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()
337 If you want to use alarm() to time out a system call you need to use an
338 eval/die pair. You can't rely on the alarm causing the system call to
339 fail with $! set to EINTR because Perl sets up signal handlers to
340 restart system calls on some systems. Using eval/die always works.
343 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
345 $nread = sysread SOCKET, $buffer, $size;
348 die if $@ && $@ ne "alarm\n"; # propagate errors
358 Returns the arctangent of Y/X in the range -PI to PI.
360 =item bind SOCKET,NAME
362 Binds a network address to a socket, just as the bind system call
363 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
364 packed address of the appropriate type for the socket. See the examples in
365 L<perlipc/"Sockets: Client/Server Communication">.
367 =item binmode FILEHANDLE
369 Arranges for the file to be read or written in "binary" mode in operating
370 systems that distinguish between binary and text files. Files that are
371 not in binary mode have CR LF sequences translated to LF on input and LF
372 translated to CR LF on output. Binmode has no effect under Unix; in DOS
373 and similarly archaic systems, it may be imperative--otherwise your
374 DOS-damaged C library may mangle your file. The key distinction between
375 systems that need binmode and those that don't is their text file
376 formats. Systems like Unix and Plan9 that delimit lines with a single
377 character, and that encode that character in C as '\n', do not need
378 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
379 is taken as the name of the filehandle.
381 =item bless REF,CLASSNAME
385 This function tells the referenced object (passed as REF) that it is now
386 an object in the CLASSNAME package--or the current package if no CLASSNAME
387 is specified, which is often the case. It returns the reference for
388 convenience, since a bless() is often the last thing in a constructor.
389 Always use the two-argument version if the function doing the blessing
390 might be inherited by a derived class. See L<perlobj> for more about the
391 blessing (and blessings) of objects.
397 Returns the context of the current subroutine call. In a scalar context,
398 returns TRUE if there is a caller, that is, if we're in a subroutine or
399 eval() or require(), and FALSE otherwise. In a list context, returns
401 ($package, $filename, $line) = caller;
403 With EXPR, it returns some extra information that the debugger uses to
404 print a stack trace. The value of EXPR indicates how many call frames
405 to go back before the current one.
407 ($package, $filename, $line,
408 $subroutine, $hasargs, $wantargs) = caller($i);
410 Furthermore, when called from within the DB package, caller returns more
411 detailed information: it sets the list variable @DB::args to be the
412 arguments with which that subroutine was invoked.
416 Changes the working directory to EXPR, if possible. If EXPR is
417 omitted, changes to home directory. Returns TRUE upon success, FALSE
418 otherwise. See example under die().
422 Changes the permissions of a list of files. The first element of the
423 list must be the numerical mode, which should probably be an octal
424 number. Returns the number of files successfully changed.
426 $cnt = chmod 0755, 'foo', 'bar';
427 chmod 0755, @executables;
435 This is a slightly safer version of chop (see below). It removes any
436 line ending that corresponds to the current value of C<$/> (also known as
437 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
438 of characters removed. It's often used to remove the newline from the
439 end of an input record when you're worried that the final record may be
440 missing its newline. When in paragraph mode (C<$/ = "">), it removes all
441 trailing newlines from the string. If VARIABLE is omitted, it chomps
445 chomp; # avoid \n on last field
450 You can actually chomp anything that's an lvalue, including an assignment:
453 chomp($answer = <STDIN>);
455 If you chomp a list, each element is chomped, and the total number of
456 characters removed is returned.
464 Chops off the last character of a string and returns the character
465 chopped. It's used primarily to remove the newline from the end of an
466 input record, but is much more efficient than C<s/\n//> because it neither
467 scans nor copies the string. If VARIABLE is omitted, chops $_.
471 chop; # avoid \n on last field
476 You can actually chop anything that's an lvalue, including an assignment:
479 chop($answer = <STDIN>);
481 If you chop a list, each element is chopped. Only the value of the
482 last chop is returned.
484 Note that chop returns the last character. To return all but the last
485 character, use C<substr($string, 0, -1)>.
489 Changes the owner (and group) of a list of files. The first two
490 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
491 Returns the number of files successfully changed.
493 $cnt = chown $uid, $gid, 'foo', 'bar';
494 chown $uid, $gid, @filenames;
496 Here's an example that looks up non-numeric uids in the passwd file:
499 chop($user = <STDIN>);
501 chop($pattern = <STDIN>);
503 ($login,$pass,$uid,$gid) = getpwnam($user)
504 or die "$user not in passwd file";
506 @ary = <${pattern}>; # expand filenames
507 chown $uid, $gid, @ary;
509 On most systems, you are not allowed to change the ownership of the
510 file unless you're the superuser, although you should be able to change
511 the group to any of your secondary groups. On insecure systems, these
512 restrictions may be relaxed, but this is not a portable assumption.
518 Returns the character represented by that NUMBER in the character set.
519 For example, C<chr(65)> is "A" in ASCII.
521 If NUMBER is omitted, uses $_.
523 =item chroot FILENAME
527 This function works as the system call by the same name: it makes the
528 named directory the new root directory for all further pathnames that
529 begin with a "/" by your process and all of its children. (It doesn't
530 change your current working directory is unaffected.) For security
531 reasons, this call is restricted to the superuser. If FILENAME is
532 omitted, does chroot to $_.
534 =item close FILEHANDLE
536 Closes the file or pipe associated with the file handle, returning TRUE
537 only if stdio successfully flushes buffers and closes the system file
538 descriptor. You don't have to close FILEHANDLE if you are immediately
539 going to do another open() on it, since open() will close it for you. (See
540 open().) However, an explicit close on an input file resets the line
541 counter ($.), while the implicit close done by open() does not. Also,
542 closing a pipe will wait for the process executing on the pipe to
543 complete, in case you want to look at the output of the pipe
544 afterwards. Closing a pipe explicitly also puts the status value of
545 the command into C<$?>. Example:
547 open(OUTPUT, '|sort >foo'); # pipe to sort
548 ... # print stuff to output
549 close OUTPUT; # wait for sort to finish
550 open(INPUT, 'foo'); # get sort's results
552 FILEHANDLE may be an expression whose value gives the real filehandle name.
554 =item closedir DIRHANDLE
556 Closes a directory opened by opendir().
558 =item connect SOCKET,NAME
560 Attempts to connect to a remote socket, just as the connect system call
561 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
562 packed address of the appropriate type for the socket. See the examples in
563 L<perlipc/"Sockets: Client/Server Communication">.
567 Actually a flow control statement rather than a function. If there is a
568 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
569 C<foreach>), it is always executed just before the conditional is about to
570 be evaluated again, just like the third part of a C<for> loop in C. Thus
571 it can be used to increment a loop variable, even when the loop has been
572 continued via the C<next> statement (which is similar to the C C<continue>
577 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
580 =item crypt PLAINTEXT,SALT
582 Encrypts a string exactly like the crypt(3) function in the C library
583 (assuming that you actually have a version there that has not been
584 extirpated as a potential munition). This can prove useful for checking
585 the password file for lousy passwords, amongst other things. Only the
586 guys wearing white hats should do this.
588 Here's an example that makes sure that whoever runs this program knows
591 $pwd = (getpwuid($<))[1];
592 $salt = substr($pwd, 0, 2);
596 chop($word = <STDIN>);
600 if (crypt($word, $salt) ne $pwd) {
606 Of course, typing in your own password to whoever asks you
609 =item dbmclose ASSOC_ARRAY
611 [This function has been superseded by the untie() function.]
613 Breaks the binding between a DBM file and an associative array.
615 =item dbmopen ASSOC,DBNAME,MODE
617 [This function has been superseded by the tie() function.]
619 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
620 associative array. ASSOC is the name of the associative array. (Unlike
621 normal open, the first argument is I<NOT> a filehandle, even though it
622 looks like one). DBNAME is the name of the database (without the F<.dir>
623 or F<.pag> extension if any). If the database does not exist, it is
624 created with protection specified by MODE (as modified by the umask()).
625 If your system only supports the older DBM functions, you may perform only
626 one dbmopen() in your program. In older versions of Perl, if your system
627 had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
628 falls back to sdbm(3).
630 If you don't have write access to the DBM file, you can only read
631 associative array variables, not set them. If you want to test whether
632 you can write, either use file tests or try setting a dummy array entry
633 inside an eval(), which will trap the error.
635 Note that functions such as keys() and values() may return huge array
636 values when used on large DBM files. You may prefer to use the each()
637 function to iterate over large DBM files. Example:
639 # print out history file offsets
640 dbmopen(%HIST,'/usr/lib/news/history',0666);
641 while (($key,$val) = each %HIST) {
642 print $key, ' = ', unpack('L',$val), "\n";
646 See also L<AnyDBM_File> for a more general description of the pros and
647 cons of the various dbm approaches, as well as L<DB_File> for a particularly
654 Returns a boolean value saying whether EXPR has a real value
655 or not. If EXPR is not present, $_ will be checked. Many operations
656 return the undefined value under exceptional conditions, such as end of
657 file, uninitialized variable, system error and such. This function
658 allows you to distinguish between an undefined
659 null scalar and a defined null scalar with operations that might return
660 a real null string, such as referencing elements of an array. You may
661 also check to see if arrays or subroutines exist. Use of defined on
662 predefined variables is not guaranteed to produce intuitive results.
664 When used on a hash array element, it tells you whether the value
665 is defined, not whether the key exists in the hash. Use exists() for that.
669 print if defined $switch{'D'};
670 print "$val\n" while defined($val = pop(@ary));
671 die "Can't readlink $sym: $!"
672 unless defined($value = readlink $sym);
673 eval '@foo = ()' if defined(@foo);
674 die "No XYZ package defined" unless defined %_XYZ;
675 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
679 Note: many folks tend to overuse defined(), and then are surprised to
680 discover that the number 0 and the null string are, in fact, defined
681 concepts. For example, if you say
685 the pattern match succeeds, and $1 is defined, despite the fact that it
686 matched "nothing". But it didn't really match nothing--rather, it
687 matched something that happened to be 0 characters long. This is all
688 very above-board and honest. When a function returns an undefined value,
689 it's an admission that it couldn't give you an honest answer. So
690 you should only use defined() when you're questioning the integrity
691 of what you're trying to do. At other times, a simple comparison to
692 0 or "" is what you want.
696 Deletes the specified value from its hash array. Returns the deleted
697 value, or the undefined value if nothing was deleted. Deleting from
698 C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM
699 file deletes the entry from the DBM file. (But deleting from a tie()d
700 hash doesn't necessarily return anything.)
702 The following deletes all the values of an associative array:
704 foreach $key (keys %ARRAY) {
708 (But it would be faster to use the undef() command.) Note that the
709 EXPR can be arbitrarily complicated as long as the final operation is
712 delete $ref->[$x][$y]{$key};
716 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
717 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
718 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> is 0,
719 exits with 255. Inside an eval(), the error message is stuffed into C<$@>,
720 and the eval() is terminated with the undefined value; this makes die()
721 the way to raise an exception.
725 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
726 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
728 If the value of EXPR does not end in a newline, the current script line
729 number and input line number (if any) are also printed, and a newline
730 is supplied. Hint: sometimes appending ", stopped" to your message
731 will cause it to make better sense when the string "at foo line 123" is
732 appended. Suppose you are running script "canasta".
