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 only
18 ever be 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 parens.) If you use the parens, 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:
61 I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
65 Each operator and function decides which sort of value it would be most
66 appropriate to return in a scalar context. Some operators return the
67 length of the list that would have been returned in a list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
73 =head2 Perl Functions by Category
75 Here are Perl's functions (including things that look like
76 functions, like some of the keywords and named operators)
77 arranged by category. Some functions appear in more
82 =item Functions for SCALARs or strings
84 chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
85 oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
86 sprintf, substr, tr///, uc, ucfirst, y///
88 =item Regular expressions and pattern matching
90 m//, pos, quotemeta, s///, split, study
92 =item Numeric functions
94 abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
97 =item Functions for real @ARRAYs
99 pop, push, shift, splice, unshift
101 =item Functions for list data
103 grep, join, map, qw/STRING/, reverse, sort, unpack
105 =item Functions for real %HASHes
107 delete, each, exists, keys, values
109 =item Input and output functions
111 binmode, close, closedir, dbmclose, dbmopen, die, eof,
112 fileno, flock, format, getc, print, printf, read, readdir,
113 rewinddir, seek, seekdir, select, syscall, sysread,
114 syswrite, tell, telldir, truncate, warn, write
116 =item Functions for fixed length data or records
118 pack, read, syscall, sysread, syswrite, unpack, vec
120 =item Functions for filehandles, files, or directories
122 I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
123 lstat, mkdir, open, opendir, readlink, rename, rmdir,
124 stat, symlink, umask, unlink, utime
126 =item Keywords related to the control flow of your perl program
128 caller, continue, die, do, dump, eval, exit, goto, last,
129 next, redo, return, sub, wantarray
131 =item Keywords related to scoping
133 caller, import, local, my, package, use
135 =item Miscellaneous functions
137 defined, dump, eval, formline, local, my, reset, scalar,
140 =item Functions for processes and process groups
142 alarm, exec, fork, getpgrp, getppid, getpriority, kill,
143 pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
146 =item Keywords related to perl modules
148 do, import, no, package, require, use
150 =item Keywords related to classes and object-orientedness
152 bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
154 =item Low-level socket functions
156 accept, bind, connect, getpeername, getsockname,
157 getsockopt, listen, recv, send, setsockopt, shutdown,
160 =item System V interprocess communication functions
162 msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
163 shmctl, shmget, shmread, shmwrite
165 =item Fetching user and group info
167 endgrent, endhostent, endnetent, endpwent, getgrent,
168 getgrgid, getgrnam, getlogin, getpwent, getpwnam,
169 getpwuid, setgrent, setpwent
171 =item Fetching network info
173 endprotoent, endservent, gethostbyaddr, gethostbyname,
174 gethostent, getnetbyaddr, getnetbyname, getnetent,
175 getprotobyname, getprotobynumber, getprotoent,
176 getservbyname, getservbyport, getservent, sethostent,
177 setnetent, setprotoent, setservent
179 =item Time-related functions
181 gmtime, localtime, time, times
183 =item Functions new in perl5
185 abs, bless, chomp, chr, exists, formline, glob, import, lc,
186 lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
187 ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
189 * - C<sub> was a keyword in perl4, but in perl5 it is an
190 operator which can be used in expressions.
192 =item Functions obsoleted in perl5
199 =head2 Alphabetical Listing of Perl Functions
210 A file test, where X is one of the letters listed below. This unary
211 operator takes one argument, either a filename or a filehandle, and
212 tests the associated file to see if something is true about it. If the
213 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
214 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
215 the undefined value if the file doesn't exist. Despite the funny
216 names, precedence is the same as any other named unary operator, and
217 the argument may be parenthesized like any other unary operator. The
218 operator may be any of:
220 -r File is readable by effective uid/gid.
221 -w File is writable by effective uid/gid.
222 -x File is executable by effective uid/gid.
223 -o File is owned by effective uid.
225 -R File is readable by real uid/gid.
226 -W File is writable by real uid/gid.
227 -X File is executable by real uid/gid.
228 -O File is owned by real uid.
231 -z File has zero size.
232 -s File has non-zero size (returns size).
234 -f File is a plain file.
235 -d File is a directory.
236 -l File is a symbolic link.
237 -p File is a named pipe (FIFO).
239 -b File is a block special file.
240 -c File is a character special file.
241 -t Filehandle is opened to a tty.
243 -u File has setuid bit set.
244 -g File has setgid bit set.
245 -k File has sticky bit set.
247 -T File is a text file.
248 -B File is a binary file (opposite of -T).
250 -M Age of file in days when script started.
251 -A Same for access time.
252 -C Same for inode change time.
254 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
255 C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
256 uids and gids of the user. There may be other reasons you can't actually
257 read, write or execute the file. Also note that, for the superuser,
258 C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
259 1 if any execute bit is set in the mode. Scripts run by the superuser may
260 thus need to do a stat() in order to determine the actual mode of the
261 file, or temporarily set the uid to something else.
267 next unless -f $_; # ignore specials
271 Note that C<-s/a/b/> does not do a negated substitution. Saying
272 C<-exp($foo)> still works as expected, however--only single letters
273 following a minus are interpreted as file tests.
275 The C<-T> and C<-B> switches work as follows. The first block or so of the
276 file is examined for odd characters such as strange control codes or
277 characters with the high bit set. If too many odd characters (E<gt>30%)
278 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
279 containing null in the first block is considered a binary file. If C<-T>
280 or C<-B> is used on a filehandle, the current stdio buffer is examined
281 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
282 file, or a file at EOF when testing a filehandle. Because you have to
283 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
284 against the file first, as in C<next unless -f $file && -T $file>.
286 If any of the file tests (or either the stat() or lstat() operators) are given the
287 special filehandle consisting of a solitary underline, then the stat
288 structure of the previous file test (or stat operator) is used, saving
289 a system call. (This doesn't work with C<-t>, and you need to remember
290 that lstat() and C<-l> will leave values in the stat structure for the
291 symbolic link, not the real file.) Example:
293 print "Can do.\n" if -r $a || -w _ || -x _;
296 print "Readable\n" if -r _;
297 print "Writable\n" if -w _;
298 print "Executable\n" if -x _;
299 print "Setuid\n" if -u _;
300 print "Setgid\n" if -g _;
301 print "Sticky\n" if -k _;
302 print "Text\n" if -T _;
303 print "Binary\n" if -B _;
309 Returns the absolute value of its argument.
310 If VALUE is omitted, uses $_.
312 =item accept NEWSOCKET,GENERICSOCKET
314 Accepts an incoming socket connect, just as the accept(2) system call
315 does. Returns the packed address if it succeeded, FALSE otherwise.
316 See example in L<perlipc/"Sockets: Client/Server Communication">.
322 Arranges to have a SIGALRM delivered to this process after the
323 specified number of seconds have elapsed. If SECONDS is not specified,
324 the value stored in $_ is used. (On some machines,
325 unfortunately, the elapsed time may be up to one second less than you
326 specified because of how seconds are counted.) Only one timer may be
327 counting at once. Each call disables the previous timer, and an
328 argument of 0 may be supplied to cancel the previous timer without
329 starting a new one. The returned value is the amount of time remaining
330 on the previous timer.
332 For delays of finer granularity than one second, you may use Perl's
333 syscall() interface to access setitimer(2) if your system supports it,
334 or else see L</select()> below. It is not advised to intermix alarm()
339 Returns the arctangent of Y/X in the range -PI to PI.
341 =item bind SOCKET,NAME
343 Binds a network address to a socket, just as the bind system call
344 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
345 packed address of the appropriate type for the socket. See the examples in
346 L<perlipc/"Sockets: Client/Server Communication">.
348 =item binmode FILEHANDLE
350 Arranges for the file to be read or written in "binary" mode in operating
351 systems that distinguish between binary and text files. Files that are
352 not in binary mode have CR LF sequences translated to LF on input and LF
353 translated to CR LF on output. Binmode has no effect under Unix; in DOS
354 and similarly archaic systems, it may be imperative--otherwise your
355 DOS-damaged C library may mangle your file. The key distinction between
356 systems that need binmode and those that don't is their text file
357 formats. Systems like Unix and Plan9 that delimit lines with a single
358 character, and that encode that character in C as '\n', do not need
359 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
360 is taken as the name of the filehandle.
362 =item bless REF,CLASSNAME
366 This function tells the referenced object (passed as REF) that it is now
367 an object in the CLASSNAME package--or the current package if no CLASSNAME
368 is specified, which is often the case. It returns the reference for
369 convenience, since a bless() is often the last thing in a constructor.
370 Always use the two-argument version if the function doing the blessing
371 might be inherited by a derived class. See L<perlobj> for more about the
372 blessing (and blessings) of objects.
378 Returns the context of the current subroutine call. In a scalar context,
379 returns TRUE if there is a caller, that is, if we're in a subroutine or
380 eval() or require(), and FALSE otherwise. In a list context, returns
382 ($package, $filename, $line) = caller;
384 With EXPR, it returns some extra information that the debugger uses to
385 print a stack trace. The value of EXPR indicates how many call frames
386 to go back before the current one.
388 ($package, $filename, $line,
389 $subroutine, $hasargs, $wantargs) = caller($i);
391 Furthermore, when called from within the DB package, caller returns more
392 detailed information: it sets the list variable @DB::args to be the
393 arguments with which that subroutine was invoked.
397 Changes the working directory to EXPR, if possible. If EXPR is
398 omitted, changes to home directory. Returns TRUE upon success, FALSE
399 otherwise. See example under die().
403 Changes the permissions of a list of files. The first element of the
404 list must be the numerical mode, which should probably be an octal
405 number. Returns the number of files successfully changed.
407 $cnt = chmod 0755, 'foo', 'bar';
408 chmod 0755, @executables;
416 This is a slightly safer version of chop (see below). It removes any
417 line ending that corresponds to the current value of C<$/> (also known as
418 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
419 of characters removed. It's often used to remove the newline from the
420 end of an input record when you're worried that the final record may be
421 missing its newline. When in paragraph mode (C<$/ = "">), it removes all
422 trailing newlines from the string. If VARIABLE is omitted, it chomps
426 chomp; # avoid \n on last field
431 You can actually chomp anything that's an lvalue, including an assignment:
434 chomp($answer = <STDIN>);
436 If you chomp a list, each element is chomped, and the total number of
437 characters removed is returned.
445 Chops off the last character of a string and returns the character
446 chopped. It's used primarily to remove the newline from the end of an
447 input record, but is much more efficient than C<s/\n//> because it neither
448 scans nor copies the string. If VARIABLE is omitted, chops $_.
452 chop; # avoid \n on last field
457 You can actually chop anything that's an lvalue, including an assignment:
460 chop($answer = <STDIN>);
462 If you chop a list, each element is chopped. Only the value of the
463 last chop is returned.
465 Note that chop returns the last character. To return all but the last
466 character, use C<substr($string, 0, -1)>.
470 Changes the owner (and group) of a list of files. The first two
471 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
472 Returns the number of files successfully changed.
474 $cnt = chown $uid, $gid, 'foo', 'bar';
475 chown $uid, $gid, @filenames;
477 Here's an example that looks up non-numeric uids in the passwd file:
480 chop($user = <STDIN>);
482 chop($pattern = <STDIN>);
484 ($login,$pass,$uid,$gid) = getpwnam($user)
485 or die "$user not in passwd file";
487 @ary = <${pattern}>; # expand filenames
488 chown $uid, $gid, @ary;
490 On most systems, you are not allowed to change the ownership of the
491 file unless you're the superuser, although you should be able to change
492 the group to any of your secondary groups. On insecure systems, these
493 restrictions may be relaxed, but this is not a portable assumption.
499 Returns the character represented by that NUMBER in the character set.
500 For example, C<chr(65)> is "A" in ASCII.
502 If NUMBER is omitted, uses $_.
504 =item chroot FILENAME
508 This function works as the system call by the same name: it makes the
509 named directory the new root directory for all further pathnames that
510 begin with a "/" by your process and all of its children. (It doesn't
511 change your current working directory is unaffected.) For security
512 reasons, this call is restricted to the superuser. If FILENAME is
513 omitted, does chroot to $_.
