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
197 =head2 Alphabetical Listing of Perl Functions
208 A file test, where X is one of the letters listed below. This unary
209 operator takes one argument, either a filename or a filehandle, and
210 tests the associated file to see if something is true about it. If the
211 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
212 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
213 the undefined value if the file doesn't exist. Despite the funny
214 names, precedence is the same as any other named unary operator, and
215 the argument may be parenthesized like any other unary operator. The
216 operator may be any of:
218 -r File is readable by effective uid/gid.
219 -w File is writable by effective uid/gid.
220 -x File is executable by effective uid/gid.
221 -o File is owned by effective uid.
223 -R File is readable by real uid/gid.
224 -W File is writable by real uid/gid.
225 -X File is executable by real uid/gid.
226 -O File is owned by real uid.
229 -z File has zero size.
230 -s File has non-zero size (returns size).
232 -f File is a plain file.
233 -d File is a directory.
234 -l File is a symbolic link.
235 -p File is a named pipe (FIFO).
237 -b File is a block special file.
238 -c File is a character special file.
239 -t Filehandle is opened to a tty.
241 -u File has setuid bit set.
242 -g File has setgid bit set.
243 -k File has sticky bit set.
245 -T File is a text file.
246 -B File is a binary file (opposite of -T).
248 -M Age of file in days when script started.
249 -A Same for access time.
250 -C Same for inode change time.
252 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
253 C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
254 uids and gids of the user. There may be other reasons you can't actually
255 read, write or execute the file. Also note that, for the superuser,
256 C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
257 1 if any execute bit is set in the mode. Scripts run by the superuser may
258 thus need to do a stat() to determine the actual mode of the
259 file, or temporarily set the uid to something else.
265 next unless -f $_; # ignore specials
269 Note that C<-s/a/b/> does not do a negated substitution. Saying
270 C<-exp($foo)> still works as expected, however--only single letters
271 following a minus are interpreted as file tests.
273 The C<-T> and C<-B> switches work as follows. The first block or so of the
274 file is examined for odd characters such as strange control codes or
275 characters with the high bit set. If too many odd characters (E<gt>30%)
276 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
277 containing null in the first block is considered a binary file. If C<-T>
278 or C<-B> is used on a filehandle, the current stdio buffer is examined
279 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
280 file, or a file at EOF when testing a filehandle. Because you have to
281 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
282 against the file first, as in C<next unless -f $file && -T $file>.
284 If any of the file tests (or either the stat() or lstat() operators) are given
285 the special filehandle consisting of a solitary underline, then the stat
286 structure of the previous file test (or stat operator) is used, saving
287 a system call. (This doesn't work with C<-t>, and you need to remember
288 that lstat() and C<-l> will leave values in the stat structure for the
289 symbolic link, not the real file.) Example:
291 print "Can do.\n" if -r $a || -w _ || -x _;
294 print "Readable\n" if -r _;
295 print "Writable\n" if -w _;
296 print "Executable\n" if -x _;
297 print "Setuid\n" if -u _;
298 print "Setgid\n" if -g _;
299 print "Sticky\n" if -k _;
300 print "Text\n" if -T _;
301 print "Binary\n" if -B _;
307 Returns the absolute value of its argument.
308 If VALUE is omitted, uses $_.
310 =item accept NEWSOCKET,GENERICSOCKET
312 Accepts an incoming socket connect, just as the accept(2) system call
313 does. Returns the packed address if it succeeded, FALSE otherwise.
314 See example in L<perlipc/"Sockets: Client/Server Communication">.
320 Arranges to have a SIGALRM delivered to this process after the
321 specified number of seconds have elapsed. If SECONDS is not specified,
322 the value stored in $_ is used. (On some machines,
323 unfortunately, the elapsed time may be up to one second less than you
324 specified because of how seconds are counted.) Only one timer may be
325 counting at once. Each call disables the previous timer, and an
326 argument of 0 may be supplied to cancel the previous timer without
327 starting a new one. The returned value is the amount of time remaining
328 on the previous timer.
330 For delays of finer granularity than one second, you may use Perl's
331 syscall() interface to access setitimer(2) if your system supports it,
332 or else see L</select()> below. It is not advised to intermix alarm()
335 If you want to use alarm() to time out a system call you need to use an
336 eval/die pair. You can't rely on the alarm causing the system call to
337 fail with $! set to EINTR because Perl sets up signal handlers to
338 restart system calls on some systems. Using eval/die always works.
341 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
343 $nread = sysread SOCKET, $buffer, $size;
346 die if $@ && $@ ne "alarm\n"; # propagate errors
356 Returns the arctangent of Y/X in the range -PI to PI.
358 For the tangent operation, you may use the POSIX::tan()
359 function, or use the familiar relation:
361 sub tan { sin($_[0]) / cos($_[0]) }
363 =item bind SOCKET,NAME
365 Binds a network address to a socket, just as the bind system call
366 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
367 packed address of the appropriate type for the socket. See the examples in
368 L<perlipc/"Sockets: Client/Server Communication">.
370 =item binmode FILEHANDLE
372 Arranges for the file to be read or written in "binary" mode in operating
373 systems that distinguish between binary and text files. Files that are
374 not in binary mode have CR LF sequences translated to LF on input and LF
375 translated to CR LF on output. Binmode has no effect under Unix; in DOS
376 and similarly archaic systems, it may be imperative--otherwise your
377 DOS-damaged C library may mangle your file. The key distinction between
378 systems that need binmode and those that don't is their text file
379 formats. Systems like Unix and Plan9 that delimit lines with a single
380 character, and that encode that character in C as '\n', do not need
381 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
382 is taken as the name of the filehandle.
384 =item bless REF,CLASSNAME
388 This function tells the thingy referenced by REF that it is now
389 an object in the CLASSNAME package--or the current package if no CLASSNAME
390 is specified, which is often the case. It returns the reference for
391 convenience, because a bless() is often the last thing in a constructor.
392 Always use the two-argument version if the function doing the blessing
393 might be inherited by a derived class. See L<perlobj> for more about the
394 blessing (and blessings) of objects.
400 Returns the context of the current subroutine call. In a scalar context,
401 returns the caller's package name if there is a caller, that is, if
402 we're in a subroutine or eval() or require(), and the undefined value
403 otherwise. In a list context, returns
405 ($package, $filename, $line) = caller;
407 With EXPR, it returns some extra information that the debugger uses to
408 print a stack trace. The value of EXPR indicates how many call frames
409 to go back before the current one.
411 ($package, $filename, $line, $subroutine,
412 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
414 Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
415 call, but C<L<eval>>. In such a case additional elements $evaltext and
416 $is_require are set: $is_require is true if the frame is created by
417 C<L<require>> or C<L<use>> statement, $evaltext contains the text of
418 C<L<eval EXPR>> statement. In particular, for C<L<eval BLOCK>>
419 statement $filename is C<"(eval)">, but $evaltext is undefined. (Note
420 also that C<L<use>> statement creates a C<L<require>> frame inside
421 an C<L<eval EXPR>>) frame.
423 Furthermore, when called from within the DB package, caller returns more
424 detailed information: it sets the list variable @DB::args to be the
425 arguments with which that subroutine was invoked.
429 Changes the working directory to EXPR, if possible. If EXPR is
430 omitted, changes to home directory. Returns TRUE upon success, FALSE
431 otherwise. See example under die().
435 Changes the permissions of a list of files. The first element of the
436 list must be the numerical mode, which should probably be an octal
437 number. Returns the number of files successfully changed.
439 $cnt = chmod 0755, 'foo', 'bar';
440 chmod 0755, @executables;
448 This is a slightly safer version of chop (see below). It removes any
449 line ending that corresponds to the current value of C<$/> (also known as
450 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
451 number of characters removed from all its arguments. It's often used to
452 remove the newline from the end of an input record when you're worried
453 that the final record may be missing its newline. When in paragraph mode
454 (C<$/ = "">), it removes all trailing newlines from the string. If
455 VARIABLE is omitted, it chomps $_. Example:
458 chomp; # avoid \n on last field
463 You can actually chomp anything that's an lvalue, including an assignment:
466 chomp($answer = <STDIN>);
468 If you chomp a list, each element is chomped, and the total number of
469 characters removed is returned.
477 Chops off the last character of a string and returns the character
478 chopped. It's used primarily to remove the newline from the end of an
479 input record, but is much more efficient than C<s/\n//> because it neither
480 scans nor copies the string. If VARIABLE is omitted, chops $_.
484 chop; # avoid \n on last field
489 You can actually chop anything that's an lvalue, including an assignment:
492 chop($answer = <STDIN>);
494 If you chop a list, each element is chopped. Only the value of the
495 last chop is returned.
497 Note that chop returns the last character. To return all but the last
498 character, use C<substr($string, 0, -1)>.
502 Changes the owner (and group) of a list of files. The first two
503 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
504 Returns the number of files successfully changed.
506 $cnt = chown $uid, $gid, 'foo', 'bar';
507 chown $uid, $gid, @filenames;
509 Here's an example that looks up non-numeric uids in the passwd file:
512 chop($user = <STDIN>);
514 chop($pattern = <STDIN>);
516 ($login,$pass,$uid,$gid) = getpwnam($user)
517 or die "$user not in passwd file";
519 @ary = <${pattern}>; # expand filenames
520 chown $uid, $gid, @ary;
522 On most systems, you are not allowed to change the ownership of the
523 file unless you're the superuser, although you should be able to change
524 the group to any of your secondary groups. On insecure systems, these
525 restrictions may be relaxed, but this is not a portable assumption.
531 Returns the character represented by that NUMBER in the character set.
532 For example, C<chr(65)> is "A" in ASCII.
534 If NUMBER is omitted, uses $_.
536 =item chroot FILENAME
540 This function works as the system call by the same name: it makes the
541 named directory the new root directory for all further pathnames that
542 begin with a "/" by your process and all of its children. (It doesn't
543 change your current working directory, which is unaffected.) For security
544 reasons, this call is restricted to the superuser. If FILENAME is
545 omitted, does chroot to $_.
547 =item close FILEHANDLE
549 Closes the file or pipe associated with the file handle, returning TRUE
550 only if stdio successfully flushes buffers and closes the system file
551 descriptor. You don't have to close FILEHANDLE if you are immediately
552 going to do another open() on it, because open() will close it for you. (See
553 open().) However, an explicit close on an input file resets the line
554 counter ($.), while the implicit close done by open() does not. Also,
555 closing a pipe will wait for the process executing on the pipe to
556 complete, in case you want to look at the output of the pipe
557 afterwards. Closing a pipe explicitly also puts the status value of
558 the command into C<$?>. Example:
560 open(OUTPUT, '|sort >foo'); # pipe to sort
561 ... # print stuff to output
562 close OUTPUT; # wait for sort to finish
563 open(INPUT, 'foo'); # get sort's results
565 FILEHANDLE may be an expression whose value gives the real filehandle name.
567 =item closedir DIRHANDLE
569 Closes a directory opened by opendir().
571 =item connect SOCKET,NAME
573 Attempts to connect to a remote socket, just as the connect system call
574 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
575 packed address of the appropriate type for the socket. See the examples in
576 L<perlipc/"Sockets: Client/Server Communication">.
580 Actually a flow control statement rather than a function. If there is a
581 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
582 C<foreach>), it is always executed just before the conditional is about to
583 be evaluated again, just like the third part of a C<for> loop in C. Thus
584 it can be used to increment a loop variable, even when the loop has been
585 continued via the C<next> statement (which is similar to the C C<continue>
590 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
593 For the inverse cosine operation, you may use the POSIX::acos()
594 function, or use this relation:
596 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
598 =item crypt PLAINTEXT,SALT
600 Encrypts a string exactly like the crypt(3) function in the C library
601 (assuming that you actually have a version there that has not been
602 extirpated as a potential munition). This can prove useful for checking
603 the password file for lousy passwords, amongst other things. Only the
604 guys wearing white hats should do this.
606 Here's an example that makes sure that whoever runs this program knows
609 $pwd = (getpwuid($<))[1];
610 $salt = substr($pwd, 0, 2);
614 chop($word = <STDIN>);
618 if (crypt($word, $salt) ne $pwd) {
624 Of course, typing in your own password to whomever asks you
627 =item dbmclose ASSOC_ARRAY
629 [This function has been superseded by the untie() function.]
631 Breaks the binding between a DBM file and an associative array.
633 =item dbmopen ASSOC,DBNAME,MODE
635 [This function has been superseded by the tie() function.]