734 die "/etc/games is no good";
735 die "/etc/games is no good, stopped";
737 produce, respectively
739 /etc/games is no good at canasta line 123.
740 /etc/games is no good, stopped at canasta line 123.
742 See also exit() and warn().
746 Not really a function. Returns the value of the last command in the
747 sequence of commands indicated by BLOCK. When modified by a loop
748 modifier, executes the BLOCK once before testing the loop condition.
749 (On other statements the loop modifiers test the conditional first.)
751 =item do SUBROUTINE(LIST)
753 A deprecated form of subroutine call. See L<perlsub>.
757 Uses the value of EXPR as a filename and executes the contents of the
758 file as a Perl script. Its primary use is to include subroutines
759 from a Perl subroutine library.
767 except that it's more efficient, more concise, keeps track of the
768 current filename for error messages, and searches all the B<-I>
769 libraries if the file isn't in the current directory (see also the @INC
770 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
771 reparse the file every time you call it, so you probably don't want to
772 do this inside a loop.
774 Note that inclusion of library modules is better done with the
775 use() and require() operators, which also do error checking
776 and raise an exception if there's a problem.
780 This causes an immediate core dump. Primarily this is so that you can
781 use the B<undump> program to turn your core dump into an executable binary
782 after having initialized all your variables at the beginning of the
783 program. When the new binary is executed it will begin by executing a
784 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
785 it as a goto with an intervening core dump and reincarnation. If LABEL
786 is omitted, restarts the program from the top. WARNING: any files
787 opened at the time of the dump will NOT be open any more when the
788 program is reincarnated, with possible resulting confusion on the part
789 of Perl. See also B<-u> option in L<perlrun>.
806 dump QUICKSTART if $ARGV[0] eq '-d';
811 =item each ASSOC_ARRAY
813 When called in a list context, returns a 2-element array consisting
814 of the key and value for the next element of an associative array,
815 so that you can iterate over it. When called in a scalar context,
816 returns the key only for the next element in the associative array.
817 Entries are returned in an apparently random order. When the array is
818 entirely read, a null array is returned in list context (which when
819 assigned produces a FALSE (0) value), and C<undef> is returned in a
820 scalar context. The next call to each() after that will start
821 iterating again. The iterator can be reset only by reading all the
822 elements from the array. You should not add elements to an array while
823 you're iterating over it. There is a single iterator for each
824 associative array, shared by all each(), keys() and values() function
825 calls in the program. The following prints out your environment like
826 the printenv(1) program, only in a different order:
828 while (($key,$value) = each %ENV) {
829 print "$key=$value\n";
832 See also keys() and values().
840 Returns 1 if the next read on FILEHANDLE will return end of file, or if
841 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
842 gives the real filehandle name. (Note that this function actually
843 reads a character and then ungetc()s it, so it is not very useful in an
844 interactive context.) Do not read from a terminal file (or call
845 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
846 as terminals may lose the end-of-file condition if you do.
848 An C<eof> without an argument uses the last file read as argument.
849 Empty parentheses () may be used to indicate
850 the pseudofile formed of the files listed on the command line, i.e.
851 C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
852 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
853 test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
855 # reset line numbering on each input file
858 close(ARGV) if (eof); # Not eof().
861 # insert dashes just before last line of last file
864 print "--------------\n";
865 close(ARGV); # close or break; is needed if we
866 # are reading from the terminal
871 Practical hint: you almost never need to use C<eof> in Perl, because the
872 input operators return undef when they run out of data.
878 EXPR is parsed and executed as if it were a little Perl program. It
879 is executed in the context of the current Perl program, so that any
880 variable settings, subroutine or format definitions remain afterwards.
881 The value returned is the value of the last expression evaluated, or a
882 return statement may be used, just as with subroutines. The last
883 expression is evaluated in scalar or array context, depending on the
886 If there is a syntax error or runtime error, or a die() statement is
887 executed, an undefined value is returned by eval(), and C<$@> is set to the
888 error message. If there was no error, C<$@> is guaranteed to be a null
889 string. If EXPR is omitted, evaluates $_. The final semicolon, if
890 any, may be omitted from the expression.
892 Note that, since eval() traps otherwise-fatal errors, it is useful for
893 determining whether a particular feature (such as socket() or symlink())
894 is implemented. It is also Perl's exception trapping mechanism, where
895 the die operator is used to raise exceptions.
897 If the code to be executed doesn't vary, you may use the eval-BLOCK
898 form to trap run-time errors without incurring the penalty of
899 recompiling each time. The error, if any, is still returned in C<$@>.
902 # make divide-by-zero non-fatal
903 eval { $answer = $a / $b; }; warn $@ if $@;
905 # same thing, but less efficient
906 eval '$answer = $a / $b'; warn $@ if $@;
908 # a compile-time error
912 eval '$answer ='; # sets $@
914 With an eval(), you should be especially careful to remember what's
915 being looked at when:
921 eval { $x }; # CASE 4
923 eval "\$$x++" # CASE 5
926 Cases 1 and 2 above behave identically: they run the code contained in the
927 variable $x. (Although case 2 has misleading double quotes making the
928 reader wonder what else might be happening (nothing is).) Cases 3 and 4
929 likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
930 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
931 is a place where normally you I<WOULD> like to use double quotes, except
932 that in that particular situation, you can just use symbolic references
933 instead, as in case 6.
937 The exec() function executes a system command I<AND NEVER RETURNS>,
938 unless the command does not exist and is executed directly instead of
939 via C</bin/sh -c> (see below). Use system() instead of exec() if you
942 If there is more than one argument in LIST, or if LIST is an array with
943 more than one value, calls execvp(3) with the arguments in LIST. If
944 there is only one scalar argument, the argument is checked for shell
945 metacharacters. If there are any, the entire argument is passed to
946 C</bin/sh -c> for parsing. If there are none, the argument is split
947 into words and passed directly to execvp(), which is more efficient.
948 Note: exec() and system() do not flush your output buffer, so you may
949 need to set C<$|> to avoid lost output. Examples:
951 exec '/bin/echo', 'Your arguments are: ', @ARGV;
952 exec "sort $outfile | uniq";
954 If you don't really want to execute the first argument, but want to lie
955 to the program you are executing about its own name, you can specify
956 the program you actually want to run as an "indirect object" (without a
957 comma) in front of the LIST. (This always forces interpretation of the
958 LIST as a multi-valued list, even if there is only a single scalar in
962 exec $shell '-sh'; # pretend it's a login shell
966 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
970 Returns TRUE if the specified hash key exists in its hash array, even
971 if the corresponding value is undefined.
973 print "Exists\n" if exists $array{$key};
974 print "Defined\n" if defined $array{$key};
975 print "True\n" if $array{$key};
977 A hash element can only be TRUE if it's defined, and defined if
978 it exists, but the reverse doesn't necessarily hold true.
980 Note that the EXPR can be arbitrarily complicated as long as the final
981 operation is a hash key lookup:
983 if (exists $ref->[$x][$y]{$key}) { ... }
987 Evaluates EXPR and exits immediately with that value. (Actually, it
988 calls any defined C<END> routines first, but the C<END> routines may not
989 abort the exit. Likewise any object destructors that need to be called
990 are called before exit.) Example:
993 exit 0 if $ans =~ /^[Xx]/;
995 See also die(). If EXPR is omitted, exits with 0 status.
1001 Returns I<e> (the natural logarithm base) to the power of EXPR.
1002 If EXPR is omitted, gives C<exp($_)>.
1004 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1006 Implements the fcntl(2) function. You'll probably have to say
1010 first to get the correct function definitions. Argument processing and
1011 value return works just like ioctl() below. Note that fcntl() will produce
1012 a fatal error if used on a machine that doesn't implement fcntl(2).
1016 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1018 =item fileno FILEHANDLE
1020 Returns the file descriptor for a filehandle. This is useful for
1021 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1022 value is taken as the name of the filehandle.
1024 =item flock FILEHANDLE,OPERATION
1026 Calls flock(2) on FILEHANDLE. See L<flock(2)> for definition of
1027 OPERATION. Returns TRUE for success, FALSE on failure. Will produce a
1028 fatal error if used on a machine that doesn't implement either flock(2) or
1029 fcntl(2). The fcntl(2) system call will be automatically used if flock(2)
1030 is missing from your system. This makes flock() the portable file locking
1031 strategy, although it will only lock entire files, not records. Note also
1032 that some versions of flock() cannot lock things over the network; you
1033 would need to use the more system-specific fcntl() for that.
1035 Here's a mailbox appender for BSD systems.
1043 flock(MBOX,$LOCK_EX);
1044 # and, in case someone appended
1045 # while we were waiting...
1050 flock(MBOX,$LOCK_UN);
1053 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1054 or die "Can't open mailbox: $!";
1057 print MBOX $msg,"\n\n";
1060 See also L<DB_File> for other flock() examples.
1064 Does a fork(2) system call. Returns the child pid to the parent process
1065 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1066 Note: unflushed buffers remain unflushed in both processes, which means
1067 you may need to set C<$|> ($AUTOFLUSH in English) or call the
1068 autoflush() FileHandle method to avoid duplicate output.
1070 If you fork() without ever waiting on your children, you will accumulate
1073 $SIG{CHLD} = sub { wait };
1075 There's also the double-fork trick (error checking on
1076 fork() returns omitted);
1078 unless ($pid = fork) {
1080 exec "what you really wanna do";
1083 ## (some_perl_code_here)
1090 See also L<perlipc> for more examples of forking and reaping
1095 Declare a picture format with use by the write() function. For
1099 Test: @<<<<<<<< @||||| @>>>>>
1100 $str, $%, '$' . int($num)
1104 $num = $cost/$quantity;
1108 See L<perlform> for many details and examples.
1111 =item formline PICTURE, LIST
1113 This is an internal function used by C<format>s, though you may call it
1114 too. It formats (see L<perlform>) a list of values according to the
1115 contents of PICTURE, placing the output into the format output
1116 accumulator, C<$^A> (or $ACCUMULATOR in English).
1117 Eventually, when a write() is done, the contents of
1118 C<$^A> are written to some filehandle, but you could also read C<$^A>
1119 yourself and then set C<$^A> back to "". Note that a format typically
1120 does one formline() per line of form, but the formline() function itself
1121 doesn't care how many newlines are embedded in the PICTURE. This means
1122 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1123 You may therefore need to use multiple formlines to implement a single
1124 record format, just like the format compiler.
1126 Be careful if you put double quotes around the picture, since an "C<@>"
1127 character may be taken to mean the beginning of an array name.
1128 formline() always returns TRUE. See L<perlform> for other examples.
1130 =item getc FILEHANDLE
1134 Returns the next character from the input file attached to FILEHANDLE,
1135 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1136 This is not particularly efficient. It cannot be used to get unbuffered
1137 single-characters, however. For that, try something more like:
1140 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1143 system "stty", '-icanon', 'eol', "\001";
1149 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1152 system "stty", 'icanon', 'eol', '^@'; # ascii null
1156 Determination of whether to whether $BSD_STYLE should be set
1157 is left as an exercise to the reader.
1159 See also the C<Term::ReadKey> module from your nearest CPAN site;
1160 details on CPAN can be found on L<perlmod/CPAN>
1164 Returns the current login from F</etc/utmp>, if any. If null, use
1167 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1169 Do not consider getlogin() for authentication: it is not as
1170 secure as getpwuid().