515 =item close FILEHANDLE
517 Closes the file or pipe associated with the file handle, returning TRUE
518 only if stdio successfully flushes buffers and closes the system file
519 descriptor. You don't have to close FILEHANDLE if you are immediately
520 going to do another open() on it, since open() will close it for you. (See
521 open().) However, an explicit close on an input file resets the line
522 counter ($.), while the implicit close done by open() does not. Also,
523 closing a pipe will wait for the process executing on the pipe to
524 complete, in case you want to look at the output of the pipe
525 afterwards. Closing a pipe explicitly also puts the status value of
526 the command into C<$?>. Example:
528 open(OUTPUT, '|sort >foo'); # pipe to sort
529 ... # print stuff to output
530 close OUTPUT; # wait for sort to finish
531 open(INPUT, 'foo'); # get sort's results
533 FILEHANDLE may be an expression whose value gives the real filehandle name.
535 =item closedir DIRHANDLE
537 Closes a directory opened by opendir().
539 =item connect SOCKET,NAME
541 Attempts to connect to a remote socket, just as the connect system call
542 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
543 packed address of the appropriate type for the socket. See the examples in
544 L<perlipc/"Sockets: Client/Server Communication">.
548 Actually a flow control statement rather than a function. If there is a
549 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
550 C<foreach>), it is always executed just before the conditional is about to
551 be evaluated again, just like the third part of a C<for> loop in C. Thus
552 it can be used to increment a loop variable, even when the loop has been
553 continued via the C<next> statement (which is similar to the C C<continue>
558 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
561 =item crypt PLAINTEXT,SALT
563 Encrypts a string exactly like the crypt(3) function in the C library
564 (assuming that you actually have a version there that has not been
565 extirpated as a potential munition). This can prove useful for checking
566 the password file for lousy passwords, amongst other things. Only the
567 guys wearing white hats should do this.
569 Here's an example that makes sure that whoever runs this program knows
572 $pwd = (getpwuid($<))[1];
573 $salt = substr($pwd, 0, 2);
577 chop($word = <STDIN>);
581 if (crypt($word, $salt) ne $pwd) {
587 Of course, typing in your own password to whoever asks you
590 =item dbmclose ASSOC_ARRAY
592 [This function has been superseded by the untie() function.]
594 Breaks the binding between a DBM file and an associative array.
596 =item dbmopen ASSOC,DBNAME,MODE
598 [This function has been superseded by the tie() function.]
600 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
601 associative array. ASSOC is the name of the associative array. (Unlike
602 normal open, the first argument is I<NOT> a filehandle, even though it
603 looks like one). DBNAME is the name of the database (without the F<.dir>
604 or F<.pag> extension if any). If the database does not exist, it is
605 created with protection specified by MODE (as modified by the umask()).
606 If your system only supports the older DBM functions, you may perform only
607 one dbmopen() in your program. In older versions of Perl, if your system
608 had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
609 falls back to sdbm(3).
611 If you don't have write access to the DBM file, you can only read
612 associative array variables, not set them. If you want to test whether
613 you can write, either use file tests or try setting a dummy array entry
614 inside an eval(), which will trap the error.
616 Note that functions such as keys() and values() may return huge array
617 values when used on large DBM files. You may prefer to use the each()
618 function to iterate over large DBM files. Example:
620 # print out history file offsets
621 dbmopen(%HIST,'/usr/lib/news/history',0666);
622 while (($key,$val) = each %HIST) {
623 print $key, ' = ', unpack('L',$val), "\n";
627 See also L<AnyDBM_File> for a more general description of the pros and
628 cons of the various dbm approaches, as well as L<DB_File> for a particularly
635 Returns a boolean value saying whether EXPR has a real value
636 or not. If EXPR is not present, $_ will be checked. Many operations
637 return the undefined value under exceptional conditions, such as end of
638 file, uninitialized variable, system error and such. This function
639 allows you to distinguish between an undefined
640 null scalar and a defined null scalar with operations that might return
641 a real null string, such as referencing elements of an array. You may
642 also check to see if arrays or subroutines exist. Use of defined on
643 predefined variables is not guaranteed to produce intuitive results.
645 When used on a hash array element, it tells you whether the value
646 is defined, not whether the key exists in the hash. Use exists() for that.
650 print if defined $switch{'D'};
651 print "$val\n" while defined($val = pop(@ary));
652 die "Can't readlink $sym: $!"
653 unless defined($value = readlink $sym);
654 eval '@foo = ()' if defined(@foo);
655 die "No XYZ package defined" unless defined %_XYZ;
656 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
660 Note: many folks tend to overuse defined(), and then are surprised to
661 discover that the number 0 and the null string are, in fact, defined
662 concepts. For example, if you say
666 the pattern match succeeds, and $1 is defined, despite the fact that it
667 matched "nothing". But it didn't really match nothing--rather, it
668 matched something that happened to be 0 characters long. This is all
669 very above-board and honest. When a function returns an undefined value,
670 it's an admission that it couldn't give you an honest answer. So
671 you should only use defined() when you're questioning the integrity
672 of what you're trying to do. At other times, a simple comparison to
673 0 or "" is what you want.
677 Deletes the specified value from its hash array. Returns the deleted
678 value, or the undefined value if nothing was deleted. Deleting from
679 C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM
680 file deletes the entry from the DBM file. (But deleting from a tie()d
681 hash doesn't necessarily return anything.)
683 The following deletes all the values of an associative array:
685 foreach $key (keys %ARRAY) {
689 (But it would be faster to use the undef() command.) Note that the
690 EXPR can be arbitrarily complicated as long as the final operation is
693 delete $ref->[$x][$y]{$key};
697 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
698 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
699 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> is 0,
700 exits with 255. Inside an eval(), the error message is stuffed into C<$@>,
701 and the eval() is terminated with the undefined value; this makes die()
702 the way to raise an exception.
706 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
707 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
709 If the value of EXPR does not end in a newline, the current script line
710 number and input line number (if any) are also printed, and a newline
711 is supplied. Hint: sometimes appending ", stopped" to your message
712 will cause it to make better sense when the string "at foo line 123" is
713 appended. Suppose you are running script "canasta".
715 die "/etc/games is no good";
716 die "/etc/games is no good, stopped";
718 produce, respectively
720 /etc/games is no good at canasta line 123.
721 /etc/games is no good, stopped at canasta line 123.
723 See also exit() and warn().
727 Not really a function. Returns the value of the last command in the
728 sequence of commands indicated by BLOCK. When modified by a loop
729 modifier, executes the BLOCK once before testing the loop condition.
730 (On other statements the loop modifiers test the conditional first.)
732 =item do SUBROUTINE(LIST)
734 A deprecated form of subroutine call. See L<perlsub>.
738 Uses the value of EXPR as a filename and executes the contents of the
739 file as a Perl script. Its primary use is to include subroutines
740 from a Perl subroutine library.
748 except that it's more efficient, more concise, keeps track of the
749 current filename for error messages, and searches all the B<-I>
750 libraries if the file isn't in the current directory (see also the @INC
751 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
752 reparse the file every time you call it, so you probably don't want to
753 do this inside a loop.
755 Note that inclusion of library modules is better done with the
756 use() and require() operators, which also do error checking
757 and raise an exception if there's a problem.
761 This causes an immediate core dump. Primarily this is so that you can
762 use the B<undump> program to turn your core dump into an executable binary
763 after having initialized all your variables at the beginning of the
764 program. When the new binary is executed it will begin by executing a
765 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
766 it as a goto with an intervening core dump and reincarnation. If LABEL
767 is omitted, restarts the program from the top. WARNING: any files
768 opened at the time of the dump will NOT be open any more when the
769 program is reincarnated, with possible resulting confusion on the part
770 of Perl. See also B<-u> option in L<perlrun>.
787 dump QUICKSTART if $ARGV[0] eq '-d';
792 =item each ASSOC_ARRAY
794 When called in a list context, returns a 2-element array consisting
795 of the key and value for the next element of an associative array,
796 so that you can iterate over it. When called in a scalar context,
797 returns the key only for the next element in the associative array.
798 Entries are returned in an apparently random order. When the array is
799 entirely read, a null array is returned in list context (which when
800 assigned produces a FALSE (0) value), and C<undef> is returned in a
801 scalar context. The next call to each() after that will start
802 iterating again. The iterator can be reset only by reading all the
803 elements from the array. You should not add elements to an array while
804 you're iterating over it. There is a single iterator for each
805 associative array, shared by all each(), keys() and values() function
806 calls in the program. The following prints out your environment like
807 the printenv(1) program, only in a different order:
809 while (($key,$value) = each %ENV) {
810 print "$key=$value\n";
813 See also keys() and values().
821 Returns 1 if the next read on FILEHANDLE will return end of file, or if
822 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
823 gives the real filehandle name. (Note that this function actually
824 reads a character and then ungetc()s it, so it is not very useful in an
825 interactive context.) Do not read from a terminal file (or call
826 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
827 as terminals may lose the end-of-file condition if you do.
829 An C<eof> without an argument uses the last file read as argument.
830 Empty parentheses () may be used to indicate
831 the pseudofile formed of the files listed on the command line, i.e.
832 C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
833 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
834 test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
836 # reset line numbering on each input file
839 close(ARGV) if (eof); # Not eof().
842 # insert dashes just before last line of last file
845 print "--------------\n";
846 close(ARGV); # close or break; is needed if we
847 # are reading from the terminal
852 Practical hint: you almost never need to use C<eof> in Perl, because the
853 input operators return undef when they run out of data.
859 EXPR is parsed and executed as if it were a little Perl program. It
860 is executed in the context of the current Perl program, so that any
861 variable settings, subroutine or format definitions remain afterwards.
862 The value returned is the value of the last expression evaluated, or a
863 return statement may be used, just as with subroutines. The last
864 expression is evaluated in scalar or array context, depending on the
867 If there is a syntax error or runtime error, or a die() statement is
868 executed, an undefined value is returned by eval(), and C<$@> is set to the
869 error message. If there was no error, C<$@> is guaranteed to be a null
870 string. If EXPR is omitted, evaluates $_. The final semicolon, if
871 any, may be omitted from the expression.
873 Note that, since eval() traps otherwise-fatal errors, it is useful for
874 determining whether a particular feature (such as socket() or symlink())
875 is implemented. It is also Perl's exception trapping mechanism, where
876 the die operator is used to raise exceptions.
878 If the code to be executed doesn't vary, you may use the eval-BLOCK
879 form to trap run-time errors without incurring the penalty of
880 recompiling each time. The error, if any, is still returned in C<$@>.
883 # make divide-by-zero non-fatal
884 eval { $answer = $a / $b; }; warn $@ if $@;
886 # same thing, but less efficient
887 eval '$answer = $a / $b'; warn $@ if $@;
889 # a compile-time error
893 eval '$answer ='; # sets $@
895 With an eval(), you should be especially careful to remember what's
896 being looked at when:
902 eval { $x }; # CASE 4
904 eval "\$$x++" # CASE 5
907 Cases 1 and 2 above behave identically: they run the code contained in the
908 variable $x. (Although case 2 has misleading double quotes making the
909 reader wonder what else might be happening (nothing is).) Cases 3 and 4
910 likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
911 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
912 is a place where normally you I<WOULD> like to use double quotes, except
913 that in that particular situation, you can just use symbolic references
914 instead, as in case 6.
918 The exec() function executes a system command I<AND NEVER RETURNS>,
919 unless the command does not exist and is executed directly instead of
920 via C</bin/sh -c> (see below). Use system() instead of exec() if you
923 If there is more than one argument in LIST, or if LIST is an array with
924 more than one value, calls execvp(3) with the arguments in LIST. If
925 there is only one scalar argument, the argument is checked for shell
926 metacharacters. If there are any, the entire argument is passed to
927 C</bin/sh -c> for parsing. If there are none, the argument is split
928 into words and passed directly to execvp(), which is more efficient.
