3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar and list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one list argument.) For instance, splice() has three scalar
19 arguments followed by a list.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parentheses.) If you use the parentheses, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+3; # Prints 6.
39 print(1+2) + 3; # Prints 3.
40 print (1+2)+3; # Also prints 3!
41 print +(1+2)+3; # Prints 6.
42 print ((1+2)+3); # Prints 6.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 non-abortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following rule:
59 =item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
63 Each operator and function decides which sort of value it would be most
64 appropriate to return in a scalar context. Some operators return the
65 length of the list that would have been returned in a list context. Some
66 operators return the first value in the list. Some operators return the
67 last value in the list. Some operators return a count of successful
68 operations. In general, they do what you want, unless you want
71 =head2 Perl Functions by Category
73 Here are Perl's functions (including things that look like
74 functions, like some of the keywords and named operators)
75 arranged by category. Some functions appear in more
80 =item Functions for SCALARs or strings
82 chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
83 oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
84 sprintf, substr, tr///, uc, ucfirst, y///
86 =item Regular expressions and pattern matching
88 m//, pos, quotemeta, s///, split, study
90 =item Numeric functions
92 abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
95 =item Functions for real @ARRAYs
97 pop, push, shift, splice, unshift
99 =item Functions for list data
101 grep, join, map, qw/STRING/, reverse, sort, unpack
103 =item Functions for real %HASHes
105 delete, each, exists, keys, values
107 =item Input and output functions
109 binmode, close, closedir, dbmclose, dbmopen, die, eof,
110 fileno, flock, format, getc, print, printf, read, readdir,
111 rewinddir, seek, seekdir, select, syscall, sysread,
112 syswrite, tell, telldir, truncate, warn, write
114 =item Functions for fixed length data or records
116 pack, read, syscall, sysread, syswrite, unpack, vec
118 =item Functions for filehandles, files, or directories
120 I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
121 lstat, mkdir, open, opendir, readlink, rename, rmdir,
122 stat, symlink, umask, unlink, utime
124 =item Keywords related to the control flow of your perl program
126 caller, continue, die, do, dump, eval, exit, goto, last,
127 next, redo, return, sub, wantarray
129 =item Keywords related to scoping
131 caller, import, local, my, package, use
133 =item Miscellaneous functions
135 defined, dump, eval, formline, local, my, reset, scalar,
138 =item Functions for processes and process groups
140 alarm, exec, fork, getpgrp, getppid, getpriority, kill,
141 pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
144 =item Keywords related to perl modules
146 do, import, no, package, require, use
148 =item Keywords related to classes and object-orientedness
150 bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
152 =item Low-level socket functions
154 accept, bind, connect, getpeername, getsockname,
155 getsockopt, listen, recv, send, setsockopt, shutdown,
158 =item System V interprocess communication functions
160 msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
161 shmctl, shmget, shmread, shmwrite
163 =item Fetching user and group info
165 endgrent, endhostent, endnetent, endpwent, getgrent,
166 getgrgid, getgrnam, getlogin, getpwent, getpwnam,
167 getpwuid, setgrent, setpwent
169 =item Fetching network info
171 endprotoent, endservent, gethostbyaddr, gethostbyname,
172 gethostent, getnetbyaddr, getnetbyname, getnetent,
173 getprotobyname, getprotobynumber, getprotoent,
174 getservbyname, getservbyport, getservent, sethostent,
175 setnetent, setprotoent, setservent
177 =item Time-related functions
179 gmtime, localtime, time, times
181 =item Functions new in perl5
183 abs, bless, chomp, chr, exists, formline, glob, import, lc,
184 lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
185 ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
187 * - C<sub> was a keyword in perl4, but in perl5 it is an
188 operator which can be used in expressions.
190 =item Functions obsoleted in perl5
197 =head2 Alphabetical Listing of Perl Functions
208 A file test, where X is one of the letters listed below. This unary
209 operator takes one argument, either a filename or a filehandle, and
210 tests the associated file to see if something is true about it. If the
211 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
212 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
213 the undefined value if the file doesn't exist. Despite the funny
214 names, precedence is the same as any other named unary operator, and
215 the argument may be parenthesized like any other unary operator. The
216 operator may be any of:
218 -r File is readable by effective uid/gid.
219 -w File is writable by effective uid/gid.
220 -x File is executable by effective uid/gid.
221 -o File is owned by effective uid.
223 -R File is readable by real uid/gid.
224 -W File is writable by real uid/gid.
225 -X File is executable by real uid/gid.
226 -O File is owned by real uid.
229 -z File has zero size.
230 -s File has non-zero size (returns size).
232 -f File is a plain file.
233 -d File is a directory.
234 -l File is a symbolic link.
235 -p File is a named pipe (FIFO).
237 -b File is a block special file.
238 -c File is a character special file.
239 -t Filehandle is opened to a tty.
241 -u File has setuid bit set.
242 -g File has setgid bit set.
243 -k File has sticky bit set.
245 -T File is a text file.
246 -B File is a binary file (opposite of -T).
248 -M Age of file in days when script started.
249 -A Same for access time.
250 -C Same for inode change time.
252 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
253 C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
254 uids and gids of the user. There may be other reasons you can't actually
255 read, write or execute the file. Also note that, for the superuser,
256 C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
257 1 if any execute bit is set in the mode. Scripts run by the superuser may
258 thus need to do a stat() to determine the actual mode of the
259 file, or temporarily set the uid to something else.
265 next unless -f $_; # ignore specials
269 Note that C<-s/a/b/> does not do a negated substitution. Saying
270 C<-exp($foo)> still works as expected, however--only single letters
271 following a minus are interpreted as file tests.
273 The C<-T> and C<-B> switches work as follows. The first block or so of the
274 file is examined for odd characters such as strange control codes or
275 characters with the high bit set. If too many odd characters (E<gt>30%)
276 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
277 containing null in the first block is considered a binary file. If C<-T>
278 or C<-B> is used on a filehandle, the current stdio buffer is examined
279 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
280 file, or a file at EOF when testing a filehandle. Because you have to
281 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
282 against the file first, as in C<next unless -f $file && -T $file>.
284 If any of the file tests (or either the stat() or lstat() operators) are given
285 the special filehandle consisting of a solitary underline, then the stat
286 structure of the previous file test (or stat operator) is used, saving
287 a system call. (This doesn't work with C<-t>, and you need to remember
288 that lstat() and C<-l> will leave values in the stat structure for the
289 symbolic link, not the real file.) Example:
291 print "Can do.\n" if -r $a || -w _ || -x _;
294 print "Readable\n" if -r _;
295 print "Writable\n" if -w _;
296 print "Executable\n" if -x _;
297 print "Setuid\n" if -u _;
298 print "Setgid\n" if -g _;
299 print "Sticky\n" if -k _;
300 print "Text\n" if -T _;
301 print "Binary\n" if -B _;
307 Returns the absolute value of its argument.
308 If VALUE is omitted, uses $_.
310 =item accept NEWSOCKET,GENERICSOCKET
312 Accepts an incoming socket connect, just as the accept(2) system call
313 does. Returns the packed address if it succeeded, FALSE otherwise.
314 See example in L<perlipc/"Sockets: Client/Server Communication">.
320 Arranges to have a SIGALRM delivered to this process after the
321 specified number of seconds have elapsed. If SECONDS is not specified,
322 the value stored in $_ is used. (On some machines,
323 unfortunately, the elapsed time may be up to one second less than you
324 specified because of how seconds are counted.) Only one timer may be
325 counting at once. Each call disables the previous timer, and an
326 argument of 0 may be supplied to cancel the previous timer without
327 starting a new one. The returned value is the amount of time remaining
328 on the previous timer.
330 For delays of finer granularity than one second, you may use Perl's
331 syscall() interface to access setitimer(2) if your system supports it,
332 or else see L</select()> below. It is not advised to intermix alarm()
335 If you want to use alarm() to time out a system call you need to use an
336 eval/die pair. You can't rely on the alarm causing the system call to
337 fail with $! set to EINTR because Perl sets up signal handlers to
338 restart system calls on some systems. Using eval/die always works.
341 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
343 $nread = sysread SOCKET, $buffer, $size;
346 die if $@ && $@ ne "alarm\n"; # propagate errors
356 Returns the arctangent of Y/X in the range -PI to PI.
358 For the tangent operation, you may use the POSIX::tan()
359 function, or use the familiar relation:
361 sub tan { sin($_[0]) / cos($_[0]) }
363 =item bind SOCKET,NAME
365 Binds a network address to a socket, just as the bind system call
366 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
367 packed address of the appropriate type for the socket. See the examples in
368 L<perlipc/"Sockets: Client/Server Communication">.
370 =item binmode FILEHANDLE
372 Arranges for the file to be read or written in "binary" mode in operating
373 systems that distinguish between binary and text files. Files that are
374 not in binary mode have CR LF sequences translated to LF on input and LF
375 translated to CR LF on output. Binmode has no effect under Unix; in DOS
376 and similarly archaic systems, it may be imperative--otherwise your
377 DOS-damaged C library may mangle your file. The key distinction between
378 systems that need binmode and those that don't is their text file
379 formats. Systems like Unix and Plan9 that delimit lines with a single
380 character, and that encode that character in C as '\n', do not need
381 C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
382 is taken as the name of the filehandle.
384 =item bless REF,CLASSNAME
388 This function tells the thingy referenced by REF that it is now
389 an object in the CLASSNAME package--or the current package if no CLASSNAME
390 is specified, which is often the case. It returns the reference for
391 convenience, because a bless() is often the last thing in a constructor.
392 Always use the two-argument version if the function doing the blessing
393 might be inherited by a derived class. See L<perlobj> for more about the
394 blessing (and blessings) of objects.
400 Returns the context of the current subroutine call. In a scalar context,
401 returns the caller's package name if there is a caller, that is, if
402 we're in a subroutine or eval() or require(), and the undefined value
403 otherwise. In a list context, returns
405 ($package, $filename, $line) = caller;
407 With EXPR, it returns some extra information that the debugger uses to
408 print a stack trace. The value of EXPR indicates how many call frames
409 to go back before the current one.
411 ($package, $filename, $line, $subroutine,
412 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
414 Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
415 call, but C<L<eval>>. In such a case additional elements $evaltext and
416 $is_require are set: $is_require is true if the frame is created by
417 C<L<require>> or C<L<use>> statement, $evaltext contains the text of
418 C<L<eval EXPR>> statement. In particular, for C<L<eval BLOCK>>
419 statement $filename is C<"(eval)">, but $evaltext is undefined. (Note
420 also that C<L<use>> statement creates a C<L<require>> frame inside
421 an C<L<eval EXPR>>) frame.
423 Furthermore, when called from within the DB package, caller returns more
424 detailed information: it sets the list variable @DB::args to be the
425 arguments with which that subroutine was invoked.
429 Changes the working directory to EXPR, if possible. If EXPR is
430 omitted, changes to home directory. Returns TRUE upon success, FALSE
431 otherwise. See example under die().
435 Changes the permissions of a list of files. The first element of the
436 list must be the numerical mode, which should probably be an octal
437 number. Returns the number of files successfully changed.
439 $cnt = chmod 0755, 'foo', 'bar';
440 chmod 0755, @executables;
448 This is a slightly safer version of chop (see below). It removes any
449 line ending that corresponds to the current value of C<$/> (also known as
450 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
451 number of characters removed from all its arguments. It's often used to
452 remove the newline from the end of an input record when you're worried
453 that the final record may be missing its newline. When in paragraph mode
454 (C<$/ = "">), it removes all trailing newlines from the string. If
455 VARIABLE is omitted, it chomps $_. Example:
458 chomp; # avoid \n on last field
463 You can actually chomp anything that's an lvalue, including an assignment:
466 chomp($answer = <STDIN>);
468 If you chomp a list, each element is chomped, and the total number of
469 characters removed is returned.
477 Chops off the last character of a string and returns the character
478 chopped. It's used primarily to remove the newline from the end of an
479 input record, but is much more efficient than C<s/\n//> because it neither
480 scans nor copies the string. If VARIABLE is omitted, chops $_.
484 chop; # avoid \n on last field
489 You can actually chop anything that's an lvalue, including an assignment:
492 chop($answer = <STDIN>);
494 If you chop a list, each element is chopped. Only the value of the
495 last chop is returned.
497 Note that chop returns the last character. To return all but the last
498 character, use C<substr($string, 0, -1)>.
502 Changes the owner (and group) of a list of files. The first two
503 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
504 Returns the number of files successfully changed.
506 $cnt = chown $uid, $gid, 'foo', 'bar';
507 chown $uid, $gid, @filenames;
509 Here's an example that looks up non-numeric uids in the passwd file:
512 chop($user = <STDIN>);
514 chop($pattern = <STDIN>);
516 ($login,$pass,$uid,$gid) = getpwnam($user)
517 or die "$user not in passwd file";
519 @ary = <${pattern}>; # expand filenames
520 chown $uid, $gid, @ary;
522 On most systems, you are not allowed to change the ownership of the
523 file unless you're the superuser, although you should be able to change
524 the group to any of your secondary groups. On insecure systems, these
525 restrictions may be relaxed, but this is not a portable assumption.
