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+3; # Prints 6.
39 print(1+2) + 3; # Prints 3.
40 print (1+2)+3; # Also prints 3!
41 print +(1+2)+3; # Prints 6.
42 print ((1+2)+3); # Prints 6.
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 non-abortive 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,
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 non-zero 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()> below. It is not advised 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($_[0]) }
361 =item bind SOCKET,NAME
363 Binds a network address to a socket, just as the bind system call
364 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
365 packed address of the appropriate type for the socket. See the examples in
366 L<perlipc/"Sockets: Client/Server Communication">.
368 =item binmode FILEHANDLE
370 Arranges for the file to be read or written in "binary" mode in operating
371 systems that distinguish between binary and text files. Files that are
372 not in binary mode have CR LF sequences translated to LF on input and LF
373 translated to CR LF on output. Binmode has no effect under Unix; in DOS
374 and similarly archaic systems, it may be imperative--otherwise your
375 DOS-damaged C library may mangle your file. The key distinction between
376 systems that need binmode and those that don't is their text file
377 formats. Systems like Unix and Plan9 that delimit lines with a single
378 character, and that encode that character in C as '\n', do not need
379 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
380 is taken as the name of the filehandle.
382 =item bless REF,CLASSNAME
386 This function tells the thingy referenced by REF that it is now
387 an object in the CLASSNAME package--or the current package if no CLASSNAME
388 is specified, which is often the case. It returns the reference for
389 convenience, because a bless() is often the last thing in a constructor.
390 Always use the two-argument version if the function doing the blessing
391 might be inherited by a derived class. See L<perlobj> for more about the
392 blessing (and blessings) of objects.
398 Returns the context of the current subroutine call. In a scalar context,
399 returns the caller's package name if there is a caller, that is, if
400 we're in a subroutine or eval() or require(), and the undefined value
401 otherwise. In a list context, returns
403 ($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 C<L<eval>>. In such a case additional elements $evaltext and
414 $is_require are set: $is_require is true if the frame is created by
415 C<L<require>> or C<L<use>> statement, $evaltext contains the text of
416 C<L<eval EXPR>> statement. In particular, for C<L<eval BLOCK>>
417 statement $filename is C<"(eval)">, but $evaltext is undefined. (Note
418 also that C<L<use>> statement creates a C<L<require>> frame inside
419 an C<L<eval EXPR>>) frame.
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 that 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. Returns the number of files successfully changed.
437 $cnt = chmod 0755, 'foo', 'bar';
438 chmod 0755, @executables;
446 This is a slightly safer version of chop (see below). It removes any
447 line ending that corresponds to the current value of C<$/> (also known as
448 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
449 number of characters removed from all its arguments. It's often used to
450 remove the newline from the end of an input record when you're worried
451 that the final record may be missing its newline. When in paragraph mode
452 (C<$/ = "">), it removes all trailing newlines from the string. If
453 VARIABLE is omitted, it chomps $_. Example:
456 chomp; # avoid \n on last field
461 You can actually chomp anything that's an lvalue, including an assignment:
464 chomp($answer = <STDIN>);
466 If you chomp a list, each element is chomped, and the total number of
467 characters removed is returned.
475 Chops off the last character of a string and returns the character
476 chopped. It's used primarily to remove the newline from the end of an
477 input record, but is much more efficient than C<s/\n//> because it neither
478 scans nor copies the string. If VARIABLE is omitted, chops $_.
482 chop; # avoid \n on last field
487 You can actually chop anything that's an lvalue, including an assignment:
490 chop($answer = <STDIN>);
492 If you chop a list, each element is chopped. Only the value of the
493 last chop is returned.
495 Note that chop returns the last character. To return all but the last
496 character, use C<substr($string, 0, -1)>.
500 Changes the owner (and group) of a list of files. The first two
501 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
502 Returns the number of files successfully changed.
504 $cnt = chown $uid, $gid, 'foo', 'bar';
505 chown $uid, $gid, @filenames;
507 Here's an example that looks up non-numeric uids in the passwd file:
510 chop($user = <STDIN>);
512 chop($pattern = <STDIN>);
514 ($login,$pass,$uid,$gid) = getpwnam($user)
515 or die "$user not in passwd file";
517 @ary = <${pattern}>; # expand filenames
518 chown $uid, $gid, @ary;
520 On most systems, you are not allowed to change the ownership of the
521 file unless you're the superuser, although you should be able to change
522 the group to any of your secondary groups. On insecure systems, these
523 restrictions may be relaxed, but this is not a portable assumption.
529 Returns the character represented by that NUMBER in the character set.
530 For example, C<chr(65)> is "A" in ASCII.
532 If NUMBER is omitted, uses $_.
534 =item chroot FILENAME
538 This function works as the system call by the same name: it makes the
539 named directory the new root directory for all further pathnames that
540 begin with a "/" by your process and all of its children. (It doesn't
541 change your current working directory, which is unaffected.) For security
542 reasons, this call is restricted to the superuser. If FILENAME is
543 omitted, does chroot to $_.
545 =item close FILEHANDLE
547 Closes the file or pipe associated with the file handle, returning TRUE
548 only if stdio successfully flushes buffers and closes the system file
549 descriptor. You don't have to close FILEHANDLE if you are immediately
550 going to do another open() on it, because open() will close it for you. (See
551 open().) However, an explicit close on an input file resets the line
552 counter ($.), while the implicit close done by open() does not. Also,
553 closing a pipe will wait for the process executing on the pipe to
554 complete, in case you want to look at the output of the pipe
555 afterwards. Closing a pipe explicitly also puts the status value of
556 the command into C<$?>. Example:
558 open(OUTPUT, '|sort >foo'); # pipe to sort
559 ... # print stuff to output
560 close OUTPUT; # wait for sort to finish
561 open(INPUT, 'foo'); # get sort's results
563 FILEHANDLE may be an expression whose value gives the real filehandle name.
565 =item closedir DIRHANDLE
567 Closes a directory opened by opendir().
569 =item connect SOCKET,NAME
571 Attempts to connect to a remote socket, just as the connect system call
572 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
573 packed address of the appropriate type for the socket. See the examples in
574 L<perlipc/"Sockets: Client/Server Communication">.
578 Actually a flow control statement rather than a function. If there is a
579 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
580 C<foreach>), it is always executed just before the conditional is about to
581 be evaluated again, just like the third part of a C<for> loop in C. Thus
582 it can be used to increment a loop variable, even when the loop has been
583 continued via the C<next> statement (which is similar to the C C<continue>
588 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
591 For the inverse cosine operation, you may use the POSIX::acos()
592 function, or use this relation:
594 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
596 =item crypt PLAINTEXT,SALT
598 Encrypts a string exactly like the crypt(3) function in the C library
599 (assuming that you actually have a version there that has not been
600 extirpated as a potential munition). This can prove useful for checking
601 the password file for lousy passwords, amongst other things. Only the
602 guys wearing white hats should do this.
604 Here's an example that makes sure that whoever runs this program knows
607 $pwd = (getpwuid($<))[1];
608 $salt = substr($pwd, 0, 2);
612 chop($word = <STDIN>);
616 if (crypt($word, $salt) ne $pwd) {
622 Of course, typing in your own password to whomever asks you
627 [This function has been superseded by the untie() function.]
629 Breaks the binding between a DBM file and a hash.
631 =item dbmopen HASH,DBNAME,MODE
633 [This function has been superseded by the tie() function.]
635 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a
636 hash. HASH is the name of the hash. (Unlike normal open, the first
637 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
638 is the name of the database (without the F<.dir> or F<.pag> extension if
639 any). If the database does not exist, it is created with protection
640 specified by MODE (as modified by the umask()). If your system supports
641 only the older DBM functions, you may perform only one dbmopen() in your
642 program. In older versions of Perl, if your system had neither DBM nor
643 ndbm, calling dbmopen() produced a fatal error; it now falls back to
646 If you don't have write access to the DBM file, you can only read hash
647 variables, not set them. If you want to test whether you can write,
648 either use file tests or try setting a dummy hash entry inside an eval(),
649 which will trap the error.
651 Note that functions such as keys() and values() may return huge array
652 values when used on large DBM files. You may prefer to use the each()
653 function to iterate over large DBM files. Example:
655 # print out history file offsets
656 dbmopen(%HIST,'/usr/lib/news/history',0666);
657 while (($key,$val) = each %HIST) {
658 print $key, ' = ', unpack('L',$val), "\n";
662 See also L<AnyDBM_File> for a more general description of the pros and
663 cons of the various dbm approaches, as well as L<DB_File> for a particularly
670 Returns a boolean value saying whether EXPR has a real value
671 or not. If EXPR is not present, $_ will be checked. Many operations
672 return the undefined value under exceptional conditions, such as end of
673 file, uninitialized variable, system error and such. This function
674 allows you to distinguish between an undefined
675 null scalar and a defined null scalar with operations that might return
676 a real null string, such as referencing elements of an array. You may
677 also check to see if arrays or subroutines exist. Use of defined on
678 predefined variables is not guaranteed to produce intuitive results.
680 When used on a hash array element, it tells you whether the value is
681 defined, not whether the key exists in the hash. Use exists() for that.
685 print if defined $switch{'D'};
686 print "$val\n" while defined($val = pop(@ary));
687 die "Can't readlink $sym: $!"
688 unless defined($value = readlink $sym);
689 eval '@foo = ()' if defined(@foo);
690 die "No XYZ package defined" unless defined %_XYZ;
691 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
695 Note: many folks tend to overuse defined(), and then are surprised to
696 discover that the number 0 and the null string are, in fact, defined
697 concepts. For example, if you say
701 the pattern match succeeds, and $1 is defined, despite the fact that it
702 matched "nothing". But it didn't really match nothing--rather, it
703 matched something that happened to be 0 characters long. This is all
704 very above-board and honest. When a function returns an undefined value,
705 it's an admission that it couldn't give you an honest answer. So
706 you should use defined() only when you're questioning the integrity
707 of what you're trying to do. At other times, a simple comparison to
708 0 or "" is what you want.
710 Another surprise is that using defined() on an entire array or
711 hash reports whether memory for that aggregate has ever been
712 allocated. So an array you set to the empty list appears undefined
713 initially, and one that once was full and that you then set to
714 the empty list still appears defined. You should instead use a
715 simple test for size:
717 if (@an_array) { print "has array elements\n" }
718 if (%a_hash) { print "has hash members\n" }
720 Using undef() on these, however, does clear their memory and then report
721 them as not defined anymore, but you shoudln't do that unless you don't
722 plan to use them again, because it saves time when you load them up
723 again to have memory already ready to be filled.
725 This counter-intuitive behaviour of defined() on aggregates may be
726 changed, fixed, or broken in a future release of Perl.
730 Deletes the specified key(s) and their associated values from a hash.
731 For each key, returns the deleted value associated with that key, or
732 the undefined value if there was no such key. Deleting from C<$ENV{}>
733 modifies the environment. Deleting from a hash tied to a DBM file
734 deletes the entry from the DBM file. (But deleting from a tie()d hash
735 doesn't necessarily return anything.)
737 The following deletes all the values of a hash:
739 foreach $key (keys %HASH) {
745 delete @HASH{keys %HASH}
747 (But both of these are slower than the undef() command.) Note that the
748 EXPR can be arbitrarily complicated as long as the final operation is a
749 hash element lookup or hash slice:
751 delete $ref->[$x][$y]{$key};
752 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
756 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
757 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
758 C<($? E<gt>E<gt> 8)> (back-tick `command` status). If C<($? E<gt>E<gt> 8)>
759 is 0, exits with 255. Inside an eval(), the error message is stuffed into
760 C<$@>, and the eval() is terminated with the undefined value; this makes
761 die() the way to raise an exception.
765 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
766 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
768 If the value of EXPR does not end in a newline, the current script line
769 number and input line number (if any) are also printed, and a newline
770 is supplied. Hint: sometimes appending ", stopped" to your message
771 will cause it to make better sense when the string "at foo line 123" is
772 appended. Suppose you are running script "canasta".
774 die "/etc/games is no good";
775 die "/etc/games is no good, stopped";
777 produce, respectively
779 /etc/games is no good at canasta line 123.
780 /etc/games is no good, stopped at canasta line 123.