1172 =item getpeername SOCKET
1174 Returns the packed sockaddr address of other end of the SOCKET connection.
1177 $hersockaddr = getpeername(SOCK);
1178 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1179 $herhostname = gethostbyaddr($iaddr, AF_INET);
1180 $herstraddr = inet_ntoa($iaddr);
1184 Returns the current process group for the specified PID. Use
1185 a PID of 0 to get the current process group for the
1186 current process. Will raise an exception if used on a machine that
1187 doesn't implement getpgrp(2). If PID is omitted, returns process
1188 group of current process. Note that the POSIX version of getpgrp()
1189 does not accept a PID argument, so only PID==0 is truly portable.
1193 Returns the process id of the parent process.
1195 =item getpriority WHICH,WHO
1197 Returns the current priority for a process, a process group, or a user.
1198 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1199 machine that doesn't implement getpriority(2).
1205 =item gethostbyname NAME
1207 =item getnetbyname NAME
1209 =item getprotobyname NAME
1215 =item getservbyname NAME,PROTO
1217 =item gethostbyaddr ADDR,ADDRTYPE
1219 =item getnetbyaddr ADDR,ADDRTYPE
1221 =item getprotobynumber NUMBER
1223 =item getservbyport PORT,PROTO
1241 =item sethostent STAYOPEN
1243 =item setnetent STAYOPEN
1245 =item setprotoent STAYOPEN
1247 =item setservent STAYOPEN
1261 These routines perform the same functions as their counterparts in the
1262 system library. Within a list context, the return values from the
1263 various get routines are as follows:
1265 ($name,$passwd,$uid,$gid,
1266 $quota,$comment,$gcos,$dir,$shell) = getpw*
1267 ($name,$passwd,$gid,$members) = getgr*
1268 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1269 ($name,$aliases,$addrtype,$net) = getnet*
1270 ($name,$aliases,$proto) = getproto*
1271 ($name,$aliases,$port,$proto) = getserv*
1273 (If the entry doesn't exist you get a null list.)
1275 Within a scalar context, you get the name, unless the function was a
1276 lookup by name, in which case you get the other thing, whatever it is.
1277 (If the entry doesn't exist you get the undefined value.) For example:
1287 The $members value returned by I<getgr*()> is a space separated list of
1288 the login names of the members of the group.
1290 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1291 C, it will be returned to you via C<$?> if the function call fails. The
1292 @addrs value returned by a successful call is a list of the raw
1293 addresses returned by the corresponding system library call. In the
1294 Internet domain, each address is four bytes long and you can unpack it
1295 by saying something like:
1297 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1299 =item getsockname SOCKET
1301 Returns the packed sockaddr address of this end of the SOCKET connection.
1304 $mysockaddr = getsockname(SOCK);
1305 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1307 =item getsockopt SOCKET,LEVEL,OPTNAME
1309 Returns the socket option requested, or undefined if there is an error.
1313 Returns the value of EXPR with filename expansions such as a shell
1314 would do. This is the internal function implementing the E<lt>*.*E<gt>
1315 operator, except it's easier to use.
1319 Converts a time as returned by the time function to a 9-element array
1320 with the time localized for the standard Greenwich timezone.
1321 Typically used as follows:
1324 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1327 All array elements are numeric, and come straight out of a struct tm.
1328 In particular this means that $mon has the range 0..11 and $wday has
1329 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1337 The goto-LABEL form finds the statement labeled with LABEL and resumes
1338 execution there. It may not be used to go into any construct that
1339 requires initialization, such as a subroutine or a foreach loop. It
1340 also can't be used to go into a construct that is optimized away. It
1341 can be used to go almost anywhere else within the dynamic scope,
1342 including out of subroutines, but it's usually better to use some other
1343 construct such as last or die. The author of Perl has never felt the
1344 need to use this form of goto (in Perl, that is--C is another matter).
1346 The goto-EXPR form expects a label name, whose scope will be resolved
1347 dynamically. This allows for computed gotos per FORTRAN, but isn't
1348 necessarily recommended if you're optimizing for maintainability:
1350 goto ("FOO", "BAR", "GLARCH")[$i];
1352 The goto-&NAME form is highly magical, and substitutes a call to the
1353 named subroutine for the currently running subroutine. This is used by
1354 AUTOLOAD subroutines that wish to load another subroutine and then
1355 pretend that the other subroutine had been called in the first place
1356 (except that any modifications to @_ in the current subroutine are
1357 propagated to the other subroutine.) After the goto, not even caller()
1358 will be able to tell that this routine was called first.
1360 =item grep BLOCK LIST
1362 =item grep EXPR,LIST
1364 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1365 $_ to each element) and returns the list value consisting of those
1366 elements for which the expression evaluated to TRUE. In a scalar
1367 context, returns the number of times the expression was TRUE.
1369 @foo = grep(!/^#/, @bar); # weed out comments
1373 @foo = grep {!/^#/} @bar; # weed out comments
1375 Note that, since $_ is a reference into the list value, it can be used
1376 to modify the elements of the array. While this is useful and
1377 supported, it can cause bizarre results if the LIST is not a named
1384 Interprets EXPR as a hex string and returns the corresponding decimal
1385 value. (To convert strings that might start with 0 or 0x see
1386 oct().) If EXPR is omitted, uses $_.
1390 There is no built-in import() function. It is merely an ordinary
1391 method (subroutine) defined (or inherited) by modules that wish to export
1392 names to another module. The use() function calls the import() method
1393 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1395 =item index STR,SUBSTR,POSITION
1397 =item index STR,SUBSTR
1399 Returns the position of the first occurrence of SUBSTR in STR at or after
1400 POSITION. If POSITION is omitted, starts searching from the beginning of
1401 the string. The return value is based at 0 (or whatever you've set the C<$[>
1402 variable to--but don't do that). If the substring is not found, returns
1403 one less than the base, ordinarily -1.
1409 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1411 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1413 Implements the ioctl(2) function. You'll probably have to say
1415 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1417 first to get the correct function definitions. If F<ioctl.ph> doesn't
1418 exist or doesn't have the correct definitions you'll have to roll your
1419 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1420 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1421 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1422 written depending on the FUNCTION--a pointer to the string value of SCALAR
1423 will be passed as the third argument of the actual ioctl call. (If SCALAR
1424 has no string value but does have a numeric value, that value will be
1425 passed rather than a pointer to the string value. To guarantee this to be
1426 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1427 functions are useful for manipulating the values of structures used by
1428 ioctl(). The following example sets the erase character to DEL.
1432 die "NO TIOCGETP" if $@ || !$getp;
1433 $sgttyb_t = "ccccs"; # 4 chars and a short
1434 if (ioctl(STDIN,$getp,$sgttyb)) {
1435 @ary = unpack($sgttyb_t,$sgttyb);
1437 $sgttyb = pack($sgttyb_t,@ary);
1438 ioctl(STDIN,&TIOCSETP,$sgttyb)
1439 || die "Can't ioctl: $!";
1442 The return value of ioctl (and fcntl) is as follows:
1444 if OS returns: then Perl returns:
1446 0 string "0 but true"
1447 anything else that number
1449 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1450 still easily determine the actual value returned by the operating
1453 ($retval = ioctl(...)) || ($retval = -1);
1454 printf "System returned %d\n", $retval;
1456 =item join EXPR,LIST
1458 Joins the separate strings of LIST or ARRAY into a single string with
1459 fields separated by the value of EXPR, and returns the string.
1462 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1464 See L<perlfunc/split>.
1466 =item keys ASSOC_ARRAY
1468 Returns a normal array consisting of all the keys of the named
1469 associative array. (In a scalar context, returns the number of keys.)
1470 The keys are returned in an apparently random order, but it is the same
1471 order as either the values() or each() function produces (given that
1472 the associative array has not been modified). Here is yet another way
1473 to print your environment:
1476 @values = values %ENV;
1477 while ($#keys >= 0) {
1478 print pop(@keys), '=', pop(@values), "\n";
1481 or how about sorted by key:
1483 foreach $key (sort(keys %ENV)) {
1484 print $key, '=', $ENV{$key}, "\n";
1487 To sort an array by value, you'll need to use a C<sort{}>
1488 function. Here's a descending numeric sort of a hash by its values:
1490 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1491 printf "%4d %s\n", $hash{$key}, $key;
1494 As an lvalue C<keys> allows you to increase the number of hash buckets
1495 allocated for the given associative array. This can gain you a measure
1496 of efficiency if you know the hash is going to get big. (This is
1497 similar to pre-extending an array by assigning a larger number to
1498 $#array.) If you say
1502 then C<%hash> will have at least 200 buckets allocated for it. These
1503 buckets will be retained even if you do C<%hash = ()>, use C<undef
1504 %hash> if you want to free the storage while C<%hash> is still in scope.
1505 You can't shrink the number of buckets allocated for the hash using
1506 C<keys> in this way (but you needn't worry about doing this by accident,
1507 as trying has no effect).
1511 Sends a signal to a list of processes. The first element of
1512 the list must be the signal to send. Returns the number of
1513 processes successfully signaled.
1515 $cnt = kill 1, $child1, $child2;
1518 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1519 process groups instead of processes. (On System V, a negative I<PROCESS>
1520 number will also kill process groups, but that's not portable.) That
1521 means you usually want to use positive not negative signals. You may also
1522 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1528 The C<last> command is like the C<break> statement in C (as used in
1529 loops); it immediately exits the loop in question. If the LABEL is
1530 omitted, the command refers to the innermost enclosing loop. The
1531 C<continue> block, if any, is not executed:
1533 LINE: while (<STDIN>) {
1534 last LINE if /^$/; # exit when done with header
1542 Returns an lowercased version of EXPR. This is the internal function
1543 implementing the \L escape in double-quoted strings.
1544 Should respect any POSIX setlocale() settings.
1546 If EXPR is omitted, uses $_.
1552 Returns the value of EXPR with the first character lowercased. This is
1553 the internal function implementing the \l escape in double-quoted strings.
1554 Should respect any POSIX setlocale() settings.
1556 If EXPR is omitted, uses $_.
1562 Returns the length in characters of the value of EXPR. If EXPR is
1563 omitted, returns length of $_.
1565 =item link OLDFILE,NEWFILE
1567 Creates a new filename linked to the old filename. Returns 1 for
1568 success, 0 otherwise.
1570 =item listen SOCKET,QUEUESIZE
1572 Does the same thing that the listen system call does. Returns TRUE if
1573 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1577 A local modifies the listed variables to be local to the enclosing block,
1578 subroutine, C<eval{}> or C<do>. If more than one value is listed, the
1579 list must be placed in parens. See L<perlsub/"Temporary Values via
1580 local()"> for details.
1582 But you really probably want to be using my() instead, because local() isn't
1583 what most people think of as "local"). See L<perlsub/"Private Variables
1584 via my()"> for details.
1586 =item localtime EXPR
1588 Converts a time as returned by the time function to a 9-element array
1589 with the time analyzed for the local timezone. Typically used as
1592 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1595 All array elements are numeric, and come straight out of a struct tm.
1596 In particular this means that $mon has the range 0..11 and $wday has
1597 the range 0..6. If EXPR is omitted, does localtime(time).
1599 In a scalar context, prints out the ctime(3) value:
1601 $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
1603 Also see the F<timelocal.pl> library, and the strftime(3) function available
1604 via the POSIX module.