531 Returns the character represented by that NUMBER in the character set.
532 For example, C<chr(65)> is "A" in ASCII.
534 If NUMBER is omitted, uses $_.
536 =item chroot FILENAME
540 This function works as the system call by the same name: it makes the
541 named directory the new root directory for all further pathnames that
542 begin with a "/" by your process and all of its children. (It doesn't
543 change your current working directory, which is unaffected.) For security
544 reasons, this call is restricted to the superuser. If FILENAME is
545 omitted, does chroot to $_.
547 =item close FILEHANDLE
549 Closes the file or pipe associated with the file handle, returning TRUE
550 only if stdio successfully flushes buffers and closes the system file
551 descriptor. You don't have to close FILEHANDLE if you are immediately
552 going to do another open() on it, because open() will close it for you. (See
553 open().) However, an explicit close on an input file resets the line
554 counter ($.), while the implicit close done by open() does not. Also,
555 closing a pipe will wait for the process executing on the pipe to
556 complete, in case you want to look at the output of the pipe
557 afterwards. Closing a pipe explicitly also puts the status value of
558 the command into C<$?>. Example:
560 open(OUTPUT, '|sort >foo'); # pipe to sort
561 ... # print stuff to output
562 close OUTPUT; # wait for sort to finish
563 open(INPUT, 'foo'); # get sort's results
565 FILEHANDLE may be an expression whose value gives the real filehandle name.
567 =item closedir DIRHANDLE
569 Closes a directory opened by opendir().
571 =item connect SOCKET,NAME
573 Attempts to connect to a remote socket, just as the connect system call
574 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
575 packed address of the appropriate type for the socket. See the examples in
576 L<perlipc/"Sockets: Client/Server Communication">.
580 Actually a flow control statement rather than a function. If there is a
581 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
582 C<foreach>), it is always executed just before the conditional is about to
583 be evaluated again, just like the third part of a C<for> loop in C. Thus
584 it can be used to increment a loop variable, even when the loop has been
585 continued via the C<next> statement (which is similar to the C C<continue>
590 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
593 For the inverse cosine operation, you may use the POSIX::acos()
594 function, or use this relation:
596 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
598 =item crypt PLAINTEXT,SALT
600 Encrypts a string exactly like the crypt(3) function in the C library
601 (assuming that you actually have a version there that has not been
602 extirpated as a potential munition). This can prove useful for checking
603 the password file for lousy passwords, amongst other things. Only the
604 guys wearing white hats should do this.
606 Here's an example that makes sure that whoever runs this program knows
609 $pwd = (getpwuid($<))[1];
610 $salt = substr($pwd, 0, 2);
614 chop($word = <STDIN>);
618 if (crypt($word, $salt) ne $pwd) {
624 Of course, typing in your own password to whomever asks you
627 =item dbmclose ASSOC_ARRAY
629 [This function has been superseded by the untie() function.]
631 Breaks the binding between a DBM file and an associative array.
633 =item dbmopen ASSOC,DBNAME,MODE
635 [This function has been superseded by the tie() function.]
637 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
638 associative array. ASSOC is the name of the associative array. (Unlike
639 normal open, the first argument is I<NOT> a filehandle, even though it
640 looks like one). DBNAME is the name of the database (without the F<.dir>
641 or F<.pag> extension if any). If the database does not exist, it is
642 created with protection specified by MODE (as modified by the umask()).
643 If your system supports only the older DBM functions, you may perform only
644 one dbmopen() in your program. In older versions of Perl, if your system
645 had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
646 falls back to sdbm(3).
648 If you don't have write access to the DBM file, you can only read
649 associative array variables, not set them. If you want to test whether
650 you can write, either use file tests or try setting a dummy array entry
651 inside an eval(), which will trap the error.
653 Note that functions such as keys() and values() may return huge array
654 values when used on large DBM files. You may prefer to use the each()
655 function to iterate over large DBM files. Example:
657 # print out history file offsets
658 dbmopen(%HIST,'/usr/lib/news/history',0666);
659 while (($key,$val) = each %HIST) {
660 print $key, ' = ', unpack('L',$val), "\n";
664 See also L<AnyDBM_File> for a more general description of the pros and
665 cons of the various dbm approaches, as well as L<DB_File> for a particularly
672 Returns a boolean value saying whether EXPR has a real value
673 or not. If EXPR is not present, $_ will be checked. Many operations
674 return the undefined value under exceptional conditions, such as end of
675 file, uninitialized variable, system error and such. This function
676 allows you to distinguish between an undefined
677 null scalar and a defined null scalar with operations that might return
678 a real null string, such as referencing elements of an array. You may
679 also check to see if arrays or subroutines exist. Use of defined on
680 predefined variables is not guaranteed to produce intuitive results.
682 When used on a hash array element, it tells you whether the value
683 is defined, not whether the key exists in the hash. Use exists() for that.
687 print if defined $switch{'D'};
688 print "$val\n" while defined($val = pop(@ary));
689 die "Can't readlink $sym: $!"
690 unless defined($value = readlink $sym);
691 eval '@foo = ()' if defined(@foo);
692 die "No XYZ package defined" unless defined %_XYZ;
693 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
697 Note: many folks tend to overuse defined(), and then are surprised to
698 discover that the number 0 and the null string are, in fact, defined
699 concepts. For example, if you say
703 the pattern match succeeds, and $1 is defined, despite the fact that it
704 matched "nothing". But it didn't really match nothing--rather, it
705 matched something that happened to be 0 characters long. This is all
706 very above-board and honest. When a function returns an undefined value,
707 it's an admission that it couldn't give you an honest answer. So
708 you should use defined() only when you're questioning the integrity
709 of what you're trying to do. At other times, a simple comparison to
710 0 or "" is what you want.
712 Another surprise is that using defined() on an entire array or
713 hash reports whether memory for that aggregate has ever been
714 allocated. So an array you set to the empty list appears undefined
715 initially, and one that once was full and that you then set to
716 the empty list still appears defined. You should instead use a
717 simple test for size:
719 if (@an_array) { print "has array elements\n" }
720 if (%a_hash) { print "has hash members\n" }
722 Using undef() on these, however, does clear their memory and then report
723 them as not defined anymore, but you shoudln't do that unless you don't
724 plan to use them again, because it saves time when you load them up
725 again to have memory already ready to be filled.
727 This counter-intuitive behaviour of defined() on aggregates may be
728 changed, fixed, or broken in a future release of Perl.
732 Deletes the specified key(s) and their associated values from a hash
733 array. For each key, returns the deleted value associated with that key,
734 or the undefined value if there was no such key. Deleting from C<$ENV{}>
735 modifies the environment. Deleting from an array tied to a DBM file
736 deletes the entry from the DBM file. (But deleting from a tie()d hash
737 doesn't necessarily return anything.)
739 The following deletes all the values of an associative array:
741 foreach $key (keys %HASH) {
747 delete @HASH{keys %HASH}
749 (But both of these are slower than the undef() command.) Note that the
750 EXPR can be arbitrarily complicated as long as the final operation is a
751 hash element lookup or hash slice:
753 delete $ref->[$x][$y]{$key};
754 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
758 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
759 the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
760 C<($? E<gt>E<gt> 8)> (back-tick `command` status). If C<($? E<gt>E<gt> 8)>
761 is 0, exits with 255. Inside an eval(), the error message is stuffed into
762 C<$@>, and the eval() is terminated with the undefined value; this makes
763 die() the way to raise an exception.
767 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
768 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
770 If the value of EXPR does not end in a newline, the current script line
771 number and input line number (if any) are also printed, and a newline
772 is supplied. Hint: sometimes appending ", stopped" to your message
773 will cause it to make better sense when the string "at foo line 123" is
774 appended. Suppose you are running script "canasta".
776 die "/etc/games is no good";
777 die "/etc/games is no good, stopped";
779 produce, respectively
781 /etc/games is no good at canasta line 123.
782 /etc/games is no good, stopped at canasta line 123.
784 See also exit() and warn().
786 You can arrange for a callback to be called just before the die() does
787 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
788 will be called with the error text and can change the error message, if
789 it sees fit, by calling die() again. See L<perlvar> for details on
790 setting C<%SIG> entries, and eval() for some examples.
794 Not really a function. Returns the value of the last command in the
795 sequence of commands indicated by BLOCK. When modified by a loop
796 modifier, executes the BLOCK once before testing the loop condition.
797 (On other statements the loop modifiers test the conditional first.)
799 =item do SUBROUTINE(LIST)
801 A deprecated form of subroutine call. See L<perlsub>.
805 Uses the value of EXPR as a filename and executes the contents of the
806 file as a Perl script. Its primary use is to include subroutines
807 from a Perl subroutine library.
815 except that it's more efficient, more concise, keeps track of the
816 current filename for error messages, and searches all the B<-I>
817 libraries if the file isn't in the current directory (see also the @INC
818 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
819 re-parse the file every time you call it, so you probably don't want to
820 do this inside a loop.
822 Note that inclusion of library modules is better done with the
823 use() and require() operators, which also do error checking
824 and raise an exception if there's a problem.
828 This causes an immediate core dump. Primarily this is so that you can
829 use the B<undump> program to turn your core dump into an executable binary
830 after having initialized all your variables at the beginning of the
831 program. When the new binary is executed it will begin by executing a
832 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
833 it as a goto with an intervening core dump and reincarnation. If LABEL
834 is omitted, restarts the program from the top. WARNING: any files
835 opened at the time of the dump will NOT be open any more when the
836 program is reincarnated, with possible resulting confusion on the part
837 of Perl. See also B<-u> option in L<perlrun>.
854 dump QUICKSTART if $ARGV[0] eq '-d';
859 =item each ASSOC_ARRAY
861 When called in a list context, returns a 2-element array consisting
862 of the key and value for the next element of an associative array,
863 so that you can iterate over it. When called in a scalar context,
864 returns the key for only the next element in the associative array.
865 Entries are returned in an apparently random order. When the array is
866 entirely read, a null array is returned in list context (which when
867 assigned produces a FALSE (0) value), and C<undef> is returned in a
868 scalar context. The next call to each() after that will start
869 iterating again. The iterator can be reset only by reading all the
870 elements from the array. You should not add elements to an array while
871 you're iterating over it. There is a single iterator for each
872 associative array, shared by all each(), keys(), and values() function
873 calls in the program. The following prints out your environment like
874 the printenv(1) program, only in a different order:
876 while (($key,$value) = each %ENV) {
877 print "$key=$value\n";
880 See also keys() and values().
888 Returns 1 if the next read on FILEHANDLE will return end of file, or if
889 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
890 gives the real filehandle name. (Note that this function actually
891 reads a character and then ungetc()s it, so it is not very useful in an
892 interactive context.) Do not read from a terminal file (or call
893 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
894 as terminals may lose the end-of-file condition if you do.
896 An C<eof> without an argument uses the last file read as argument.
897 Empty parentheses () may be used to indicate
898 the pseudo file formed of the files listed on the command line, i.e.,
899 C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
900 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
901 test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
903 # reset line numbering on each input file
906 close(ARGV) if (eof); # Not eof().
909 # insert dashes just before last line of last file
912 print "--------------\n";
913 close(ARGV); # close or break; is needed if we
914 # are reading from the terminal
919 Practical hint: you almost never need to use C<eof> in Perl, because the
920 input operators return undef when they run out of data.
926 EXPR is parsed and executed as if it were a little Perl program. It
927 is executed in the context of the current Perl program, so that any
928 variable settings or subroutine and format definitions remain afterwards.
929 The value returned is the value of the last expression evaluated, or a
930 return statement may be used, just as with subroutines. The last
931 expression is evaluated in scalar or array context, depending on the
934 If there is a syntax error or runtime error, or a die() statement is
935 executed, an undefined value is returned by eval(), and C<$@> is set to the
936 error message. If there was no error, C<$@> is guaranteed to be a null
937 string. If EXPR is omitted, evaluates C<$_>. The final semicolon, if
938 any, may be omitted from the expression. Beware that using eval()
939 neither silences perl from printing warnings to STDERR, nor does it
940 stuff the text of warning messages into C<$@>. To do either of those,
941 you have to use the C<$SIG{__WARN__}> facility. See warn() and L<perlvar>.
943 Note that, because eval() traps otherwise-fatal errors, it is useful for
944 determining whether a particular feature (such as socket() or symlink())
945 is implemented. It is also Perl's exception trapping mechanism, where
946 the die operator is used to raise exceptions.
948 If the code to be executed doesn't vary, you may use the eval-BLOCK
949 form to trap run-time errors without incurring the penalty of
950 recompiling each time. The error, if any, is still returned in C<$@>.
953 # make divide-by-zero non-fatal
954 eval { $answer = $a / $b; }; warn $@ if $@;
956 # same thing, but less efficient
957 eval '$answer = $a / $b'; warn $@ if $@;
959 # a compile-time error
963 eval '$answer ='; # sets $@
965 When using the eval{} form as an exception trap in libraries, you may
966 wish not to trigger any C<__DIE__> hooks that user code may have
967 installed. You can use the C<local $SIG{__DIE__}> construct for this
968 purpose, as shown in this example:
970 # a very private exception trap for divide-by-zero
971 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
973 This is especially significant, given that C<__DIE__> hooks can call
974 die() again, which has the effect of changing their error messages:
976 # __DIE__ hooks may modify error messages
978 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
979 eval { die "foo foofs here" };
980 print $@ if $@; # prints "bar barfs here"
983 With an eval(), you should be especially careful to remember what's
984 being looked at when:
990 eval { $x }; # CASE 4
992 eval "\$$x++" # CASE 5
995 Cases 1 and 2 above behave identically: they run the code contained in the
996 variable $x. (Although case 2 has misleading double quotes making the
997 reader wonder what else might be happening (nothing is).) Cases 3 and 4
998 likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
999 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
1000 is a place where normally you I<WOULD> like to use double quotes, except
1001 that in that particular situation, you can just use symbolic references
1002 instead, as in case 6.
