For example, here is a loop which inserts index producing entries
3380 before any line containing a certain pattern:
3384 print ".IX foo\n" if /\bfoo\b/;
3385 print ".IX bar\n" if /\bbar\b/;
3386 print ".IX blurfl\n" if /\bblurfl\b/;
3391 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3392 will be looked at, because "f" is rarer than "o". In general, this is
3393 a big win except in pathological cases. The only question is whether
3394 it saves you more time than it took to build the linked list in the
3397 Note that if you have to look for strings that you don't know till
3398 runtime, you can build an entire loop as a string and eval that to
3399 avoid recompiling all your patterns all the time. Together with
3400 undefining $/ to input entire files as one record, this can be very
3401 fast, often faster than specialized programs like fgrep(1). The following
3402 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3403 out the names of those files that contain a match:
3405 $search = 'while (<>) { study;';
3406 foreach $word (@words) {
3407 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3412 eval $search; # this screams
3413 $/ = "\n"; # put back to normal input delimiter
3414 foreach $file (sort keys(%seen)) {
3422 =item sub NAME BLOCK
3424 This is subroutine definition, not a real function I<per se>. With just a
3425 NAME (and possibly prototypes), it's just a forward declaration. Without
3426 a NAME, it's an anonymous function declaration, and does actually return a
3427 value: the CODE ref of the closure you just created. See L<perlsub> and
3428 L<perlref> for details.
3430 =item substr EXPR,OFFSET,LEN
3432 =item substr EXPR,OFFSET
3434 Extracts a substring out of EXPR and returns it. First character is at
3435 offset 0, or whatever you've set C<$[> to (but don't do that).
3436 If OFFSET is negative (or more precisely, less than C<$[>), starts
3437 that far from the end of the string. If LEN is omitted, returns
3438 everything to the end of the string. If LEN is negative, leaves that
3439 many characters off the end of the string.
3441 If you specify a substring which is partly outside the string, the part
3442 within the string is returned. If the substring is totally outside
3443 the string a warning is produced.
3445 You can use the substr() function
3446 as an lvalue, in which case EXPR must be an lvalue. If you assign
3447 something shorter than LEN, the string will shrink, and if you assign
3448 something longer than LEN, the string will grow to accommodate it. To
3449 keep the string the same length you may need to pad or chop your value
3452 =item symlink OLDFILE,NEWFILE
3454 Creates a new filename symbolically linked to the old filename.
3455 Returns 1 for success, 0 otherwise. On systems that don't support
3456 symbolic links, produces a fatal error at run time. To check for that,
3459 $symlink_exists = (eval {symlink("","")};, $@ eq '');
3463 Calls the system call specified as the first element of the list,
3464 passing the remaining elements as arguments to the system call. If
3465 unimplemented, produces a fatal error. The arguments are interpreted
3466 as follows: if a given argument is numeric, the argument is passed as
3467 an int. If not, the pointer to the string value is passed. You are
3468 responsible to make sure a string is pre-extended long enough to
3469 receive any result that might be written into a string. If your
3470 integer arguments are not literals and have never been interpreted in a
3471 numeric context, you may need to add 0 to them to force them to look
3474 require 'syscall.ph'; # may need to run h2ph
3475 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3477 Note that Perl supports passing of up to only 14 arguments to your system call,
3478 which in practice should usually suffice.
3480 Syscall returns whatever value returned by the system call it calls.
3481 If the system call fails, syscall returns -1 and sets C<$!> (errno).
3482 Note that some system calls can legitimately return -1. The proper
3483 way to handle such calls is to assign C<$!=0;> before the call and
3484 check the value of <$!> if syscall returns -1.
3486 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3487 number of the read end of the pipe it creates. There is no way
3488 to retrieve the file number of the other end. You can avoid this
3489 problem by using C<pipe> instead.
3491 =item sysopen FILEHANDLE,FILENAME,MODE
3493 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3495 Opens the file whose filename is given by FILENAME, and associates it
3496 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3497 the name of the real filehandle wanted. This function calls the
3498 underlying operating system's C<open> function with the parameters
3499 FILENAME, MODE, PERMS.
3501 The possible values and flag bits of the MODE parameter are
3502 system-dependent; they are available via the standard module C<Fcntl>.