929 Note: exec() and system() do not flush your output buffer, so you may
930 need to set C<$|> to avoid lost output. Examples:
932 exec '/bin/echo', 'Your arguments are: ', @ARGV;
933 exec "sort $outfile | uniq";
935 If you don't really want to execute the first argument, but want to lie
936 to the program you are executing about its own name, you can specify
937 the program you actually want to run as an "indirect object" (without a
938 comma) in front of the LIST. (This always forces interpretation of the
939 LIST as a multi-valued list, even if there is only a single scalar in
943 exec $shell '-sh'; # pretend it's a login shell
947 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
951 Returns TRUE if the specified hash key exists in its hash array, even
952 if the corresponding value is undefined.
954 print "Exists\n" if exists $array{$key};
955 print "Defined\n" if defined $array{$key};
956 print "True\n" if $array{$key};
958 A hash element can only be TRUE if it's defined, and defined if
959 it exists, but the reverse doesn't necessarily hold true.
961 Note that the EXPR can be arbitrarily complicated as long as the final
962 operation is a hash key lookup:
964 if (exists $ref->[$x][$y]{$key}) { ... }
968 Evaluates EXPR and exits immediately with that value. (Actually, it
969 calls any defined C<END> routines first, but the C<END> routines may not
970 abort the exit. Likewise any object destructors that need to be called
971 are called before exit.) Example:
974 exit 0 if $ans =~ /^[Xx]/;
976 See also die(). If EXPR is omitted, exits with 0 status.
982 Returns I<e> (the natural logarithm base) to the power of EXPR.
983 If EXPR is omitted, gives C<exp($_)>.
985 =item fcntl FILEHANDLE,FUNCTION,SCALAR
987 Implements the fcntl(2) function. You'll probably have to say
991 first to get the correct function definitions. Argument processing and
992 value return works just like ioctl() below. Note that fcntl() will produce
993 a fatal error if used on a machine that doesn't implement fcntl(2).
997 fcntl($filehandle, F_GETLK, $packed_return_buffer);
999 =item fileno FILEHANDLE
1001 Returns the file descriptor for a filehandle. This is useful for
1002 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1003 value is taken as the name of the filehandle.
1005 =item flock FILEHANDLE,OPERATION
1007 Calls flock(2) on FILEHANDLE. See L<flock(2)> for definition of
1008 OPERATION. Returns TRUE for success, FALSE on failure. Will produce a
1009 fatal error if used on a machine that doesn't implement either flock(2) or
1010 fcntl(2). The fcntl(2) system call will be automatically used if flock(2)
1011 is missing from your system. This makes flock() the portable file locking
1012 strategy, although it will only lock entire files, not records. Note also
1013 that some versions of flock() cannot lock things over the network; you
1014 would need to use the more system-specific fcntl() for that.
1016 Here's a mailbox appender for BSD systems.
1024 flock(MBOX,$LOCK_EX);
1025 # and, in case someone appended
1026 # while we were waiting...
1031 flock(MBOX,$LOCK_UN);
1034 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1035 or die "Can't open mailbox: $!";
1038 print MBOX $msg,"\n\n";
1041 See also L<DB_File> for other flock() examples.
1045 Does a fork(2) system call. Returns the child pid to the parent process
1046 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1047 Note: unflushed buffers remain unflushed in both processes, which means
1048 you may need to set C<$|> ($AUTOFLUSH in English) or call the
1049 autoflush() FileHandle method to avoid duplicate output.
1051 If you fork() without ever waiting on your children, you will accumulate
1054 $SIG{CHLD} = sub { wait };
1056 There's also the double-fork trick (error checking on
1057 fork() returns omitted);
1059 unless ($pid = fork) {
1061 exec "what you really wanna do";
1064 ## (some_perl_code_here)
1071 See also L<perlipc> for more examples of forking and reaping
1076 Declare a picture format with use by the write() function. For
1080 Test: @<<<<<<<< @||||| @>>>>>
1081 $str, $%, '$' . int($num)
1085 $num = $cost/$quantity;
1089 See L<perlform> for many details and examples.
1092 =item formline PICTURE, LIST
1094 This is an internal function used by C<format>s, though you may call it
1095 too. It formats (see L<perlform>) a list of values according to the
1096 contents of PICTURE, placing the output into the format output
1097 accumulator, C<$^A> (or $ACCUMULATOR in English).
1098 Eventually, when a write() is done, the contents of
1099 C<$^A> are written to some filehandle, but you could also read C<$^A>
1100 yourself and then set C<$^A> back to "". Note that a format typically
1101 does one formline() per line of form, but the formline() function itself
1102 doesn't care how many newlines are embedded in the PICTURE. This means
1103 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1104 You may therefore need to use multiple formlines to implement a single
1105 record format, just like the format compiler.
1107 Be careful if you put double quotes around the picture, since an "C<@>"
1108 character may be taken to mean the beginning of an array name.
1109 formline() always returns TRUE. See L<perlform> for other examples.
1111 =item getc FILEHANDLE
1115 Returns the next character from the input file attached to FILEHANDLE,
1116 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1117 This is not particularly efficient. It cannot be used to get unbuffered
1118 single-characters, however. For that, try something more like:
1121 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1124 system "stty", '-icanon', 'eol', "\001";
1130 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1133 system "stty", 'icanon', 'eol', '^@'; # ascii null
1137 Determination of whether to whether $BSD_STYLE should be set
1138 is left as an exercise to the reader.
1140 See also the C<Term::ReadKey> module from your nearest CPAN site;
1141 details on CPAN can be found on L<perlmod/CPAN>
1145 Returns the current login from F</etc/utmp>, if any. If null, use
1148 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1150 Do not consider getlogin() for authentication: it is not as
1151 secure as getpwuid().
1153 =item getpeername SOCKET
1155 Returns the packed sockaddr address of other end of the SOCKET connection.
1158 $hersockaddr = getpeername(SOCK);
1159 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1160 $herhostname = gethostbyaddr($iaddr, AF_INET);
1161 $herstraddr = inet_ntoa($iaddr);
1165 Returns the current process group for the specified PID. Use
1166 a PID of 0 to get the current process group for the
1167 current process. Will raise an exception if used on a machine that
1168 doesn't implement getpgrp(2). If PID is omitted, returns process
1169 group of current process. Note that the POSIX version of getpgrp()
1170 does not accept a PID argument, so only PID==0 is truly portable.
1174 Returns the process id of the parent process.
1176 =item getpriority WHICH,WHO
1178 Returns the current priority for a process, a process group, or a user.
1179 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1180 machine that doesn't implement getpriority(2).
1186 =item gethostbyname NAME
1188 =item getnetbyname NAME
1190 =item getprotobyname NAME
1196 =item getservbyname NAME,PROTO
1198 =item gethostbyaddr ADDR,ADDRTYPE
1200 =item getnetbyaddr ADDR,ADDRTYPE
1202 =item getprotobynumber NUMBER
1204 =item getservbyport PORT,PROTO
1222 =item sethostent STAYOPEN
1224 =item setnetent STAYOPEN
1226 =item setprotoent STAYOPEN
1228 =item setservent STAYOPEN
1242 These routines perform the same functions as their counterparts in the
1243 system library. Within a list context, the return values from the
1244 various get routines are as follows:
1246 ($name,$passwd,$uid,$gid,
1247 $quota,$comment,$gcos,$dir,$shell) = getpw*
1248 ($name,$passwd,$gid,$members) = getgr*
1249 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1250 ($name,$aliases,$addrtype,$net) = getnet*
1251 ($name,$aliases,$proto) = getproto*
1252 ($name,$aliases,$port,$proto) = getserv*
1254 (If the entry doesn't exist you get a null list.)
1256 Within a scalar context, you get the name, unless the function was a
1257 lookup by name, in which case you get the other thing, whatever it is.
1258 (If the entry doesn't exist you get the undefined value.) For example:
1268 The $members value returned by I<getgr*()> is a space separated list of
1269 the login names of the members of the group.
1271 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1272 C, it will be returned to you via C<$?> if the function call fails. The
1273 @addrs value returned by a successful call is a list of the raw
1274 addresses returned by the corresponding system library call. In the
1275 Internet domain, each address is four bytes long and you can unpack it
1276 by saying something like:
1278 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1280 =item getsockname SOCKET
1282 Returns the packed sockaddr address of this end of the SOCKET connection.
1285 $mysockaddr = getsockname(SOCK);
1286 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1288 =item getsockopt SOCKET,LEVEL,OPTNAME
1290 Returns the socket option requested, or undefined if there is an error.
1294 Returns the value of EXPR with filename expansions such as a shell
1295 would do. This is the internal function implementing the E<lt>*.*E<gt>
1296 operator, except it's easier to use.
1300 Converts a time as returned by the time function to a 9-element array
1301 with the time localized for the standard Greenwich timezone.
1302 Typically used as follows:
1305 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1308 All array elements are numeric, and come straight out of a struct tm.
1309 In particular this means that $mon has the range 0..11 and $wday has
1310 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1318 The goto-LABEL form finds the statement labeled with LABEL and resumes
1319 execution there. It may not be used to go into any construct that
1320 requires initialization, such as a subroutine or a foreach loop. It
1321 also can't be used to go into a construct that is optimized away. It
1322 can be used to go almost anywhere else within the dynamic scope,
1323 including out of subroutines, but it's usually better to use some other
1324 construct such as last or die. The author of Perl has never felt the
1325 need to use this form of goto (in Perl, that is--C is another matter).
1327 The goto-EXPR form expects a label name, whose scope will be resolved
1328 dynamically. This allows for computed gotos per FORTRAN, but isn't
1329 necessarily recommended if you're optimizing for maintainability:
1331 goto ("FOO", "BAR", "GLARCH")[$i];
1333 The goto-&NAME form is highly magical, and substitutes a call to the
1334 named subroutine for the currently running subroutine. This is used by
1335 AUTOLOAD subroutines that wish to load another subroutine and then
1336 pretend that the other subroutine had been called in the first place
1337 (except that any modifications to @_ in the current subroutine are
1338 propagated to the other subroutine.) After the goto, not even caller()
1339 will be able to tell that this routine was called first.
1341 =item grep BLOCK LIST
1343 =item grep EXPR,LIST
1345 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1346 $_ to each element) and returns the list value consisting of those
1347 elements for which the expression evaluated to TRUE. In a scalar
1348 context, returns the number of times the expression was TRUE.
1350 @foo = grep(!/^#/, @bar); # weed out comments
1354 @foo = grep {!/^#/} @bar; # weed out comments
1356 Note that, since $_ is a reference into the list value, it can be used
1357 to modify the elements of the array. While this is useful and
1358 supported, it can cause bizarre results if the LIST is not a named
1365 Interprets EXPR as a hex string and returns the corresponding decimal
1366 value. (To convert strings that might start with 0 or 0x see
1367 oct().) If EXPR is omitted, uses $_.
1371 There is no built-in import() function. It is merely an ordinary
1372 method (subroutine) defined (or inherited) by modules that wish to export
1373 names to another module. The use() function calls the import() method
1374 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1376 =item index STR,SUBSTR,POSITION
1378 =item index STR,SUBSTR
1380 Returns the position of the first occurrence of SUBSTR in STR at or after
1381 POSITION. If POSITION is omitted, starts searching from the beginning of
1382 the string. The return value is based at 0 (or whatever you've set the C<$[>
1383 variable to--but don't do that). If the substring is not found, returns
1384 one less than the base, ordinarily -1.
1390 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1392 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1394 Implements the ioctl(2) function. You'll probably have to say
1396 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1398 first to get the correct function definitions. If F<ioctl.ph> doesn't
1399 exist or doesn't have the correct definitions you'll have to roll your
1400 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1401 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1402 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1403 written depending on the FUNCTION--a pointer to the string value of SCALAR
1404 will be passed as the third argument of the actual ioctl call. (If SCALAR
1405 has no string value but does have a numeric value, that value will be
1406 passed rather than a pointer to the string value. To guarantee this to be
1407 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1408 functions are useful for manipulating the values of structures used by
1409 ioctl(). The following example sets the erase character to DEL.