637 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
638 associative array. ASSOC is the name of the associative array. (Unlike
639 normal open, the first argument is I<NOT> a filehandle, even though it
640 looks like one). DBNAME is the name of the database (without the F<.dir>
641 or F<.pag> extension if any). If the database does not exist, it is
642 created with protection specified by MODE (as modified by the umask()).
643 If your system supports only the older DBM functions, you may perform only
644 one dbmopen() in your program. In older versions of Perl, if your system
645 had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
646 falls back to sdbm(3).
648 If you don't have write access to the DBM file, you can only read
649 associative array variables, not set them. If you want to test whether
650 you can write, either use file tests or try setting a dummy array entry
651 inside an eval(), which will trap the error.
653 Note that functions such as keys() and values() may return huge array
654 values when used on large DBM files. You may prefer to use the each()
655 function to iterate over large DBM files. Example:
657 # print out history file offsets
658 dbmopen(%HIST,'/usr/lib/news/history',0666);
659 while (($key,$val) = each %HIST) {
660 print $key, ' = ', unpack('L',$val), "\n";
664 See also L<AnyDBM_File> for a more general description of the pros and
665 cons of the various dbm approaches, as well as L<DB_File> for a particularly
672 Returns a boolean value saying whether EXPR has a real value
673 or not. If EXPR is not present, $_ will be checked. Many operations
674 return the undefined value under exceptional conditions, such as end of
675 file, uninitialized variable, system error and such. This function
676 allows you to distinguish between an undefined
677 null scalar and a defined null scalar with operations that might return
678 a real null string, such as referencing elements of an array. You may
679 also check to see if arrays or subroutines exist. Use of defined on
680 predefined variables is not guaranteed to produce intuitive results.
682 When used on a hash array element, it tells you whether the value
683 is defined, not whether the key exists in the hash. Use exists() for that.
687 print if defined $switch{'D'};
688 print "$val\n" while defined($val = pop(@ary));
689 die "Can't readlink $sym: $!"
690 unless defined($value = readlink $sym);
691 eval '@foo = ()' if defined(@foo);
692 die "No XYZ package defined" unless defined %_XYZ;
693 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
697 Note: many folks tend to overuse defined(), and then are surprised to
698 discover that the number 0 and the null string are, in fact, defined
699 concepts. For example, if you say
703 the pattern match succeeds, and $1 is defined, despite the fact that it
704 matched "nothing". But it didn't really match nothing--rather, it
705 matched something that happened to be 0 characters long. This is all
706 very above-board and honest. When a function returns an undefined value,
707 it's an admission that it couldn't give you an honest answer. So
708 you should use defined() only when you're questioning the integrity
709 of what you're trying to do. At other times, a simple comparison to
710 0 or "" is what you want.
712 Another surprise is that using defined() on an entire array or
713 hash reports whether memory for that aggregate has ever been
714 allocated. So an array you set to the empty list appears undefined
715 initially, and one that once was full and that you then set to
716 the empty list still appears defined. You should instead use a
717 simple test for size:
719 if (@an_array) { print "has array elements\n" }
720 if (%a_hash) { print "has hash members\n" }
722 Using undef() on these, however, does clear their memory and then report
723 them as not defined anymore, but you shoudln't do that unless you don't
724 plan to use them again, because it saves time when you load them up
725 again to have memory already ready to be filled.
727 This counter-intuitive behaviour of defined() on aggregates may be
728 changed, fixed, or broken in a future release of Perl.
732 Deletes the specified key(s) and their associated values from a hash
733 array. For each key, returns the deleted value associated with that key,
734 or the undefined value if there was no such key. Deleting from C<$ENV{}>
735 modifies the environment. Deleting from an array tied to a DBM file
736 deletes the entry from the DBM file. (But deleting from a tie()d hash
737 doesn't necessarily return anything.)
739 The following deletes all the values of an associative array:
741 foreach $key (keys %HASH) {
747 delete @HASH{keys %HASH}
749 (But both of these are slower than the undef() command.) Note that the
750 EXPR can be arbitrarily complicated as long as the final operation is a
751 hash element lookup or hash slice:
753 delete $ref->[$x][$y]{$key};
754 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
758 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
759 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
760 C<($? E<gt>E<gt> 8)> (back-tick `command` status). If C<($? E<gt>E<gt> 8)>
761 is 0, exits with 255. Inside an eval(), the error message is stuffed into
762 C<$@>, and the eval() is terminated with the undefined value; this makes
763 die() the way to raise an exception.
767 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
768 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
770 If the value of EXPR does not end in a newline, the current script line
771 number and input line number (if any) are also printed, and a newline
772 is supplied. Hint: sometimes appending ", stopped" to your message
773 will cause it to make better sense when the string "at foo line 123" is
774 appended. Suppose you are running script "canasta".
776 die "/etc/games is no good";
777 die "/etc/games is no good, stopped";
779 produce, respectively
781 /etc/games is no good at canasta line 123.
782 /etc/games is no good, stopped at canasta line 123.
784 See also exit() and warn().
788 Not really a function. Returns the value of the last command in the
789 sequence of commands indicated by BLOCK. When modified by a loop
790 modifier, executes the BLOCK once before testing the loop condition.
791 (On other statements the loop modifiers test the conditional first.)
793 =item do SUBROUTINE(LIST)
795 A deprecated form of subroutine call. See L<perlsub>.
799 Uses the value of EXPR as a filename and executes the contents of the
800 file as a Perl script. Its primary use is to include subroutines
801 from a Perl subroutine library.
809 except that it's more efficient, more concise, keeps track of the
810 current filename for error messages, and searches all the B<-I>
811 libraries if the file isn't in the current directory (see also the @INC
812 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
813 re-parse the file every time you call it, so you probably don't want to
814 do this inside a loop.
816 Note that inclusion of library modules is better done with the
817 use() and require() operators, which also do error checking
818 and raise an exception if there's a problem.
822 This causes an immediate core dump. Primarily this is so that you can
823 use the B<undump> program to turn your core dump into an executable binary
824 after having initialized all your variables at the beginning of the
825 program. When the new binary is executed it will begin by executing a
826 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
827 it as a goto with an intervening core dump and reincarnation. If LABEL
828 is omitted, restarts the program from the top. WARNING: any files
829 opened at the time of the dump will NOT be open any more when the
830 program is reincarnated, with possible resulting confusion on the part
831 of Perl. See also B<-u> option in L<perlrun>.
848 dump QUICKSTART if $ARGV[0] eq '-d';
853 =item each ASSOC_ARRAY
855 When called in a list context, returns a 2-element array consisting
856 of the key and value for the next element of an associative array,
857 so that you can iterate over it. When called in a scalar context,
858 returns the key for only the next element in the associative array.
859 Entries are returned in an apparently random order. When the array is
860 entirely read, a null array is returned in list context (which when
861 assigned produces a FALSE (0) value), and C<undef> is returned in a
862 scalar context. The next call to each() after that will start
863 iterating again. The iterator can be reset only by reading all the
864 elements from the array. You should not add elements to an array while
865 you're iterating over it. There is a single iterator for each
866 associative array, shared by all each(), keys(), and values() function
867 calls in the program. The following prints out your environment like
868 the printenv(1) program, only in a different order:
870 while (($key,$value) = each %ENV) {
871 print "$key=$value\n";
874 See also keys() and values().
882 Returns 1 if the next read on FILEHANDLE will return end of file, or if
883 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
884 gives the real filehandle name. (Note that this function actually
885 reads a character and then ungetc()s it, so it is not very useful in an
886 interactive context.) Do not read from a terminal file (or call
887 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
888 as terminals may lose the end-of-file condition if you do.
890 An C<eof> without an argument uses the last file read as argument.
891 Empty parentheses () may be used to indicate
892 the pseudo file formed of the files listed on the command line, i.e.,
893 C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
894 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
895 test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
897 # reset line numbering on each input file
900 close(ARGV) if (eof); # Not eof().
903 # insert dashes just before last line of last file
906 print "--------------\n";
907 close(ARGV); # close or break; is needed if we
908 # are reading from the terminal
913 Practical hint: you almost never need to use C<eof> in Perl, because the
914 input operators return undef when they run out of data.
920 EXPR is parsed and executed as if it were a little Perl program. It
921 is executed in the context of the current Perl program, so that any
922 variable settings or subroutine and format definitions remain afterwards.
923 The value returned is the value of the last expression evaluated, or a
924 return statement may be used, just as with subroutines. The last
925 expression is evaluated in scalar or array context, depending on the
928 If there is a syntax error or runtime error, or a die() statement is
929 executed, an undefined value is returned by eval(), and C<$@> is set to the
930 error message. If there was no error, C<$@> is guaranteed to be a null
931 string. If EXPR is omitted, evaluates $_. The final semicolon, if
932 any, may be omitted from the expression.
934 Note that, because eval() traps otherwise-fatal errors, it is useful for
935 determining whether a particular feature (such as socket() or symlink())
936 is implemented. It is also Perl's exception trapping mechanism, where
937 the die operator is used to raise exceptions.
939 If the code to be executed doesn't vary, you may use the eval-BLOCK
940 form to trap run-time errors without incurring the penalty of
941 recompiling each time. The error, if any, is still returned in C<$@>.
944 # make divide-by-zero non-fatal
945 eval { $answer = $a / $b; }; warn $@ if $@;
947 # same thing, but less efficient
948 eval '$answer = $a / $b'; warn $@ if $@;
950 # a compile-time error
954 eval '$answer ='; # sets $@
956 With an eval(), you should be especially careful to remember what's
957 being looked at when:
963 eval { $x }; # CASE 4
965 eval "\$$x++" # CASE 5
968 Cases 1 and 2 above behave identically: they run the code contained in the
969 variable $x. (Although case 2 has misleading double quotes making the
970 reader wonder what else might be happening (nothing is).) Cases 3 and 4
971 likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
972 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
973 is a place where normally you I<WOULD> like to use double quotes, except
974 that in that particular situation, you can just use symbolic references
975 instead, as in case 6.
979 The exec() function executes a system command I<AND NEVER RETURNS>,
980 unless the command does not exist and is executed directly instead of
981 via C</bin/sh -c> (see below). Use system() instead of exec() if you
984 If there is more than one argument in LIST, or if LIST is an array with
985 more than one value, calls execvp(3) with the arguments in LIST. If
986 there is only one scalar argument, the argument is checked for shell
987 metacharacters. If there are any, the entire argument is passed to
988 C</bin/sh -c> for parsing. If there are none, the argument is split
989 into words and passed directly to execvp(), which is more efficient.
990 Note: exec() and system() do not flush your output buffer, so you may
991 need to set C<$|> to avoid lost output. Examples:
993 exec '/bin/echo', 'Your arguments are: ', @ARGV;
994 exec "sort $outfile | uniq";
996 If you don't really want to execute the first argument, but want to lie
997 to the program you are executing about its own name, you can specify
998 the program you actually want to run as an "indirect object" (without a
999 comma) in front of the LIST. (This always forces interpretation of the
1000 LIST as a multi-valued list, even if there is only a single scalar in
1003 $shell = '/bin/csh';
1004 exec $shell '-sh'; # pretend it's a login shell
1008 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1012 Returns TRUE if the specified hash key exists in its hash array, even
1013 if the corresponding value is undefined.
1015 print "Exists\n" if exists $array{$key};
1016 print "Defined\n" if defined $array{$key};
1017 print "True\n" if $array{$key};
1019 A hash element can be TRUE only if it's defined, and defined if
1020 it exists, but the reverse doesn't necessarily hold true.
1022 Note that the EXPR can be arbitrarily complicated as long as the final
1023 operation is a hash key lookup:
1025 if (exists $ref->[$x][$y]{$key}) { ... }
1029 Evaluates EXPR and exits immediately with that value. (Actually, it
1030 calls any defined C<END> routines first, but the C<END> routines may not
1031 abort the exit. Likewise any object destructors that need to be called
1032 are called before exit.) Example:
1035 exit 0 if $ans =~ /^[Xx]/;
1037 See also die(). If EXPR is omitted, exits with 0 status.
1039 You shouldn't use exit() to abort a subroutine if there's any chance that
1040 someone might want to trap whatever error happened. Use die() instead,
1041 which can be trapped by an eval().
1047 Returns I<e> (the natural logarithm base) to the power of EXPR.
1048 If EXPR is omitted, gives C<exp($_)>.
1050 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1052 Implements the fcntl(2) function. You'll probably have to say
1056 first to get the correct function definitions. Argument processing and
1057 value return works just like ioctl() below. Note that fcntl() will produce
1058 a fatal error if used on a machine that doesn't implement fcntl(2).
