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
554 descriptor. If the file handle came from a piped open C<close> will
555 additionally return FALSE if one of the other system calls involved
556 fails or if the program exits with non-zero status. (If the problem was
557 that the program exited non-zero $! will be set to 0.)
558 You don't have to close FILEHANDLE if you are immediately
559 going to do another open() on it, because open() will close it for you. (See
560 open().) However, an explicit close on an input file resets the line
561 counter ($.), while the implicit close done by open() does not. Also,
562 closing a pipe will wait for the process executing on the pipe to
563 complete, in case you want to look at the output of the pipe
564 afterwards. Closing a pipe explicitly also puts the status value of
565 the command into C<$?>. Example:
567 open(OUTPUT, '|sort >foo'); # pipe to sort
568 ... # print stuff to output
569 close OUTPUT; # wait for sort to finish
570 open(INPUT, 'foo'); # get sort's results
572 FILEHANDLE may be an expression whose value gives the real filehandle name.
574 =item closedir DIRHANDLE
576 Closes a directory opened by opendir().
578 =item connect SOCKET,NAME
580 Attempts to connect to a remote socket, just as the connect system call
581 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
582 packed address of the appropriate type for the socket. See the examples in
583 L<perlipc/"Sockets: Client/Server Communication">.
587 Actually a flow control statement rather than a function. If there is a
588 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
589 C<foreach>), it is always executed just before the conditional is about to
590 be evaluated again, just like the third part of a C<for> loop in C. Thus
591 it can be used to increment a loop variable, even when the loop has been
592 continued via the C<next> statement (which is similar to the C C<continue>
597 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
600 For the inverse cosine operation, you may use the POSIX::acos()
601 function, or use this relation:
603 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
605 =item crypt PLAINTEXT,SALT
607 Encrypts a string exactly like the crypt(3) function in the C library
608 (assuming that you actually have a version there that has not been
609 extirpated as a potential munition). This can prove useful for checking
610 the password file for lousy passwords, amongst other things. Only the
611 guys wearing white hats should do this.
613 Note that crypt is intended to be a one-way function, much like breaking
614 eggs to make an omelette. There is no (known) corresponding decrypt
615 function. As a result, this function isn't all that useful for
616 cryptography. (For that, see your nearby CPAN mirror.)
618 Here's an example that makes sure that whoever runs this program knows
621 $pwd = (getpwuid($<))[1];
622 $salt = substr($pwd, 0, 2);
626 chop($word = <STDIN>);
630 if (crypt($word, $salt) ne $pwd) {
636 Of course, typing in your own password to whomever asks you
641 [This function has been superseded by the untie() function.]
643 Breaks the binding between a DBM file and a hash.
645 =item dbmopen HASH,DBNAME,MODE
647 [This function has been superseded by the tie() function.]
649 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a
650 hash. HASH is the name of the hash. (Unlike normal open, the first
651 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
652 is the name of the database (without the F<.dir> or F<.pag> extension if
653 any). If the database does not exist, it is created with protection
654 specified by MODE (as modified by the umask()). If your system supports
655 only the older DBM functions, you may perform only one dbmopen() in your
656 program. In older versions of Perl, if your system had neither DBM nor
657 ndbm, calling dbmopen() produced a fatal error; it now falls back to
660 If you don't have write access to the DBM file, you can only read hash
661 variables, not set them. If you want to test whether you can write,
662 either use file tests or try setting a dummy hash entry inside an eval(),
663 which will trap the error.
665 Note that functions such as keys() and values() may return huge array
666 values when used on large DBM files. You may prefer to use the each()
667 function to iterate over large DBM files. Example:
669 # print out history file offsets
670 dbmopen(%HIST,'/usr/lib/news/history',0666);
671 while (($key,$val) = each %HIST) {
672 print $key, ' = ', unpack('L',$val), "\n";
676 See also L<AnyDBM_File> for a more general description of the pros and
677 cons of the various dbm approaches, as well as L<DB_File> for a particularly
684 Returns a Boolean value telling whether EXPR has a value other than
685 the undefined value C<undef>. If EXPR is not present, C<$_> will be
688 Many operations return C<undef> to indicate failure, end of file,
689 system error, uninitialized variable, and other exceptional
690 conditions. This function allows you to distinguish C<undef> from
691 other values. (A simple Boolean test will not distinguish among
692 C<undef>, zero, the empty string, and "0", which are all equally
693 false.) Note that since C<undef> is a valid scalar, its presence
694 doesn't I<necessarily> indicate an exceptional condition: pop()
695 returns C<undef> when its argument is an empty array, I<or> when the
696 element to return happens to be C<undef>.
698 You may also use defined() to check whether a subroutine exists. On
699 the other hand, use of defined() upon aggregates (hashes and arrays)
700 is not guaranteed to produce intuitive results, and should probably be
703 When used on a hash element, it tells you whether the value is defined,
704 not whether the key exists in the hash. Use L</exists> for the latter
709 print if defined $switch{'D'};
710 print "$val\n" while defined($val = pop(@ary));
711 die "Can't readlink $sym: $!"
712 unless defined($value = readlink $sym);
713 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
714 $debugging = 0 unless defined $debugging;
716 Note: Many folks tend to overuse defined(), and then are surprised to
717 discover that the number 0 and "" (the zero-length string) are, in fact,
718 defined values. For example, if you say
722 the pattern match succeeds, and $1 is defined, despite the fact that it
723 matched "nothing". But it didn't really match nothing--rather, it
724 matched something that happened to be 0 characters long. This is all
725 very above-board and honest. When a function returns an undefined value,
726 it's an admission that it couldn't give you an honest answer. So you
727 should use defined() only when you're questioning the integrity of what
728 you're trying to do. At other times, a simple comparison to 0 or "" is
731 Currently, using defined() on an entire array or hash reports whether
732 memory for that aggregate has ever been allocated. So an array you set
733 to the empty list appears undefined initially, and one that once was full
734 and that you then set to the empty list still appears defined. You
735 should instead use a simple test for size:
737 if (@an_array) { print "has array elements\n" }
738 if (%a_hash) { print "has hash members\n" }
740 Using undef() on these, however, does clear their memory and then report
741 them as not defined anymore, but you shoudln't do that unless you don't
742 plan to use them again, because it saves time when you load them up
743 again to have memory already ready to be filled.
745 This counterintuitive behaviour of defined() on aggregates may be
746 changed, fixed, or broken in a future release of Perl.
748 See also L</undef>, L</exists>, L</ref>.
752 Deletes the specified key(s) and their associated values from a hash.
753 For each key, returns the deleted value associated with that key, or
754 the undefined value if there was no such key. Deleting from C<$ENV{}>
755 modifies the environment. Deleting from a hash tied to a DBM file
756 deletes the entry from the DBM file. (But deleting from a tie()d hash
757 doesn't necessarily return anything.)
759 The following deletes all the values of a hash:
761 foreach $key (keys %HASH) {
767 delete @HASH{keys %HASH}
769 (But both of these are slower than the undef() command.) Note that the
770 EXPR can be arbitrarily complicated as long as the final operation is a
771 hash element lookup or hash slice:
773 delete $ref->[$x][$y]{$key};
774 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
778 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
779 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
780 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
781 is 0, exits with 255. Inside an eval(), the error message is stuffed into
782 C<$@>, and the eval() is terminated with the undefined value; this makes
783 die() the way to raise an exception.
787 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
788 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
790 If the value of EXPR does not end in a newline, the current script line
791 number and input line number (if any) are also printed, and a newline
792 is supplied. Hint: sometimes appending ", stopped" to your message
793 will cause it to make better sense when the string "at foo line 123" is
794 appended. Suppose you are running script "canasta".
796 die "/etc/games is no good";
797 die "/etc/games is no good, stopped";
799 produce, respectively
801 /etc/games is no good at canasta line 123.
802 /etc/games is no good, stopped at canasta line 123.
804 See also exit() and warn().
806 You can arrange for a callback to be called just before the die() does
807 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
808 will be called with the error text and can change the error message, if
809 it sees fit, by calling die() again. See L<perlvar> for details on
810 setting C<%SIG> entries, and eval() for some examples.
814 Not really a function. Returns the value of the last command in the
815 sequence of commands indicated by BLOCK. When modified by a loop
816 modifier, executes the BLOCK once before testing the loop condition.
817 (On other statements the loop modifiers test the conditional first.)
819 =item do SUBROUTINE(LIST)
821 A deprecated form of subroutine call. See L<perlsub>.
825 Uses the value of EXPR as a filename and executes the contents of the
826 file as a Perl script. Its primary use is to include subroutines
827 from a Perl subroutine library.
835 except that it's more efficient, more concise, keeps track of the
836 current filename for error messages, and searches all the B<-I>
837 libraries if the file isn't in the current directory (see also the @INC
838 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
839 reparse the file every time you call it, so you probably don't want to
840 do this inside a loop.
842 Note that inclusion of library modules is better done with the
843 use() and require() operators, which also do error checking
844 and raise an exception if there's a problem.
848 This causes an immediate core dump. Primarily this is so that you can
849 use the B<undump> program to turn your core dump into an executable binary
850 after having initialized all your variables at the beginning of the
851 program. When the new binary is executed it will begin by executing a
852 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
853 it as a goto with an intervening core dump and reincarnation. If LABEL
854 is omitted, restarts the program from the top. WARNING: any files
855 opened at the time of the dump will NOT be open any more when the
856 program is reincarnated, with possible resulting confusion on the part
857 of Perl. See also B<-u> option in L<perlrun>.
874 dump QUICKSTART if $ARGV[0] eq '-d';
881 When called in a list context, returns a 2-element array consisting of the
882 key and value for the next element of a hash, so that you can iterate over
883 it. When called in a scalar context, returns the key for only the next
884 element in the hash. (Note: Keys may be "0" or "", which are logically
885 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
888 Entries are returned in an apparently random order. When the hash is
889 entirely read, a null array is returned in list context (which when
890 assigned produces a FALSE (0) value), and C<undef> is returned in a
891 scalar context. The next call to each() after that will start iterating
892 again. There is a single iterator for each hash, shared by all each(),
893 keys(), and values() function calls in the program; it can be reset by
894 reading all the elements from the hash, or by evaluating C<keys HASH> or
895 C<values HASH>. If you add or delete elements of a hash while you're
896 iterating over it, you may get entries skipped or duplicated, so don't.
898 The following prints out your environment like the printenv(1) program,
899 only in a different order:
901 while (($key,$value) = each %ENV) {
902 print "$key=$value\n";
905 See also keys() and values().
913 Returns 1 if the next read on FILEHANDLE will return end of file, or if
914 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
915 gives the real filehandle name. (Note that this function actually
916 reads a character and then ungetc()s it, so it is not very useful in an
917 interactive context.) Do not read from a terminal file (or call
918 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
919 as terminals may lose the end-of-file condition if you do.
921 An C<eof> without an argument uses the last file read as argument.
922 Empty parentheses () may be used to indicate the pseudo file formed of
923 the files listed on the command line, i.e., C<eof()> is reasonable to
924 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
925 last file. Use C<eof(ARGV)> or eof without the parentheses to test
926 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
928 # reset line numbering on each input file
931 close(ARGV) if (eof); # Not eof().
934 # insert dashes just before last line of last file
937 print "--------------\n";
938 close(ARGV); # close or break; is needed if we
939 # are reading from the terminal
944 Practical hint: you almost never need to use C<eof> in Perl, because the
945 input operators return undef when they run out of data.
951 EXPR is parsed and executed as if it were a little Perl program. It
952 is executed in the context of the current Perl program, so that any
953 variable settings or subroutine and format definitions remain afterwards.
954 The value returned is the value of the last expression evaluated, or a
955 return statement may be used, just as with subroutines. The last
956 expression is evaluated in scalar or array context, depending on the
959 If there is a syntax error or runtime error, or a die() statement is
960 executed, an undefined value is returned by eval(), and C<$@> is set to the
961 error message. If there was no error, C<$@> is guaranteed to be a null
962 string. If EXPR is omitted, evaluates C<$_>. The final semicolon, if
963 any, may be omitted from the expression. Beware that using eval()
964 neither silences perl from printing warnings to STDERR, nor does it
965 stuff the text of warning messages into C<$@>. To do either of those,
966 you have to use the C<$SIG{__WARN__}> facility. See warn() and L<perlvar>.
968 Note that, because eval() traps otherwise-fatal errors, it is useful for
969 determining whether a particular feature (such as socket() or symlink())
970 is implemented. It is also Perl's exception trapping mechanism, where
971 the die operator is used to raise exceptions.