782 See also exit() and warn().
784 You can arrange for a callback to be called just before the die() does
785 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
786 will be called with the error text and can change the error message, if
787 it sees fit, by calling die() again. See L<perlvar> for details on
788 setting C<%SIG> entries, and eval() for some examples.
792 Not really a function. Returns the value of the last command in the
793 sequence of commands indicated by BLOCK. When modified by a loop
794 modifier, executes the BLOCK once before testing the loop condition.
795 (On other statements the loop modifiers test the conditional first.)
797 =item do SUBROUTINE(LIST)
799 A deprecated form of subroutine call. See L<perlsub>.
803 Uses the value of EXPR as a filename and executes the contents of the
804 file as a Perl script. Its primary use is to include subroutines
805 from a Perl subroutine library.
813 except that it's more efficient, more concise, keeps track of the
814 current filename for error messages, and searches all the B<-I>
815 libraries if the file isn't in the current directory (see also the @INC
816 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
817 re-parse the file every time you call it, so you probably don't want to
818 do this inside a loop.
820 Note that inclusion of library modules is better done with the
821 use() and require() operators, which also do error checking
822 and raise an exception if there's a problem.
826 This causes an immediate core dump. Primarily this is so that you can
827 use the B<undump> program to turn your core dump into an executable binary
828 after having initialized all your variables at the beginning of the
829 program. When the new binary is executed it will begin by executing a
830 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
831 it as a goto with an intervening core dump and reincarnation. If LABEL
832 is omitted, restarts the program from the top. WARNING: any files
833 opened at the time of the dump will NOT be open any more when the
834 program is reincarnated, with possible resulting confusion on the part
835 of Perl. See also B<-u> option in L<perlrun>.
852 dump QUICKSTART if $ARGV[0] eq '-d';
859 When called in a list context, returns a 2-element array consisting of the
860 key and value for the next element of a hash, so that you can iterate over
861 it. When called in a scalar context, returns the key for only the next
862 element in the hash. Entries are returned in an apparently random order.
863 When the hash is entirely read, a null array is returned in list context
864 (which when assigned produces a FALSE (0) value), and C<undef> is returned
865 in a scalar context. The next call to each() after that will start
866 iterating again. There is a single iterator for each hash, shared by all
867 each(), keys(), and values() function calls in the program; it can be
868 reset by reading all the elements from the hash, or by evaluating C<keys
869 HASH> or C<values HASH> in a scalar context. You should not add elements
870 to an hash while you're iterating over it.
872 The following prints out your environment like the printenv(1) program,
873 only in a different order:
875 while (($key,$value) = each %ENV) {
876 print "$key=$value\n";
879 See also keys() and values().
887 Returns 1 if the next read on FILEHANDLE will return end of file, or if
888 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
889 gives the real filehandle name. (Note that this function actually
890 reads a character and then ungetc()s it, so it is not very useful in an
891 interactive context.) Do not read from a terminal file (or call
892 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
893 as terminals may lose the end-of-file condition if you do.
895 An C<eof> without an argument uses the last file read as argument.
896 Empty parentheses () may be used to indicate
897 the pseudo file formed of the files listed on the command line, i.e.,
898 C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
899 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
900 test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
902 # reset line numbering on each input file
905 close(ARGV) if (eof); # Not eof().
908 # insert dashes just before last line of last file
911 print "--------------\n";
912 close(ARGV); # close or break; is needed if we
913 # are reading from the terminal
918 Practical hint: you almost never need to use C<eof> in Perl, because the
919 input operators return undef when they run out of data.
925 EXPR is parsed and executed as if it were a little Perl program. It
926 is executed in the context of the current Perl program, so that any
927 variable settings or subroutine and format definitions remain afterwards.
928 The value returned is the value of the last expression evaluated, or a
929 return statement may be used, just as with subroutines. The last
930 expression is evaluated in scalar or array context, depending on the
933 If there is a syntax error or runtime error, or a die() statement is
934 executed, an undefined value is returned by eval(), and C<$@> is set to the
935 error message. If there was no error, C<$@> is guaranteed to be a null
936 string. If EXPR is omitted, evaluates C<$_>. The final semicolon, if
937 any, may be omitted from the expression. Beware that using eval()
938 neither silences perl from printing warnings to STDERR, nor does it
939 stuff the text of warning messages into C<$@>. To do either of those,
940 you have to use the C<$SIG{__WARN__}> facility. See warn() and L<perlvar>.
942 Note that, because eval() traps otherwise-fatal errors, it is useful for
943 determining whether a particular feature (such as socket() or symlink())
944 is implemented. It is also Perl's exception trapping mechanism, where
945 the die operator is used to raise exceptions.
947 If the code to be executed doesn't vary, you may use the eval-BLOCK
948 form to trap run-time errors without incurring the penalty of
949 recompiling each time. The error, if any, is still returned in C<$@>.
952 # make divide-by-zero non-fatal
953 eval { $answer = $a / $b; }; warn $@ if $@;
955 # same thing, but less efficient
956 eval '$answer = $a / $b'; warn $@ if $@;
958 # a compile-time error
962 eval '$answer ='; # sets $@
964 When using the eval{} form as an exception trap in libraries, you may
965 wish not to trigger any C<__DIE__> hooks that user code may have
966 installed. You can use the C<local $SIG{__DIE__}> construct for this
967 purpose, as shown in this example:
969 # a very private exception trap for divide-by-zero
970 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
972 This is especially significant, given that C<__DIE__> hooks can call
973 die() again, which has the effect of changing their error messages:
975 # __DIE__ hooks may modify error messages
977 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
978 eval { die "foo foofs here" };
979 print $@ if $@; # prints "bar barfs here"
982 With an eval(), you should be especially careful to remember what's
983 being looked at when:
989 eval { $x }; # CASE 4
991 eval "\$$x++" # CASE 5
994 Cases 1 and 2 above behave identically: they run the code contained in the
995 variable $x. (Although case 2 has misleading double quotes making the
996 reader wonder what else might be happening (nothing is).) Cases 3 and 4
997 likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
998 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
999 is a place where normally you I<WOULD> like to use double quotes, except
1000 that in that particular situation, you can just use symbolic references
1001 instead, as in case 6.
1005 The exec() function executes a system command I<AND NEVER RETURNS>,
1006 unless the command does not exist and is executed directly instead of
1007 via C</bin/sh -c> (see below). Use system() instead of exec() if you
1010 If there is more than one argument in LIST, or if LIST is an array with
1011 more than one value, calls execvp(3) with the arguments in LIST. If
1012 there is only one scalar argument, the argument is checked for shell
1013 metacharacters. If there are any, the entire argument is passed to
1014 C</bin/sh -c> for parsing. If there are none, the argument is split
1015 into words and passed directly to execvp(), which is more efficient.
1016 Note: exec() and system() do not flush your output buffer, so you may
1017 need to set C<$|> to avoid lost output. Examples:
1019 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1020 exec "sort $outfile | uniq";
1022 If you don't really want to execute the first argument, but want to lie
1023 to the program you are executing about its own name, you can specify
1024 the program you actually want to run as an "indirect object" (without a
1025 comma) in front of the LIST. (This always forces interpretation of the
1026 LIST as a multi-valued list, even if there is only a single scalar in
1029 $shell = '/bin/csh';
1030 exec $shell '-sh'; # pretend it's a login shell
1034 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1038 Returns TRUE if the specified hash key exists in its hash array, even
1039 if the corresponding value is undefined.
1041 print "Exists\n" if exists $array{$key};
1042 print "Defined\n" if defined $array{$key};
1043 print "True\n" if $array{$key};
1045 A hash element can be TRUE only if it's defined, and defined if
1046 it exists, but the reverse doesn't necessarily hold true.
1048 Note that the EXPR can be arbitrarily complicated as long as the final
1049 operation is a hash key lookup:
1051 if (exists $ref->[$x][$y]{$key}) { ... }
1055 Evaluates EXPR and exits immediately with that value. (Actually, it
1056 calls any defined C<END> routines first, but the C<END> routines may not
1057 abort the exit. Likewise any object destructors that need to be called
1058 are called before exit.) Example:
1061 exit 0 if $ans =~ /^[Xx]/;
1063 See also die(). If EXPR is omitted, exits with 0 status. The only
1064 univerally portable values for EXPR are 0 for success and 1 for error;
1065 all other values are subject to unpredictable interpretation depending
1066 on the environment in which the Perl program is running.
1068 You shouldn't use exit() to abort a subroutine if there's any chance that
1069 someone might want to trap whatever error happened. Use die() instead,
1070 which can be trapped by an eval().
1076 Returns I<e> (the natural logarithm base) to the power of EXPR.
1077 If EXPR is omitted, gives C<exp($_)>.
1079 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1081 Implements the fcntl(2) function. You'll probably have to say
1085 first to get the correct function definitions. Argument processing and
1086 value return works just like ioctl() below. Note that fcntl() will produce
1087 a fatal error if used on a machine that doesn't implement fcntl(2).
1091 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1093 =item fileno FILEHANDLE
1095 Returns the file descriptor for a filehandle. This is useful for
1096 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1097 value is taken as the name of the filehandle.
1099 =item flock FILEHANDLE,OPERATION
1101 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1102 success, FALSE on failure. Will produce a fatal error if used on a
1103 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1104 flock() is Perl's portable file locking interface, although it will lock
1105 only entire files, not records.
1107 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1108 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1109 you can use the symbolic names if you pull them in with an explicit
1110 request to the Fcntl module. The names can be requested as a group with
1111 the :flock tag (or they can be requested individually, of course).
1112 LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and
1113 LOCK_UN releases a previously requested lock. If LOCK_NB is added to
1114 LOCK_SH or LOCK_EX then flock() will return immediately rather than
1115 blocking waiting for the lock (check the return status to see if you got
1118 Note that the emulation built with lockf(3) doesn't provide shared
1119 locks, and it requires that FILEHANDLE be open with write intent. These
1120 are the semantics that lockf(3) implements. Most (all?) systems
1121 implement lockf(3) in terms of fcntl(2) locking, though, so the
1122 differing semantics shouldn't bite too many people.
1124 Note also that some versions of flock() cannot lock things over the
1125 network; you would need to use the more system-specific fcntl() for
1126 that. If you like you can force Perl to ignore your system's flock(2)
1127 function, and so provide its own fcntl(2)-based emulation, by passing
1128 the switch C<-Ud_flock> to the F<Configure> program when you configure
1131 Here's a mailbox appender for BSD systems.
1133 use Fcntl ':flock'; # import LOCK_* constants
1136 flock(MBOX,LOCK_EX);
1137 # and, in case someone appended
1138 # while we were waiting...
1143 flock(MBOX,LOCK_UN);
1146 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1147 or die "Can't open mailbox: $!";
1150 print MBOX $msg,"\n\n";
1153 See also L<DB_File> for other flock() examples.
1157 Does a fork(2) system call. Returns the child pid to the parent process
1158 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1159 Note: unflushed buffers remain unflushed in both processes, which means
1160 you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1161 method of IO::Handle to avoid duplicate output.
1163 If you fork() without ever waiting on your children, you will accumulate
1166 $SIG{CHLD} = sub { wait };
1168 There's also the double-fork trick (error checking on
1169 fork() returns omitted);
1171 unless ($pid = fork) {
1173 exec "what you really wanna do";
1176 ## (some_perl_code_here)
1183 See also L<perlipc> for more examples of forking and reaping
1186 Note that if your forked child inherits system file descriptors like
1187 STDIN and STDOUT that are actually connected by a pipe or socket, even
1188 if you exit, the remote server (such as, say, httpd or rsh) won't think
1189 you're done. You should reopen those to /dev/null if it's any issue.
1193 Declare a picture format with use by the write() function. For
1197 Test: @<<<<<<<< @||||| @>>>>>
1198 $str, $%, '$' . int($num)
1202 $num = $cost/$quantity;
1206 See L<perlform> for many details and examples.
1209 =item formline PICTURE, LIST
1211 This is an internal function used by C<format>s, though you may call it
1212 too. It formats (see L<perlform>) a list of values according to the
1213 contents of PICTURE, placing the output into the format output
1214 accumulator, C<$^A> (or $ACCUMULATOR in English).