1062 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1064 =item fileno FILEHANDLE
1066 Returns the file descriptor for a filehandle. This is useful for
1067 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1068 value is taken as the name of the filehandle.
1070 =item flock FILEHANDLE,OPERATION
1072 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1073 success, FALSE on failure. Will produce a fatal error if used on a
1074 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1075 flock() is Perl's portable file locking interface, although it will lock
1076 only entire files, not records.
1078 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1079 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1080 you can use the symbolic names if you pull them in with an explicit
1081 request to the Fcntl module. The names can be requested as a group with
1082 the :flock tag (or they can be requested individually, of course).
1083 LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and
1084 LOCK_UN releases a previously requested lock. If LOCK_NB is added to
1085 LOCK_SH or LOCK_EX then flock() will return immediately rather than
1086 blocking waiting for the lock (check the return status to see if you got
1089 Note that the emulation built with lockf(3) doesn't provide shared
1090 locks, and it requires that FILEHANDLE be open with write intent. These
1091 are the semantics that lockf(3) implements. Most (all?) systems
1092 implement lockf(3) in terms of fcntl(2) locking, though, so the
1093 differing semantics shouldn't bite too many people.
1095 Note also that some versions of flock() cannot lock things over the
1096 network; you would need to use the more system-specific fcntl() for
1097 that. If you like you can force Perl to ignore your system's flock(2)
1098 function, and so provide its own fcntl(2)-based emulation, by passing
1099 the switch C<-Ud_flock> to the F<Configure> program when you configure
1102 Here's a mailbox appender for BSD systems.
1104 use Fcntl ':flock'; # import LOCK_* constants
1107 flock(MBOX,LOCK_EX);
1108 # and, in case someone appended
1109 # while we were waiting...
1114 flock(MBOX,LOCK_UN);
1117 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1118 or die "Can't open mailbox: $!";
1121 print MBOX $msg,"\n\n";
1124 See also L<DB_File> for other flock() examples.
1128 Does a fork(2) system call. Returns the child pid to the parent process
1129 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1130 Note: unflushed buffers remain unflushed in both processes, which means
1131 you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1132 method of IO::Handle to avoid duplicate output.
1134 If you fork() without ever waiting on your children, you will accumulate
1137 $SIG{CHLD} = sub { wait };
1139 There's also the double-fork trick (error checking on
1140 fork() returns omitted);
1142 unless ($pid = fork) {
1144 exec "what you really wanna do";
1147 ## (some_perl_code_here)
1154 See also L<perlipc> for more examples of forking and reaping
1157 Note that if your forked child inherits system file descriptors like
1158 STDIN and STDOUT that are actually connected by a pipe or socket, even
1159 if you exit, the remote server (such as, say, httpd or rsh) won't think
1160 you're done. You should reopen those to /dev/null if it's any issue.
1164 Declare a picture format with use by the write() function. For
1168 Test: @<<<<<<<< @||||| @>>>>>
1169 $str, $%, '$' . int($num)
1173 $num = $cost/$quantity;
1177 See L<perlform> for many details and examples.
1180 =item formline PICTURE, LIST
1182 This is an internal function used by C<format>s, though you may call it
1183 too. It formats (see L<perlform>) a list of values according to the
1184 contents of PICTURE, placing the output into the format output
1185 accumulator, C<$^A> (or $ACCUMULATOR in English).
1186 Eventually, when a write() is done, the contents of
1187 C<$^A> are written to some filehandle, but you could also read C<$^A>
1188 yourself and then set C<$^A> back to "". Note that a format typically
1189 does one formline() per line of form, but the formline() function itself
1190 doesn't care how many newlines are embedded in the PICTURE. This means
1191 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1192 You may therefore need to use multiple formlines to implement a single
1193 record format, just like the format compiler.
1195 Be careful if you put double quotes around the picture, because an "C<@>"
1196 character may be taken to mean the beginning of an array name.
1197 formline() always returns TRUE. See L<perlform> for other examples.
1199 =item getc FILEHANDLE
1203 Returns the next character from the input file attached to FILEHANDLE,
1204 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1205 This is not particularly efficient. It cannot be used to get unbuffered
1206 single-characters, however. For that, try something more like:
1209 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1212 system "stty", '-icanon', 'eol', "\001";
1218 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1221 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1225 Determination of whether to whether $BSD_STYLE should be set
1226 is left as an exercise to the reader.
1228 The POSIX::getattr() function can do this more portably on systems
1229 alleging POSIX compliance.
1230 See also the C<Term::ReadKey> module from your nearest CPAN site;
1231 details on CPAN can be found on L<perlmod/CPAN>.
1235 Returns the current login from F</etc/utmp>, if any. If null, use
1238 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1240 Do not consider getlogin() for authentication: it is not as
1241 secure as getpwuid().
1243 =item getpeername SOCKET
1245 Returns the packed sockaddr address of other end of the SOCKET connection.
1248 $hersockaddr = getpeername(SOCK);
1249 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1250 $herhostname = gethostbyaddr($iaddr, AF_INET);
1251 $herstraddr = inet_ntoa($iaddr);
1255 Returns the current process group for the specified PID. Use
1256 a PID of 0 to get the current process group for the
1257 current process. Will raise an exception if used on a machine that
1258 doesn't implement getpgrp(2). If PID is omitted, returns process
1259 group of current process. Note that the POSIX version of getpgrp()
1260 does not accept a PID argument, so only PID==0 is truly portable.
1264 Returns the process id of the parent process.
1266 =item getpriority WHICH,WHO
1268 Returns the current priority for a process, a process group, or a user.
1269 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1270 machine that doesn't implement getpriority(2).
1276 =item gethostbyname NAME
1278 =item getnetbyname NAME
1280 =item getprotobyname NAME
1286 =item getservbyname NAME,PROTO
1288 =item gethostbyaddr ADDR,ADDRTYPE
1290 =item getnetbyaddr ADDR,ADDRTYPE
1292 =item getprotobynumber NUMBER
1294 =item getservbyport PORT,PROTO
1312 =item sethostent STAYOPEN
1314 =item setnetent STAYOPEN
1316 =item setprotoent STAYOPEN
1318 =item setservent STAYOPEN
1332 These routines perform the same functions as their counterparts in the
1333 system library. Within a list context, the return values from the
1334 various get routines are as follows:
1336 ($name,$passwd,$uid,$gid,
1337 $quota,$comment,$gcos,$dir,$shell) = getpw*
1338 ($name,$passwd,$gid,$members) = getgr*
1339 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1340 ($name,$aliases,$addrtype,$net) = getnet*
1341 ($name,$aliases,$proto) = getproto*
1342 ($name,$aliases,$port,$proto) = getserv*
1344 (If the entry doesn't exist you get a null list.)
1346 Within a scalar context, you get the name, unless the function was a
1347 lookup by name, in which case you get the other thing, whatever it is.
1348 (If the entry doesn't exist you get the undefined value.) For example:
1358 The $members value returned by I<getgr*()> is a space separated list of
1359 the login names of the members of the group.
1361 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1362 C, it will be returned to you via C<$?> if the function call fails. The
1363 @addrs value returned by a successful call is a list of the raw
1364 addresses returned by the corresponding system library call. In the
1365 Internet domain, each address is four bytes long and you can unpack it
1366 by saying something like:
1368 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1370 =item getsockname SOCKET
1372 Returns the packed sockaddr address of this end of the SOCKET connection.
1375 $mysockaddr = getsockname(SOCK);
1376 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1378 =item getsockopt SOCKET,LEVEL,OPTNAME
1380 Returns the socket option requested, or undefined if there is an error.
1384 Returns the value of EXPR with filename expansions such as a shell
1385 would do. This is the internal function implementing the E<lt>*.*E<gt>
1386 operator, except it's easier to use.
1390 Converts a time as returned by the time function to a 9-element array
1391 with the time localized for the standard Greenwich time zone.
1392 Typically used as follows:
1395 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1398 All array elements are numeric, and come straight out of a struct tm.
1399 In particular this means that $mon has the range 0..11 and $wday has
1400 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1408 The goto-LABEL form finds the statement labeled with LABEL and resumes
1409 execution there. It may not be used to go into any construct that
1410 requires initialization, such as a subroutine or a foreach loop. It
1411 also can't be used to go into a construct that is optimized away. It
1412 can be used to go almost anywhere else within the dynamic scope,
1413 including out of subroutines, but it's usually better to use some other
1414 construct such as last or die. The author of Perl has never felt the
1415 need to use this form of goto (in Perl, that is--C is another matter).
1417 The goto-EXPR form expects a label name, whose scope will be resolved
1418 dynamically. This allows for computed gotos per FORTRAN, but isn't
1419 necessarily recommended if you're optimizing for maintainability:
1421 goto ("FOO", "BAR", "GLARCH")[$i];
1423 The goto-&NAME form is highly magical, and substitutes a call to the
1424 named subroutine for the currently running subroutine. This is used by
1425 AUTOLOAD subroutines that wish to load another subroutine and then
1426 pretend that the other subroutine had been called in the first place
1427 (except that any modifications to @_ in the current subroutine are
1428 propagated to the other subroutine.) After the goto, not even caller()
1429 will be able to tell that this routine was called first.
1431 =item grep BLOCK LIST
1433 =item grep EXPR,LIST
1435 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1436 $_ to each element) and returns the list value consisting of those
1437 elements for which the expression evaluated to TRUE. In a scalar
1438 context, returns the number of times the expression was TRUE.
1440 @foo = grep(!/^#/, @bar); # weed out comments
1444 @foo = grep {!/^#/} @bar; # weed out comments
1446 Note that, because $_ is a reference into the list value, it can be used
1447 to modify the elements of the array. While this is useful and
1448 supported, it can cause bizarre results if the LIST is not a named
1455 Interprets EXPR as a hex string and returns the corresponding decimal
1456 value. (To convert strings that might start with 0 or 0x see
1457 oct().) If EXPR is omitted, uses $_.
1461 There is no built-in import() function. It is merely an ordinary
1462 method (subroutine) defined (or inherited) by modules that wish to export
1463 names to another module. The use() function calls the import() method
1464 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1466 =item index STR,SUBSTR,POSITION
1468 =item index STR,SUBSTR
1470 Returns the position of the first occurrence of SUBSTR in STR at or after
1471 POSITION. If POSITION is omitted, starts searching from the beginning of
1472 the string. The return value is based at 0 (or whatever you've set the C<$[>
1473 variable to--but don't do that). If the substring is not found, returns
1474 one less than the base, ordinarily -1.
1480 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1482 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1484 Implements the ioctl(2) function. You'll probably have to say
1486 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1488 first to get the correct function definitions. If F<ioctl.ph> doesn't
1489 exist or doesn't have the correct definitions you'll have to roll your
1490 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1491 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1492 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1493 written depending on the FUNCTION--a pointer to the string value of SCALAR
1494 will be passed as the third argument of the actual ioctl call. (If SCALAR
1495 has no string value but does have a numeric value, that value will be
1496 passed rather than a pointer to the string value. To guarantee this to be
1497 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1498 functions are useful for manipulating the values of structures used by
1499 ioctl(). The following example sets the erase character to DEL.
1503 die "NO TIOCGETP" if $@ || !$getp;
1504 $sgttyb_t = "ccccs"; # 4 chars and a short
1505 if (ioctl(STDIN,$getp,$sgttyb)) {
1506 @ary = unpack($sgttyb_t,$sgttyb);
1508 $sgttyb = pack($sgttyb_t,@ary);
1509 ioctl(STDIN,&TIOCSETP,$sgttyb)
1510 || die "Can't ioctl: $!";
1513 The return value of ioctl (and fcntl) is as follows:
1515 if OS returns: then Perl returns:
1517 0 string "0 but true"
1518 anything else that number
1520 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1521 still easily determine the actual value returned by the operating
1524 ($retval = ioctl(...)) || ($retval = -1);
1525 printf "System returned %d\n", $retval;
1527 =item join EXPR,LIST
1529 Joins the separate strings of LIST or ARRAY into a single string with
1530 fields separated by the value of EXPR, and returns the string.
1533 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1535 See L<perlfunc/split>.
1537 =item keys ASSOC_ARRAY
1539 Returns a normal array consisting of all the keys of the named
1540 associative array. (In a scalar context, returns the number of keys.)