973 If the code to be executed doesn't vary, you may use the eval-BLOCK
974 form to trap run-time errors without incurring the penalty of
975 recompiling each time. The error, if any, is still returned in C<$@>.
978 # make divide-by-zero nonfatal
979 eval { $answer = $a / $b; }; warn $@ if $@;
981 # same thing, but less efficient
982 eval '$answer = $a / $b'; warn $@ if $@;
984 # a compile-time error
988 eval '$answer ='; # sets $@
990 When using the eval{} form as an exception trap in libraries, you may
991 wish not to trigger any C<__DIE__> hooks that user code may have
992 installed. You can use the C<local $SIG{__DIE__}> construct for this
993 purpose, as shown in this example:
995 # a very private exception trap for divide-by-zero
996 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
998 This is especially significant, given that C<__DIE__> hooks can call
999 die() again, which has the effect of changing their error messages:
1001 # __DIE__ hooks may modify error messages
1003 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1004 eval { die "foo foofs here" };
1005 print $@ if $@; # prints "bar barfs here"
1008 With an eval(), you should be especially careful to remember what's
1009 being looked at when:
1015 eval { $x }; # CASE 4
1017 eval "\$$x++" # CASE 5
1020 Cases 1 and 2 above behave identically: they run the code contained in
1021 the variable $x. (Although case 2 has misleading double quotes making
1022 the reader wonder what else might be happening (nothing is).) Cases 3
1023 and 4 likewise behave in the same way: they run the code '$x', which
1024 does nothing but return the value of C<$x>. (Case 4 is preferred for
1025 purely visual reasons, but it also has the advantage of compiling at
1026 compile-time instead of at run-time.) Case 5 is a place where
1027 normally you I<WOULD> like to use double quotes, except that in this
1028 particular situation, you can just use symbolic references instead, as
1033 The exec() function executes a system command I<AND NEVER RETURNS>,
1034 unless the command does not exist and is executed directly instead of
1035 via your system's command shell (see below). Use system() instead of
1036 exec() if you want it to return.
1038 If there is more than one argument in LIST, or if LIST is an array with
1039 more than one value, calls execvp(3) with the arguments in LIST. If
1040 there is only one scalar argument, the argument is checked for shell
1041 metacharacters, and if there are any, the entire argument is passed to
1042 the system's command shell for parsing (this is C</bin/sh -c> on Unix
1043 platforms, but varies on other platforms). If there are no shell
1044 metacharacters in the argument, it is split into words and passed
1045 directly to execvp(), which is more efficient. Note: exec() and
1046 system() do not flush your output buffer, so you may need to set C<$|>
1047 to avoid lost output. Examples:
1049 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1050 exec "sort $outfile | uniq";
1052 If you don't really want to execute the first argument, but want to lie
1053 to the program you are executing about its own name, you can specify
1054 the program you actually want to run as an "indirect object" (without a
1055 comma) in front of the LIST. (This always forces interpretation of the
1056 LIST as a multivalued list, even if there is only a single scalar in
1059 $shell = '/bin/csh';
1060 exec $shell '-sh'; # pretend it's a login shell
1064 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1066 When the arguments get executed via the system shell, results will
1067 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1072 Returns TRUE if the specified hash key exists in its hash array, even
1073 if the corresponding value is undefined.
1075 print "Exists\n" if exists $array{$key};
1076 print "Defined\n" if defined $array{$key};
1077 print "True\n" if $array{$key};
1079 A hash element can be TRUE only if it's defined, and defined if
1080 it exists, but the reverse doesn't necessarily hold true.
1082 Note that the EXPR can be arbitrarily complicated as long as the final
1083 operation is a hash key lookup:
1085 if (exists $ref->[$x][$y]{$key}) { ... }
1089 Evaluates EXPR and exits immediately with that value. (Actually, it
1090 calls any defined C<END> routines first, but the C<END> routines may not
1091 abort the exit. Likewise any object destructors that need to be called
1092 are called before exit.) Example:
1095 exit 0 if $ans =~ /^[Xx]/;
1097 See also die(). If EXPR is omitted, exits with 0 status. The only
1098 universally portable values for EXPR are 0 for success and 1 for error;
1099 all other values are subject to unpredictable interpretation depending
1100 on the environment in which the Perl program is running.
1102 You shouldn't use exit() to abort a subroutine if there's any chance that
1103 someone might want to trap whatever error happened. Use die() instead,
1104 which can be trapped by an eval().
1110 Returns I<e> (the natural logarithm base) to the power of EXPR.
1111 If EXPR is omitted, gives C<exp($_)>.
1113 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1115 Implements the fcntl(2) function. You'll probably have to say
1119 first to get the correct function definitions. Argument processing and
1120 value return works just like ioctl() below. Note that fcntl() will produce
1121 a fatal error if used on a machine that doesn't implement fcntl(2).
1125 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1127 =item fileno FILEHANDLE
1129 Returns the file descriptor for a filehandle. This is useful for
1130 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1131 value is taken as the name of the filehandle.
1133 =item flock FILEHANDLE,OPERATION
1135 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1136 success, FALSE on failure. Produces a fatal error if used on a machine
1137 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1138 is Perl's portable file locking interface, although it locks only entire
1141 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1142 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1143 you can use the symbolic names if import them from the Fcntl module,
1144 either individually, or as a group using the ':flock' tag. LOCK_SH
1145 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1146 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1147 LOCK_EX then flock() will return immediately rather than blocking
1148 waiting for the lock (check the return status to see if you got it).
1150 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1151 before (un)locking it.
1153 Note that the emulation built with lockf(3) doesn't provide shared
1154 locks, and it requires that FILEHANDLE be open with write intent. These
1155 are the semantics that lockf(3) implements. Most (all?) systems
1156 implement lockf(3) in terms of fcntl(2) locking, though, so the
1157 differing semantics shouldn't bite too many people.
1159 Note also that some versions of flock() cannot lock things over the
1160 network; you would need to use the more system-specific fcntl() for
1161 that. If you like you can force Perl to ignore your system's flock(2)
1162 function, and so provide its own fcntl(2)-based emulation, by passing
1163 the switch C<-Ud_flock> to the F<Configure> program when you configure
1166 Here's a mailbox appender for BSD systems.
1168 use Fcntl ':flock'; # import LOCK_* constants
1171 flock(MBOX,LOCK_EX);
1172 # and, in case someone appended
1173 # while we were waiting...
1178 flock(MBOX,LOCK_UN);
1181 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1182 or die "Can't open mailbox: $!";
1185 print MBOX $msg,"\n\n";
1188 See also L<DB_File> for other flock() examples.
1192 Does a fork(2) system call. Returns the child pid to the parent process
1193 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1194 Note: unflushed buffers remain unflushed in both processes, which means
1195 you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1196 method of IO::Handle to avoid duplicate output.
1198 If you fork() without ever waiting on your children, you will accumulate
1201 $SIG{CHLD} = sub { wait };
1203 There's also the double-fork trick (error checking on
1204 fork() returns omitted);
1206 unless ($pid = fork) {
1208 exec "what you really wanna do";
1211 ## (some_perl_code_here)
1218 See also L<perlipc> for more examples of forking and reaping
1221 Note that if your forked child inherits system file descriptors like
1222 STDIN and STDOUT that are actually connected by a pipe or socket, even
1223 if you exit, the remote server (such as, say, httpd or rsh) won't think
1224 you're done. You should reopen those to /dev/null if it's any issue.
1228 Declare a picture format with use by the write() function. For
1232 Test: @<<<<<<<< @||||| @>>>>>
1233 $str, $%, '$' . int($num)
1237 $num = $cost/$quantity;
1241 See L<perlform> for many details and examples.
1244 =item formline PICTURE,LIST
1246 This is an internal function used by C<format>s, though you may call it
1247 too. It formats (see L<perlform>) a list of values according to the
1248 contents of PICTURE, placing the output into the format output
1249 accumulator, C<$^A> (or $ACCUMULATOR in English).
1250 Eventually, when a write() is done, the contents of
1251 C<$^A> are written to some filehandle, but you could also read C<$^A>
1252 yourself and then set C<$^A> back to "". Note that a format typically
1253 does one formline() per line of form, but the formline() function itself
1254 doesn't care how many newlines are embedded in the PICTURE. This means
1255 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1256 You may therefore need to use multiple formlines to implement a single
1257 record format, just like the format compiler.
1259 Be careful if you put double quotes around the picture, because an "C<@>"
1260 character may be taken to mean the beginning of an array name.
1261 formline() always returns TRUE. See L<perlform> for other examples.
1263 =item getc FILEHANDLE
1267 Returns the next character from the input file attached to FILEHANDLE,
1268 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1269 This is not particularly efficient. It cannot be used to get unbuffered
1270 single-characters, however. For that, try something more like:
1273 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1276 system "stty", '-icanon', 'eol', "\001";
1282 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1285 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1289 Determination of whether $BSD_STYLE should be set
1290 is left as an exercise to the reader.
1292 The POSIX::getattr() function can do this more portably on systems
1293 alleging POSIX compliance.
1294 See also the C<Term::ReadKey> module from your nearest CPAN site;
1295 details on CPAN can be found on L<perlmod/CPAN>.
1299 Returns the current login from F</etc/utmp>, if any. If null, use
1302 $login = getlogin || getpwuid($<) || "Kilroy";
1304 Do not consider getlogin() for authentication: it is not as
1305 secure as getpwuid().
1307 =item getpeername SOCKET
1309 Returns the packed sockaddr address of other end of the SOCKET connection.
1312 $hersockaddr = getpeername(SOCK);
1313 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1314 $herhostname = gethostbyaddr($iaddr, AF_INET);
1315 $herstraddr = inet_ntoa($iaddr);
1319 Returns the current process group for the specified PID. Use
1320 a PID of 0 to get the current process group for the
1321 current process. Will raise an exception if used on a machine that
1322 doesn't implement getpgrp(2). If PID is omitted, returns process
1323 group of current process. Note that the POSIX version of getpgrp()
1324 does not accept a PID argument, so only PID==0 is truly portable.
1328 Returns the process id of the parent process.
1330 =item getpriority WHICH,WHO
1332 Returns the current priority for a process, a process group, or a user.
1333 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1334 machine that doesn't implement getpriority(2).
1340 =item gethostbyname NAME
1342 =item getnetbyname NAME
1344 =item getprotobyname NAME
1350 =item getservbyname NAME,PROTO
1352 =item gethostbyaddr ADDR,ADDRTYPE
1354 =item getnetbyaddr ADDR,ADDRTYPE
1356 =item getprotobynumber NUMBER
1358 =item getservbyport PORT,PROTO
1376 =item sethostent STAYOPEN
1378 =item setnetent STAYOPEN
1380 =item setprotoent STAYOPEN
1382 =item setservent STAYOPEN
1396 These routines perform the same functions as their counterparts in the
1397 system library. Within a list context, the return values from the
1398 various get routines are as follows:
1400 ($name,$passwd,$uid,$gid,
1401 $quota,$comment,$gcos,$dir,$shell) = getpw*
1402 ($name,$passwd,$gid,$members) = getgr*
1403 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1404 ($name,$aliases,$addrtype,$net) = getnet*
1405 ($name,$aliases,$proto) = getproto*
1406 ($name,$aliases,$port,$proto) = getserv*
1408 (If the entry doesn't exist you get a null list.)
1410 Within a scalar context, you get the name, unless the function was a
1411 lookup by name, in which case you get the other thing, whatever it is.
1412 (If the entry doesn't exist you get the undefined value.) For example:
1422 The $members value returned by I<getgr*()> is a space separated list of
1423 the login names of the members of the group.
1425 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1426 C, it will be returned to you via C<$?> if the function call fails. The
1427 @addrs value returned by a successful call is a list of the raw
1428 addresses returned by the corresponding system library call. In the
1429 Internet domain, each address is four bytes long and you can unpack it
1430 by saying something like:
1432 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1434 =item getsockname SOCKET
1436 Returns the packed sockaddr address of this end of the SOCKET connection.
1439 $mysockaddr = getsockname(SOCK);
1440 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1442 =item getsockopt SOCKET,LEVEL,OPTNAME
1444 Returns the socket option requested, or undefined if there is an error.
1450 Returns the value of EXPR with filename expansions such as a shell would
1451 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1452 operator, but you can use it directly. If EXPR is omitted, $_ is used.
1453 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1454 L<perlop/"I/O Operators">.
1458 Converts a time as returned by the time function to a 9-element array
1459 with the time localized for the standard Greenwich time zone.
1460 Typically used as follows:
1463 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1466 All array elements are numeric, and come straight out of a struct tm.