1610 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1613 =item lstat FILEHANDLE
1619 Does the same thing as the stat() function, but stats a symbolic link
1620 instead of the file the symbolic link points to. If symbolic links are
1621 unimplemented on your system, a normal stat() is done.
1623 If EXPR is omitted, stats $_.
1627 The match operator. See L<perlop>.
1629 =item map BLOCK LIST
1633 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1634 element) and returns the list value composed of the results of each such
1635 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1636 may produce zero, one, or more elements in the returned value.
1638 @chars = map(chr, @nums);
1640 translates a list of numbers to the corresponding characters. And
1642 %hash = map { getkey($_) => $_ } @array;
1644 is just a funny way to write
1647 foreach $_ (@array) {
1648 $hash{getkey($_)} = $_;
1651 =item mkdir FILENAME,MODE
1653 Creates the directory specified by FILENAME, with permissions specified
1654 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1655 it returns 0 and sets C<$!> (errno).
1657 =item msgctl ID,CMD,ARG
1659 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1660 must be a variable which will hold the returned msqid_ds structure.
1661 Returns like ioctl: the undefined value for error, "0 but true" for
1662 zero, or the actual return value otherwise.
1664 =item msgget KEY,FLAGS
1666 Calls the System V IPC function msgget(2). Returns the message queue id,
1667 or the undefined value if there is an error.
1669 =item msgsnd ID,MSG,FLAGS
1671 Calls the System V IPC function msgsnd to send the message MSG to the
1672 message queue ID. MSG must begin with the long integer message type,
1673 which may be created with C<pack("l", $type)>. Returns TRUE if
1674 successful, or FALSE if there is an error.
1676 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1678 Calls the System V IPC function msgrcv to receive a message from
1679 message queue ID into variable VAR with a maximum message size of
1680 SIZE. Note that if a message is received, the message type will be the
1681 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1682 of the message type. Returns TRUE if successful, or FALSE if there is
1687 A "my" declares the listed variables to be local (lexically) to the
1688 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1689 more than one value is listed, the list must be placed in parens. See
1690 L<perlsub/"Private Variables via my()"> for details.
1696 The C<next> command is like the C<continue> statement in C; it starts
1697 the next iteration of the loop:
1699 LINE: while (<STDIN>) {
1700 next LINE if /^#/; # discard comments
1704 Note that if there were a C<continue> block on the above, it would get
1705 executed even on discarded lines. If the LABEL is omitted, the command
1706 refers to the innermost enclosing loop.
1708 =item no Module LIST
1710 See the "use" function, which "no" is the opposite of.
1716 Interprets EXPR as an octal string and returns the corresponding
1717 decimal value. (If EXPR happens to start off with 0x, interprets it as
1718 a hex string instead.) The following will handle decimal, octal, and
1719 hex in the standard Perl or C notation:
1721 $val = oct($val) if $val =~ /^0/;
1723 If EXPR is omitted, uses $_.
1725 =item open FILEHANDLE,EXPR
1727 =item open FILEHANDLE
1729 Opens the file whose filename is given by EXPR, and associates it with
1730 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name
1731 of the real filehandle wanted. If EXPR is omitted, the scalar variable of
1732 the same name as the FILEHANDLE contains the filename. If the filename
1733 begins with "E<lt>" or nothing, the file is opened for input. If the filename
1734 begins with "E<gt>", the file is opened for output. If the filename begins
1735 with "E<gt>E<gt>", the file is opened for appending. You can put a '+' in
1736 front of the 'E<gt>' or 'E<lt>' to indicate that you want both read and write
1737 access to the file; thus '+E<lt>' is usually preferred for read/write
1738 updates--the '+E<gt>' mode would clobber the file first. These correspond to
1739 the fopen(3) modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
1741 If the filename begins with "|", the filename is interpreted
1742 as a command to which output is to be piped, and if the filename ends with
1743 a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
1744 for more examples of this. as command which pipes input to us. (You may
1745 not have a raw open() to a command that pipes both in I<and> out, but see L<open2>,
1746 L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)
1748 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1749 non-zero upon success, the undefined value otherwise. If the open
1750 involved a pipe, the return value happens to be the pid of the
1753 If you're unfortunate enough to be running Perl on a system that
1754 distinguishes between text files and binary files (modern operating
1755 systems don't care), then you should check out L</binmode> for tips for
1756 dealing with this. The key distinction between systems that need binmode
1757 and those that don't is their text file formats. Systems like Unix and
1758 Plan9 that delimit lines with a single character, and that encode that
1759 character in C as '\n', do not need C<binmode>. The rest need it.
1764 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1765 while (<ARTICLE>) {...
1767 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1769 open(DBASE, '+<dbase.mine'); # open for update
1771 open(ARTICLE, "caesar <$article |"); # decrypt article
1773 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1775 # process argument list of files along with any includes
1777 foreach $file (@ARGV) {
1778 process($file, 'fh00');
1782 local($filename, $input) = @_;
1783 $input++; # this is a string increment
1784 unless (open($input, $filename)) {
1785 print STDERR "Can't open $filename: $!\n";
1789 while (<$input>) { # note use of indirection
1790 if (/^#include "(.*)"/) {
1791 process($1, $input);
1798 You may also, in the Bourne shell tradition, specify an EXPR beginning
1799 with "E<gt>&", in which case the rest of the string is interpreted as the
1800 name of a filehandle (or file descriptor, if numeric) which is to be
1801 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1802 +E<gt>E<gt> and +E<lt>. The
1803 mode you specify should match the mode of the original filehandle.
1804 (Duping a filehandle does not take into account any existing contents of
1806 Here is a script that saves, redirects, and restores STDOUT and
1810 open(SAVEOUT, ">&STDOUT");
1811 open(SAVEERR, ">&STDERR");
1813 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1814 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1816 select(STDERR); $| = 1; # make unbuffered
1817 select(STDOUT); $| = 1; # make unbuffered
1819 print STDOUT "stdout 1\n"; # this works for
1820 print STDERR "stderr 1\n"; # subprocesses too
1825 open(STDOUT, ">&SAVEOUT");
1826 open(STDERR, ">&SAVEERR");
1828 print STDOUT "stdout 2\n";
1829 print STDERR "stderr 2\n";
1832 If you specify "E<lt>&=N", where N is a number, then Perl will do an
1833 equivalent of C's fdopen() of that file descriptor; this is more
1834 parsimonious of file descriptors. For example:
1836 open(FILEHANDLE, "<&=$fd")
1838 If you open a pipe on the command "-", i.e. either "|-" or "-|", then
1839 there is an implicit fork done, and the return value of open is the pid
1840 of the child within the parent process, and 0 within the child
1841 process. (Use C<defined($pid)> to determine whether the open was successful.)
1842 The filehandle behaves normally for the parent, but i/o to that
1843 filehandle is piped from/to the STDOUT/STDIN of the child process.
1844 In the child process the filehandle isn't opened--i/o happens from/to
1845 the new STDOUT or STDIN. Typically this is used like the normal
1846 piped open when you want to exercise more control over just how the
1847 pipe command gets executed, such as when you are running setuid, and
1848 don't want to have to scan shell commands for metacharacters.
1849 The following pairs are more or less equivalent:
1851 open(FOO, "|tr '[a-z]' '[A-Z]'");
1852 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1854 open(FOO, "cat -n '$file'|");
1855 open(FOO, "-|") || exec 'cat', '-n', $file;
1857 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1859 Explicitly closing any piped filehandle causes the parent process to
1860 wait for the child to finish, and returns the status value in C<$?>.
1861 Note: on any operation which may do a fork, unflushed buffers remain
1862 unflushed in both processes, which means you may need to set C<$|> to
1863 avoid duplicate output.
1865 Using the FileHandle constructor from the FileHandle package,
1866 you can generate anonymous filehandles which have the scope of whatever
1867 variables hold references to them, and automatically close whenever
1868 and however you leave that scope:
1872 sub read_myfile_munged {
1874 my $handle = new FileHandle;
1875 open($handle, "myfile") or die "myfile: $!";
1877 or return (); # Automatically closed here.
1878 mung $first or die "mung failed"; # Or here.
1879 return $first, <$handle> if $ALL; # Or here.
1883 The filename that is passed to open will have leading and trailing
1884 whitespace deleted. In order to open a file with arbitrary weird
1885 characters in it, it's necessary to protect any leading and trailing
1888 $file =~ s#^(\s)#./$1#;
1889 open(FOO, "< $file\0");
1891 If you want a "real" C open() (see L<open(2)> on your system), then
1892 you should use the sysopen() function. This is another way to
1893 protect your filenames from interpretation. For example:
1896 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
1897 or die "sysopen $path: $!";
1898 HANDLE->autoflush(1);
1899 HANDLE->print("stuff $$\n");
1901 print "File contains: ", <HANDLE>;
1903 See L</seek()> for some details about mixing reading and writing.
1905 =item opendir DIRHANDLE,EXPR
1907 Opens a directory named EXPR for processing by readdir(), telldir(),
1908 seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
1909 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1915 Returns the numeric ascii value of the first character of EXPR. If
1916 EXPR is omitted, uses $_.
1918 =item pack TEMPLATE,LIST
1920 Takes an array or list of values and packs it into a binary structure,
1921 returning the string containing the structure. The TEMPLATE is a
1922 sequence of characters that give the order and type of values, as
1925 A An ascii string, will be space padded.
1926 a An ascii string, will be null padded.
1927 b A bit string (ascending bit order, like vec()).
1928 B A bit string (descending bit order).
1929 h A hex string (low nybble first).
1930 H A hex string (high nybble first).
1932 c A signed char value.
1933 C An unsigned char value.
1934 s A signed short value.
1935 S An unsigned short value.
1936 i A signed integer value.
1937 I An unsigned integer value.
1938 l A signed long value.
1939 L An unsigned long value.
1941 n A short in "network" order.
1942 N A long in "network" order.
1943 v A short in "VAX" (little-endian) order.
1944 V A long in "VAX" (little-endian) order.
1946 f A single-precision float in the native format.
1947 d A double-precision float in the native format.
1949 p A pointer to a null-terminated string.
1950 P A pointer to a structure (fixed-length string).
1952 u A uuencoded string.
1954 w A BER compressed integer. Bytes give an unsigned integer base
1955 128, most significant digit first, with as few digits as
1956 possible, and with the bit 8 of each byte except the last set
1961 @ Null fill to absolute position.
1963 Each letter may optionally be followed by a number which gives a repeat
1964 count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
1965 pack function will gobble up that many values from the LIST. A * for the
1966 repeat count means to use however many items are left. The "a" and "A"
1967 types gobble just one value, but pack it as a string of length count,
1968 padding with nulls or spaces as necessary. (When unpacking, "A" strips
1969 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
1970 fields pack a string that many bits long. The "h" and "H" fields pack a
1971 string that many nybbles long. The "P" packs a pointer to a structure of
1972 the size indicated by the length. Real numbers (floats and doubles) are
1973 in the native machine format only; due to the multiplicity of floating
1974 formats around, and the lack of a standard "network" representation, no
1975 facility for interchange has been made. This means that packed floating
1976 point data written on one machine may not be readable on another - even if
1977 both use IEEE floating point arithmetic (as the endian-ness of the memory
1978 representation is not part of the IEEE spec). Note that Perl uses doubles
1979 internally for all numeric calculation, and converting from double into
1980 float and thence back to double again will lose precision (i.e.