1541 The keys are returned in an apparently random order, but it is the same
1542 order as either the values() or each() function produces (given that
1543 the associative array has not been modified). Here is yet another way
1544 to print your environment:
1547 @values = values %ENV;
1548 while ($#keys >= 0) {
1549 print pop(@keys), '=', pop(@values), "\n";
1552 or how about sorted by key:
1554 foreach $key (sort(keys %ENV)) {
1555 print $key, '=', $ENV{$key}, "\n";
1558 To sort an array by value, you'll need to use a C<sort{}>
1559 function. Here's a descending numeric sort of a hash by its values:
1561 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1562 printf "%4d %s\n", $hash{$key}, $key;
1565 As an lvalue C<keys> allows you to increase the number of hash buckets
1566 allocated for the given associative array. This can gain you a measure
1567 of efficiency if you know the hash is going to get big. (This is
1568 similar to pre-extending an array by assigning a larger number to
1569 $#array.) If you say
1573 then C<%hash> will have at least 200 buckets allocated for it. These
1574 buckets will be retained even if you do C<%hash = ()>, use C<undef
1575 %hash> if you want to free the storage while C<%hash> is still in scope.
1576 You can't shrink the number of buckets allocated for the hash using
1577 C<keys> in this way (but you needn't worry about doing this by accident,
1578 as trying has no effect).
1582 Sends a signal to a list of processes. The first element of
1583 the list must be the signal to send. Returns the number of
1584 processes successfully signaled.
1586 $cnt = kill 1, $child1, $child2;
1589 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1590 process groups instead of processes. (On System V, a negative I<PROCESS>
1591 number will also kill process groups, but that's not portable.) That
1592 means you usually want to use positive not negative signals. You may also
1593 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1599 The C<last> command is like the C<break> statement in C (as used in
1600 loops); it immediately exits the loop in question. If the LABEL is
1601 omitted, the command refers to the innermost enclosing loop. The
1602 C<continue> block, if any, is not executed:
1604 LINE: while (<STDIN>) {
1605 last LINE if /^$/; # exit when done with header
1613 Returns an lowercased version of EXPR. This is the internal function
1614 implementing the \L escape in double-quoted strings.
1615 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1617 If EXPR is omitted, uses $_.
1623 Returns the value of EXPR with the first character lowercased. This is
1624 the internal function implementing the \l escape in double-quoted strings.
1625 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1627 If EXPR is omitted, uses $_.
1633 Returns the length in characters of the value of EXPR. If EXPR is
1634 omitted, returns length of $_.
1636 =item link OLDFILE,NEWFILE
1638 Creates a new filename linked to the old filename. Returns 1 for
1639 success, 0 otherwise.
1641 =item listen SOCKET,QUEUESIZE
1643 Does the same thing that the listen system call does. Returns TRUE if
1644 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1648 A local modifies the listed variables to be local to the enclosing block,
1649 subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1650 list must be placed in parentheses. See L<perlsub/"Temporary Values via
1651 local()"> for details.
1653 But you really probably want to be using my() instead, because local() isn't
1654 what most people think of as "local"). See L<perlsub/"Private Variables
1655 via my()"> for details.
1657 =item localtime EXPR
1659 Converts a time as returned by the time function to a 9-element array
1660 with the time analyzed for the local time zone. Typically used as
1663 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1666 All array elements are numeric, and come straight out of a struct tm.
1667 In particular this means that $mon has the range 0..11 and $wday has
1668 the range 0..6. If EXPR is omitted, does localtime(time).
1670 In a scalar context, prints out the ctime(3) value:
1672 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
1674 Also see the F<timelocal.pl> library, and the strftime(3) function available
1675 via the POSIX module.
1681 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1684 =item lstat FILEHANDLE
1690 Does the same thing as the stat() function, but stats a symbolic link
1691 instead of the file the symbolic link points to. If symbolic links are
1692 unimplemented on your system, a normal stat() is done.
1694 If EXPR is omitted, stats $_.
1698 The match operator. See L<perlop>.
1700 =item map BLOCK LIST
1704 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1705 element) and returns the list value composed of the results of each such
1706 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1707 may produce zero, one, or more elements in the returned value.
1709 @chars = map(chr, @nums);
1711 translates a list of numbers to the corresponding characters. And
1713 %hash = map { getkey($_) => $_ } @array;
1715 is just a funny way to write
1718 foreach $_ (@array) {
1719 $hash{getkey($_)} = $_;
1722 =item mkdir FILENAME,MODE
1724 Creates the directory specified by FILENAME, with permissions specified
1725 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1726 it returns 0 and sets C<$!> (errno).
1728 =item msgctl ID,CMD,ARG
1730 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1731 must be a variable which will hold the returned msqid_ds structure.
1732 Returns like ioctl: the undefined value for error, "0 but true" for
1733 zero, or the actual return value otherwise.
1735 =item msgget KEY,FLAGS
1737 Calls the System V IPC function msgget(2). Returns the message queue id,
1738 or the undefined value if there is an error.
1740 =item msgsnd ID,MSG,FLAGS
1742 Calls the System V IPC function msgsnd to send the message MSG to the
1743 message queue ID. MSG must begin with the long integer message type,
1744 which may be created with C<pack("l", $type)>. Returns TRUE if
1745 successful, or FALSE if there is an error.
1747 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1749 Calls the System V IPC function msgrcv to receive a message from
1750 message queue ID into variable VAR with a maximum message size of
1751 SIZE. Note that if a message is received, the message type will be the
1752 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1753 of the message type. Returns TRUE if successful, or FALSE if there is
1758 A "my" declares the listed variables to be local (lexically) to the
1759 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1760 more than one value is listed, the list must be placed in parentheses. See
1761 L<perlsub/"Private Variables via my()"> for details.
1767 The C<next> command is like the C<continue> statement in C; it starts
1768 the next iteration of the loop:
1770 LINE: while (<STDIN>) {
1771 next LINE if /^#/; # discard comments
1775 Note that if there were a C<continue> block on the above, it would get
1776 executed even on discarded lines. If the LABEL is omitted, the command
1777 refers to the innermost enclosing loop.
1779 =item no Module LIST
1781 See the "use" function, which "no" is the opposite of.
1787 Interprets EXPR as an octal string and returns the corresponding
1788 decimal value. (If EXPR happens to start off with 0x, interprets it as
1789 a hex string instead.) The following will handle decimal, octal, and
1790 hex in the standard Perl or C notation:
1792 $val = oct($val) if $val =~ /^0/;
1794 If EXPR is omitted, uses $_.
1796 =item open FILEHANDLE,EXPR
1798 =item open FILEHANDLE
1800 Opens the file whose filename is given by EXPR, and associates it with
1801 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1802 name of the real filehandle wanted. If EXPR is omitted, the scalar
1803 variable of the same name as the FILEHANDLE contains the filename.
1804 (Note that lexical variables--those declared with C<my>--will not work
1805 for this purpose; so if you're using C<my>, specify EXPR in your call
1808 If the filename begins with '<' or nothing, the file is opened for input.
1809 If the filename begins with '>', the file is truncated and opened for
1810 output. If the filename begins with '>>', the file is opened for
1811 appending. You can put a '+' in front of the '>' or '<' to indicate that
1812 you want both read and write access to the file; thus '+<' is almost
1813 always preferred for read/write updates--the '+>' mode would clobber the
1814 file first. The prefix and the filename may be separated with spaces.
1815 These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1816 'w+', 'a', and 'a+'.
1818 If the filename begins with "|", the filename is interpreted as a command
1819 to which output is to be piped, and if the filename ends with a "|", the
1820 filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1821 examples of this. as command which pipes input to us. (You may not have
1822 a raw open() to a command that pipes both in I<and> out, but see
1823 L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1826 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1827 non-zero upon success, the undefined value otherwise. If the open
1828 involved a pipe, the return value happens to be the pid of the
1831 If you're unfortunate enough to be running Perl on a system that
1832 distinguishes between text files and binary files (modern operating
1833 systems don't care), then you should check out L</binmode> for tips for
1834 dealing with this. The key distinction between systems that need binmode
1835 and those that don't is their text file formats. Systems like Unix and
1836 Plan9 that delimit lines with a single character, and that encode that
1837 character in C as '\n', do not need C<binmode>. The rest need it.
1842 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1843 while (<ARTICLE>) {...
1845 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1847 open(DBASE, '+<dbase.mine'); # open for update
1849 open(ARTICLE, "caesar <$article |"); # decrypt article
1851 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1853 # process argument list of files along with any includes
1855 foreach $file (@ARGV) {
1856 process($file, 'fh00');
1860 local($filename, $input) = @_;
1861 $input++; # this is a string increment
1862 unless (open($input, $filename)) {
1863 print STDERR "Can't open $filename: $!\n";
1867 while (<$input>) { # note use of indirection
1868 if (/^#include "(.*)"/) {
1869 process($1, $input);
1876 You may also, in the Bourne shell tradition, specify an EXPR beginning
1877 with "E<gt>&", in which case the rest of the string is interpreted as the
1878 name of a filehandle (or file descriptor, if numeric) which is to be
1879 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1880 +E<gt>E<gt>, and +E<lt>. The
1881 mode you specify should match the mode of the original filehandle.
1882 (Duping a filehandle does not take into account any existing contents of
1884 Here is a script that saves, redirects, and restores STDOUT and
1888 open(SAVEOUT, ">&STDOUT");
1889 open(SAVEERR, ">&STDERR");
1891 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1892 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1894 select(STDERR); $| = 1; # make unbuffered
1895 select(STDOUT); $| = 1; # make unbuffered
1897 print STDOUT "stdout 1\n"; # this works for
1898 print STDERR "stderr 1\n"; # subprocesses too
1903 open(STDOUT, ">&SAVEOUT");
1904 open(STDERR, ">&SAVEERR");
1906 print STDOUT "stdout 2\n";
1907 print STDERR "stderr 2\n";
1910 If you specify "E<lt>&=N", where N is a number, then Perl will do an
1911 equivalent of C's fdopen() of that file descriptor; this is more
1912 parsimonious of file descriptors. For example:
1914 open(FILEHANDLE, "<&=$fd")
1916 If you open a pipe on the command "-", i.e., either "|-" or "-|", then
1917 there is an implicit fork done, and the return value of open is the pid
1918 of the child within the parent process, and 0 within the child
1919 process. (Use C<defined($pid)> to determine whether the open was successful.)
1920 The filehandle behaves normally for the parent, but i/o to that
1921 filehandle is piped from/to the STDOUT/STDIN of the child process.
1922 In the child process the filehandle isn't opened--i/o happens from/to
1923 the new STDOUT or STDIN. Typically this is used like the normal
1924 piped open when you want to exercise more control over just how the
1925 pipe command gets executed, such as when you are running setuid, and
1926 don't want to have to scan shell commands for metacharacters.
1927 The following pairs are more or less equivalent:
1929 open(FOO, "|tr '[a-z]' '[A-Z]'");
1930 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1932 open(FOO, "cat -n '$file'|");
1933 open(FOO, "-|") || exec 'cat', '-n', $file;
1935 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1937 Explicitly closing any piped filehandle causes the parent process to
1938 wait for the child to finish, and returns the status value in C<$?>.
1939 Note: on any operation which may do a fork, unflushed buffers remain
1940 unflushed in both processes, which means you may need to set C<$|> to
1941 avoid duplicate output.
1943 Using the constructor from the IO::Handle package (or one of its
1944 subclasses, such as IO::File or IO::Socket),
1945 you can generate anonymous filehandles which have the scope of whatever
1946 variables hold references to them, and automatically close whenever
1947 and however you leave that scope:
1951 sub read_myfile_munged {
1953 my $handle = new IO::File;
1954 open($handle, "myfile") or die "myfile: $!";
1956 or return (); # Automatically closed here.
1957 mung $first or die "mung failed"; # Or here.
1958 return $first, <$handle> if $ALL; # Or here.
1962 The filename that is passed to open will have leading and trailing
1963 whitespace deleted. To open a file with arbitrary weird
1964 characters in it, it's necessary to protect any leading and trailing
1967 $file =~ s#^(\s)#./$1#;
1968 open(FOO, "< $file\0");
1970 If you want a "real" C open() (see L<open(2)> on your system), then
1971 you should use the sysopen() function. This is another way to
1972 protect your filenames from interpretation. For example:
1975 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
1976 or die "sysopen $path: $!";
1977 HANDLE->autoflush(1);
1978 HANDLE->print("stuff $$\n");
1980 print "File contains: ", <HANDLE>;
1982 See L</seek()> for some details about mixing reading and writing.
1984 =item opendir DIRHANDLE,EXPR
1986 Opens a directory named EXPR for processing by readdir(), telldir(),
1987 seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
1988 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1994 Returns the numeric ascii value of the first character of EXPR. If
1995 EXPR is omitted, uses $_.
1997 =item pack TEMPLATE,LIST
1999 Takes an array or list of values and packs it into a binary structure,
2000 returning the string containing the structure. The TEMPLATE is a
2001 sequence of characters that give the order and type of values, as
2004 A An ascii string, will be space padded.
2005 a An ascii string, will be null padded.