1006 The exec() function executes a system command I<AND NEVER RETURNS>,
1007 unless the command does not exist and is executed directly instead of
1008 via C</bin/sh -c> (see below). Use system() instead of exec() if you
1011 If there is more than one argument in LIST, or if LIST is an array with
1012 more than one value, calls execvp(3) with the arguments in LIST. If
1013 there is only one scalar argument, the argument is checked for shell
1014 metacharacters. If there are any, the entire argument is passed to
1015 C</bin/sh -c> for parsing. If there are none, the argument is split
1016 into words and passed directly to execvp(), which is more efficient.
1017 Note: exec() and system() do not flush your output buffer, so you may
1018 need to set C<$|> to avoid lost output. Examples:
1020 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1021 exec "sort $outfile | uniq";
1023 If you don't really want to execute the first argument, but want to lie
1024 to the program you are executing about its own name, you can specify
1025 the program you actually want to run as an "indirect object" (without a
1026 comma) in front of the LIST. (This always forces interpretation of the
1027 LIST as a multi-valued list, even if there is only a single scalar in
1030 $shell = '/bin/csh';
1031 exec $shell '-sh'; # pretend it's a login shell
1035 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1039 Returns TRUE if the specified hash key exists in its hash array, even
1040 if the corresponding value is undefined.
1042 print "Exists\n" if exists $array{$key};
1043 print "Defined\n" if defined $array{$key};
1044 print "True\n" if $array{$key};
1046 A hash element can be TRUE only if it's defined, and defined if
1047 it exists, but the reverse doesn't necessarily hold true.
1049 Note that the EXPR can be arbitrarily complicated as long as the final
1050 operation is a hash key lookup:
1052 if (exists $ref->[$x][$y]{$key}) { ... }
1056 Evaluates EXPR and exits immediately with that value. (Actually, it
1057 calls any defined C<END> routines first, but the C<END> routines may not
1058 abort the exit. Likewise any object destructors that need to be called
1059 are called before exit.) Example:
1062 exit 0 if $ans =~ /^[Xx]/;
1064 See also die(). If EXPR is omitted, exits with 0 status.
1066 You shouldn't use exit() to abort a subroutine if there's any chance that
1067 someone might want to trap whatever error happened. Use die() instead,
1068 which can be trapped by an eval().
1074 Returns I<e> (the natural logarithm base) to the power of EXPR.
1075 If EXPR is omitted, gives C<exp($_)>.
1077 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1079 Implements the fcntl(2) function. You'll probably have to say
1083 first to get the correct function definitions. Argument processing and
1084 value return works just like ioctl() below. Note that fcntl() will produce
1085 a fatal error if used on a machine that doesn't implement fcntl(2).
1089 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1091 =item fileno FILEHANDLE
1093 Returns the file descriptor for a filehandle. This is useful for
1094 constructing bitmaps for select(). If FILEHANDLE is an expression, the
1095 value is taken as the name of the filehandle.
1097 =item flock FILEHANDLE,OPERATION
1099 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1100 success, FALSE on failure. Will produce a fatal error if used on a
1101 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1102 flock() is Perl's portable file locking interface, although it will lock
1103 only entire files, not records.
1105 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1106 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1107 you can use the symbolic names if you pull them in with an explicit
1108 request to the Fcntl module. The names can be requested as a group with
1109 the :flock tag (or they can be requested individually, of course).
1110 LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and
1111 LOCK_UN releases a previously requested lock. If LOCK_NB is added to
1112 LOCK_SH or LOCK_EX then flock() will return immediately rather than
1113 blocking waiting for the lock (check the return status to see if you got
1116 Note that the emulation built with lockf(3) doesn't provide shared
1117 locks, and it requires that FILEHANDLE be open with write intent. These
1118 are the semantics that lockf(3) implements. Most (all?) systems
1119 implement lockf(3) in terms of fcntl(2) locking, though, so the
1120 differing semantics shouldn't bite too many people.
1122 Note also that some versions of flock() cannot lock things over the
1123 network; you would need to use the more system-specific fcntl() for
1124 that. If you like you can force Perl to ignore your system's flock(2)
1125 function, and so provide its own fcntl(2)-based emulation, by passing
1126 the switch C<-Ud_flock> to the F<Configure> program when you configure
1129 Here's a mailbox appender for BSD systems.
1131 use Fcntl ':flock'; # import LOCK_* constants
1134 flock(MBOX,LOCK_EX);
1135 # and, in case someone appended
1136 # while we were waiting...
1141 flock(MBOX,LOCK_UN);
1144 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1145 or die "Can't open mailbox: $!";
1148 print MBOX $msg,"\n\n";
1151 See also L<DB_File> for other flock() examples.
1155 Does a fork(2) system call. Returns the child pid to the parent process
1156 and 0 to the child process, or C<undef> if the fork is unsuccessful.
1157 Note: unflushed buffers remain unflushed in both processes, which means
1158 you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1159 method of IO::Handle to avoid duplicate output.
1161 If you fork() without ever waiting on your children, you will accumulate
1164 $SIG{CHLD} = sub { wait };
1166 There's also the double-fork trick (error checking on
1167 fork() returns omitted);
1169 unless ($pid = fork) {
1171 exec "what you really wanna do";
1174 ## (some_perl_code_here)
1181 See also L<perlipc> for more examples of forking and reaping
1184 Note that if your forked child inherits system file descriptors like
1185 STDIN and STDOUT that are actually connected by a pipe or socket, even
1186 if you exit, the remote server (such as, say, httpd or rsh) won't think
1187 you're done. You should reopen those to /dev/null if it's any issue.
1191 Declare a picture format with use by the write() function. For
1195 Test: @<<<<<<<< @||||| @>>>>>
1196 $str, $%, '$' . int($num)
1200 $num = $cost/$quantity;
1204 See L<perlform> for many details and examples.
1207 =item formline PICTURE, LIST
1209 This is an internal function used by C<format>s, though you may call it
1210 too. It formats (see L<perlform>) a list of values according to the
1211 contents of PICTURE, placing the output into the format output
1212 accumulator, C<$^A> (or $ACCUMULATOR in English).
1213 Eventually, when a write() is done, the contents of
1214 C<$^A> are written to some filehandle, but you could also read C<$^A>
1215 yourself and then set C<$^A> back to "". Note that a format typically
1216 does one formline() per line of form, but the formline() function itself
1217 doesn't care how many newlines are embedded in the PICTURE. This means
1218 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1219 You may therefore need to use multiple formlines to implement a single
1220 record format, just like the format compiler.
1222 Be careful if you put double quotes around the picture, because an "C<@>"
1223 character may be taken to mean the beginning of an array name.
1224 formline() always returns TRUE. See L<perlform> for other examples.
1226 =item getc FILEHANDLE
1230 Returns the next character from the input file attached to FILEHANDLE,
1231 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
1232 This is not particularly efficient. It cannot be used to get unbuffered
1233 single-characters, however. For that, try something more like:
1236 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1239 system "stty", '-icanon', 'eol', "\001";
1245 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1248 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1252 Determination of whether to whether $BSD_STYLE should be set
1253 is left as an exercise to the reader.
1255 The POSIX::getattr() function can do this more portably on systems
1256 alleging POSIX compliance.
1257 See also the C<Term::ReadKey> module from your nearest CPAN site;
1258 details on CPAN can be found on L<perlmod/CPAN>.
1262 Returns the current login from F</etc/utmp>, if any. If null, use
1265 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1267 Do not consider getlogin() for authentication: it is not as
1268 secure as getpwuid().
1270 =item getpeername SOCKET
1272 Returns the packed sockaddr address of other end of the SOCKET connection.
1275 $hersockaddr = getpeername(SOCK);
1276 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1277 $herhostname = gethostbyaddr($iaddr, AF_INET);
1278 $herstraddr = inet_ntoa($iaddr);
1282 Returns the current process group for the specified PID. Use
1283 a PID of 0 to get the current process group for the
1284 current process. Will raise an exception if used on a machine that
1285 doesn't implement getpgrp(2). If PID is omitted, returns process
1286 group of current process. Note that the POSIX version of getpgrp()
1287 does not accept a PID argument, so only PID==0 is truly portable.
1291 Returns the process id of the parent process.
1293 =item getpriority WHICH,WHO
1295 Returns the current priority for a process, a process group, or a user.
1296 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1297 machine that doesn't implement getpriority(2).
1303 =item gethostbyname NAME
1305 =item getnetbyname NAME
1307 =item getprotobyname NAME
1313 =item getservbyname NAME,PROTO
1315 =item gethostbyaddr ADDR,ADDRTYPE
1317 =item getnetbyaddr ADDR,ADDRTYPE
1319 =item getprotobynumber NUMBER
1321 =item getservbyport PORT,PROTO
1339 =item sethostent STAYOPEN
1341 =item setnetent STAYOPEN
1343 =item setprotoent STAYOPEN
1345 =item setservent STAYOPEN
1359 These routines perform the same functions as their counterparts in the
1360 system library. Within a list context, the return values from the
1361 various get routines are as follows:
1363 ($name,$passwd,$uid,$gid,
1364 $quota,$comment,$gcos,$dir,$shell) = getpw*
1365 ($name,$passwd,$gid,$members) = getgr*
1366 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1367 ($name,$aliases,$addrtype,$net) = getnet*
1368 ($name,$aliases,$proto) = getproto*
1369 ($name,$aliases,$port,$proto) = getserv*
1371 (If the entry doesn't exist you get a null list.)
1373 Within a scalar context, you get the name, unless the function was a
1374 lookup by name, in which case you get the other thing, whatever it is.
1375 (If the entry doesn't exist you get the undefined value.) For example:
1385 The $members value returned by I<getgr*()> is a space separated list of
1386 the login names of the members of the group.
1388 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1389 C, it will be returned to you via C<$?> if the function call fails. The
1390 @addrs value returned by a successful call is a list of the raw
1391 addresses returned by the corresponding system library call. In the
1392 Internet domain, each address is four bytes long and you can unpack it
1393 by saying something like:
1395 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1397 =item getsockname SOCKET
1399 Returns the packed sockaddr address of this end of the SOCKET connection.
1402 $mysockaddr = getsockname(SOCK);
1403 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1405 =item getsockopt SOCKET,LEVEL,OPTNAME
1407 Returns the socket option requested, or undefined if there is an error.
1411 Returns the value of EXPR with filename expansions such as a shell
1412 would do. This is the internal function implementing the E<lt>*.*E<gt>
1413 operator, except it's easier to use.
1417 Converts a time as returned by the time function to a 9-element array
1418 with the time localized for the standard Greenwich time zone.
1419 Typically used as follows:
1422 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1425 All array elements are numeric, and come straight out of a struct tm.
1426 In particular this means that $mon has the range 0..11 and $wday has
1427 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1435 The goto-LABEL form finds the statement labeled with LABEL and resumes
1436 execution there. It may not be used to go into any construct that
1437 requires initialization, such as a subroutine or a foreach loop. It
1438 also can't be used to go into a construct that is optimized away. It
1439 can be used to go almost anywhere else within the dynamic scope,
1440 including out of subroutines, but it's usually better to use some other
1441 construct such as last or die. The author of Perl has never felt the
1442 need to use this form of goto (in Perl, that is--C is another matter).
1444 The goto-EXPR form expects a label name, whose scope will be resolved
1445 dynamically. This allows for computed gotos per FORTRAN, but isn't
1446 necessarily recommended if you're optimizing for maintainability:
1448 goto ("FOO", "BAR", "GLARCH")[$i];
1450 The goto-&NAME form is highly magical, and substitutes a call to the
1451 named subroutine for the currently running subroutine. This is used by
1452 AUTOLOAD subroutines that wish to load another subroutine and then
1453 pretend that the other subroutine had been called in the first place
1454 (except that any modifications to @_ in the current subroutine are
1455 propagated to the other subroutine.) After the goto, not even caller()
1456 will be able to tell that this routine was called first.
1458 =item grep BLOCK LIST
1460 =item grep EXPR,LIST
1462 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1463 $_ to each element) and returns the list value consisting of those
1464 elements for which the expression evaluated to TRUE. In a scalar
1465 context, returns the number of times the expression was TRUE.
1467 @foo = grep(!/^#/, @bar); # weed out comments
1471 @foo = grep {!/^#/} @bar; # weed out comments
1473 Note that, because $_ is a reference into the list value, it can be used
1474 to modify the elements of the array. While this is useful and
1475 supported, it can cause bizarre results if the LIST is not a named
1482 Interprets EXPR as a hex string and returns the corresponding decimal
1483 value. (To convert strings that might start with 0 or 0x see
1484 oct().) If EXPR is omitted, uses $_.