1215 Eventually, when a write() is done, the contents of
1216 C<$^A> are written to some filehandle, but you could also read C<$^A>
1217 yourself and then set C<$^A> back to "". Note that a format typically
1218 does one formline() per line of form, but the formline() function itself
1219 doesn't care how many newlines are embedded in the PICTURE. This means
1220 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1221 You may therefore need to use multiple formlines to implement a single
1222 record format, just like the format compiler.
1224 Be careful if you put double quotes around the picture, because an "C<@>"
1225 character may be taken to mean the beginning of an array name.
1226 formline() always returns TRUE. See L<perlform> for other examples.
1228 =item getc FILEHANDLE
1232 Returns the next character from the input file attached to FILEHANDLE,
1233 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1234 This is not particularly efficient. It cannot be used to get unbuffered
1235 single-characters, however. For that, try something more like:
1238 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1241 system "stty", '-icanon', 'eol', "\001";
1247 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1250 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1254 Determination of whether $BSD_STYLE should be set
1255 is left as an exercise to the reader.
1257 The POSIX::getattr() function can do this more portably on systems
1258 alleging POSIX compliance.
1259 See also the C<Term::ReadKey> module from your nearest CPAN site;
1260 details on CPAN can be found on L<perlmod/CPAN>.
1264 Returns the current login from F</etc/utmp>, if any. If null, use
1267 $login = getlogin || getpwuid($<) || "Kilroy";
1269 Do not consider getlogin() for authentication: it is not as
1270 secure as getpwuid().
1272 =item getpeername SOCKET
1274 Returns the packed sockaddr address of other end of the SOCKET connection.
1277 $hersockaddr = getpeername(SOCK);
1278 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1279 $herhostname = gethostbyaddr($iaddr, AF_INET);
1280 $herstraddr = inet_ntoa($iaddr);
1284 Returns the current process group for the specified PID. Use
1285 a PID of 0 to get the current process group for the
1286 current process. Will raise an exception if used on a machine that
1287 doesn't implement getpgrp(2). If PID is omitted, returns process
1288 group of current process. Note that the POSIX version of getpgrp()
1289 does not accept a PID argument, so only PID==0 is truly portable.
1293 Returns the process id of the parent process.
1295 =item getpriority WHICH,WHO
1297 Returns the current priority for a process, a process group, or a user.
1298 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1299 machine that doesn't implement getpriority(2).
1305 =item gethostbyname NAME
1307 =item getnetbyname NAME
1309 =item getprotobyname NAME
1315 =item getservbyname NAME,PROTO
1317 =item gethostbyaddr ADDR,ADDRTYPE
1319 =item getnetbyaddr ADDR,ADDRTYPE
1321 =item getprotobynumber NUMBER
1323 =item getservbyport PORT,PROTO
1341 =item sethostent STAYOPEN
1343 =item setnetent STAYOPEN
1345 =item setprotoent STAYOPEN
1347 =item setservent STAYOPEN
1361 These routines perform the same functions as their counterparts in the
1362 system library. Within a list context, the return values from the
1363 various get routines are as follows:
1365 ($name,$passwd,$uid,$gid,
1366 $quota,$comment,$gcos,$dir,$shell) = getpw*
1367 ($name,$passwd,$gid,$members) = getgr*
1368 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1369 ($name,$aliases,$addrtype,$net) = getnet*
1370 ($name,$aliases,$proto) = getproto*
1371 ($name,$aliases,$port,$proto) = getserv*
1373 (If the entry doesn't exist you get a null list.)
1375 Within a scalar context, you get the name, unless the function was a
1376 lookup by name, in which case you get the other thing, whatever it is.
1377 (If the entry doesn't exist you get the undefined value.) For example:
1387 The $members value returned by I<getgr*()> is a space separated list of
1388 the login names of the members of the group.
1390 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1391 C, it will be returned to you via C<$?> if the function call fails. The
1392 @addrs value returned by a successful call is a list of the raw
1393 addresses returned by the corresponding system library call. In the
1394 Internet domain, each address is four bytes long and you can unpack it
1395 by saying something like:
1397 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1399 =item getsockname SOCKET
1401 Returns the packed sockaddr address of this end of the SOCKET connection.
1404 $mysockaddr = getsockname(SOCK);
1405 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1407 =item getsockopt SOCKET,LEVEL,OPTNAME
1409 Returns the socket option requested, or undefined if there is an error.
1415 Returns the value of EXPR with filename expansions such as a shell
1416 would do. This is the internal function implementing the E<lt>*.*E<gt>
1417 operator, except it's easier to use.
1418 If EXPR is omitted, $_ is used.
1422 Converts a time as returned by the time function to a 9-element array
1423 with the time localized for the standard Greenwich time zone.
1424 Typically used as follows:
1427 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1430 All array elements are numeric, and come straight out of a struct tm.
1431 In particular this means that $mon has the range 0..11 and $wday has
1432 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1434 In a scalar context, prints out the ctime(3) value:
1436 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1438 Also see the F<timegm.pl> library, and the strftime(3) function available
1439 via the POSIX module.
1447 The goto-LABEL form finds the statement labeled with LABEL and resumes
1448 execution there. It may not be used to go into any construct that
1449 requires initialization, such as a subroutine or a foreach loop. It
1450 also can't be used to go into a construct that is optimized away,
1451 or to get out of a block or subroutine given to sort().
1452 It can be used to go almost anywhere else within the dynamic scope,
1453 including out of subroutines, but it's usually better to use some other
1454 construct such as last or die. The author of Perl has never felt the
1455 need to use this form of goto (in Perl, that is--C is another matter).
1457 The goto-EXPR form expects a label name, whose scope will be resolved
1458 dynamically. This allows for computed gotos per FORTRAN, but isn't
1459 necessarily recommended if you're optimizing for maintainability:
1461 goto ("FOO", "BAR", "GLARCH")[$i];
1463 The goto-&NAME form is highly magical, and substitutes a call to the
1464 named subroutine for the currently running subroutine. This is used by
1465 AUTOLOAD subroutines that wish to load another subroutine and then
1466 pretend that the other subroutine had been called in the first place
1467 (except that any modifications to @_ in the current subroutine are
1468 propagated to the other subroutine.) After the goto, not even caller()
1469 will be able to tell that this routine was called first.
1471 =item grep BLOCK LIST
1473 =item grep EXPR,LIST
1475 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1476 $_ to each element) and returns the list value consisting of those
1477 elements for which the expression evaluated to TRUE. In a scalar
1478 context, returns the number of times the expression was TRUE.
1480 @foo = grep(!/^#/, @bar); # weed out comments
1484 @foo = grep {!/^#/} @bar; # weed out comments
1486 Note that, because $_ is a reference into the list value, it can be used
1487 to modify the elements of the array. While this is useful and
1488 supported, it can cause bizarre results if the LIST is not a named
1495 Interprets EXPR as a hex string and returns the corresponding decimal
1496 value. (To convert strings that might start with 0 or 0x see
1497 oct().) If EXPR is omitted, uses $_.
1501 There is no built-in import() function. It is merely an ordinary
1502 method (subroutine) defined (or inherited) by modules that wish to export
1503 names to another module. The use() function calls the import() method
1504 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1506 =item index STR,SUBSTR,POSITION
1508 =item index STR,SUBSTR
1510 Returns the position of the first occurrence of SUBSTR in STR at or after
1511 POSITION. If POSITION is omitted, starts searching from the beginning of
1512 the string. The return value is based at 0 (or whatever you've set the C<$[>
1513 variable to--but don't do that). If the substring is not found, returns
1514 one less than the base, ordinarily -1.
1520 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1522 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1524 Implements the ioctl(2) function. You'll probably have to say
1526 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1528 first to get the correct function definitions. If F<ioctl.ph> doesn't
1529 exist or doesn't have the correct definitions you'll have to roll your
1530 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1531 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1532 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1533 written depending on the FUNCTION--a pointer to the string value of SCALAR
1534 will be passed as the third argument of the actual ioctl call. (If SCALAR
1535 has no string value but does have a numeric value, that value will be
1536 passed rather than a pointer to the string value. To guarantee this to be
1537 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1538 functions are useful for manipulating the values of structures used by
1539 ioctl(). The following example sets the erase character to DEL.
1543 die "NO TIOCGETP" if $@ || !$getp;
1544 $sgttyb_t = "ccccs"; # 4 chars and a short
1545 if (ioctl(STDIN,$getp,$sgttyb)) {
1546 @ary = unpack($sgttyb_t,$sgttyb);
1548 $sgttyb = pack($sgttyb_t,@ary);
1549 ioctl(STDIN,&TIOCSETP,$sgttyb)
1550 || die "Can't ioctl: $!";
1553 The return value of ioctl (and fcntl) is as follows:
1555 if OS returns: then Perl returns:
1557 0 string "0 but true"
1558 anything else that number
1560 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1561 still easily determine the actual value returned by the operating
1564 ($retval = ioctl(...)) || ($retval = -1);
1565 printf "System returned %d\n", $retval;
1567 =item join EXPR,LIST
1569 Joins the separate strings of LIST or ARRAY into a single string with
1570 fields separated by the value of EXPR, and returns the string.
1573 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1575 See L<perlfunc/split>.
1579 Returns a normal array consisting of all the keys of the named hash. (In
1580 a scalar context, returns the number of keys.) The keys are returned in
1581 an apparently random order, but it is the same order as either the
1582 values() or each() function produces (given that the hash has not been
1583 modified). As a side effect, it resets HASH's iterator.
1585 Here is yet another way to print your environment:
1588 @values = values %ENV;
1589 while ($#keys >= 0) {
1590 print pop(@keys), '=', pop(@values), "\n";
1593 or how about sorted by key:
1595 foreach $key (sort(keys %ENV)) {
1596 print $key, '=', $ENV{$key}, "\n";
1599 To sort an array by value, you'll need to use a C<sort{}> function.
1600 Here's a descending numeric sort of a hash by its values:
1602 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1603 printf "%4d %s\n", $hash{$key}, $key;
1606 As an lvalue C<keys> allows you to increase the number of hash buckets
1607 allocated for the given hash. This can gain you a measure of efficiency if
1608 you know the hash is going to get big. (This is similar to pre-extending
1609 an array by assigning a larger number to $#array.) If you say
1613 then C<%hash> will have at least 200 buckets allocated for it. These
1614 buckets will be retained even if you do C<%hash = ()>, use C<undef
1615 %hash> if you want to free the storage while C<%hash> is still in scope.
1616 You can't shrink the number of buckets allocated for the hash using
1617 C<keys> in this way (but you needn't worry about doing this by accident,
1618 as trying has no effect).
1622 Sends a signal to a list of processes. The first element of
1623 the list must be the signal to send. Returns the number of
1624 processes successfully signaled.
1626 $cnt = kill 1, $child1, $child2;
1629 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1630 process groups instead of processes. (On System V, a negative I<PROCESS>
1631 number will also kill process groups, but that's not portable.) That
1632 means you usually want to use positive not negative signals. You may also
1633 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1639 The C<last> command is like the C<break> statement in C (as used in
1640 loops); it immediately exits the loop in question. If the LABEL is
1641 omitted, the command refers to the innermost enclosing loop. The
1642 C<continue> block, if any, is not executed:
1644 LINE: while (<STDIN>) {
1645 last LINE if /^$/; # exit when done with header
1653 Returns an lowercased version of EXPR. This is the internal function
1654 implementing the \L escape in double-quoted strings.
1655 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1657 If EXPR is omitted, uses $_.
1663 Returns the value of EXPR with the first character lowercased. This is
1664 the internal function implementing the \l escape in double-quoted strings.
1665 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1667 If EXPR is omitted, uses $_.
1673 Returns the length in characters of the value of EXPR. If EXPR is
1674 omitted, returns length of $_.
1676 =item link OLDFILE,NEWFILE
1678 Creates a new filename linked to the old filename. Returns 1 for
1679 success, 0 otherwise.
1681 =item listen SOCKET,QUEUESIZE
1683 Does the same thing that the listen system call does. Returns TRUE if
1684 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1688 A local modifies the listed variables to be local to the enclosing block,
1689 subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1690 list must be placed in parentheses. See L<perlsub/"Temporary Values via
1691 local()"> for details.