2006 b A bit string (ascending bit order, like vec()).
2007 B A bit string (descending bit order).
2008 h A hex string (low nybble first).
2009 H A hex string (high nybble first).
2011 c A signed char value.
2012 C An unsigned char value.
2013 s A signed short value.
2014 S An unsigned short value.
2015 i A signed integer value.
2016 I An unsigned integer value.
2017 l A signed long value.
2018 L An unsigned long value.
2020 n A short in "network" order.
2021 N A long in "network" order.
2022 v A short in "VAX" (little-endian) order.
2023 V A long in "VAX" (little-endian) order.
2025 f A single-precision float in the native format.
2026 d A double-precision float in the native format.
2028 p A pointer to a null-terminated string.
2029 P A pointer to a structure (fixed-length string).
2031 u A uuencoded string.
2033 w A BER compressed integer. Bytes give an unsigned integer base
2034 128, most significant digit first, with as few digits as
2035 possible, and with the bit 8 of each byte except the last set
2040 @ Null fill to absolute position.
2042 Each letter may optionally be followed by a number which gives a repeat
2043 count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
2044 pack function will gobble up that many values from the LIST. A * for the
2045 repeat count means to use however many items are left. The "a" and "A"
2046 types gobble just one value, but pack it as a string of length count,
2047 padding with nulls or spaces as necessary. (When unpacking, "A" strips
2048 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2049 fields pack a string that many bits long. The "h" and "H" fields pack a
2050 string that many nybbles long. The "P" packs a pointer to a structure of
2051 the size indicated by the length. Real numbers (floats and doubles) are
2052 in the native machine format only; due to the multiplicity of floating
2053 formats around, and the lack of a standard "network" representation, no
2054 facility for interchange has been made. This means that packed floating
2055 point data written on one machine may not be readable on another - even if
2056 both use IEEE floating point arithmetic (as the endian-ness of the memory
2057 representation is not part of the IEEE spec). Note that Perl uses doubles
2058 internally for all numeric calculation, and converting from double into
2059 float and thence back to double again will lose precision (i.e.,
2060 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2064 $foo = pack("cccc",65,66,67,68);
2066 $foo = pack("c4",65,66,67,68);
2069 $foo = pack("ccxxcc",65,66,67,68);
2072 $foo = pack("s2",1,2);
2073 # "\1\0\2\0" on little-endian
2074 # "\0\1\0\2" on big-endian
2076 $foo = pack("a4","abcd","x","y","z");
2079 $foo = pack("aaaa","abcd","x","y","z");
2082 $foo = pack("a14","abcdefg");
2083 # "abcdefg\0\0\0\0\0\0\0"
2085 $foo = pack("i9pl", gmtime);
2086 # a real struct tm (on my system anyway)
2089 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2092 The same template may generally also be used in the unpack function.
2094 =item package NAMESPACE
2096 Declares the compilation unit as being in the given namespace. The scope
2097 of the package declaration is from the declaration itself through the end of
2098 the enclosing block (the same scope as the local() operator). All further
2099 unqualified dynamic identifiers will be in this namespace. A package
2100 statement affects only dynamic variables--including those you've used
2101 local() on--but I<not> lexical variables created with my(). Typically it
2102 would be the first declaration in a file to be included by the C<require>
2103 or C<use> operator. You can switch into a package in more than one place;
2104 it influences merely which symbol table is used by the compiler for the
2105 rest of that block. You can refer to variables and filehandles in other
2106 packages by prefixing the identifier with the package name and a double
2107 colon: C<$Package::Variable>. If the package name is null, the C<main>
2108 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2110 See L<perlmod/"Packages"> for more information about packages, modules,
2111 and classes. See L<perlsub> for other scoping issues.
2113 =item pipe READHANDLE,WRITEHANDLE
2115 Opens a pair of connected pipes like the corresponding system call.
2116 Note that if you set up a loop of piped processes, deadlock can occur
2117 unless you are very careful. In addition, note that Perl's pipes use
2118 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2119 after each command, depending on the application.
2121 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2122 for examples of such things.
2128 Pops and returns the last value of the array, shortening the array by
2129 1. Has a similar effect to
2131 $tmp = $ARRAY[$#ARRAY--];
2133 If there are no elements in the array, returns the undefined value.
2134 If ARRAY is omitted, pops the
2135 @ARGV array in the main program, and the @_ array in subroutines, just
2142 Returns the offset of where the last C<m//g> search left off for the variable
2143 is in question ($_ is used when the variable is not specified). May be
2144 modified to change that offset. Such modification will also influence
2145 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2148 =item print FILEHANDLE LIST
2154 Prints a string or a comma-separated list of strings. Returns TRUE
2155 if successful. FILEHANDLE may be a scalar variable name, in which case
2156 the variable contains the name of or a reference to the filehandle, thus introducing one
2157 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2158 token is a term, it may be misinterpreted as an operator unless you
2159 interpose a + or put parentheses around the arguments.) If FILEHANDLE is
2160 omitted, prints by default to standard output (or to the last selected
2161 output channel--see L</select>). If LIST is also omitted, prints $_ to
2162 STDOUT. To set the default output channel to something other than
2163 STDOUT use the select operation. Note that, because print takes a
2164 LIST, anything in the LIST is evaluated in a list context, and any
2165 subroutine that you call will have one or more of its expressions
2166 evaluated in a list context. Also be careful not to follow the print
2167 keyword with a left parenthesis unless you want the corresponding right
2168 parenthesis to terminate the arguments to the print--interpose a + or
2169 put parentheses around all the arguments.
2171 Note that if you're storing FILEHANDLES in an array or other expression,
2172 you will have to use a block returning its value instead:
2174 print { $files[$i] } "stuff\n";
2175 print { $OK ? STDOUT : STDERR } "stuff\n";
2177 =item printf FILEHANDLE FORMAT, LIST
2179 =item printf FORMAT, LIST
2181 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2182 of the list will be interpreted as the printf format. If C<use locale> is
2183 in effect, the character used for the decimal point in formatted real numbers
2184 is affected by the LC_NUMERIC locale. See L<perllocale>.
2186 Don't fall into the trap of using a printf() when a simple
2187 print() would do. The print() is more efficient, and less
2190 =item prototype FUNCTION
2192 Returns the prototype of a function as a string (or C<undef> if the
2193 function has no prototype). FUNCTION is a reference to, or the name of,
2194 the function whose prototype you want to retrieve.
2196 =item push ARRAY,LIST
2198 Treats ARRAY as a stack, and pushes the values of LIST
2199 onto the end of ARRAY. The length of ARRAY increases by the length of
2200 LIST. Has the same effect as
2203 $ARRAY[++$#ARRAY] = $value;
2206 but is more efficient. Returns the new number of elements in the array.
2216 Generalized quotes. See L<perlop>.
2218 =item quotemeta EXPR
2222 Returns the value of EXPR with with all non-alphanumeric
2223 characters backslashed. (That is, all characters not matching
2224 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2225 returned string, regardless of any locale settings.)
2226 This is the internal function implementing
2227 the \Q escape in double-quoted strings.
2229 If EXPR is omitted, uses $_.
2235 Returns a random fractional number between 0 and the value of EXPR.
2236 (EXPR should be positive.) If EXPR is omitted, returns a value between
2237 0 and 1. This function produces repeatable sequences unless srand()
2238 is invoked. See also srand().
2240 (Note: if your rand function consistently returns numbers that are too
2241 large or too small, then your version of Perl was probably compiled
2242 with the wrong number of RANDBITS. As a workaround, you can usually
2243 multiply EXPR by the correct power of 2 to get the range you want.
2244 This will make your script unportable, however. It's better to recompile
2247 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2249 =item read FILEHANDLE,SCALAR,LENGTH
2251 Attempts to read LENGTH bytes of data into variable SCALAR from the
2252 specified FILEHANDLE. Returns the number of bytes actually read, or
2253 undef if there was an error. SCALAR will be grown or shrunk to the
2254 length actually read. An OFFSET may be specified to place the read
2255 data at some other place than the beginning of the string. This call
2256 is actually implemented in terms of stdio's fread call. To get a true
2257 read system call, see sysread().
2259 =item readdir DIRHANDLE
2261 Returns the next directory entry for a directory opened by opendir().
2262 If used in a list context, returns all the rest of the entries in the
2263 directory. If there are no more entries, returns an undefined value in
2264 a scalar context or a null list in a list context.
2266 If you're planning to filetest the return values out of a readdir(), you'd
2267 better prepend the directory in question. Otherwise, because we didn't
2268 chdir() there, it would have been testing the wrong file.
2270 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2271 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2278 Returns the value of a symbolic link, if symbolic links are
2279 implemented. If not, gives a fatal error. If there is some system
2280 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2283 =item recv SOCKET,SCALAR,LEN,FLAGS
2285 Receives a message on a socket. Attempts to receive LENGTH bytes of
2286 data into variable SCALAR from the specified SOCKET filehandle.
2287 Actually does a C recvfrom(), so that it can returns the address of the
2288 sender. Returns the undefined value if there's an error. SCALAR will
2289 be grown or shrunk to the length actually read. Takes the same flags
2290 as the system call of the same name.
2291 See L<perlipc/"UDP: Message Passing"> for examples.
2297 The C<redo> command restarts the loop block without evaluating the
2298 conditional again. The C<continue> block, if any, is not executed. If
2299 the LABEL is omitted, the command refers to the innermost enclosing
2300 loop. This command is normally used by programs that want to lie to
2301 themselves about what was just input:
2303 # a simpleminded Pascal comment stripper
2304 # (warning: assumes no { or } in strings)
2305 LINE: while (<STDIN>) {
2306 while (s|({.*}.*){.*}|$1 |) {}
2311 if (/}/) { # end of comment?
2324 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2325 is not specified, $_ will be used. The value returned depends on the
2326 type of thing the reference is a reference to.
2327 Builtin types include:
2336 If the referenced object has been blessed into a package, then that package
2337 name is returned instead. You can think of ref() as a typeof() operator.
2339 if (ref($r) eq "HASH") {
2340 print "r is a reference to an associative array.\n";
2343 print "r is not a reference at all.\n";
2346 See also L<perlref>.
2348 =item rename OLDNAME,NEWNAME
2350 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2351 not work across file system boundaries.
2357 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2358 supplied. If EXPR is numeric, demands that the current version of Perl
2359 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2361 Otherwise, demands that a library file be included if it hasn't already
2362 been included. The file is included via the do-FILE mechanism, which is
2363 essentially just a variety of eval(). Has semantics similar to the following
2367 local($filename) = @_;
2368 return 1 if $INC{$filename};
2369 local($realfilename,$result);
2371 foreach $prefix (@INC) {
2372 $realfilename = "$prefix/$filename";
2373 if (-f $realfilename) {
2374 $result = do $realfilename;
2378 die "Can't find $filename in \@INC";
2381 die "$filename did not return true value" unless $result;
2382 $INC{$filename} = $realfilename;
2386 Note that the file will not be included twice under the same specified
2387 name. The file must return TRUE as the last statement to indicate
2388 successful execution of any initialization code, so it's customary to
2389 end such a file with "1;" unless you're sure it'll return TRUE
2390 otherwise. But it's better just to put the "C<1;>", in case you add more
2393 If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2394 replaces "F<::>" with "F</>" in the filename for you,
2395 to make it easy to load standard modules. This form of loading of
2396 modules does not risk altering your namespace.
2398 For a yet-more-powerful import facility, see L</use> and
2405 Generally used in a C<continue> block at the end of a loop to clear
2406 variables and reset ?? searches so that they work again. The
2407 expression is interpreted as a list of single characters (hyphens
2408 allowed for ranges). All variables and arrays beginning with one of
2409 those letters are reset to their pristine state. If the expression is
2410 omitted, one-match searches (?pattern?) are reset to match again. Resets
2411 only variables or searches in the current package. Always returns
2414 reset 'X'; # reset all X variables
2415 reset 'a-z'; # reset lower case variables
2416 reset; # just reset ?? searches
2418 Resetting "A-Z" is not recommended because you'll wipe out your
2419 ARGV and ENV arrays. Resets only package variables--lexical variables
2420 are unaffected, but they clean themselves up on scope exit anyway,
2421 so you'll probably want to use them instead. See L</my>.