1413 die "NO TIOCGETP" if $@ || !$getp;
1414 $sgttyb_t = "ccccs"; # 4 chars and a short
1415 if (ioctl(STDIN,$getp,$sgttyb)) {
1416 @ary = unpack($sgttyb_t,$sgttyb);
1418 $sgttyb = pack($sgttyb_t,@ary);
1419 ioctl(STDIN,&TIOCSETP,$sgttyb)
1420 || die "Can't ioctl: $!";
1423 The return value of ioctl (and fcntl) is as follows:
1425 if OS returns: then Perl returns:
1427 0 string "0 but true"
1428 anything else that number
1430 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1431 still easily determine the actual value returned by the operating
1434 ($retval = ioctl(...)) || ($retval = -1);
1435 printf "System returned %d\n", $retval;
1437 =item join EXPR,LIST
1439 Joins the separate strings of LIST or ARRAY into a single string with
1440 fields separated by the value of EXPR, and returns the string.
1443 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1445 See L<perlfunc/split>.
1447 =item keys ASSOC_ARRAY
1449 Returns a normal array consisting of all the keys of the named
1450 associative array. (In a scalar context, returns the number of keys.)
1451 The keys are returned in an apparently random order, but it is the same
1452 order as either the values() or each() function produces (given that
1453 the associative array has not been modified). Here is yet another way
1454 to print your environment:
1457 @values = values %ENV;
1458 while ($#keys >= 0) {
1459 print pop(@keys), '=', pop(@values), "\n";
1462 or how about sorted by key:
1464 foreach $key (sort(keys %ENV)) {
1465 print $key, '=', $ENV{$key}, "\n";
1468 To sort an array by value, you'll need to use a C<sort{}>
1469 function. Here's a descending numeric sort of a hash by its values:
1471 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1472 printf "%4d %s\n", $hash{$key}, $key;
1475 As an lvalue C<keys> allows you to increase the number of hash buckets
1476 allocated for the given associative array. This can gain you a measure
1477 of efficiency if you know the hash is going to get big. (This is
1478 similar to pre-extending an array by assigning a larger number to
1479 $#array.) If you say
1483 then C<%hash> will have at least 200 buckets allocated for it. These
1484 buckets will be retained even if you do C<%hash = ()>, use C<undef
1485 %hash> if you want to free the storage while C<%hash> is still in scope.
1486 You can't shrink the number of buckets allocated for the hash using
1487 C<keys> in this way (but you needn't worry about doing this by accident,
1488 as trying has no effect).
1492 Sends a signal to a list of processes. The first element of
1493 the list must be the signal to send. Returns the number of
1494 processes successfully signaled.
1496 $cnt = kill 1, $child1, $child2;
1499 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1500 process groups instead of processes. (On System V, a negative I<PROCESS>
1501 number will also kill process groups, but that's not portable.) That
1502 means you usually want to use positive not negative signals. You may also
1503 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1509 The C<last> command is like the C<break> statement in C (as used in
1510 loops); it immediately exits the loop in question. If the LABEL is
1511 omitted, the command refers to the innermost enclosing loop. The
1512 C<continue> block, if any, is not executed:
1514 LINE: while (<STDIN>) {
1515 last LINE if /^$/; # exit when done with header
1523 Returns an lowercased version of EXPR. This is the internal function
1524 implementing the \L escape in double-quoted strings.
1525 Should respect any POSIX setlocale() settings.
1527 If EXPR is omitted, uses $_.
1533 Returns the value of EXPR with the first character lowercased. This is
1534 the internal function implementing the \l escape in double-quoted strings.
1535 Should respect any POSIX setlocale() settings.
1537 If EXPR is omitted, uses $_.
1543 Returns the length in characters of the value of EXPR. If EXPR is
1544 omitted, returns length of $_.
1546 =item link OLDFILE,NEWFILE
1548 Creates a new filename linked to the old filename. Returns 1 for
1549 success, 0 otherwise.
1551 =item listen SOCKET,QUEUESIZE
1553 Does the same thing that the listen system call does. Returns TRUE if
1554 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1558 A local modifies the listed variables to be local to the enclosing block,
1559 subroutine, C<eval{}> or C<do>. If more than one value is listed, the
1560 list must be placed in parens. See L<perlsub/"Temporary Values via
1561 local()"> for details.
1563 But you really probably want to be using my() instead, because local() isn't
1564 what most people think of as "local"). See L<perlsub/"Private Variables
1565 via my()"> for details.
1567 =item localtime EXPR
1569 Converts a time as returned by the time function to a 9-element array
1570 with the time analyzed for the local timezone. Typically used as
1573 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1576 All array elements are numeric, and come straight out of a struct tm.
1577 In particular this means that $mon has the range 0..11 and $wday has
1578 the range 0..6. If EXPR is omitted, does localtime(time).
1580 In a scalar context, prints out the ctime(3) value:
1582 $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
1584 Also see the F<timelocal.pl> library, and the strftime(3) function available
1585 via the POSIX module.
1591 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1594 =item lstat FILEHANDLE
1600 Does the same thing as the stat() function, but stats a symbolic link
1601 instead of the file the symbolic link points to. If symbolic links are
1602 unimplemented on your system, a normal stat() is done.
1604 If EXPR is omitted, stats $_.
1608 The match operator. See L<perlop>.
1610 =item map BLOCK LIST
1614 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1615 element) and returns the list value composed of the results of each such
1616 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1617 may produce zero, one, or more elements in the returned value.
1619 @chars = map(chr, @nums);
1621 translates a list of numbers to the corresponding characters. And
1623 %hash = map { getkey($_) => $_ } @array;
1625 is just a funny way to write
1628 foreach $_ (@array) {
1629 $hash{getkey($_)} = $_;
1632 =item mkdir FILENAME,MODE
1634 Creates the directory specified by FILENAME, with permissions specified
1635 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1636 it returns 0 and sets C<$!> (errno).
1638 =item msgctl ID,CMD,ARG
1640 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1641 must be a variable which will hold the returned msqid_ds structure.
1642 Returns like ioctl: the undefined value for error, "0 but true" for
1643 zero, or the actual return value otherwise.
1645 =item msgget KEY,FLAGS
1647 Calls the System V IPC function msgget(2). Returns the message queue id,
1648 or the undefined value if there is an error.
1650 =item msgsnd ID,MSG,FLAGS
1652 Calls the System V IPC function msgsnd to send the message MSG to the
1653 message queue ID. MSG must begin with the long integer message type,
1654 which may be created with C<pack("l", $type)>. Returns TRUE if
1655 successful, or FALSE if there is an error.
1657 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1659 Calls the System V IPC function msgrcv to receive a message from
1660 message queue ID into variable VAR with a maximum message size of
1661 SIZE. Note that if a message is received, the message type will be the
1662 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1663 of the message type. Returns TRUE if successful, or FALSE if there is
1668 A "my" declares the listed variables to be local (lexically) to the
1669 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1670 more than one value is listed, the list must be placed in parens. See
1671 L<perlsub/"Private Variables via my()"> for details.
1677 The C<next> command is like the C<continue> statement in C; it starts
1678 the next iteration of the loop:
1680 LINE: while (<STDIN>) {
1681 next LINE if /^#/; # discard comments
1685 Note that if there were a C<continue> block on the above, it would get
1686 executed even on discarded lines. If the LABEL is omitted, the command
1687 refers to the innermost enclosing loop.
1689 =item no Module LIST
1691 See the "use" function, which "no" is the opposite of.
1697 Interprets EXPR as an octal string and returns the corresponding
1698 decimal value. (If EXPR happens to start off with 0x, interprets it as
1699 a hex string instead.) The following will handle decimal, octal, and
1700 hex in the standard Perl or C notation:
1702 $val = oct($val) if $val =~ /^0/;
1704 If EXPR is omitted, uses $_.
1706 =item open FILEHANDLE,EXPR
1708 =item open FILEHANDLE
1710 Opens the file whose filename is given by EXPR, and associates it with
1711 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name
1712 of the real filehandle wanted. If EXPR is omitted, the scalar variable of
1713 the same name as the FILEHANDLE contains the filename. If the filename
1714 begins with "E<lt>" or nothing, the file is opened for input. If the filename
1715 begins with "E<gt>", the file is opened for output. If the filename begins
1716 with "E<gt>E<gt>", the file is opened for appending. You can put a '+' in
1717 front of the 'E<gt>' or 'E<lt>' to indicate that you want both read and write
1718 access to the file; thus '+E<lt>' is usually preferred for read/write
1719 updates--the '+E<gt>' mode would clobber the file first. These correspond to
1720 the fopen(3) modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
1722 If the filename begins with "|", the filename is interpreted
1723 as a command to which output is to be piped, and if the filename ends with
1724 a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
1725 for more examples of this. as command which pipes input to us. (You may
1726 not have a raw open() to a command that pipes both in I<and> out, but see L<open2>,
1727 L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)
1729 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1730 non-zero upon success, the undefined value otherwise. If the open
1731 involved a pipe, the return value happens to be the pid of the
1734 If you're unfortunate enough to be running Perl on a system that
1735 distinguishes between text files and binary files (modern operating
1736 systems don't care), then you should check out L</binmode> for tips for
1737 dealing with this. The key distinction between systems that need binmode
1738 and those that don't is their text file formats. Systems like Unix and
1739 Plan9 that delimit lines with a single character, and that encode that
1740 character in C as '\n', do not need C<binmode>. The rest need it.
1745 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1746 while (<ARTICLE>) {...
1748 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1750 open(DBASE, '+<dbase.mine'); # open for update
1752 open(ARTICLE, "caesar <$article |"); # decrypt article
1754 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1756 # process argument list of files along with any includes
1758 foreach $file (@ARGV) {
1759 process($file, 'fh00');
1763 local($filename, $input) = @_;
1764 $input++; # this is a string increment
1765 unless (open($input, $filename)) {
1766 print STDERR "Can't open $filename: $!\n";
1770 while (<$input>) { # note use of indirection
1771 if (/^#include "(.*)"/) {
1772 process($1, $input);
1779 You may also, in the Bourne shell tradition, specify an EXPR beginning
1780 with "E<gt>&", in which case the rest of the string is interpreted as the
1781 name of a filehandle (or file descriptor, if numeric) which is to be
1782 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1783 +E<gt>E<gt> and +E<lt>. The
1784 mode you specify should match the mode of the original filehandle.
1785 (Duping a filehandle does not take into account any existing contents of
1787 Here is a script that saves, redirects, and restores STDOUT and
1791 open(SAVEOUT, ">&STDOUT");
1792 open(SAVEERR, ">&STDERR");
1794 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1795 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1797 select(STDERR); $| = 1; # make unbuffered
1798 select(STDOUT); $| = 1; # make unbuffered
1800 print STDOUT "stdout 1\n"; # this works for
1801 print STDERR "stderr 1\n"; # subprocesses too
1806 open(STDOUT, ">&SAVEOUT");
1807 open(STDERR, ">&SAVEERR");
1809 print STDOUT "stdout 2\n";
1810 print STDERR "stderr 2\n";
1813 If you specify "E<lt>&=N", where N is a number, then Perl will do an
1814 equivalent of C's fdopen() of that file descriptor; this is more
1815 parsimonious of file descriptors. For example:
1817 open(FILEHANDLE, "<&=$fd")
1819 If you open a pipe on the command "-", i.e. either "|-" or "-|", then
1820 there is an implicit fork done, and the return value of open is the pid
1821 of the child within the parent process, and 0 within the child
1822 process. (Use C<defined($pid)> to determine whether the open was successful.)
1823 The filehandle behaves normally for the parent, but i/o to that
1824 filehandle is piped from/to the STDOUT/STDIN of the child process.
1825 In the child process the filehandle isn't opened--i/o happens from/to
1826 the new STDOUT or STDIN. Typically this is used like the normal
1827 piped open when you want to exercise more control over just how the
1828 pipe command gets executed, such as when you are running setuid, and
1829 don't want to have to scan shell commands for metacharacters.
1830 The following pairs are more or less equivalent:
1832 open(FOO, "|tr '[a-z]' '[A-Z]'");
1833 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1835 open(FOO, "cat -n '$file'|");
1836 open(FOO, "-|") || exec 'cat', '-n', $file;
1838 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1840 Explicitly closing any piped filehandle causes the parent process to
1841 wait for the child to finish, and returns the status value in C<$?>.
1842 Note: on any operation which may do a fork, unflushed buffers remain
1843 unflushed in both processes, which means you may need to set C<$|> to
1844 avoid duplicate output.
