3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar and list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one list argument.) For instance, splice() has three scalar
19 arguments followed by a list.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parentheses.) If you use the parentheses, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+3; # Prints 6.
39 print(1+2) + 3; # Prints 3.
40 print (1+2)+3; # Also prints 3!
41 print +(1+2)+3; # Prints 6.
42 print ((1+2)+3); # Prints 6.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 non-abortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following rule:
59 =item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
63 Each operator and function decides which sort of value it would be most
64 appropriate to return in a scalar context. Some operators return the
65 length of the list that would have been returned in a list context. Some
66 operators return the first value in the list. Some operators return the
67 last value in the list. Some operators return a count of successful
68 operations. In general, they do what you want, unless you want
71 =head2 Perl Functions by Category
73 Here are Perl's functions (including things that look like
74 functions, like some of the keywords and named operators)
75 arranged by category. Some functions appear in more
80 =item Functions for SCALARs or strings
82 chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
83 oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
84 sprintf, substr, tr///, uc, ucfirst, y///
86 =item Regular expressions and pattern matching
88 m//, pos, quotemeta, s///, split, study
90 =item Numeric functions
92 abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
95 =item Functions for real @ARRAYs
97 pop, push, shift, splice, unshift
99 =item Functions for list data
101 grep, join, map, qw/STRING/, reverse, sort, unpack
103 =item Functions for real %HASHes
105 delete, each, exists, keys, values
107 =item Input and output functions
109 binmode, close, closedir, dbmclose, dbmopen, die, eof,
110 fileno, flock, format, getc, print, printf, read, readdir,
111 rewinddir, seek, seekdir, select, syscall, sysread,
112 syswrite, tell, telldir, truncate, warn, write
114 =item Functions for fixed length data or records
116 pack, read, syscall, sysread, syswrite, unpack, vec
118 =item Functions for filehandles, files, or directories
120 I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
121 lstat, mkdir, open, opendir, readlink, rename, rmdir,
122 stat, symlink, umask, unlink, utime
124 =item Keywords related to the control flow of your perl program
126 caller, continue, die, do, dump, eval, exit, goto, last,
127 next, redo, return, sub, wantarray
129 =item Keywords related to scoping
131 caller, import, local, my, package, use
133 =item Miscellaneous functions
135 defined, dump, eval, formline, local, my, reset, scalar,
138 =item Functions for processes and process groups
140 alarm, exec, fork, getpgrp, getppid, getpriority, kill,
141 pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
144 =item Keywords related to perl modules
146 do, import, no, package, require, use
148 =item Keywords related to classes and object-orientedness
150 bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
152 =item Low-level socket functions
154 accept, bind, connect, getpeername, getsockname,
155 getsockopt, listen, recv, send, setsockopt, shutdown,
158 =item System V interprocess communication functions
160 msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
161 shmctl, shmget, shmread, shmwrite
163 =item Fetching user and group info
165 endgrent, endhostent, endnetent, endpwent, getgrent,
166 getgrgid, getgrnam, getlogin, getpwent, getpwnam,
167 getpwuid, setgrent, setpwent
169 =item Fetching network info
171 endprotoent, endservent, gethostbyaddr, gethostbyname,
172 gethostent, getnetbyaddr, getnetbyname, getnetent,
173 getprotobyname, getprotobynumber, getprotoent,
174 getservbyname, getservbyport, getservent, sethostent,
175 setnetent, setprotoent, setservent
177 =item Time-related functions
179 gmtime, localtime, time, times
181 =item Functions new in perl5
183 abs, bless, chomp, chr, exists, formline, glob, import, lc,
184 lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
185 ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
187 * - C<sub> was a keyword in perl4, but in perl5 it is an
188 operator which can be used in expressions.
190 =item Functions obsoleted in perl5
197 =head2 Alphabetical Listing of Perl Functions
208 A file test, where X is one of the letters listed below. This unary
209 operator takes one argument, either a filename or a filehandle, and
210 tests the associated file to see if something is true about it. If the
211 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
212 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
213 the undefined value if the file doesn't exist. Despite the funny
214 names, precedence is the same as any other named unary operator, and
215 the argument may be parenthesized like any other unary operator. The
216 operator may be any of:
218 -r File is readable by effective uid/gid.
219 -w File is writable by effective uid/gid.
220 -x File is executable by effective uid/gid.
221 -o File is owned by effective uid.
223 -R File is readable by real uid/gid.
224 -W File is writable by real uid/gid.
225 -X File is executable by real uid/gid.
226 -O File is owned by real uid.
229 -z File has zero size.
230 -s File has non-zero size (returns size).
232 -f File is a plain file.
233 -d File is a directory.
234 -l File is a symbolic link.
235 -p File is a named pipe (FIFO).
237 -b File is a block special file.
238 -c File is a character special file.
239 -t Filehandle is opened to a tty.
241 -u File has setuid bit set.
242 -g File has setgid bit set.
243 -k File has sticky bit set.
245 -T File is a text file.
246 -B File is a binary file (opposite of -T).
248 -M Age of file in days when script started.
249 -A Same for access time.
250 -C Same for inode change time.
252 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
253 C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
254 uids and gids of the user. There may be other reasons you can't actually
255 read, write or execute the file. Also note that, for the superuser,
256 C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
257 1 if any execute bit is set in the mode. Scripts run by the superuser may
258 thus need to do a stat() to determine the actual mode of the
259 file, or temporarily set the uid to something else.
265 next unless -f $_; # ignore specials
269 Note that C<-s/a/b/> does not do a negated substitution. Saying
270 C<-exp($foo)> still works as expected, however--only single letters
271 following a minus are interpreted as file tests.
273 The C<-T> and C<-B> switches work as follows. The first block or so of the
274 file is examined for odd characters such as strange control codes or
275 characters with the high bit set. If too many odd characters (E<gt>30%)
276 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
277 containing null in the first block is considered a binary file. If C<-T>
278 or C<-B> is used on a filehandle, the current stdio buffer is examined
279 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
280 file, or a file at EOF when testing a filehandle. Because you have to
281 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
282 against the file first, as in C<next unless -f $file && -T $file>.
284 If any of the file tests (or either the stat() or lstat() operators) are given
285 the special filehandle consisting of a solitary underline, then the stat
286 structure of the previous file test (or stat operator) is used, saving
287 a system call. (This doesn't work with C<-t>, and you need to remember
288 that lstat() and C<-l> will leave values in the stat structure for the
289 symbolic link, not the real file.) Example:
291 print "Can do.\n" if -r $a || -w _ || -x _;
294 print "Readable\n" if -r _;
295 print "Writable\n" if -w _;
296 print "Executable\n" if -x _;
297 print "Setuid\n" if -u _;
298 print "Setgid\n" if -g _;
299 print "Sticky\n" if -k _;
300 print "Text\n" if -T _;
301 print "Binary\n" if -B _;
307 Returns the absolute value of its argument.
308 If VALUE is omitted, uses $_.
310 =item accept NEWSOCKET,GENERICSOCKET
312 Accepts an incoming socket connect, just as the accept(2) system call
313 does. Returns the packed address if it succeeded, FALSE otherwise.
314 See example in L<perlipc/"Sockets: Client/Server Communication">.
320 Arranges to have a SIGALRM delivered to this process after the
321 specified number of seconds have elapsed. If SECONDS is not specified,
322 the value stored in $_ is used. (On some machines,
323 unfortunately, the elapsed time may be up to one second less than you
324 specified because of how seconds are counted.) Only one timer may be
325 counting at once. Each call disables the previous timer, and an
326 argument of 0 may be supplied to cancel the previous timer without
327 starting a new one. The returned value is the amount of time remaining
328 on the previous timer.
330 For delays of finer granularity than one second, you may use Perl's
331 syscall() interface to access setitimer(2) if your system supports it,
332 or else see L</select()> below. It is not advised to intermix alarm()
335 If you want to use alarm() to time out a system call you need to use an
336 eval/die pair. You can't rely on the alarm causing the system call to
337 fail with $! set to EINTR because Perl sets up signal handlers to
338 restart system calls on some systems. Using eval/die always works.
341 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
343 $nread = sysread SOCKET, $buffer, $size;
346 die if $@ && $@ ne "alarm\n"; # propagate errors
356 Returns the arctangent of Y/X in the range -PI to PI.
358 For the tangent operation, you may use the POSIX::tan()
359 function, or use the familiar relation:
361 sub tan { sin($_[0]) / cos($_[0]) }
363 =item bind SOCKET,NAME
365 Binds a network address to a socket, just as the bind system call
366 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
367 packed address of the appropriate type for the socket. See the examples in
368 L<perlipc/"Sockets: Client/Server Communication">.
370 =item binmode FILEHANDLE
372 Arranges for the file to be read or written in "binary" mode in operating
373 systems that distinguish between binary and text files. Files that are
374 not in binary mode have CR LF sequences translated to LF on input and LF
375 translated to CR LF on output. Binmode has no effect under Unix; in DOS
376 and similarly archaic systems, it may be imperative--otherwise your
377 DOS-damaged C library may mangle your file. The key distinction between
378 systems that need binmode and those that don't is their text file
379 formats. Systems like Unix and Plan9 that delimit lines with a single
380 character, and that encode that character in C as '\n', do not need
381 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
382 is taken as the name of the filehandle.
384 =item bless REF,CLASSNAME
388 This function tells the thingy referenced by REF that it is now
389 an object in the CLASSNAME package--or the current package if no CLASSNAME
390 is specified, which is often the case. It returns the reference for
391 convenience, because a bless() is often the last thing in a constructor.
392 Always use the two-argument version if the function doing the blessing
393 might be inherited by a derived class. See L<perlobj> for more about the
394 blessing (and blessings) of objects.
400 Returns the context of the current subroutine call. In a scalar context,
401 returns the caller's package name if there is a caller, that is, if
402 we're in a subroutine or eval() or require(), and the undefined value
403 otherwise. In a list context, returns
405 ($package, $filename, $line) = caller;
407 With EXPR, it returns some extra information that the debugger uses to
408 print a stack trace. The value of EXPR indicates how many call frames
409 to go back before the current one.
411 ($package, $filename, $line,
412 $subroutine, $hasargs, $wantarray) = caller($i);
414 Furthermore, when called from within the DB package, caller returns more
415 detailed information: it sets the list variable @DB::args to be the
416 arguments with which that subroutine was invoked.
420 Changes the working directory to EXPR, if possible. If EXPR is
421 omitted, changes to home directory. Returns TRUE upon success, FALSE
422 otherwise. See example under die().
426 Changes the permissions of a list of files. The first element of the
427 list must be the numerical mode, which should probably be an octal
428 number. Returns the number of files successfully changed.
430 $cnt = chmod 0755, 'foo', 'bar';
431 chmod 0755, @executables;
439 This is a slightly safer version of chop (see below). It removes any
440 line ending that corresponds to the current value of C<$/> (also known as
441 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
442 number of characters removed from all its arguments. It's often used to
443 remove the newline from the end of an input record when you're worried
444 that the final record may be missing its newline. When in paragraph mode
445 (C<$/ = "">), it removes all trailing newlines from the string. If
446 VARIABLE is omitted, it chomps $_. Example:
449 chomp; # avoid \n on last field
454 You can actually chomp anything that's an lvalue, including an assignment:
457 chomp($answer = <STDIN>);
459 If you chomp a list, each element is chomped, and the total number of
460 characters removed is returned.
468 Chops off the last character of a string and returns the character
469 chopped. It's used primarily to remove the newline from the end of an
470 input record, but is much more efficient than C<s/\n//> because it neither
471 scans nor copies the string. If VARIABLE is omitted, chops $_.
475 chop; # avoid \n on last field
480 You can actually chop anything that's an lvalue, including an assignment:
483 chop($answer = <STDIN>);
485 If you chop a list, each element is chopped. Only the value of the
486 last chop is returned.
488 Note that chop returns the last character. To return all but the last
489 character, use C<substr($string, 0, -1)>.
493 Changes the owner (and group) of a list of files. The first two
494 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
495 Returns the number of files successfully changed.
497 $cnt = chown $uid, $gid, 'foo', 'bar';
498 chown $uid, $gid, @filenames;
500 Here's an example that looks up non-numeric uids in the passwd file:
503 chop($user = <STDIN>);
505 chop($pattern = <STDIN>);
507 ($login,$pass,$uid,$gid) = getpwnam($user)
508 or die "$user not in passwd file";
510 @ary = <${pattern}>; # expand filenames
511 chown $uid, $gid, @ary;
513 On most systems, you are not allowed to change the ownership of the
514 file unless you're the superuser, although you should be able to change
515 the group to any of your secondary groups. On insecure systems, these
516 restrictions may be relaxed, but this is not a portable assumption.