2425 Returns from a subroutine or eval with the value specified. (Note that
2426 in the absence of a return a subroutine or eval() will automatically
2427 return the value of the last expression evaluated.)
2431 In a list context, returns a list value consisting of the elements
2432 of LIST in the opposite order. In a scalar context, returns a string
2433 value consisting of the bytes of the first element of LIST in the
2436 print reverse <>; # line tac
2439 print scalar reverse scalar <>; # byte tac
2441 =item rewinddir DIRHANDLE
2443 Sets the current position to the beginning of the directory for the
2444 readdir() routine on DIRHANDLE.
2446 =item rindex STR,SUBSTR,POSITION
2448 =item rindex STR,SUBSTR
2450 Works just like index except that it returns the position of the LAST
2451 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2452 last occurrence at or before that position.
2454 =item rmdir FILENAME
2458 Deletes the directory specified by FILENAME if it is empty. If it
2459 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2460 FILENAME is omitted, uses $_.
2464 The substitution operator. See L<perlop>.
2468 Forces EXPR to be interpreted in a scalar context and returns the value
2471 @counts = ( scalar @a, scalar @b, scalar @c );
2473 There is no equivalent operator to force an expression to
2474 be interpolated in a list context because it's in practice never
2475 needed. If you really wanted to do so, however, you could use
2476 the construction C<@{[ (some expression) ]}>, but usually a simple
2477 C<(some expression)> suffices.
2479 =item seek FILEHANDLE,POSITION,WHENCE
2481 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2482 call of stdio. FILEHANDLE may be an expression whose value gives the name
2483 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2484 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2485 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2486 this from POSIX module. Returns 1 upon success, 0 otherwise.
2488 On some systems you have to do a seek whenever you switch between reading
2489 and writing. Amongst other things, this may have the effect of calling
2490 stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2495 This is also useful for applications emulating C<tail -f>. Once you hit
2496 EOF on your read, and then sleep for a while, you might have to stick in a
2497 seek() to reset things. First the simple trick listed above to clear the
2498 filepointer. The seek() doesn't change the current position, but it
2499 I<does> clear the end-of-file condition on the handle, so that the next
2500 C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2502 If that doesn't work (some stdios are particularly cantankerous), then
2503 you may need something more like this:
2506 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2507 # search for some stuff and put it into files
2509 sleep($for_a_while);
2510 seek(FILE, $curpos, 0);
2513 =item seekdir DIRHANDLE,POS
2515 Sets the current position for the readdir() routine on DIRHANDLE. POS
2516 must be a value returned by telldir(). Has the same caveats about
2517 possible directory compaction as the corresponding system library
2520 =item select FILEHANDLE
2524 Returns the currently selected filehandle. Sets the current default
2525 filehandle for output, if FILEHANDLE is supplied. This has two
2526 effects: first, a C<write> or a C<print> without a filehandle will
2527 default to this FILEHANDLE. Second, references to variables related to
2528 output will refer to this output channel. For example, if you have to
2529 set the top of form format for more than one output channel, you might
2537 FILEHANDLE may be an expression whose value gives the name of the
2538 actual filehandle. Thus:
2540 $oldfh = select(STDERR); $| = 1; select($oldfh);
2542 Some programmers may prefer to think of filehandles as objects with
2543 methods, preferring to write the last example as:
2546 STDERR->autoflush(1);
2548 =item select RBITS,WBITS,EBITS,TIMEOUT
2550 This calls the select(2) system call with the bit masks specified, which
2551 can be constructed using fileno() and vec(), along these lines:
2553 $rin = $win = $ein = '';
2554 vec($rin,fileno(STDIN),1) = 1;
2555 vec($win,fileno(STDOUT),1) = 1;
2558 If you want to select on many filehandles you might wish to write a
2562 local(@fhlist) = split(' ',$_[0]);
2565 vec($bits,fileno($_),1) = 1;
2569 $rin = fhbits('STDIN TTY SOCK');
2573 ($nfound,$timeleft) =
2574 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2576 or to block until something becomes ready just do this
2578 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2580 Most systems do not bother to return anything useful in $timeleft, so
2581 calling select() in a scalar context just returns $nfound.
2583 Any of the bit masks can also be undef. The timeout, if specified, is
2584 in seconds, which may be fractional. Note: not all implementations are
2585 capable of returning the $timeleft. If not, they always return
2586 $timeleft equal to the supplied $timeout.
2588 You can effect a sleep of 250 milliseconds this way:
2590 select(undef, undef, undef, 0.25);
2592 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2593 with select(). You have to use sysread() instead.
2595 =item semctl ID,SEMNUM,CMD,ARG
2597 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2598 &GETALL, then ARG must be a variable which will hold the returned
2599 semid_ds structure or semaphore value array. Returns like ioctl: the
2600 undefined value for error, "0 but true" for zero, or the actual return
2603 =item semget KEY,NSEMS,FLAGS
2605 Calls the System V IPC function semget. Returns the semaphore id, or
2606 the undefined value if there is an error.
2608 =item semop KEY,OPSTRING
2610 Calls the System V IPC function semop to perform semaphore operations
2611 such as signaling and waiting. OPSTRING must be a packed array of
2612 semop structures. Each semop structure can be generated with
2613 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2614 operations is implied by the length of OPSTRING. Returns TRUE if
2615 successful, or FALSE if there is an error. As an example, the
2616 following code waits on semaphore $semnum of semaphore id $semid:
2618 $semop = pack("sss", $semnum, -1, 0);
2619 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2621 To signal the semaphore, replace "-1" with "1".
2623 =item send SOCKET,MSG,FLAGS,TO
2625 =item send SOCKET,MSG,FLAGS
2627 Sends a message on a socket. Takes the same flags as the system call
2628 of the same name. On unconnected sockets you must specify a
2629 destination to send TO, in which case it does a C sendto(). Returns
2630 the number of characters sent, or the undefined value if there is an
2632 See L<perlipc/"UDP: Message Passing"> for examples.
2634 =item setpgrp PID,PGRP
2636 Sets the current process group for the specified PID, 0 for the current
2637 process. Will produce a fatal error if used on a machine that doesn't
2638 implement setpgrp(2). If the arguments are omitted, it defaults to
2639 0,0. Note that the POSIX version of setpgrp() does not accept any
2640 arguments, so only setpgrp 0,0 is portable.
2642 =item setpriority WHICH,WHO,PRIORITY
2644 Sets the current priority for a process, a process group, or a user.
2645 (See setpriority(2).) Will produce a fatal error if used on a machine
2646 that doesn't implement setpriority(2).
2648 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2650 Sets the socket option requested. Returns undefined if there is an
2651 error. OPTVAL may be specified as undef if you don't want to pass an
2658 Shifts the first value of the array off and returns it, shortening the
2659 array by 1 and moving everything down. If there are no elements in the
2660 array, returns the undefined value. If ARRAY is omitted, shifts the
2661 @ARGV array in the main program, and the @_ array in subroutines.
2662 (This is determined lexically.) See also unshift(), push(), and pop().
2663 Shift() and unshift() do the same thing to the left end of an array
2664 that push() and pop() do to the right end.
2666 =item shmctl ID,CMD,ARG
2668 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2669 must be a variable which will hold the returned shmid_ds structure.
2670 Returns like ioctl: the undefined value for error, "0 but true" for
2671 zero, or the actual return value otherwise.
2673 =item shmget KEY,SIZE,FLAGS
2675 Calls the System V IPC function shmget. Returns the shared memory
2676 segment id, or the undefined value if there is an error.
2678 =item shmread ID,VAR,POS,SIZE
2680 =item shmwrite ID,STRING,POS,SIZE
2682 Reads or writes the System V shared memory segment ID starting at
2683 position POS for size SIZE by attaching to it, copying in/out, and
2684 detaching from it. When reading, VAR must be a variable which will
2685 hold the data read. When writing, if STRING is too long, only SIZE
2686 bytes are used; if STRING is too short, nulls are written to fill out
2687 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2689 =item shutdown SOCKET,HOW
2691 Shuts down a socket connection in the manner indicated by HOW, which
2692 has the same interpretation as in the system call of the same name.
2698 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2701 For the inverse sine operation, you may use the POSIX::sin()
2702 function, or use this relation:
2704 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2710 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2711 May be interrupted by sending the process a SIGALRM. Returns the
2712 number of seconds actually slept. You probably cannot mix alarm() and
2713 sleep() calls, because sleep() is often implemented using alarm().
2715 On some older systems, it may sleep up to a full second less than what
2716 you requested, depending on how it counts seconds. Most modern systems
2717 always sleep the full amount.
2719 For delays of finer granularity than one second, you may use Perl's
2720 syscall() interface to access setitimer(2) if your system supports it,
2721 or else see L</select()> below.
2723 See also the POSIX module's sigpause() function.
2725 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2727 Opens a socket of the specified kind and attaches it to filehandle
2728 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
2729 system call of the same name. You should "use Socket;" first to get
2730 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2732 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2734 Creates an unnamed pair of sockets in the specified domain, of the
2735 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
2736 for the system call of the same name. If unimplemented, yields a fatal
2737 error. Returns TRUE if successful.
2739 =item sort SUBNAME LIST
2741 =item sort BLOCK LIST
2745 Sorts the LIST and returns the sorted list value. Nonexistent values
2746 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2747 in standard string comparison order. If SUBNAME is specified, it
2748 gives the name of a subroutine that returns an integer less than, equal
2749 to, or greater than 0, depending on how the elements of the array are
2750 to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
2751 routines.) SUBNAME may be a scalar variable name, in which case the
2752 value provides the name of the subroutine to use. In place of a
2753 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2756 In the interests of efficiency the normal calling code for subroutines is
2757 bypassed, with the following effects: the subroutine may not be a
2758 recursive subroutine, and the two elements to be compared are passed into
2759 the subroutine not via @_ but as the package global variables $a and
2760 $b (see example below). They are passed by reference, so don't
2761 modify $a and $b. And don't try to declare them as lexicals either.
2763 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2764 current collation locale. See L<perllocale>.
2769 @articles = sort @files;
2771 # same thing, but with explicit sort routine
2772 @articles = sort {$a cmp $b} @files;
2774 # now case-insensitively
2775 @articles = sort { uc($a) cmp uc($b)} @files;
2777 # same thing in reversed order
2778 @articles = sort {$b cmp $a} @files;
2780 # sort numerically ascending
2781 @articles = sort {$a <=> $b} @files;
2783 # sort numerically descending
2784 @articles = sort {$b <=> $a} @files;
2786 # sort using explicit subroutine name
2788 $age{$a} <=> $age{$b}; # presuming integers
2790 @sortedclass = sort byage @class;
2792 # this sorts the %age associative arrays by value
2793 # instead of key using an in-line function
2794 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2796 sub backwards { $b cmp $a; }
2797 @harry = ('dog','cat','x','Cain','Abel');
2798 @george = ('gone','chased','yz','Punished','Axed');
2800 # prints AbelCaincatdogx
2801 print sort backwards @harry;
2802 # prints xdogcatCainAbel
2803 print sort @george, 'to', @harry;
2804 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2806 # inefficiently sort by descending numeric compare using
2807 # the first integer after the first = sign, or the
2808 # whole record case-insensitively otherwise
2811 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2816 # same thing, but much more efficiently;
2817 # we'll build auxiliary indices instead
2821 push @nums, /=(\d+)/;
2826 $nums[$b] <=> $nums[$a]
2828 $caps[$a] cmp $caps[$b]
2832 # same thing using a Schwartzian Transform (no temps)
2833 @new = map { $_->[0] }
2834 sort { $b->[1] <=> $a->[1]
2837 } map { [$_, /=(\d+)/, uc($_)] } @old;
2839 If you're using strict, you I<MUST NOT> declare $a
2840 and $b as lexicals. They are package globals. That means
2841 if you're in the C<main> package, it's
2843 @articles = sort {$main::b <=> $main::a} @files;
2847 @articles = sort {$::b <=> $::a} @files;
2849 but if you're in the C<FooPack> package, it's
2851 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2853 The comparison function is required to behave. If it returns
2854 inconsistent results (sometimes saying $x[1] is less than $x[2] and
2855 sometimes saying the opposite, for example) the Perl interpreter will
2856 probably crash and dump core. This is entirely due to and dependent
2857 upon your system's qsort(3) library routine; this routine often avoids
2858 sanity checks in the interest of speed.