1981 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
1985 $foo = pack("cccc",65,66,67,68);
1987 $foo = pack("c4",65,66,67,68);
1990 $foo = pack("ccxxcc",65,66,67,68);
1993 $foo = pack("s2",1,2);
1994 # "\1\0\2\0" on little-endian
1995 # "\0\1\0\2" on big-endian
1997 $foo = pack("a4","abcd","x","y","z");
2000 $foo = pack("aaaa","abcd","x","y","z");
2003 $foo = pack("a14","abcdefg");
2004 # "abcdefg\0\0\0\0\0\0\0"
2006 $foo = pack("i9pl", gmtime);
2007 # a real struct tm (on my system anyway)
2010 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2013 The same template may generally also be used in the unpack function.
2015 =item package NAMESPACE
2017 Declares the compilation unit as being in the given namespace. The scope
2018 of the package declaration is from the declaration itself through the end of
2019 the enclosing block (the same scope as the local() operator). All further
2020 unqualified dynamic identifiers will be in this namespace. A package
2021 statement only affects dynamic variables--including those you've used
2022 local() on--but I<not> lexical variables created with my(). Typically it
2023 would be the first declaration in a file to be included by the C<require>
2024 or C<use> operator. You can switch into a package in more than one place;
2025 it merely influences which symbol table is used by the compiler for the
2026 rest of that block. You can refer to variables and filehandles in other
2027 packages by prefixing the identifier with the package name and a double
2028 colon: C<$Package::Variable>. If the package name is null, the C<main>
2029 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2031 See L<perlmod/"Packages"> for more information about packages, modules,
2032 and classes. See L<perlsub> for other scoping issues.
2034 =item pipe READHANDLE,WRITEHANDLE
2036 Opens a pair of connected pipes like the corresponding system call.
2037 Note that if you set up a loop of piped processes, deadlock can occur
2038 unless you are very careful. In addition, note that Perl's pipes use
2039 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2040 after each command, depending on the application.
2042 See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
2043 for examples of such things.
2047 Pops and returns the last value of the array, shortening the array by
2048 1. Has a similar effect to
2050 $tmp = $ARRAY[$#ARRAY--];
2052 If there are no elements in the array, returns the undefined value.
2053 If ARRAY is omitted, pops the
2054 @ARGV array in the main program, and the @_ array in subroutines, just
2061 Returns the offset of where the last C<m//g> search left off for the variable
2062 is in question ($_ is used when the variable is not specified). May be
2063 modified to change that offset.
2065 =item print FILEHANDLE LIST
2071 Prints a string or a comma-separated list of strings. Returns TRUE
2072 if successful. FILEHANDLE may be a scalar variable name, in which case
2073 the variable contains the name of or a reference to the filehandle, thus introducing one
2074 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2075 token is a term, it may be misinterpreted as an operator unless you
2076 interpose a + or put parens around the arguments.) If FILEHANDLE is
2077 omitted, prints by default to standard output (or to the last selected
2078 output channel--see L</select>). If LIST is also omitted, prints $_ to
2079 STDOUT. To set the default output channel to something other than
2080 STDOUT use the select operation. Note that, because print takes a
2081 LIST, anything in the LIST is evaluated in a list context, and any
2082 subroutine that you call will have one or more of its expressions
2083 evaluated in a list context. Also be careful not to follow the print
2084 keyword with a left parenthesis unless you want the corresponding right
2085 parenthesis to terminate the arguments to the print--interpose a + or
2086 put parens around all the arguments.
2088 Note that if you're storing FILEHANDLES in an array or other expression,
2089 you will have to use a block returning its value instead:
2091 print { $files[$i] } "stuff\n";
2092 print { $OK ? STDOUT : STDERR } "stuff\n";
2094 =item printf FILEHANDLE LIST
2098 Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
2099 of the list will be interpreted as the printf format.
2101 =item prototype FUNCTION
2103 Returns the prototype of a function as a string (or C<undef> if the
2104 function has no prototype). FUNCTION is a reference to the the
2105 function whose prototype you want to retrieve.
2107 =item push ARRAY,LIST
2109 Treats ARRAY as a stack, and pushes the values of LIST
2110 onto the end of ARRAY. The length of ARRAY increases by the length of
2111 LIST. Has the same effect as
2114 $ARRAY[++$#ARRAY] = $value;
2117 but is more efficient. Returns the new number of elements in the array.
2127 Generalized quotes. See L<perlop>.
2129 =item quotemeta EXPR
2133 Returns the value of EXPR with with all regular expression
2134 metacharacters backslashed. This is the internal function implementing
2135 the \Q escape in double-quoted strings.
2137 If EXPR is omitted, uses $_.
2143 Returns a random fractional number between 0 and the value of EXPR.
2144 (EXPR should be positive.) If EXPR is omitted, returns a value between
2145 0 and 1. This function produces repeatable sequences unless srand()
2146 is invoked. See also srand().
2148 (Note: if your rand function consistently returns numbers that are too
2149 large or too small, then your version of Perl was probably compiled
2150 with the wrong number of RANDBITS. As a workaround, you can usually
2151 multiply EXPR by the correct power of 2 to get the range you want.
2152 This will make your script unportable, however. It's better to recompile
2155 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2157 =item read FILEHANDLE,SCALAR,LENGTH
2159 Attempts to read LENGTH bytes of data into variable SCALAR from the
2160 specified FILEHANDLE. Returns the number of bytes actually read, or
2161 undef if there was an error. SCALAR will be grown or shrunk to the
2162 length actually read. An OFFSET may be specified to place the read
2163 data at some other place than the beginning of the string. This call
2164 is actually implemented in terms of stdio's fread call. To get a true
2165 read system call, see sysread().
2167 =item readdir DIRHANDLE
2169 Returns the next directory entry for a directory opened by opendir().
2170 If used in a list context, returns all the rest of the entries in the
2171 directory. If there are no more entries, returns an undefined value in
2172 a scalar context or a null list in a list context.
2174 If you're planning to filetest the return values out of a readdir(), you'd
2175 better prepend the directory in question. Otherwise, since we didn't
2176 chdir() there, it would have been testing the wrong file.
2178 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2179 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2186 Returns the value of a symbolic link, if symbolic links are
2187 implemented. If not, gives a fatal error. If there is some system
2188 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2191 =item recv SOCKET,SCALAR,LEN,FLAGS
2193 Receives a message on a socket. Attempts to receive LENGTH bytes of
2194 data into variable SCALAR from the specified SOCKET filehandle.
2195 Actually does a C recvfrom(), so that it can returns the address of the
2196 sender. Returns the undefined value if there's an error. SCALAR will
2197 be grown or shrunk to the length actually read. Takes the same flags
2198 as the system call of the same name.
2199 See L<perlipc/"UDP: Message Passing"> for examples.
2205 The C<redo> command restarts the loop block without evaluating the
2206 conditional again. The C<continue> block, if any, is not executed. If
2207 the LABEL is omitted, the command refers to the innermost enclosing
2208 loop. This command is normally used by programs that want to lie to
2209 themselves about what was just input:
2211 # a simpleminded Pascal comment stripper
2212 # (warning: assumes no { or } in strings)
2213 LINE: while (<STDIN>) {
2214 while (s|({.*}.*){.*}|$1 |) {}
2219 if (/}/) { # end of comment?
2232 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2233 is not specified, $_ will be used. The value returned depends on the
2234 type of thing the reference is a reference to.
2235 Builtin types include:
2244 If the referenced object has been blessed into a package, then that package
2245 name is returned instead. You can think of ref() as a typeof() operator.
2247 if (ref($r) eq "HASH") {
2248 print "r is a reference to an associative array.\n";
2251 print "r is not a reference at all.\n";
2254 See also L<perlref>.
2256 =item rename OLDNAME,NEWNAME
2258 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2259 not work across filesystem boundaries.
2265 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2266 supplied. If EXPR is numeric, demands that the current version of Perl
2267 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2269 Otherwise, demands that a library file be included if it hasn't already
2270 been included. The file is included via the do-FILE mechanism, which is
2271 essentially just a variety of eval(). Has semantics similar to the following
2275 local($filename) = @_;
2276 return 1 if $INC{$filename};
2277 local($realfilename,$result);
2279 foreach $prefix (@INC) {
2280 $realfilename = "$prefix/$filename";
2281 if (-f $realfilename) {
2282 $result = do $realfilename;
2286 die "Can't find $filename in \@INC";
2289 die "$filename did not return true value" unless $result;
2290 $INC{$filename} = $realfilename;
2294 Note that the file will not be included twice under the same specified
2295 name. The file must return TRUE as the last statement to indicate
2296 successful execution of any initialization code, so it's customary to
2297 end such a file with "1;" unless you're sure it'll return TRUE
2298 otherwise. But it's better just to put the "C<1;>", in case you add more
2301 If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2302 replaces "F<::>" with "F</>" in the filename for you,
2303 to make it easy to load standard modules. This form of loading of
2304 modules does not risk altering your namespace.
2306 For a yet-more-powerful import facility, see L</use> and
2313 Generally used in a C<continue> block at the end of a loop to clear
2314 variables and reset ?? searches so that they work again. The
2315 expression is interpreted as a list of single characters (hyphens
2316 allowed for ranges). All variables and arrays beginning with one of
2317 those letters are reset to their pristine state. If the expression is
2318 omitted, one-match searches (?pattern?) are reset to match again. Only
2319 resets variables or searches in the current package. Always returns
2322 reset 'X'; # reset all X variables
2323 reset 'a-z'; # reset lower case variables
2324 reset; # just reset ?? searches
2326 Resetting "A-Z" is not recommended since you'll wipe out your
2327 ARGV and ENV arrays. Only resets package variables--lexical variables
2328 are unaffected, but they clean themselves up on scope exit anyway,
2329 so you'll probably want to use them instead. See L</my>.
2333 Returns from a subroutine or eval with the value specified. (Note that
2334 in the absence of a return a subroutine or eval() will automatically
2335 return the value of the last expression evaluated.)
2339 In a list context, returns a list value consisting of the elements
2340 of LIST in the opposite order. In a scalar context, returns a string
2341 value consisting of the bytes of the first element of LIST in the
2344 print reverse <>; # line tac
2347 print scalar reverse scalar <>; # byte tac
2349 =item rewinddir DIRHANDLE
2351 Sets the current position to the beginning of the directory for the
2352 readdir() routine on DIRHANDLE.
2354 =item rindex STR,SUBSTR,POSITION
2356 =item rindex STR,SUBSTR
2358 Works just like index except that it returns the position of the LAST
2359 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2360 last occurrence at or before that position.
2362 =item rmdir FILENAME
2366 Deletes the directory specified by FILENAME if it is empty. If it
2367 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2368 FILENAME is omitted, uses $_.
2372 The substitution operator. See L<perlop>.
2376 Forces EXPR to be interpreted in a scalar context and returns the value
2379 @counts = ( scalar @a, scalar @b, scalar @c );
2381 There is no equivalent operator to force an expression to
2382 be interpolated in a list context because it's in practice never
2383 needed. If you really wanted to do so, however, you could use
2384 the construction C<@{[ (some expression) ]}>, but usually a simple
2385 C<(some expression)> suffices.
2387 =item seek FILEHANDLE,POSITION,WHENCE
2389 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2390 call of stdio. FILEHANDLE may be an expression whose value gives the name
2391 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2392 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2393 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2394 this from POSIX module. Returns 1 upon success, 0 otherwise.