522 Returns the character represented by that NUMBER in the character set.
523 For example, C<chr(65)> is "A" in ASCII.
525 If NUMBER is omitted, uses $_.
527 =item chroot FILENAME
531 This function works as the system call by the same name: it makes the
532 named directory the new root directory for all further pathnames that
533 begin with a "/" by your process and all of its children. (It doesn't
534 change your current working directory, which is unaffected.) For security
535 reasons, this call is restricted to the superuser. If FILENAME is
536 omitted, does chroot to $_.
538 =item close FILEHANDLE
540 Closes the file or pipe associated with the file handle, returning TRUE
541 only if stdio successfully flushes buffers and closes the system file
542 descriptor. You don't have to close FILEHANDLE if you are immediately
543 going to do another open() on it, because open() will close it for you. (See
544 open().) However, an explicit close on an input file resets the line
545 counter ($.), while the implicit close done by open() does not. Also,
546 closing a pipe will wait for the process executing on the pipe to
547 complete, in case you want to look at the output of the pipe
548 afterwards. Closing a pipe explicitly also puts the status value of
549 the command into C<$?>. Example:
551 open(OUTPUT, '|sort >foo'); # pipe to sort
552 ... # print stuff to output
553 close OUTPUT; # wait for sort to finish
554 open(INPUT, 'foo'); # get sort's results
556 FILEHANDLE may be an expression whose value gives the real filehandle name.
558 =item closedir DIRHANDLE
560 Closes a directory opened by opendir().
562 =item connect SOCKET,NAME
564 Attempts to connect to a remote socket, just as the connect system call
565 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
566 packed address of the appropriate type for the socket. See the examples in
567 L<perlipc/"Sockets: Client/Server Communication">.
571 Actually a flow control statement rather than a function. If there is a
572 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
573 C<foreach>), it is always executed just before the conditional is about to
574 be evaluated again, just like the third part of a C<for> loop in C. Thus
575 it can be used to increment a loop variable, even when the loop has been
576 continued via the C<next> statement (which is similar to the C C<continue>
581 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
584 For the inverse cosine operation, you may use the POSIX::acos()
585 function, or use this relation:
587 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
589 =item crypt PLAINTEXT,SALT
591 Encrypts a string exactly like the crypt(3) function in the C library
592 (assuming that you actually have a version there that has not been
593 extirpated as a potential munition). This can prove useful for checking
594 the password file for lousy passwords, amongst other things. Only the
595 guys wearing white hats should do this.
597 Here's an example that makes sure that whoever runs this program knows
600 $pwd = (getpwuid($<))[1];
601 $salt = substr($pwd, 0, 2);
605 chop($word = <STDIN>);
609 if (crypt($word, $salt) ne $pwd) {
615 Of course, typing in your own password to whomever asks you
618 =item dbmclose ASSOC_ARRAY
620 [This function has been superseded by the untie() function.]
622 Breaks the binding between a DBM file and an associative array.
624 =item dbmopen ASSOC,DBNAME,MODE
626 [This function has been superseded by the tie() function.]
628 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
629 associative array. ASSOC is the name of the associative array. (Unlike
630 normal open, the first argument is I<NOT> a filehandle, even though it
631 looks like one). DBNAME is the name of the database (without the F<.dir>
632 or F<.pag> extension if any). If the database does not exist, it is
633 created with protection specified by MODE (as modified by the umask()).
634 If your system supports only the older DBM functions, you may perform only
635 one dbmopen() in your program. In older versions of Perl, if your system
636 had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
637 falls back to sdbm(3).
639 If you don't have write access to the DBM file, you can only read
640 associative array variables, not set them. If you want to test whether
641 you can write, either use file tests or try setting a dummy array entry
642 inside an eval(), which will trap the error.
644 Note that functions such as keys() and values() may return huge array
645 values when used on large DBM files. You may prefer to use the each()
646 function to iterate over large DBM files. Example:
648 # print out history file offsets
649 dbmopen(%HIST,'/usr/lib/news/history',0666);
650 while (($key,$val) = each %HIST) {
651 print $key, ' = ', unpack('L',$val), "\n";
655 See also L<AnyDBM_File> for a more general description of the pros and
656 cons of the various dbm approaches, as well as L<DB_File> for a particularly
663 Returns a boolean value saying whether EXPR has a real value
664 or not. If EXPR is not present, $_ will be checked. Many operations
665 return the undefined value under exceptional conditions, such as end of
666 file, uninitialized variable, system error and such. This function
667 allows you to distinguish between an undefined
668 null scalar and a defined null scalar with operations that might return
669 a real null string, such as referencing elements of an array. You may
670 also check to see if arrays or subroutines exist. Use of defined on
671 predefined variables is not guaranteed to produce intuitive results.
673 When used on a hash array element, it tells you whether the value
674 is defined, not whether the key exists in the hash. Use exists() for that.
678 print if defined $switch{'D'};
679 print "$val\n" while defined($val = pop(@ary));
680 die "Can't readlink $sym: $!"
681 unless defined($value = readlink $sym);
682 eval '@foo = ()' if defined(@foo);
683 die "No XYZ package defined" unless defined %_XYZ;
684 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
688 Note: many folks tend to overuse defined(), and then are surprised to
689 discover that the number 0 and the null string are, in fact, defined
690 concepts. For example, if you say
694 the pattern match succeeds, and $1 is defined, despite the fact that it
695 matched "nothing". But it didn't really match nothing--rather, it
696 matched something that happened to be 0 characters long. This is all
697 very above-board and honest. When a function returns an undefined value,
698 it's an admission that it couldn't give you an honest answer. So
699 you should use defined() only when you're questioning the integrity
700 of what you're trying to do. At other times, a simple comparison to
701 0 or "" is what you want.
703 Another surprise is that using defined() on an entire array or
704 hash reports whether memory for that aggregate has ever been
705 allocated. So an array you set to the empty list appears undefined
706 initially, and one that once was full and that you then set to
707 the empty list still appears defined. You should instead use a
708 simple test for size:
710 if (@an_array) { print "has array elements\n" }
711 if (%a_hash) { print "has hash members\n" }
713 Using undef() on these, however, does clear their memory and then report
714 them as not defined anymore, but you shoudln't do that unless you don't
715 plan to use them again, because it saves time when you load them up
716 again to have memory already ready to be filled.
718 This counter-intuitive behaviour of defined() on aggregates may be
719 changed, fixed, or broken in a future release of Perl.
723 Deletes the specified key(s) and their associated values from a hash
724 array. For each key, returns the deleted value associated with that key,
725 or the undefined value if there was no such key. Deleting from C<$ENV{}>
726 modifies the environment. Deleting from an array tied to a DBM file
727 deletes the entry from the DBM file. (But deleting from a tie()d hash
728 doesn't necessarily return anything.)
730 The following deletes all the values of an associative array:
732 foreach $key (keys %HASH) {
738 delete @HASH{keys %HASH}
740 (But both of these are slower than the undef() command.) Note that the
741 EXPR can be arbitrarily complicated as long as the final operation is a
742 hash element lookup or hash slice:
744 delete $ref->[$x][$y]{$key};
745 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
749 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
750 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
751 C<($? E<gt>E<gt> 8)> (back-tick `command` status). If C<($? E<gt>E<gt> 8)>
752 is 0, exits with 255. Inside an eval(), the error message is stuffed into
753 C<$@>, and the eval() is terminated with the undefined value; this makes
754 die() the way to raise an exception.
758 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
759 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
761 If the value of EXPR does not end in a newline, the current script line
762 number and input line number (if any) are also printed, and a newline
763 is supplied. Hint: sometimes appending ", stopped" to your message
764 will cause it to make better sense when the string "at foo line 123" is
765 appended. Suppose you are running script "canasta".
767 die "/etc/games is no good";
768 die "/etc/games is no good, stopped";
770 produce, respectively
772 /etc/games is no good at canasta line 123.
773 /etc/games is no good, stopped at canasta line 123.
775 See also exit() and warn().
779 Not really a function. Returns the value of the last command in the
780 sequence of commands indicated by BLOCK. When modified by a loop
781 modifier, executes the BLOCK once before testing the loop condition.
782 (On other statements the loop modifiers test the conditional first.)
784 =item do SUBROUTINE(LIST)
786 A deprecated form of subroutine call. See L<perlsub>.
790 Uses the value of EXPR as a filename and executes the contents of the
791 file as a Perl script. Its primary use is to include subroutines
792 from a Perl subroutine library.
800 except that it's more efficient, more concise, keeps track of the
801 current filename for error messages, and searches all the B<-I>
802 libraries if the file isn't in the current directory (see also the @INC
803 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
804 re-parse the file every time you call it, so you probably don't want to
805 do this inside a loop.
807 Note that inclusion of library modules is better done with the
808 use() and require() operators, which also do error checking
809 and raise an exception if there's a problem.
813 This causes an immediate core dump. Primarily this is so that you can
814 use the B<undump> program to turn your core dump into an executable binary
815 after having initialized all your variables at the beginning of the
816 program. When the new binary is executed it will begin by executing a
817 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
818 it as a goto with an intervening core dump and reincarnation. If LABEL
819 is omitted, restarts the program from the top. WARNING: any files
820 opened at the time of the dump will NOT be open any more when the
821 program is reincarnated, with possible resulting confusion on the part
822 of Perl. See also B<-u> option in L<perlrun>.
839 dump QUICKSTART if $ARGV[0] eq '-d';
844 =item each ASSOC_ARRAY
846 When called in a list context, returns a 2-element array consisting
847 of the key and value for the next element of an associative array,
848 so that you can iterate over it. When called in a scalar context,
849 returns the key for only the next element in the associative array.
850 Entries are returned in an apparently random order. When the array is
851 entirely read, a null array is returned in list context (which when
852 assigned produces a FALSE (0) value), and C<undef> is returned in a
853 scalar context. The next call to each() after that will start
854 iterating again. The iterator can be reset only by reading all the
855 elements from the array. You should not add elements to an array while
856 you're iterating over it. There is a single iterator for each
857 associative array, shared by all each(), keys(), and values() function
858 calls in the program. The following prints out your environment like
859 the printenv(1) program, only in a different order:
861 while (($key,$value) = each %ENV) {
862 print "$key=$value\n";
865 See also keys() and values().
873 Returns 1 if the next read on FILEHANDLE will return end of file, or if
874 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
875 gives the real filehandle name. (Note that this function actually
876 reads a character and then ungetc()s it, so it is not very useful in an
877 interactive context.) Do not read from a terminal file (or call
878 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
879 as terminals may lose the end-of-file condition if you do.
881 An C<eof> without an argument uses the last file read as argument.
882 Empty parentheses () may be used to indicate
883 the pseudo file formed of the files listed on the command line, i.e.,
884 C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
885 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
886 test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
888 # reset line numbering on each input file
891 close(ARGV) if (eof); # Not eof().
894 # insert dashes just before last line of last file
897 print "--------------\n";
898 close(ARGV); # close or break; is needed if we
899 # are reading from the terminal
904 Practical hint: you almost never need to use C<eof> in Perl, because the
905 input operators return undef when they run out of data.
911 EXPR is parsed and executed as if it were a little Perl program. It
912 is executed in the context of the current Perl program, so that any
913 variable settings or subroutine and format definitions remain afterwards.
914 The value returned is the value of the last expression evaluated, or a
915 return statement may be used, just as with subroutines. The last
916 expression is evaluated in scalar or array context, depending on the
919 If there is a syntax error or runtime error, or a die() statement is
920 executed, an undefined value is returned by eval(), and C<$@> is set to the
921 error message. If there was no error, C<$@> is guaranteed to be a null
922 string. If EXPR is omitted, evaluates $_. The final semicolon, if
923 any, may be omitted from the expression.
925 Note that, because eval() traps otherwise-fatal errors, it is useful for
926 determining whether a particular feature (such as socket() or symlink())
927 is implemented. It is also Perl's exception trapping mechanism, where
928 the die operator is used to raise exceptions.
930 If the code to be executed doesn't vary, you may use the eval-BLOCK
931 form to trap run-time errors without incurring the penalty of
932 recompiling each time. The error, if any, is still returned in C<$@>.
935 # make divide-by-zero non-fatal
936 eval { $answer = $a / $b; }; warn $@ if $@;
938 # same thing, but less efficient
939 eval '$answer = $a / $b'; warn $@ if $@;
941 # a compile-time error
945 eval '$answer ='; # sets $@
947 With an eval(), you should be especially careful to remember what's
948 being looked at when:
954 eval { $x }; # CASE 4
956 eval "\$$x++" # CASE 5
959 Cases 1 and 2 above behave identically: they run the code contained in the
960 variable $x. (Although case 2 has misleading double quotes making the
961 reader wonder what else might be happening (nothing is).) Cases 3 and 4
962 likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
963 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
964 is a place where normally you I<WOULD> like to use double quotes, except
965 that in that particular situation, you can just use symbolic references
966 instead, as in case 6.