2860 =item splice ARRAY,OFFSET,LENGTH,LIST
2862 =item splice ARRAY,OFFSET,LENGTH
2864 =item splice ARRAY,OFFSET
2866 Removes the elements designated by OFFSET and LENGTH from an array, and
2867 replaces them with the elements of LIST, if any. Returns the elements
2868 removed from the array. The array grows or shrinks as necessary. If
2869 LENGTH is omitted, removes everything from OFFSET onward. The
2870 following equivalences hold (assuming C<$[ == 0>):
2872 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2873 pop(@a) splice(@a,-1)
2874 shift(@a) splice(@a,0,1)
2875 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2876 $a[$x] = $y splice(@a,$x,1,$y);
2878 Example, assuming array lengths are passed before arrays:
2880 sub aeq { # compare two list values
2881 local(@a) = splice(@_,0,shift);
2882 local(@b) = splice(@_,0,shift);
2883 return 0 unless @a == @b; # same len?
2885 return 0 if pop(@a) ne pop(@b);
2889 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2891 =item split /PATTERN/,EXPR,LIMIT
2893 =item split /PATTERN/,EXPR
2895 =item split /PATTERN/
2899 Splits a string into an array of strings, and returns it.
2901 If not in a list context, returns the number of fields found and splits into
2902 the @_ array. (In a list context, you can force the split into @_ by
2903 using C<??> as the pattern delimiters, but it still returns the array
2904 value.) The use of implicit split to @_ is deprecated, however.
2906 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2907 splits on whitespace (after skipping any leading whitespace). Anything
2908 matching PATTERN is taken to be a delimiter separating the fields. (Note
2909 that the delimiter may be longer than one character.) If LIMIT is
2910 specified and is not negative, splits into no more than that many fields
2911 (though it may split into fewer). If LIMIT is unspecified, trailing null
2912 fields are stripped (which potential users of pop() would do well to
2913 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2914 LIMIT had been specified.
2916 A pattern matching the null string (not to be confused with
2917 a null pattern C<//>, which is just one member of the set of patterns
2918 matching a null string) will split the value of EXPR into separate
2919 characters at each point it matches that way. For example:
2921 print join(':', split(/ */, 'hi there'));
2923 produces the output 'h:i:t:h:e:r:e'.
2925 The LIMIT parameter can be used to split a line partially
2927 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2929 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2930 one larger than the number of variables in the list, to avoid
2931 unnecessary work. For the list above LIMIT would have been 4 by
2932 default. In time critical applications it behooves you not to split
2933 into more fields than you really need.
2935 If the PATTERN contains parentheses, additional array elements are
2936 created from each matching substring in the delimiter.
2938 split(/([,-])/, "1-10,20", 3);
2940 produces the list value
2942 (1, '-', 10, ',', 20)
2944 If you had the entire header of a normal Unix email message in $header,
2945 you could split it up into fields and their values this way:
2947 $header =~ s/\n\s+/ /g; # fix continuation lines
2948 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2950 The pattern C</PATTERN/> may be replaced with an expression to specify
2951 patterns that vary at runtime. (To do runtime compilation only once,
2952 use C</$variable/o>.)
2954 As a special case, specifying a PATTERN of space (C<' '>) will split on
2955 white space just as split with no arguments does. Thus, split(' ') can
2956 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2957 will give you as many null initial fields as there are leading spaces.
2958 A split on /\s+/ is like a split(' ') except that any leading
2959 whitespace produces a null first field. A split with no arguments
2960 really does a C<split(' ', $_)> internally.
2964 open(passwd, '/etc/passwd');
2966 ($login, $passwd, $uid, $gid, $gcos,
2967 $home, $shell) = split(/:/);
2971 (Note that $shell above will still have a newline on it. See L</chop>,
2972 L</chomp>, and L</join>.)
2974 =item sprintf FORMAT, LIST
2976 Returns a string formatted by the usual printf conventions of the C
2977 language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2978 (The * character for an indirectly specified length is not
2979 supported, but you can get the same effect by interpolating a variable
2980 into the pattern.) If C<use locale> is
2981 in effect, the character used for the decimal point in formatted real numbers
2982 is affected by the LC_NUMERIC locale. See L<perllocale>.
2983 Some C libraries' implementations of sprintf() can
2984 dump core when fed ludicrous arguments.
2990 Return the square root of EXPR. If EXPR is omitted, returns square
2995 Sets the random number seed for the C<rand> operator. If EXPR is omitted,
2996 uses a semi-random value based on the current time and process ID, among
2997 other things. In versions of Perl prior to 5.004 the default seed was
2998 just the current time(). This isn't a particularly good seed, so many
2999 old programs supply their own seed value (often C<time ^ $$> or C<time ^
3000 ($$ + ($$ << 15))>), but that isn't necessary any more.
3002 You need something much more random than the default seed for
3003 cryptographic purposes, though. Checksumming the compressed output of
3004 one or more rapidly changing operating system status programs is the
3005 usual method. For example:
3007 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3009 If you're particularly concerned with this, see the Math::TrulyRandom
3012 Do I<not> call srand() multiple times in your program unless you know
3013 exactly what you're doing and why you're doing it. The point of the
3014 function is to "seed" the rand() function so that rand() can produce
3015 a different sequence each time you run your program. Just do it once at the
3016 top of your program, or you I<won't> get random numbers out of rand()!
3018 Frequently called programs (like CGI scripts) that simply use
3022 for a seed can fall prey to the mathematical property that
3026 one-third of the time. So don't do that.
3028 =item stat FILEHANDLE
3034 Returns a 13-element array giving the status info for a file, either the
3035 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
3036 stats $_. Returns a null list if the stat fails. Typically used as
3040 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3041 $atime,$mtime,$ctime,$blksize,$blocks)
3044 Not all fields are supported on all filesystem types. Here are the
3045 meaning of the fields:
3047 dev device number of filesystem
3049 mode file mode (type and permissions)
3050 nlink number of (hard) links to the file
3051 uid numeric user ID of file's owner
3052 gid numeric group ID of file's owner
3053 rdev the device identifier (special files only)
3054 size total size of file, in bytes
3055 atime last access time since the epoch
3056 mtime last modify time since the epoch
3057 ctime inode change time (NOT creation type!) since the epoch
3058 blksize preferred block size for file system I/O
3059 blocks actual number of blocks allocated
3061 (The epoch was at 00:00 January 1, 1970 GMT.)
3063 If stat is passed the special filehandle consisting of an underline, no
3064 stat is done, but the current contents of the stat structure from the
3065 last stat or filetest are returned. Example:
3067 if (-x $file && (($d) = stat(_)) && $d < 0) {
3068 print "$file is executable NFS file\n";
3071 (This works on machines only for which the device number is negative under NFS.)
3077 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3078 doing many pattern matches on the string before it is next modified.
3079 This may or may not save time, depending on the nature and number of
3080 patterns you are searching on, and on the distribution of character
3081 frequencies in the string to be searched--you probably want to compare
3082 run times with and without it to see which runs faster. Those loops
3083 which scan for many short constant strings (including the constant
3084 parts of more complex patterns) will benefit most. You may have only
3085 one study active at a time--if you study a different scalar the first
3086 is "unstudied". (The way study works is this: a linked list of every
3087 character in the string to be searched is made, so we know, for
3088 example, where all the 'k' characters are. From each search string,
3089 the rarest character is selected, based on some static frequency tables
3090 constructed from some C programs and English text. Only those places
3091 that contain this "rarest" character are examined.)
3093 For example, here is a loop which inserts index producing entries
3094 before any line containing a certain pattern:
3098 print ".IX foo\n" if /\bfoo\b/;
3099 print ".IX bar\n" if /\bbar\b/;
3100 print ".IX blurfl\n" if /\bblurfl\b/;
3105 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3106 will be looked at, because "f" is rarer than "o". In general, this is
3107 a big win except in pathological cases. The only question is whether
3108 it saves you more time than it took to build the linked list in the
3111 Note that if you have to look for strings that you don't know till
3112 runtime, you can build an entire loop as a string and eval that to
3113 avoid recompiling all your patterns all the time. Together with
3114 undefining $/ to input entire files as one record, this can be very
3115 fast, often faster than specialized programs like fgrep(1). The following
3116 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3117 out the names of those files that contain a match:
3119 $search = 'while (<>) { study;';
3120 foreach $word (@words) {
3121 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3126 eval $search; # this screams
3127 $/ = "\n"; # put back to normal input delimiter
3128 foreach $file (sort keys(%seen)) {
3136 =item sub NAME BLOCK
3138 This is subroutine definition, not a real function I<per se>. With just a
3139 NAME (and possibly prototypes), it's just a forward declaration. Without
3140 a NAME, it's an anonymous function declaration, and does actually return a
3141 value: the CODE ref of the closure you just created. See L<perlsub> and
3142 L<perlref> for details.
3144 =item substr EXPR,OFFSET,LEN
3146 =item substr EXPR,OFFSET
3148 Extracts a substring out of EXPR and returns it. First character is at
3149 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
3150 that far from the end of the string. If LEN is omitted, returns
3151 everything to the end of the string. If LEN is negative, leaves that
3152 many characters off the end of the string.
3154 You can use the substr() function
3155 as an lvalue, in which case EXPR must be an lvalue. If you assign
3156 something shorter than LEN, the string will shrink, and if you assign
3157 something longer than LEN, the string will grow to accommodate it. To
3158 keep the string the same length you may need to pad or chop your value
3161 =item symlink OLDFILE,NEWFILE
3163 Creates a new filename symbolically linked to the old filename.
3164 Returns 1 for success, 0 otherwise. On systems that don't support
3165 symbolic links, produces a fatal error at run time. To check for that,
3168 $symlink_exists = (eval 'symlink("","");', $@ eq '');
3172 Calls the system call specified as the first element of the list,
3173 passing the remaining elements as arguments to the system call. If
3174 unimplemented, produces a fatal error. The arguments are interpreted
3175 as follows: if a given argument is numeric, the argument is passed as
3176 an int. If not, the pointer to the string value is passed. You are
3177 responsible to make sure a string is pre-extended long enough to
3178 receive any result that might be written into a string. If your
3179 integer arguments are not literals and have never been interpreted in a
3180 numeric context, you may need to add 0 to them to force them to look
3183 require 'syscall.ph'; # may need to run h2ph
3184 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3186 Note that Perl supports passing of up to only 14 arguments to your system call,
3187 which in practice should usually suffice.
3189 =item sysopen FILEHANDLE,FILENAME,MODE
3191 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3193 Opens the file whose filename is given by FILENAME, and associates it
3194 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3195 the name of the real filehandle wanted. This function calls the
3196 underlying operating system's C<open> function with the parameters
3197 FILENAME, MODE, PERMS.
3199 The possible values and flag bits of the MODE parameter are
3200 system-dependent; they are available via the standard module C<Fcntl>.
3201 However, for historical reasons, some values are universal: zero means
3202 read-only, one means write-only, and two means read/write.
3204 If the file named by FILENAME does not exist and the C<open> call
3205 creates it (typically because MODE includes the O_CREAT flag), then
3206 the value of PERMS specifies the permissions of the newly created
3207 file. If PERMS is omitted, the default value is 0666, which allows
3208 read and write for all. This default is reasonable: see C<umask>.
3210 The IO::File module provides a more object-oriented approach, if you're
3211 into that kind of thing.
3213 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3215 =item sysread FILEHANDLE,SCALAR,LENGTH
3217 Attempts to read LENGTH bytes of data into variable SCALAR from the
3218 specified FILEHANDLE, using the system call read(2). It bypasses
3219 stdio, so mixing this with other kinds of reads may cause confusion.
3220 Returns the number of bytes actually read, or undef if there was an
3221 error. SCALAR will be grown or shrunk so that the last byte actually
3222 read is the last byte of the scalar after the read.
3224 An OFFSET may be specified to place the read data at some place in the
3225 string other than the beginning. A negative OFFSET specifies
3226 placement at that many bytes counting backwards from the end of the
3227 string. A positive OFFSET greater than the length of SCALAR results
3228 in the string being padded to the required size with "\0" bytes before
3229 the result of the read is appended.
3233 Does exactly the same thing as "exec LIST" except that a fork is done
3234 first, and the parent process waits for the child process to complete.
