3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar and list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can only
18 ever be one list argument.) For instance, splice() has three scalar
19 arguments followed by a list.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parens.) If you use the parens, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+3; # Prints 6.
39 print(1+2) + 3; # Prints 3.
40 print (1+2)+3; # Also prints 3!
41 print +(1+2)+3; # Prints 6.
42 print ((1+2)+3); # Prints 6.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 non-abortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following rule:
61 I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
65 Each operator and function decides which sort of value it would be most
66 appropriate to return in a scalar context. Some operators return the
67 length of the list that would have been returned in a list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
81 A file test, where X is one of the letters listed below. This unary
82 operator takes one argument, either a filename or a filehandle, and
83 tests the associated file to see if something is true about it. If the
84 argument is omitted, tests $_, except for C<-t>, which tests STDIN.
85 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
86 the undefined value if the file doesn't exist. Despite the funny
87 names, precedence is the same as any other named unary operator, and
88 the argument may be parenthesized like any other unary operator. The
89 operator may be any of:
91 -r File is readable by effective uid/gid.
92 -w File is writable by effective uid/gid.
93 -x File is executable by effective uid/gid.
94 -o File is owned by effective uid.
96 -R File is readable by real uid/gid.
97 -W File is writable by real uid/gid.
98 -X File is executable by real uid/gid.
99 -O File is owned by real uid.
102 -z File has zero size.
103 -s File has non-zero size (returns size).
105 -f File is a plain file.
106 -d File is a directory.
107 -l File is a symbolic link.
108 -p File is a named pipe (FIFO).
110 -b File is a block special file.
111 -c File is a character special file.
112 -t Filehandle is opened to a tty.
114 -u File has setuid bit set.
115 -g File has setgid bit set.
116 -k File has sticky bit set.
118 -T File is a text file.
119 -B File is a binary file (opposite of -T).
121 -M Age of file in days when script started.
122 -A Same for access time.
123 -C Same for inode change time.
125 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
126 C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
127 uids and gids of the user. There may be other reasons you can't actually
128 read, write or execute the file. Also note that, for the superuser,
129 C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
130 1 if any execute bit is set in the mode. Scripts run by the superuser may
131 thus need to do a stat() in order to determine the actual mode of the
132 file, or temporarily set the uid to something else.
138 next unless -f $_; # ignore specials
142 Note that C<-s/a/b/> does not do a negated substitution. Saying
143 C<-exp($foo)> still works as expected, however--only single letters
144 following a minus are interpreted as file tests.
146 The C<-T> and C<-B> switches work as follows. The first block or so of the
147 file is examined for odd characters such as strange control codes or
148 characters with the high bit set. If too many odd characters (>30%)
149 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
150 containing null in the first block is considered a binary file. If C<-T>
151 or C<-B> is used on a filehandle, the current stdio buffer is examined
152 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
153 file, or a file at EOF when testing a filehandle. Because you have to
154 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
155 against the file first, as in C<next unless -f $file && -T $file>.
157 If any of the file tests (or either the stat() or lstat() operators) are given the
158 special filehandle consisting of a solitary underline, then the stat
159 structure of the previous file test (or stat operator) is used, saving
160 a system call. (This doesn't work with C<-t>, and you need to remember
161 that lstat() and C<-l> will leave values in the stat structure for the
162 symbolic link, not the real file.) Example:
164 print "Can do.\n" if -r $a || -w _ || -x _;
167 print "Readable\n" if -r _;
168 print "Writable\n" if -w _;
169 print "Executable\n" if -x _;
170 print "Setuid\n" if -u _;
171 print "Setgid\n" if -g _;
172 print "Sticky\n" if -k _;
173 print "Text\n" if -T _;
174 print "Binary\n" if -B _;
178 Returns the absolute value of its argument.
180 =item accept NEWSOCKET,GENERICSOCKET
182 Accepts an incoming socket connect, just as the accept(2) system call
183 does. Returns the packed address if it succeeded, FALSE otherwise.
184 See example in L<perlipc/"Sockets: Client/Server Communication">.
188 Arranges to have a SIGALRM delivered to this process after the
189 specified number of seconds have elapsed. (On some machines,
190 unfortunately, the elapsed time may be up to one second less than you
191 specified because of how seconds are counted.) Only one timer may be
192 counting at once. Each call disables the previous timer, and an
193 argument of 0 may be supplied to cancel the previous timer without
194 starting a new one. The returned value is the amount of time remaining
195 on the previous timer.