3503 However, for historical reasons, some values are universal: zero means
3504 read-only, one means write-only, and two means read/write.
3506 If the file named by FILENAME does not exist and the C<open> call
3507 creates it (typically because MODE includes the O_CREAT flag), then
3508 the value of PERMS specifies the permissions of the newly created
3509 file. If PERMS is omitted, the default value is 0666, which allows
3510 read and write for all. This default is reasonable: see C<umask>.
3512 The IO::File module provides a more object-oriented approach, if you're
3513 into that kind of thing.
3515 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3517 =item sysread FILEHANDLE,SCALAR,LENGTH
3519 Attempts to read LENGTH bytes of data into variable SCALAR from the
3520 specified FILEHANDLE, using the system call read(2). It bypasses
3521 stdio, so mixing this with other kinds of reads, print(), write(),
3522 seek(), or tell() can cause confusion because stdio usually buffers
3523 data. Returns the number of bytes actually read, or undef if there
3524 was an error. SCALAR will be grown or shrunk so that the last byte
3525 actually read is the last byte of the scalar after the read.
3527 An OFFSET may be specified to place the read data at some place in the
3528 string other than the beginning. A negative OFFSET specifies
3529 placement at that many bytes counting backwards from the end of the
3530 string. A positive OFFSET greater than the length of SCALAR results
3531 in the string being padded to the required size with "\0" bytes before
3532 the result of the read is appended.
3534 =item sysseek FILEHANDLE,POSITION,WHENCE
3536 Sets FILEHANDLE's system position using the system call lseek(2). It
3537 bypasses stdio, so mixing this with reads (other than sysread()),
3538 print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may
3539 be an expression whose value gives the name of the filehandle. The
3540 values for WHENCE are 0 to set the new position to POSITION, 1 to set
3541 the it to the current position plus POSITION, and 2 to set it to EOF
3542 plus POSITION (typically negative). For WHENCE, you may use the
3543 constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable
3544 or the POSIX module.
3546 Returns the new position, or the undefined value on failure. A position
3547 of zero is returned as the string "0 but true"; thus sysseek() returns
3548 TRUE on success and FALSE on failure, yet you can still easily determine
3553 Does exactly the same thing as "exec LIST" except that a fork is done
3554 first, and the parent process waits for the child process to complete.
3555 Note that argument processing varies depending on the number of
3556 arguments. The return value is the exit status of the program as
3557 returned by the wait() call. To get the actual exit value divide by
3558 256. See also L</exec>. This is I<NOT> what you want to use to capture
3559 the output from a command, for that you should use merely backticks or
3560 qx//, as described in L<perlop/"`STRING`">.
3562 Because system() and backticks block SIGINT and SIGQUIT, killing the
3563 program they're running doesn't actually interrupt your program.
3565 @args = ("command", "arg1", "arg2");
3567 or die "system @args failed: $?"
3569 Here's a more elaborate example of analysing the return value from
3570 system() on a Unix system to check for all possibilities, including for
3571 signals and core dumps.
3573 $rc = 0xffff & system @args;
3574 printf "system(%s) returned %#04x: ", "@args", $rc;
3576 print "ran with normal exit\n";
3578 elsif ($rc == 0xff00) {
3579 print "command failed: $!\n";
3581 elsif ($rc > 0x80) {
3583 print "ran with non-zero exit status $rc\n";
3589 print "core dump from ";
3591 print "signal $rc\n"
3595 When the arguments get executed via the system shell, results will
3596 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
3599 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3601 =item syswrite FILEHANDLE,SCALAR,LENGTH
3603 Attempts to write LENGTH bytes of data from variable SCALAR to the
3604 specified FILEHANDLE, using the system call write(2). It bypasses
3605 stdio, so mixing this with reads (other than sysread()), print(),
3606 write(), seek(), or tell() may cause confusion because stdio usually
3607 buffers data. Returns the number of bytes actually written, or undef
3608 if there was an error. If the LENGTH is greater than the available
3609 data in the SCALAR after the OFFSET, only as much data as is available
3612 An OFFSET may be specified to write the data from some part of the
3613 string other than the beginning. A negative OFFSET specifies writing
3614 that many bytes counting backwards from the end of the string. In the
3615 case the SCALAR is empty you can use OFFSET but only zero offset.