1467 In particular this means that $mon has the range 0..11 and $wday has
1468 the range 0..6 with sunday as day 0. Also, $year is the number of
1469 years since 1900, I<not> simply the last two digits of the year.
1471 If EXPR is omitted, does C<gmtime(time())>.
1473 In a scalar context, returns the ctime(3) value:
1475 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1477 Also see the timegm() function provided by the Time::Local module,
1478 and the strftime(3) function available via the POSIX module.
1486 The goto-LABEL form finds the statement labeled with LABEL and resumes
1487 execution there. It may not be used to go into any construct that
1488 requires initialization, such as a subroutine or a foreach loop. It
1489 also can't be used to go into a construct that is optimized away,
1490 or to get out of a block or subroutine given to sort().
1491 It can be used to go almost anywhere else within the dynamic scope,
1492 including out of subroutines, but it's usually better to use some other
1493 construct such as last or die. The author of Perl has never felt the
1494 need to use this form of goto (in Perl, that is--C is another matter).
1496 The goto-EXPR form expects a label name, whose scope will be resolved
1497 dynamically. This allows for computed gotos per FORTRAN, but isn't
1498 necessarily recommended if you're optimizing for maintainability:
1500 goto ("FOO", "BAR", "GLARCH")[$i];
1502 The goto-&NAME form is highly magical, and substitutes a call to the
1503 named subroutine for the currently running subroutine. This is used by
1504 AUTOLOAD subroutines that wish to load another subroutine and then
1505 pretend that the other subroutine had been called in the first place
1506 (except that any modifications to @_ in the current subroutine are
1507 propagated to the other subroutine.) After the goto, not even caller()
1508 will be able to tell that this routine was called first.
1510 =item grep BLOCK LIST
1512 =item grep EXPR,LIST
1514 This is similar in spirit to, but not the same as, grep(1)
1515 and its relatives. In particular, it is not limited to using
1516 regular expressions.
1518 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1519 $_ to each element) and returns the list value consisting of those
1520 elements for which the expression evaluated to TRUE. In a scalar
1521 context, returns the number of times the expression was TRUE.
1523 @foo = grep(!/^#/, @bar); # weed out comments
1527 @foo = grep {!/^#/} @bar; # weed out comments
1529 Note that, because $_ is a reference into the list value, it can be used
1530 to modify the elements of the array. While this is useful and
1531 supported, it can cause bizarre results if the LIST is not a named
1532 array. Similarly, grep returns aliases into the original list,
1533 much like the way that L<Foreach Loops>'s index variable aliases the list
1534 elements. That is, modifying an element of a list returned by grep
1535 actually modifies the element in the original list.
1541 Interprets EXPR as a hex string and returns the corresponding
1542 value. (To convert strings that might start with either 0 or 0x
1543 see L</oct>.) If EXPR is omitted, uses $_.
1545 print hex '0xAf'; # prints '175'
1546 print hex 'aF'; # same
1550 There is no builtin import() function. It is merely an ordinary
1551 method (subroutine) defined (or inherited) by modules that wish to export
1552 names to another module. The use() function calls the import() method
1553 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1555 =item index STR,SUBSTR,POSITION
1557 =item index STR,SUBSTR
1559 Returns the position of the first occurrence of SUBSTR in STR at or after
1560 POSITION. If POSITION is omitted, starts searching from the beginning of
1561 the string. The return value is based at 0 (or whatever you've set the C<$[>
1562 variable to--but don't do that). If the substring is not found, returns
1563 one less than the base, ordinarily -1.
1569 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1571 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1573 Implements the ioctl(2) function. You'll probably have to say
1575 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1577 first to get the correct function definitions. If F<ioctl.ph> doesn't
1578 exist or doesn't have the correct definitions you'll have to roll your
1579 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1580 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1581 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1582 written depending on the FUNCTION--a pointer to the string value of SCALAR
1583 will be passed as the third argument of the actual ioctl call. (If SCALAR
1584 has no string value but does have a numeric value, that value will be
1585 passed rather than a pointer to the string value. To guarantee this to be
1586 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1587 functions are useful for manipulating the values of structures used by
1588 ioctl(). The following example sets the erase character to DEL.
1592 die "NO TIOCGETP" if $@ || !$getp;
1593 $sgttyb_t = "ccccs"; # 4 chars and a short
1594 if (ioctl(STDIN,$getp,$sgttyb)) {
1595 @ary = unpack($sgttyb_t,$sgttyb);
1597 $sgttyb = pack($sgttyb_t,@ary);
1598 ioctl(STDIN,&TIOCSETP,$sgttyb)
1599 || die "Can't ioctl: $!";
1602 The return value of ioctl (and fcntl) is as follows:
1604 if OS returns: then Perl returns:
1606 0 string "0 but true"
1607 anything else that number
1609 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1610 still easily determine the actual value returned by the operating
1613 ($retval = ioctl(...)) || ($retval = -1);
1614 printf "System returned %d\n", $retval;
1616 =item join EXPR,LIST
1618 Joins the separate strings of LIST into a single string with
1619 fields separated by the value of EXPR, and returns the string.
1622 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1624 See L<perlfunc/split>.
1628 Returns a normal array consisting of all the keys of the named hash. (In
1629 a scalar context, returns the number of keys.) The keys are returned in
1630 an apparently random order, but it is the same order as either the
1631 values() or each() function produces (given that the hash has not been
1632 modified). As a side effect, it resets HASH's iterator.
1634 Here is yet another way to print your environment:
1637 @values = values %ENV;
1638 while ($#keys >= 0) {
1639 print pop(@keys), '=', pop(@values), "\n";
1642 or how about sorted by key:
1644 foreach $key (sort(keys %ENV)) {
1645 print $key, '=', $ENV{$key}, "\n";
1648 To sort an array by value, you'll need to use a C<sort> function.
1649 Here's a descending numeric sort of a hash by its values:
1651 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1652 printf "%4d %s\n", $hash{$key}, $key;
1655 As an lvalue C<keys> allows you to increase the number of hash buckets
1656 allocated for the given hash. This can gain you a measure of efficiency if
1657 you know the hash is going to get big. (This is similar to pre-extending
1658 an array by assigning a larger number to $#array.) If you say
1662 then C<%hash> will have at least 200 buckets allocated for it. These
1663 buckets will be retained even if you do C<%hash = ()>, use C<undef
1664 %hash> if you want to free the storage while C<%hash> is still in scope.
1665 You can't shrink the number of buckets allocated for the hash using
1666 C<keys> in this way (but you needn't worry about doing this by accident,
1667 as trying has no effect).
1671 Sends a signal to a list of processes. The first element of
1672 the list must be the signal to send. Returns the number of
1673 processes successfully signaled.
1675 $cnt = kill 1, $child1, $child2;
1678 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1679 process groups instead of processes. (On System V, a negative I<PROCESS>
1680 number will also kill process groups, but that's not portable.) That
1681 means you usually want to use positive not negative signals. You may also
1682 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1688 The C<last> command is like the C<break> statement in C (as used in
1689 loops); it immediately exits the loop in question. If the LABEL is
1690 omitted, the command refers to the innermost enclosing loop. The
1691 C<continue> block, if any, is not executed:
1693 LINE: while (<STDIN>) {
1694 last LINE if /^$/; # exit when done with header
1702 Returns an lowercased version of EXPR. This is the internal function
1703 implementing the \L escape in double-quoted strings.
1704 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1706 If EXPR is omitted, uses $_.
1712 Returns the value of EXPR with the first character lowercased. This is
1713 the internal function implementing the \l escape in double-quoted strings.
1714 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1716 If EXPR is omitted, uses $_.
1722 Returns the length in characters of the value of EXPR. If EXPR is
1723 omitted, returns length of $_.
1725 =item link OLDFILE,NEWFILE
1727 Creates a new filename linked to the old filename. Returns 1 for
1728 success, 0 otherwise.
1730 =item listen SOCKET,QUEUESIZE
1732 Does the same thing that the listen system call does. Returns TRUE if
1733 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1737 A local modifies the listed variables to be local to the enclosing block,
1738 subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1739 list must be placed in parentheses. See L<perlsub/"Temporary Values via
1740 local()"> for details, including issues with tied arrays and hashes.
1742 But you really probably want to be using my() instead, because local() isn't
1743 what most people think of as "local"). See L<perlsub/"Private Variables
1744 via my()"> for details.
1746 =item localtime EXPR
1748 Converts a time as returned by the time function to a 9-element array
1749 with the time analyzed for the local time zone. Typically used as
1753 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1756 All array elements are numeric, and come straight out of a struct tm.
1757 In particular this means that $mon has the range 0..11 and $wday has
1758 the range 0..6 with sunday as day 0. Also, $year is the number of
1759 years since 1900, that is, $year is 123 in year 2023.
1761 If EXPR is omitted, uses the current time (C<localtime(time)>).
1763 In a scalar context, returns the ctime(3) value:
1765 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
1767 Also see the Time::Local module, and the strftime(3) and mktime(3)
1768 function available via the POSIX module.
1774 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1777 =item lstat FILEHANDLE
1783 Does the same thing as the stat() function, but stats a symbolic link
1784 instead of the file the symbolic link points to. If symbolic links are
1785 unimplemented on your system, a normal stat() is done.
1787 If EXPR is omitted, stats $_.
1791 The match operator. See L<perlop>.
1793 =item map BLOCK LIST
1797 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1798 element) and returns the list value composed of the results of each such
1799 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1800 may produce zero, one, or more elements in the returned value.
1802 @chars = map(chr, @nums);
1804 translates a list of numbers to the corresponding characters. And
1806 %hash = map { getkey($_) => $_ } @array;
1808 is just a funny way to write
1811 foreach $_ (@array) {
1812 $hash{getkey($_)} = $_;
1815 =item mkdir FILENAME,MODE
1817 Creates the directory specified by FILENAME, with permissions specified
1818 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1819 it returns 0 and sets C<$!> (errno).
1821 =item msgctl ID,CMD,ARG
1823 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1824 must be a variable which will hold the returned msqid_ds structure.
1825 Returns like ioctl: the undefined value for error, "0 but true" for
1826 zero, or the actual return value otherwise.
1828 =item msgget KEY,FLAGS
1830 Calls the System V IPC function msgget(2). Returns the message queue id,
1831 or the undefined value if there is an error.
1833 =item msgsnd ID,MSG,FLAGS
1835 Calls the System V IPC function msgsnd to send the message MSG to the
1836 message queue ID. MSG must begin with the long integer message type,
1837 which may be created with C<pack("l", $type)>. Returns TRUE if
1838 successful, or FALSE if there is an error.
1840 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1842 Calls the System V IPC function msgrcv to receive a message from
1843 message queue ID into variable VAR with a maximum message size of
1844 SIZE. Note that if a message is received, the message type will be the
1845 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1846 of the message type. Returns TRUE if successful, or FALSE if there is
1851 A "my" declares the listed variables to be local (lexically) to the
1852 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1853 more than one value is listed, the list must be placed in parentheses. See
1854 L<perlsub/"Private Variables via my()"> for details.
1860 The C<next> command is like the C<continue> statement in C; it starts
1861 the next iteration of the loop:
1863 LINE: while (<STDIN>) {
1864 next LINE if /^#/; # discard comments
1868 Note that if there were a C<continue> block on the above, it would get
1869 executed even on discarded lines. If the LABEL is omitted, the command
1870 refers to the innermost enclosing loop.
1872 =item no Module LIST
1874 See the "use" function, which "no" is the opposite of.
1880 Interprets EXPR as an octal string and returns the corresponding
1881 value. (If EXPR happens to start off with 0x, interprets it as
1882 a hex string instead.) The following will handle decimal, octal, and
1883 hex in the standard Perl or C notation:
1885 $val = oct($val) if $val =~ /^0/;
1887 If EXPR is omitted, uses $_. This function is commonly used when
1888 a string such as "644" needs to be converted into a file mode, for
1889 example. (Although perl will automatically convert strings into
1890 numbers as needed, this automatic conversion assumes base 10.)
1892 =item open FILEHANDLE,EXPR
1894 =item open FILEHANDLE
1896 Opens the file whose filename is given by EXPR, and associates it with
1897 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1898 name of the real filehandle wanted. If EXPR is omitted, the scalar
1899 variable of the same name as the FILEHANDLE contains the filename.
1900 (Note that lexical variables--those declared with C<my>--will not work
1901 for this purpose; so if you're using C<my>, specify EXPR in your call
1904 If the filename begins with '<' or nothing, the file is opened for input.
1905 If the filename begins with '>', the file is truncated and opened for
1906 output. If the filename begins with '>>', the file is opened for
1907 appending. You can put a '+' in front of the '>' or '<' to indicate that
1908 you want both read and write access to the file; thus '+<' is almost
1909 always preferred for read/write updates--the '+>' mode would clobber the
1910 file first. The prefix and the filename may be separated with spaces.