1488 There is no built-in import() function. It is merely an ordinary
1489 method (subroutine) defined (or inherited) by modules that wish to export
1490 names to another module. The use() function calls the import() method
1491 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1493 =item index STR,SUBSTR,POSITION
1495 =item index STR,SUBSTR
1497 Returns the position of the first occurrence of SUBSTR in STR at or after
1498 POSITION. If POSITION is omitted, starts searching from the beginning of
1499 the string. The return value is based at 0 (or whatever you've set the C<$[>
1500 variable to--but don't do that). If the substring is not found, returns
1501 one less than the base, ordinarily -1.
1507 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1509 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1511 Implements the ioctl(2) function. You'll probably have to say
1513 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1515 first to get the correct function definitions. If F<ioctl.ph> doesn't
1516 exist or doesn't have the correct definitions you'll have to roll your
1517 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1518 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1519 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1520 written depending on the FUNCTION--a pointer to the string value of SCALAR
1521 will be passed as the third argument of the actual ioctl call. (If SCALAR
1522 has no string value but does have a numeric value, that value will be
1523 passed rather than a pointer to the string value. To guarantee this to be
1524 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1525 functions are useful for manipulating the values of structures used by
1526 ioctl(). The following example sets the erase character to DEL.
1530 die "NO TIOCGETP" if $@ || !$getp;
1531 $sgttyb_t = "ccccs"; # 4 chars and a short
1532 if (ioctl(STDIN,$getp,$sgttyb)) {
1533 @ary = unpack($sgttyb_t,$sgttyb);
1535 $sgttyb = pack($sgttyb_t,@ary);
1536 ioctl(STDIN,&TIOCSETP,$sgttyb)
1537 || die "Can't ioctl: $!";
1540 The return value of ioctl (and fcntl) is as follows:
1542 if OS returns: then Perl returns:
1544 0 string "0 but true"
1545 anything else that number
1547 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1548 still easily determine the actual value returned by the operating
1551 ($retval = ioctl(...)) || ($retval = -1);
1552 printf "System returned %d\n", $retval;
1554 =item join EXPR,LIST
1556 Joins the separate strings of LIST or ARRAY into a single string with
1557 fields separated by the value of EXPR, and returns the string.
1560 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1562 See L<perlfunc/split>.
1564 =item keys ASSOC_ARRAY
1566 Returns a normal array consisting of all the keys of the named
1567 associative array. (In a scalar context, returns the number of keys.)
1568 The keys are returned in an apparently random order, but it is the same
1569 order as either the values() or each() function produces (given that
1570 the associative array has not been modified). Here is yet another way
1571 to print your environment:
1574 @values = values %ENV;
1575 while ($#keys >= 0) {
1576 print pop(@keys), '=', pop(@values), "\n";
1579 or how about sorted by key:
1581 foreach $key (sort(keys %ENV)) {
1582 print $key, '=', $ENV{$key}, "\n";
1585 To sort an array by value, you'll need to use a C<sort{}>
1586 function. Here's a descending numeric sort of a hash by its values:
1588 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1589 printf "%4d %s\n", $hash{$key}, $key;
1592 As an lvalue C<keys> allows you to increase the number of hash buckets
1593 allocated for the given associative array. This can gain you a measure
1594 of efficiency if you know the hash is going to get big. (This is
1595 similar to pre-extending an array by assigning a larger number to
1596 $#array.) If you say
1600 then C<%hash> will have at least 200 buckets allocated for it. These
1601 buckets will be retained even if you do C<%hash = ()>, use C<undef
1602 %hash> if you want to free the storage while C<%hash> is still in scope.
1603 You can't shrink the number of buckets allocated for the hash using
1604 C<keys> in this way (but you needn't worry about doing this by accident,
1605 as trying has no effect).
1609 Sends a signal to a list of processes. The first element of
1610 the list must be the signal to send. Returns the number of
1611 processes successfully signaled.
1613 $cnt = kill 1, $child1, $child2;
1616 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1617 process groups instead of processes. (On System V, a negative I<PROCESS>
1618 number will also kill process groups, but that's not portable.) That
1619 means you usually want to use positive not negative signals. You may also
1620 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1626 The C<last> command is like the C<break> statement in C (as used in
1627 loops); it immediately exits the loop in question. If the LABEL is
1628 omitted, the command refers to the innermost enclosing loop. The
1629 C<continue> block, if any, is not executed:
1631 LINE: while (<STDIN>) {
1632 last LINE if /^$/; # exit when done with header
1640 Returns an lowercased version of EXPR. This is the internal function
1641 implementing the \L escape in double-quoted strings.
1642 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1644 If EXPR is omitted, uses $_.
1650 Returns the value of EXPR with the first character lowercased. This is
1651 the internal function implementing the \l escape in double-quoted strings.
1652 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1654 If EXPR is omitted, uses $_.
1660 Returns the length in characters of the value of EXPR. If EXPR is
1661 omitted, returns length of $_.
1663 =item link OLDFILE,NEWFILE
1665 Creates a new filename linked to the old filename. Returns 1 for
1666 success, 0 otherwise.
1668 =item listen SOCKET,QUEUESIZE
1670 Does the same thing that the listen system call does. Returns TRUE if
1671 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1675 A local modifies the listed variables to be local to the enclosing block,
1676 subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1677 list must be placed in parentheses. See L<perlsub/"Temporary Values via
1678 local()"> for details.
1680 But you really probably want to be using my() instead, because local() isn't
1681 what most people think of as "local"). See L<perlsub/"Private Variables
1682 via my()"> for details.
1684 =item localtime EXPR
1686 Converts a time as returned by the time function to a 9-element array
1687 with the time analyzed for the local time zone. Typically used as
1690 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1693 All array elements are numeric, and come straight out of a struct tm.
1694 In particular this means that $mon has the range 0..11 and $wday has
1695 the range 0..6. If EXPR is omitted, does localtime(time).
1697 In a scalar context, prints out the ctime(3) value:
1699 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
1701 Also see the F<timelocal.pl> library, and the strftime(3) function available
1702 via the POSIX module.
1708 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1711 =item lstat FILEHANDLE
1717 Does the same thing as the stat() function, but stats a symbolic link
1718 instead of the file the symbolic link points to. If symbolic links are
1719 unimplemented on your system, a normal stat() is done.
1721 If EXPR is omitted, stats $_.
1725 The match operator. See L<perlop>.
1727 =item map BLOCK LIST
1731 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1732 element) and returns the list value composed of the results of each such
1733 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1734 may produce zero, one, or more elements in the returned value.
1736 @chars = map(chr, @nums);
1738 translates a list of numbers to the corresponding characters. And
1740 %hash = map { getkey($_) => $_ } @array;
1742 is just a funny way to write
1745 foreach $_ (@array) {
1746 $hash{getkey($_)} = $_;
1749 =item mkdir FILENAME,MODE
1751 Creates the directory specified by FILENAME, with permissions specified
1752 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1753 it returns 0 and sets C<$!> (errno).
1755 =item msgctl ID,CMD,ARG
1757 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1758 must be a variable which will hold the returned msqid_ds structure.
1759 Returns like ioctl: the undefined value for error, "0 but true" for
1760 zero, or the actual return value otherwise.
1762 =item msgget KEY,FLAGS
1764 Calls the System V IPC function msgget(2). Returns the message queue id,
1765 or the undefined value if there is an error.
1767 =item msgsnd ID,MSG,FLAGS
1769 Calls the System V IPC function msgsnd to send the message MSG to the
1770 message queue ID. MSG must begin with the long integer message type,
1771 which may be created with C<pack("l", $type)>. Returns TRUE if
1772 successful, or FALSE if there is an error.
1774 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1776 Calls the System V IPC function msgrcv to receive a message from
1777 message queue ID into variable VAR with a maximum message size of
1778 SIZE. Note that if a message is received, the message type will be the
1779 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1780 of the message type. Returns TRUE if successful, or FALSE if there is
1785 A "my" declares the listed variables to be local (lexically) to the
1786 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1787 more than one value is listed, the list must be placed in parentheses. See
1788 L<perlsub/"Private Variables via my()"> for details.
1794 The C<next> command is like the C<continue> statement in C; it starts
1795 the next iteration of the loop:
1797 LINE: while (<STDIN>) {
1798 next LINE if /^#/; # discard comments
1802 Note that if there were a C<continue> block on the above, it would get
1803 executed even on discarded lines. If the LABEL is omitted, the command
1804 refers to the innermost enclosing loop.
1806 =item no Module LIST
1808 See the "use" function, which "no" is the opposite of.
1814 Interprets EXPR as an octal string and returns the corresponding
1815 decimal value. (If EXPR happens to start off with 0x, interprets it as
1816 a hex string instead.) The following will handle decimal, octal, and
1817 hex in the standard Perl or C notation:
1819 $val = oct($val) if $val =~ /^0/;
1821 If EXPR is omitted, uses $_.
1823 =item open FILEHANDLE,EXPR
1825 =item open FILEHANDLE
1827 Opens the file whose filename is given by EXPR, and associates it with
1828 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1829 name of the real filehandle wanted. If EXPR is omitted, the scalar
1830 variable of the same name as the FILEHANDLE contains the filename.
1831 (Note that lexical variables--those declared with C<my>--will not work
1832 for this purpose; so if you're using C<my>, specify EXPR in your call
1835 If the filename begins with '<' or nothing, the file is opened for input.
1836 If the filename begins with '>', the file is truncated and opened for
1837 output. If the filename begins with '>>', the file is opened for
1838 appending. You can put a '+' in front of the '>' or '<' to indicate that
1839 you want both read and write access to the file; thus '+<' is almost
1840 always preferred for read/write updates--the '+>' mode would clobber the
1841 file first. The prefix and the filename may be separated with spaces.
1842 These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1843 'w+', 'a', and 'a+'.
1845 If the filename begins with "|", the filename is interpreted as a command
1846 to which output is to be piped, and if the filename ends with a "|", the
1847 filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1848 examples of this. as command which pipes input to us. (You may not have
1849 a raw open() to a command that pipes both in I<and> out, but see
1850 L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1853 Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
1854 non-zero upon success, the undefined value otherwise. If the open
1855 involved a pipe, the return value happens to be the pid of the
1858 If you're unfortunate enough to be running Perl on a system that
1859 distinguishes between text files and binary files (modern operating
1860 systems don't care), then you should check out L</binmode> for tips for
1861 dealing with this. The key distinction between systems that need binmode
1862 and those that don't is their text file formats. Systems like Unix and
1863 Plan9 that delimit lines with a single character, and that encode that
1864 character in C as '\n', do not need C<binmode>. The rest need it.
1869 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1870 while (<ARTICLE>) {...
1872 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1874 open(DBASE, '+<dbase.mine'); # open for update
1876 open(ARTICLE, "caesar <$article |"); # decrypt article
1878 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1880 # process argument list of files along with any includes
1882 foreach $file (@ARGV) {
1883 process($file, 'fh00');
1887 local($filename, $input) = @_;
1888 $input++; # this is a string increment
1889 unless (open($input, $filename)) {
1890 print STDERR "Can't open $filename: $!\n";
1894 while (<$input>) { # note use of indirection
1895 if (/^#include "(.*)"/) {
1896 process($1, $input);
1903 You may also, in the Bourne shell tradition, specify an EXPR beginning
1904 with "E<gt>&", in which case the rest of the string is interpreted as the
1905 name of a filehandle (or file descriptor, if numeric) which is to be
1906 duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1907 +E<gt>E<gt>, and +E<lt>. The
1908 mode you specify should match the mode of the original filehandle.
1909 (Duping a filehandle does not take into account any existing contents of
1911 Here is a script that saves, redirects, and restores STDOUT and
1915 open(SAVEOUT, ">&STDOUT");
1916 open(SAVEERR, ">&STDERR");
1918 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1919 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1921 select(STDERR); $| = 1; # make unbuffered
1922 select(STDOUT); $| = 1; # make unbuffered
1924 print STDOUT "stdout 1\n"; # this works for
1925 print STDERR "stderr 1\n"; # subprocesses too
1930 open(STDOUT, ">&SAVEOUT");
1931 open(STDERR, ">&SAVEERR");
1933 print STDOUT "stdout 2\n";
1934 print STDERR "stderr 2\n";
1937 If you specify "E<lt>&=N", where N is a number, then Perl will do an
1938 equivalent of C's fdopen() of that file descriptor; this is more
1939 parsimonious of file descriptors. For example:
1941 open(FILEHANDLE, "<&=$fd")
1943 If you open a pipe on the command "-", i.e., either "|-" or "-|", then
1944 there is an implicit fork done, and the return value of open is the pid
1945 of the child within the parent process, and 0 within the child
1946 process. (Use C<defined($pid)> to determine whether the open was successful.)
1947 The filehandle behaves normally for the parent, but i/o to that
1948 filehandle is piped from/to the STDOUT/STDIN of the child process.
1949 In the child process the filehandle isn't opened--i/o happens from/to
1950 the new STDOUT or STDIN. Typically this is used like the normal
1951 piped open when you want to exercise more control over just how the
1952 pipe command gets executed, such as when you are running setuid, and
1953 don't want to have to scan shell commands for metacharacters.