1693 But you really probably want to be using my() instead, because local() isn't
1694 what most people think of as "local"). See L<perlsub/"Private Variables
1695 via my()"> for details.
1697 =item localtime EXPR
1699 Converts a time as returned by the time function to a 9-element array
1700 with the time analyzed for the local time zone. Typically used as
1703 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1706 All array elements are numeric, and come straight out of a struct tm.
1707 In particular this means that $mon has the range 0..11 and $wday has
1708 the range 0..6 and $year is year-1900, that is, $year is 123 in year
1709 2023. If EXPR is omitted, uses the current time ("localtime(time)").
1711 In a scalar context, returns the ctime(3) value:
1713 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
1715 Also see the Time::Local module, and the strftime(3) function available
1716 via the POSIX module.
1722 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1725 =item lstat FILEHANDLE
1731 Does the same thing as the stat() function, but stats a symbolic link
1732 instead of the file the symbolic link points to. If symbolic links are
1733 unimplemented on your system, a normal stat() is done.
1735 If EXPR is omitted, stats $_.
1739 The match operator. See L<perlop>.
1741 =item map BLOCK LIST
1745 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1746 element) and returns the list value composed of the results of each such
1747 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1748 may produce zero, one, or more elements in the returned value.
1750 @chars = map(chr, @nums);
1752 translates a list of numbers to the corresponding characters. And
1754 %hash = map { getkey($_) => $_ } @array;
1756 is just a funny way to write
1759 foreach $_ (@array) {
1760 $hash{getkey($_)} = $_;
1763 =item mkdir FILENAME,MODE
1765 Creates the directory specified by FILENAME, with permissions specified
1766 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1767 it returns 0 and sets C<$!> (errno).
1769 =item msgctl ID,CMD,ARG
1771 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1772 must be a variable which will hold the returned msqid_ds structure.
1773 Returns like ioctl: the undefined value for error, "0 but true" for
1774 zero, or the actual return value otherwise.
1776 =item msgget KEY,FLAGS
1778 Calls the System V IPC function msgget(2). Returns the message queue id,
1779 or the undefined value if there is an error.
1781 =item msgsnd ID,MSG,FLAGS
1783 Calls the System V IPC function msgsnd to send the message MSG to the
1784 message queue ID. MSG must begin with the long integer message type,
1785 which may be created with C<pack("l", $type)>. Returns TRUE if
1786 successful, or FALSE if there is an error.
1788 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1790 Calls the System V IPC function msgrcv to receive a message from
1791 message queue ID into variable VAR with a maximum message size of
1792 SIZE. Note that if a message is received, the message type will be the
1793 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1794 of the message type. Returns TRUE if successful, or FALSE if there is
1799 A "my" declares the listed variables to be local (lexically) to the
1800 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1801 more than one value is listed, the list must be placed in parentheses. See
1802 L<perlsub/"Private Variables via my()"> for details.
1808 The C<next> command is like the C<continue> statement in C; it starts
1809 the next iteration of the loop:
1811 LINE: while (<STDIN>) {
1812 next LINE if /^#/; # discard comments
1816 Note that if there were a C<continue> block on the above, it would get
1817 executed even on discarded lines. If the LABEL is omitted, the command
1818 refers to the innermost enclosing loop.
1820 =item no Module LIST
1822 See the "use" function, which "no" is the opposite of.
1828 Interprets EXPR as an octal string and returns the corresponding
1829 decimal value. (If EXPR happens to start off with 0x, interprets it as
1830 a hex string instead.) The following will handle decimal, octal, and
1831 hex in the standard Perl or C notation:
1833 $val = oct($val) if $val =~ /^0/;
1835 If EXPR is omitted, uses $_.
1837 =item open FILEHANDLE,EXPR
1839 =item open FILEHANDLE
1841 Opens the file whose filename is given by EXPR, and associates it with
1842 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1843 name of the real filehandle wanted. If EXPR is omitted, the scalar
1844 variable of the same name as the FILEHANDLE contains the filename.
1845 (Note that lexical variables--those declared with C<my>--will not work
1846 for this purpose; so if you're using C<my>, specify EXPR in your call
1849 If the filename begins with '<' or nothing, the file is opened for input.
1850 If the filename begins with '>', the file is truncated and opened for
1851 output. If the filename begins with '>>', the file is opened for
1852 appending. You can put a '+' in front of the '>' or '<' to indicate that
1853 you want both read and write access to the file; thus '+<' is almost
1854 always preferred for read/write updates--the '+>' mode would clobber the
1855 file first. The prefix and the filename may be separated with spaces.
1856 These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1857 'w+', 'a', and 'a+'.
1859 If the filename begins with "|", the filename is interpreted as a command
1860 to which output is to be piped, and if the filename ends with a "|", the
1861 filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1862 examples of this. as command which pipes input to us. (You may not have
1863 a raw open() to a command that pipes both in I<and> out, but see
1864 L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1867 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1868 non-zero upon success, the undefined value otherwise. If the open
1869 involved a pipe, the return value happens to be the pid of the
1872 If you're unfortunate enough to be running Perl on a system that
1873 distinguishes between text files and binary files (modern operating
1874 systems don't care), then you should check out L</binmode> for tips for
1875 dealing with this. The key distinction between systems that need binmode
1876 and those that don't is their text file formats. Systems like Unix and
1877 Plan9 that delimit lines with a single character, and that encode that
1878 character in C as '\n', do not need C<binmode>. The rest need it.
1883 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1884 while (<ARTICLE>) {...
1886 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1888 open(DBASE, '+<dbase.mine'); # open for update
1890 open(ARTICLE, "caesar <$article |"); # decrypt article
1892 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1894 # process argument list of files along with any includes
1896 foreach $file (@ARGV) {
1897 process($file, 'fh00');
1901 local($filename, $input) = @_;
1902 $input++; # this is a string increment
1903 unless (open($input, $filename)) {
1904 print STDERR "Can't open $filename: $!\n";
1908 while (<$input>) { # note use of indirection
1909 if (/^#include "(.*)"/) {
1910 process($1, $input);
1917 You may also, in the Bourne shell tradition, specify an EXPR beginning
1918 with "E<gt>&", in which case the rest of the string is interpreted as the
1919 name of a filehandle (or file descriptor, if numeric) which is to be
1920 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1921 +E<gt>E<gt>, and +E<lt>. The
1922 mode you specify should match the mode of the original filehandle.
1923 (Duping a filehandle does not take into account any existing contents of
1925 Here is a script that saves, redirects, and restores STDOUT and
1929 open(SAVEOUT, ">&STDOUT");
1930 open(SAVEERR, ">&STDERR");
1932 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1933 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1935 select(STDERR); $| = 1; # make unbuffered
1936 select(STDOUT); $| = 1; # make unbuffered
1938 print STDOUT "stdout 1\n"; # this works for
1939 print STDERR "stderr 1\n"; # subprocesses too
1944 open(STDOUT, ">&SAVEOUT");
1945 open(STDERR, ">&SAVEERR");
1947 print STDOUT "stdout 2\n";
1948 print STDERR "stderr 2\n";
1951 If you specify "E<lt>&=N", where N is a number, then Perl will do an
1952 equivalent of C's fdopen() of that file descriptor; this is more
1953 parsimonious of file descriptors. For example:
1955 open(FILEHANDLE, "<&=$fd")
1957 If you open a pipe on the command "-", i.e., either "|-" or "-|", then
1958 there is an implicit fork done, and the return value of open is the pid
1959 of the child within the parent process, and 0 within the child
1960 process. (Use C<defined($pid)> to determine whether the open was successful.)
1961 The filehandle behaves normally for the parent, but i/o to that
1962 filehandle is piped from/to the STDOUT/STDIN of the child process.
1963 In the child process the filehandle isn't opened--i/o happens from/to
1964 the new STDOUT or STDIN. Typically this is used like the normal
1965 piped open when you want to exercise more control over just how the
1966 pipe command gets executed, such as when you are running setuid, and
1967 don't want to have to scan shell commands for metacharacters.
1968 The following pairs are more or less equivalent:
1970 open(FOO, "|tr '[a-z]' '[A-Z]'");
1971 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1973 open(FOO, "cat -n '$file'|");
1974 open(FOO, "-|") || exec 'cat', '-n', $file;
1976 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1978 Explicitly closing any piped filehandle causes the parent process to
1979 wait for the child to finish, and returns the status value in C<$?>.
1980 Note: on any operation which may do a fork, unflushed buffers remain
1981 unflushed in both processes, which means you may need to set C<$|> to
1982 avoid duplicate output.
1984 Using the constructor from the IO::Handle package (or one of its
1985 subclasses, such as IO::File or IO::Socket),
1986 you can generate anonymous filehandles which have the scope of whatever
1987 variables hold references to them, and automatically close whenever
1988 and however you leave that scope:
1992 sub read_myfile_munged {
1994 my $handle = new IO::File;
1995 open($handle, "myfile") or die "myfile: $!";
1997 or return (); # Automatically closed here.
1998 mung $first or die "mung failed"; # Or here.
1999 return $first, <$handle> if $ALL; # Or here.
2003 The filename that is passed to open will have leading and trailing
2004 whitespace deleted. To open a file with arbitrary weird
2005 characters in it, it's necessary to protect any leading and trailing
2008 $file =~ s#^(\s)#./$1#;
2009 open(FOO, "< $file\0");
2011 If you want a "real" C open() (see L<open(2)> on your system), then
2012 you should use the sysopen() function. This is another way to
2013 protect your filenames from interpretation. For example:
2016 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2017 or die "sysopen $path: $!";
2018 HANDLE->autoflush(1);
2019 HANDLE->print("stuff $$\n");
2021 print "File contains: ", <HANDLE>;
2023 See L</seek()> for some details about mixing reading and writing.
2025 =item opendir DIRHANDLE,EXPR
2027 Opens a directory named EXPR for processing by readdir(), telldir(),
2028 seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
2029 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2035 Returns the numeric ascii value of the first character of EXPR. If
2036 EXPR is omitted, uses $_.
2038 =item pack TEMPLATE,LIST
2040 Takes an array or list of values and packs it into a binary structure,
2041 returning the string containing the structure. The TEMPLATE is a
2042 sequence of characters that give the order and type of values, as
2045 A An ascii string, will be space padded.
2046 a An ascii string, will be null padded.
2047 b A bit string (ascending bit order, like vec()).
2048 B A bit string (descending bit order).
2049 h A hex string (low nybble first).
2050 H A hex string (high nybble first).
2052 c A signed char value.
2053 C An unsigned char value.
2054 s A signed short value.
2055 S An unsigned short value.
2056 i A signed integer value.
2057 I An unsigned integer value.
2058 l A signed long value.
2059 L An unsigned long value.
2061 n A short in "network" order.
2062 N A long in "network" order.
2063 v A short in "VAX" (little-endian) order.
2064 V A long in "VAX" (little-endian) order.
2066 f A single-precision float in the native format.
2067 d A double-precision float in the native format.
2069 p A pointer to a null-terminated string.
2070 P A pointer to a structure (fixed-length string).
2072 u A uuencoded string.
2074 w A BER compressed integer. Bytes give an unsigned integer base
2075 128, most significant digit first, with as few digits as
2076 possible, and with the bit 8 of each byte except the last set
2081 @ Null fill to absolute position.
2083 Each letter may optionally be followed by a number which gives a repeat
2084 count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
2085 pack function will gobble up that many values from the LIST. A * for the
2086 repeat count means to use however many items are left. The "a" and "A"
2087 types gobble just one value, but pack it as a string of length count,
2088 padding with nulls or spaces as necessary. (When unpacking, "A" strips
2089 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2090 fields pack a string that many bits long. The "h" and "H" fields pack a
2091 string that many nybbles long. The "P" packs a pointer to a structure of
2092 the size indicated by the length. Real numbers (floats and doubles) are
2093 in the native machine format only; due to the multiplicity of floating
2094 formats around, and the lack of a standard "network" representation, no
2095 facility for interchange has been made. This means that packed floating
2096 point data written on one machine may not be readable on another - even if
2097 both use IEEE floating point arithmetic (as the endian-ness of the memory
2098 representation is not part of the IEEE spec). Note that Perl uses doubles
2099 internally for all numeric calculation, and converting from double into
2100 float and thence back to double again will lose precision (i.e.,
2101 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2105 $foo = pack("cccc",65,66,67,68);
2107 $foo = pack("c4",65,66,67,68);
2110 $foo = pack("ccxxcc",65,66,67,68);
2113 $foo = pack("s2",1,2);
2114 # "\1\0\2\0" on little-endian
2115 # "\0\1\0\2" on big-endian
2117 $foo = pack("a4","abcd","x","y","z");
2120 $foo = pack("aaaa","abcd","x","y","z");
2123 $foo = pack("a14","abcdefg");
2124 # "abcdefg\0\0\0\0\0\0\0"
2126 $foo = pack("i9pl", gmtime);
2127 # a real struct tm (on my system anyway)
2130 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2133 The same template may generally also be used in the unpack function.