970 The exec() function executes a system command I<AND NEVER RETURNS>,
971 unless the command does not exist and is executed directly instead of
972 via C</bin/sh -c> (see below). Use system() instead of exec() if you
975 If there is more than one argument in LIST, or if LIST is an array with
976 more than one value, calls execvp(3) with the arguments in LIST. If
977 there is only one scalar argument, the argument is checked for shell
978 metacharacters. If there are any, the entire argument is passed to
979 C</bin/sh -c> for parsing. If there are none, the argument is split
980 into words and passed directly to execvp(), which is more efficient.
981 Note: exec() and system() do not flush your output buffer, so you may
982 need to set C<$|> to avoid lost output. Examples:
984 exec '/bin/echo', 'Your arguments are: ', @ARGV;
985 exec "sort $outfile | uniq";
987 If you don't really want to execute the first argument, but want to lie
988 to the program you are executing about its own name, you can specify
989 the program you actually want to run as an "indirect object" (without a
990 comma) in front of the LIST. (This always forces interpretation of the
991 LIST as a multi-valued list, even if there is only a single scalar in
995 exec $shell '-sh'; # pretend it's a login shell
999 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1003 Returns TRUE if the specified hash key exists in its hash array, even
1004 if the corresponding value is undefined.
1006 print "Exists\n" if exists $array{$key};
1007 print "Defined\n" if defined $array{$key};
1008 print "True\n" if $array{$key};
1010 A hash element can be TRUE only if it's defined, and defined if
1011 it exists, but the reverse doesn't necessarily hold true.
1013 Note that the EXPR can be arbitrarily complicated as long as the final
1014 operation is a hash key lookup:
1016 if (exists $ref->[$x][$y]{$key}) { ... }
1020 Evaluates EXPR and exits immediately with that value. (Actually, it
1021 calls any defined C<END> routines first, but the C<END> routines may not
1022 abort the exit. Likewise any object destructors that need to be called
1023 are called before exit.) Example:
1026 exit 0 if $ans =~ /^[Xx]/;
1028 See also die(). If EXPR is omitted, exits with 0 status.
1030 You shouldn't use exit() to abort a subroutine if there's any chance that
1031 someone might want to trap whatever error happened. Use die() instead,
1032 which can be trapped by an eval().
1038 Returns I<e> (the natural logarithm base) to the power of EXPR.
1039 If EXPR is omitted, gives C<exp($_)>.
1041 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1043 Implements the fcntl(2) function. You'll probably have to say
1047 first to get the correct function definitions. Argument processing and
1048 value return works just like ioctl() below. Note that fcntl() will produce
1049 a fatal error if used on a machine that doesn't implement fcntl(2).
1053 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1055 =item fileno FILEHANDLE
1057 Returns the file descriptor for a filehandle. This is useful for
1058 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1059 value is taken as the name of the filehandle.
1061 =item flock FILEHANDLE,OPERATION
1063 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1064 success, FALSE on failure. Will produce a fatal error if used on a
1065 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1066 flock() is Perl's portable file locking interface, although it will lock
1067 only entire files, not records.
1069 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1070 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1071 you can use the symbolic names if you pull them in with an explicit
1072 request to the Fcntl module. The names can be requested as a group with
1073 the :flock tag (or they can be requested individually, of course).
1074 LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and
1075 LOCK_UN releases a previously requested lock. If LOCK_NB is added to
1076 LOCK_SH or LOCK_EX then flock() will return immediately rather than
1077 blocking waiting for the lock (check the return status to see if you got
1080 Note that the emulation built with lockf(3) doesn't provide shared
1081 locks, and it requires that FILEHANDLE be open with write intent. These
1082 are the semantics that lockf(3) implements. Most (all?) systems
1083 implement lockf(3) in terms of fcntl(2) locking, though, so the
1084 differing semantics shouldn't bite too many people.
1086 Note also that some versions of flock() cannot lock things over the
1087 network; you would need to use the more system-specific fcntl() for
1088 that. If you like you can force Perl to ignore your system's flock(2)
1089 function, and so provide its own fcntl(2)-based emulation, by passing
1090 the switch C<-Ud_flock> to the F<Configure> program when you configure
1093 Here's a mailbox appender for BSD systems.
1095 use Fcntl ':flock'; # import LOCK_* constants
1098 flock(MBOX,LOCK_EX);
1099 # and, in case someone appended
1100 # while we were waiting...
1105 flock(MBOX,LOCK_UN);
1108 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1109 or die "Can't open mailbox: $!";
1112 print MBOX $msg,"\n\n";
1115 See also L<DB_File> for other flock() examples.
1119 Does a fork(2) system call. Returns the child pid to the parent process
1120 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1121 Note: unflushed buffers remain unflushed in both processes, which means
1122 you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1123 method of IO::Handle to avoid duplicate output.
1125 If you fork() without ever waiting on your children, you will accumulate
1128 $SIG{CHLD} = sub { wait };
1130 There's also the double-fork trick (error checking on
1131 fork() returns omitted);
1133 unless ($pid = fork) {
1135 exec "what you really wanna do";
1138 ## (some_perl_code_here)
1145 See also L<perlipc> for more examples of forking and reaping
1148 Note that if your forked child inherits system file descriptors like
1149 STDIN and STDOUT that are actually connected by a pipe or socket, even
1150 if you exit, the remote server (such as, say, httpd or rsh) won't think
1151 you're done. You should reopen those to /dev/null if it's any issue.
1155 Declare a picture format with use by the write() function. For
1159 Test: @<<<<<<<< @||||| @>>>>>
1160 $str, $%, '$' . int($num)
1164 $num = $cost/$quantity;
1168 See L<perlform> for many details and examples.
1171 =item formline PICTURE, LIST
1173 This is an internal function used by C<format>s, though you may call it
1174 too. It formats (see L<perlform>) a list of values according to the
1175 contents of PICTURE, placing the output into the format output
1176 accumulator, C<$^A> (or $ACCUMULATOR in English).
1177 Eventually, when a write() is done, the contents of
1178 C<$^A> are written to some filehandle, but you could also read C<$^A>
1179 yourself and then set C<$^A> back to "". Note that a format typically
1180 does one formline() per line of form, but the formline() function itself
1181 doesn't care how many newlines are embedded in the PICTURE. This means
1182 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1183 You may therefore need to use multiple formlines to implement a single
1184 record format, just like the format compiler.
1186 Be careful if you put double quotes around the picture, because an "C<@>"
1187 character may be taken to mean the beginning of an array name.
1188 formline() always returns TRUE. See L<perlform> for other examples.
1190 =item getc FILEHANDLE
1194 Returns the next character from the input file attached to FILEHANDLE,
1195 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1196 This is not particularly efficient. It cannot be used to get unbuffered
1197 single-characters, however. For that, try something more like:
1200 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1203 system "stty", '-icanon', 'eol', "\001";
1209 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1212 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1216 Determination of whether to whether $BSD_STYLE should be set
1217 is left as an exercise to the reader.
1219 The POSIX::getattr() function can do this more portably on systems
1220 alleging POSIX compliance.
1221 See also the C<Term::ReadKey> module from your nearest CPAN site;
1222 details on CPAN can be found on L<perlmod/CPAN>.
1226 Returns the current login from F</etc/utmp>, if any. If null, use
1229 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1231 Do not consider getlogin() for authentication: it is not as
1232 secure as getpwuid().
1234 =item getpeername SOCKET
1236 Returns the packed sockaddr address of other end of the SOCKET connection.
1239 $hersockaddr = getpeername(SOCK);
1240 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1241 $herhostname = gethostbyaddr($iaddr, AF_INET);
1242 $herstraddr = inet_ntoa($iaddr);
1246 Returns the current process group for the specified PID. Use
1247 a PID of 0 to get the current process group for the
1248 current process. Will raise an exception if used on a machine that
1249 doesn't implement getpgrp(2). If PID is omitted, returns process
1250 group of current process. Note that the POSIX version of getpgrp()
1251 does not accept a PID argument, so only PID==0 is truly portable.
1255 Returns the process id of the parent process.
1257 =item getpriority WHICH,WHO
1259 Returns the current priority for a process, a process group, or a user.
1260 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1261 machine that doesn't implement getpriority(2).
1267 =item gethostbyname NAME
1269 =item getnetbyname NAME
1271 =item getprotobyname NAME
1277 =item getservbyname NAME,PROTO
1279 =item gethostbyaddr ADDR,ADDRTYPE
1281 =item getnetbyaddr ADDR,ADDRTYPE
1283 =item getprotobynumber NUMBER
1285 =item getservbyport PORT,PROTO
1303 =item sethostent STAYOPEN
1305 =item setnetent STAYOPEN
1307 =item setprotoent STAYOPEN
1309 =item setservent STAYOPEN
1323 These routines perform the same functions as their counterparts in the
1324 system library. Within a list context, the return values from the
1325 various get routines are as follows:
1327 ($name,$passwd,$uid,$gid,
1328 $quota,$comment,$gcos,$dir,$shell) = getpw*
1329 ($name,$passwd,$gid,$members) = getgr*
1330 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1331 ($name,$aliases,$addrtype,$net) = getnet*
1332 ($name,$aliases,$proto) = getproto*
1333 ($name,$aliases,$port,$proto) = getserv*
1335 (If the entry doesn't exist you get a null list.)
1337 Within a scalar context, you get the name, unless the function was a
1338 lookup by name, in which case you get the other thing, whatever it is.
1339 (If the entry doesn't exist you get the undefined value.) For example:
1349 The $members value returned by I<getgr*()> is a space separated list of
1350 the login names of the members of the group.
1352 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1353 C, it will be returned to you via C<$?> if the function call fails. The
1354 @addrs value returned by a successful call is a list of the raw
1355 addresses returned by the corresponding system library call. In the
1356 Internet domain, each address is four bytes long and you can unpack it
1357 by saying something like:
1359 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1361 =item getsockname SOCKET
1363 Returns the packed sockaddr address of this end of the SOCKET connection.
1366 $mysockaddr = getsockname(SOCK);
1367 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1369 =item getsockopt SOCKET,LEVEL,OPTNAME
1371 Returns the socket option requested, or undefined if there is an error.
1375 Returns the value of EXPR with filename expansions such as a shell
1376 would do. This is the internal function implementing the E<lt>*.*E<gt>
1377 operator, except it's easier to use.
1381 Converts a time as returned by the time function to a 9-element array
1382 with the time localized for the standard Greenwich time zone.
1383 Typically used as follows:
1386 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1389 All array elements are numeric, and come straight out of a struct tm.
1390 In particular this means that $mon has the range 0..11 and $wday has
1391 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1399 The goto-LABEL form finds the statement labeled with LABEL and resumes
1400 execution there. It may not be used to go into any construct that
1401 requires initialization, such as a subroutine or a foreach loop. It
1402 also can't be used to go into a construct that is optimized away. It
1403 can be used to go almost anywhere else within the dynamic scope,
1404 including out of subroutines, but it's usually better to use some other
1405 construct such as last or die. The author of Perl has never felt the
1406 need to use this form of goto (in Perl, that is--C is another matter).
1408 The goto-EXPR form expects a label name, whose scope will be resolved
1409 dynamically. This allows for computed gotos per FORTRAN, but isn't
1410 necessarily recommended if you're optimizing for maintainability:
1412 goto ("FOO", "BAR", "GLARCH")[$i];
1414 The goto-&NAME form is highly magical, and substitutes a call to the
1415 named subroutine for the currently running subroutine. This is used by
1416 AUTOLOAD subroutines that wish to load another subroutine and then
1417 pretend that the other subroutine had been called in the first place
1418 (except that any modifications to @_ in the current subroutine are
1419 propagated to the other subroutine.) After the goto, not even caller()
1420 will be able to tell that this routine was called first.
1422 =item grep BLOCK LIST
1424 =item grep EXPR,LIST
1426 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1427 $_ to each element) and returns the list value consisting of those
1428 elements for which the expression evaluated to TRUE. In a scalar
1429 context, returns the number of times the expression was TRUE.
1431 @foo = grep(!/^#/, @bar); # weed out comments
1435 @foo = grep {!/^#/} @bar; # weed out comments
1437 Note that, because $_ is a reference into the list value, it can be used
1438 to modify the elements of the array. While this is useful and
1439 supported, it can cause bizarre results if the LIST is not a named
1446 Interprets EXPR as a hex string and returns the corresponding decimal
1447 value. (To convert strings that might start with 0 or 0x see
1448 oct().) If EXPR is omitted, uses $_.
1452 There is no built-in import() function. It is merely an ordinary
1453 method (subroutine) defined (or inherited) by modules that wish to export
1454 names to another module. The use() function calls the import() method
1455 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1457 =item index STR,SUBSTR,POSITION
1459 =item index STR,SUBSTR
1461 Returns the position of the first occurrence of SUBSTR in STR at or after
1462 POSITION. If POSITION is omitted, starts searching from the beginning of
1463 the string. The return value is based at 0 (or whatever you've set the C<$[>
1464 variable to--but don't do that). If the substring is not found, returns
1465 one less than the base, ordinarily -1.