1911 These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1912 'w+', 'a', and 'a+'.
1914 If the filename begins with "|", the filename is interpreted as a command
1915 to which output is to be piped, and if the filename ends with a "|", the
1916 filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1917 examples of this. as command which pipes input to us. (You may not have
1918 a raw open() to a command that pipes both in I<and> out, but see
1919 L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1922 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1923 nonzero upon success, the undefined value otherwise. If the open
1924 involved a pipe, the return value happens to be the pid of the
1927 If you're unfortunate enough to be running Perl on a system that
1928 distinguishes between text files and binary files (modern operating
1929 systems don't care), then you should check out L</binmode> for tips for
1930 dealing with this. The key distinction between systems that need binmode
1931 and those that don't is their text file formats. Systems like Unix and
1932 Plan9 that delimit lines with a single character, and that encode that
1933 character in C as '\n', do not need C<binmode>. The rest need it.
1938 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1939 while (<ARTICLE>) {...
1941 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1943 open(DBASE, '+<dbase.mine'); # open for update
1945 open(ARTICLE, "caesar <$article |"); # decrypt article
1947 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1949 # process argument list of files along with any includes
1951 foreach $file (@ARGV) {
1952 process($file, 'fh00');
1956 local($filename, $input) = @_;
1957 $input++; # this is a string increment
1958 unless (open($input, $filename)) {
1959 print STDERR "Can't open $filename: $!\n";
1963 while (<$input>) { # note use of indirection
1964 if (/^#include "(.*)"/) {
1965 process($1, $input);
1972 You may also, in the Bourne shell tradition, specify an EXPR beginning
1973 with "E<gt>&", in which case the rest of the string is interpreted as the
1974 name of a filehandle (or file descriptor, if numeric) which is to be
1975 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1976 +E<gt>E<gt>, and +E<lt>. The
1977 mode you specify should match the mode of the original filehandle.
1978 (Duping a filehandle does not take into account any existing contents of
1980 Here is a script that saves, redirects, and restores STDOUT and
1984 open(SAVEOUT, ">&STDOUT");
1985 open(SAVEERR, ">&STDERR");
1987 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1988 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1990 select(STDERR); $| = 1; # make unbuffered
1991 select(STDOUT); $| = 1; # make unbuffered
1993 print STDOUT "stdout 1\n"; # this works for
1994 print STDERR "stderr 1\n"; # subprocesses too
1999 open(STDOUT, ">&SAVEOUT");
2000 open(STDERR, ">&SAVEERR");
2002 print STDOUT "stdout 2\n";
2003 print STDERR "stderr 2\n";
2006 If you specify "E<lt>&=N", where N is a number, then Perl will do an
2007 equivalent of C's fdopen() of that file descriptor; this is more
2008 parsimonious of file descriptors. For example:
2010 open(FILEHANDLE, "<&=$fd")
2012 If you open a pipe on the command "-", i.e., either "|-" or "-|", then
2013 there is an implicit fork done, and the return value of open is the pid
2014 of the child within the parent process, and 0 within the child
2015 process. (Use C<defined($pid)> to determine whether the open was successful.)
2016 The filehandle behaves normally for the parent, but i/o to that
2017 filehandle is piped from/to the STDOUT/STDIN of the child process.
2018 In the child process the filehandle isn't opened--i/o happens from/to
2019 the new STDOUT or STDIN. Typically this is used like the normal
2020 piped open when you want to exercise more control over just how the
2021 pipe command gets executed, such as when you are running setuid, and
2022 don't want to have to scan shell commands for metacharacters.
2023 The following pairs are more or less equivalent:
2025 open(FOO, "|tr '[a-z]' '[A-Z]'");
2026 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2028 open(FOO, "cat -n '$file'|");
2029 open(FOO, "-|") || exec 'cat', '-n', $file;
2031 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2033 NOTE: On any operation which may do a fork, unflushed buffers remain
2034 unflushed in both processes, which means you may need to set C<$|> to
2035 avoid duplicate output.
2037 Closing any piped filehandle causes the parent process to wait for the
2038 child to finish, and returns the status value in C<$?>.
2040 Using the constructor from the IO::Handle package (or one of its
2041 subclasses, such as IO::File or IO::Socket),
2042 you can generate anonymous filehandles which have the scope of whatever
2043 variables hold references to them, and automatically close whenever
2044 and however you leave that scope:
2048 sub read_myfile_munged {
2050 my $handle = new IO::File;
2051 open($handle, "myfile") or die "myfile: $!";
2053 or return (); # Automatically closed here.
2054 mung $first or die "mung failed"; # Or here.
2055 return $first, <$handle> if $ALL; # Or here.
2059 The filename that is passed to open will have leading and trailing
2060 whitespace deleted. To open a file with arbitrary weird
2061 characters in it, it's necessary to protect any leading and trailing
2064 $file =~ s#^(\s)#./$1#;
2065 open(FOO, "< $file\0");
2067 If you want a "real" C open() (see L<open(2)> on your system), then
2068 you should use the sysopen() function. This is another way to
2069 protect your filenames from interpretation. For example:
2072 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2073 or die "sysopen $path: $!";
2074 HANDLE->autoflush(1);
2075 HANDLE->print("stuff $$\n");
2077 print "File contains: ", <HANDLE>;
2079 See L</seek()> for some details about mixing reading and writing.
2081 =item opendir DIRHANDLE,EXPR
2083 Opens a directory named EXPR for processing by readdir(), telldir(),
2084 seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
2085 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2091 Returns the numeric ascii value of the first character of EXPR. If
2092 EXPR is omitted, uses $_. For the reverse, see L</chr>.
2094 =item pack TEMPLATE,LIST
2096 Takes an array or list of values and packs it into a binary structure,
2097 returning the string containing the structure. The TEMPLATE is a
2098 sequence of characters that give the order and type of values, as
2101 A An ascii string, will be space padded.
2102 a An ascii string, will be null padded.
2103 b A bit string (ascending bit order, like vec()).
2104 B A bit string (descending bit order).
2105 h A hex string (low nybble first).
2106 H A hex string (high nybble first).
2108 c A signed char value.
2109 C An unsigned char value.
2111 s A signed short value.
2112 S An unsigned short value.
2113 (This 'short' is _exactly_ 16 bits, which may differ from
2114 what a local C compiler calls 'short'.)
2116 i A signed integer value.
2117 I An unsigned integer value.
2118 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2119 depends on what a local C compiler calls 'int', and may
2120 even be larger than the 'long' described in the next item.)
2122 l A signed long value.
2123 L An unsigned long value.
2124 (This 'long' is _exactly_ 32 bits, which may differ from
2125 what a local C compiler calls 'long'.)
2127 n A short in "network" (big-endian) order.
2128 N A long in "network" (big-endian) order.
2129 v A short in "VAX" (little-endian) order.
2130 V A long in "VAX" (little-endian) order.
2131 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2132 _exactly_ 32 bits, respectively.)
2134 f A single-precision float in the native format.
2135 d A double-precision float in the native format.
2137 p A pointer to a null-terminated string.
2138 P A pointer to a structure (fixed-length string).
2140 u A uuencoded string.
2142 w A BER compressed integer. Its bytes represent an unsigned
2143 integer in base 128, most significant digit first, with as few
2144 digits as possible. Bit eight (the high bit) is set on each
2145 byte except the last.
2149 @ Null fill to absolute position.
2151 Each letter may optionally be followed by a number which gives a repeat
2152 count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
2153 pack function will gobble up that many values from the LIST. A * for the
2154 repeat count means to use however many items are left. The "a" and "A"
2155 types gobble just one value, but pack it as a string of length count,
2156 padding with nulls or spaces as necessary. (When unpacking, "A" strips
2157 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2158 fields pack a string that many bits long. The "h" and "H" fields pack a
2159 string that many nybbles long. The "p" type packs a pointer to a null-
2160 terminated string. You are responsible for ensuring the string is not a
2161 temporary value (which can potentially get deallocated before you get
2162 around to using the packed result). The "P" packs a pointer to a structure
2163 of the size indicated by the length. Real numbers (floats and doubles) are
2164 in the native machine format only; due to the multiplicity of floating
2165 formats around, and the lack of a standard "network" representation, no
2166 facility for interchange has been made. This means that packed floating
2167 point data written on one machine may not be readable on another - even if
2168 both use IEEE floating point arithmetic (as the endian-ness of the memory
2169 representation is not part of the IEEE spec). Note that Perl uses doubles
2170 internally for all numeric calculation, and converting from double into
2171 float and thence back to double again will lose precision (i.e.,
2172 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2176 $foo = pack("cccc",65,66,67,68);
2178 $foo = pack("c4",65,66,67,68);
2181 $foo = pack("ccxxcc",65,66,67,68);
2184 $foo = pack("s2",1,2);
2185 # "\1\0\2\0" on little-endian
2186 # "\0\1\0\2" on big-endian
2188 $foo = pack("a4","abcd","x","y","z");
2191 $foo = pack("aaaa","abcd","x","y","z");
2194 $foo = pack("a14","abcdefg");
2195 # "abcdefg\0\0\0\0\0\0\0"
2197 $foo = pack("i9pl", gmtime);
2198 # a real struct tm (on my system anyway)
2201 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2204 The same template may generally also be used in the unpack function.
2206 =item package NAMESPACE
2208 Declares the compilation unit as being in the given namespace. The scope
2209 of the package declaration is from the declaration itself through the end of
2210 the enclosing block (the same scope as the local() operator). All further
2211 unqualified dynamic identifiers will be in this namespace. A package
2212 statement affects only dynamic variables--including those you've used
2213 local() on--but I<not> lexical variables created with my(). Typically it
2214 would be the first declaration in a file to be included by the C<require>
2215 or C<use> operator. You can switch into a package in more than one place;
2216 it influences merely which symbol table is used by the compiler for the
2217 rest of that block. You can refer to variables and filehandles in other
2218 packages by prefixing the identifier with the package name and a double
2219 colon: C<$Package::Variable>. If the package name is null, the C<main>
2220 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2222 See L<perlmod/"Packages"> for more information about packages, modules,
2223 and classes. See L<perlsub> for other scoping issues.
2225 =item pipe READHANDLE,WRITEHANDLE
2227 Opens a pair of connected pipes like the corresponding system call.
2228 Note that if you set up a loop of piped processes, deadlock can occur
2229 unless you are very careful. In addition, note that Perl's pipes use
2230 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2231 after each command, depending on the application.
2233 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2234 for examples of such things.
2240 Pops and returns the last value of the array, shortening the array by
2241 1. Has a similar effect to
2243 $tmp = $ARRAY[$#ARRAY--];
2245 If there are no elements in the array, returns the undefined value.
2246 If ARRAY is omitted, pops the
2247 @ARGV array in the main program, and the @_ array in subroutines, just
2254 Returns the offset of where the last C<m//g> search left off for the variable
2255 is in question ($_ is used when the variable is not specified). May be
2256 modified to change that offset. Such modification will also influence
2257 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2260 =item print FILEHANDLE LIST
2266 Prints a string or a comma-separated list of strings. Returns TRUE
2267 if successful. FILEHANDLE may be a scalar variable name, in which case
2268 the variable contains the name of or a reference to the filehandle, thus introducing one
2269 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2270 token is a term, it may be misinterpreted as an operator unless you
2271 interpose a + or put parentheses around the arguments.) If FILEHANDLE is
2272 omitted, prints by default to standard output (or to the last selected
2273 output channel--see L</select>). If LIST is also omitted, prints $_ to
2274 STDOUT. To set the default output channel to something other than
2275 STDOUT use the select operation. Note that, because print takes a
2276 LIST, anything in the LIST is evaluated in a list context, and any
2277 subroutine that you call will have one or more of its expressions
2278 evaluated in a list context. Also be careful not to follow the print
2279 keyword with a left parenthesis unless you want the corresponding right
2280 parenthesis to terminate the arguments to the print--interpose a + or
2281 put parentheses around all the arguments.
2283 Note that if you're storing FILEHANDLES in an array or other expression,
2284 you will have to use a block returning its value instead:
2286 print { $files[$i] } "stuff\n";
2287 print { $OK ? STDOUT : STDERR } "stuff\n";
2289 =item printf FILEHANDLE FORMAT, LIST
2291 =item printf FORMAT, LIST
2293 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2294 of the list will be interpreted as the printf format. If C<use locale> is
2295 in effect, the character used for the decimal point in formatted real numbers
2296 is affected by the LC_NUMERIC locale. See L<perllocale>.