3617 =item tell FILEHANDLE
3621 Returns the current position for FILEHANDLE. FILEHANDLE may be an
3622 expression whose value gives the name of the actual filehandle. If
3623 FILEHANDLE is omitted, assumes the file last read.
3625 =item telldir DIRHANDLE
3627 Returns the current position of the readdir() routines on DIRHANDLE.
3628 Value may be given to seekdir() to access a particular location in a
3629 directory. Has the same caveats about possible directory compaction as
3630 the corresponding system library routine.
3632 =item tie VARIABLE,CLASSNAME,LIST
3634 This function binds a variable to a package class that will provide the
3635 implementation for the variable. VARIABLE is the name of the variable
3636 to be enchanted. CLASSNAME is the name of a class implementing objects
3637 of correct type. Any additional arguments are passed to the "new"
3638 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3639 Typically these are arguments such as might be passed to the dbm_open()
3640 function of C. The object returned by the "new" method is also
3641 returned by the tie() function, which would be useful if you want to
3642 access other methods in CLASSNAME.
3644 Note that functions such as keys() and values() may return huge array
3645 values when used on large objects, like DBM files. You may prefer to
3646 use the each() function to iterate over such. Example:
3648 # print out history file offsets
3650 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3651 while (($key,$val) = each %HIST) {
3652 print $key, ' = ', unpack('L',$val), "\n";
3656 A class implementing a hash should have the following methods:
3658 TIEHASH classname, LIST
3661 STORE this, key, value
3665 NEXTKEY this, lastkey
3667 A class implementing an ordinary array should have the following methods:
3669 TIEARRAY classname, LIST
3672 STORE this, key, value
3675 A class implementing a scalar should have the following methods:
3677 TIESCALAR classname, LIST
3682 Unlike dbmopen(), the tie() function will not use or require a module
3683 for you--you need to do that explicitly yourself. See L<DB_File>
3684 or the F<Config> module for interesting tie() implementations.
3688 Returns a reference to the object underlying VARIABLE (the same value
3689 that was originally returned by the tie() call which bound the variable
3690 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3695 Returns the number of non-leap seconds since whatever time the system
3696 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3697 and 00:00:00 UTC, January 1, 1970 for most other systems).
3698 Suitable for feeding to gmtime() and localtime().
3702 Returns a four-element array giving the user and system times, in
3703 seconds, for this process and the children of this process.
3705 ($user,$system,$cuser,$csystem) = times;
3709 The translation operator. Same as y///. See L<perlop>.
3711 =item truncate FILEHANDLE,LENGTH
3713 =item truncate EXPR,LENGTH
3715 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3716 specified length. Produces a fatal error if truncate isn't implemented
3723 Returns an uppercased version of EXPR. This is the internal function
3724 implementing the \U escape in double-quoted strings.
3725 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3727 If EXPR is omitted, uses $_.
3733 Returns the value of EXPR with the first character uppercased. This is
3734 the internal function implementing the \u escape in double-quoted strings.
3735 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3737 If EXPR is omitted, uses $_.
3743 Sets the umask for the process to EXPR and returns the previous value.
3744 If EXPR is omitted, merely returns the current umask. Remember that a
3745 umask is a number, usually given in octal; it is I<not> a string of octal
3746 digits. See also L</oct>, if all you have is a string.
3752 Undefines the value of EXPR, which must be an lvalue. Use only on a
3753 scalar value, an entire array, an entire hash, or a subroutine name (using
3754 "&"). (Using undef() will probably not do what you expect on most
3755 predefined variables or DBM list values, so don't do that.) Always
3756 returns the undefined value. You can omit the EXPR, in which case
3757 nothing is undefined, but you still get an undefined value that you
3758 could, for instance, return from a subroutine, assign to a variable or
3759 pass as a parameter. Examples:
3762 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
3766 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
3767 select undef, undef, undef, 0.25;
3768 ($a, $b, undef, $c) = &foo; # Ignore third value returned
3774 Deletes a list of files. Returns the number of files successfully
3777 $cnt = unlink 'a', 'b', 'c';
3781 Note: unlink will not delete directories unless you are superuser and
3782 the B<-U> flag is supplied to Perl. Even if these conditions are
3783 met, be warned that unlinking a directory can inflict damage on your
3784 filesystem. Use rmdir instead.