1846 Using the FileHandle constructor from the FileHandle package,
1847 you can generate anonymous filehandles which have the scope of whatever
1848 variables hold references to them, and automatically close whenever
1849 and however you leave that scope:
1853 sub read_myfile_munged {
1855 my $handle = new FileHandle;
1856 open($handle, "myfile") or die "myfile: $!";
1858 or return (); # Automatically closed here.
1859 mung $first or die "mung failed"; # Or here.
1860 return $first, <$handle> if $ALL; # Or here.
1864 The filename that is passed to open will have leading and trailing
1865 whitespace deleted. In order to open a file with arbitrary weird
1866 characters in it, it's necessary to protect any leading and trailing
1869 $file =~ s#^(\s)#./$1#;
1870 open(FOO, "< $file\0");
1872 If you want a "real" C open() (see L<open(2)> on your system), then
1873 you should use the sysopen() function. This is another way to
1874 protect your filenames from interpretation. For example:
1877 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
1878 or die "sysopen $path: $!";
1879 HANDLE->autoflush(1);
1880 HANDLE->print("stuff $$\n");
1882 print "File contains: ", <HANDLE>;
1884 See L</seek()> for some details about mixing reading and writing.
1886 =item opendir DIRHANDLE,EXPR
1888 Opens a directory named EXPR for processing by readdir(), telldir(),
1889 seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
1890 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1896 Returns the numeric ascii value of the first character of EXPR. If
1897 EXPR is omitted, uses $_.
1899 =item pack TEMPLATE,LIST
1901 Takes an array or list of values and packs it into a binary structure,
1902 returning the string containing the structure. The TEMPLATE is a
1903 sequence of characters that give the order and type of values, as
1906 A An ascii string, will be space padded.
1907 a An ascii string, will be null padded.
1908 b A bit string (ascending bit order, like vec()).
1909 B A bit string (descending bit order).
1910 h A hex string (low nybble first).
1911 H A hex string (high nybble first).
1913 c A signed char value.
1914 C An unsigned char value.
1915 s A signed short value.
1916 S An unsigned short value.
1917 i A signed integer value.
1918 I An unsigned integer value.
1919 l A signed long value.
1920 L An unsigned long value.
1922 n A short in "network" order.
1923 N A long in "network" order.
1924 v A short in "VAX" (little-endian) order.
1925 V A long in "VAX" (little-endian) order.
1927 f A single-precision float in the native format.
1928 d A double-precision float in the native format.
1930 p A pointer to a null-terminated string.
1931 P A pointer to a structure (fixed-length string).
1933 u A uuencoded string.
1935 w A BER compressed integer. Bytes give an unsigned integer base
1936 128, most significant digit first, with as few digits as
1937 possible, and with the bit 8 of each byte except the last set
1942 @ Null fill to absolute position.
1944 Each letter may optionally be followed by a number which gives a repeat
1945 count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
1946 pack function will gobble up that many values from the LIST. A * for the
1947 repeat count means to use however many items are left. The "a" and "A"
1948 types gobble just one value, but pack it as a string of length count,
1949 padding with nulls or spaces as necessary. (When unpacking, "A" strips
1950 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
1951 fields pack a string that many bits long. The "h" and "H" fields pack a
1952 string that many nybbles long. The "P" packs a pointer to a structure of
1953 the size indicated by the length. Real numbers (floats and doubles) are
1954 in the native machine format only; due to the multiplicity of floating
1955 formats around, and the lack of a standard "network" representation, no
1956 facility for interchange has been made. This means that packed floating
1957 point data written on one machine may not be readable on another - even if
1958 both use IEEE floating point arithmetic (as the endian-ness of the memory
1959 representation is not part of the IEEE spec). Note that Perl uses doubles
1960 internally for all numeric calculation, and converting from double into
1961 float and thence back to double again will lose precision (i.e.
1962 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
1966 $foo = pack("cccc",65,66,67,68);
1968 $foo = pack("c4",65,66,67,68);
1971 $foo = pack("ccxxcc",65,66,67,68);
1974 $foo = pack("s2",1,2);
1975 # "\1\0\2\0" on little-endian
1976 # "\0\1\0\2" on big-endian
1978 $foo = pack("a4","abcd","x","y","z");
1981 $foo = pack("aaaa","abcd","x","y","z");
1984 $foo = pack("a14","abcdefg");
1985 # "abcdefg\0\0\0\0\0\0\0"
1987 $foo = pack("i9pl", gmtime);
1988 # a real struct tm (on my system anyway)
1991 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
1994 The same template may generally also be used in the unpack function.
1996 =item package NAMESPACE
1998 Declares the compilation unit as being in the given namespace. The scope
1999 of the package declaration is from the declaration itself through the end of
2000 the enclosing block (the same scope as the local() operator). All further
2001 unqualified dynamic identifiers will be in this namespace. A package
2002 statement only affects dynamic variables--including those you've used
2003 local() on--but I<not> lexical variables created with my(). Typically it
2004 would be the first declaration in a file to be included by the C<require>
2005 or C<use> operator. You can switch into a package in more than one place;
2006 it merely influences which symbol table is used by the compiler for the
2007 rest of that block. You can refer to variables and filehandles in other
2008 packages by prefixing the identifier with the package name and a double
2009 colon: C<$Package::Variable>. If the package name is null, the C<main>
2010 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2012 See L<perlmod/"Packages"> for more information about packages, modules,
2013 and classes. See L<perlsub> for other scoping issues.
2015 =item pipe READHANDLE,WRITEHANDLE
2017 Opens a pair of connected pipes like the corresponding system call.
2018 Note that if you set up a loop of piped processes, deadlock can occur
2019 unless you are very careful. In addition, note that Perl's pipes use
2020 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2021 after each command, depending on the application.
2023 See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
2024 for examples of such things.
2028 Pops and returns the last value of the array, shortening the array by
2029 1. Has a similar effect to
2031 $tmp = $ARRAY[$#ARRAY--];
2033 If there are no elements in the array, returns the undefined value.
2034 If ARRAY is omitted, pops the
2035 @ARGV array in the main program, and the @_ array in subroutines, just
2042 Returns the offset of where the last C<m//g> search left off for the variable
2043 is in question ($_ is used when the variable is not specified). May be
2044 modified to change that offset.
2046 =item print FILEHANDLE LIST
2052 Prints a string or a comma-separated list of strings. Returns TRUE
2053 if successful. FILEHANDLE may be a scalar variable name, in which case
2054 the variable contains the name of or a reference to the filehandle, thus introducing one
2055 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2056 token is a term, it may be misinterpreted as an operator unless you
2057 interpose a + or put parens around the arguments.) If FILEHANDLE is
2058 omitted, prints by default to standard output (or to the last selected
2059 output channel--see L</select>). If LIST is also omitted, prints $_ to
2060 STDOUT. To set the default output channel to something other than
2061 STDOUT use the select operation. Note that, because print takes a
2062 LIST, anything in the LIST is evaluated in a list context, and any
2063 subroutine that you call will have one or more of its expressions
2064 evaluated in a list context. Also be careful not to follow the print
2065 keyword with a left parenthesis unless you want the corresponding right
2066 parenthesis to terminate the arguments to the print--interpose a + or
2067 put parens around all the arguments.
2069 Note that if you're storing FILEHANDLES in an array or other expression,
2070 you will have to use a block returning its value instead:
2072 print { $files[$i] } "stuff\n";
2073 print { $OK ? STDOUT : STDERR } "stuff\n";
2075 =item printf FILEHANDLE LIST
2079 Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
2080 of the list will be interpreted as the printf format.
2082 =item prototype FUNCTION
2084 Returns the prototype of a function as a string (or C<undef> if the
2085 function has no prototype). FUNCTION is a reference to the the
2086 function whose prototype you want to retrieve.
2088 =item push ARRAY,LIST
2090 Treats ARRAY as a stack, and pushes the values of LIST
2091 onto the end of ARRAY. The length of ARRAY increases by the length of
2092 LIST. Has the same effect as
2095 $ARRAY[++$#ARRAY] = $value;
2098 but is more efficient. Returns the new number of elements in the array.
2108 Generalized quotes. See L<perlop>.
2110 =item quotemeta EXPR
2114 Returns the value of EXPR with with all regular expression
2115 metacharacters backslashed. This is the internal function implementing
2116 the \Q escape in double-quoted strings.
2118 If EXPR is omitted, uses $_.
2124 Returns a random fractional number between 0 and the value of EXPR.
2125 (EXPR should be positive.) If EXPR is omitted, returns a value between
2126 0 and 1. This function produces repeatable sequences unless srand()
2127 is invoked. See also srand().
2129 (Note: if your rand function consistently returns numbers that are too
2130 large or too small, then your version of Perl was probably compiled
2131 with the wrong number of RANDBITS. As a workaround, you can usually
2132 multiply EXPR by the correct power of 2 to get the range you want.
2133 This will make your script unportable, however. It's better to recompile
2136 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2138 =item read FILEHANDLE,SCALAR,LENGTH
2140 Attempts to read LENGTH bytes of data into variable SCALAR from the
2141 specified FILEHANDLE. Returns the number of bytes actually read, or
2142 undef if there was an error. SCALAR will be grown or shrunk to the
2143 length actually read. An OFFSET may be specified to place the read
2144 data at some other place than the beginning of the string. This call
2145 is actually implemented in terms of stdio's fread call. To get a true
2146 read system call, see sysread().
2148 =item readdir DIRHANDLE
2150 Returns the next directory entry for a directory opened by opendir().
2151 If used in a list context, returns all the rest of the entries in the
2152 directory. If there are no more entries, returns an undefined value in
2153 a scalar context or a null list in a list context.
2155 If you're planning to filetest the return values out of a readdir(), you'd
2156 better prepend the directory in question. Otherwise, since we didn't
2157 chdir() there, it would have been testing the wrong file.
2159 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2160 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2167 Returns the value of a symbolic link, if symbolic links are
2168 implemented. If not, gives a fatal error. If there is some system
2169 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2172 =item recv SOCKET,SCALAR,LEN,FLAGS
2174 Receives a message on a socket. Attempts to receive LENGTH bytes of
2175 data into variable SCALAR from the specified SOCKET filehandle.
2176 Actually does a C recvfrom(), so that it can returns the address of the
2177 sender. Returns the undefined value if there's an error. SCALAR will
2178 be grown or shrunk to the length actually read. Takes the same flags
2179 as the system call of the same name.
2180 See L<perlipc/"UDP: Message Passing"> for examples.
2186 The C<redo> command restarts the loop block without evaluating the
2187 conditional again. The C<continue> block, if any, is not executed. If
2188 the LABEL is omitted, the command refers to the innermost enclosing
2189 loop. This command is normally used by programs that want to lie to
2190 themselves about what was just input:
2192 # a simpleminded Pascal comment stripper
2193 # (warning: assumes no { or } in strings)
2194 LINE: while (<STDIN>) {
2195 while (s|({.*}.*){.*}|$1 |) {}
2200 if (/}/) { # end of comment?
2213 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2214 is not specified, $_ will be used. The value returned depends on the
2215 type of thing the reference is a reference to.
2216 Builtin types include:
2225 If the referenced object has been blessed into a package, then that package
2226 name is returned instead. You can think of ref() as a typeof() operator.
2228 if (ref($r) eq "HASH") {
2229 print "r is a reference to an associative array.\n";
2232 print "r is not a reference at all.\n";
2235 See also L<perlref>.
2237 =item rename OLDNAME,NEWNAME
2239 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2240 not work across filesystem boundaries.
2246 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2247 supplied. If EXPR is numeric, demands that the current version of Perl
2248 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2250 Otherwise, demands that a library file be included if it hasn't already
2251 been included. The file is included via the do-FILE mechanism, which is
2252 essentially just a variety of eval(). Has semantics similar to the following
2256 local($filename) = @_;
2257 return 1 if $INC{$filename};
2258 local($realfilename,$result);
2260 foreach $prefix (@INC) {
2261 $realfilename = "$prefix/$filename";
2262 if (-f $realfilename) {
2263 $result = do $realfilename;
2267 die "Can't find $filename in \@INC";
2270 die "$filename did not return true value" unless $result;
2271 $INC{$filename} = $realfilename;
2275 Note that the file will not be included twice under the same specified
2276 name. The file must return TRUE as the last statement to indicate
2277 successful execution of any initialization code, so it's customary to
2278 end such a file with "1;" unless you're sure it'll return TRUE
2279 otherwise. But it's better just to put the "C<1;>", in case you add more
2282 If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2283 replaces "F<::>" with "F</>" in the filename for you,
2284 to make it easy to load standard modules. This form of loading of
2285 modules does not risk altering your namespace.