3235 Note that argument processing varies depending on the number of
3236 arguments. The return value is the exit status of the program as
3237 returned by the wait() call. To get the actual exit value divide by
3238 256. See also L</exec>. This is I<NOT> what you want to use to capture
3239 the output from a command, for that you should use merely back-ticks or
3240 qx//, as described in L<perlop/"`STRING`">.
3242 Because system() and back-ticks block SIGINT and SIGQUIT, killing the
3243 program they're running doesn't actually interrupt your program.
3245 @args = ("command", "arg1", "arg2");
3247 or die "system @args failed: $?"
3249 Here's a more elaborate example of analysing the return value from
3250 system() on a UNIX system to check for all possibilities, including for
3251 signals and coredumps.
3253 $rc = 0xffff & system @args;
3254 printf "system(%s) returned %#04x: ", "@args", $rc;
3256 print "ran with normal exit\n";
3258 elsif ($rc == 0xff00) {
3259 print "command failed: $!\n";
3261 elsif ($rc > 0x80) {
3263 print "ran with non-zero exit status $rc\n";
3269 print "coredump from ";
3271 print "signal $rc\n"
3275 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3277 =item syswrite FILEHANDLE,SCALAR,LENGTH
3279 Attempts to write LENGTH bytes of data from variable SCALAR to the
3280 specified FILEHANDLE, using the system call write(2). It bypasses
3281 stdio, so mixing this with prints may cause confusion. Returns the
3282 number of bytes actually written, or undef if there was an error.
3283 If the length is greater than the available data, only as much data as
3284 is available will be written.
3286 An OFFSET may be specified to write the data from some part of the
3287 string other than the beginning. A negative OFFSET specifies writing
3288 from that many bytes counting backwards from the end of the string.
3290 =item tell FILEHANDLE
3294 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3295 expression whose value gives the name of the actual filehandle. If
3296 FILEHANDLE is omitted, assumes the file last read.
3298 =item telldir DIRHANDLE
3300 Returns the current position of the readdir() routines on DIRHANDLE.
3301 Value may be given to seekdir() to access a particular location in a
3302 directory. Has the same caveats about possible directory compaction as
3303 the corresponding system library routine.
3305 =item tie VARIABLE,CLASSNAME,LIST
3307 This function binds a variable to a package class that will provide the
3308 implementation for the variable. VARIABLE is the name of the variable
3309 to be enchanted. CLASSNAME is the name of a class implementing objects
3310 of correct type. Any additional arguments are passed to the "new"
3311 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3312 Typically these are arguments such as might be passed to the dbm_open()
3313 function of C. The object returned by the "new" method is also
3314 returned by the tie() function, which would be useful if you want to
3315 access other methods in CLASSNAME.
3317 Note that functions such as keys() and values() may return huge array
3318 values when used on large objects, like DBM files. You may prefer to
3319 use the each() function to iterate over such. Example:
3321 # print out history file offsets
3323 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3324 while (($key,$val) = each %HIST) {
3325 print $key, ' = ', unpack('L',$val), "\n";
3329 A class implementing an associative array should have the following
3332 TIEHASH classname, LIST
3335 STORE this, key, value
3339 NEXTKEY this, lastkey
3341 A class implementing an ordinary array should have the following methods:
3343 TIEARRAY classname, LIST
3346 STORE this, key, value
3349 A class implementing a scalar should have the following methods:
3351 TIESCALAR classname, LIST
3356 Unlike dbmopen(), the tie() function will not use or require a module
3357 for you--you need to do that explicitly yourself. See L<DB_File>
3358 or the F<Config> module for interesting tie() implementations.
3362 Returns a reference to the object underlying VARIABLE (the same value
3363 that was originally returned by the tie() call which bound the variable
3364 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3369 Returns the number of non-leap seconds since whatever time the system
3370 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3371 and 00:00:00 UTC, January 1, 1970 for most other systems).
3372 Suitable for feeding to gmtime() and localtime().
3376 Returns a four-element array giving the user and system times, in
3377 seconds, for this process and the children of this process.
3379 ($user,$system,$cuser,$csystem) = times;
3383 The translation operator. See L<perlop>.
3385 =item truncate FILEHANDLE,LENGTH
3387 =item truncate EXPR,LENGTH
3389 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3390 specified length. Produces a fatal error if truncate isn't implemented
3397 Returns an uppercased version of EXPR. This is the internal function
3398 implementing the \U escape in double-quoted strings.
3399 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3401 If EXPR is omitted, uses $_.
3407 Returns the value of EXPR with the first character uppercased. This is
3408 the internal function implementing the \u escape in double-quoted strings.
3409 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3411 If EXPR is omitted, uses $_.
3417 Sets the umask for the process and returns the old one. If EXPR is
3418 omitted, returns merely the current umask.
3424 Undefines the value of EXPR, which must be an lvalue. Use on only a
3425 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3426 will probably not do what you expect on most predefined variables or
3427 DBM list values, so don't do that.) Always returns the undefined value. You can omit
3428 the EXPR, in which case nothing is undefined, but you still get an
3429 undefined value that you could, for instance, return from a
3430 subroutine. Examples:
3433 undef $bar{'blurfl'};
3437 return (wantarray ? () : undef) if $they_blew_it;
3443 Deletes a list of files. Returns the number of files successfully
3446 $cnt = unlink 'a', 'b', 'c';
3450 Note: unlink will not delete directories unless you are superuser and
3451 the B<-U> flag is supplied to Perl. Even if these conditions are
3452 met, be warned that unlinking a directory can inflict damage on your
3453 filesystem. Use rmdir instead.
3455 If LIST is omitted, uses $_.
3457 =item unpack TEMPLATE,EXPR
3459 Unpack does the reverse of pack: it takes a string representing a
3460 structure and expands it out into a list value, returning the array
3461 value. (In a scalar context, it returns merely the first value
3462 produced.) The TEMPLATE has the same format as in the pack function.
3463 Here's a subroutine that does substring:
3466 local($what,$where,$howmuch) = @_;
3467 unpack("x$where a$howmuch", $what);
3472 sub ordinal { unpack("c",$_[0]); } # same as ord()
3474 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3475 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3476 themselves. Default is a 16-bit checksum. For example, the following
3477 computes the same number as the System V sum program:
3480 $checksum += unpack("%16C*", $_);
3484 The following efficiently counts the number of set bits in a bit vector:
3486 $setbits = unpack("%32b*", $selectmask);
3488 =item untie VARIABLE
3490 Breaks the binding between a variable and a package. (See tie().)
3492 =item unshift ARRAY,LIST
3494 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3495 depending on how you look at it. Prepends list to the front of the
3496 array, and returns the new number of elements in the array.
3498 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3500 Note the LIST is prepended whole, not one element at a time, so the
3501 prepended elements stay in the same order. Use reverse to do the
3504 =item use Module LIST
3508 =item use Module VERSION LIST
3512 Imports some semantics into the current package from the named module,
3513 generally by aliasing certain subroutine or variable names into your
3514 package. It is exactly equivalent to
3516 BEGIN { require Module; import Module LIST; }
3518 except that Module I<must> be a bare word.
3520 If the first argument to C<use> is a number, it is treated as a version
3521 number instead of a module name. If the version of the Perl interpreter
3522 is less than VERSION, then an error message is printed and Perl exits
3523 immediately. This is often useful if you need to check the current
3524 Perl version before C<use>ing library modules which have changed in
3525 incompatible ways from older versions of Perl. (We try not to do
3526 this more than we have to.)
3528 The BEGIN forces the require and import to happen at compile time. The
3529 require makes sure the module is loaded into memory if it hasn't been
3530 yet. The import is not a builtin--it's just an ordinary static method
3531 call into the "Module" package to tell the module to import the list of
3532 features back into the current package. The module can implement its
3533 import method any way it likes, though most modules just choose to
3534 derive their import method via inheritance from the Exporter class that
3535 is defined in the Exporter module. See L<Exporter>. If no import
3536 method can be found then the error is currently silently ignored. This
3537 may change to a fatal error in a future version.
3539 If you don't want your namespace altered, explicitly supply an empty list:
3543 That is exactly equivalent to
3545 BEGIN { require Module; }
3547 If the VERSION argument is present between Module and LIST, then the
3548 C<use> will call the VERSION method in class Module with the given
3549 version as an argument. The default VERSION method, inherited from
3550 the Universal class, croaks if the given version is larger than the
3551 value of the variable $Module::VERSION. (Note that there is not a
3552 comma after VERSION!)
3554 Because this is a wide-open interface, pragmas (compiler directives)
3555 are also implemented this way. Currently implemented pragmas are:
3559 use sigtrap qw(SEGV BUS);
3560 use strict qw(subs vars refs);
3561 use subs qw(afunc blurfl);
3563 These pseudo-modules import semantics into the current block scope, unlike
3564 ordinary modules, which import symbols into the current package (which are
3565 effective through the end of the file).
3567 There's a corresponding "no" command that unimports meanings imported
3568 by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
3573 If no unimport method can be found the call fails with a fatal error.
3575 See L<perlmod> for a list of standard modules and pragmas.
3579 Changes the access and modification times on each file of a list of
3580 files. The first two elements of the list must be the NUMERICAL access
3581 and modification times, in that order. Returns the number of files
3582 successfully changed. The inode modification time of each file is set
3583 to the current time. Example of a "touch" command:
3587 utime $now, $now, @ARGV;
3589 =item values ASSOC_ARRAY
3591 Returns a normal array consisting of all the values of the named
3592 associative array. (In a scalar context, returns the number of
3593 values.) The values are returned in an apparently random order, but it
3594 is the same order as either the keys() or each() function would produce
3595 on the same array. See also keys(), each(), and sort().
3597 =item vec EXPR,OFFSET,BITS
3599 Treats the string in EXPR as a vector of unsigned integers, and
3600 returns the value of the bit field specified by OFFSET. BITS specifies
3601 the number of bits that are reserved for each entry in the bit
3602 vector. This must be a power of two from 1 to 32. vec() may also be
3603 assigned to, in which case parentheses are needed to give the expression
3604 the correct precedence as in
3606 vec($image, $max_x * $x + $y, 8) = 3;
3608 Vectors created with vec() can also be manipulated with the logical
3609 operators |, &, and ^, which will assume a bit vector operation is
3610 desired when both operands are strings.
3612 To transform a bit vector into a string or array of 0's and 1's, use these:
3614 $bits = unpack("b*", $vector);
3615 @bits = split(//, unpack("b*", $vector));
3617 If you know the exact length in bits, it can be used in place of the *.
3621 Waits for a child process to terminate and returns the pid of the
3622 deceased process, or -1 if there are no child processes. The status is
3625 =item waitpid PID,FLAGS
3627 Waits for a particular child process to terminate and returns the pid
3628 of the deceased process, or -1 if there is no such child process. The
3629 status is returned in C<$?>. If you say
3631 use POSIX ":sys_wait_h";
3633 waitpid(-1,&WNOHANG);
3635 then you can do a non-blocking wait for any process. Non-blocking wait
3636 is available on machines supporting either the waitpid(2) or
3637 wait4(2) system calls. However, waiting for a particular pid with
3638 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3639 by remembering the status values of processes that have exited but have
3640 not been harvested by the Perl script yet.)
3644 Returns TRUE if the context of the currently executing subroutine is
3645 looking for a list value. Returns FALSE if the context is looking
3648 return wantarray ? () : undef;
3652 Produces a message on STDERR just like die(), but doesn't exit or
3655 =item write FILEHANDLE
3661 Writes a formatted record (possibly multi-line) to the specified file,
3662 using the format associated with that file. By default the format for
3663 a file is the one having the same name is the filehandle, but the
3664 format for the current output channel (see the select() function) may be set
3665 explicitly by assigning the name of the format to the C<$~> variable.
3667 Top of form processing is handled automatically: if there is
3668 insufficient room on the current page for the formatted record, the
3669 page is advanced by writing a form feed, a special top-of-page format
3670 is used to format the new page header, and then the record is written.
3671 By default the top-of-page format is the name of the filehandle with
3672 "_TOP" appended, but it may be dynamically set to the format of your
3673 choice by assigning the name to the C<$^> variable while the filehandle is
3674 selected. The number of lines remaining on the current page is in
3675 variable C<$->, which can be set to 0 to force a new page.
3677 If FILEHANDLE is unspecified, output goes to the current default output
3678 channel, which starts out as STDOUT but may be changed by the
3679 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3680 is evaluated and the resulting string is used to look up the name of
3681 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3683 Note that write is I<NOT> the opposite of read. Unfortunately.
3687 The translation operator. See L<perlop>.