3786 If LIST is omitted, uses $_.
3788 =item unpack TEMPLATE,EXPR
3790 Unpack does the reverse of pack: it takes a string representing a
3791 structure and expands it out into a list value, returning the array
3792 value. (In a scalar context, it returns merely the first value
3793 produced.) The TEMPLATE has the same format as in the pack function.
3794 Here's a subroutine that does substring:
3797 local($what,$where,$howmuch) = @_;
3798 unpack("x$where a$howmuch", $what);
3803 sub ordinal { unpack("c",$_[0]); } # same as ord()
3805 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3806 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3807 themselves. Default is a 16-bit checksum. For example, the following
3808 computes the same number as the System V sum program:
3811 $checksum += unpack("%16C*", $_);
3815 The following efficiently counts the number of set bits in a bit vector:
3817 $setbits = unpack("%32b*", $selectmask);
3819 =item untie VARIABLE
3821 Breaks the binding between a variable and a package. (See tie().)
3823 =item unshift ARRAY,LIST
3825 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3826 depending on how you look at it. Prepends list to the front of the
3827 array, and returns the new number of elements in the array.
3829 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3831 Note the LIST is prepended whole, not one element at a time, so the
3832 prepended elements stay in the same order. Use reverse to do the
3835 =item use Module LIST
3839 =item use Module VERSION LIST
3843 Imports some semantics into the current package from the named module,
3844 generally by aliasing certain subroutine or variable names into your
3845 package. It is exactly equivalent to
3847 BEGIN { require Module; import Module LIST; }
3849 except that Module I<must> be a bareword.
3851 If the first argument to C<use> is a number, it is treated as a version
3852 number instead of a module name. If the version of the Perl interpreter
3853 is less than VERSION, then an error message is printed and Perl exits
3854 immediately. This is often useful if you need to check the current
3855 Perl version before C<use>ing library modules which have changed in
3856 incompatible ways from older versions of Perl. (We try not to do
3857 this more than we have to.)
3859 The BEGIN forces the require and import to happen at compile time. The
3860 require makes sure the module is loaded into memory if it hasn't been
3861 yet. The import is not a builtin--it's just an ordinary static method
3862 call into the "Module" package to tell the module to import the list of
3863 features back into the current package. The module can implement its
3864 import method any way it likes, though most modules just choose to
3865 derive their import method via inheritance from the Exporter class that
3866 is defined in the Exporter module. See L<Exporter>. If no import
3867 method can be found then the error is currently silently ignored. This
3868 may change to a fatal error in a future version.
3870 If you don't want your namespace altered, explicitly supply an empty list:
3874 That is exactly equivalent to
3876 BEGIN { require Module; }
3878 If the VERSION argument is present between Module and LIST, then the
3879 C<use> will call the VERSION method in class Module with the given
3880 version as an argument. The default VERSION method, inherited from
3881 the Universal class, croaks if the given version is larger than the
3882 value of the variable $Module::VERSION. (Note that there is not a
3883 comma after VERSION!)
3885 Because this is a wide-open interface, pragmas (compiler directives)
3886 are also implemented this way. Currently implemented pragmas are:
3890 use sigtrap qw(SEGV BUS);
3891 use strict qw(subs vars refs);
3892 use subs qw(afunc blurfl);
3894 These pseudo-modules import semantics into the current block scope, unlike
3895 ordinary modules, which import symbols into the current package (which are
3896 effective through the end of the file).
3898 There's a corresponding "no" command that unimports meanings imported
3899 by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
3904 If no unimport method can be found the call fails with a fatal error.
3906 See L<perlmod> for a list of standard modules and pragmas.
3910 Changes the access and modification times on each file of a list of
3911 files. The first two elements of the list must be the NUMERICAL access
3912 and modification times, in that order. Returns the number of files
3913 successfully changed. The inode modification time of each file is set
3914 to the current time. Example of a "touch" command:
3918 utime $now, $now, @ARGV;
3922 Returns a normal array consisting of all the values of the named hash.
3923 (In a scalar context, returns the number of values.) The values are
3924 returned in an apparently random order, but it is the same order as either
3925 the keys() or each() function would produce on the same hash. As a side
3926 effect, it resets HASH's iterator. See also keys(), each(), and sort().