1471 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1473 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1475 Implements the ioctl(2) function. You'll probably have to say
1477 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1479 first to get the correct function definitions. If F<ioctl.ph> doesn't
1480 exist or doesn't have the correct definitions you'll have to roll your
1481 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1482 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1483 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1484 written depending on the FUNCTION--a pointer to the string value of SCALAR
1485 will be passed as the third argument of the actual ioctl call. (If SCALAR
1486 has no string value but does have a numeric value, that value will be
1487 passed rather than a pointer to the string value. To guarantee this to be
1488 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1489 functions are useful for manipulating the values of structures used by
1490 ioctl(). The following example sets the erase character to DEL.
1494 die "NO TIOCGETP" if $@ || !$getp;
1495 $sgttyb_t = "ccccs"; # 4 chars and a short
1496 if (ioctl(STDIN,$getp,$sgttyb)) {
1497 @ary = unpack($sgttyb_t,$sgttyb);
1499 $sgttyb = pack($sgttyb_t,@ary);
1500 ioctl(STDIN,&TIOCSETP,$sgttyb)
1501 || die "Can't ioctl: $!";
1504 The return value of ioctl (and fcntl) is as follows:
1506 if OS returns: then Perl returns:
1508 0 string "0 but true"
1509 anything else that number
1511 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1512 still easily determine the actual value returned by the operating
1515 ($retval = ioctl(...)) || ($retval = -1);
1516 printf "System returned %d\n", $retval;
1518 =item join EXPR,LIST
1520 Joins the separate strings of LIST or ARRAY into a single string with
1521 fields separated by the value of EXPR, and returns the string.
1524 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1526 See L<perlfunc/split>.
1528 =item keys ASSOC_ARRAY
1530 Returns a normal array consisting of all the keys of the named
1531 associative array. (In a scalar context, returns the number of keys.)
1532 The keys are returned in an apparently random order, but it is the same
1533 order as either the values() or each() function produces (given that
1534 the associative array has not been modified). Here is yet another way
1535 to print your environment:
1538 @values = values %ENV;
1539 while ($#keys >= 0) {
1540 print pop(@keys), '=', pop(@values), "\n";
1543 or how about sorted by key:
1545 foreach $key (sort(keys %ENV)) {
1546 print $key, '=', $ENV{$key}, "\n";
1549 To sort an array by value, you'll need to use a C<sort{}>
1550 function. Here's a descending numeric sort of a hash by its values:
1552 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1553 printf "%4d %s\n", $hash{$key}, $key;
1556 As an lvalue C<keys> allows you to increase the number of hash buckets
1557 allocated for the given associative array. This can gain you a measure
1558 of efficiency if you know the hash is going to get big. (This is
1559 similar to pre-extending an array by assigning a larger number to
1560 $#array.) If you say
1564 then C<%hash> will have at least 200 buckets allocated for it. These
1565 buckets will be retained even if you do C<%hash = ()>, use C<undef
1566 %hash> if you want to free the storage while C<%hash> is still in scope.
1567 You can't shrink the number of buckets allocated for the hash using
1568 C<keys> in this way (but you needn't worry about doing this by accident,
1569 as trying has no effect).
1573 Sends a signal to a list of processes. The first element of
1574 the list must be the signal to send. Returns the number of
1575 processes successfully signaled.
1577 $cnt = kill 1, $child1, $child2;
1580 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1581 process groups instead of processes. (On System V, a negative I<PROCESS>
1582 number will also kill process groups, but that's not portable.) That
1583 means you usually want to use positive not negative signals. You may also
1584 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1590 The C<last> command is like the C<break> statement in C (as used in
1591 loops); it immediately exits the loop in question. If the LABEL is
1592 omitted, the command refers to the innermost enclosing loop. The
1593 C<continue> block, if any, is not executed:
1595 LINE: while (<STDIN>) {
1596 last LINE if /^$/; # exit when done with header
1604 Returns an lowercased version of EXPR. This is the internal function
1605 implementing the \L escape in double-quoted strings.
1606 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1608 If EXPR is omitted, uses $_.
1614 Returns the value of EXPR with the first character lowercased. This is
1615 the internal function implementing the \l escape in double-quoted strings.
1616 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1618 If EXPR is omitted, uses $_.
1624 Returns the length in characters of the value of EXPR. If EXPR is
1625 omitted, returns length of $_.
1627 =item link OLDFILE,NEWFILE
1629 Creates a new filename linked to the old filename. Returns 1 for
1630 success, 0 otherwise.
1632 =item listen SOCKET,QUEUESIZE
1634 Does the same thing that the listen system call does. Returns TRUE if
1635 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1639 A local modifies the listed variables to be local to the enclosing block,
1640 subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1641 list must be placed in parentheses. See L<perlsub/"Temporary Values via
1642 local()"> for details.
1644 But you really probably want to be using my() instead, because local() isn't
1645 what most people think of as "local"). See L<perlsub/"Private Variables
1646 via my()"> for details.
1648 =item localtime EXPR
1650 Converts a time as returned by the time function to a 9-element array
1651 with the time analyzed for the local time zone. Typically used as
1654 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1657 All array elements are numeric, and come straight out of a struct tm.
1658 In particular this means that $mon has the range 0..11 and $wday has
1659 the range 0..6. If EXPR is omitted, does localtime(time).
1661 In a scalar context, prints out the ctime(3) value:
1663 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
1665 Also see the F<timelocal.pl> library, and the strftime(3) function available
1666 via the POSIX module.
1672 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1675 =item lstat FILEHANDLE
1681 Does the same thing as the stat() function, but stats a symbolic link
1682 instead of the file the symbolic link points to. If symbolic links are
1683 unimplemented on your system, a normal stat() is done.
1685 If EXPR is omitted, stats $_.
1689 The match operator. See L<perlop>.
1691 =item map BLOCK LIST
1695 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1696 element) and returns the list value composed of the results of each such
1697 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1698 may produce zero, one, or more elements in the returned value.
1700 @chars = map(chr, @nums);
1702 translates a list of numbers to the corresponding characters. And
1704 %hash = map { getkey($_) => $_ } @array;
1706 is just a funny way to write
1709 foreach $_ (@array) {
1710 $hash{getkey($_)} = $_;
1713 =item mkdir FILENAME,MODE
1715 Creates the directory specified by FILENAME, with permissions specified
1716 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1717 it returns 0 and sets C<$!> (errno).
1719 =item msgctl ID,CMD,ARG
1721 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1722 must be a variable which will hold the returned msqid_ds structure.
1723 Returns like ioctl: the undefined value for error, "0 but true" for
1724 zero, or the actual return value otherwise.
1726 =item msgget KEY,FLAGS
1728 Calls the System V IPC function msgget(2). Returns the message queue id,
1729 or the undefined value if there is an error.
1731 =item msgsnd ID,MSG,FLAGS
1733 Calls the System V IPC function msgsnd to send the message MSG to the
1734 message queue ID. MSG must begin with the long integer message type,
1735 which may be created with C<pack("l", $type)>. Returns TRUE if
1736 successful, or FALSE if there is an error.
1738 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1740 Calls the System V IPC function msgrcv to receive a message from
1741 message queue ID into variable VAR with a maximum message size of
1742 SIZE. Note that if a message is received, the message type will be the
1743 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1744 of the message type. Returns TRUE if successful, or FALSE if there is
1749 A "my" declares the listed variables to be local (lexically) to the
1750 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1751 more than one value is listed, the list must be placed in parentheses. See
1752 L<perlsub/"Private Variables via my()"> for details.
1758 The C<next> command is like the C<continue> statement in C; it starts
1759 the next iteration of the loop:
1761 LINE: while (<STDIN>) {
1762 next LINE if /^#/; # discard comments
1766 Note that if there were a C<continue> block on the above, it would get
1767 executed even on discarded lines. If the LABEL is omitted, the command
1768 refers to the innermost enclosing loop.
1770 =item no Module LIST
1772 See the "use" function, which "no" is the opposite of.
1778 Interprets EXPR as an octal string and returns the corresponding
1779 decimal value. (If EXPR happens to start off with 0x, interprets it as
1780 a hex string instead.) The following will handle decimal, octal, and
1781 hex in the standard Perl or C notation:
1783 $val = oct($val) if $val =~ /^0/;
1785 If EXPR is omitted, uses $_.
1787 =item open FILEHANDLE,EXPR
1789 =item open FILEHANDLE
1791 Opens the file whose filename is given by EXPR, and associates it with
1792 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1793 name of the real filehandle wanted. If EXPR is omitted, the scalar
1794 variable of the same name as the FILEHANDLE contains the filename.
1795 (Note that lexical variables--those declared with C<my>--will not work
1796 for this purpose; so if you're using C<my>, specify EXPR in your call
1799 If the filename begins with '<' or nothing, the file is opened for input.
1800 If the filename begins with '>', the file is truncated and opened for
1801 output. If the filename begins with '>>', the file is opened for
1802 appending. You can put a '+' in front of the '>' or '<' to indicate that
1803 you want both read and write access to the file; thus '+<' is almost
1804 always preferred for read/write updates--the '+>' mode would clobber the
1805 file first. The prefix and the filename may be separated with spaces.
1806 These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1807 'w+', 'a', and 'a+'.
1809 If the filename begins with "|", the filename is interpreted as a command
1810 to which output is to be piped, and if the filename ends with a "|", the
1811 filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1812 examples of this. as command which pipes input to us. (You may not have
1813 a raw open() to a command that pipes both in I<and> out, but see
1814 L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1817 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1818 non-zero upon success, the undefined value otherwise. If the open
1819 involved a pipe, the return value happens to be the pid of the
1822 If you're unfortunate enough to be running Perl on a system that
1823 distinguishes between text files and binary files (modern operating
1824 systems don't care), then you should check out L</binmode> for tips for
1825 dealing with this. The key distinction between systems that need binmode
1826 and those that don't is their text file formats. Systems like Unix and
1827 Plan9 that delimit lines with a single character, and that encode that
1828 character in C as '\n', do not need C<binmode>. The rest need it.
1833 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1834 while (<ARTICLE>) {...
1836 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1838 open(DBASE, '+<dbase.mine'); # open for update
1840 open(ARTICLE, "caesar <$article |"); # decrypt article
1842 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1844 # process argument list of files along with any includes
1846 foreach $file (@ARGV) {
1847 process($file, 'fh00');
1851 local($filename, $input) = @_;
1852 $input++; # this is a string increment
1853 unless (open($input, $filename)) {
1854 print STDERR "Can't open $filename: $!\n";
1858 while (<$input>) { # note use of indirection
1859 if (/^#include "(.*)"/) {
1860 process($1, $input);
1867 You may also, in the Bourne shell tradition, specify an EXPR beginning
1868 with "E<gt>&", in which case the rest of the string is interpreted as the
1869 name of a filehandle (or file descriptor, if numeric) which is to be
1870 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1871 +E<gt>E<gt>, and +E<lt>. The
1872 mode you specify should match the mode of the original filehandle.
1873 (Duping a filehandle does not take into account any existing contents of
1875 Here is a script that saves, redirects, and restores STDOUT and
1879 open(SAVEOUT, ">&STDOUT");
1880 open(SAVEERR, ">&STDERR");
1882 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1883 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1885 select(STDERR); $| = 1; # make unbuffered
1886 select(STDOUT); $| = 1; # make unbuffered
1888 print STDOUT "stdout 1\n"; # this works for
1889 print STDERR "stderr 1\n"; # subprocesses too
1894 open(STDOUT, ">&SAVEOUT");
1895 open(STDERR, ">&SAVEERR");
1897 print STDOUT "stdout 2\n";
1898 print STDERR "stderr 2\n";
1901 If you specify "E<lt>&=N", where N is a number, then Perl will do an
1902 equivalent of C's fdopen() of that file descriptor; this is more
1903 parsimonious of file descriptors. For example:
1905 open(FILEHANDLE, "<&=$fd")
1907 If you open a pipe on the command "-", i.e., either "|-" or "-|", then
1908 there is an implicit fork done, and the return value of open is the pid
1909 of the child within the parent process, and 0 within the child
1910 process. (Use C<defined($pid)> to determine whether the open was successful.)
1911 The filehandle behaves normally for the parent, but i/o to that
1912 filehandle is piped from/to the STDOUT/STDIN of the child process.
1913 In the child process the filehandle isn't opened--i/o happens from/to
1914 the new STDOUT or STDIN. Typically this is used like the normal
1915 piped open when you want to exercise more control over just how the
1916 pipe command gets executed, such as when you are running setuid, and
1917 don't want to have to scan shell commands for metacharacters.
1918 The following pairs are more or less equivalent:
1920 open(FOO, "|tr '[a-z]' '[A-Z]'");
1921 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1923 open(FOO, "cat -n '$file'|");
1924 open(FOO, "-|") || exec 'cat', '-n', $file;
1926 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1928 Explicitly closing any piped filehandle causes the parent process to
1929 wait for the child to finish, and returns the status value in C<$?>.