2287 For a yet-more-powerful import facility, see L</use> and
2294 Generally used in a C<continue> block at the end of a loop to clear
2295 variables and reset ?? searches so that they work again. The
2296 expression is interpreted as a list of single characters (hyphens
2297 allowed for ranges). All variables and arrays beginning with one of
2298 those letters are reset to their pristine state. If the expression is
2299 omitted, one-match searches (?pattern?) are reset to match again. Only
2300 resets variables or searches in the current package. Always returns
2303 reset 'X'; # reset all X variables
2304 reset 'a-z'; # reset lower case variables
2305 reset; # just reset ?? searches
2307 Resetting "A-Z" is not recommended since you'll wipe out your
2308 ARGV and ENV arrays. Only resets package variables--lexical variables
2309 are unaffected, but they clean themselves up on scope exit anyway,
2310 so you'll probably want to use them instead. See L</my>.
2314 Returns from a subroutine or eval with the value specified. (Note that
2315 in the absence of a return a subroutine or eval() will automatically
2316 return the value of the last expression evaluated.)
2320 In a list context, returns a list value consisting of the elements
2321 of LIST in the opposite order. In a scalar context, returns a string
2322 value consisting of the bytes of the first element of LIST in the
2325 print reverse <>; # line tac
2328 print scalar reverse scalar <>; # byte tac
2330 =item rewinddir DIRHANDLE
2332 Sets the current position to the beginning of the directory for the
2333 readdir() routine on DIRHANDLE.
2335 =item rindex STR,SUBSTR,POSITION
2337 =item rindex STR,SUBSTR
2339 Works just like index except that it returns the position of the LAST
2340 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2341 last occurrence at or before that position.
2343 =item rmdir FILENAME
2347 Deletes the directory specified by FILENAME if it is empty. If it
2348 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2349 FILENAME is omitted, uses $_.
2353 The substitution operator. See L<perlop>.
2357 Forces EXPR to be interpreted in a scalar context and returns the value
2360 @counts = ( scalar @a, scalar @b, scalar @c );
2362 There is no equivalent operator to force an expression to
2363 be interpolated in a list context because it's in practice never
2364 needed. If you really wanted to do so, however, you could use
2365 the construction C<@{[ (some expression) ]}>, but usually a simple
2366 C<(some expression)> suffices.
2368 =item seek FILEHANDLE,POSITION,WHENCE
2370 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2371 call of stdio. FILEHANDLE may be an expression whose value gives the name
2372 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2373 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2374 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2375 this from POSIX module. Returns 1 upon success, 0 otherwise.
2377 On some systems you have to do a seek whenever you switch between reading
2378 and writing. Amongst other things, this may have the effect of calling
2379 stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2384 This is also useful for applications emulating C<tail -f>. Once you hit
2385 EOF on your read, and then sleep for a while, you might have to stick in a
2386 seek() to reset things. First the simple trick listed above to clear the
2387 filepointer. The seek() doesn't change the current position, but it
2388 I<does> clear the end-of-file condition on the handle, so that the next
2389 C<E<lt>FILEE<gt>> makes Perl try again to read something. Hopefully.
2391 If that doesn't work (some stdios are particularly cantankerous), then
2392 you may need something more like this:
2395 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2396 # search for some stuff and put it into files
2398 sleep($for_a_while);
2399 seek(FILE, $curpos, 0);
2402 =item seekdir DIRHANDLE,POS
2404 Sets the current position for the readdir() routine on DIRHANDLE. POS
2405 must be a value returned by telldir(). Has the same caveats about
2406 possible directory compaction as the corresponding system library
2409 =item select FILEHANDLE
2413 Returns the currently selected filehandle. Sets the current default
2414 filehandle for output, if FILEHANDLE is supplied. This has two
2415 effects: first, a C<write> or a C<print> without a filehandle will
2416 default to this FILEHANDLE. Second, references to variables related to
2417 output will refer to this output channel. For example, if you have to
2418 set the top of form format for more than one output channel, you might
2426 FILEHANDLE may be an expression whose value gives the name of the
2427 actual filehandle. Thus:
2429 $oldfh = select(STDERR); $| = 1; select($oldfh);
2431 Some programmers may prefer to think of filehandles as objects with
2432 methods, preferring to write the last example as:
2435 STDERR->autoflush(1);
2437 =item select RBITS,WBITS,EBITS,TIMEOUT
2439 This calls the select(2) system call with the bitmasks specified, which
2440 can be constructed using fileno() and vec(), along these lines:
2442 $rin = $win = $ein = '';
2443 vec($rin,fileno(STDIN),1) = 1;
2444 vec($win,fileno(STDOUT),1) = 1;
2447 If you want to select on many filehandles you might wish to write a
2451 local(@fhlist) = split(' ',$_[0]);
2454 vec($bits,fileno($_),1) = 1;
2458 $rin = fhbits('STDIN TTY SOCK');
2462 ($nfound,$timeleft) =
2463 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2465 or to block until something becomes ready just do this
2467 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2469 Most systems do not both to return anything useful in $timeleft, so
2470 calling select() in a scalar context just returns $nfound.
2472 Any of the bitmasks can also be undef. The timeout, if specified, is
2473 in seconds, which may be fractional. Note: not all implementations are
2474 capable of returning the $timeleft. If not, they always return
2475 $timeleft equal to the supplied $timeout.
2477 You can effect a 250-millisecond sleep this way:
2479 select(undef, undef, undef, 0.25);
2481 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2482 with select(). You have to use sysread() instead.
2484 =item semctl ID,SEMNUM,CMD,ARG
2486 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2487 &GETALL, then ARG must be a variable which will hold the returned
2488 semid_ds structure or semaphore value array. Returns like ioctl: the
2489 undefined value for error, "0 but true" for zero, or the actual return
2492 =item semget KEY,NSEMS,FLAGS
2494 Calls the System V IPC function semget. Returns the semaphore id, or
2495 the undefined value if there is an error.
2497 =item semop KEY,OPSTRING
2499 Calls the System V IPC function semop to perform semaphore operations
2500 such as signaling and waiting. OPSTRING must be a packed array of
2501 semop structures. Each semop structure can be generated with
2502 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2503 operations is implied by the length of OPSTRING. Returns TRUE if
2504 successful, or FALSE if there is an error. As an example, the
2505 following code waits on semaphore $semnum of semaphore id $semid:
2507 $semop = pack("sss", $semnum, -1, 0);
2508 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2510 To signal the semaphore, replace "-1" with "1".
2512 =item send SOCKET,MSG,FLAGS,TO
2514 =item send SOCKET,MSG,FLAGS
2516 Sends a message on a socket. Takes the same flags as the system call
2517 of the same name. On unconnected sockets you must specify a
2518 destination to send TO, in which case it does a C sendto(). Returns
2519 the number of characters sent, or the undefined value if there is an
2521 See L<perlipc/"UDP: Message Passing"> for examples.
2523 =item setpgrp PID,PGRP
2525 Sets the current process group for the specified PID, 0 for the current
2526 process. Will produce a fatal error if used on a machine that doesn't
2527 implement setpgrp(2). If the arguments are ommitted, it defaults to
2528 0,0. Note that the POSIX version of setpgrp() does not accept any
2529 arguments, so only setpgrp 0,0 is portable.
2531 =item setpriority WHICH,WHO,PRIORITY
2533 Sets the current priority for a process, a process group, or a user.
2534 (See setpriority(2).) Will produce a fatal error if used on a machine
2535 that doesn't implement setpriority(2).
2537 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2539 Sets the socket option requested. Returns undefined if there is an
2540 error. OPTVAL may be specified as undef if you don't want to pass an
2547 Shifts the first value of the array off and returns it, shortening the
2548 array by 1 and moving everything down. If there are no elements in the
2549 array, returns the undefined value. If ARRAY is omitted, shifts the
2550 @ARGV array in the main program, and the @_ array in subroutines.
2551 (This is determined lexically.) See also unshift(), push(), and pop().
2552 Shift() and unshift() do the same thing to the left end of an array
2553 that push() and pop() do to the right end.
2555 =item shmctl ID,CMD,ARG
2557 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2558 must be a variable which will hold the returned shmid_ds structure.
2559 Returns like ioctl: the undefined value for error, "0 but true" for
2560 zero, or the actual return value otherwise.
2562 =item shmget KEY,SIZE,FLAGS
2564 Calls the System V IPC function shmget. Returns the shared memory
2565 segment id, or the undefined value if there is an error.
2567 =item shmread ID,VAR,POS,SIZE
2569 =item shmwrite ID,STRING,POS,SIZE
2571 Reads or writes the System V shared memory segment ID starting at
2572 position POS for size SIZE by attaching to it, copying in/out, and
2573 detaching from it. When reading, VAR must be a variable which will
2574 hold the data read. When writing, if STRING is too long, only SIZE
2575 bytes are used; if STRING is too short, nulls are written to fill out
2576 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2578 =item shutdown SOCKET,HOW
2580 Shuts down a socket connection in the manner indicated by HOW, which
2581 has the same interpretation as in the system call of the same name.
2587 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2594 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2595 May be interrupted by sending the process a SIGALRM. Returns the
2596 number of seconds actually slept. You probably cannot mix alarm() and
2597 sleep() calls, since sleep() is often implemented using alarm().
2599 On some older systems, it may sleep up to a full second less than what
2600 you requested, depending on how it counts seconds. Most modern systems
2601 always sleep the full amount.
2603 For delays of finer granularity than one second, you may use Perl's
2604 syscall() interface to access setitimer(2) if your system supports it,
2605 or else see L</select()> below.
2607 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2609 Opens a socket of the specified kind and attaches it to filehandle
2610 SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
2611 system call of the same name. You should "use Socket;" first to get
2612 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2614 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2616 Creates an unnamed pair of sockets in the specified domain, of the
2617 specified type. DOMAIN, TYPE and PROTOCOL are specified the same as
2618 for the system call of the same name. If unimplemented, yields a fatal
2619 error. Returns TRUE if successful.
2621 =item sort SUBNAME LIST
2623 =item sort BLOCK LIST
2627 Sorts the LIST and returns the sorted list value. Nonexistent values
2628 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2629 in standard string comparison order. If SUBNAME is specified, it
2630 gives the name of a subroutine that returns an integer less than, equal
2631 to, or greater than 0, depending on how the elements of the array are
2632 to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
2633 routines.) SUBNAME may be a scalar variable name, in which case the
2634 value provides the name of the subroutine to use. In place of a
2635 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2638 In the interests of efficiency the normal calling code for subroutines is
2639 bypassed, with the following effects: the subroutine may not be a
2640 recursive subroutine, and the two elements to be compared are passed into
2641 the subroutine not via @_ but as the package global variables $a and
2642 $b (see example below). They are passed by reference, so don't
2643 modify $a and $b. And don't try to declare them as lexicals either.