3928 =item vec EXPR,OFFSET,BITS
3930 Treats the string in EXPR as a vector of unsigned integers, and
3931 returns the value of the bit field specified by OFFSET. BITS specifies
3932 the number of bits that are reserved for each entry in the bit
3933 vector. This must be a power of two from 1 to 32. vec() may also be
3934 assigned to, in which case parentheses are needed to give the expression
3935 the correct precedence as in
3937 vec($image, $max_x * $x + $y, 8) = 3;
3939 Vectors created with vec() can also be manipulated with the logical
3940 operators |, &, and ^, which will assume a bit vector operation is
3941 desired when both operands are strings.
3943 To transform a bit vector into a string or array of 0's and 1's, use these:
3945 $bits = unpack("b*", $vector);
3946 @bits = split(//, unpack("b*", $vector));
3948 If you know the exact length in bits, it can be used in place of the *.
3952 Waits for a child process to terminate and returns the pid of the
3953 deceased process, or -1 if there are no child processes. The status is
3956 =item waitpid PID,FLAGS
3958 Waits for a particular child process to terminate and returns the pid
3959 of the deceased process, or -1 if there is no such child process. The
3960 status is returned in C<$?>. If you say
3962 use POSIX ":sys_wait_h";
3964 waitpid(-1,&WNOHANG);
3966 then you can do a non-blocking wait for any process. Non-blocking wait
3967 is available on machines supporting either the waitpid(2) or
3968 wait4(2) system calls. However, waiting for a particular pid with
3969 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3970 by remembering the status values of processes that have exited but have
3971 not been harvested by the Perl script yet.)
3975 Returns TRUE if the context of the currently executing subroutine is
3976 looking for a list value. Returns FALSE if the context is looking
3977 for a scalar. Returns the undefined value if the context is looking
3978 for no value (void context).
3980 return unless defined wantarray; # don't bother doing more
3981 my @a = complex_calculation();
3982 return wantarray ? @a : "@a";
3986 Produces a message on STDERR just like die(), but doesn't exit or throw
3989 No message is printed if there is a C<$SIG{__WARN__}> handler
3990 installed. It is the handler's responsibility to deal with the message
3991 as it sees fit (like, for instance, converting it into a die()). Most
3992 handlers must therefore make arrangements to actually display the
3993 warnings that they are not prepared to deal with, by calling warn()
3994 again in the handler. Note that this is quite safe and will not
3995 produce an endless loop, since C<__WARN__> hooks are not called from
3998 You will find this behavior is slightly different from that of
3999 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4000 instead call die() again to change it).
4002 Using a C<__WARN__> handler provides a powerful way to silence all
4003 warnings (even the so-called mandatory ones). An example:
4005 # wipe out *all* compile-time warnings
4006 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4008 my $foo = 20; # no warning about duplicate my $foo,
4009 # but hey, you asked for it!
4010 # no compile-time or run-time warnings before here
4013 # run-time warnings enabled after here
4014 warn "\$foo is alive and $foo!"; # does show up
4016 See L<perlvar> for details on setting C<%SIG> entries, and for more
4019 =item write FILEHANDLE
4025 Writes a formatted record (possibly multi-line) to the specified file,
4026 using the format associated with that file. By default the format for
4027 a file is the one having the same name as the filehandle, but the
4028 format for the current output channel (see the select() function) may be set
4029 explicitly by assigning the name of the format to the C<$~> variable.
4031 Top of form processing is handled automatically: if there is
4032 insufficient room on the current page for the formatted record, the
4033 page is advanced by writing a form feed, a special top-of-page format
4034 is used to format the new page header, and then the record is written.
4035 By default the top-of-page format is the name of the filehandle with
4036 "_TOP" appended, but it may be dynamically set to the format of your
4037 choice by assigning the name to the C<$^> variable while the filehandle is
4038 selected. The number of lines remaining on the current page is in
4039 variable C<$->, which can be set to 0 to force a new page.
4041 If FILEHANDLE is unspecified, output goes to the current default output
4042 channel, which starts out as STDOUT but may be changed by the
4043 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
4044 is evaluated and the resulting string is used to look up the name of
4045 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4047 Note that write is I<NOT> the opposite of read. Unfortunately.
4051 The translation operator. Same as tr///. See L<perlop>.
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