1954 The following pairs are more or less equivalent:
1956 open(FOO, "|tr '[a-z]' '[A-Z]'");
1957 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1959 open(FOO, "cat -n '$file'|");
1960 open(FOO, "-|") || exec 'cat', '-n', $file;
1962 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1964 Explicitly closing any piped filehandle causes the parent process to
1965 wait for the child to finish, and returns the status value in C<$?>.
1966 Note: on any operation which may do a fork, unflushed buffers remain
1967 unflushed in both processes, which means you may need to set C<$|> to
1968 avoid duplicate output.
1970 Using the constructor from the IO::Handle package (or one of its
1971 subclasses, such as IO::File or IO::Socket),
1972 you can generate anonymous filehandles which have the scope of whatever
1973 variables hold references to them, and automatically close whenever
1974 and however you leave that scope:
1978 sub read_myfile_munged {
1980 my $handle = new IO::File;
1981 open($handle, "myfile") or die "myfile: $!";
1983 or return (); # Automatically closed here.
1984 mung $first or die "mung failed"; # Or here.
1985 return $first, <$handle> if $ALL; # Or here.
1989 The filename that is passed to open will have leading and trailing
1990 whitespace deleted. To open a file with arbitrary weird
1991 characters in it, it's necessary to protect any leading and trailing
1994 $file =~ s#^(\s)#./$1#;
1995 open(FOO, "< $file\0");
1997 If you want a "real" C open() (see L<open(2)> on your system), then
1998 you should use the sysopen() function. This is another way to
1999 protect your filenames from interpretation. For example:
2002 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2003 or die "sysopen $path: $!";
2004 HANDLE->autoflush(1);
2005 HANDLE->print("stuff $$\n");
2007 print "File contains: ", <HANDLE>;
2009 See L</seek()> for some details about mixing reading and writing.
2011 =item opendir DIRHANDLE,EXPR
2013 Opens a directory named EXPR for processing by readdir(), telldir(),
2014 seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
2015 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2021 Returns the numeric ascii value of the first character of EXPR. If
2022 EXPR is omitted, uses $_.
2024 =item pack TEMPLATE,LIST
2026 Takes an array or list of values and packs it into a binary structure,
2027 returning the string containing the structure. The TEMPLATE is a
2028 sequence of characters that give the order and type of values, as
2031 A An ascii string, will be space padded.
2032 a An ascii string, will be null padded.
2033 b A bit string (ascending bit order, like vec()).
2034 B A bit string (descending bit order).
2035 h A hex string (low nybble first).
2036 H A hex string (high nybble first).
2038 c A signed char value.
2039 C An unsigned char value.
2040 s A signed short value.
2041 S An unsigned short value.
2042 i A signed integer value.
2043 I An unsigned integer value.
2044 l A signed long value.
2045 L An unsigned long value.
2047 n A short in "network" order.
2048 N A long in "network" order.
2049 v A short in "VAX" (little-endian) order.
2050 V A long in "VAX" (little-endian) order.
2052 f A single-precision float in the native format.
2053 d A double-precision float in the native format.
2055 p A pointer to a null-terminated string.
2056 P A pointer to a structure (fixed-length string).
2058 u A uuencoded string.
2060 w A BER compressed integer. Bytes give an unsigned integer base
2061 128, most significant digit first, with as few digits as
2062 possible, and with the bit 8 of each byte except the last set
2067 @ Null fill to absolute position.
2069 Each letter may optionally be followed by a number which gives a repeat
2070 count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
2071 pack function will gobble up that many values from the LIST. A * for the
2072 repeat count means to use however many items are left. The "a" and "A"
2073 types gobble just one value, but pack it as a string of length count,
2074 padding with nulls or spaces as necessary. (When unpacking, "A" strips
2075 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2076 fields pack a string that many bits long. The "h" and "H" fields pack a
2077 string that many nybbles long. The "P" packs a pointer to a structure of
2078 the size indicated by the length. Real numbers (floats and doubles) are
2079 in the native machine format only; due to the multiplicity of floating
2080 formats around, and the lack of a standard "network" representation, no
2081 facility for interchange has been made. This means that packed floating
2082 point data written on one machine may not be readable on another - even if
2083 both use IEEE floating point arithmetic (as the endian-ness of the memory
2084 representation is not part of the IEEE spec). Note that Perl uses doubles
2085 internally for all numeric calculation, and converting from double into
2086 float and thence back to double again will lose precision (i.e.,
2087 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2091 $foo = pack("cccc",65,66,67,68);
2093 $foo = pack("c4",65,66,67,68);
2096 $foo = pack("ccxxcc",65,66,67,68);
2099 $foo = pack("s2",1,2);
2100 # "\1\0\2\0" on little-endian
2101 # "\0\1\0\2" on big-endian
2103 $foo = pack("a4","abcd","x","y","z");
2106 $foo = pack("aaaa","abcd","x","y","z");
2109 $foo = pack("a14","abcdefg");
2110 # "abcdefg\0\0\0\0\0\0\0"
2112 $foo = pack("i9pl", gmtime);
2113 # a real struct tm (on my system anyway)
2116 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2119 The same template may generally also be used in the unpack function.
2121 =item package NAMESPACE
2123 Declares the compilation unit as being in the given namespace. The scope
2124 of the package declaration is from the declaration itself through the end of
2125 the enclosing block (the same scope as the local() operator). All further
2126 unqualified dynamic identifiers will be in this namespace. A package
2127 statement affects only dynamic variables--including those you've used
2128 local() on--but I<not> lexical variables created with my(). Typically it
2129 would be the first declaration in a file to be included by the C<require>
2130 or C<use> operator. You can switch into a package in more than one place;
2131 it influences merely which symbol table is used by the compiler for the
2132 rest of that block. You can refer to variables and filehandles in other
2133 packages by prefixing the identifier with the package name and a double
2134 colon: C<$Package::Variable>. If the package name is null, the C<main>
2135 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2137 See L<perlmod/"Packages"> for more information about packages, modules,
2138 and classes. See L<perlsub> for other scoping issues.
2140 =item pipe READHANDLE,WRITEHANDLE
2142 Opens a pair of connected pipes like the corresponding system call.
2143 Note that if you set up a loop of piped processes, deadlock can occur
2144 unless you are very careful. In addition, note that Perl's pipes use
2145 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2146 after each command, depending on the application.
2148 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2149 for examples of such things.
2155 Pops and returns the last value of the array, shortening the array by
2156 1. Has a similar effect to
2158 $tmp = $ARRAY[$#ARRAY--];
2160 If there are no elements in the array, returns the undefined value.
2161 If ARRAY is omitted, pops the
2162 @ARGV array in the main program, and the @_ array in subroutines, just
2169 Returns the offset of where the last C<m//g> search left off for the variable
2170 is in question ($_ is used when the variable is not specified). May be
2171 modified to change that offset. Such modification will also influence
2172 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2175 =item print FILEHANDLE LIST
2181 Prints a string or a comma-separated list of strings. Returns TRUE
2182 if successful. FILEHANDLE may be a scalar variable name, in which case
2183 the variable contains the name of or a reference to the filehandle, thus introducing one
2184 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2185 token is a term, it may be misinterpreted as an operator unless you
2186 interpose a + or put parentheses around the arguments.) If FILEHANDLE is
2187 omitted, prints by default to standard output (or to the last selected
2188 output channel--see L</select>). If LIST is also omitted, prints $_ to
2189 STDOUT. To set the default output channel to something other than
2190 STDOUT use the select operation. Note that, because print takes a
2191 LIST, anything in the LIST is evaluated in a list context, and any
2192 subroutine that you call will have one or more of its expressions
2193 evaluated in a list context. Also be careful not to follow the print
2194 keyword with a left parenthesis unless you want the corresponding right
2195 parenthesis to terminate the arguments to the print--interpose a + or
2196 put parentheses around all the arguments.
2198 Note that if you're storing FILEHANDLES in an array or other expression,
2199 you will have to use a block returning its value instead:
2201 print { $files[$i] } "stuff\n";
2202 print { $OK ? STDOUT : STDERR } "stuff\n";
2204 =item printf FILEHANDLE FORMAT, LIST
2206 =item printf FORMAT, LIST
2208 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2209 of the list will be interpreted as the printf format. If C<use locale> is
2210 in effect, the character used for the decimal point in formatted real numbers
2211 is affected by the LC_NUMERIC locale. See L<perllocale>.
2213 Don't fall into the trap of using a printf() when a simple
2214 print() would do. The print() is more efficient, and less
2217 =item prototype FUNCTION
2219 Returns the prototype of a function as a string (or C<undef> if the
2220 function has no prototype). FUNCTION is a reference to, or the name of,
2221 the function whose prototype you want to retrieve.
2223 =item push ARRAY,LIST
2225 Treats ARRAY as a stack, and pushes the values of LIST
2226 onto the end of ARRAY. The length of ARRAY increases by the length of
2227 LIST. Has the same effect as
2230 $ARRAY[++$#ARRAY] = $value;
2233 but is more efficient. Returns the new number of elements in the array.
2243 Generalized quotes. See L<perlop>.
2245 =item quotemeta EXPR
2249 Returns the value of EXPR with with all non-alphanumeric
2250 characters backslashed. (That is, all characters not matching
2251 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2252 returned string, regardless of any locale settings.)
2253 This is the internal function implementing
2254 the \Q escape in double-quoted strings.
2256 If EXPR is omitted, uses $_.
2262 Returns a random fractional number between 0 and the value of EXPR.
2263 (EXPR should be positive.) If EXPR is omitted, returns a value between
2264 0 and 1. This function produces repeatable sequences unless srand()
2265 is invoked. See also srand().
2267 (Note: if your rand function consistently returns numbers that are too
2268 large or too small, then your version of Perl was probably compiled
2269 with the wrong number of RANDBITS. As a workaround, you can usually
2270 multiply EXPR by the correct power of 2 to get the range you want.
2271 This will make your script unportable, however. It's better to recompile
2274 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2276 =item read FILEHANDLE,SCALAR,LENGTH
2278 Attempts to read LENGTH bytes of data into variable SCALAR from the
2279 specified FILEHANDLE. Returns the number of bytes actually read, or
2280 undef if there was an error. SCALAR will be grown or shrunk to the
2281 length actually read. An OFFSET may be specified to place the read
2282 data at some other place than the beginning of the string. This call
2283 is actually implemented in terms of stdio's fread call. To get a true
2284 read system call, see sysread().
2286 =item readdir DIRHANDLE
2288 Returns the next directory entry for a directory opened by opendir().
2289 If used in a list context, returns all the rest of the entries in the
2290 directory. If there are no more entries, returns an undefined value in
2291 a scalar context or a null list in a list context.
2293 If you're planning to filetest the return values out of a readdir(), you'd
2294 better prepend the directory in question. Otherwise, because we didn't
2295 chdir() there, it would have been testing the wrong file.
2297 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2298 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2305 Returns the value of a symbolic link, if symbolic links are
2306 implemented. If not, gives a fatal error. If there is some system
2307 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2310 =item recv SOCKET,SCALAR,LEN,FLAGS
2312 Receives a message on a socket. Attempts to receive LENGTH bytes of
2313 data into variable SCALAR from the specified SOCKET filehandle.
2314 Actually does a C recvfrom(), so that it can returns the address of the
2315 sender. Returns the undefined value if there's an error. SCALAR will
2316 be grown or shrunk to the length actually read. Takes the same flags
2317 as the system call of the same name.
2318 See L<perlipc/"UDP: Message Passing"> for examples.
2324 The C<redo> command restarts the loop block without evaluating the
2325 conditional again. The C<continue> block, if any, is not executed. If
2326 the LABEL is omitted, the command refers to the innermost enclosing
2327 loop. This command is normally used by programs that want to lie to
2328 themselves about what was just input:
2330 # a simpleminded Pascal comment stripper
2331 # (warning: assumes no { or } in strings)
2332 LINE: while (<STDIN>) {
2333 while (s|({.*}.*){.*}|$1 |) {}
2338 if (/}/) { # end of comment?
2351 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2352 is not specified, $_ will be used. The value returned depends on the
2353 type of thing the reference is a reference to.
2354 Builtin types include:
2363 If the referenced object has been blessed into a package, then that package
2364 name is returned instead. You can think of ref() as a typeof() operator.
2366 if (ref($r) eq "HASH") {
2367 print "r is a reference to an associative array.\n";
2370 print "r is not a reference at all.\n";
2373 See also L<perlref>.
2375 =item rename OLDNAME,NEWNAME
2377 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2378 not work across file system boundaries.
2384 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2385 supplied. If EXPR is numeric, demands that the current version of Perl
2386 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2388 Otherwise, demands that a library file be included if it hasn't already
2389 been included. The file is included via the do-FILE mechanism, which is
2390 essentially just a variety of eval(). Has semantics similar to the following
2394 local($filename) = @_;
2395 return 1 if $INC{$filename};
2396 local($realfilename,$result);
2398 foreach $prefix (@INC) {
2399 $realfilename = "$prefix/$filename";
2400 if (-f $realfilename) {
2401 $result = do $realfilename;
2405 die "Can't find $filename in \@INC";
2408 die "$filename did not return true value" unless $result;
2409 $INC{$filename} = $realfilename;
2413 Note that the file will not be included twice under the same specified
2414 name. The file must return TRUE as the last statement to indicate
2415 successful execution of any initialization code, so it's customary to
2416 end such a file with "1;" unless you're sure it'll return TRUE
2417 otherwise. But it's better just to put the "C<1;>", in case you add more
2420 If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2421 replaces "F<::>" with "F</>" in the filename for you,
2422 to make it easy to load standard modules. This form of loading of
2423 modules does not risk altering your namespace.