2648 @articles = sort @files;
2650 # same thing, but with explicit sort routine
2651 @articles = sort {$a cmp $b} @files;
2653 # now case-insensitively
2654 @articles = sort { uc($a) cmp uc($b)} @files;
2656 # same thing in reversed order
2657 @articles = sort {$b cmp $a} @files;
2659 # sort numerically ascending
2660 @articles = sort {$a <=> $b} @files;
2662 # sort numerically descending
2663 @articles = sort {$b <=> $a} @files;
2665 # sort using explicit subroutine name
2667 $age{$a} <=> $age{$b}; # presuming integers
2669 @sortedclass = sort byage @class;
2671 # this sorts the %age associative arrays by value
2672 # instead of key using an inline function
2673 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2675 sub backwards { $b cmp $a; }
2676 @harry = ('dog','cat','x','Cain','Abel');
2677 @george = ('gone','chased','yz','Punished','Axed');
2679 # prints AbelCaincatdogx
2680 print sort backwards @harry;
2681 # prints xdogcatCainAbel
2682 print sort @george, 'to', @harry;
2683 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2685 # inefficiently sort by descending numeric compare using
2686 # the first integer after the first = sign, or the
2687 # whole record case-insensitively otherwise
2690 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2695 # same thing, but much more efficiently;
2696 # we'll build auxiliary indices instead
2700 push @nums, /=(\d+)/;
2705 $nums[$b] <=> $nums[$a]
2707 $caps[$a] cmp $caps[$b]
2711 # same thing using a Schwartzian Transform (no temps)
2712 @new = map { $_->[0] }
2713 sort { $b->[1] <=> $a->[1]
2716 } map { [$_, /=(\d+)/, uc($_)] } @old;
2718 If you're using strict, you I<MUST NOT> declare $a
2719 and $b as lexicals. They are package globals. That means
2720 if you're in the C<main> package, it's
2722 @articles = sort {$main::b <=> $main::a} @files;
2726 @articles = sort {$::b <=> $::a} @files;
2728 but if you're in the C<FooPack> package, it's
2730 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2732 The comparison function is required to behave. If it returns
2733 inconsistent results (sometimes saying $x[1] is less than $x[2] and
2734 sometimes saying the opposite, for example) the Perl interpreter will
2735 probably crash and dump core. This is entirely due to and dependent
2736 upon your system's qsort(3) library routine; this routine often avoids
2737 sanity checks in the interest of speed.
2739 =item splice ARRAY,OFFSET,LENGTH,LIST
2741 =item splice ARRAY,OFFSET,LENGTH
2743 =item splice ARRAY,OFFSET
2745 Removes the elements designated by OFFSET and LENGTH from an array, and
2746 replaces them with the elements of LIST, if any. Returns the elements
2747 removed from the array. The array grows or shrinks as necessary. If
2748 LENGTH is omitted, removes everything from OFFSET onward. The
2749 following equivalencies hold (assuming C<$[ == 0>):
2751 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2752 pop(@a) splice(@a,-1)
2753 shift(@a) splice(@a,0,1)
2754 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2755 $a[$x] = $y splice(@a,$x,1,$y);
2757 Example, assuming array lengths are passed before arrays:
2759 sub aeq { # compare two list values
2760 local(@a) = splice(@_,0,shift);
2761 local(@b) = splice(@_,0,shift);
2762 return 0 unless @a == @b; # same len?
2764 return 0 if pop(@a) ne pop(@b);
2768 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2770 =item split /PATTERN/,EXPR,LIMIT
2772 =item split /PATTERN/,EXPR
2774 =item split /PATTERN/
2778 Splits a string into an array of strings, and returns it.
2780 If not in a list context, returns the number of fields found and splits into
2781 the @_ array. (In a list context, you can force the split into @_ by
2782 using C<??> as the pattern delimiters, but it still returns the array
2783 value.) The use of implicit split to @_ is deprecated, however.
2785 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2786 splits on whitespace (after skipping any leading whitespace). Anything
2787 matching PATTERN is taken to be a delimiter separating the fields. (Note
2788 that the delimiter may be longer than one character.) If LIMIT is
2789 specified and is not negative, splits into no more than that many fields
2790 (though it may split into fewer). If LIMIT is unspecified, trailing null
2791 fields are stripped (which potential users of pop() would do well to
2792 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2793 LIMIT had been specified.
2795 A pattern matching the null string (not to be confused with
2796 a null pattern C<//>, which is just one member of the set of patterns
2797 matching a null string) will split the value of EXPR into separate
2798 characters at each point it matches that way. For example:
2800 print join(':', split(/ */, 'hi there'));
2802 produces the output 'h:i:t:h:e:r:e'.
2804 The LIMIT parameter can be used to partially split a line
2806 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2808 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2809 one larger than the number of variables in the list, to avoid
2810 unnecessary work. For the list above LIMIT would have been 4 by
2811 default. In time critical applications it behooves you not to split
2812 into more fields than you really need.
2814 If the PATTERN contains parentheses, additional array elements are
2815 created from each matching substring in the delimiter.
2817 split(/([,-])/, "1-10,20", 3);
2819 produces the list value
2821 (1, '-', 10, ',', 20)
2823 If you had the entire header of a normal Unix email message in $header,
2824 you could split it up into fields and their values this way:
2826 $header =~ s/\n\s+/ /g; # fix continuation lines
2827 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2829 The pattern C</PATTERN/> may be replaced with an expression to specify
2830 patterns that vary at runtime. (To do runtime compilation only once,
2831 use C</$variable/o>.)
2833 As a special case, specifying a PATTERN of space (C<' '>) will split on
2834 white space just as split with no arguments does. Thus, split(' ') can
2835 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2836 will give you as many null initial fields as there are leading spaces.
2837 A split on /\s+/ is like a split(' ') except that any leading
2838 whitespace produces a null first field. A split with no arguments
2839 really does a C<split(' ', $_)> internally.
2843 open(passwd, '/etc/passwd');
2845 ($login, $passwd, $uid, $gid, $gcos,
2846 $home, $shell) = split(/:/);
2850 (Note that $shell above will still have a newline on it. See L</chop>,
2851 L</chomp>, and L</join>.)
2853 =item sprintf FORMAT,LIST
2855 Returns a string formatted by the usual printf conventions of the C
2856 language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2857 (The * character for an indirectly specified length is not
2858 supported, but you can get the same effect by interpolating a variable
2859 into the pattern.) Some C libraries' implementations of sprintf() can
2860 dump core when fed ludicrous arguments.
2866 Return the square root of EXPR. If EXPR is omitted, returns square
2871 Sets the random number seed for the C<rand> operator. If EXPR is omitted,
2872 uses a semirandom value based on the current time and process ID, among
2873 other things. Of course, you'd need something much more random than that for
2874 cryptographic purposes, since it's easy to guess the current time.
2875 Checksumming the compressed output of rapidly changing operating system
2876 status programs is the usual method. Examples are posted regularly to
2877 the comp.security.unix newsgroup.
2879 =item stat FILEHANDLE
2885 Returns a 13-element array giving the status info for a file, either the
2886 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
2887 stats $_. Returns a null list if the stat fails. Typically used as
2891 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2892 $atime,$mtime,$ctime,$blksize,$blocks)
2895 Not all fields are supported on all filesystem types. Here are the
2896 meaning of the fields:
2898 dev device number of filesystem
2900 mode file mode (type and permissions)
2901 nlink number of (hard) links to the file
2902 uid numeric user ID of file's owner
2903 gid numer group ID of file's owner
2904 rdev the device identifier (special files only)
2905 size total size of file, in bytes
2906 atime last access time since the epoch
2907 mtime last modify time since the epoch
2908 ctime inode change time (NOT creation type!) since the epoch
2909 blksize preferred blocksize for file system I/O
2910 blocks actual number of blocks allocated
2912 (The epoch was at 00:00 January 1, 1970 GMT.)
2914 If stat is passed the special filehandle consisting of an underline, no
2915 stat is done, but the current contents of the stat structure from the
2916 last stat or filetest are returned. Example:
2918 if (-x $file && (($d) = stat(_)) && $d < 0) {
2919 print "$file is executable NFS file\n";
2922 (This only works on machines for which the device number is negative under NFS.)
2928 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
2929 doing many pattern matches on the string before it is next modified.
2930 This may or may not save time, depending on the nature and number of
2931 patterns you are searching on, and on the distribution of character
2932 frequencies in the string to be searched--you probably want to compare
2933 runtimes with and without it to see which runs faster. Those loops
2934 which scan for many short constant strings (including the constant
2935 parts of more complex patterns) will benefit most. You may have only
2936 one study active at a time--if you study a different scalar the first
2937 is "unstudied". (The way study works is this: a linked list of every
2938 character in the string to be searched is made, so we know, for
2939 example, where all the 'k' characters are. From each search string,
2940 the rarest character is selected, based on some static frequency tables
2941 constructed from some C programs and English text. Only those places
2942 that contain this "rarest" character are examined.)
2944 For example, here is a loop which inserts index producing entries
2945 before any line containing a certain pattern:
2949 print ".IX foo\n" if /\bfoo\b/;
2950 print ".IX bar\n" if /\bbar\b/;
2951 print ".IX blurfl\n" if /\bblurfl\b/;
2956 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
2957 will be looked at, because "f" is rarer than "o". In general, this is
2958 a big win except in pathological cases. The only question is whether
2959 it saves you more time than it took to build the linked list in the
2962 Note that if you have to look for strings that you don't know till
2963 runtime, you can build an entire loop as a string and eval that to
2964 avoid recompiling all your patterns all the time. Together with
2965 undefining $/ to input entire files as one record, this can be very
2966 fast, often faster than specialized programs like fgrep(1). The following
2967 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
2968 out the names of those files that contain a match:
2970 $search = 'while (<>) { study;';
2971 foreach $word (@words) {
2972 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
2977 eval $search; # this screams
2978 $/ = "\n"; # put back to normal input delim
2979 foreach $file (sort keys(%seen)) {
2987 =item sub NAME BLOCK
2989 This is subroutine definition, not a real function I<per se>. With just a
2990 NAME (and possibly prototypes), it's just a forward declaration. Without
2991 a NAME, it's an anonymous function declaration, and does actually return a
2992 value: the CODE ref of the closure you just created. See L<perlsub> and
2993 L<perlref> for details.
2995 =item substr EXPR,OFFSET,LEN
2997 =item substr EXPR,OFFSET
2999 Extracts a substring out of EXPR and returns it. First character is at
3000 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
3001 that far from the end of the string. If LEN is omitted, returns
3002 everything to the end of the string. If LEN is negative, leaves that
3003 many characters off the end of the string.
3005 You can use the substr() function
3006 as an lvalue, in which case EXPR must be an lvalue. If you assign
3007 something shorter than LEN, the string will shrink, and if you assign
3008 something longer than LEN, the string will grow to accommodate it. To
3009 keep the string the same length you may need to pad or chop your value
3012 =item symlink OLDFILE,NEWFILE
3014 Creates a new filename symbolically linked to the old filename.
3015 Returns 1 for success, 0 otherwise. On systems that don't support
3016 symbolic links, produces a fatal error at run time. To check for that,
3019 $symlink_exists = (eval 'symlink("","");', $@ eq '');
3023 Calls the system call specified as the first element of the list,
3024 passing the remaining elements as arguments to the system call. If
3025 unimplemented, produces a fatal error. The arguments are interpreted
3026 as follows: if a given argument is numeric, the argument is passed as
3027 an int. If not, the pointer to the string value is passed. You are
3028 responsible to make sure a string is pre-extended long enough to
3029 receive any result that might be written into a string. If your
3030 integer arguments are not literals and have never been interpreted in a
3031 numeric context, you may need to add 0 to them to force them to look
3034 require 'syscall.ph'; # may need to run h2ph
3035 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3037 Note that Perl only supports passing of up to 14 arguments to your system call,
3038 which in practice should usually suffice.
3040 =item sysopen FILEHANDLE,FILENAME,MODE
3042 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3044 Opens the file whose filename is given by FILENAME, and associates it
3045 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3046 the name of the real filehandle wanted. This function calls the
3047 underlying operating system's C<open> function with the parameters
3048 FILENAME, MODE, PERMS.
3050 The possible values and flag bits of the MODE parameter are
3051 system-dependent; they are available via the standard module C<Fcntl>.
3052 However, for historical reasons, some values are universal: zero means
3053 read-only, one means write-only, and two means read/write.
3055 If the file named by FILENAME does not exist and the C<open> call
3056 creates it (typically because MODE includes the O_CREAT flag), then
3057 the value of PERMS specifies the permissions of the newly created
3058 file. If PERMS is omitted, the default value is 0666, which allows
3059 read and write for all. This default is reasonable: see C<umask>.
3061 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3063 =item sysread FILEHANDLE,SCALAR,LENGTH
3065 Attempts to read LENGTH bytes of data into variable SCALAR from the
3066 specified FILEHANDLE, using the system call read(2). It bypasses
3067 stdio, so mixing this with other kinds of reads may cause confusion.
3068 Returns the number of bytes actually read, or undef if there was an
3069 error. SCALAR will be grown or shrunk to the length actually read.