2298 Don't fall into the trap of using a printf() when a simple
2299 print() would do. The print() is more efficient, and less
2302 =item prototype FUNCTION
2304 Returns the prototype of a function as a string (or C<undef> if the
2305 function has no prototype). FUNCTION is a reference to, or the name of,
2306 the function whose prototype you want to retrieve.
2308 =item push ARRAY,LIST
2310 Treats ARRAY as a stack, and pushes the values of LIST
2311 onto the end of ARRAY. The length of ARRAY increases by the length of
2312 LIST. Has the same effect as
2315 $ARRAY[++$#ARRAY] = $value;
2318 but is more efficient. Returns the new number of elements in the array.
2328 Generalized quotes. See L<perlop>.
2330 =item quotemeta EXPR
2334 Returns the value of EXPR with all non-alphanumeric
2335 characters backslashed. (That is, all characters not matching
2336 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2337 returned string, regardless of any locale settings.)
2338 This is the internal function implementing
2339 the \Q escape in double-quoted strings.
2341 If EXPR is omitted, uses $_.
2347 Returns a random fractional number greater than or equal to 0 and less
2348 than the value of EXPR. (EXPR should be positive.) If EXPR is
2349 omitted, the value 1 is used. Automatically calls srand() unless
2350 srand() has already been called. See also srand().
2352 (Note: If your rand function consistently returns numbers that are too
2353 large or too small, then your version of Perl was probably compiled
2354 with the wrong number of RANDBITS.)
2356 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2358 =item read FILEHANDLE,SCALAR,LENGTH
2360 Attempts to read LENGTH bytes of data into variable SCALAR from the
2361 specified FILEHANDLE. Returns the number of bytes actually read, or
2362 undef if there was an error. SCALAR will be grown or shrunk to the
2363 length actually read. An OFFSET may be specified to place the read
2364 data at some other place than the beginning of the string. This call
2365 is actually implemented in terms of stdio's fread call. To get a true
2366 read system call, see sysread().
2368 =item readdir DIRHANDLE
2370 Returns the next directory entry for a directory opened by opendir().
2371 If used in a list context, returns all the rest of the entries in the
2372 directory. If there are no more entries, returns an undefined value in
2373 a scalar context or a null list in a list context.
2375 If you're planning to filetest the return values out of a readdir(), you'd
2376 better prepend the directory in question. Otherwise, because we didn't
2377 chdir() there, it would have been testing the wrong file.
2379 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2380 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2385 Reads from the file handle EXPR. In scalar context, a single line
2386 is read and returned. In list context, reads until end-of-file is
2387 reached and returns a list of lines (however you've defined lines
2388 with $/ or $INPUT_RECORD_SEPARATOR).
2389 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2390 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2391 operator is discussed in more detail in L<perlop/"I/O Operators">.
2397 Returns the value of a symbolic link, if symbolic links are
2398 implemented. If not, gives a fatal error. If there is some system
2399 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2404 EXPR is interpolated and then executed as a system command.
2405 The collected standard output of the command is returned.
2406 In scalar context, it comes back as a single (potentially
2407 multi-line) string. In list context, returns a list of lines
2408 (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).
2409 This is the internal function implementing the C<qx/EXPR/>
2410 operator, but you can use it directly. The C<qx/EXPR/>
2411 operator is discussed in more detail in L<perlop/"I/O Operators">.
2413 =item recv SOCKET,SCALAR,LEN,FLAGS
2415 Receives a message on a socket. Attempts to receive LENGTH bytes of
2416 data into variable SCALAR from the specified SOCKET filehandle.
2417 Actually does a C recvfrom(), so that it can returns the address of the
2418 sender. Returns the undefined value if there's an error. SCALAR will
2419 be grown or shrunk to the length actually read. Takes the same flags
2420 as the system call of the same name.
2421 See L<perlipc/"UDP: Message Passing"> for examples.
2427 The C<redo> command restarts the loop block without evaluating the
2428 conditional again. The C<continue> block, if any, is not executed. If
2429 the LABEL is omitted, the command refers to the innermost enclosing
2430 loop. This command is normally used by programs that want to lie to
2431 themselves about what was just input:
2433 # a simpleminded Pascal comment stripper
2434 # (warning: assumes no { or } in strings)
2435 LINE: while (<STDIN>) {
2436 while (s|({.*}.*){.*}|$1 |) {}
2441 if (/}/) { # end of comment?
2454 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2455 is not specified, $_ will be used. The value returned depends on the
2456 type of thing the reference is a reference to.
2457 Builtin types include:
2466 If the referenced object has been blessed into a package, then that package
2467 name is returned instead. You can think of ref() as a typeof() operator.
2469 if (ref($r) eq "HASH") {
2470 print "r is a reference to a hash.\n";
2473 print "r is not a reference at all.\n";
2476 See also L<perlref>.
2478 =item rename OLDNAME,NEWNAME
2480 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2481 not work across file system boundaries.
2487 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2488 supplied. If EXPR is numeric, demands that the current version of Perl
2489 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2491 Otherwise, demands that a library file be included if it hasn't already
2492 been included. The file is included via the do-FILE mechanism, which is
2493 essentially just a variety of eval(). Has semantics similar to the following
2497 local($filename) = @_;
2498 return 1 if $INC{$filename};
2499 local($realfilename,$result);
2501 foreach $prefix (@INC) {
2502 $realfilename = "$prefix/$filename";
2503 if (-f $realfilename) {
2504 $result = do $realfilename;
2508 die "Can't find $filename in \@INC";
2511 die "$filename did not return true value" unless $result;
2512 $INC{$filename} = $realfilename;
2516 Note that the file will not be included twice under the same specified
2517 name. The file must return TRUE as the last statement to indicate
2518 successful execution of any initialization code, so it's customary to
2519 end such a file with "1;" unless you're sure it'll return TRUE
2520 otherwise. But it's better just to put the "C<1;>", in case you add more
2523 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
2524 replaces "F<::>" with "F</>" in the filename for you,
2525 to make it easy to load standard modules. This form of loading of
2526 modules does not risk altering your namespace.
2528 For a yet-more-powerful import facility, see L</use> and
2535 Generally used in a C<continue> block at the end of a loop to clear
2536 variables and reset ?? searches so that they work again. The
2537 expression is interpreted as a list of single characters (hyphens
2538 allowed for ranges). All variables and arrays beginning with one of
2539 those letters are reset to their pristine state. If the expression is
2540 omitted, one-match searches (?pattern?) are reset to match again. Resets
2541 only variables or searches in the current package. Always returns
2544 reset 'X'; # reset all X variables
2545 reset 'a-z'; # reset lower case variables
2546 reset; # just reset ?? searches
2548 Resetting "A-Z" is not recommended because you'll wipe out your
2549 ARGV and ENV arrays. Resets only package variables--lexical variables
2550 are unaffected, but they clean themselves up on scope exit anyway,
2551 so you'll probably want to use them instead. See L</my>.
2557 Returns from a subroutine, eval(), or do FILE with the value of the
2558 given EXPR. Evaluation of EXPR may be in a list, scalar, or void
2559 context, depending on how the return value will be used, and the context
2560 may vary from one execution to the next (see wantarray()). If no EXPR
2561 is given, returns an empty list in a list context, an undefined value in
2562 a scalar context, or nothing in a void context.
2564 (Note that in the absence of a return, a subroutine, eval, or do FILE
2565 will automatically return the value of the last expression evaluated.)
2569 In a list context, returns a list value consisting of the elements
2570 of LIST in the opposite order. In a scalar context, concatenates the
2571 elements of LIST, and returns a string value consisting of those bytes,
2572 but in the opposite order.
2574 print reverse <>; # line tac, last line first
2576 undef $/; # for efficiency of <>
2577 print scalar reverse <>; # byte tac, last line tsrif
2579 This operator is also handy for inverting a hash, although there are some
2580 caveats. If a value is duplicated in the original hash, only one of those
2581 can be represented as a key in the inverted hash. Also, this has to
2582 unwind one hash and build a whole new one, which may take some time
2585 %by_name = reverse %by_address; # Invert the hash
2587 =item rewinddir DIRHANDLE
2589 Sets the current position to the beginning of the directory for the
2590 readdir() routine on DIRHANDLE.
2592 =item rindex STR,SUBSTR,POSITION
2594 =item rindex STR,SUBSTR
2596 Works just like index except that it returns the position of the LAST
2597 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2598 last occurrence at or before that position.
2600 =item rmdir FILENAME
2604 Deletes the directory specified by FILENAME if it is empty. If it
2605 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2606 FILENAME is omitted, uses $_.
2610 The substitution operator. See L<perlop>.
2614 Forces EXPR to be interpreted in a scalar context and returns the value
2617 @counts = ( scalar @a, scalar @b, scalar @c );
2619 There is no equivalent operator to force an expression to
2620 be interpolated in a list context because it's in practice never
2621 needed. If you really wanted to do so, however, you could use
2622 the construction C<@{[ (some expression) ]}>, but usually a simple
2623 C<(some expression)> suffices.
2625 =item seek FILEHANDLE,POSITION,WHENCE
2627 Sets FILEHANDLE's position, just like the fseek() call of stdio.
2628 FILEHANDLE may be an expression whose value gives the name of the
2629 filehandle. The values for WHENCE are 0 to set the new position to
2630 POSITION, 1 to set it to the current position plus POSITION, and 2 to
2631 set it to EOF plus POSITION (typically negative). For WHENCE you may
2632 use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2633 IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2635 If you want to position file for sysread() or syswrite(), don't use
2636 seek() -- buffering makes its effect on the file's system position
2637 unpredictable and non-portable. Use sysseek() instead.
2639 On some systems you have to do a seek whenever you switch between reading
2640 and writing. Amongst other things, this may have the effect of calling
2641 stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2646 This is also useful for applications emulating C<tail -f>. Once you hit
2647 EOF on your read, and then sleep for a while, you might have to stick in a
2648 seek() to reset things. The seek() doesn't change the current position,
2649 but it I<does> clear the end-of-file condition on the handle, so that the
2650 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2652 If that doesn't work (some stdios are particularly cantankerous), then
2653 you may need something more like this:
2656 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2657 # search for some stuff and put it into files
2659 sleep($for_a_while);
2660 seek(FILE, $curpos, 0);
2663 =item seekdir DIRHANDLE,POS
2665 Sets the current position for the readdir() routine on DIRHANDLE. POS
2666 must be a value returned by telldir(). Has the same caveats about
2667 possible directory compaction as the corresponding system library
2670 =item select FILEHANDLE
2674 Returns the currently selected filehandle. Sets the current default
2675 filehandle for output, if FILEHANDLE is supplied. This has two
2676 effects: first, a C<write> or a C<print> without a filehandle will
2677 default to this FILEHANDLE. Second, references to variables related to
2678 output will refer to this output channel. For example, if you have to
2679 set the top of form format for more than one output channel, you might
2687 FILEHANDLE may be an expression whose value gives the name of the
2688 actual filehandle. Thus:
2690 $oldfh = select(STDERR); $| = 1; select($oldfh);
2692 Some programmers may prefer to think of filehandles as objects with
2693 methods, preferring to write the last example as:
2696 STDERR->autoflush(1);
2698 =item select RBITS,WBITS,EBITS,TIMEOUT
2700 This calls the select(2) system call with the bit masks specified, which
2701 can be constructed using fileno() and vec(), along these lines:
2703 $rin = $win = $ein = '';
2704 vec($rin,fileno(STDIN),1) = 1;
2705 vec($win,fileno(STDOUT),1) = 1;
2708 If you want to select on many filehandles you might wish to write a
2712 local(@fhlist) = split(' ',$_[0]);
2715 vec($bits,fileno($_),1) = 1;
2719 $rin = fhbits('STDIN TTY SOCK');
2723 ($nfound,$timeleft) =
2724 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2726 or to block until something becomes ready just do this
2728 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2730 Most systems do not bother to return anything useful in $timeleft, so
2731 calling select() in a scalar context just returns $nfound.
2733 Any of the bit masks can also be undef. The timeout, if specified, is
2734 in seconds, which may be fractional. Note: not all implementations are
2735 capable of returning the $timeleft. If not, they always return
2736 $timeleft equal to the supplied $timeout.
2738 You can effect a sleep of 250 milliseconds this way:
2740 select(undef, undef, undef, 0.25);
2742 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2743 with select(). You have to use sysread() instead.