2425 For a yet-more-powerful import facility, see L</use> and
2432 Generally used in a C<continue> block at the end of a loop to clear
2433 variables and reset ?? searches so that they work again. The
2434 expression is interpreted as a list of single characters (hyphens
2435 allowed for ranges). All variables and arrays beginning with one of
2436 those letters are reset to their pristine state. If the expression is
2437 omitted, one-match searches (?pattern?) are reset to match again. Resets
2438 only variables or searches in the current package. Always returns
2441 reset 'X'; # reset all X variables
2442 reset 'a-z'; # reset lower case variables
2443 reset; # just reset ?? searches
2445 Resetting "A-Z" is not recommended because you'll wipe out your
2446 ARGV and ENV arrays. Resets only package variables--lexical variables
2447 are unaffected, but they clean themselves up on scope exit anyway,
2448 so you'll probably want to use them instead. See L</my>.
2452 Returns from a subroutine or eval with the value specified. (Note that
2453 in the absence of a return a subroutine or eval() will automatically
2454 return the value of the last expression evaluated.)
2458 In a list context, returns a list value consisting of the elements
2459 of LIST in the opposite order. In a scalar context, returns a string
2460 value consisting of the bytes of the first element of LIST in the
2463 print reverse <>; # line tac
2466 print scalar reverse scalar <>; # byte tac
2468 =item rewinddir DIRHANDLE
2470 Sets the current position to the beginning of the directory for the
2471 readdir() routine on DIRHANDLE.
2473 =item rindex STR,SUBSTR,POSITION
2475 =item rindex STR,SUBSTR
2477 Works just like index except that it returns the position of the LAST
2478 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2479 last occurrence at or before that position.
2481 =item rmdir FILENAME
2485 Deletes the directory specified by FILENAME if it is empty. If it
2486 succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
2487 FILENAME is omitted, uses $_.
2491 The substitution operator. See L<perlop>.
2495 Forces EXPR to be interpreted in a scalar context and returns the value
2498 @counts = ( scalar @a, scalar @b, scalar @c );
2500 There is no equivalent operator to force an expression to
2501 be interpolated in a list context because it's in practice never
2502 needed. If you really wanted to do so, however, you could use
2503 the construction C<@{[ (some expression) ]}>, but usually a simple
2504 C<(some expression)> suffices.
2506 =item seek FILEHANDLE,POSITION,WHENCE
2508 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2509 call of stdio. FILEHANDLE may be an expression whose value gives the name
2510 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2511 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2512 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2513 this from POSIX module. Returns 1 upon success, 0 otherwise.
2515 On some systems you have to do a seek whenever you switch between reading
2516 and writing. Amongst other things, this may have the effect of calling
2517 stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2522 This is also useful for applications emulating C<tail -f>. Once you hit
2523 EOF on your read, and then sleep for a while, you might have to stick in a
2524 seek() to reset things. First the simple trick listed above to clear the
2525 filepointer. The seek() doesn't change the current position, but it
2526 I<does> clear the end-of-file condition on the handle, so that the next
2527 C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
2529 If that doesn't work (some stdios are particularly cantankerous), then
2530 you may need something more like this:
2533 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2534 # search for some stuff and put it into files
2536 sleep($for_a_while);
2537 seek(FILE, $curpos, 0);
2540 =item seekdir DIRHANDLE,POS
2542 Sets the current position for the readdir() routine on DIRHANDLE. POS
2543 must be a value returned by telldir(). Has the same caveats about
2544 possible directory compaction as the corresponding system library
2547 =item select FILEHANDLE
2551 Returns the currently selected filehandle. Sets the current default
2552 filehandle for output, if FILEHANDLE is supplied. This has two
2553 effects: first, a C<write> or a C<print> without a filehandle will
2554 default to this FILEHANDLE. Second, references to variables related to
2555 output will refer to this output channel. For example, if you have to
2556 set the top of form format for more than one output channel, you might
2564 FILEHANDLE may be an expression whose value gives the name of the
2565 actual filehandle. Thus:
2567 $oldfh = select(STDERR); $| = 1; select($oldfh);
2569 Some programmers may prefer to think of filehandles as objects with
2570 methods, preferring to write the last example as:
2573 STDERR->autoflush(1);
2575 =item select RBITS,WBITS,EBITS,TIMEOUT
2577 This calls the select(2) system call with the bit masks specified, which
2578 can be constructed using fileno() and vec(), along these lines:
2580 $rin = $win = $ein = '';
2581 vec($rin,fileno(STDIN),1) = 1;
2582 vec($win,fileno(STDOUT),1) = 1;
2585 If you want to select on many filehandles you might wish to write a
2589 local(@fhlist) = split(' ',$_[0]);
2592 vec($bits,fileno($_),1) = 1;
2596 $rin = fhbits('STDIN TTY SOCK');
2600 ($nfound,$timeleft) =
2601 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2603 or to block until something becomes ready just do this
2605 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2607 Most systems do not bother to return anything useful in $timeleft, so
2608 calling select() in a scalar context just returns $nfound.
2610 Any of the bit masks can also be undef. The timeout, if specified, is
2611 in seconds, which may be fractional. Note: not all implementations are
2612 capable of returning the $timeleft. If not, they always return
2613 $timeleft equal to the supplied $timeout.
2615 You can effect a sleep of 250 milliseconds this way:
2617 select(undef, undef, undef, 0.25);
2619 B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
2620 with select(). You have to use sysread() instead.
2622 =item semctl ID,SEMNUM,CMD,ARG
2624 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2625 &GETALL, then ARG must be a variable which will hold the returned
2626 semid_ds structure or semaphore value array. Returns like ioctl: the
2627 undefined value for error, "0 but true" for zero, or the actual return
2630 =item semget KEY,NSEMS,FLAGS
2632 Calls the System V IPC function semget. Returns the semaphore id, or
2633 the undefined value if there is an error.
2635 =item semop KEY,OPSTRING
2637 Calls the System V IPC function semop to perform semaphore operations
2638 such as signaling and waiting. OPSTRING must be a packed array of
2639 semop structures. Each semop structure can be generated with
2640 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2641 operations is implied by the length of OPSTRING. Returns TRUE if
2642 successful, or FALSE if there is an error. As an example, the
2643 following code waits on semaphore $semnum of semaphore id $semid:
2645 $semop = pack("sss", $semnum, -1, 0);
2646 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2648 To signal the semaphore, replace "-1" with "1".
2650 =item send SOCKET,MSG,FLAGS,TO
2652 =item send SOCKET,MSG,FLAGS
2654 Sends a message on a socket. Takes the same flags as the system call
2655 of the same name. On unconnected sockets you must specify a
2656 destination to send TO, in which case it does a C sendto(). Returns
2657 the number of characters sent, or the undefined value if there is an
2659 See L<perlipc/"UDP: Message Passing"> for examples.
2661 =item setpgrp PID,PGRP
2663 Sets the current process group for the specified PID, 0 for the current
2664 process. Will produce a fatal error if used on a machine that doesn't
2665 implement setpgrp(2). If the arguments are omitted, it defaults to
2666 0,0. Note that the POSIX version of setpgrp() does not accept any
2667 arguments, so only setpgrp 0,0 is portable.
2669 =item setpriority WHICH,WHO,PRIORITY
2671 Sets the current priority for a process, a process group, or a user.
2672 (See setpriority(2).) Will produce a fatal error if used on a machine
2673 that doesn't implement setpriority(2).
2675 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2677 Sets the socket option requested. Returns undefined if there is an
2678 error. OPTVAL may be specified as undef if you don't want to pass an
2685 Shifts the first value of the array off and returns it, shortening the
2686 array by 1 and moving everything down. If there are no elements in the
2687 array, returns the undefined value. If ARRAY is omitted, shifts the
2688 @ARGV array in the main program, and the @_ array in subroutines.
2689 (This is determined lexically.) See also unshift(), push(), and pop().
2690 Shift() and unshift() do the same thing to the left end of an array
2691 that push() and pop() do to the right end.
2693 =item shmctl ID,CMD,ARG
2695 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2696 must be a variable which will hold the returned shmid_ds structure.
2697 Returns like ioctl: the undefined value for error, "0 but true" for
2698 zero, or the actual return value otherwise.
2700 =item shmget KEY,SIZE,FLAGS
2702 Calls the System V IPC function shmget. Returns the shared memory
2703 segment id, or the undefined value if there is an error.
2705 =item shmread ID,VAR,POS,SIZE
2707 =item shmwrite ID,STRING,POS,SIZE
2709 Reads or writes the System V shared memory segment ID starting at
2710 position POS for size SIZE by attaching to it, copying in/out, and
2711 detaching from it. When reading, VAR must be a variable which will
2712 hold the data read. When writing, if STRING is too long, only SIZE
2713 bytes are used; if STRING is too short, nulls are written to fill out
2714 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2716 =item shutdown SOCKET,HOW
2718 Shuts down a socket connection in the manner indicated by HOW, which
2719 has the same interpretation as in the system call of the same name.
2725 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2728 For the inverse sine operation, you may use the POSIX::sin()
2729 function, or use this relation:
2731 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2737 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2738 May be interrupted by sending the process a SIGALRM. Returns the
2739 number of seconds actually slept. You probably cannot mix alarm() and
2740 sleep() calls, because sleep() is often implemented using alarm().
2742 On some older systems, it may sleep up to a full second less than what
2743 you requested, depending on how it counts seconds. Most modern systems
2744 always sleep the full amount.
2746 For delays of finer granularity than one second, you may use Perl's
2747 syscall() interface to access setitimer(2) if your system supports it,
2748 or else see L</select()> below.
2750 See also the POSIX module's sigpause() function.
2752 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2754 Opens a socket of the specified kind and attaches it to filehandle
2755 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
2756 system call of the same name. You should "use Socket;" first to get
2757 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2759 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2761 Creates an unnamed pair of sockets in the specified domain, of the
2762 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
2763 for the system call of the same name. If unimplemented, yields a fatal
2764 error. Returns TRUE if successful.
2766 =item sort SUBNAME LIST
2768 =item sort BLOCK LIST
2772 Sorts the LIST and returns the sorted list value. Nonexistent values
2773 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2774 in standard string comparison order. If SUBNAME is specified, it
2775 gives the name of a subroutine that returns an integer less than, equal
2776 to, or greater than 0, depending on how the elements of the array are
2777 to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
2778 routines.) SUBNAME may be a scalar variable name, in which case the
2779 value provides the name of the subroutine to use. In place of a
2780 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2783 In the interests of efficiency the normal calling code for subroutines is
2784 bypassed, with the following effects: the subroutine may not be a
2785 recursive subroutine, and the two elements to be compared are passed into
2786 the subroutine not via @_ but as the package global variables $a and
2787 $b (see example below). They are passed by reference, so don't
2788 modify $a and $b. And don't try to declare them as lexicals either.
2790 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2791 current collation locale. See L<perllocale>.
2796 @articles = sort @files;
2798 # same thing, but with explicit sort routine
2799 @articles = sort {$a cmp $b} @files;
2801 # now case-insensitively
2802 @articles = sort { uc($a) cmp uc($b)} @files;
2804 # same thing in reversed order
2805 @articles = sort {$b cmp $a} @files;
2807 # sort numerically ascending
2808 @articles = sort {$a <=> $b} @files;
2810 # sort numerically descending
2811 @articles = sort {$b <=> $a} @files;
2813 # sort using explicit subroutine name
2815 $age{$a} <=> $age{$b}; # presuming integers
2817 @sortedclass = sort byage @class;
2819 # this sorts the %age associative arrays by value
2820 # instead of key using an in-line function
2821 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2823 sub backwards { $b cmp $a; }
2824 @harry = ('dog','cat','x','Cain','Abel');
2825 @george = ('gone','chased','yz','Punished','Axed');
2827 # prints AbelCaincatdogx
2828 print sort backwards @harry;
2829 # prints xdogcatCainAbel
2830 print sort @george, 'to', @harry;
2831 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2833 # inefficiently sort by descending numeric compare using
2834 # the first integer after the first = sign, or the
2835 # whole record case-insensitively otherwise
2838 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2843 # same thing, but much more efficiently;
2844 # we'll build auxiliary indices instead
2848 push @nums, /=(\d+)/;
2853 $nums[$b] <=> $nums[$a]
2855 $caps[$a] cmp $caps[$b]
2859 # same thing using a Schwartzian Transform (no temps)
2860 @new = map { $_->[0] }
2861 sort { $b->[1] <=> $a->[1]
2864 } map { [$_, /=(\d+)/, uc($_)] } @old;
2866 If you're using strict, you I<MUST NOT> declare $a
2867 and $b as lexicals. They are package globals. That means
2868 if you're in the C<main> package, it's
2870 @articles = sort {$main::b <=> $main::a} @files;
2874 @articles = sort {$::b <=> $::a} @files;
2876 but if you're in the C<FooPack> package, it's
2878 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2880 The comparison function is required to behave. If it returns
2881 inconsistent results (sometimes saying $x[1] is less than $x[2] and
2882 sometimes saying the opposite, for example) the Perl interpreter will
2883 probably crash and dump core. This is entirely due to and dependent
2884 upon your system's qsort(3) library routine; this routine often avoids
2885 sanity checks in the interest of speed.