1930 Note: on any operation which may do a fork, unflushed buffers remain
1931 unflushed in both processes, which means you may need to set C<$|> to
1932 avoid duplicate output.
1934 Using the constructor from the IO::Handle package (or one of its
1935 subclasses, such as IO::File or IO::Socket),
1936 you can generate anonymous filehandles which have the scope of whatever
1937 variables hold references to them, and automatically close whenever
1938 and however you leave that scope:
1942 sub read_myfile_munged {
1944 my $handle = new IO::File;
1945 open($handle, "myfile") or die "myfile: $!";
1947 or return (); # Automatically closed here.
1948 mung $first or die "mung failed"; # Or here.
1949 return $first, <$handle> if $ALL; # Or here.
1953 The filename that is passed to open will have leading and trailing
1954 whitespace deleted. To open a file with arbitrary weird
1955 characters in it, it's necessary to protect any leading and trailing
1958 $file =~ s#^(\s)#./$1#;
1959 open(FOO, "< $file\0");
1961 If you want a "real" C open() (see L<open(2)> on your system), then
1962 you should use the sysopen() function. This is another way to
1963 protect your filenames from interpretation. For example:
1966 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
1967 or die "sysopen $path: $!";
1968 HANDLE->autoflush(1);
1969 HANDLE->print("stuff $$\n");
1971 print "File contains: ", <HANDLE>;
1973 See L</seek()> for some details about mixing reading and writing.
1975 =item opendir DIRHANDLE,EXPR
1977 Opens a directory named EXPR for processing by readdir(), telldir(),
1978 seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
1979 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1985 Returns the numeric ascii value of the first character of EXPR. If
1986 EXPR is omitted, uses $_.
1988 =item pack TEMPLATE,LIST
1990 Takes an array or list of values and packs it into a binary structure,
1991 returning the string containing the structure. The TEMPLATE is a
1992 sequence of characters that give the order and type of values, as
1995 A An ascii string, will be space padded.
1996 a An ascii string, will be null padded.
1997 b A bit string (ascending bit order, like vec()).
1998 B A bit string (descending bit order).
1999 h A hex string (low nybble first).
2000 H A hex string (high nybble first).
2002 c A signed char value.
2003 C An unsigned char value.
2004 s A signed short value.
2005 S An unsigned short value.
2006 i A signed integer value.
2007 I An unsigned integer value.
2008 l A signed long value.
2009 L An unsigned long value.
2011 n A short in "network" order.
2012 N A long in "network" order.
2013 v A short in "VAX" (little-endian) order.
2014 V A long in "VAX" (little-endian) order.
2016 f A single-precision float in the native format.
2017 d A double-precision float in the native format.
2019 p A pointer to a null-terminated string.
2020 P A pointer to a structure (fixed-length string).
2022 u A uuencoded string.
2024 w A BER compressed integer. Bytes give an unsigned integer base
2025 128, most significant digit first, with as few digits as
2026 possible, and with the bit 8 of each byte except the last set
2031 @ Null fill to absolute position.
2033 Each letter may optionally be followed by a number which gives a repeat
2034 count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
2035 pack function will gobble up that many values from the LIST. A * for the
2036 repeat count means to use however many items are left. The "a" and "A"
2037 types gobble just one value, but pack it as a string of length count,
2038 padding with nulls or spaces as necessary. (When unpacking, "A" strips
2039 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2040 fields pack a string that many bits long. The "h" and "H" fields pack a
2041 string that many nybbles long. The "P" packs a pointer to a structure of
2042 the size indicated by the length. Real numbers (floats and doubles) are
2043 in the native machine format only; due to the multiplicity of floating
2044 formats around, and the lack of a standard "network" representation, no
2045 facility for interchange has been made. This means that packed floating
2046 point data written on one machine may not be readable on another - even if
2047 both use IEEE floating point arithmetic (as the endian-ness of the memory
2048 representation is not part of the IEEE spec). Note that Perl uses doubles
2049 internally for all numeric calculation, and converting from double into
2050 float and thence back to double again will lose precision (i.e.,
2051 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2055 $foo = pack("cccc",65,66,67,68);
2057 $foo = pack("c4",65,66,67,68);
2060 $foo = pack("ccxxcc",65,66,67,68);
2063 $foo = pack("s2",1,2);
2064 # "\1\0\2\0" on little-endian
2065 # "\0\1\0\2" on big-endian
2067 $foo = pack("a4","abcd","x","y","z");
2070 $foo = pack("aaaa","abcd","x","y","z");
2073 $foo = pack("a14","abcdefg");
2074 # "abcdefg\0\0\0\0\0\0\0"
2076 $foo = pack("i9pl", gmtime);
2077 # a real struct tm (on my system anyway)
2080 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2083 The same template may generally also be used in the unpack function.
2085 =item package NAMESPACE
2087 Declares the compilation unit as being in the given namespace. The scope
2088 of the package declaration is from the declaration itself through the end of
2089 the enclosing block (the same scope as the local() operator). All further
2090 unqualified dynamic identifiers will be in this namespace. A package
2091 statement affects only dynamic variables--including those you've used
2092 local() on--but I<not> lexical variables created with my(). Typically it
2093 would be the first declaration in a file to be included by the C<require>
2094 or C<use> operator. You can switch into a package in more than one place;
2095 it influences merely which symbol table is used by the compiler for the
2096 rest of that block. You can refer to variables and filehandles in other
2097 packages by prefixing the identifier with the package name and a double
2098 colon: C<$Package::Variable>. If the package name is null, the C<main>
2099 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2101 See L<perlmod/"Packages"> for more information about packages, modules,
2102 and classes. See L<perlsub> for other scoping issues.
2104 =item pipe READHANDLE,WRITEHANDLE
2106 Opens a pair of connected pipes like the corresponding system call.
2107 Note that if you set up a loop of piped processes, deadlock can occur
2108 unless you are very careful. In addition, note that Perl's pipes use
2109 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2110 after each command, depending on the application.
2112 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2113 for examples of such things.
2119 Pops and returns the last value of the array, shortening the array by
2120 1. Has a similar effect to
2122 $tmp = $ARRAY[$#ARRAY--];
2124 If there are no elements in the array, returns the undefined value.
2125 If ARRAY is omitted, pops the
2126 @ARGV array in the main program, and the @_ array in subroutines, just
2133 Returns the offset of where the last C<m//g> search left off for the variable
2134 is in question ($_ is used when the variable is not specified). May be
2135 modified to change that offset. Such modification will also influence
2136 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2139 =item print FILEHANDLE LIST
2145 Prints a string or a comma-separated list of strings. Returns TRUE
2146 if successful. FILEHANDLE may be a scalar variable name, in which case
2147 the variable contains the name of or a reference to the filehandle, thus introducing one
2148 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2149 token is a term, it may be misinterpreted as an operator unless you
2150 interpose a + or put parentheses around the arguments.) If FILEHANDLE is
2151 omitted, prints by default to standard output (or to the last selected
2152 output channel--see L</select>). If LIST is also omitted, prints $_ to
2153 STDOUT. To set the default output channel to something other than
2154 STDOUT use the select operation. Note that, because print takes a
2155 LIST, anything in the LIST is evaluated in a list context, and any
2156 subroutine that you call will have one or more of its expressions
2157 evaluated in a list context. Also be careful not to follow the print
2158 keyword with a left parenthesis unless you want the corresponding right
2159 parenthesis to terminate the arguments to the print--interpose a + or
2160 put parentheses around all the arguments.
2162 Note that if you're storing FILEHANDLES in an array or other expression,
2163 you will have to use a block returning its value instead:
2165 print { $files[$i] } "stuff\n";
2166 print { $OK ? STDOUT : STDERR } "stuff\n";
2168 =item printf FILEHANDLE FORMAT, LIST
2170 =item printf FORMAT, LIST
2172 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2173 of the list will be interpreted as the printf format. If C<use locale> is
2174 in effect, the character used for the decimal point in formatted real numbers
2175 is affected by the LC_NUMERIC locale. See L<perllocale>.
2177 Don't fall into the trap of using a printf() when a simple
2178 print() would do. The print() is more efficient, and less
2181 =item prototype FUNCTION
2183 Returns the prototype of a function as a string (or C<undef> if the
2184 function has no prototype). FUNCTION is a reference to, or the name of,
2185 the function whose prototype you want to retrieve.
2187 =item push ARRAY,LIST
2189 Treats ARRAY as a stack, and pushes the values of LIST
2190 onto the end of ARRAY. The length of ARRAY increases by the length of
2191 LIST. Has the same effect as
2194 $ARRAY[++$#ARRAY] = $value;
2197 but is more efficient. Returns the new number of elements in the array.
2207 Generalized quotes. See L<perlop>.
2209 =item quotemeta EXPR
2213 Returns the value of EXPR with with all non-alphanumeric
2214 characters backslashed. (That is, all characters not matching
2215 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2216 returned string, regardless of any locale settings.)
2217 This is the internal function implementing
2218 the \Q escape in double-quoted strings.
2220 If EXPR is omitted, uses $_.
2226 Returns a random fractional number between 0 and the value of EXPR.
2227 (EXPR should be positive.) If EXPR is omitted, returns a value between
2228 0 and 1. This function produces repeatable sequences unless srand()
2229 is invoked. See also srand().
2231 (Note: if your rand function consistently returns numbers that are too
2232 large or too small, then your version of Perl was probably compiled
2233 with the wrong number of RANDBITS. As a workaround, you can usually
2234 multiply EXPR by the correct power of 2 to get the range you want.
2235 This will make your script unportable, however. It's better to recompile
2238 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2240 =item read FILEHANDLE,SCALAR,LENGTH
2242 Attempts to read LENGTH bytes of data into variable SCALAR from the
2243 specified FILEHANDLE. Returns the number of bytes actually read, or
2244 undef if there was an error. SCALAR will be grown or shrunk to the
2245 length actually read. An OFFSET may be specified to place the read
2246 data at some other place than the beginning of the string. This call
2247 is actually implemented in terms of stdio's fread call. To get a true
2248 read system call, see sysread().
2250 =item readdir DIRHANDLE
2252 Returns the next directory entry for a directory opened by opendir().
2253 If used in a list context, returns all the rest of the entries in the
2254 directory. If there are no more entries, returns an undefined value in
2255 a scalar context or a null list in a list context.
2257 If you're planning to filetest the return values out of a readdir(), you'd
2258 better prepend the directory in question. Otherwise, because we didn't
2259 chdir() there, it would have been testing the wrong file.
2261 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2262 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2269 Returns the value of a symbolic link, if symbolic links are
2270 implemented. If not, gives a fatal error. If there is some system
2271 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2274 =item recv SOCKET,SCALAR,LEN,FLAGS
2276 Receives a message on a socket. Attempts to receive LENGTH bytes of
2277 data into variable SCALAR from the specified SOCKET filehandle.
2278 Actually does a C recvfrom(), so that it can returns the address of the
2279 sender. Returns the undefined value if there's an error. SCALAR will
2280 be grown or shrunk to the length actually read. Takes the same flags
2281 as the system call of the same name.
2282 See L<perlipc/"UDP: Message Passing"> for examples.
2288 The C<redo> command restarts the loop block without evaluating the
2289 conditional again. The C<continue> block, if any, is not executed. If
2290 the LABEL is omitted, the command refers to the innermost enclosing
2291 loop. This command is normally used by programs that want to lie to
2292 themselves about what was just input:
2294 # a simpleminded Pascal comment stripper
2295 # (warning: assumes no { or } in strings)
2296 LINE: while (<STDIN>) {
2297 while (s|({.*}.*){.*}|$1 |) {}
2302 if (/}/) { # end of comment?
2315 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2316 is not specified, $_ will be used. The value returned depends on the
2317 type of thing the reference is a reference to.
2318 Builtin types include:
2327 If the referenced object has been blessed into a package, then that package
2328 name is returned instead. You can think of ref() as a typeof() operator.
2330 if (ref($r) eq "HASH") {
2331 print "r is a reference to an associative array.\n";
2334 print "r is not a reference at all.\n";
2337 See also L<perlref>.
2339 =item rename OLDNAME,NEWNAME
2341 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2342 not work across file system boundaries.