2745 =item semctl ID,SEMNUM,CMD,ARG
2747 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2748 &GETALL, then ARG must be a variable which will hold the returned
2749 semid_ds structure or semaphore value array. Returns like ioctl: the
2750 undefined value for error, "0 but true" for zero, or the actual return
2753 =item semget KEY,NSEMS,FLAGS
2755 Calls the System V IPC function semget. Returns the semaphore id, or
2756 the undefined value if there is an error.
2758 =item semop KEY,OPSTRING
2760 Calls the System V IPC function semop to perform semaphore operations
2761 such as signaling and waiting. OPSTRING must be a packed array of
2762 semop structures. Each semop structure can be generated with
2763 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2764 operations is implied by the length of OPSTRING. Returns TRUE if
2765 successful, or FALSE if there is an error. As an example, the
2766 following code waits on semaphore $semnum of semaphore id $semid:
2768 $semop = pack("sss", $semnum, -1, 0);
2769 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2771 To signal the semaphore, replace "-1" with "1".
2773 =item send SOCKET,MSG,FLAGS,TO
2775 =item send SOCKET,MSG,FLAGS
2777 Sends a message on a socket. Takes the same flags as the system call
2778 of the same name. On unconnected sockets you must specify a
2779 destination to send TO, in which case it does a C sendto(). Returns
2780 the number of characters sent, or the undefined value if there is an
2782 See L<perlipc/"UDP: Message Passing"> for examples.
2784 =item setpgrp PID,PGRP
2786 Sets the current process group for the specified PID, 0 for the current
2787 process. Will produce a fatal error if used on a machine that doesn't
2788 implement setpgrp(2). If the arguments are omitted, it defaults to
2789 0,0. Note that the POSIX version of setpgrp() does not accept any
2790 arguments, so only setpgrp 0,0 is portable.
2792 =item setpriority WHICH,WHO,PRIORITY
2794 Sets the current priority for a process, a process group, or a user.
2795 (See setpriority(2).) Will produce a fatal error if used on a machine
2796 that doesn't implement setpriority(2).
2798 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2800 Sets the socket option requested. Returns undefined if there is an
2801 error. OPTVAL may be specified as undef if you don't want to pass an
2808 Shifts the first value of the array off and returns it, shortening the
2809 array by 1 and moving everything down. If there are no elements in the
2810 array, returns the undefined value. If ARRAY is omitted, shifts the
2811 @ARGV array in the main program, and the @_ array in subroutines.
2812 (This is determined lexically.) See also unshift(), push(), and pop().
2813 Shift() and unshift() do the same thing to the left end of an array
2814 that pop() and push() do to the right end.
2816 =item shmctl ID,CMD,ARG
2818 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2819 must be a variable which will hold the returned shmid_ds structure.
2820 Returns like ioctl: the undefined value for error, "0 but true" for
2821 zero, or the actual return value otherwise.
2823 =item shmget KEY,SIZE,FLAGS
2825 Calls the System V IPC function shmget. Returns the shared memory
2826 segment id, or the undefined value if there is an error.
2828 =item shmread ID,VAR,POS,SIZE
2830 =item shmwrite ID,STRING,POS,SIZE
2832 Reads or writes the System V shared memory segment ID starting at
2833 position POS for size SIZE by attaching to it, copying in/out, and
2834 detaching from it. When reading, VAR must be a variable which will
2835 hold the data read. When writing, if STRING is too long, only SIZE
2836 bytes are used; if STRING is too short, nulls are written to fill out
2837 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2839 =item shutdown SOCKET,HOW
2841 Shuts down a socket connection in the manner indicated by HOW, which
2842 has the same interpretation as in the system call of the same name.
2848 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2851 For the inverse sine operation, you may use the POSIX::asin()
2852 function, or use this relation:
2854 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2860 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2861 May be interrupted by sending the process a SIGALRM. Returns the
2862 number of seconds actually slept. You probably cannot mix alarm() and
2863 sleep() calls, because sleep() is often implemented using alarm().
2865 On some older systems, it may sleep up to a full second less than what
2866 you requested, depending on how it counts seconds. Most modern systems
2867 always sleep the full amount.
2869 For delays of finer granularity than one second, you may use Perl's
2870 syscall() interface to access setitimer(2) if your system supports it,
2871 or else see L</select()> below.
2873 See also the POSIX module's sigpause() function.
2875 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2877 Opens a socket of the specified kind and attaches it to filehandle
2878 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
2879 system call of the same name. You should "use Socket;" first to get
2880 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2882 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2884 Creates an unnamed pair of sockets in the specified domain, of the
2885 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
2886 for the system call of the same name. If unimplemented, yields a fatal
2887 error. Returns TRUE if successful.
2889 =item sort SUBNAME LIST
2891 =item sort BLOCK LIST
2895 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
2896 is omitted, sorts in standard string comparison order. If SUBNAME is
2897 specified, it gives the name of a subroutine that returns an integer
2898 less than, equal to, or greater than 0, depending on how the elements
2899 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
2900 operators are extremely useful in such routines.) SUBNAME may be a
2901 scalar variable name, in which case the value provides the name of the
2902 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
2903 an anonymous, in-line sort subroutine.
2905 In the interests of efficiency the normal calling code for subroutines is
2906 bypassed, with the following effects: the subroutine may not be a
2907 recursive subroutine, and the two elements to be compared are passed into
2908 the subroutine not via @_ but as the package global variables $a and
2909 $b (see example below). They are passed by reference, so don't
2910 modify $a and $b. And don't try to declare them as lexicals either.
2912 You also cannot exit out of the sort block or subroutine using any of the
2913 loop control operators described in L<perlsyn> or with goto().
2915 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2916 current collation locale. See L<perllocale>.
2921 @articles = sort @files;
2923 # same thing, but with explicit sort routine
2924 @articles = sort {$a cmp $b} @files;
2926 # now case-insensitively
2927 @articles = sort {uc($a) cmp uc($b)} @files;
2929 # same thing in reversed order
2930 @articles = sort {$b cmp $a} @files;
2932 # sort numerically ascending
2933 @articles = sort {$a <=> $b} @files;
2935 # sort numerically descending
2936 @articles = sort {$b <=> $a} @files;
2938 # sort using explicit subroutine name
2940 $age{$a} <=> $age{$b}; # presuming numeric
2942 @sortedclass = sort byage @class;
2944 # this sorts the %age hash by value instead of key
2945 # using an in-line function
2946 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2948 sub backwards { $b cmp $a; }
2949 @harry = ('dog','cat','x','Cain','Abel');
2950 @george = ('gone','chased','yz','Punished','Axed');
2952 # prints AbelCaincatdogx
2953 print sort backwards @harry;
2954 # prints xdogcatCainAbel
2955 print sort @george, 'to', @harry;
2956 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2958 # inefficiently sort by descending numeric compare using
2959 # the first integer after the first = sign, or the
2960 # whole record case-insensitively otherwise
2963 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2968 # same thing, but much more efficiently;
2969 # we'll build auxiliary indices instead
2973 push @nums, /=(\d+)/;
2978 $nums[$b] <=> $nums[$a]
2980 $caps[$a] cmp $caps[$b]
2984 # same thing using a Schwartzian Transform (no temps)
2985 @new = map { $_->[0] }
2986 sort { $b->[1] <=> $a->[1]
2989 } map { [$_, /=(\d+)/, uc($_)] } @old;
2991 If you're using strict, you I<MUST NOT> declare $a
2992 and $b as lexicals. They are package globals. That means
2993 if you're in the C<main> package, it's
2995 @articles = sort {$main::b <=> $main::a} @files;
2999 @articles = sort {$::b <=> $::a} @files;
3001 but if you're in the C<FooPack> package, it's
3003 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3005 The comparison function is required to behave. If it returns
3006 inconsistent results (sometimes saying $x[1] is less than $x[2] and
3007 sometimes saying the opposite, for example) the Perl interpreter will
3008 probably crash and dump core. This is entirely due to and dependent
3009 upon your system's qsort(3) library routine; this routine often avoids
3010 sanity checks in the interest of speed.
3012 =item splice ARRAY,OFFSET,LENGTH,LIST
3014 =item splice ARRAY,OFFSET,LENGTH
3016 =item splice ARRAY,OFFSET
3018 Removes the elements designated by OFFSET and LENGTH from an array, and
3019 replaces them with the elements of LIST, if any. Returns the elements
3020 removed from the array. The array grows or shrinks as necessary. If
3021 LENGTH is omitted, removes everything from OFFSET onward. The
3022 following equivalences hold (assuming C<$[ == 0>):
3024 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
3025 pop(@a) splice(@a,-1)
3026 shift(@a) splice(@a,0,1)
3027 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3028 $a[$x] = $y splice(@a,$x,1,$y);
3030 Example, assuming array lengths are passed before arrays:
3032 sub aeq { # compare two list values
3033 local(@a) = splice(@_,0,shift);
3034 local(@b) = splice(@_,0,shift);
3035 return 0 unless @a == @b; # same len?
3037 return 0 if pop(@a) ne pop(@b);
3041 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3043 =item split /PATTERN/,EXPR,LIMIT
3045 =item split /PATTERN/,EXPR
3047 =item split /PATTERN/
3051 Splits a string into an array of strings, and returns it.
3053 If not in a list context, returns the number of fields found and splits into
3054 the @_ array. (In a list context, you can force the split into @_ by
3055 using C<??> as the pattern delimiters, but it still returns the array
3056 value.) The use of implicit split to @_ is deprecated, however.
3058 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
3059 splits on whitespace (after skipping any leading whitespace). Anything
3060 matching PATTERN is taken to be a delimiter separating the fields. (Note
3061 that the delimiter may be longer than one character.) If LIMIT is
3062 specified and is not negative, splits into no more than that many fields
3063 (though it may split into fewer). If LIMIT is unspecified, trailing null
3064 fields are stripped (which potential users of pop() would do well to
3065 remember). If LIMIT is negative, it is treated as if an arbitrarily large
3066 LIMIT had been specified.
3068 A pattern matching the null string (not to be confused with
3069 a null pattern C<//>, which is just one member of the set of patterns
3070 matching a null string) will split the value of EXPR into separate
3071 characters at each point it matches that way. For example:
3073 print join(':', split(/ */, 'hi there'));
3075 produces the output 'h:i:t:h:e:r:e'.
3077 The LIMIT parameter can be used to split a line partially
3079 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3081 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3082 one larger than the number of variables in the list, to avoid
3083 unnecessary work. For the list above LIMIT would have been 4 by
3084 default. In time critical applications it behooves you not to split
3085 into more fields than you really need.
3087 If the PATTERN contains parentheses, additional array elements are
3088 created from each matching substring in the delimiter.
3090 split(/([,-])/, "1-10,20", 3);
3092 produces the list value
3094 (1, '-', 10, ',', 20)
3096 If you had the entire header of a normal Unix email message in $header,
3097 you could split it up into fields and their values this way:
3099 $header =~ s/\n\s+/ /g; # fix continuation lines
3100 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
3102 The pattern C</PATTERN/> may be replaced with an expression to specify
3103 patterns that vary at runtime. (To do runtime compilation only once,
3104 use C</$variable/o>.)
3106 As a special case, specifying a PATTERN of space (C<' '>) will split on
3107 white space just as split with no arguments does. Thus, split(' ') can
3108 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3109 will give you as many null initial fields as there are leading spaces.
3110 A split on /\s+/ is like a split(' ') except that any leading
3111 whitespace produces a null first field. A split with no arguments
3112 really does a C<split(' ', $_)> internally.
3116 open(passwd, '/etc/passwd');
3118 ($login, $passwd, $uid, $gid, $gcos,
3119 $home, $shell) = split(/:/);
3123 (Note that $shell above will still have a newline on it. See L</chop>,
3124 L</chomp>, and L</join>.)
3126 =item sprintf FORMAT, LIST
3128 Returns a string formatted by the usual printf conventions of the
3129 C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3130 on your system for an explanation of the general principles.
3132 Perl does all of its own sprintf() formatting -- it emulates the C
3133 function sprintf(), but it doesn't use it (except for floating-point
3134 numbers, and even then only the standard modifiers are allowed). As a
3135 result, any non-standard extensions in your local sprintf() are not
3136 available from Perl.