2396 On some systems you have to do a seek whenever you switch between reading
2397 and writing. Amongst other things, this may have the effect of calling
2398 stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2403 This is also useful for applications emulating C<tail -f>. Once you hit
2404 EOF on your read, and then sleep for a while, you might have to stick in a
2405 seek() to reset things. First the simple trick listed above to clear the
2406 filepointer. The seek() doesn't change the current position, but it
2407 I<does> clear the end-of-file condition on the handle, so that the next
2408 C<E<lt>FILEE<gt>> makes Perl try again to read something. Hopefully.
2410 If that doesn't work (some stdios are particularly cantankerous), then
2411 you may need something more like this:
2414 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2415 # search for some stuff and put it into files
2417 sleep($for_a_while);
2418 seek(FILE, $curpos, 0);
2421 =item seekdir DIRHANDLE,POS
2423 Sets the current position for the readdir() routine on DIRHANDLE. POS
2424 must be a value returned by telldir(). Has the same caveats about
2425 possible directory compaction as the corresponding system library
2428 =item select FILEHANDLE
2432 Returns the currently selected filehandle. Sets the current default
2433 filehandle for output, if FILEHANDLE is supplied. This has two
2434 effects: first, a C<write> or a C<print> without a filehandle will
2435 default to this FILEHANDLE. Second, references to variables related to
2436 output will refer to this output channel. For example, if you have to
2437 set the top of form format for more than one output channel, you might
2445 FILEHANDLE may be an expression whose value gives the name of the
2446 actual filehandle. Thus:
2448 $oldfh = select(STDERR); $| = 1; select($oldfh);
2450 Some programmers may prefer to think of filehandles as objects with
2451 methods, preferring to write the last example as:
2454 STDERR->autoflush(1);
2456 =item select RBITS,WBITS,EBITS,TIMEOUT
2458 This calls the select(2) system call with the bitmasks specified, which
2459 can be constructed using fileno() and vec(), along these lines:
2461 $rin = $win = $ein = '';
2462 vec($rin,fileno(STDIN),1) = 1;
2463 vec($win,fileno(STDOUT),1) = 1;
2466 If you want to select on many filehandles you might wish to write a
2470 local(@fhlist) = split(' ',$_[0]);
2473 vec($bits,fileno($_),1) = 1;
2477 $rin = fhbits('STDIN TTY SOCK');
2481 ($nfound,$timeleft) =
2482 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2484 or to block until something becomes ready just do this
2486 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2488 Most systems do not both to return anything useful in $timeleft, so
2489 calling select() in a scalar context just returns $nfound.
2491 Any of the bitmasks can also be undef. The timeout, if specified, is
2492 in seconds, which may be fractional. Note: not all implementations are
2493 capable of returning the $timeleft. If not, they always return
2494 $timeleft equal to the supplied $timeout.
2496 You can effect a sleep of 250 milliseconds this way:
2498 select(undef, undef, undef, 0.25);
2500 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2501 with select(). You have to use sysread() instead.
2503 =item semctl ID,SEMNUM,CMD,ARG
2505 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2506 &GETALL, then ARG must be a variable which will hold the returned
2507 semid_ds structure or semaphore value array. Returns like ioctl: the
2508 undefined value for error, "0 but true" for zero, or the actual return
2511 =item semget KEY,NSEMS,FLAGS
2513 Calls the System V IPC function semget. Returns the semaphore id, or
2514 the undefined value if there is an error.
2516 =item semop KEY,OPSTRING
2518 Calls the System V IPC function semop to perform semaphore operations
2519 such as signaling and waiting. OPSTRING must be a packed array of
2520 semop structures. Each semop structure can be generated with
2521 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2522 operations is implied by the length of OPSTRING. Returns TRUE if
2523 successful, or FALSE if there is an error. As an example, the
2524 following code waits on semaphore $semnum of semaphore id $semid:
2526 $semop = pack("sss", $semnum, -1, 0);
2527 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2529 To signal the semaphore, replace "-1" with "1".
2531 =item send SOCKET,MSG,FLAGS,TO
2533 =item send SOCKET,MSG,FLAGS
2535 Sends a message on a socket. Takes the same flags as the system call
2536 of the same name. On unconnected sockets you must specify a
2537 destination to send TO, in which case it does a C sendto(). Returns
2538 the number of characters sent, or the undefined value if there is an
2540 See L<perlipc/"UDP: Message Passing"> for examples.
2542 =item setpgrp PID,PGRP
2544 Sets the current process group for the specified PID, 0 for the current
2545 process. Will produce a fatal error if used on a machine that doesn't
2546 implement setpgrp(2). If the arguments are ommitted, it defaults to
2547 0,0. Note that the POSIX version of setpgrp() does not accept any
2548 arguments, so only setpgrp 0,0 is portable.
2550 =item setpriority WHICH,WHO,PRIORITY
2552 Sets the current priority for a process, a process group, or a user.
2553 (See setpriority(2).) Will produce a fatal error if used on a machine
2554 that doesn't implement setpriority(2).
2556 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2558 Sets the socket option requested. Returns undefined if there is an
2559 error. OPTVAL may be specified as undef if you don't want to pass an
2566 Shifts the first value of the array off and returns it, shortening the
2567 array by 1 and moving everything down. If there are no elements in the
2568 array, returns the undefined value. If ARRAY is omitted, shifts the
2569 @ARGV array in the main program, and the @_ array in subroutines.
2570 (This is determined lexically.) See also unshift(), push(), and pop().
2571 Shift() and unshift() do the same thing to the left end of an array
2572 that push() and pop() do to the right end.
2574 =item shmctl ID,CMD,ARG
2576 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2577 must be a variable which will hold the returned shmid_ds structure.
2578 Returns like ioctl: the undefined value for error, "0 but true" for
2579 zero, or the actual return value otherwise.
2581 =item shmget KEY,SIZE,FLAGS
2583 Calls the System V IPC function shmget. Returns the shared memory
2584 segment id, or the undefined value if there is an error.
2586 =item shmread ID,VAR,POS,SIZE
2588 =item shmwrite ID,STRING,POS,SIZE
2590 Reads or writes the System V shared memory segment ID starting at
2591 position POS for size SIZE by attaching to it, copying in/out, and
2592 detaching from it. When reading, VAR must be a variable which will
2593 hold the data read. When writing, if STRING is too long, only SIZE
2594 bytes are used; if STRING is too short, nulls are written to fill out
2595 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2597 =item shutdown SOCKET,HOW
2599 Shuts down a socket connection in the manner indicated by HOW, which
2600 has the same interpretation as in the system call of the same name.
2606 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2613 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2614 May be interrupted by sending the process a SIGALRM. Returns the
2615 number of seconds actually slept. You probably cannot mix alarm() and
2616 sleep() calls, since sleep() is often implemented using alarm().
2618 On some older systems, it may sleep up to a full second less than what
2619 you requested, depending on how it counts seconds. Most modern systems
2620 always sleep the full amount.
2622 For delays of finer granularity than one second, you may use Perl's
2623 syscall() interface to access setitimer(2) if your system supports it,
2624 or else see L</select()> below.
2626 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2628 Opens a socket of the specified kind and attaches it to filehandle
2629 SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
2630 system call of the same name. You should "use Socket;" first to get
2631 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2633 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2635 Creates an unnamed pair of sockets in the specified domain, of the
2636 specified type. DOMAIN, TYPE and PROTOCOL are specified the same as
2637 for the system call of the same name. If unimplemented, yields a fatal
2638 error. Returns TRUE if successful.
2640 =item sort SUBNAME LIST
2642 =item sort BLOCK LIST
2646 Sorts the LIST and returns the sorted list value. Nonexistent values
2647 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2648 in standard string comparison order. If SUBNAME is specified, it
2649 gives the name of a subroutine that returns an integer less than, equal
2650 to, or greater than 0, depending on how the elements of the array are
2651 to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
2652 routines.) SUBNAME may be a scalar variable name, in which case the
2653 value provides the name of the subroutine to use. In place of a
2654 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2657 In the interests of efficiency the normal calling code for subroutines is
2658 bypassed, with the following effects: the subroutine may not be a
2659 recursive subroutine, and the two elements to be compared are passed into
2660 the subroutine not via @_ but as the package global variables $a and
2661 $b (see example below). They are passed by reference, so don't
2662 modify $a and $b. And don't try to declare them as lexicals either.
2667 @articles = sort @files;
2669 # same thing, but with explicit sort routine
2670 @articles = sort {$a cmp $b} @files;
2672 # now case-insensitively
2673 @articles = sort { uc($a) cmp uc($b)} @files;
2675 # same thing in reversed order
2676 @articles = sort {$b cmp $a} @files;
2678 # sort numerically ascending
2679 @articles = sort {$a <=> $b} @files;
2681 # sort numerically descending
2682 @articles = sort {$b <=> $a} @files;
2684 # sort using explicit subroutine name
2686 $age{$a} <=> $age{$b}; # presuming integers
2688 @sortedclass = sort byage @class;
2690 # this sorts the %age associative arrays by value
2691 # instead of key using an inline function
2692 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2694 sub backwards { $b cmp $a; }
2695 @harry = ('dog','cat','x','Cain','Abel');
2696 @george = ('gone','chased','yz','Punished','Axed');
2698 # prints AbelCaincatdogx
2699 print sort backwards @harry;
2700 # prints xdogcatCainAbel
2701 print sort @george, 'to', @harry;
2702 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2704 # inefficiently sort by descending numeric compare using
2705 # the first integer after the first = sign, or the
2706 # whole record case-insensitively otherwise
2709 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2714 # same thing, but much more efficiently;
2715 # we'll build auxiliary indices instead
2719 push @nums, /=(\d+)/;
2724 $nums[$b] <=> $nums[$a]
2726 $caps[$a] cmp $caps[$b]
2730 # same thing using a Schwartzian Transform (no temps)
2731 @new = map { $_->[0] }
2732 sort { $b->[1] <=> $a->[1]
2735 } map { [$_, /=(\d+)/, uc($_)] } @old;
2737 If you're using strict, you I<MUST NOT> declare $a
2738 and $b as lexicals. They are package globals. That means
2739 if you're in the C<main> package, it's
2741 @articles = sort {$main::b <=> $main::a} @files;
2745 @articles = sort {$::b <=> $::a} @files;
2747 but if you're in the C<FooPack> package, it's
2749 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2751 The comparison function is required to behave. If it returns
2752 inconsistent results (sometimes saying $x[1] is less than $x[2] and
2753 sometimes saying the opposite, for example) the Perl interpreter will
2754 probably crash and dump core. This is entirely due to and dependent
2755 upon your system's qsort(3) library routine; this routine often avoids
2756 sanity checks in the interest of speed.