2135 =item package NAMESPACE
2137 Declares the compilation unit as being in the given namespace. The scope
2138 of the package declaration is from the declaration itself through the end of
2139 the enclosing block (the same scope as the local() operator). All further
2140 unqualified dynamic identifiers will be in this namespace. A package
2141 statement affects only dynamic variables--including those you've used
2142 local() on--but I<not> lexical variables created with my(). Typically it
2143 would be the first declaration in a file to be included by the C<require>
2144 or C<use> operator. You can switch into a package in more than one place;
2145 it influences merely which symbol table is used by the compiler for the
2146 rest of that block. You can refer to variables and filehandles in other
2147 packages by prefixing the identifier with the package name and a double
2148 colon: C<$Package::Variable>. If the package name is null, the C<main>
2149 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2151 See L<perlmod/"Packages"> for more information about packages, modules,
2152 and classes. See L<perlsub> for other scoping issues.
2154 =item pipe READHANDLE,WRITEHANDLE
2156 Opens a pair of connected pipes like the corresponding system call.
2157 Note that if you set up a loop of piped processes, deadlock can occur
2158 unless you are very careful. In addition, note that Perl's pipes use
2159 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2160 after each command, depending on the application.
2162 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2163 for examples of such things.
2169 Pops and returns the last value of the array, shortening the array by
2170 1. Has a similar effect to
2172 $tmp = $ARRAY[$#ARRAY--];
2174 If there are no elements in the array, returns the undefined value.
2175 If ARRAY is omitted, pops the
2176 @ARGV array in the main program, and the @_ array in subroutines, just
2183 Returns the offset of where the last C<m//g> search left off for the variable
2184 is in question ($_ is used when the variable is not specified). May be
2185 modified to change that offset. Such modification will also influence
2186 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2189 =item print FILEHANDLE LIST
2195 Prints a string or a comma-separated list of strings. Returns TRUE
2196 if successful. FILEHANDLE may be a scalar variable name, in which case
2197 the variable contains the name of or a reference to the filehandle, thus introducing one
2198 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2199 token is a term, it may be misinterpreted as an operator unless you
2200 interpose a + or put parentheses around the arguments.) If FILEHANDLE is
2201 omitted, prints by default to standard output (or to the last selected
2202 output channel--see L</select>). If LIST is also omitted, prints $_ to
2203 STDOUT. To set the default output channel to something other than
2204 STDOUT use the select operation. Note that, because print takes a
2205 LIST, anything in the LIST is evaluated in a list context, and any
2206 subroutine that you call will have one or more of its expressions
2207 evaluated in a list context. Also be careful not to follow the print
2208 keyword with a left parenthesis unless you want the corresponding right
2209 parenthesis to terminate the arguments to the print--interpose a + or
2210 put parentheses around all the arguments.
2212 Note that if you're storing FILEHANDLES in an array or other expression,
2213 you will have to use a block returning its value instead:
2215 print { $files[$i] } "stuff\n";
2216 print { $OK ? STDOUT : STDERR } "stuff\n";
2218 =item printf FILEHANDLE FORMAT, LIST
2220 =item printf FORMAT, LIST
2222 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2223 of the list will be interpreted as the printf format. If C<use locale> is
2224 in effect, the character used for the decimal point in formatted real numbers
2225 is affected by the LC_NUMERIC locale. See L<perllocale>.
2227 Don't fall into the trap of using a printf() when a simple
2228 print() would do. The print() is more efficient, and less
2231 =item prototype FUNCTION
2233 Returns the prototype of a function as a string (or C<undef> if the
2234 function has no prototype). FUNCTION is a reference to, or the name of,
2235 the function whose prototype you want to retrieve.
2237 =item push ARRAY,LIST
2239 Treats ARRAY as a stack, and pushes the values of LIST
2240 onto the end of ARRAY. The length of ARRAY increases by the length of
2241 LIST. Has the same effect as
2244 $ARRAY[++$#ARRAY] = $value;
2247 but is more efficient. Returns the new number of elements in the array.
2257 Generalized quotes. See L<perlop>.
2259 =item quotemeta EXPR
2263 Returns the value of EXPR with with all non-alphanumeric
2264 characters backslashed. (That is, all characters not matching
2265 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2266 returned string, regardless of any locale settings.)
2267 This is the internal function implementing
2268 the \Q escape in double-quoted strings.
2270 If EXPR is omitted, uses $_.
2276 Returns a random fractional number between 0 and the value of EXPR.
2277 (EXPR should be positive.) If EXPR is omitted, returns a value between
2278 0 and 1. Automatically calls srand() unless srand() has already been
2279 called. See also srand().
2281 (Note: if your rand function consistently returns numbers that are too
2282 large or too small, then your version of Perl was probably compiled
2283 with the wrong number of RANDBITS. As a workaround, you can usually
2284 multiply EXPR by the correct power of 2 to get the range you want.
2285 This will make your script unportable, however. It's better to recompile
2288 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2290 =item read FILEHANDLE,SCALAR,LENGTH
2292 Attempts to read LENGTH bytes of data into variable SCALAR from the
2293 specified FILEHANDLE. Returns the number of bytes actually read, or
2294 undef if there was an error. SCALAR will be grown or shrunk to the
2295 length actually read. An OFFSET may be specified to place the read
2296 data at some other place than the beginning of the string. This call
2297 is actually implemented in terms of stdio's fread call. To get a true
2298 read system call, see sysread().
2300 =item readdir DIRHANDLE
2302 Returns the next directory entry for a directory opened by opendir().
2303 If used in a list context, returns all the rest of the entries in the
2304 directory. If there are no more entries, returns an undefined value in
2305 a scalar context or a null list in a list context.
2307 If you're planning to filetest the return values out of a readdir(), you'd
2308 better prepend the directory in question. Otherwise, because we didn't
2309 chdir() there, it would have been testing the wrong file.
2311 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2312 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2319 Returns the value of a symbolic link, if symbolic links are
2320 implemented. If not, gives a fatal error. If there is some system
2321 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2324 =item recv SOCKET,SCALAR,LEN,FLAGS
2326 Receives a message on a socket. Attempts to receive LENGTH bytes of
2327 data into variable SCALAR from the specified SOCKET filehandle.
2328 Actually does a C recvfrom(), so that it can returns the address of the
2329 sender. Returns the undefined value if there's an error. SCALAR will
2330 be grown or shrunk to the length actually read. Takes the same flags
2331 as the system call of the same name.
2332 See L<perlipc/"UDP: Message Passing"> for examples.
2338 The C<redo> command restarts the loop block without evaluating the
2339 conditional again. The C<continue> block, if any, is not executed. If
2340 the LABEL is omitted, the command refers to the innermost enclosing
2341 loop. This command is normally used by programs that want to lie to
2342 themselves about what was just input:
2344 # a simpleminded Pascal comment stripper
2345 # (warning: assumes no { or } in strings)
2346 LINE: while (<STDIN>) {
2347 while (s|({.*}.*){.*}|$1 |) {}
2352 if (/}/) { # end of comment?
2365 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2366 is not specified, $_ will be used. The value returned depends on the
2367 type of thing the reference is a reference to.
2368 Builtin types include:
2377 If the referenced object has been blessed into a package, then that package
2378 name is returned instead. You can think of ref() as a typeof() operator.
2380 if (ref($r) eq "HASH") {
2381 print "r is a reference to a hash.\n";
2384 print "r is not a reference at all.\n";
2387 See also L<perlref>.
2389 =item rename OLDNAME,NEWNAME
2391 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2392 not work across file system boundaries.
2398 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2399 supplied. If EXPR is numeric, demands that the current version of Perl
2400 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2402 Otherwise, demands that a library file be included if it hasn't already
2403 been included. The file is included via the do-FILE mechanism, which is
2404 essentially just a variety of eval(). Has semantics similar to the following
2408 local($filename) = @_;
2409 return 1 if $INC{$filename};
2410 local($realfilename,$result);
2412 foreach $prefix (@INC) {
2413 $realfilename = "$prefix/$filename";
2414 if (-f $realfilename) {
2415 $result = do $realfilename;
2419 die "Can't find $filename in \@INC";
2422 die "$filename did not return true value" unless $result;
2423 $INC{$filename} = $realfilename;
2427 Note that the file will not be included twice under the same specified
2428 name. The file must return TRUE as the last statement to indicate
2429 successful execution of any initialization code, so it's customary to
2430 end such a file with "1;" unless you're sure it'll return TRUE
2431 otherwise. But it's better just to put the "C<1;>", in case you add more
2434 If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2435 replaces "F<::>" with "F</>" in the filename for you,
2436 to make it easy to load standard modules. This form of loading of
2437 modules does not risk altering your namespace.
2439 For a yet-more-powerful import facility, see L</use> and
2446 Generally used in a C<continue> block at the end of a loop to clear
2447 variables and reset ?? searches so that they work again. The
2448 expression is interpreted as a list of single characters (hyphens
2449 allowed for ranges). All variables and arrays beginning with one of
2450 those letters are reset to their pristine state. If the expression is
2451 omitted, one-match searches (?pattern?) are reset to match again. Resets
2452 only variables or searches in the current package. Always returns
2455 reset 'X'; # reset all X variables
2456 reset 'a-z'; # reset lower case variables
2457 reset; # just reset ?? searches
2459 Resetting "A-Z" is not recommended because you'll wipe out your
2460 ARGV and ENV arrays. Resets only package variables--lexical variables
2461 are unaffected, but they clean themselves up on scope exit anyway,
2462 so you'll probably want to use them instead. See L</my>.
2466 Returns from a subroutine or eval with the value specified. (Note that
2467 in the absence of a return a subroutine or eval() will automatically
2468 return the value of the last expression evaluated.)
2472 In a list context, returns a list value consisting of the elements
2473 of LIST in the opposite order. In a scalar context, returns a string
2474 value consisting of the bytes of the first element of LIST in the
2477 print reverse <>; # line tac
2480 print scalar reverse scalar <>; # byte tac
2482 =item rewinddir DIRHANDLE
2484 Sets the current position to the beginning of the directory for the
2485 readdir() routine on DIRHANDLE.
2487 =item rindex STR,SUBSTR,POSITION
2489 =item rindex STR,SUBSTR
2491 Works just like index except that it returns the position of the LAST
2492 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2493 last occurrence at or before that position.
2495 =item rmdir FILENAME
2499 Deletes the directory specified by FILENAME if it is empty. If it
2500 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2501 FILENAME is omitted, uses $_.
2505 The substitution operator. See L<perlop>.
2509 Forces EXPR to be interpreted in a scalar context and returns the value
2512 @counts = ( scalar @a, scalar @b, scalar @c );
2514 There is no equivalent operator to force an expression to
2515 be interpolated in a list context because it's in practice never
2516 needed. If you really wanted to do so, however, you could use
2517 the construction C<@{[ (some expression) ]}>, but usually a simple
2518 C<(some expression)> suffices.
2520 =item seek FILEHANDLE,POSITION,WHENCE
2522 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2523 call of stdio. FILEHANDLE may be an expression whose value gives the name
2524 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2525 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2526 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2527 this from POSIX module. Returns 1 upon success, 0 otherwise.