3138 Perl's sprintf() permits the following universally-known conversions:
3141 %c a character with the given number
3143 %d a signed integer, in decimal
3144 %u an unsigned integer, in decimal
3145 %o an unsigned integer, in octal
3146 %x an unsigned integer, in hexadecimal
3147 %e a floating-point number, in scientific notation
3148 %f a floating-point number, in fixed decimal notation
3149 %g a floating-point number, in %e or %f notation
3151 In addition, Perl permits the following widely-supported conversions:
3153 %X like %x, but using upper-case letters
3154 %E like %e, but using an upper-case "E"
3155 %G like %g, but with an upper-case "E" (if applicable)
3156 %p a pointer (outputs the Perl value's address in hexadecimal)
3157 %n special: *stores* the number of characters output so far
3158 into the next variable in the parameter list
3160 Finally, for backward (and we do mean "backward") compatibility, Perl
3161 permits these unnecessary but widely-supported conversions:
3164 %D a synonym for %ld
3165 %U a synonym for %lu
3166 %O a synonym for %lo
3169 Perl permits the following universally-known flags between the C<%>
3170 and the conversion letter:
3172 space prefix positive number with a space
3173 + prefix positive number with a plus sign
3174 - left-justify within the field
3175 0 use zeros, not spaces, to right-justify
3176 # prefix octal with "0", hex with "0x"
3177 number minimum field width
3178 .number "precision": digits after decimal point for floating-point,
3179 max length for string, minimum length for integer
3180 l interpret integer as C type "long" or "unsigned long"
3181 h interpret integer as C type "short" or "unsigned short"
3183 There is also one Perl-specific flag:
3185 V interpret integer as Perl's standard integer type
3187 Where a number would appear in the flags, an asterisk ("*") may be
3188 used instead, in which case Perl uses the next item in the parameter
3189 list as the given number (that is, as the field width or precision).
3190 If a field width obtained through "*" is negative, it has the same
3191 effect as the '-' flag: left-justification.
3193 If C<use locale> is in effect, the character used for the decimal
3194 point in formatted real numbers is affected by the LC_NUMERIC locale.
3201 Return the square root of EXPR. If EXPR is omitted, returns square
3208 Sets the random number seed for the C<rand> operator. If EXPR is
3209 omitted, uses a semi-random value based on the current time and process
3210 ID, among other things. In versions of Perl prior to 5.004 the default
3211 seed was just the current time(). This isn't a particularly good seed,
3212 so many old programs supply their own seed value (often C<time ^ $$> or
3213 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
3215 In fact, it's usually not necessary to call srand() at all, because if
3216 it is not called explicitly, it is called implicitly at the first use of
3217 the C<rand> operator. However, this was not the case in version of Perl
3218 before 5.004, so if your script will run under older Perl versions, it
3219 should call srand().
3221 Note that you need something much more random than the default seed for
3222 cryptographic purposes. Checksumming the compressed output of one or more
3223 rapidly changing operating system status programs is the usual method. For
3226 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3228 If you're particularly concerned with this, see the Math::TrulyRandom
3231 Do I<not> call srand() multiple times in your program unless you know
3232 exactly what you're doing and why you're doing it. The point of the
3233 function is to "seed" the rand() function so that rand() can produce
3234 a different sequence each time you run your program. Just do it once at the
3235 top of your program, or you I<won't> get random numbers out of rand()!
3237 Frequently called programs (like CGI scripts) that simply use
3241 for a seed can fall prey to the mathematical property that
3245 one-third of the time. So don't do that.
3247 =item stat FILEHANDLE
3253 Returns a 13-element array giving the status info for a file, either the
3254 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
3255 stats $_. Returns a null list if the stat fails. Typically used as
3259 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3260 $atime,$mtime,$ctime,$blksize,$blocks)
3263 Not all fields are supported on all filesystem types. Here are the
3264 meaning of the fields:
3266 0 dev device number of filesystem
3268 2 mode file mode (type and permissions)
3269 3 nlink number of (hard) links to the file
3270 4 uid numeric user ID of file's owner
3271 5 gid numeric group ID of file's owner
3272 6 rdev the device identifier (special files only)
3273 7 size total size of file, in bytes
3274 8 atime last access time since the epoch
3275 9 mtime last modify time since the epoch
3276 10 ctime inode change time (NOT creation time!) since the epoch
3277 11 blksize preferred block size for file system I/O
3278 12 blocks actual number of blocks allocated
3280 (The epoch was at 00:00 January 1, 1970 GMT.)
3282 If stat is passed the special filehandle consisting of an underline, no
3283 stat is done, but the current contents of the stat structure from the
3284 last stat or filetest are returned. Example:
3286 if (-x $file && (($d) = stat(_)) && $d < 0) {
3287 print "$file is executable NFS file\n";
3290 (This works on machines only for which the device number is negative under NFS.)
3296 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3297 doing many pattern matches on the string before it is next modified.
3298 This may or may not save time, depending on the nature and number of
3299 patterns you are searching on, and on the distribution of character
3300 frequencies in the string to be searched -- you probably want to compare
3301 run times with and without it to see which runs faster. Those loops
3302 which scan for many short constant strings (including the constant
3303 parts of more complex patterns) will benefit most. You may have only
3304 one study active at a time -- if you study a different scalar the first
3305 is "unstudied". (The way study works is this: a linked list of every
3306 character in the string to be searched is made, so we know, for
3307 example, where all the 'k' characters are. From each search string,
3308 the rarest character is selected, based on some static frequency tables
3309 constructed from some C programs and English text. Only those places
3310 that contain this "rarest" character are examined.)
3312 For example, here is a loop which inserts index producing entries
3313 before any line containing a certain pattern:
3317 print ".IX foo\n" if /\bfoo\b/;
3318 print ".IX bar\n" if /\bbar\b/;
3319 print ".IX blurfl\n" if /\bblurfl\b/;
3324 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3325 will be looked at, because "f" is rarer than "o". In general, this is
3326 a big win except in pathological cases. The only question is whether
3327 it saves you more time than it took to build the linked list in the
3330 Note that if you have to look for strings that you don't know till
3331 runtime, you can build an entire loop as a string and eval that to
3332 avoid recompiling all your patterns all the time. Together with
3333 undefining $/ to input entire files as one record, this can be very
3334 fast, often faster than specialized programs like fgrep(1). The following
3335 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3336 out the names of those files that contain a match:
3338 $search = 'while (<>) { study;';
3339 foreach $word (@words) {
3340 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3345 eval $search; # this screams
3346 $/ = "\n"; # put back to normal input delimiter
3347 foreach $file (sort keys(%seen)) {
3355 =item sub NAME BLOCK
3357 This is subroutine definition, not a real function I<per se>. With just a
3358 NAME (and possibly prototypes), it's just a forward declaration. Without
3359 a NAME, it's an anonymous function declaration, and does actually return a
3360 value: the CODE ref of the closure you just created. See L<perlsub> and
3361 L<perlref> for details.
3363 =item substr EXPR,OFFSET,LEN
3365 =item substr EXPR,OFFSET
3367 Extracts a substring out of EXPR and returns it. First character is at
3368 offset 0, or whatever you've set C<$[> to (but don't do that).
3369 If OFFSET is negative (or more precisely, less than C<$[>), starts
3370 that far from the end of the string. If LEN is omitted, returns
3371 everything to the end of the string. If LEN is negative, leaves that
3372 many characters off the end of the string.
3374 If you specify a substring which is partly outside the string, the part
3375 within the string is returned. If the substring is totally outside
3376 the string a warning is produced.
3378 You can use the substr() function
3379 as an lvalue, in which case EXPR must be an lvalue. If you assign
3380 something shorter than LEN, the string will shrink, and if you assign
3381 something longer than LEN, the string will grow to accommodate it. To
3382 keep the string the same length you may need to pad or chop your value
3385 =item symlink OLDFILE,NEWFILE
3387 Creates a new filename symbolically linked to the old filename.
3388 Returns 1 for success, 0 otherwise. On systems that don't support
3389 symbolic links, produces a fatal error at run time. To check for that,
3392 $symlink_exists = (eval {symlink("","")};, $@ eq '');
3396 Calls the system call specified as the first element of the list,
3397 passing the remaining elements as arguments to the system call. If
3398 unimplemented, produces a fatal error. The arguments are interpreted
3399 as follows: if a given argument is numeric, the argument is passed as
3400 an int. If not, the pointer to the string value is passed. You are
3401 responsible to make sure a string is pre-extended long enough to
3402 receive any result that might be written into a string. If your
3403 integer arguments are not literals and have never been interpreted in a
3404 numeric context, you may need to add 0 to them to force them to look
3407 require 'syscall.ph'; # may need to run h2ph
3408 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3410 Note that Perl supports passing of up to only 14 arguments to your system call,
3411 which in practice should usually suffice.
3413 =item sysopen FILEHANDLE,FILENAME,MODE
3415 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3417 Opens the file whose filename is given by FILENAME, and associates it
3418 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3419 the name of the real filehandle wanted. This function calls the
3420 underlying operating system's C<open> function with the parameters
3421 FILENAME, MODE, PERMS.
3423 The possible values and flag bits of the MODE parameter are
3424 system-dependent; they are available via the standard module C<Fcntl>.
3425 However, for historical reasons, some values are universal: zero means
3426 read-only, one means write-only, and two means read/write.
3428 If the file named by FILENAME does not exist and the C<open> call
3429 creates it (typically because MODE includes the O_CREAT flag), then
3430 the value of PERMS specifies the permissions of the newly created
3431 file. If PERMS is omitted, the default value is 0666, which allows
3432 read and write for all. This default is reasonable: see C<umask>.
3434 The IO::File module provides a more object-oriented approach, if you're
3435 into that kind of thing.
3437 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3439 =item sysread FILEHANDLE,SCALAR,LENGTH
3441 Attempts to read LENGTH bytes of data into variable SCALAR from the
3442 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
3443 so mixing this with other kinds of reads, print(), write(), seek(), or
3444 tell() can cause confusion. Returns the number of bytes actually read,
3445 or undef if there was an error. SCALAR will be grown or shrunk so that
3446 the last byte actually read is the last byte of the scalar after the
3449 An OFFSET may be specified to place the read data at some place in the
3450 string other than the beginning. A negative OFFSET specifies
3451 placement at that many bytes counting backwards from the end of the
3452 string. A positive OFFSET greater than the length of SCALAR results
3453 in the string being padded to the required size with "\0" bytes before
3454 the result of the read is appended.
3456 =item sysseek FILEHANDLE,POSITION,WHENCE
3458 Sets FILEHANDLE's system position using the system call lseek(2). It
3459 bypasses stdio, so mixing this with reads (other than sysread()),
3460 print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may
3461 be an expression whose value gives the name of the filehandle. The
3462 values for WHENCE are 0 to set the new position to POSITION, 1 to set
3463 the it to the current position plus POSITION, and 2 to set it to EOF
3464 plus POSITION (typically negative). For WHENCE, you may use the
3465 constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable
3466 or the POSIX module.
3468 Returns the new position, or the undefined value on failure. A position
3469 of zero is returned as the string "0 but true"; thus sysseek() returns
3470 TRUE on success and FALSE on failure, yet you can still easily determine
3475 Does exactly the same thing as "exec LIST" except that a fork is done
3476 first, and the parent process waits for the child process to complete.
3477 Note that argument processing varies depending on the number of
3478 arguments. The return value is the exit status of the program as
3479 returned by the wait() call. To get the actual exit value divide by
3480 256. See also L</exec>. This is I<NOT> what you want to use to capture
3481 the output from a command, for that you should use merely backticks or
3482 qx//, as described in L<perlop/"`STRING`">.
3484 Because system() and backticks block SIGINT and SIGQUIT, killing the
3485 program they're running doesn't actually interrupt your program.
3487 @args = ("command", "arg1", "arg2");
3489 or die "system @args failed: $?"
3491 Here's a more elaborate example of analysing the return value from
3492 system() on a Unix system to check for all possibilities, including for
3493 signals and core dumps.
3495 $rc = 0xffff & system @args;
3496 printf "system(%s) returned %#04x: ", "@args", $rc;
3498 print "ran with normal exit\n";
3500 elsif ($rc == 0xff00) {
3501 print "command failed: $!\n";
3503 elsif ($rc > 0x80) {
3505 print "ran with non-zero exit status $rc\n";
3511 print "core dump from ";
3513 print "signal $rc\n"
3517 When the arguments get executed via the system shell, results will
3518 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
3521 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3523 =item syswrite FILEHANDLE,SCALAR,LENGTH
3525 Attempts to write LENGTH bytes of data from variable SCALAR to the
3526 specified FILEHANDLE, using the system call write(2). It bypasses
3527 stdio, so mixing this with reads (other than sysread()), print(),
3528 write(), seek(), or tell() may cause confusion. Returns the number of
3529 bytes actually written, or undef if there was an error. If the length
3530 is greater than the available data, only as much data as is available
3533 An OFFSET may be specified to write the data from some part of the
3534 string other than the beginning. A negative OFFSET specifies writing
3535 that many bytes counting backwards from the end of the string.