2758 =item splice ARRAY,OFFSET,LENGTH,LIST
2760 =item splice ARRAY,OFFSET,LENGTH
2762 =item splice ARRAY,OFFSET
2764 Removes the elements designated by OFFSET and LENGTH from an array, and
2765 replaces them with the elements of LIST, if any. Returns the elements
2766 removed from the array. The array grows or shrinks as necessary. If
2767 LENGTH is omitted, removes everything from OFFSET onward. The
2768 following equivalencies hold (assuming C<$[ == 0>):
2770 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2771 pop(@a) splice(@a,-1)
2772 shift(@a) splice(@a,0,1)
2773 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2774 $a[$x] = $y splice(@a,$x,1,$y);
2776 Example, assuming array lengths are passed before arrays:
2778 sub aeq { # compare two list values
2779 local(@a) = splice(@_,0,shift);
2780 local(@b) = splice(@_,0,shift);
2781 return 0 unless @a == @b; # same len?
2783 return 0 if pop(@a) ne pop(@b);
2787 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2789 =item split /PATTERN/,EXPR,LIMIT
2791 =item split /PATTERN/,EXPR
2793 =item split /PATTERN/
2797 Splits a string into an array of strings, and returns it.
2799 If not in a list context, returns the number of fields found and splits into
2800 the @_ array. (In a list context, you can force the split into @_ by
2801 using C<??> as the pattern delimiters, but it still returns the array
2802 value.) The use of implicit split to @_ is deprecated, however.
2804 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2805 splits on whitespace (after skipping any leading whitespace). Anything
2806 matching PATTERN is taken to be a delimiter separating the fields. (Note
2807 that the delimiter may be longer than one character.) If LIMIT is
2808 specified and is not negative, splits into no more than that many fields
2809 (though it may split into fewer). If LIMIT is unspecified, trailing null
2810 fields are stripped (which potential users of pop() would do well to
2811 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2812 LIMIT had been specified.
2814 A pattern matching the null string (not to be confused with
2815 a null pattern C<//>, which is just one member of the set of patterns
2816 matching a null string) will split the value of EXPR into separate
2817 characters at each point it matches that way. For example:
2819 print join(':', split(/ */, 'hi there'));
2821 produces the output 'h:i:t:h:e:r:e'.
2823 The LIMIT parameter can be used to partially split a line
2825 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2827 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2828 one larger than the number of variables in the list, to avoid
2829 unnecessary work. For the list above LIMIT would have been 4 by
2830 default. In time critical applications it behooves you not to split
2831 into more fields than you really need.
2833 If the PATTERN contains parentheses, additional array elements are
2834 created from each matching substring in the delimiter.
2836 split(/([,-])/, "1-10,20", 3);
2838 produces the list value
2840 (1, '-', 10, ',', 20)
2842 If you had the entire header of a normal Unix email message in $header,
2843 you could split it up into fields and their values this way:
2845 $header =~ s/\n\s+/ /g; # fix continuation lines
2846 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2848 The pattern C</PATTERN/> may be replaced with an expression to specify
2849 patterns that vary at runtime. (To do runtime compilation only once,
2850 use C</$variable/o>.)
2852 As a special case, specifying a PATTERN of space (C<' '>) will split on
2853 white space just as split with no arguments does. Thus, split(' ') can
2854 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2855 will give you as many null initial fields as there are leading spaces.
2856 A split on /\s+/ is like a split(' ') except that any leading
2857 whitespace produces a null first field. A split with no arguments
2858 really does a C<split(' ', $_)> internally.
2862 open(passwd, '/etc/passwd');
2864 ($login, $passwd, $uid, $gid, $gcos,
2865 $home, $shell) = split(/:/);
2869 (Note that $shell above will still have a newline on it. See L</chop>,
2870 L</chomp>, and L</join>.)
2872 =item sprintf FORMAT,LIST
2874 Returns a string formatted by the usual printf conventions of the C
2875 language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2876 (The * character for an indirectly specified length is not
2877 supported, but you can get the same effect by interpolating a variable
2878 into the pattern.) Some C libraries' implementations of sprintf() can
2879 dump core when fed ludicrous arguments.
2885 Return the square root of EXPR. If EXPR is omitted, returns square
2890 Sets the random number seed for the C<rand> operator. If EXPR is omitted,
2891 uses a semirandom value based on the current time and process ID, among
2892 other things. Of course, you'd need something much more random than that for
2893 cryptographic purposes, since it's easy to guess the current time.
2894 Checksumming the compressed output of rapidly changing operating system
2895 status programs is the usual method. Examples are posted regularly to
2896 the comp.security.unix newsgroup.
2898 =item stat FILEHANDLE
2904 Returns a 13-element array giving the status info for a file, either the
2905 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
2906 stats $_. Returns a null list if the stat fails. Typically used as
2910 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2911 $atime,$mtime,$ctime,$blksize,$blocks)
2914 Not all fields are supported on all filesystem types. Here are the
2915 meaning of the fields:
2917 dev device number of filesystem
2919 mode file mode (type and permissions)
2920 nlink number of (hard) links to the file
2921 uid numeric user ID of file's owner
2922 gid numer group ID of file's owner
2923 rdev the device identifier (special files only)
2924 size total size of file, in bytes
2925 atime last access time since the epoch
2926 mtime last modify time since the epoch
2927 ctime inode change time (NOT creation type!) since the epoch
2928 blksize preferred blocksize for file system I/O
2929 blocks actual number of blocks allocated
2931 (The epoch was at 00:00 January 1, 1970 GMT.)
2933 If stat is passed the special filehandle consisting of an underline, no
2934 stat is done, but the current contents of the stat structure from the
2935 last stat or filetest are returned. Example:
2937 if (-x $file && (($d) = stat(_)) && $d < 0) {
2938 print "$file is executable NFS file\n";
2941 (This only works on machines for which the device number is negative under NFS.)
2947 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
2948 doing many pattern matches on the string before it is next modified.
2949 This may or may not save time, depending on the nature and number of
2950 patterns you are searching on, and on the distribution of character
2951 frequencies in the string to be searched--you probably want to compare
2952 runtimes with and without it to see which runs faster. Those loops
2953 which scan for many short constant strings (including the constant
2954 parts of more complex patterns) will benefit most. You may have only
2955 one study active at a time--if you study a different scalar the first
2956 is "unstudied". (The way study works is this: a linked list of every
2957 character in the string to be searched is made, so we know, for
2958 example, where all the 'k' characters are. From each search string,
2959 the rarest character is selected, based on some static frequency tables
2960 constructed from some C programs and English text. Only those places
2961 that contain this "rarest" character are examined.)
2963 For example, here is a loop which inserts index producing entries
2964 before any line containing a certain pattern:
2968 print ".IX foo\n" if /\bfoo\b/;
2969 print ".IX bar\n" if /\bbar\b/;
2970 print ".IX blurfl\n" if /\bblurfl\b/;
2975 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
2976 will be looked at, because "f" is rarer than "o". In general, this is
2977 a big win except in pathological cases. The only question is whether
2978 it saves you more time than it took to build the linked list in the
2981 Note that if you have to look for strings that you don't know till
2982 runtime, you can build an entire loop as a string and eval that to
2983 avoid recompiling all your patterns all the time. Together with
2984 undefining $/ to input entire files as one record, this can be very
2985 fast, often faster than specialized programs like fgrep(1). The following
2986 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
2987 out the names of those files that contain a match:
2989 $search = 'while (<>) { study;';
2990 foreach $word (@words) {
2991 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
2996 eval $search; # this screams
2997 $/ = "\n"; # put back to normal input delim
2998 foreach $file (sort keys(%seen)) {
3006 =item sub NAME BLOCK
3008 This is subroutine definition, not a real function I<per se>. With just a
3009 NAME (and possibly prototypes), it's just a forward declaration. Without
3010 a NAME, it's an anonymous function declaration, and does actually return a
3011 value: the CODE ref of the closure you just created. See L<perlsub> and
3012 L<perlref> for details.
3014 =item substr EXPR,OFFSET,LEN
3016 =item substr EXPR,OFFSET
3018 Extracts a substring out of EXPR and returns it. First character is at
3019 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
3020 that far from the end of the string. If LEN is omitted, returns
3021 everything to the end of the string. If LEN is negative, leaves that
3022 many characters off the end of the string.
3024 You can use the substr() function
3025 as an lvalue, in which case EXPR must be an lvalue. If you assign
3026 something shorter than LEN, the string will shrink, and if you assign
3027 something longer than LEN, the string will grow to accommodate it. To
3028 keep the string the same length you may need to pad or chop your value
3031 =item symlink OLDFILE,NEWFILE
3033 Creates a new filename symbolically linked to the old filename.
3034 Returns 1 for success, 0 otherwise. On systems that don't support
3035 symbolic links, produces a fatal error at run time. To check for that,
3038 $symlink_exists = (eval 'symlink("","");', $@ eq '');
3042 Calls the system call specified as the first element of the list,
3043 passing the remaining elements as arguments to the system call. If
3044 unimplemented, produces a fatal error. The arguments are interpreted
3045 as follows: if a given argument is numeric, the argument is passed as
3046 an int. If not, the pointer to the string value is passed. You are
3047 responsible to make sure a string is pre-extended long enough to
3048 receive any result that might be written into a string. If your
3049 integer arguments are not literals and have never been interpreted in a
3050 numeric context, you may need to add 0 to them to force them to look
3053 require 'syscall.ph'; # may need to run h2ph
3054 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3056 Note that Perl only supports passing of up to 14 arguments to your system call,
3057 which in practice should usually suffice.
3059 =item sysopen FILEHANDLE,FILENAME,MODE
3061 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3063 Opens the file whose filename is given by FILENAME, and associates it
3064 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3065 the name of the real filehandle wanted. This function calls the
3066 underlying operating system's C<open> function with the parameters
3067 FILENAME, MODE, PERMS.
3069 The possible values and flag bits of the MODE parameter are
3070 system-dependent; they are available via the standard module C<Fcntl>.
3071 However, for historical reasons, some values are universal: zero means
3072 read-only, one means write-only, and two means read/write.
3074 If the file named by FILENAME does not exist and the C<open> call
3075 creates it (typically because MODE includes the O_CREAT flag), then
3076 the value of PERMS specifies the permissions of the newly created
3077 file. If PERMS is omitted, the default value is 0666, which allows
3078 read and write for all. This default is reasonable: see C<umask>.
3080 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3082 =item sysread FILEHANDLE,SCALAR,LENGTH
3084 Attempts to read LENGTH bytes of data into variable SCALAR from the
3085 specified FILEHANDLE, using the system call read(2). It bypasses
3086 stdio, so mixing this with other kinds of reads may cause confusion.
3087 Returns the number of bytes actually read, or undef if there was an
3088 error. SCALAR will be grown or shrunk so that the last byte actually
3089 read is the last byte of the scalar after the read.
3091 An OFFSET may be specified to place the read data at some place in the
3092 string other than the beginning. A negative OFFSET specifies
3093 placement at that many bytes counting backwards from the end of the
3094 string. A positive OFFSET greater than the length of SCALAR results
3095 in the string being padded to the required size with "\0" bytes before
3096 the result of the read is appended.
3100 Does exactly the same thing as "exec LIST" except that a fork is done
3101 first, and the parent process waits for the child process to complete.
3102 Note that argument processing varies depending on the number of
3103 arguments. The return value is the exit status of the program as
3104 returned by the wait() call. To get the actual exit value divide by
3105 256. See also L</exec>. This is I<NOT> what you want to use to capture
3106 the output from a command, for that you should merely use backticks, as
3107 described in L<perlop/"`STRING`">.