2348 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2349 supplied. If EXPR is numeric, demands that the current version of Perl
2350 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2352 Otherwise, demands that a library file be included if it hasn't already
2353 been included. The file is included via the do-FILE mechanism, which is
2354 essentially just a variety of eval(). Has semantics similar to the following
2358 local($filename) = @_;
2359 return 1 if $INC{$filename};
2360 local($realfilename,$result);
2362 foreach $prefix (@INC) {
2363 $realfilename = "$prefix/$filename";
2364 if (-f $realfilename) {
2365 $result = do $realfilename;
2369 die "Can't find $filename in \@INC";
2372 die "$filename did not return true value" unless $result;
2373 $INC{$filename} = $realfilename;
2377 Note that the file will not be included twice under the same specified
2378 name. The file must return TRUE as the last statement to indicate
2379 successful execution of any initialization code, so it's customary to
2380 end such a file with "1;" unless you're sure it'll return TRUE
2381 otherwise. But it's better just to put the "C<1;>", in case you add more
2384 If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2385 replaces "F<::>" with "F</>" in the filename for you,
2386 to make it easy to load standard modules. This form of loading of
2387 modules does not risk altering your namespace.
2389 For a yet-more-powerful import facility, see L</use> and
2396 Generally used in a C<continue> block at the end of a loop to clear
2397 variables and reset ?? searches so that they work again. The
2398 expression is interpreted as a list of single characters (hyphens
2399 allowed for ranges). All variables and arrays beginning with one of
2400 those letters are reset to their pristine state. If the expression is
2401 omitted, one-match searches (?pattern?) are reset to match again. Resets
2402 only variables or searches in the current package. Always returns
2405 reset 'X'; # reset all X variables
2406 reset 'a-z'; # reset lower case variables
2407 reset; # just reset ?? searches
2409 Resetting "A-Z" is not recommended because you'll wipe out your
2410 ARGV and ENV arrays. Resets only package variables--lexical variables
2411 are unaffected, but they clean themselves up on scope exit anyway,
2412 so you'll probably want to use them instead. See L</my>.
2416 Returns from a subroutine or eval with the value specified. (Note that
2417 in the absence of a return a subroutine or eval() will automatically
2418 return the value of the last expression evaluated.)
2422 In a list context, returns a list value consisting of the elements
2423 of LIST in the opposite order. In a scalar context, returns a string
2424 value consisting of the bytes of the first element of LIST in the
2427 print reverse <>; # line tac
2430 print scalar reverse scalar <>; # byte tac
2432 =item rewinddir DIRHANDLE
2434 Sets the current position to the beginning of the directory for the
2435 readdir() routine on DIRHANDLE.
2437 =item rindex STR,SUBSTR,POSITION
2439 =item rindex STR,SUBSTR
2441 Works just like index except that it returns the position of the LAST
2442 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2443 last occurrence at or before that position.
2445 =item rmdir FILENAME
2449 Deletes the directory specified by FILENAME if it is empty. If it
2450 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2451 FILENAME is omitted, uses $_.
2455 The substitution operator. See L<perlop>.
2459 Forces EXPR to be interpreted in a scalar context and returns the value
2462 @counts = ( scalar @a, scalar @b, scalar @c );
2464 There is no equivalent operator to force an expression to
2465 be interpolated in a list context because it's in practice never
2466 needed. If you really wanted to do so, however, you could use
2467 the construction C<@{[ (some expression) ]}>, but usually a simple
2468 C<(some expression)> suffices.
2470 =item seek FILEHANDLE,POSITION,WHENCE
2472 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2473 call of stdio. FILEHANDLE may be an expression whose value gives the name
2474 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2475 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2476 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2477 this from POSIX module. Returns 1 upon success, 0 otherwise.
2479 On some systems you have to do a seek whenever you switch between reading
2480 and writing. Amongst other things, this may have the effect of calling
2481 stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2486 This is also useful for applications emulating C<tail -f>. Once you hit
2487 EOF on your read, and then sleep for a while, you might have to stick in a
2488 seek() to reset things. First the simple trick listed above to clear the
2489 filepointer. The seek() doesn't change the current position, but it
2490 I<does> clear the end-of-file condition on the handle, so that the next
2491 C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2493 If that doesn't work (some stdios are particularly cantankerous), then
2494 you may need something more like this:
2497 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2498 # search for some stuff and put it into files
2500 sleep($for_a_while);
2501 seek(FILE, $curpos, 0);
2504 =item seekdir DIRHANDLE,POS
2506 Sets the current position for the readdir() routine on DIRHANDLE. POS
2507 must be a value returned by telldir(). Has the same caveats about
2508 possible directory compaction as the corresponding system library
2511 =item select FILEHANDLE
2515 Returns the currently selected filehandle. Sets the current default
2516 filehandle for output, if FILEHANDLE is supplied. This has two
2517 effects: first, a C<write> or a C<print> without a filehandle will
2518 default to this FILEHANDLE. Second, references to variables related to
2519 output will refer to this output channel. For example, if you have to
2520 set the top of form format for more than one output channel, you might
2528 FILEHANDLE may be an expression whose value gives the name of the
2529 actual filehandle. Thus:
2531 $oldfh = select(STDERR); $| = 1; select($oldfh);
2533 Some programmers may prefer to think of filehandles as objects with
2534 methods, preferring to write the last example as:
2537 STDERR->autoflush(1);
2539 =item select RBITS,WBITS,EBITS,TIMEOUT
2541 This calls the select(2) system call with the bit masks specified, which
2542 can be constructed using fileno() and vec(), along these lines:
2544 $rin = $win = $ein = '';
2545 vec($rin,fileno(STDIN),1) = 1;
2546 vec($win,fileno(STDOUT),1) = 1;
2549 If you want to select on many filehandles you might wish to write a
2553 local(@fhlist) = split(' ',$_[0]);
2556 vec($bits,fileno($_),1) = 1;
2560 $rin = fhbits('STDIN TTY SOCK');
2564 ($nfound,$timeleft) =
2565 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2567 or to block until something becomes ready just do this
2569 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2571 Most systems do not bother to return anything useful in $timeleft, so
2572 calling select() in a scalar context just returns $nfound.
2574 Any of the bit masks can also be undef. The timeout, if specified, is
2575 in seconds, which may be fractional. Note: not all implementations are
2576 capable of returning the $timeleft. If not, they always return
2577 $timeleft equal to the supplied $timeout.
2579 You can effect a sleep of 250 milliseconds this way:
2581 select(undef, undef, undef, 0.25);
2583 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2584 with select(). You have to use sysread() instead.
2586 =item semctl ID,SEMNUM,CMD,ARG
2588 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2589 &GETALL, then ARG must be a variable which will hold the returned
2590 semid_ds structure or semaphore value array. Returns like ioctl: the
2591 undefined value for error, "0 but true" for zero, or the actual return
2594 =item semget KEY,NSEMS,FLAGS
2596 Calls the System V IPC function semget. Returns the semaphore id, or
2597 the undefined value if there is an error.
2599 =item semop KEY,OPSTRING
2601 Calls the System V IPC function semop to perform semaphore operations
2602 such as signaling and waiting. OPSTRING must be a packed array of
2603 semop structures. Each semop structure can be generated with
2604 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2605 operations is implied by the length of OPSTRING. Returns TRUE if
2606 successful, or FALSE if there is an error. As an example, the
2607 following code waits on semaphore $semnum of semaphore id $semid:
2609 $semop = pack("sss", $semnum, -1, 0);
2610 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2612 To signal the semaphore, replace "-1" with "1".
2614 =item send SOCKET,MSG,FLAGS,TO
2616 =item send SOCKET,MSG,FLAGS
2618 Sends a message on a socket. Takes the same flags as the system call
2619 of the same name. On unconnected sockets you must specify a
2620 destination to send TO, in which case it does a C sendto(). Returns
2621 the number of characters sent, or the undefined value if there is an
2623 See L<perlipc/"UDP: Message Passing"> for examples.
2625 =item setpgrp PID,PGRP
2627 Sets the current process group for the specified PID, 0 for the current
2628 process. Will produce a fatal error if used on a machine that doesn't
2629 implement setpgrp(2). If the arguments are omitted, it defaults to
2630 0,0. Note that the POSIX version of setpgrp() does not accept any
2631 arguments, so only setpgrp 0,0 is portable.
2633 =item setpriority WHICH,WHO,PRIORITY
2635 Sets the current priority for a process, a process group, or a user.
2636 (See setpriority(2).) Will produce a fatal error if used on a machine
2637 that doesn't implement setpriority(2).
2639 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2641 Sets the socket option requested. Returns undefined if there is an
2642 error. OPTVAL may be specified as undef if you don't want to pass an
2649 Shifts the first value of the array off and returns it, shortening the
2650 array by 1 and moving everything down. If there are no elements in the
2651 array, returns the undefined value. If ARRAY is omitted, shifts the
2652 @ARGV array in the main program, and the @_ array in subroutines.
2653 (This is determined lexically.) See also unshift(), push(), and pop().
2654 Shift() and unshift() do the same thing to the left end of an array
2655 that push() and pop() do to the right end.
2657 =item shmctl ID,CMD,ARG
2659 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2660 must be a variable which will hold the returned shmid_ds structure.
2661 Returns like ioctl: the undefined value for error, "0 but true" for
2662 zero, or the actual return value otherwise.
2664 =item shmget KEY,SIZE,FLAGS
2666 Calls the System V IPC function shmget. Returns the shared memory
2667 segment id, or the undefined value if there is an error.
2669 =item shmread ID,VAR,POS,SIZE
2671 =item shmwrite ID,STRING,POS,SIZE
2673 Reads or writes the System V shared memory segment ID starting at
2674 position POS for size SIZE by attaching to it, copying in/out, and
2675 detaching from it. When reading, VAR must be a variable which will
2676 hold the data read. When writing, if STRING is too long, only SIZE
2677 bytes are used; if STRING is too short, nulls are written to fill out
2678 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2680 =item shutdown SOCKET,HOW
2682 Shuts down a socket connection in the manner indicated by HOW, which
2683 has the same interpretation as in the system call of the same name.
2689 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2692 For the inverse sine operation, you may use the POSIX::sin()
2693 function, or use this relation:
2695 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2701 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2702 May be interrupted by sending the process a SIGALRM. Returns the
2703 number of seconds actually slept. You probably cannot mix alarm() and
2704 sleep() calls, because sleep() is often implemented using alarm().
2706 On some older systems, it may sleep up to a full second less than what
2707 you requested, depending on how it counts seconds. Most modern systems
2708 always sleep the full amount.
2710 For delays of finer granularity than one second, you may use Perl's
2711 syscall() interface to access setitimer(2) if your system supports it,
2712 or else see L</select()> below.
2714 See also the POSIX module's sigpause() function.
2716 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2718 Opens a socket of the specified kind and attaches it to filehandle
2719 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
2720 system call of the same name. You should "use Socket;" first to get
2721 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2723 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2725 Creates an unnamed pair of sockets in the specified domain, of the
2726 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
2727 for the system call of the same name. If unimplemented, yields a fatal
2728 error. Returns TRUE if successful.
2730 =item sort SUBNAME LIST
2732 =item sort BLOCK LIST
2736 Sorts the LIST and returns the sorted list value. Nonexistent values
2737 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2738 in standard string comparison order. If SUBNAME is specified, it
2739 gives the name of a subroutine that returns an integer less than, equal
2740 to, or greater than 0, depending on how the elements of the array are
2741 to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
2742 routines.) SUBNAME may be a scalar variable name, in which case the
2743 value provides the name of the subroutine to use. In place of a
2744 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2747 In the interests of efficiency the normal calling code for subroutines is
2748 bypassed, with the following effects: the subroutine may not be a
2749 recursive subroutine, and the two elements to be compared are passed into
2750 the subroutine not via @_ but as the package global variables $a and
2751 $b (see example below). They are passed by reference, so don't
2752 modify $a and $b. And don't try to declare them as lexicals either.
2754 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2755 current collation locale. See L<perllocale>.
2760 @articles = sort @files;
2762 # same thing, but with explicit sort routine
2763 @articles = sort {$a cmp $b} @files;
2765 # now case-insensitively
2766 @articles = sort { uc($a) cmp uc($b)} @files;
2768 # same thing in reversed order
2769 @articles = sort {$b cmp $a} @files;
2771 # sort numerically ascending
2772 @articles = sort {$a <=> $b} @files;
2774 # sort numerically descending
2775 @articles = sort {$b <=> $a} @files;
2777 # sort using explicit subroutine name
2779 $age{$a} <=> $age{$b}; # presuming integers
2781 @sortedclass = sort byage @class;
2783 # this sorts the %age associative arrays by value
2784 # instead of key using an in-line function
2785 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2787 sub backwards { $b cmp $a; }
2788 @harry = ('dog','cat','x','Cain','Abel');
2789 @george = ('gone','chased','yz','Punished','Axed');
2791 # prints AbelCaincatdogx
2792 print sort backwards @harry;
2793 # prints xdogcatCainAbel
2794 print sort @george, 'to', @harry;
2795 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2797 # inefficiently sort by descending numeric compare using
2798 # the first integer after the first = sign, or the
2799 # whole record case-insensitively otherwise
2802 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2807 # same thing, but much more efficiently;
2808 # we'll build auxiliary indices instead
2812 push @nums, /=(\d+)/;
2817 $nums[$b] <=> $nums[$a]
2819 $caps[$a] cmp $caps[$b]
2823 # same thing using a Schwartzian Transform (no temps)
2824 @new = map { $_->[0] }
2825 sort { $b->[1] <=> $a->[1]
2828 } map { [$_, /=(\d+)/, uc($_)] } @old;
2830 If you're using strict, you I<MUST NOT> declare $a
2831 and $b as lexicals. They are package globals. That means
2832 if you're in the C<main> package, it's
2834 @articles = sort {$main::b <=> $main::a} @files;
2838 @articles = sort {$::b <=> $::a} @files;
2840 but if you're in the C<FooPack> package, it's
2842 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2844 The comparison function is required to behave. If it returns
2845 inconsistent results (sometimes saying $x[1] is less than $x[2] and
2846 sometimes saying the opposite, for example) the Perl interpreter will
2847 probably crash and dump core. This is entirely due to and dependent
2848 upon your system's qsort(3) library routine; this routine often avoids
2849 sanity checks in the interest of speed.