3070 In the case of growing the new data area will be padded with "\0" bytes.
3071 An OFFSET may be specified to place the read data at some other
3072 place than the beginning of the string. A negative OFFSET means
3073 placing the read data at that many bytes counting backwards from the end
3078 Does exactly the same thing as "exec LIST" except that a fork is done
3079 first, and the parent process waits for the child process to complete.
3080 Note that argument processing varies depending on the number of
3081 arguments. The return value is the exit status of the program as
3082 returned by the wait() call. To get the actual exit value divide by
3083 256. See also L</exec>. This is I<NOT> what you want to use to capture
3084 the output from a command, for that you should merely use backticks, as
3085 described in L<perlop/"`STRING`">.
3087 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3089 =item syswrite FILEHANDLE,SCALAR,LENGTH
3091 Attempts to write LENGTH bytes of data from variable SCALAR to the
3092 specified FILEHANDLE, using the system call write(2). It bypasses
3093 stdio, so mixing this with prints may cause confusion. Returns the
3094 number of bytes actually written, or undef if there was an error.
3095 If the length is greater than the available data, only as much data as
3096 is available will be written. An OFFSET may be specified to write the
3097 data from some other place than the beginning of the string.
3098 A negative OFFSET means starting the writing from that many bytes
3099 counting backwards from the end of the string.
3101 =item tell FILEHANDLE
3105 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3106 expression whose value gives the name of the actual filehandle. If
3107 FILEHANDLE is omitted, assumes the file last read.
3109 =item telldir DIRHANDLE
3111 Returns the current position of the readdir() routines on DIRHANDLE.
3112 Value may be given to seekdir() to access a particular location in a
3113 directory. Has the same caveats about possible directory compaction as
3114 the corresponding system library routine.
3116 =item tie VARIABLE,CLASSNAME,LIST
3118 This function binds a variable to a package class that will provide the
3119 implementation for the variable. VARIABLE is the name of the variable
3120 to be enchanted. CLASSNAME is the name of a class implementing objects
3121 of correct type. Any additional arguments are passed to the "new"
3122 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3123 Typically these are arguments such as might be passed to the dbm_open()
3124 function of C. The object returned by the "new" method is also
3125 returned by the tie() function, which would be useful if you want to
3126 access other methods in CLASSNAME.
3128 Note that functions such as keys() and values() may return huge array
3129 values when used on large objects, like DBM files. You may prefer to
3130 use the each() function to iterate over such. Example:
3132 # print out history file offsets
3134 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3135 while (($key,$val) = each %HIST) {
3136 print $key, ' = ', unpack('L',$val), "\n";
3140 A class implementing an associative array should have the following
3143 TIEHASH classname, LIST
3146 STORE this, key, value
3150 NEXTKEY this, lastkey
3152 A class implementing an ordinary array should have the following methods:
3154 TIEARRAY classname, LIST
3157 STORE this, key, value
3160 A class implementing a scalar should have the following methods:
3162 TIESCALAR classname, LIST
3167 Unlike dbmopen(), the tie() function will not use or require a module
3168 for you--you need to do that explicitly yourself. See L<DB_File>
3169 or the F<Config> module for interesting tie() implementations.
3173 Returns a reference to the object underlying VARIABLE (the same value
3174 that was originally returned by the tie() call which bound the variable
3175 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3180 Returns the number of non-leap seconds since whatever time the system
3181 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3182 and 00:00:00 UTC, January 1, 1970 for most other systems).
3183 Suitable for feeding to gmtime() and localtime().
3187 Returns a four-element array giving the user and system times, in
3188 seconds, for this process and the children of this process.
3190 ($user,$system,$cuser,$csystem) = times;
3194 The translation operator. See L<perlop>.
3196 =item truncate FILEHANDLE,LENGTH
3198 =item truncate EXPR,LENGTH
3200 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3201 specified length. Produces a fatal error if truncate isn't implemented
3208 Returns an uppercased version of EXPR. This is the internal function
3209 implementing the \U escape in double-quoted strings.
3210 Should respect any POSIX setlocale() settings.
3212 If EXPR is omitted, uses $_.
3218 Returns the value of EXPR with the first character uppercased. This is
3219 the internal function implementing the \u escape in double-quoted strings.
3220 Should respect any POSIX setlocale() settings.
3222 If EXPR is omitted, uses $_.
3228 Sets the umask for the process and returns the old one. If EXPR is
3229 omitted, merely returns current umask.
3235 Undefines the value of EXPR, which must be an lvalue. Use only on a
3236 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3237 will probably not do what you expect on most predefined variables or
3238 DBM list values, so don't do that.) Always returns the undefined value. You can omit
3239 the EXPR, in which case nothing is undefined, but you still get an
3240 undefined value that you could, for instance, return from a
3241 subroutine. Examples:
3244 undef $bar{'blurfl'};
3248 return (wantarray ? () : undef) if $they_blew_it;
3254 Deletes a list of files. Returns the number of files successfully
3257 $cnt = unlink 'a', 'b', 'c';
3261 Note: unlink will not delete directories unless you are superuser and
3262 the B<-U> flag is supplied to Perl. Even if these conditions are
3263 met, be warned that unlinking a directory can inflict damage on your
3264 filesystem. Use rmdir instead.
3266 If LIST is omitted, uses $_.
3268 =item unpack TEMPLATE,EXPR
3270 Unpack does the reverse of pack: it takes a string representing a
3271 structure and expands it out into a list value, returning the array
3272 value. (In a scalar context, it merely returns the first value
3273 produced.) The TEMPLATE has the same format as in the pack function.
3274 Here's a subroutine that does substring:
3277 local($what,$where,$howmuch) = @_;
3278 unpack("x$where a$howmuch", $what);
3283 sub ordinal { unpack("c",$_[0]); } # same as ord()
3285 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3286 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3287 themselves. Default is a 16-bit checksum. For example, the following
3288 computes the same number as the System V sum program:
3291 $checksum += unpack("%16C*", $_);
3295 The following efficiently counts the number of set bits in a bit vector:
3297 $setbits = unpack("%32b*", $selectmask);
3299 =item untie VARIABLE
3301 Breaks the binding between a variable and a package. (See tie().)
3303 =item unshift ARRAY,LIST
3305 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3306 depending on how you look at it. Prepends list to the front of the
3307 array, and returns the new number of elements in the array.
3309 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3311 Note the LIST is prepended whole, not one element at a time, so the
3312 prepended elements stay in the same order. Use reverse to do the
3315 =item use Module LIST
3319 =item use Module VERSION LIST
3323 Imports some semantics into the current package from the named module,
3324 generally by aliasing certain subroutine or variable names into your
3325 package. It is exactly equivalent to
3327 BEGIN { require Module; import Module LIST; }
3329 except that Module I<must> be a bare word.
3331 If the first argument to C<use> is a number, it is treated as a version
3332 number instead of a module name. If the version of the Perl interpreter
3333 is less than VERSION, then an error message is printed and Perl exits
3334 immediately. This is often useful if you need to check the current
3335 Perl version before C<use>ing library modules which have changed in
3336 incompatible ways from older versions of Perl. (We try not to do
3337 this more than we have to.)
3339 The BEGIN forces the require and import to happen at compile time. The
3340 require makes sure the module is loaded into memory if it hasn't been
3341 yet. The import is not a builtin--it's just an ordinary static method
3342 call into the "Module" package to tell the module to import the list of
3343 features back into the current package. The module can implement its
3344 import method any way it likes, though most modules just choose to
3345 derive their import method via inheritance from the Exporter class that
3346 is defined in the Exporter module. See L<Exporter>. If no import
3347 method can be found then the error is currently silently ignored. This
3348 may change to a fatal error in a future version.
3350 If you don't want your namespace altered, explicitly supply an empty list:
3354 That is exactly equivalent to
3356 BEGIN { require Module; }
3358 If the VERSION argument is present between Module and LIST, then the
3359 C<use> will fail if the C<$VERSION> variable in package Module is
3362 Because this is a wide-open interface, pragmas (compiler directives)
3363 are also implemented this way. Currently implemented pragmas are:
3367 use sigtrap qw(SEGV BUS);
3368 use strict qw(subs vars refs);
3369 use subs qw(afunc blurfl);
3371 These pseudomodules import semantics into the current block scope, unlike
3372 ordinary modules, which import symbols into the current package (which are
3373 effective through the end of the file).
3375 There's a corresponding "no" command that unimports meanings imported
3376 by use, i.e. it calls C<unimport Module LIST> instead of C<import>.
3381 If no unimport method can be found the call fails with a fatal error.
3383 See L<perlmod> for a list of standard modules and pragmas.
3387 Changes the access and modification times on each file of a list of
3388 files. The first two elements of the list must be the NUMERICAL access
3389 and modification times, in that order. Returns the number of files
3390 successfully changed. The inode modification time of each file is set
3391 to the current time. Example of a "touch" command:
3395 utime $now, $now, @ARGV;
3397 =item values ASSOC_ARRAY
3399 Returns a normal array consisting of all the values of the named
3400 associative array. (In a scalar context, returns the number of
3401 values.) The values are returned in an apparently random order, but it
3402 is the same order as either the keys() or each() function would produce
3403 on the same array. See also keys(), each(), and sort().
3405 =item vec EXPR,OFFSET,BITS
3407 Treats the string in EXPR as a vector of unsigned integers, and
3408 returns the value of the bitfield specified by OFFSET. BITS specifies
3409 the number of bits that are reserved for each entry in the bit
3410 vector. This must be a power of two from 1 to 32. vec() may also be
3411 assigned to, in which case parens are needed to give the expression
3412 the correct precedence as in
3414 vec($image, $max_x * $x + $y, 8) = 3;
3416 Vectors created with vec() can also be manipulated with the logical
3417 operators |, & and ^, which will assume a bit vector operation is
3418 desired when both operands are strings.
3420 To transform a bit vector into a string or array of 0's and 1's, use these:
3422 $bits = unpack("b*", $vector);
3423 @bits = split(//, unpack("b*", $vector));
3425 If you know the exact length in bits, it can be used in place of the *.
3429 Waits for a child process to terminate and returns the pid of the
3430 deceased process, or -1 if there are no child processes. The status is
3433 =item waitpid PID,FLAGS
3435 Waits for a particular child process to terminate and returns the pid
3436 of the deceased process, or -1 if there is no such child process. The
3437 status is returned in C<$?>. If you say
3439 use POSIX ":wait_h";
3441 waitpid(-1,&WNOHANG);
3443 then you can do a non-blocking wait for any process. Non-blocking wait
3444 is only available on machines supporting either the waitpid(2) or
3445 wait4(2) system calls. However, waiting for a particular pid with
3446 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3447 by remembering the status values of processes that have exited but have
3448 not been harvested by the Perl script yet.)
3452 Returns TRUE if the context of the currently executing subroutine is
3453 looking for a list value. Returns FALSE if the context is looking
3456 return wantarray ? () : undef;
3460 Produces a message on STDERR just like die(), but doesn't exit or
3463 =item write FILEHANDLE
3469 Writes a formatted record (possibly multi-line) to the specified file,
3470 using the format associated with that file. By default the format for
3471 a file is the one having the same name is the filehandle, but the
3472 format for the current output channel (see the select() function) may be set
3473 explicitly by assigning the name of the format to the C<$~> variable.
3475 Top of form processing is handled automatically: if there is
3476 insufficient room on the current page for the formatted record, the
3477 page is advanced by writing a form feed, a special top-of-page format
3478 is used to format the new page header, and then the record is written.
3479 By default the top-of-page format is the name of the filehandle with
3480 "_TOP" appended, but it may be dynamically set to the format of your
3481 choice by assigning the name to the C<$^> variable while the filehandle is
3482 selected. The number of lines remaining on the current page is in
3483 variable C<$->, which can be set to 0 to force a new page.
3485 If FILEHANDLE is unspecified, output goes to the current default output
3486 channel, which starts out as STDOUT but may be changed by the
3487 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3488 is evaluated and the resulting string is used to look up the name of
3489 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3491 Note that write is I<NOT> the opposite of read. Unfortunately.
3495 The translation operator. See L<perlop>.
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