3109 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3111 =item syswrite FILEHANDLE,SCALAR,LENGTH
3113 Attempts to write LENGTH bytes of data from variable SCALAR to the
3114 specified FILEHANDLE, using the system call write(2). It bypasses
3115 stdio, so mixing this with prints may cause confusion. Returns the
3116 number of bytes actually written, or undef if there was an error.
3117 If the length is greater than the available data, only as much data as
3118 is available will be written.
3120 An OFFSET may be specified to write the data from some part of the
3121 string other than the beginning. A negative OFFSET specifies writing
3122 from that many bytes counting backwards from the end of the string.
3124 =item tell FILEHANDLE
3128 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3129 expression whose value gives the name of the actual filehandle. If
3130 FILEHANDLE is omitted, assumes the file last read.
3132 =item telldir DIRHANDLE
3134 Returns the current position of the readdir() routines on DIRHANDLE.
3135 Value may be given to seekdir() to access a particular location in a
3136 directory. Has the same caveats about possible directory compaction as
3137 the corresponding system library routine.
3139 =item tie VARIABLE,CLASSNAME,LIST
3141 This function binds a variable to a package class that will provide the
3142 implementation for the variable. VARIABLE is the name of the variable
3143 to be enchanted. CLASSNAME is the name of a class implementing objects
3144 of correct type. Any additional arguments are passed to the "new"
3145 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3146 Typically these are arguments such as might be passed to the dbm_open()
3147 function of C. The object returned by the "new" method is also
3148 returned by the tie() function, which would be useful if you want to
3149 access other methods in CLASSNAME.
3151 Note that functions such as keys() and values() may return huge array
3152 values when used on large objects, like DBM files. You may prefer to
3153 use the each() function to iterate over such. Example:
3155 # print out history file offsets
3157 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3158 while (($key,$val) = each %HIST) {
3159 print $key, ' = ', unpack('L',$val), "\n";
3163 A class implementing an associative array should have the following
3166 TIEHASH classname, LIST
3169 STORE this, key, value
3173 NEXTKEY this, lastkey
3175 A class implementing an ordinary array should have the following methods:
3177 TIEARRAY classname, LIST
3180 STORE this, key, value
3183 A class implementing a scalar should have the following methods:
3185 TIESCALAR classname, LIST
3190 Unlike dbmopen(), the tie() function will not use or require a module
3191 for you--you need to do that explicitly yourself. See L<DB_File>
3192 or the F<Config> module for interesting tie() implementations.
3196 Returns a reference to the object underlying VARIABLE (the same value
3197 that was originally returned by the tie() call which bound the variable
3198 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3203 Returns the number of non-leap seconds since whatever time the system
3204 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3205 and 00:00:00 UTC, January 1, 1970 for most other systems).
3206 Suitable for feeding to gmtime() and localtime().
3210 Returns a four-element array giving the user and system times, in
3211 seconds, for this process and the children of this process.
3213 ($user,$system,$cuser,$csystem) = times;
3217 The translation operator. See L<perlop>.
3219 =item truncate FILEHANDLE,LENGTH
3221 =item truncate EXPR,LENGTH
3223 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3224 specified length. Produces a fatal error if truncate isn't implemented
3231 Returns an uppercased version of EXPR. This is the internal function
3232 implementing the \U escape in double-quoted strings.
3233 Should respect any POSIX setlocale() settings.
3235 If EXPR is omitted, uses $_.
3241 Returns the value of EXPR with the first character uppercased. This is
3242 the internal function implementing the \u escape in double-quoted strings.
3243 Should respect any POSIX setlocale() settings.
3245 If EXPR is omitted, uses $_.
3251 Sets the umask for the process and returns the old one. If EXPR is
3252 omitted, merely returns current umask.
3258 Undefines the value of EXPR, which must be an lvalue. Use only on a
3259 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3260 will probably not do what you expect on most predefined variables or
3261 DBM list values, so don't do that.) Always returns the undefined value. You can omit
3262 the EXPR, in which case nothing is undefined, but you still get an
3263 undefined value that you could, for instance, return from a
3264 subroutine. Examples:
3267 undef $bar{'blurfl'};
3271 return (wantarray ? () : undef) if $they_blew_it;
3277 Deletes a list of files. Returns the number of files successfully
3280 $cnt = unlink 'a', 'b', 'c';
3284 Note: unlink will not delete directories unless you are superuser and
3285 the B<-U> flag is supplied to Perl. Even if these conditions are
3286 met, be warned that unlinking a directory can inflict damage on your
3287 filesystem. Use rmdir instead.
3289 If LIST is omitted, uses $_.
3291 =item unpack TEMPLATE,EXPR
3293 Unpack does the reverse of pack: it takes a string representing a
3294 structure and expands it out into a list value, returning the array
3295 value. (In a scalar context, it merely returns the first value
3296 produced.) The TEMPLATE has the same format as in the pack function.
3297 Here's a subroutine that does substring:
3300 local($what,$where,$howmuch) = @_;
3301 unpack("x$where a$howmuch", $what);
3306 sub ordinal { unpack("c",$_[0]); } # same as ord()
3308 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3309 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3310 themselves. Default is a 16-bit checksum. For example, the following
3311 computes the same number as the System V sum program:
3314 $checksum += unpack("%16C*", $_);
3318 The following efficiently counts the number of set bits in a bit vector:
3320 $setbits = unpack("%32b*", $selectmask);
3322 =item untie VARIABLE
3324 Breaks the binding between a variable and a package. (See tie().)
3326 =item unshift ARRAY,LIST
3328 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3329 depending on how you look at it. Prepends list to the front of the
3330 array, and returns the new number of elements in the array.
3332 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3334 Note the LIST is prepended whole, not one element at a time, so the
3335 prepended elements stay in the same order. Use reverse to do the
3338 =item use Module LIST
3342 =item use Module VERSION LIST
3346 Imports some semantics into the current package from the named module,
3347 generally by aliasing certain subroutine or variable names into your
3348 package. It is exactly equivalent to
3350 BEGIN { require Module; import Module LIST; }
3352 except that Module I<must> be a bare word.
3354 If the first argument to C<use> is a number, it is treated as a version
3355 number instead of a module name. If the version of the Perl interpreter
3356 is less than VERSION, then an error message is printed and Perl exits
3357 immediately. This is often useful if you need to check the current
3358 Perl version before C<use>ing library modules which have changed in
3359 incompatible ways from older versions of Perl. (We try not to do
3360 this more than we have to.)
3362 The BEGIN forces the require and import to happen at compile time. The
3363 require makes sure the module is loaded into memory if it hasn't been
3364 yet. The import is not a builtin--it's just an ordinary static method
3365 call into the "Module" package to tell the module to import the list of
3366 features back into the current package. The module can implement its
3367 import method any way it likes, though most modules just choose to
3368 derive their import method via inheritance from the Exporter class that
3369 is defined in the Exporter module. See L<Exporter>. If no import
3370 method can be found then the error is currently silently ignored. This
3371 may change to a fatal error in a future version.
3373 If you don't want your namespace altered, explicitly supply an empty list:
3377 That is exactly equivalent to
3379 BEGIN { require Module; }
3381 If the VERSION argument is present between Module and LIST, then the
3382 C<use> will fail if the C<$VERSION> variable in package Module is
3385 Because this is a wide-open interface, pragmas (compiler directives)
3386 are also implemented this way. Currently implemented pragmas are:
3390 use sigtrap qw(SEGV BUS);
3391 use strict qw(subs vars refs);
3392 use subs qw(afunc blurfl);
3394 These pseudomodules import semantics into the current block scope, unlike
3395 ordinary modules, which import symbols into the current package (which are
3396 effective through the end of the file).
3398 There's a corresponding "no" command that unimports meanings imported
3399 by use, i.e. it calls C<unimport Module LIST> instead of C<import>.
3404 If no unimport method can be found the call fails with a fatal error.
3406 See L<perlmod> for a list of standard modules and pragmas.
3410 Changes the access and modification times on each file of a list of
3411 files. The first two elements of the list must be the NUMERICAL access
3412 and modification times, in that order. Returns the number of files
3413 successfully changed. The inode modification time of each file is set
3414 to the current time. Example of a "touch" command:
3418 utime $now, $now, @ARGV;
3420 =item values ASSOC_ARRAY
3422 Returns a normal array consisting of all the values of the named
3423 associative array. (In a scalar context, returns the number of
3424 values.) The values are returned in an apparently random order, but it
3425 is the same order as either the keys() or each() function would produce
3426 on the same array. See also keys(), each(), and sort().
3428 =item vec EXPR,OFFSET,BITS
3430 Treats the string in EXPR as a vector of unsigned integers, and
3431 returns the value of the bitfield specified by OFFSET. BITS specifies
3432 the number of bits that are reserved for each entry in the bit
3433 vector. This must be a power of two from 1 to 32. vec() may also be
3434 assigned to, in which case parens are needed to give the expression
3435 the correct precedence as in
3437 vec($image, $max_x * $x + $y, 8) = 3;
3439 Vectors created with vec() can also be manipulated with the logical
3440 operators |, & and ^, which will assume a bit vector operation is
3441 desired when both operands are strings.
3443 To transform a bit vector into a string or array of 0's and 1's, use these:
3445 $bits = unpack("b*", $vector);
3446 @bits = split(//, unpack("b*", $vector));
3448 If you know the exact length in bits, it can be used in place of the *.
3452 Waits for a child process to terminate and returns the pid of the
3453 deceased process, or -1 if there are no child processes. The status is
3456 =item waitpid PID,FLAGS
3458 Waits for a particular child process to terminate and returns the pid
3459 of the deceased process, or -1 if there is no such child process. The
3460 status is returned in C<$?>. If you say
3462 use POSIX ":wait_h";
3464 waitpid(-1,&WNOHANG);
3466 then you can do a non-blocking wait for any process. Non-blocking wait
3467 is only available on machines supporting either the waitpid(2) or
3468 wait4(2) system calls. However, waiting for a particular pid with
3469 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3470 by remembering the status values of processes that have exited but have
3471 not been harvested by the Perl script yet.)
3475 Returns TRUE if the context of the currently executing subroutine is
3476 looking for a list value. Returns FALSE if the context is looking
3479 return wantarray ? () : undef;
3483 Produces a message on STDERR just like die(), but doesn't exit or
3486 =item write FILEHANDLE
3492 Writes a formatted record (possibly multi-line) to the specified file,
3493 using the format associated with that file. By default the format for
3494 a file is the one having the same name is the filehandle, but the
3495 format for the current output channel (see the select() function) may be set
3496 explicitly by assigning the name of the format to the C<$~> variable.
3498 Top of form processing is handled automatically: if there is
3499 insufficient room on the current page for the formatted record, the
3500 page is advanced by writing a form feed, a special top-of-page format
3501 is used to format the new page header, and then the record is written.
3502 By default the top-of-page format is the name of the filehandle with
3503 "_TOP" appended, but it may be dynamically set to the format of your
3504 choice by assigning the name to the C<$^> variable while the filehandle is
3505 selected. The number of lines remaining on the current page is in
3506 variable C<$->, which can be set to 0 to force a new page.
3508 If FILEHANDLE is unspecified, output goes to the current default output
3509 channel, which starts out as STDOUT but may be changed by the
3510 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3511 is evaluated and the resulting string is used to look up the name of
3512 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3514 Note that write is I<NOT> the opposite of read. Unfortunately.
3518 The translation operator. See L<perlop>.