2851 =item splice ARRAY,OFFSET,LENGTH,LIST
2853 =item splice ARRAY,OFFSET,LENGTH
2855 =item splice ARRAY,OFFSET
2857 Removes the elements designated by OFFSET and LENGTH from an array, and
2858 replaces them with the elements of LIST, if any. Returns the elements
2859 removed from the array. The array grows or shrinks as necessary. If
2860 LENGTH is omitted, removes everything from OFFSET onward. The
2861 following equivalences hold (assuming C<$[ == 0>):
2863 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2864 pop(@a) splice(@a,-1)
2865 shift(@a) splice(@a,0,1)
2866 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2867 $a[$x] = $y splice(@a,$x,1,$y);
2869 Example, assuming array lengths are passed before arrays:
2871 sub aeq { # compare two list values
2872 local(@a) = splice(@_,0,shift);
2873 local(@b) = splice(@_,0,shift);
2874 return 0 unless @a == @b; # same len?
2876 return 0 if pop(@a) ne pop(@b);
2880 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2882 =item split /PATTERN/,EXPR,LIMIT
2884 =item split /PATTERN/,EXPR
2886 =item split /PATTERN/
2890 Splits a string into an array of strings, and returns it.
2892 If not in a list context, returns the number of fields found and splits into
2893 the @_ array. (In a list context, you can force the split into @_ by
2894 using C<??> as the pattern delimiters, but it still returns the array
2895 value.) The use of implicit split to @_ is deprecated, however.
2897 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2898 splits on whitespace (after skipping any leading whitespace). Anything
2899 matching PATTERN is taken to be a delimiter separating the fields. (Note
2900 that the delimiter may be longer than one character.) If LIMIT is
2901 specified and is not negative, splits into no more than that many fields
2902 (though it may split into fewer). If LIMIT is unspecified, trailing null
2903 fields are stripped (which potential users of pop() would do well to
2904 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2905 LIMIT had been specified.
2907 A pattern matching the null string (not to be confused with
2908 a null pattern C<//>, which is just one member of the set of patterns
2909 matching a null string) will split the value of EXPR into separate
2910 characters at each point it matches that way. For example:
2912 print join(':', split(/ */, 'hi there'));
2914 produces the output 'h:i:t:h:e:r:e'.
2916 The LIMIT parameter can be used to split a line partially
2918 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2920 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2921 one larger than the number of variables in the list, to avoid
2922 unnecessary work. For the list above LIMIT would have been 4 by
2923 default. In time critical applications it behooves you not to split
2924 into more fields than you really need.
2926 If the PATTERN contains parentheses, additional array elements are
2927 created from each matching substring in the delimiter.
2929 split(/([,-])/, "1-10,20", 3);
2931 produces the list value
2933 (1, '-', 10, ',', 20)
2935 If you had the entire header of a normal Unix email message in $header,
2936 you could split it up into fields and their values this way:
2938 $header =~ s/\n\s+/ /g; # fix continuation lines
2939 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2941 The pattern C</PATTERN/> may be replaced with an expression to specify
2942 patterns that vary at runtime. (To do runtime compilation only once,
2943 use C</$variable/o>.)
2945 As a special case, specifying a PATTERN of space (C<' '>) will split on
2946 white space just as split with no arguments does. Thus, split(' ') can
2947 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2948 will give you as many null initial fields as there are leading spaces.
2949 A split on /\s+/ is like a split(' ') except that any leading
2950 whitespace produces a null first field. A split with no arguments
2951 really does a C<split(' ', $_)> internally.
2955 open(passwd, '/etc/passwd');
2957 ($login, $passwd, $uid, $gid, $gcos,
2958 $home, $shell) = split(/:/);
2962 (Note that $shell above will still have a newline on it. See L</chop>,
2963 L</chomp>, and L</join>.)
2965 =item sprintf FORMAT, LIST
2967 Returns a string formatted by the usual printf conventions of the C
2968 language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2969 (The * character for an indirectly specified length is not
2970 supported, but you can get the same effect by interpolating a variable
2971 into the pattern.) If C<use locale> is
2972 in effect, the character used for the decimal point in formatted real numbers
2973 is affected by the LC_NUMERIC locale. See L<perllocale>.
2974 Some C libraries' implementations of sprintf() can
2975 dump core when fed ludicrous arguments.
2981 Return the square root of EXPR. If EXPR is omitted, returns square
2986 Sets the random number seed for the C<rand> operator. If EXPR is omitted,
2987 uses a semi-random value based on the current time and process ID, among
2990 Simply seeding with time() and the process ID isn't particularly random,
2991 especially if they vary together.
2993 Try something like this instead:
2995 srand( time() ^ ($$ + ($$ << 15)) );
2997 Of course, you'd need something much more random than that for
2998 serious cryptographic purposes, since it's easy to guess the current time.
2999 Checksumming the compressed output of one or more rapidly changing operating
3000 system status programs is the usual method. For example:
3002 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3004 Do I<not>fP call srand() multiple times in your program unless you know
3005 exactly what you're doing and why you're doing it. The point of the
3006 function is to "seed" the rand() function so that rand() can produce
3007 a different sequence each time you run your program. Just do it once at the
3008 top of your program, or you I<won't> get random numbers out of rand()!
3010 Frequently called programs (like CGI scripts) that simply use
3014 for a seed can fall prey to the mathematical property that
3018 one-third of the time. If you're particularly concerned with this,
3019 see the Math::TrulyRandom module in CPAN.
3021 =item stat FILEHANDLE
3027 Returns a 13-element array giving the status info for a file, either the
3028 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
3029 stats $_. Returns a null list if the stat fails. Typically used as
3033 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3034 $atime,$mtime,$ctime,$blksize,$blocks)
3037 Not all fields are supported on all filesystem types. Here are the
3038 meaning of the fields:
3040 dev device number of filesystem
3042 mode file mode (type and permissions)
3043 nlink number of (hard) links to the file
3044 uid numeric user ID of file's owner
3045 gid numeric group ID of file's owner
3046 rdev the device identifier (special files only)
3047 size total size of file, in bytes
3048 atime last access time since the epoch
3049 mtime last modify time since the epoch
3050 ctime inode change time (NOT creation type!) since the epoch
3051 blksize preferred block size for file system I/O
3052 blocks actual number of blocks allocated
3054 (The epoch was at 00:00 January 1, 1970 GMT.)
3056 If stat is passed the special filehandle consisting of an underline, no
3057 stat is done, but the current contents of the stat structure from the
3058 last stat or filetest are returned. Example:
3060 if (-x $file && (($d) = stat(_)) && $d < 0) {
3061 print "$file is executable NFS file\n";
3064 (This works on machines only for which the device number is negative under NFS.)
3070 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3071 doing many pattern matches on the string before it is next modified.
3072 This may or may not save time, depending on the nature and number of
3073 patterns you are searching on, and on the distribution of character
3074 frequencies in the string to be searched--you probably want to compare
3075 run times with and without it to see which runs faster. Those loops
3076 which scan for many short constant strings (including the constant
3077 parts of more complex patterns) will benefit most. You may have only
3078 one study active at a time--if you study a different scalar the first
3079 is "unstudied". (The way study works is this: a linked list of every
3080 character in the string to be searched is made, so we know, for
3081 example, where all the 'k' characters are. From each search string,
3082 the rarest character is selected, based on some static frequency tables
3083 constructed from some C programs and English text. Only those places
3084 that contain this "rarest" character are examined.)
3086 For example, here is a loop which inserts index producing entries
3087 before any line containing a certain pattern:
3091 print ".IX foo\n" if /\bfoo\b/;
3092 print ".IX bar\n" if /\bbar\b/;
3093 print ".IX blurfl\n" if /\bblurfl\b/;
3098 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3099 will be looked at, because "f" is rarer than "o". In general, this is
3100 a big win except in pathological cases. The only question is whether
3101 it saves you more time than it took to build the linked list in the
3104 Note that if you have to look for strings that you don't know till
3105 runtime, you can build an entire loop as a string and eval that to
3106 avoid recompiling all your patterns all the time. Together with
3107 undefining $/ to input entire files as one record, this can be very
3108 fast, often faster than specialized programs like fgrep(1). The following
3109 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3110 out the names of those files that contain a match:
3112 $search = 'while (<>) { study;';
3113 foreach $word (@words) {
3114 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3119 eval $search; # this screams
3120 $/ = "\n"; # put back to normal input delimiter
3121 foreach $file (sort keys(%seen)) {
3129 =item sub NAME BLOCK
3131 This is subroutine definition, not a real function I<per se>. With just a
3132 NAME (and possibly prototypes), it's just a forward declaration. Without
3133 a NAME, it's an anonymous function declaration, and does actually return a
3134 value: the CODE ref of the closure you just created. See L<perlsub> and
3135 L<perlref> for details.
3137 =item substr EXPR,OFFSET,LEN
3139 =item substr EXPR,OFFSET
3141 Extracts a substring out of EXPR and returns it. First character is at
3142 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
3143 that far from the end of the string. If LEN is omitted, returns
3144 everything to the end of the string. If LEN is negative, leaves that
3145 many characters off the end of the string.
3147 You can use the substr() function
3148 as an lvalue, in which case EXPR must be an lvalue. If you assign
3149 something shorter than LEN, the string will shrink, and if you assign
3150 something longer than LEN, the string will grow to accommodate it. To
3151 keep the string the same length you may need to pad or chop your value
3154 =item symlink OLDFILE,NEWFILE
3156 Creates a new filename symbolically linked to the old filename.
3157 Returns 1 for success, 0 otherwise. On systems that don't support
3158 symbolic links, produces a fatal error at run time. To check for that,
3161 $symlink_exists = (eval 'symlink("","");', $@ eq '');
3165 Calls the system call specified as the first element of the list,
3166 passing the remaining elements as arguments to the system call. If
3167 unimplemented, produces a fatal error. The arguments are interpreted
3168 as follows: if a given argument is numeric, the argument is passed as
3169 an int. If not, the pointer to the string value is passed. You are
3170 responsible to make sure a string is pre-extended long enough to
3171 receive any result that might be written into a string. If your
3172 integer arguments are not literals and have never been interpreted in a
3173 numeric context, you may need to add 0 to them to force them to look
3176 require 'syscall.ph'; # may need to run h2ph
3177 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3179 Note that Perl supports passing of up to only 14 arguments to your system call,
3180 which in practice should usually suffice.
3182 =item sysopen FILEHANDLE,FILENAME,MODE
3184 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3186 Opens the file whose filename is given by FILENAME, and associates it
3187 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3188 the name of the real filehandle wanted. This function calls the
3189 underlying operating system's C<open> function with the parameters
3190 FILENAME, MODE, PERMS.
3192 The possible values and flag bits of the MODE parameter are
3193 system-dependent; they are available via the standard module C<Fcntl>.
3194 However, for historical reasons, some values are universal: zero means
3195 read-only, one means write-only, and two means read/write.
3197 If the file named by FILENAME does not exist and the C<open> call
3198 creates it (typically because MODE includes the O_CREAT flag), then
3199 the value of PERMS specifies the permissions of the newly created
3200 file. If PERMS is omitted, the default value is 0666, which allows
3201 read and write for all. This default is reasonable: see C<umask>.
3203 The IO::File module provides a more object-oriented approach, if you're
3204 into that kind of thing.
3206 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3208 =item sysread FILEHANDLE,SCALAR,LENGTH
3210 Attempts to read LENGTH bytes of data into variable SCALAR from the
3211 specified FILEHANDLE, using the system call read(2). It bypasses
3212 stdio, so mixing this with other kinds of reads may cause confusion.
3213 Returns the number of bytes actually read, or undef if there was an
3214 error. SCALAR will be grown or shrunk so that the last byte actually
3215 read is the last byte of the scalar after the read.