2529 On some systems you have to do a seek whenever you switch between reading
2530 and writing. Amongst other things, this may have the effect of calling
2531 stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2536 This is also useful for applications emulating C<tail -f>. Once you hit
2537 EOF on your read, and then sleep for a while, you might have to stick in a
2538 seek() to reset things. First the simple trick listed above to clear the
2539 filepointer. The seek() doesn't change the current position, but it
2540 I<does> clear the end-of-file condition on the handle, so that the next
2541 C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2543 If that doesn't work (some stdios are particularly cantankerous), then
2544 you may need something more like this:
2547 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2548 # search for some stuff and put it into files
2550 sleep($for_a_while);
2551 seek(FILE, $curpos, 0);
2554 =item seekdir DIRHANDLE,POS
2556 Sets the current position for the readdir() routine on DIRHANDLE. POS
2557 must be a value returned by telldir(). Has the same caveats about
2558 possible directory compaction as the corresponding system library
2561 =item select FILEHANDLE
2565 Returns the currently selected filehandle. Sets the current default
2566 filehandle for output, if FILEHANDLE is supplied. This has two
2567 effects: first, a C<write> or a C<print> without a filehandle will
2568 default to this FILEHANDLE. Second, references to variables related to
2569 output will refer to this output channel. For example, if you have to
2570 set the top of form format for more than one output channel, you might
2578 FILEHANDLE may be an expression whose value gives the name of the
2579 actual filehandle. Thus:
2581 $oldfh = select(STDERR); $| = 1; select($oldfh);
2583 Some programmers may prefer to think of filehandles as objects with
2584 methods, preferring to write the last example as:
2587 STDERR->autoflush(1);
2589 =item select RBITS,WBITS,EBITS,TIMEOUT
2591 This calls the select(2) system call with the bit masks specified, which
2592 can be constructed using fileno() and vec(), along these lines:
2594 $rin = $win = $ein = '';
2595 vec($rin,fileno(STDIN),1) = 1;
2596 vec($win,fileno(STDOUT),1) = 1;
2599 If you want to select on many filehandles you might wish to write a
2603 local(@fhlist) = split(' ',$_[0]);
2606 vec($bits,fileno($_),1) = 1;
2610 $rin = fhbits('STDIN TTY SOCK');
2614 ($nfound,$timeleft) =
2615 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2617 or to block until something becomes ready just do this
2619 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2621 Most systems do not bother to return anything useful in $timeleft, so
2622 calling select() in a scalar context just returns $nfound.
2624 Any of the bit masks can also be undef. The timeout, if specified, is
2625 in seconds, which may be fractional. Note: not all implementations are
2626 capable of returning the $timeleft. If not, they always return
2627 $timeleft equal to the supplied $timeout.
2629 You can effect a sleep of 250 milliseconds this way:
2631 select(undef, undef, undef, 0.25);
2633 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2634 with select(). You have to use sysread() instead.
2636 =item semctl ID,SEMNUM,CMD,ARG
2638 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2639 &GETALL, then ARG must be a variable which will hold the returned
2640 semid_ds structure or semaphore value array. Returns like ioctl: the
2641 undefined value for error, "0 but true" for zero, or the actual return
2644 =item semget KEY,NSEMS,FLAGS
2646 Calls the System V IPC function semget. Returns the semaphore id, or
2647 the undefined value if there is an error.
2649 =item semop KEY,OPSTRING
2651 Calls the System V IPC function semop to perform semaphore operations
2652 such as signaling and waiting. OPSTRING must be a packed array of
2653 semop structures. Each semop structure can be generated with
2654 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2655 operations is implied by the length of OPSTRING. Returns TRUE if
2656 successful, or FALSE if there is an error. As an example, the
2657 following code waits on semaphore $semnum of semaphore id $semid:
2659 $semop = pack("sss", $semnum, -1, 0);
2660 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2662 To signal the semaphore, replace "-1" with "1".
2664 =item send SOCKET,MSG,FLAGS,TO
2666 =item send SOCKET,MSG,FLAGS
2668 Sends a message on a socket. Takes the same flags as the system call
2669 of the same name. On unconnected sockets you must specify a
2670 destination to send TO, in which case it does a C sendto(). Returns
2671 the number of characters sent, or the undefined value if there is an
2673 See L<perlipc/"UDP: Message Passing"> for examples.
2675 =item setpgrp PID,PGRP
2677 Sets the current process group for the specified PID, 0 for the current
2678 process. Will produce a fatal error if used on a machine that doesn't
2679 implement setpgrp(2). If the arguments are omitted, it defaults to
2680 0,0. Note that the POSIX version of setpgrp() does not accept any
2681 arguments, so only setpgrp 0,0 is portable.
2683 =item setpriority WHICH,WHO,PRIORITY
2685 Sets the current priority for a process, a process group, or a user.
2686 (See setpriority(2).) Will produce a fatal error if used on a machine
2687 that doesn't implement setpriority(2).
2689 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2691 Sets the socket option requested. Returns undefined if there is an
2692 error. OPTVAL may be specified as undef if you don't want to pass an
2699 Shifts the first value of the array off and returns it, shortening the
2700 array by 1 and moving everything down. If there are no elements in the
2701 array, returns the undefined value. If ARRAY is omitted, shifts the
2702 @ARGV array in the main program, and the @_ array in subroutines.
2703 (This is determined lexically.) See also unshift(), push(), and pop().
2704 Shift() and unshift() do the same thing to the left end of an array
2705 that push() and pop() do to the right end.
2707 =item shmctl ID,CMD,ARG
2709 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2710 must be a variable which will hold the returned shmid_ds structure.
2711 Returns like ioctl: the undefined value for error, "0 but true" for
2712 zero, or the actual return value otherwise.
2714 =item shmget KEY,SIZE,FLAGS
2716 Calls the System V IPC function shmget. Returns the shared memory
2717 segment id, or the undefined value if there is an error.
2719 =item shmread ID,VAR,POS,SIZE
2721 =item shmwrite ID,STRING,POS,SIZE
2723 Reads or writes the System V shared memory segment ID starting at
2724 position POS for size SIZE by attaching to it, copying in/out, and
2725 detaching from it. When reading, VAR must be a variable which will
2726 hold the data read. When writing, if STRING is too long, only SIZE
2727 bytes are used; if STRING is too short, nulls are written to fill out
2728 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2730 =item shutdown SOCKET,HOW
2732 Shuts down a socket connection in the manner indicated by HOW, which
2733 has the same interpretation as in the system call of the same name.
2739 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2742 For the inverse sine operation, you may use the POSIX::sin()
2743 function, or use this relation:
2745 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2751 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2752 May be interrupted by sending the process a SIGALRM. Returns the
2753 number of seconds actually slept. You probably cannot mix alarm() and
2754 sleep() calls, because sleep() is often implemented using alarm().
2756 On some older systems, it may sleep up to a full second less than what
2757 you requested, depending on how it counts seconds. Most modern systems
2758 always sleep the full amount.
2760 For delays of finer granularity than one second, you may use Perl's
2761 syscall() interface to access setitimer(2) if your system supports it,
2762 or else see L</select()> below.
2764 See also the POSIX module's sigpause() function.
2766 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2768 Opens a socket of the specified kind and attaches it to filehandle
2769 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
2770 system call of the same name. You should "use Socket;" first to get
2771 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2773 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2775 Creates an unnamed pair of sockets in the specified domain, of the
2776 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
2777 for the system call of the same name. If unimplemented, yields a fatal
2778 error. Returns TRUE if successful.
2780 =item sort SUBNAME LIST
2782 =item sort BLOCK LIST
2786 Sorts the LIST and returns the sorted list value. Nonexistent values
2787 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2788 in standard string comparison order. If SUBNAME is specified, it
2789 gives the name of a subroutine that returns an integer less than, equal
2790 to, or greater than 0, depending on how the elements of the array are
2791 to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
2792 routines.) SUBNAME may be a scalar variable name, in which case the
2793 value provides the name of the subroutine to use. In place of a
2794 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2797 In the interests of efficiency the normal calling code for subroutines is
2798 bypassed, with the following effects: the subroutine may not be a
2799 recursive subroutine, and the two elements to be compared are passed into
2800 the subroutine not via @_ but as the package global variables $a and
2801 $b (see example below). They are passed by reference, so don't
2802 modify $a and $b. And don't try to declare them as lexicals either.
2804 You also cannot exit out of the sort block or subroutine using any of the
2805 loop control operators described in L<perlsyn> or with goto().
2807 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2808 current collation locale. See L<perllocale>.
2813 @articles = sort @files;
2815 # same thing, but with explicit sort routine
2816 @articles = sort {$a cmp $b} @files;
2818 # now case-insensitively
2819 @articles = sort { uc($a) cmp uc($b)} @files;
2821 # same thing in reversed order
2822 @articles = sort {$b cmp $a} @files;
2824 # sort numerically ascending
2825 @articles = sort {$a <=> $b} @files;
2827 # sort numerically descending
2828 @articles = sort {$b <=> $a} @files;
2830 # sort using explicit subroutine name
2832 $age{$a} <=> $age{$b}; # presuming integers
2834 @sortedclass = sort byage @class;
2836 # this sorts the %age hash by value instead of key
2837 # using an in-line function
2838 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2840 sub backwards { $b cmp $a; }
2841 @harry = ('dog','cat','x','Cain','Abel');
2842 @george = ('gone','chased','yz','Punished','Axed');
2844 # prints AbelCaincatdogx
2845 print sort backwards @harry;
2846 # prints xdogcatCainAbel
2847 print sort @george, 'to', @harry;
2848 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2850 # inefficiently sort by descending numeric compare using
2851 # the first integer after the first = sign, or the
2852 # whole record case-insensitively otherwise
2855 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2860 # same thing, but much more efficiently;
2861 # we'll build auxiliary indices instead
2865 push @nums, /=(\d+)/;
2870 $nums[$b] <=> $nums[$a]
2872 $caps[$a] cmp $caps[$b]
2876 # same thing using a Schwartzian Transform (no temps)
2877 @new = map { $_->[0] }
2878 sort { $b->[1] <=> $a->[1]
2881 } map { [$_, /=(\d+)/, uc($_)] } @old;
2883 If you're using strict, you I<MUST NOT> declare $a
2884 and $b as lexicals. They are package globals. That means
2885 if you're in the C<main> package, it's
2887 @articles = sort {$main::b <=> $main::a} @files;
2891 @articles = sort {$::b <=> $::a} @files;
2893 but if you're in the C<FooPack> package, it's
2895 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2897 The comparison function is required to behave. If it returns
2898 inconsistent results (sometimes saying $x[1] is less than $x[2] and
2899 sometimes saying the opposite, for example) the Perl interpreter will
2900 probably crash and dump core. This is entirely due to and dependent
2901 upon your system's qsort(3) library routine; this routine often avoids
2902 sanity checks in the interest of speed.
2904 =item splice ARRAY,OFFSET,LENGTH,LIST
2906 =item splice ARRAY,OFFSET,LENGTH
2908 =item splice ARRAY,OFFSET
2910 Removes the elements designated by OFFSET and LENGTH from an array, and
2911 replaces them with the elements of LIST, if any. Returns the elements
2912 removed from the array. The array grows or shrinks as necessary. If
2913 LENGTH is omitted, removes everything from OFFSET onward. The
2914 following equivalences hold (assuming C<$[ == 0>):
2916 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2917 pop(@a) splice(@a,-1)
2918 shift(@a) splice(@a,0,1)
2919 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2920 $a[$x] = $y splice(@a,$x,1,$y);
2922 Example, assuming array lengths are passed before arrays:
2924 sub aeq { # compare two list values
2925 local(@a) = splice(@_,0,shift);
2926 local(@b) = splice(@_,0,shift);
2927 return 0 unless @a == @b; # same len?
2929 return 0 if pop(@a) ne pop(@b);
2933 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2935 =item split /PATTERN/,EXPR,LIMIT
2937 =item split /PATTERN/,EXPR
2939 =item split /PATTERN/
2943 Splits a string into an array of strings, and returns it.
2945 If not in a list context, returns the number of fields found and splits into
2946 the @_ array. (In a list context, you can force the split into @_ by
2947 using C<??> as the pattern delimiters, but it still returns the array
2948 value.) The use of implicit split to @_ is deprecated, however.