2887 =item splice ARRAY,OFFSET,LENGTH,LIST
2889 =item splice ARRAY,OFFSET,LENGTH
2891 =item splice ARRAY,OFFSET
2893 Removes the elements designated by OFFSET and LENGTH from an array, and
2894 replaces them with the elements of LIST, if any. Returns the elements
2895 removed from the array. The array grows or shrinks as necessary. If
2896 LENGTH is omitted, removes everything from OFFSET onward. The
2897 following equivalences hold (assuming C<$[ == 0>):
2899 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2900 pop(@a) splice(@a,-1)
2901 shift(@a) splice(@a,0,1)
2902 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2903 $a[$x] = $y splice(@a,$x,1,$y);
2905 Example, assuming array lengths are passed before arrays:
2907 sub aeq { # compare two list values
2908 local(@a) = splice(@_,0,shift);
2909 local(@b) = splice(@_,0,shift);
2910 return 0 unless @a == @b; # same len?
2912 return 0 if pop(@a) ne pop(@b);
2916 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2918 =item split /PATTERN/,EXPR,LIMIT
2920 =item split /PATTERN/,EXPR
2922 =item split /PATTERN/
2926 Splits a string into an array of strings, and returns it.
2928 If not in a list context, returns the number of fields found and splits into
2929 the @_ array. (In a list context, you can force the split into @_ by
2930 using C<??> as the pattern delimiters, but it still returns the array
2931 value.) The use of implicit split to @_ is deprecated, however.
2933 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2934 splits on whitespace (after skipping any leading whitespace). Anything
2935 matching PATTERN is taken to be a delimiter separating the fields. (Note
2936 that the delimiter may be longer than one character.) If LIMIT is
2937 specified and is not negative, splits into no more than that many fields
2938 (though it may split into fewer). If LIMIT is unspecified, trailing null
2939 fields are stripped (which potential users of pop() would do well to
2940 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2941 LIMIT had been specified.
2943 A pattern matching the null string (not to be confused with
2944 a null pattern C<//>, which is just one member of the set of patterns
2945 matching a null string) will split the value of EXPR into separate
2946 characters at each point it matches that way. For example:
2948 print join(':', split(/ */, 'hi there'));
2950 produces the output 'h:i:t:h:e:r:e'.
2952 The LIMIT parameter can be used to split a line partially
2954 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2956 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2957 one larger than the number of variables in the list, to avoid
2958 unnecessary work. For the list above LIMIT would have been 4 by
2959 default. In time critical applications it behooves you not to split
2960 into more fields than you really need.
2962 If the PATTERN contains parentheses, additional array elements are
2963 created from each matching substring in the delimiter.
2965 split(/([,-])/, "1-10,20", 3);
2967 produces the list value
2969 (1, '-', 10, ',', 20)
2971 If you had the entire header of a normal Unix email message in $header,
2972 you could split it up into fields and their values this way:
2974 $header =~ s/\n\s+/ /g; # fix continuation lines
2975 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2977 The pattern C</PATTERN/> may be replaced with an expression to specify
2978 patterns that vary at runtime. (To do runtime compilation only once,
2979 use C</$variable/o>.)
2981 As a special case, specifying a PATTERN of space (C<' '>) will split on
2982 white space just as split with no arguments does. Thus, split(' ') can
2983 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2984 will give you as many null initial fields as there are leading spaces.
2985 A split on /\s+/ is like a split(' ') except that any leading
2986 whitespace produces a null first field. A split with no arguments
2987 really does a C<split(' ', $_)> internally.
2991 open(passwd, '/etc/passwd');
2993 ($login, $passwd, $uid, $gid, $gcos,
2994 $home, $shell) = split(/:/);
2998 (Note that $shell above will still have a newline on it. See L</chop>,
2999 L</chomp>, and L</join>.)
3001 =item sprintf FORMAT, LIST
3003 Returns a string formatted by the usual printf conventions of the C
3004 language. See L<sprintf(3)> or L<printf(3)> on your system for details.
3005 (The * character for an indirectly specified length is not
3006 supported, but you can get the same effect by interpolating a variable
3007 into the pattern.) If C<use locale> is
3008 in effect, the character used for the decimal point in formatted real numbers
3009 is affected by the LC_NUMERIC locale. See L<perllocale>.
3010 Some C libraries' implementations of sprintf() can
3011 dump core when fed ludicrous arguments.
3017 Return the square root of EXPR. If EXPR is omitted, returns square
3022 Sets the random number seed for the C<rand> operator. If EXPR is omitted,
3023 uses a semi-random value based on the current time and process ID, among
3024 other things. In versions of Perl prior to 5.004 the default seed was
3025 just the current time(). This isn't a particularly good seed, so many
3026 old programs supply their own seed value (often C<time ^ $$> or C<time ^
3027 ($$ + ($$ << 15))>), but that isn't necessary any more.
3029 You need something much more random than the default seed for
3030 cryptographic purposes, though. Checksumming the compressed output of
3031 one or more rapidly changing operating system status programs is the
3032 usual method. For example:
3034 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3036 If you're particularly concerned with this, see the Math::TrulyRandom
3039 Do I<not> call srand() multiple times in your program unless you know
3040 exactly what you're doing and why you're doing it. The point of the
3041 function is to "seed" the rand() function so that rand() can produce
3042 a different sequence each time you run your program. Just do it once at the
3043 top of your program, or you I<won't> get random numbers out of rand()!
3045 Frequently called programs (like CGI scripts) that simply use
3049 for a seed can fall prey to the mathematical property that
3053 one-third of the time. So don't do that.
3055 =item stat FILEHANDLE
3061 Returns a 13-element array giving the status info for a file, either the
3062 file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
3063 stats $_. Returns a null list if the stat fails. Typically used as
3067 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3068 $atime,$mtime,$ctime,$blksize,$blocks)
3071 Not all fields are supported on all filesystem types. Here are the
3072 meaning of the fields:
3074 dev device number of filesystem
3076 mode file mode (type and permissions)
3077 nlink number of (hard) links to the file
3078 uid numeric user ID of file's owner
3079 gid numeric group ID of file's owner
3080 rdev the device identifier (special files only)
3081 size total size of file, in bytes
3082 atime last access time since the epoch
3083 mtime last modify time since the epoch
3084 ctime inode change time (NOT creation time!) since the epoch
3085 blksize preferred block size for file system I/O
3086 blocks actual number of blocks allocated
3088 (The epoch was at 00:00 January 1, 1970 GMT.)
3090 If stat is passed the special filehandle consisting of an underline, no
3091 stat is done, but the current contents of the stat structure from the
3092 last stat or filetest are returned. Example:
3094 if (-x $file && (($d) = stat(_)) && $d < 0) {
3095 print "$file is executable NFS file\n";
3098 (This works on machines only for which the device number is negative under NFS.)
3104 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3105 doing many pattern matches on the string before it is next modified.
3106 This may or may not save time, depending on the nature and number of
3107 patterns you are searching on, and on the distribution of character
3108 frequencies in the string to be searched--you probably want to compare
3109 run times with and without it to see which runs faster. Those loops
3110 which scan for many short constant strings (including the constant
3111 parts of more complex patterns) will benefit most. You may have only
3112 one study active at a time--if you study a different scalar the first
3113 is "unstudied". (The way study works is this: a linked list of every
3114 character in the string to be searched is made, so we know, for
3115 example, where all the 'k' characters are. From each search string,
3116 the rarest character is selected, based on some static frequency tables
3117 constructed from some C programs and English text. Only those places
3118 that contain this "rarest" character are examined.)
3120 For example, here is a loop which inserts index producing entries
3121 before any line containing a certain pattern:
3125 print ".IX foo\n" if /\bfoo\b/;
3126 print ".IX bar\n" if /\bbar\b/;
3127 print ".IX blurfl\n" if /\bblurfl\b/;
3132 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3133 will be looked at, because "f" is rarer than "o". In general, this is
3134 a big win except in pathological cases. The only question is whether
3135 it saves you more time than it took to build the linked list in the
3138 Note that if you have to look for strings that you don't know till
3139 runtime, you can build an entire loop as a string and eval that to
3140 avoid recompiling all your patterns all the time. Together with
3141 undefining $/ to input entire files as one record, this can be very
3142 fast, often faster than specialized programs like fgrep(1). The following
3143 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3144 out the names of those files that contain a match:
3146 $search = 'while (<>) { study;';
3147 foreach $word (@words) {
3148 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3153 eval $search; # this screams
3154 $/ = "\n"; # put back to normal input delimiter
3155 foreach $file (sort keys(%seen)) {
3163 =item sub NAME BLOCK
3165 This is subroutine definition, not a real function I<per se>. With just a
3166 NAME (and possibly prototypes), it's just a forward declaration. Without
3167 a NAME, it's an anonymous function declaration, and does actually return a
3168 value: the CODE ref of the closure you just created. See L<perlsub> and
3169 L<perlref> for details.
3171 =item substr EXPR,OFFSET,LEN
3173 =item substr EXPR,OFFSET
3175 Extracts a substring out of EXPR and returns it. First character is at
3176 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
3177 that far from the end of the string. If LEN is omitted, returns
3178 everything to the end of the string. If LEN is negative, leaves that
3179 many characters off the end of the string.
3181 You can use the substr() function
3182 as an lvalue, in which case EXPR must be an lvalue. If you assign
3183 something shorter than LEN, the string will shrink, and if you assign
3184 something longer than LEN, the string will grow to accommodate it. To
3185 keep the string the same length you may need to pad or chop your value
3188 =item symlink OLDFILE,NEWFILE
3190 Creates a new filename symbolically linked to the old filename.
3191 Returns 1 for success, 0 otherwise. On systems that don't support
3192 symbolic links, produces a fatal error at run time. To check for that,
3195 $symlink_exists = (eval 'symlink("","");', $@ eq '');
3199 Calls the system call specified as the first element of the list,
3200 passing the remaining elements as arguments to the system call. If
3201 unimplemented, produces a fatal error. The arguments are interpreted
3202 as follows: if a given argument is numeric, the argument is passed as
3203 an int. If not, the pointer to the string value is passed. You are
3204 responsible to make sure a string is pre-extended long enough to
3205 receive any result that might be written into a string. If your
3206 integer arguments are not literals and have never been interpreted in a
3207 numeric context, you may need to add 0 to them to force them to look
3210 require 'syscall.ph'; # may need to run h2ph
3211 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3213 Note that Perl supports passing of up to only 14 arguments to your system call,
3214 which in practice should usually suffice.
3216 =item sysopen FILEHANDLE,FILENAME,MODE
3218 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3220 Opens the file whose filename is given by FILENAME, and associates it
3221 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3222 the name of the real filehandle wanted. This function calls the
3223 underlying operating system's C<open> function with the parameters
3224 FILENAME, MODE, PERMS.
3226 The possible values and flag bits of the MODE parameter are
3227 system-dependent; they are available via the standard module C<Fcntl>.
3228 However, for historical reasons, some values are universal: zero means
3229 read-only, one means write-only, and two means read/write.
3231 If the file named by FILENAME does not exist and the C<open> call
3232 creates it (typically because MODE includes the O_CREAT flag), then
3233 the value of PERMS specifies the permissions of the newly created
3234 file. If PERMS is omitted, the default value is 0666, which allows
3235 read and write for all. This default is reasonable: see C<umask>.
3237 The IO::File module provides a more object-oriented approach, if you're
3238 into that kind of thing.
3240 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3242 =item sysread FILEHANDLE,SCALAR,LENGTH
3244 Attempts to read LENGTH bytes of data into variable SCALAR from the
3245 specified FILEHANDLE, using the system call read(2). It bypasses
3246 stdio, so mixing this with other kinds of reads may cause confusion.
3247 Returns the number of bytes actually read, or undef if there was an
3248 error. SCALAR will be grown or shrunk so that the last byte actually
3249 read is the last byte of the scalar after the read.
3251 An OFFSET may be specified to place the read data at some place in the
3252 string other than the beginning. A negative OFFSET specifies
3253 placement at that many bytes counting backwards from the end of the
3254 string. A positive OFFSET greater than the length of SCALAR results
3255 in the string being padded to the required size with "\0" bytes before
3256 the result of the read is appended.
3260 Does exactly the same thing as "exec LIST" except that a fork is done
3261 first, and the parent process waits for the child process to complete.
3262 Note that argument processing varies depending on the number of
3263 arguments. The return value is the exit status of the program as
3264 returned by the wait() call. To get the actual exit value divide by
3265 256. See also L</exec>. This is I<NOT> what you want to use to capture
3266 the output from a command, for that you should use merely back-ticks or
3267 qx//, as described in L<perlop/"`STRING`">.
3269 Because system() and back-ticks block SIGINT and SIGQUIT, killing the
3270 program they're running doesn't actually interrupt your program.