3217 An OFFSET may be specified to place the read data at some place in the
3218 string other than the beginning. A negative OFFSET specifies
3219 placement at that many bytes counting backwards from the end of the
3220 string. A positive OFFSET greater than the length of SCALAR results
3221 in the string being padded to the required size with "\0" bytes before
3222 the result of the read is appended.
3226 Does exactly the same thing as "exec LIST" except that a fork is done
3227 first, and the parent process waits for the child process to complete.
3228 Note that argument processing varies depending on the number of
3229 arguments. The return value is the exit status of the program as
3230 returned by the wait() call. To get the actual exit value divide by
3231 256. See also L</exec>. This is I<NOT> what you want to use to capture
3232 the output from a command, for that you should use merely back-ticks or
3233 qx//, as described in L<perlop/"`STRING`">.
3235 Because system() and back-ticks block SIGINT and SIGQUIT, killing the
3236 program they're running doesn't actually interrupt your program.
3238 @args = ("command", "arg1", "arg2");
3240 or die "system @args failed: $?"
3242 Here's a more elaborate example of analysing the return value from
3243 system() on a UNIX system to check for all possibilities, including for
3244 signals and coredumps.
3246 $rc = 0xffff & system @args;
3247 printf "system(%s) returned %#04x: ", "@args", $rc;
3249 print "ran with normal exit\n";
3251 elsif ($rc == 0xff00) {
3252 print "command failed: $!\n";
3254 elsif ($rc > 0x80) {
3256 print "ran with non-zero exit status $rc\n";
3262 print "coredump from ";
3264 print "signal $rc\n"
3268 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3270 =item syswrite FILEHANDLE,SCALAR,LENGTH
3272 Attempts to write LENGTH bytes of data from variable SCALAR to the
3273 specified FILEHANDLE, using the system call write(2). It bypasses
3274 stdio, so mixing this with prints may cause confusion. Returns the
3275 number of bytes actually written, or undef if there was an error.
3276 If the length is greater than the available data, only as much data as
3277 is available will be written.
3279 An OFFSET may be specified to write the data from some part of the
3280 string other than the beginning. A negative OFFSET specifies writing
3281 from that many bytes counting backwards from the end of the string.
3283 =item tell FILEHANDLE
3287 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3288 expression whose value gives the name of the actual filehandle. If
3289 FILEHANDLE is omitted, assumes the file last read.
3291 =item telldir DIRHANDLE
3293 Returns the current position of the readdir() routines on DIRHANDLE.
3294 Value may be given to seekdir() to access a particular location in a
3295 directory. Has the same caveats about possible directory compaction as
3296 the corresponding system library routine.
3298 =item tie VARIABLE,CLASSNAME,LIST
3300 This function binds a variable to a package class that will provide the
3301 implementation for the variable. VARIABLE is the name of the variable
3302 to be enchanted. CLASSNAME is the name of a class implementing objects
3303 of correct type. Any additional arguments are passed to the "new"
3304 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3305 Typically these are arguments such as might be passed to the dbm_open()
3306 function of C. The object returned by the "new" method is also
3307 returned by the tie() function, which would be useful if you want to
3308 access other methods in CLASSNAME.
3310 Note that functions such as keys() and values() may return huge array
3311 values when used on large objects, like DBM files. You may prefer to
3312 use the each() function to iterate over such. Example:
3314 # print out history file offsets
3316 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3317 while (($key,$val) = each %HIST) {
3318 print $key, ' = ', unpack('L',$val), "\n";
3322 A class implementing an associative array should have the following
3325 TIEHASH classname, LIST
3328 STORE this, key, value
3332 NEXTKEY this, lastkey
3334 A class implementing an ordinary array should have the following methods:
3336 TIEARRAY classname, LIST
3339 STORE this, key, value
3342 A class implementing a scalar should have the following methods:
3344 TIESCALAR classname, LIST
3349 Unlike dbmopen(), the tie() function will not use or require a module
3350 for you--you need to do that explicitly yourself. See L<DB_File>
3351 or the F<Config> module for interesting tie() implementations.
3355 Returns a reference to the object underlying VARIABLE (the same value
3356 that was originally returned by the tie() call which bound the variable
3357 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3362 Returns the number of non-leap seconds since whatever time the system
3363 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3364 and 00:00:00 UTC, January 1, 1970 for most other systems).
3365 Suitable for feeding to gmtime() and localtime().
3369 Returns a four-element array giving the user and system times, in
3370 seconds, for this process and the children of this process.
3372 ($user,$system,$cuser,$csystem) = times;
3376 The translation operator. See L<perlop>.
3378 =item truncate FILEHANDLE,LENGTH
3380 =item truncate EXPR,LENGTH
3382 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3383 specified length. Produces a fatal error if truncate isn't implemented
3390 Returns an uppercased version of EXPR. This is the internal function
3391 implementing the \U escape in double-quoted strings.
3392 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3394 If EXPR is omitted, uses $_.
3400 Returns the value of EXPR with the first character uppercased. This is
3401 the internal function implementing the \u escape in double-quoted strings.
3402 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3404 If EXPR is omitted, uses $_.
3410 Sets the umask for the process and returns the old one. If EXPR is
3411 omitted, returns merely the current umask.
3417 Undefines the value of EXPR, which must be an lvalue. Use on only a
3418 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3419 will probably not do what you expect on most predefined variables or
3420 DBM list values, so don't do that.) Always returns the undefined value. You can omit
3421 the EXPR, in which case nothing is undefined, but you still get an
3422 undefined value that you could, for instance, return from a
3423 subroutine. Examples:
3426 undef $bar{'blurfl'};
3430 return (wantarray ? () : undef) if $they_blew_it;
3436 Deletes a list of files. Returns the number of files successfully
3439 $cnt = unlink 'a', 'b', 'c';
3443 Note: unlink will not delete directories unless you are superuser and
3444 the B<-U> flag is supplied to Perl. Even if these conditions are
3445 met, be warned that unlinking a directory can inflict damage on your
3446 filesystem. Use rmdir instead.
3448 If LIST is omitted, uses $_.
3450 =item unpack TEMPLATE,EXPR
3452 Unpack does the reverse of pack: it takes a string representing a
3453 structure and expands it out into a list value, returning the array
3454 value. (In a scalar context, it returns merely the first value
3455 produced.) The TEMPLATE has the same format as in the pack function.
3456 Here's a subroutine that does substring:
3459 local($what,$where,$howmuch) = @_;
3460 unpack("x$where a$howmuch", $what);
3465 sub ordinal { unpack("c",$_[0]); } # same as ord()
3467 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3468 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3469 themselves. Default is a 16-bit checksum. For example, the following
3470 computes the same number as the System V sum program:
3473 $checksum += unpack("%16C*", $_);
3477 The following efficiently counts the number of set bits in a bit vector:
3479 $setbits = unpack("%32b*", $selectmask);
3481 =item untie VARIABLE
3483 Breaks the binding between a variable and a package. (See tie().)
3485 =item unshift ARRAY,LIST
3487 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3488 depending on how you look at it. Prepends list to the front of the
3489 array, and returns the new number of elements in the array.
3491 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3493 Note the LIST is prepended whole, not one element at a time, so the
3494 prepended elements stay in the same order. Use reverse to do the
3497 =item use Module LIST
3501 =item use Module VERSION LIST
3505 Imports some semantics into the current package from the named module,
3506 generally by aliasing certain subroutine or variable names into your
3507 package. It is exactly equivalent to
3509 BEGIN { require Module; import Module LIST; }
3511 except that Module I<must> be a bare word.
3513 If the first argument to C<use> is a number, it is treated as a version
3514 number instead of a module name. If the version of the Perl interpreter
3515 is less than VERSION, then an error message is printed and Perl exits
3516 immediately. This is often useful if you need to check the current
3517 Perl version before C<use>ing library modules which have changed in
3518 incompatible ways from older versions of Perl. (We try not to do
3519 this more than we have to.)
3521 The BEGIN forces the require and import to happen at compile time. The
3522 require makes sure the module is loaded into memory if it hasn't been
3523 yet. The import is not a builtin--it's just an ordinary static method
3524 call into the "Module" package to tell the module to import the list of
3525 features back into the current package. The module can implement its
3526 import method any way it likes, though most modules just choose to
3527 derive their import method via inheritance from the Exporter class that
3528 is defined in the Exporter module. See L<Exporter>. If no import
3529 method can be found then the error is currently silently ignored. This
3530 may change to a fatal error in a future version.
3532 If you don't want your namespace altered, explicitly supply an empty list:
3536 That is exactly equivalent to
3538 BEGIN { require Module; }
3540 If the VERSION argument is present between Module and LIST, then the
3541 C<use> will call the VERSION method in class Module with the given
3542 version as an argument. The default VERSION method, inherited from
3543 the Universal class, croaks if the given version is larger than the
3544 value of the variable $Module::VERSION. (Note that there is not a
3545 comma after VERSION!)
3547 Because this is a wide-open interface, pragmas (compiler directives)
3548 are also implemented this way. Currently implemented pragmas are:
3552 use sigtrap qw(SEGV BUS);
3553 use strict qw(subs vars refs);
3554 use subs qw(afunc blurfl);
3556 These pseudo-modules import semantics into the current block scope, unlike
3557 ordinary modules, which import symbols into the current package (which are
3558 effective through the end of the file).
3560 There's a corresponding "no" command that unimports meanings imported
3561 by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
3566 If no unimport method can be found the call fails with a fatal error.
3568 See L<perlmod> for a list of standard modules and pragmas.
3572 Changes the access and modification times on each file of a list of
3573 files. The first two elements of the list must be the NUMERICAL access
3574 and modification times, in that order. Returns the number of files
3575 successfully changed. The inode modification time of each file is set
3576 to the current time. Example of a "touch" command:
3580 utime $now, $now, @ARGV;
3582 =item values ASSOC_ARRAY
3584 Returns a normal array consisting of all the values of the named
3585 associative array. (In a scalar context, returns the number of
3586 values.) The values are returned in an apparently random order, but it
3587 is the same order as either the keys() or each() function would produce
3588 on the same array. See also keys(), each(), and sort().
3590 =item vec EXPR,OFFSET,BITS
3592 Treats the string in EXPR as a vector of unsigned integers, and
3593 returns the value of the bit field specified by OFFSET. BITS specifies
3594 the number of bits that are reserved for each entry in the bit
3595 vector. This must be a power of two from 1 to 32. vec() may also be
3596 assigned to, in which case parentheses are needed to give the expression
3597 the correct precedence as in
3599 vec($image, $max_x * $x + $y, 8) = 3;
3601 Vectors created with vec() can also be manipulated with the logical
3602 operators |, &, and ^, which will assume a bit vector operation is
3603 desired when both operands are strings.
3605 To transform a bit vector into a string or array of 0's and 1's, use these:
3607 $bits = unpack("b*", $vector);
3608 @bits = split(//, unpack("b*", $vector));
3610 If you know the exact length in bits, it can be used in place of the *.
3614 Waits for a child process to terminate and returns the pid of the
3615 deceased process, or -1 if there are no child processes. The status is
3618 =item waitpid PID,FLAGS
3620 Waits for a particular child process to terminate and returns the pid
3621 of the deceased process, or -1 if there is no such child process. The
3622 status is returned in C<$?>. If you say
3624 use POSIX ":sys_wait_h";
3626 waitpid(-1,&WNOHANG);
3628 then you can do a non-blocking wait for any process. Non-blocking wait
3629 is available on machines supporting either the waitpid(2) or
3630 wait4(2) system calls. However, waiting for a particular pid with
3631 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3632 by remembering the status values of processes that have exited but have
3633 not been harvested by the Perl script yet.)
3637 Returns TRUE if the context of the currently executing subroutine is
3638 looking for a list value. Returns FALSE if the context is looking
3641 return wantarray ? () : undef;
3645 Produces a message on STDERR just like die(), but doesn't exit or
3648 =item write FILEHANDLE
3654 Writes a formatted record (possibly multi-line) to the specified file,
3655 using the format associated with that file. By default the format for
3656 a file is the one having the same name is the filehandle, but the
3657 format for the current output channel (see the select() function) may be set
3658 explicitly by assigning the name of the format to the C<$~> variable.
3660 Top of form processing is handled automatically: if there is
3661 insufficient room on the current page for the formatted record, the
3662 page is advanced by writing a form feed, a special top-of-page format
3663 is used to format the new page header, and then the record is written.
3664 By default the top-of-page format is the name of the filehandle with
3665 "_TOP" appended, but it may be dynamically set to the format of your
3666 choice by assigning the name to the C<$^> variable while the filehandle is
3667 selected. The number of lines remaining on the current page is in
3668 variable C<$->, which can be set to 0 to force a new page.
3670 If FILEHANDLE is unspecified, output goes to the current default output
3671 channel, which starts out as STDOUT but may be changed by the
3672 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3673 is evaluated and the resulting string is used to look up the name of
3674 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3676 Note that write is I<NOT> the opposite of read. Unfortunately.
3680 The translation operator. See L<perlop>.