2950 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2951 splits on whitespace (after skipping any leading whitespace). Anything
2952 matching PATTERN is taken to be a delimiter separating the fields. (Note
2953 that the delimiter may be longer than one character.) If LIMIT is
2954 specified and is not negative, splits into no more than that many fields
2955 (though it may split into fewer). If LIMIT is unspecified, trailing null
2956 fields are stripped (which potential users of pop() would do well to
2957 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2958 LIMIT had been specified.
2960 A pattern matching the null string (not to be confused with
2961 a null pattern C<//>, which is just one member of the set of patterns
2962 matching a null string) will split the value of EXPR into separate
2963 characters at each point it matches that way. For example:
2965 print join(':', split(/ */, 'hi there'));
2967 produces the output 'h:i:t:h:e:r:e'.
2969 The LIMIT parameter can be used to split a line partially
2971 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2973 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2974 one larger than the number of variables in the list, to avoid
2975 unnecessary work. For the list above LIMIT would have been 4 by
2976 default. In time critical applications it behooves you not to split
2977 into more fields than you really need.
2979 If the PATTERN contains parentheses, additional array elements are
2980 created from each matching substring in the delimiter.
2982 split(/([,-])/, "1-10,20", 3);
2984 produces the list value
2986 (1, '-', 10, ',', 20)
2988 If you had the entire header of a normal Unix email message in $header,
2989 you could split it up into fields and their values this way:
2991 $header =~ s/\n\s+/ /g; # fix continuation lines
2992 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2994 The pattern C</PATTERN/> may be replaced with an expression to specify
2995 patterns that vary at runtime. (To do runtime compilation only once,
2996 use C</$variable/o>.)
2998 As a special case, specifying a PATTERN of space (C<' '>) will split on
2999 white space just as split with no arguments does. Thus, split(' ') can
3000 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3001 will give you as many null initial fields as there are leading spaces.
3002 A split on /\s+/ is like a split(' ') except that any leading
3003 whitespace produces a null first field. A split with no arguments
3004 really does a C<split(' ', $_)> internally.
3008 open(passwd, '/etc/passwd');
3010 ($login, $passwd, $uid, $gid, $gcos,
3011 $home, $shell) = split(/:/);
3015 (Note that $shell above will still have a newline on it. See L</chop>,
3016 L</chomp>, and L</join>.)
3018 =item sprintf FORMAT, LIST
3020 Returns a string formatted by the usual printf conventions of the C
3021 language. See L<sprintf(3)> or L<printf(3)> on your system for details.
3022 (The * character for an indirectly specified length is not
3023 supported, but you can get the same effect by interpolating a variable
3024 into the pattern.) If C<use locale> is
3025 in effect, the character used for the decimal point in formatted real numbers
3026 is affected by the LC_NUMERIC locale. See L<perllocale>.
3027 Some C libraries' implementations of sprintf() can
3028 dump core when fed ludicrous arguments.
3034 Return the square root of EXPR. If EXPR is omitted, returns square
3041 Sets the random number seed for the C<rand> operator. If EXPR is
3042 omitted, uses a semi-random value based on the current time and process
3043 ID, among other things. In versions of Perl prior to 5.004 the default
3044 seed was just the current time(). This isn't a particularly good seed,
3045 so many old programs supply their own seed value (often C<time ^ $$> or
3046 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
3048 In fact, it's usually not necessary to call srand() at all, because if
3049 it is not called explicitly, it is called implicitly at the first use of
3050 the C<rand> operator.
3052 However, you need something much more random than the default seed for
3053 cryptographic purposes. Checksumming the compressed output of one or
3054 more rapidly changing operating system status programs is the usual
3055 method. For example:
3057 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3059 If you're particularly concerned with this, see the Math::TrulyRandom
3062 Do I<not> call srand() multiple times in your program unless you know
3063 exactly what you're doing and why you're doing it. The point of the
3064 function is to "seed" the rand() function so that rand() can produce
3065 a different sequence each time you run your program. Just do it once at the
3066 top of your program, or you I<won't> get random numbers out of rand()!
3068 Frequently called programs (like CGI scripts) that simply use
3072 for a seed can fall prey to the mathematical property that
3076 one-third of the time. So don't do that.
3078 =item stat FILEHANDLE
3084 Returns a 13-element array giving the status info for a file, either the
3085 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
3086 stats $_. Returns a null list if the stat fails. Typically used as
3090 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3091 $atime,$mtime,$ctime,$blksize,$blocks)
3094 Not all fields are supported on all filesystem types. Here are the
3095 meaning of the fields:
3097 dev device number of filesystem
3099 mode file mode (type and permissions)
3100 nlink number of (hard) links to the file
3101 uid numeric user ID of file's owner
3102 gid numeric group ID of file's owner
3103 rdev the device identifier (special files only)
3104 size total size of file, in bytes
3105 atime last access time since the epoch
3106 mtime last modify time since the epoch
3107 ctime inode change time (NOT creation time!) since the epoch
3108 blksize preferred block size for file system I/O
3109 blocks actual number of blocks allocated
3111 (The epoch was at 00:00 January 1, 1970 GMT.)
3113 If stat is passed the special filehandle consisting of an underline, no
3114 stat is done, but the current contents of the stat structure from the
3115 last stat or filetest are returned. Example:
3117 if (-x $file && (($d) = stat(_)) && $d < 0) {
3118 print "$file is executable NFS file\n";
3121 (This works on machines only for which the device number is negative under NFS.)
3127 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3128 doing many pattern matches on the string before it is next modified.
3129 This may or may not save time, depending on the nature and number of
3130 patterns you are searching on, and on the distribution of character
3131 frequencies in the string to be searched--you probably want to compare
3132 run times with and without it to see which runs faster. Those loops
3133 which scan for many short constant strings (including the constant
3134 parts of more complex patterns) will benefit most. You may have only
3135 one study active at a time--if you study a different scalar the first
3136 is "unstudied". (The way study works is this: a linked list of every
3137 character in the string to be searched is made, so we know, for
3138 example, where all the 'k' characters are. From each search string,
3139 the rarest character is selected, based on some static frequency tables
3140 constructed from some C programs and English text. Only those places
3141 that contain this "rarest" character are examined.)
3143 For example, here is a loop which inserts index producing entries
3144 before any line containing a certain pattern:
3148 print ".IX foo\n" if /\bfoo\b/;
3149 print ".IX bar\n" if /\bbar\b/;
3150 print ".IX blurfl\n" if /\bblurfl\b/;
3155 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3156 will be looked at, because "f" is rarer than "o". In general, this is
3157 a big win except in pathological cases. The only question is whether
3158 it saves you more time than it took to build the linked list in the
3161 Note that if you have to look for strings that you don't know till
3162 runtime, you can build an entire loop as a string and eval that to
3163 avoid recompiling all your patterns all the time. Together with
3164 undefining $/ to input entire files as one record, this can be very
3165 fast, often faster than specialized programs like fgrep(1). The following
3166 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3167 out the names of those files that contain a match:
3169 $search = 'while (<>) { study;';
3170 foreach $word (@words) {
3171 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3176 eval $search; # this screams
3177 $/ = "\n"; # put back to normal input delimiter
3178 foreach $file (sort keys(%seen)) {
3186 =item sub NAME BLOCK
3188 This is subroutine definition, not a real function I<per se>. With just a
3189 NAME (and possibly prototypes), it's just a forward declaration. Without
3190 a NAME, it's an anonymous function declaration, and does actually return a
3191 value: the CODE ref of the closure you just created. See L<perlsub> and
3192 L<perlref> for details.
3194 =item substr EXPR,OFFSET,LEN
3196 =item substr EXPR,OFFSET
3198 Extracts a substring out of EXPR and returns it. First character is at
3199 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
3200 that far from the end of the string. If LEN is omitted, returns
3201 everything to the end of the string. If LEN is negative, leaves that
3202 many characters off the end of the string.
3204 You can use the substr() function
3205 as an lvalue, in which case EXPR must be an lvalue. If you assign
3206 something shorter than LEN, the string will shrink, and if you assign
3207 something longer than LEN, the string will grow to accommodate it. To
3208 keep the string the same length you may need to pad or chop your value
3211 =item symlink OLDFILE,NEWFILE
3213 Creates a new filename symbolically linked to the old filename.
3214 Returns 1 for success, 0 otherwise. On systems that don't support
3215 symbolic links, produces a fatal error at run time. To check for that,
3218 $symlink_exists = (eval 'symlink("","");', $@ eq '');
3222 Calls the system call specified as the first element of the list,
3223 passing the remaining elements as arguments to the system call. If
3224 unimplemented, produces a fatal error. The arguments are interpreted
3225 as follows: if a given argument is numeric, the argument is passed as
3226 an int. If not, the pointer to the string value is passed. You are
3227 responsible to make sure a string is pre-extended long enough to
3228 receive any result that might be written into a string. If your
3229 integer arguments are not literals and have never been interpreted in a
3230 numeric context, you may need to add 0 to them to force them to look
3233 require 'syscall.ph'; # may need to run h2ph
3234 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3236 Note that Perl supports passing of up to only 14 arguments to your system call,
3237 which in practice should usually suffice.
3239 =item sysopen FILEHANDLE,FILENAME,MODE
3241 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3243 Opens the file whose filename is given by FILENAME, and associates it
3244 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3245 the name of the real filehandle wanted. This function calls the
3246 underlying operating system's C<open> function with the parameters
3247 FILENAME, MODE, PERMS.
3249 The possible values and flag bits of the MODE parameter are
3250 system-dependent; they are available via the standard module C<Fcntl>.
3251 However, for historical reasons, some values are universal: zero means
3252 read-only, one means write-only, and two means read/write.
3254 If the file named by FILENAME does not exist and the C<open> call
3255 creates it (typically because MODE includes the O_CREAT flag), then
3256 the value of PERMS specifies the permissions of the newly created
3257 file. If PERMS is omitted, the default value is 0666, which allows
3258 read and write for all. This default is reasonable: see C<umask>.
3260 The IO::File module provides a more object-oriented approach, if you're
3261 into that kind of thing.
3263 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3265 =item sysread FILEHANDLE,SCALAR,LENGTH
3267 Attempts to read LENGTH bytes of data into variable SCALAR from the
3268 specified FILEHANDLE, using the system call read(2). It bypasses
3269 stdio, so mixing this with other kinds of reads may cause confusion.
3270 Returns the number of bytes actually read, or undef if there was an
3271 error. SCALAR will be grown or shrunk so that the last byte actually
3272 read is the last byte of the scalar after the read.
3274 An OFFSET may be specified to place the read data at some place in the
3275 string other than the beginning. A negative OFFSET specifies
3276 placement at that many bytes counting backwards from the end of the
3277 string. A positive OFFSET greater than the length of SCALAR results
3278 in the string being padded to the required size with "\0" bytes before
3279 the result of the read is appended.
3283 Does exactly the same thing as "exec LIST" except that a fork is done
3284 first, and the parent process waits for the child process to complete.
3285 Note that argument processing varies depending on the number of
3286 arguments. The return value is the exit status of the program as
3287 returned by the wait() call. To get the actual exit value divide by
3288 256. See also L</exec>. This is I<NOT> what you want to use to capture
3289 the output from a command, for that you should use merely back-ticks or
3290 qx//, as described in L<perlop/"`STRING`">.
3292 Because system() and back-ticks block SIGINT and SIGQUIT, killing the
3293 program they're running doesn't actually interrupt your program.
3295 @args = ("command", "arg1", "arg2");
3297 or die "system @args failed: $?"
3299 Here's a more elaborate example of analysing the return value from
3300 system() on a UNIX system to check for all possibilities, including for
3301 signals and coredumps.
3303 $rc = 0xffff & system @args;
3304 printf "system(%s) returned %#04x: ", "@args", $rc;
3306 print "ran with normal exit\n";
3308 elsif ($rc == 0xff00) {
3309 print "command failed: $!\n";
3311 elsif ($rc > 0x80) {
3313 print "ran with non-zero exit status $rc\n";
3319 print "coredump from ";
3321 print "signal $rc\n"
3325 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3327 =item syswrite FILEHANDLE,SCALAR,LENGTH
3329 Attempts to write LENGTH bytes of data from variable SCALAR to the
3330 specified FILEHANDLE, using the system call write(2). It bypasses
3331 stdio, so mixing this with prints may cause confusion. Returns the
3332 number of bytes actually written, or undef if there was an error.