3272 @args = ("command", "arg1", "arg2");
3274 or die "system @args failed: $?"
3276 Here's a more elaborate example of analysing the return value from
3277 system() on a UNIX system to check for all possibilities, including for
3278 signals and coredumps.
3280 $rc = 0xffff & system @args;
3281 printf "system(%s) returned %#04x: ", "@args", $rc;
3283 print "ran with normal exit\n";
3285 elsif ($rc == 0xff00) {
3286 print "command failed: $!\n";
3288 elsif ($rc > 0x80) {
3290 print "ran with non-zero exit status $rc\n";
3296 print "coredump from ";
3298 print "signal $rc\n"
3302 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3304 =item syswrite FILEHANDLE,SCALAR,LENGTH
3306 Attempts to write LENGTH bytes of data from variable SCALAR to the
3307 specified FILEHANDLE, using the system call write(2). It bypasses
3308 stdio, so mixing this with prints may cause confusion. Returns the
3309 number of bytes actually written, or undef if there was an error.
3310 If the length is greater than the available data, only as much data as
3311 is available will be written.
3313 An OFFSET may be specified to write the data from some part of the
3314 string other than the beginning. A negative OFFSET specifies writing
3315 from that many bytes counting backwards from the end of the string.
3317 =item tell FILEHANDLE
3321 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3322 expression whose value gives the name of the actual filehandle. If
3323 FILEHANDLE is omitted, assumes the file last read.
3325 =item telldir DIRHANDLE
3327 Returns the current position of the readdir() routines on DIRHANDLE.
3328 Value may be given to seekdir() to access a particular location in a
3329 directory. Has the same caveats about possible directory compaction as
3330 the corresponding system library routine.
3332 =item tie VARIABLE,CLASSNAME,LIST
3334 This function binds a variable to a package class that will provide the
3335 implementation for the variable. VARIABLE is the name of the variable
3336 to be enchanted. CLASSNAME is the name of a class implementing objects
3337 of correct type. Any additional arguments are passed to the "new"
3338 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3339 Typically these are arguments such as might be passed to the dbm_open()
3340 function of C. The object returned by the "new" method is also
3341 returned by the tie() function, which would be useful if you want to
3342 access other methods in CLASSNAME.
3344 Note that functions such as keys() and values() may return huge array
3345 values when used on large objects, like DBM files. You may prefer to
3346 use the each() function to iterate over such. Example:
3348 # print out history file offsets
3350 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
3351 while (($key,$val) = each %HIST) {
3352 print $key, ' = ', unpack('L',$val), "\n";
3356 A class implementing an associative array should have the following
3359 TIEHASH classname, LIST
3362 STORE this, key, value
3366 NEXTKEY this, lastkey
3368 A class implementing an ordinary array should have the following methods:
3370 TIEARRAY classname, LIST
3373 STORE this, key, value
3376 A class implementing a scalar should have the following methods:
3378 TIESCALAR classname, LIST
3383 Unlike dbmopen(), the tie() function will not use or require a module
3384 for you--you need to do that explicitly yourself. See L<DB_File>
3385 or the F<Config> module for interesting tie() implementations.
3389 Returns a reference to the object underlying VARIABLE (the same value
3390 that was originally returned by the tie() call which bound the variable
3391 to a package.) Returns the undefined value if VARIABLE isn't tied to a
3396 Returns the number of non-leap seconds since whatever time the system
3397 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3398 and 00:00:00 UTC, January 1, 1970 for most other systems).
3399 Suitable for feeding to gmtime() and localtime().
3403 Returns a four-element array giving the user and system times, in
3404 seconds, for this process and the children of this process.
3406 ($user,$system,$cuser,$csystem) = times;
3410 The translation operator. See L<perlop>.
3412 =item truncate FILEHANDLE,LENGTH
3414 =item truncate EXPR,LENGTH
3416 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3417 specified length. Produces a fatal error if truncate isn't implemented
3424 Returns an uppercased version of EXPR. This is the internal function
3425 implementing the \U escape in double-quoted strings.
3426 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3428 If EXPR is omitted, uses $_.
3434 Returns the value of EXPR with the first character uppercased. This is
3435 the internal function implementing the \u escape in double-quoted strings.
3436 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
3438 If EXPR is omitted, uses $_.
3444 Sets the umask for the process and returns the old one. If EXPR is
3445 omitted, returns merely the current umask.
3451 Undefines the value of EXPR, which must be an lvalue. Use on only a
3452 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3453 will probably not do what you expect on most predefined variables or
3454 DBM list values, so don't do that.) Always returns the undefined value. You can omit
3455 the EXPR, in which case nothing is undefined, but you still get an
3456 undefined value that you could, for instance, return from a
3457 subroutine. Examples:
3460 undef $bar{'blurfl'};
3464 return (wantarray ? () : undef) if $they_blew_it;
3470 Deletes a list of files. Returns the number of files successfully
3473 $cnt = unlink 'a', 'b', 'c';
3477 Note: unlink will not delete directories unless you are superuser and
3478 the B<-U> flag is supplied to Perl. Even if these conditions are
3479 met, be warned that unlinking a directory can inflict damage on your
3480 filesystem. Use rmdir instead.
3482 If LIST is omitted, uses $_.
3484 =item unpack TEMPLATE,EXPR
3486 Unpack does the reverse of pack: it takes a string representing a
3487 structure and expands it out into a list value, returning the array
3488 value. (In a scalar context, it returns merely the first value
3489 produced.) The TEMPLATE has the same format as in the pack function.
3490 Here's a subroutine that does substring:
3493 local($what,$where,$howmuch) = @_;
3494 unpack("x$where a$howmuch", $what);
3499 sub ordinal { unpack("c",$_[0]); } # same as ord()
3501 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3502 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
3503 themselves. Default is a 16-bit checksum. For example, the following
3504 computes the same number as the System V sum program:
3507 $checksum += unpack("%16C*", $_);
3511 The following efficiently counts the number of set bits in a bit vector:
3513 $setbits = unpack("%32b*", $selectmask);
3515 =item untie VARIABLE
3517 Breaks the binding between a variable and a package. (See tie().)
3519 =item unshift ARRAY,LIST
3521 Does the opposite of a C<shift>. Or the opposite of a C<push>,
3522 depending on how you look at it. Prepends list to the front of the
3523 array, and returns the new number of elements in the array.
3525 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3527 Note the LIST is prepended whole, not one element at a time, so the
3528 prepended elements stay in the same order. Use reverse to do the
3531 =item use Module LIST
3535 =item use Module VERSION LIST
3539 Imports some semantics into the current package from the named module,
3540 generally by aliasing certain subroutine or variable names into your
3541 package. It is exactly equivalent to
3543 BEGIN { require Module; import Module LIST; }
3545 except that Module I<must> be a bare word.
3547 If the first argument to C<use> is a number, it is treated as a version
3548 number instead of a module name. If the version of the Perl interpreter
3549 is less than VERSION, then an error message is printed and Perl exits
3550 immediately. This is often useful if you need to check the current
3551 Perl version before C<use>ing library modules which have changed in
3552 incompatible ways from older versions of Perl. (We try not to do
3553 this more than we have to.)
3555 The BEGIN forces the require and import to happen at compile time. The
3556 require makes sure the module is loaded into memory if it hasn't been
3557 yet. The import is not a builtin--it's just an ordinary static method
3558 call into the "Module" package to tell the module to import the list of
3559 features back into the current package. The module can implement its
3560 import method any way it likes, though most modules just choose to
3561 derive their import method via inheritance from the Exporter class that
3562 is defined in the Exporter module. See L<Exporter>. If no import
3563 method can be found then the error is currently silently ignored. This
3564 may change to a fatal error in a future version.
3566 If you don't want your namespace altered, explicitly supply an empty list:
3570 That is exactly equivalent to
3572 BEGIN { require Module; }
3574 If the VERSION argument is present between Module and LIST, then the
3575 C<use> will call the VERSION method in class Module with the given
3576 version as an argument. The default VERSION method, inherited from
3577 the Universal class, croaks if the given version is larger than the
3578 value of the variable $Module::VERSION. (Note that there is not a
3579 comma after VERSION!)
3581 Because this is a wide-open interface, pragmas (compiler directives)
3582 are also implemented this way. Currently implemented pragmas are:
3586 use sigtrap qw(SEGV BUS);
3587 use strict qw(subs vars refs);
3588 use subs qw(afunc blurfl);
3590 These pseudo-modules import semantics into the current block scope, unlike
3591 ordinary modules, which import symbols into the current package (which are
3592 effective through the end of the file).
3594 There's a corresponding "no" command that unimports meanings imported
3595 by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
3600 If no unimport method can be found the call fails with a fatal error.
3602 See L<perlmod> for a list of standard modules and pragmas.
3606 Changes the access and modification times on each file of a list of
3607 files. The first two elements of the list must be the NUMERICAL access
3608 and modification times, in that order. Returns the number of files
3609 successfully changed. The inode modification time of each file is set
3610 to the current time. Example of a "touch" command:
3614 utime $now, $now, @ARGV;
3616 =item values ASSOC_ARRAY
3618 Returns a normal array consisting of all the values of the named
3619 associative array. (In a scalar context, returns the number of
3620 values.) The values are returned in an apparently random order, but it
3621 is the same order as either the keys() or each() function would produce
3622 on the same array. See also keys(), each(), and sort().
3624 =item vec EXPR,OFFSET,BITS
3626 Treats the string in EXPR as a vector of unsigned integers, and
3627 returns the value of the bit field specified by OFFSET. BITS specifies
3628 the number of bits that are reserved for each entry in the bit
3629 vector. This must be a power of two from 1 to 32. vec() may also be
3630 assigned to, in which case parentheses are needed to give the expression
3631 the correct precedence as in
3633 vec($image, $max_x * $x + $y, 8) = 3;
3635 Vectors created with vec() can also be manipulated with the logical
3636 operators |, &, and ^, which will assume a bit vector operation is
3637 desired when both operands are strings.
3639 To transform a bit vector into a string or array of 0's and 1's, use these:
3641 $bits = unpack("b*", $vector);
3642 @bits = split(//, unpack("b*", $vector));
3644 If you know the exact length in bits, it can be used in place of the *.
3648 Waits for a child process to terminate and returns the pid of the
3649 deceased process, or -1 if there are no child processes. The status is
3652 =item waitpid PID,FLAGS
3654 Waits for a particular child process to terminate and returns the pid
3655 of the deceased process, or -1 if there is no such child process. The
3656 status is returned in C<$?>. If you say
3658 use POSIX ":sys_wait_h";
3660 waitpid(-1,&WNOHANG);
3662 then you can do a non-blocking wait for any process. Non-blocking wait
3663 is available on machines supporting either the waitpid(2) or
3664 wait4(2) system calls. However, waiting for a particular pid with
3665 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3666 by remembering the status values of processes that have exited but have
3667 not been harvested by the Perl script yet.)
3671 Returns TRUE if the context of the currently executing subroutine is
3672 looking for a list value. Returns FALSE if the context is looking
3675 return wantarray ? () : undef;
3679 Produces a message on STDERR just like die(), but doesn't exit or throw
3682 No message is printed if there is a C<$SIG{__WARN__}> handler
3683 installed. It is the handler's responsibility to deal with the message
3684 as it sees fit (like, for instance, converting it into a die()). Most
3685 handlers must therefore make arrangements to actually display the
3686 warnings that they are not prepared to deal with, by calling warn()
3687 again in the handler. Note that this is quite safe and will not
3688 produce an endless loop, since C<__WARN__> hooks are not called from
3691 You will find this behavior is slightly different from that of
3692 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
3693 instead call die() again to change it).
3695 Using a C<__WARN__> handler provides a powerful way to silence all
3696 warnings (even the so-called mandatory ones). An example:
3698 # wipe out *all* compile-time warnings
3699 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
3701 my $foo = 20; # no warning about duplicate my $foo,
3702 # but hey, you asked for it!
3703 # no compile-time or run-time warnings before here
3706 # run-time warnings enabled after here
3707 warn "\$foo is alive and $foo!"; # does show up
3709 See L<perlvar> for details on setting C<%SIG> entries, and for more
3712 =item write FILEHANDLE
3718 Writes a formatted record (possibly multi-line) to the specified file,
3719 using the format associated with that file. By default the format for
3720 a file is the one having the same name is the filehandle, but the
3721 format for the current output channel (see the select() function) may be set
3722 explicitly by assigning the name of the format to the C<$~> variable.
3724 Top of form processing is handled automatically: if there is
3725 insufficient room on the current page for the formatted record, the
3726 page is advanced by writing a form feed, a special top-of-page format
3727 is used to format the new page header, and then the record is written.
3728 By default the top-of-page format is the name of the filehandle with
3729 "_TOP" appended, but it may be dynamically set to the format of your
3730 choice by assigning the name to the C<$^> variable while the filehandle is
3731 selected. The number of lines remaining on the current page is in
3732 variable C<$->, which can be set to 0 to force a new page.
3734 If FILEHANDLE is unspecified, output goes to the current default output
3735 channel, which starts out as STDOUT but may be changed by the
3736 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3737 is evaluated and the resulting string is used to look up the name of
3738 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3740 Note that write is I<NOT> the opposite of read. Unfortunately.
3744 The translation operator. See L<perlop>.