3537 =item tell FILEHANDLE
3541 Returns the current position for FILEHANDLE. FILEHANDLE may be an
3542 expression whose value gives the name of the actual filehandle. If
3543 FILEHANDLE is omitted, assumes the file last read.
3545 =item telldir DIRHANDLE
3547 Returns the current position of the readdir() routines on DIRHANDLE.
3548 Value may be given to seekdir() to access a particular location in a
3549 directory. Has the same caveats about possible directory compaction as
3550 the corresponding system library routine.
3552 =item tie VARIABLE,CLASSNAME,LIST
3554 This function binds a variable to a package class that will provide the
3555 implementation for the variable. VARIABLE is the name of the variable
3556 to be enchanted. CLASSNAME is the name of a class implementing objects
3557 of correct type. Any additional arguments are passed to the "new"
3558 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3559 Typically these are arguments such as might be passed to the dbm_open()
3560 function of C. The object returned by the "new" method is also
3561 returned by the tie() function, which would be useful if you want to
3562 access other methods in CLASSNAME.
3564 Note that functions such as keys() and values() may return huge array
3565 values when used on large objects, like DBM files. You may prefer to
3566 use the each() function to iterate over such. Example:
3568 # print out history file offsets
3570 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3571 while (($key,$val) = each %HIST) {
3572 print $key, ' = ', unpack('L',$val), "\n";
3576 A class implementing a hash should have the following methods:
3578 TIEHASH classname, LIST
3581 STORE this, key, value
3585 NEXTKEY this, lastkey
3587 A class implementing an ordinary array should have the following methods:
3589 TIEARRAY classname, LIST
3592 STORE this, key, value
3595 A class implementing a scalar should have the following methods:
3597 TIESCALAR classname, LIST
3602 Unlike dbmopen(), the tie() function will not use or require a module
3603 for you--you need to do that explicitly yourself. See L<DB_File>
3604 or the F<Config> module for interesting tie() implementations.
3608 Returns a reference to the object underlying VARIABLE (the same value
3609 that was originally returned by the tie() call which bound the variable
3610 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3615 Returns the number of non-leap seconds since whatever time the system
3616 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3617 and 00:00:00 UTC, January 1, 1970 for most other systems).
3618 Suitable for feeding to gmtime() and localtime().
3622 Returns a four-element array giving the user and system times, in
3623 seconds, for this process and the children of this process.
3625 ($user,$system,$cuser,$csystem) = times;
3629 The translation operator. Same as y///. See L<perlop>.
3631 =item truncate FILEHANDLE,LENGTH
3633 =item truncate EXPR,LENGTH
3635 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3636 specified length. Produces a fatal error if truncate isn't implemented
3643 Returns an uppercased version of EXPR. This is the internal function
3644 implementing the \U escape in double-quoted strings.
3645 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3647 If EXPR is omitted, uses $_.
3653 Returns the value of EXPR with the first character uppercased. This is
3654 the internal function implementing the \u escape in double-quoted strings.
3655 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3657 If EXPR is omitted, uses $_.
3663 Sets the umask for the process to EXPR and returns the previous value.
3664 If EXPR is omitted, merely returns the current umask. Remember that a
3665 umask is a number, usually given in octal; it is I<not> a string of octal
3666 digits. See also L</oct>, if all you have is a string.
3672 Undefines the value of EXPR, which must be an lvalue. Use only on a
3673 scalar value, an entire array, an entire hash, or a subroutine name (using
3674 "&"). (Using undef() will probably not do what you expect on most
3675 predefined variables or DBM list values, so don't do that.) Always
3676 returns the undefined value. You can omit the EXPR, in which case
3677 nothing is undefined, but you still get an undefined value that you
3678 could, for instance, return from a subroutine, assign to a variable or
3679 pass as a parameter. Examples:
3682 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
3686 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
3687 select undef, undef, undef, 0.25;
3688 ($a, $b, undef, $c) = &foo; # Ignore third value returned
3694 Deletes a list of files. Returns the number of files successfully
3697 $cnt = unlink 'a', 'b', 'c';
3701 Note: unlink will not delete directories unless you are superuser and
3702 the B<-U> flag is supplied to Perl. Even if these conditions are
3703 met, be warned that unlinking a directory can inflict damage on your
3704 filesystem. Use rmdir instead.
3706 If LIST is omitted, uses $_.
3708 =item unpack TEMPLATE,EXPR
3710 Unpack does the reverse of pack: it takes a string representing a
3711 structure and expands it out into a list value, returning the array
3712 value. (In a scalar context, it returns merely the first value
3713 produced.) The TEMPLATE has the same format as in the pack function.
3714 Here's a subroutine that does substring:
3717 local($what,$where,$howmuch) = @_;
3718 unpack("x$where a$howmuch", $what);
3723 sub ordinal { unpack("c",$_[0]); } # same as ord()
3725 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3726 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3727 themselves. Default is a 16-bit checksum. For example, the following
3728 computes the same number as the System V sum program:
3731 $checksum += unpack("%16C*", $_);
3735 The following efficiently counts the number of set bits in a bit vector:
3737 $setbits = unpack("%32b*", $selectmask);
3739 =item untie VARIABLE
3741 Breaks the binding between a variable and a package. (See tie().)
3743 =item unshift ARRAY,LIST
3745 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3746 depending on how you look at it. Prepends list to the front of the
3747 array, and returns the new number of elements in the array.
3749 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3751 Note the LIST is prepended whole, not one element at a time, so the
3752 prepended elements stay in the same order. Use reverse to do the
3755 =item use Module LIST
3759 =item use Module VERSION LIST
3763 Imports some semantics into the current package from the named module,
3764 generally by aliasing certain subroutine or variable names into your
3765 package. It is exactly equivalent to
3767 BEGIN { require Module; import Module LIST; }
3769 except that Module I<must> be a bareword.
3771 If the first argument to C<use> is a number, it is treated as a version
3772 number instead of a module name. If the version of the Perl interpreter
3773 is less than VERSION, then an error message is printed and Perl exits
3774 immediately. This is often useful if you need to check the current
3775 Perl version before C<use>ing library modules which have changed in
3776 incompatible ways from older versions of Perl. (We try not to do
3777 this more than we have to.)
3779 The BEGIN forces the require and import to happen at compile time. The
3780 require makes sure the module is loaded into memory if it hasn't been
3781 yet. The import is not a builtin--it's just an ordinary static method
3782 call into the "Module" package to tell the module to import the list of
3783 features back into the current package. The module can implement its
3784 import method any way it likes, though most modules just choose to
3785 derive their import method via inheritance from the Exporter class that
3786 is defined in the Exporter module. See L<Exporter>. If no import
3787 method can be found then the error is currently silently ignored. This
3788 may change to a fatal error in a future version.
3790 If you don't want your namespace altered, explicitly supply an empty list:
3794 That is exactly equivalent to
3796 BEGIN { require Module; }
3798 If the VERSION argument is present between Module and LIST, then the
3799 C<use> will call the VERSION method in class Module with the given
3800 version as an argument. The default VERSION method, inherited from
3801 the Universal class, croaks if the given version is larger than the
3802 value of the variable $Module::VERSION. (Note that there is not a
3803 comma after VERSION!)
3805 Because this is a wide-open interface, pragmas (compiler directives)
3806 are also implemented this way. Currently implemented pragmas are:
3810 use sigtrap qw(SEGV BUS);
3811 use strict qw(subs vars refs);
3812 use subs qw(afunc blurfl);
3814 These pseudo-modules import semantics into the current block scope, unlike
3815 ordinary modules, which import symbols into the current package (which are
3816 effective through the end of the file).
3818 There's a corresponding "no" command that unimports meanings imported
3819 by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
3824 If no unimport method can be found the call fails with a fatal error.
3826 See L<perlmod> for a list of standard modules and pragmas.
3830 Changes the access and modification times on each file of a list of
3831 files. The first two elements of the list must be the NUMERICAL access
3832 and modification times, in that order. Returns the number of files
3833 successfully changed. The inode modification time of each file is set
3834 to the current time. Example of a "touch" command:
3838 utime $now, $now, @ARGV;
3842 Returns a normal array consisting of all the values of the named hash.
3843 (In a scalar context, returns the number of values.) The values are
3844 returned in an apparently random order, but it is the same order as either
3845 the keys() or each() function would produce on the same hash. As a side
3846 effect, it resets HASH's iterator. See also keys(), each(), and sort().
3848 =item vec EXPR,OFFSET,BITS
3850 Treats the string in EXPR as a vector of unsigned integers, and
3851 returns the value of the bit field specified by OFFSET. BITS specifies
3852 the number of bits that are reserved for each entry in the bit
3853 vector. This must be a power of two from 1 to 32. vec() may also be
3854 assigned to, in which case parentheses are needed to give the expression
3855 the correct precedence as in
3857 vec($image, $max_x * $x + $y, 8) = 3;
3859 Vectors created with vec() can also be manipulated with the logical
3860 operators |, &, and ^, which will assume a bit vector operation is
3861 desired when both operands are strings.
3863 To transform a bit vector into a string or array of 0's and 1's, use these:
3865 $bits = unpack("b*", $vector);
3866 @bits = split(//, unpack("b*", $vector));
3868 If you know the exact length in bits, it can be used in place of the *.
3872 Waits for a child process to terminate and returns the pid of the
3873 deceased process, or -1 if there are no child processes. The status is
3876 =item waitpid PID,FLAGS
3878 Waits for a particular child process to terminate and returns the pid
3879 of the deceased process, or -1 if there is no such child process. The
3880 status is returned in C<$?>. If you say
3882 use POSIX ":sys_wait_h";
3884 waitpid(-1,&WNOHANG);
3886 then you can do a non-blocking wait for any process. Non-blocking wait
3887 is available on machines supporting either the waitpid(2) or
3888 wait4(2) system calls. However, waiting for a particular pid with
3889 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3890 by remembering the status values of processes that have exited but have
3891 not been harvested by the Perl script yet.)
3895 Returns TRUE if the context of the currently executing subroutine is
3896 looking for a list value. Returns FALSE if the context is looking
3897 for a scalar. Returns the undefined value if the context is looking
3898 for no value (void context).
3900 return unless defined wantarray; # don't bother doing more
3901 my @a = complex_calculation();
3902 return wantarray ? @a : "@a";
3906 Produces a message on STDERR just like die(), but doesn't exit or throw
3909 No message is printed if there is a C<$SIG{__WARN__}> handler
3910 installed. It is the handler's responsibility to deal with the message
3911 as it sees fit (like, for instance, converting it into a die()). Most
3912 handlers must therefore make arrangements to actually display the
3913 warnings that they are not prepared to deal with, by calling warn()
3914 again in the handler. Note that this is quite safe and will not
3915 produce an endless loop, since C<__WARN__> hooks are not called from
3918 You will find this behavior is slightly different from that of
3919 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
3920 instead call die() again to change it).
3922 Using a C<__WARN__> handler provides a powerful way to silence all
3923 warnings (even the so-called mandatory ones). An example:
3925 # wipe out *all* compile-time warnings
3926 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
3928 my $foo = 20; # no warning about duplicate my $foo,
3929 # but hey, you asked for it!
3930 # no compile-time or run-time warnings before here
3933 # run-time warnings enabled after here
3934 warn "\$foo is alive and $foo!"; # does show up
3936 See L<perlvar> for details on setting C<%SIG> entries, and for more
3939 =item write FILEHANDLE
3945 Writes a formatted record (possibly multi-line) to the specified file,
3946 using the format associated with that file. By default the format for
3947 a file is the one having the same name as the filehandle, but the
3948 format for the current output channel (see the select() function) may be set
3949 explicitly by assigning the name of the format to the C<$~> variable.
3951 Top of form processing is handled automatically: if there is
3952 insufficient room on the current page for the formatted record, the
3953 page is advanced by writing a form feed, a special top-of-page format
3954 is used to format the new page header, and then the record is written.
3955 By default the top-of-page format is the name of the filehandle with
3956 "_TOP" appended, but it may be dynamically set to the format of your
3957 choice by assigning the name to the C<$^> variable while the filehandle is
3958 selected. The number of lines remaining on the current page is in
3959 variable C<$->, which can be set to 0 to force a new page.
3961 If FILEHANDLE is unspecified, output goes to the current default output
3962 channel, which starts out as STDOUT but may be changed by the
3963 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3964 is evaluated and the resulting string is used to look up the name of
3965 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3967 Note that write is I<NOT> the opposite of read. Unfortunately.
3971 The translation operator. Same as tr///. See L<perlop>.
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