3333 If the length is greater than the available data, only as much data as
3334 is available will be written.
3336 An OFFSET may be specified to write the data from some part of the
3337 string other than the beginning. A negative OFFSET specifies writing
3338 from that many bytes counting backwards from the end of the string.
3340 =item tell FILEHANDLE
3344 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3345 expression whose value gives the name of the actual filehandle. If
3346 FILEHANDLE is omitted, assumes the file last read.
3348 =item telldir DIRHANDLE
3350 Returns the current position of the readdir() routines on DIRHANDLE.
3351 Value may be given to seekdir() to access a particular location in a
3352 directory. Has the same caveats about possible directory compaction as
3353 the corresponding system library routine.
3355 =item tie VARIABLE,CLASSNAME,LIST
3357 This function binds a variable to a package class that will provide the
3358 implementation for the variable. VARIABLE is the name of the variable
3359 to be enchanted. CLASSNAME is the name of a class implementing objects
3360 of correct type. Any additional arguments are passed to the "new"
3361 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3362 Typically these are arguments such as might be passed to the dbm_open()
3363 function of C. The object returned by the "new" method is also
3364 returned by the tie() function, which would be useful if you want to
3365 access other methods in CLASSNAME.
3367 Note that functions such as keys() and values() may return huge array
3368 values when used on large objects, like DBM files. You may prefer to
3369 use the each() function to iterate over such. Example:
3371 # print out history file offsets
3373 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3374 while (($key,$val) = each %HIST) {
3375 print $key, ' = ', unpack('L',$val), "\n";
3379 A class implementing a hash should have the following methods:
3381 TIEHASH classname, LIST
3384 STORE this, key, value
3388 NEXTKEY this, lastkey
3390 A class implementing an ordinary array should have the following methods:
3392 TIEARRAY classname, LIST
3395 STORE this, key, value
3398 A class implementing a scalar should have the following methods:
3400 TIESCALAR classname, LIST
3405 Unlike dbmopen(), the tie() function will not use or require a module
3406 for you--you need to do that explicitly yourself. See L<DB_File>
3407 or the F<Config> module for interesting tie() implementations.
3411 Returns a reference to the object underlying VARIABLE (the same value
3412 that was originally returned by the tie() call which bound the variable
3413 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3418 Returns the number of non-leap seconds since whatever time the system
3419 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3420 and 00:00:00 UTC, January 1, 1970 for most other systems).
3421 Suitable for feeding to gmtime() and localtime().
3425 Returns a four-element array giving the user and system times, in
3426 seconds, for this process and the children of this process.
3428 ($user,$system,$cuser,$csystem) = times;
3432 The translation operator. See L<perlop>.
3434 =item truncate FILEHANDLE,LENGTH
3436 =item truncate EXPR,LENGTH
3438 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3439 specified length. Produces a fatal error if truncate isn't implemented
3446 Returns an uppercased version of EXPR. This is the internal function
3447 implementing the \U escape in double-quoted strings.
3448 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3450 If EXPR is omitted, uses $_.
3456 Returns the value of EXPR with the first character uppercased. This is
3457 the internal function implementing the \u escape in double-quoted strings.
3458 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3460 If EXPR is omitted, uses $_.
3466 Sets the umask for the process and returns the old one. If EXPR is
3467 omitted, returns merely the current umask.
3473 Undefines the value of EXPR, which must be an lvalue. Use on only a
3474 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3475 will probably not do what you expect on most predefined variables or
3476 DBM list values, so don't do that.) Always returns the undefined value. You can omit
3477 the EXPR, in which case nothing is undefined, but you still get an
3478 undefined value that you could, for instance, return from a
3479 subroutine. Examples:
3482 undef $bar{'blurfl'};
3486 return (wantarray ? () : undef) if $they_blew_it;
3492 Deletes a list of files. Returns the number of files successfully
3495 $cnt = unlink 'a', 'b', 'c';
3499 Note: unlink will not delete directories unless you are superuser and
3500 the B<-U> flag is supplied to Perl. Even if these conditions are
3501 met, be warned that unlinking a directory can inflict damage on your
3502 filesystem. Use rmdir instead.
3504 If LIST is omitted, uses $_.
3506 =item unpack TEMPLATE,EXPR
3508 Unpack does the reverse of pack: it takes a string representing a
3509 structure and expands it out into a list value, returning the array
3510 value. (In a scalar context, it returns merely the first value
3511 produced.) The TEMPLATE has the same format as in the pack function.
3512 Here's a subroutine that does substring:
3515 local($what,$where,$howmuch) = @_;
3516 unpack("x$where a$howmuch", $what);
3521 sub ordinal { unpack("c",$_[0]); } # same as ord()
3523 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3524 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3525 themselves. Default is a 16-bit checksum. For example, the following
3526 computes the same number as the System V sum program:
3529 $checksum += unpack("%16C*", $_);
3533 The following efficiently counts the number of set bits in a bit vector:
3535 $setbits = unpack("%32b*", $selectmask);
3537 =item untie VARIABLE
3539 Breaks the binding between a variable and a package. (See tie().)
3541 =item unshift ARRAY,LIST
3543 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3544 depending on how you look at it. Prepends list to the front of the
3545 array, and returns the new number of elements in the array.
3547 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3549 Note the LIST is prepended whole, not one element at a time, so the
3550 prepended elements stay in the same order. Use reverse to do the
3553 =item use Module LIST
3557 =item use Module VERSION LIST
3561 Imports some semantics into the current package from the named module,
3562 generally by aliasing certain subroutine or variable names into your
3563 package. It is exactly equivalent to
3565 BEGIN { require Module; import Module LIST; }
3567 except that Module I<must> be a bare word.
3569 If the first argument to C<use> is a number, it is treated as a version
3570 number instead of a module name. If the version of the Perl interpreter
3571 is less than VERSION, then an error message is printed and Perl exits
3572 immediately. This is often useful if you need to check the current
3573 Perl version before C<use>ing library modules which have changed in
3574 incompatible ways from older versions of Perl. (We try not to do
3575 this more than we have to.)
3577 The BEGIN forces the require and import to happen at compile time. The
3578 require makes sure the module is loaded into memory if it hasn't been
3579 yet. The import is not a builtin--it's just an ordinary static method
3580 call into the "Module" package to tell the module to import the list of
3581 features back into the current package. The module can implement its
3582 import method any way it likes, though most modules just choose to
3583 derive their import method via inheritance from the Exporter class that
3584 is defined in the Exporter module. See L<Exporter>. If no import
3585 method can be found then the error is currently silently ignored. This
3586 may change to a fatal error in a future version.
3588 If you don't want your namespace altered, explicitly supply an empty list:
3592 That is exactly equivalent to
3594 BEGIN { require Module; }
3596 If the VERSION argument is present between Module and LIST, then the
3597 C<use> will call the VERSION method in class Module with the given
3598 version as an argument. The default VERSION method, inherited from
3599 the Universal class, croaks if the given version is larger than the
3600 value of the variable $Module::VERSION. (Note that there is not a
3601 comma after VERSION!)
3603 Because this is a wide-open interface, pragmas (compiler directives)
3604 are also implemented this way. Currently implemented pragmas are:
3608 use sigtrap qw(SEGV BUS);
3609 use strict qw(subs vars refs);
3610 use subs qw(afunc blurfl);
3612 These pseudo-modules import semantics into the current block scope, unlike
3613 ordinary modules, which import symbols into the current package (which are
3614 effective through the end of the file).
3616 There's a corresponding "no" command that unimports meanings imported
3617 by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
3622 If no unimport method can be found the call fails with a fatal error.
3624 See L<perlmod> for a list of standard modules and pragmas.
3628 Changes the access and modification times on each file of a list of
3629 files. The first two elements of the list must be the NUMERICAL access
3630 and modification times, in that order. Returns the number of files
3631 successfully changed. The inode modification time of each file is set
3632 to the current time. Example of a "touch" command:
3636 utime $now, $now, @ARGV;
3640 Returns a normal array consisting of all the values of the named hash.
3641 (In a scalar context, returns the number of values.) The values are
3642 returned in an apparently random order, but it is the same order as either
3643 the keys() or each() function would produce on the same hash. As a side
3644 effect, it resets HASH's iterator. See also keys(), each(), and sort().
3646 =item vec EXPR,OFFSET,BITS
3648 Treats the string in EXPR as a vector of unsigned integers, and
3649 returns the value of the bit field specified by OFFSET. BITS specifies
3650 the number of bits that are reserved for each entry in the bit
3651 vector. This must be a power of two from 1 to 32. vec() may also be
3652 assigned to, in which case parentheses are needed to give the expression
3653 the correct precedence as in
3655 vec($image, $max_x * $x + $y, 8) = 3;
3657 Vectors created with vec() can also be manipulated with the logical
3658 operators |, &, and ^, which will assume a bit vector operation is
3659 desired when both operands are strings.
3661 To transform a bit vector into a string or array of 0's and 1's, use these:
3663 $bits = unpack("b*", $vector);
3664 @bits = split(//, unpack("b*", $vector));
3666 If you know the exact length in bits, it can be used in place of the *.
3670 Waits for a child process to terminate and returns the pid of the
3671 deceased process, or -1 if there are no child processes. The status is
3674 =item waitpid PID,FLAGS
3676 Waits for a particular child process to terminate and returns the pid
3677 of the deceased process, or -1 if there is no such child process. The
3678 status is returned in C<$?>. If you say
3680 use POSIX ":sys_wait_h";
3682 waitpid(-1,&WNOHANG);
3684 then you can do a non-blocking wait for any process. Non-blocking wait
3685 is available on machines supporting either the waitpid(2) or
3686 wait4(2) system calls. However, waiting for a particular pid with
3687 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3688 by remembering the status values of processes that have exited but have
3689 not been harvested by the Perl script yet.)
3693 Returns TRUE if the context of the currently executing subroutine is
3694 looking for a list value. Returns FALSE if the context is looking
3697 return wantarray ? () : undef;
3701 Produces a message on STDERR just like die(), but doesn't exit or throw
3704 No message is printed if there is a C<$SIG{__WARN__}> handler
3705 installed. It is the handler's responsibility to deal with the message
3706 as it sees fit (like, for instance, converting it into a die()). Most
3707 handlers must therefore make arrangements to actually display the
3708 warnings that they are not prepared to deal with, by calling warn()
3709 again in the handler. Note that this is quite safe and will not
3710 produce an endless loop, since C<__WARN__> hooks are not called from
3713 You will find this behavior is slightly different from that of
3714 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
3715 instead call die() again to change it).
3717 Using a C<__WARN__> handler provides a powerful way to silence all
3718 warnings (even the so-called mandatory ones). An example:
3720 # wipe out *all* compile-time warnings
3721 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
3723 my $foo = 20; # no warning about duplicate my $foo,
3724 # but hey, you asked for it!
3725 # no compile-time or run-time warnings before here
3728 # run-time warnings enabled after here
3729 warn "\$foo is alive and $foo!"; # does show up
3731 See L<perlvar> for details on setting C<%SIG> entries, and for more
3734 =item write FILEHANDLE
3740 Writes a formatted record (possibly multi-line) to the specified file,
3741 using the format associated with that file. By default the format for
3742 a file is the one having the same name is the filehandle, but the
3743 format for the current output channel (see the select() function) may be set
3744 explicitly by assigning the name of the format to the C<$~> variable.
3746 Top of form processing is handled automatically: if there is
3747 insufficient room on the current page for the formatted record, the
3748 page is advanced by writing a form feed, a special top-of-page format
3749 is used to format the new page header, and then the record is written.
3750 By default the top-of-page format is the name of the filehandle with
3751 "_TOP" appended, but it may be dynamically set to the format of your
3752 choice by assigning the name to the C<$^> variable while the filehandle is
3753 selected. The number of lines remaining on the current page is in
3754 variable C<$->, which can be set to 0 to force a new page.
3756 If FILEHANDLE is unspecified, output goes to the current default output
3757 channel, which starts out as STDOUT but may be changed by the
3758 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3759 is evaluated and the resulting string is used to look up the name of
3760 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3762 Note that write is I<NOT> the opposite of read. Unfortunately.
3766 The translation operator. See L<perlop>.