197 For delays of finer granularity than one second, you may use Perl's
198 syscall() interface to access setitimer(2) if your system supports it,
199 or else see L</select()> below. It is not advised to intermix alarm()
204 Returns the arctangent of Y/X in the range -PI to PI.
206 =item bind SOCKET,NAME
208 Binds a network address to a socket, just as the bind system call
209 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
210 packed address of the appropriate type for the socket. See the examples in
211 L<perlipc/"Sockets: Client/Server Communication">.
213 =item binmode FILEHANDLE
215 Arranges for the file to be read or written in "binary" mode in
216 operating systems that distinguish between binary and text files.
217 Files that are not in binary mode have CR LF sequences translated to LF
218 on input and LF translated to CR LF on output. Binmode has no effect
219 under Unix; in DOS, it may be imperative--otherwise your DOS C library
220 may mangle your file. If FILEHANDLE is an expression,
221 the value is taken as the name of the filehandle.
223 =item bless REF,CLASSNAME
227 This function tells the referenced object (passed as REF) that it is now
228 an object in the CLASSNAME package--or the current package if no CLASSNAME
229 is specified, which is often the case. It returns the reference for
230 convenience, since a bless() is often the last thing in a constructor.
231 Always use the two-argument version if the function doing the blessing
232 might be inherited by a derived class. See L<perlobj> for more about the
233 blessing (and blessings) of objects.
239 Returns the context of the current subroutine call. In a scalar context,
240 returns TRUE if there is a caller, that is, if we're in a subroutine or
241 eval() or require(), and FALSE otherwise. In a list context, returns
243 ($package, $filename, $line) = caller;
245 With EXPR, it returns some extra information that the debugger uses to
246 print a stack trace. The value of EXPR indicates how many call frames
247 to go back before the current one.
249 ($package, $filename, $line,
250 $subroutine, $hasargs, $wantargs) = caller($i);
252 Furthermore, when called from within the DB package, caller returns more
253 detailed information: it sets the list variable @DB::args to be the
254 arguments with which that subroutine was invoked.
258 Changes the working directory to EXPR, if possible. If EXPR is
259 omitted, changes to home directory. Returns TRUE upon success, FALSE
260 otherwise. See example under die().
264 Changes the permissions of a list of files. The first element of the
265 list must be the numerical mode, which should probably be an octal
266 number. Returns the number of files successfully changed.
268 $cnt = chmod 0755, 'foo', 'bar';
269 chmod 0755, @executables;
277 This is a slightly safer version of chop (see below). It removes any
278 line ending that corresponds to the current value of C<$/> (also known as
279 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
280 of characters removed. It's often used to remove the newline from the
281 end of an input record when you're worried that the final record may be
282 missing its newline. When in paragraph mode (C<$/ = "">), it removes all
283 trailing newlines from the string. If VARIABLE is omitted, it chomps
287 chomp; # avoid \n on last field
292 You can actually chomp anything that's an lvalue, including an assignment:
295 chomp($answer = <STDIN>);
297 If you chomp a list, each element is chomped, and the total number of
298 characters removed is returned.
306 Chops off the last character of a string and returns the character
307 chopped. It's used primarily to remove the newline from the end of an
308 input record, but is much more efficient than C<s/\n//> because it neither
309 scans nor copies the string. If VARIABLE is omitted, chops $_.
313 chop; # avoid \n on last field
318 You can actually chop anything that's an lvalue, including an assignment:
321 chop($answer = <STDIN>);
323 If you chop a list, each element is chopped. Only the value of the
324 last chop is returned.
326 Note that chop returns the last character. To return all but the last
327 character, use C<substr($string, 0, -1)>.
331 Changes the owner (and group) of a list of files. The first two
332 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
333 Returns the number of files successfully changed.
335 $cnt = chown $uid, $gid, 'foo', 'bar';
336 chown $uid, $gid, @filenames;
338 Here's an example that looks up non-numeric uids in the passwd file:
341 chop($user = <STDIN>);
343 chop($pattern = <STDIN>);
345 ($login,$pass,$uid,$gid) = getpwnam($user)
346 or die "$user not in passwd file";
348 @ary = <${pattern}>; # expand filenames
349 chown $uid, $gid, @ary;
351 On most systems, you are not allowed to change the ownership of the
352 file unless you're the superuser, although you should be able to change
353 the group to any of your secondary groups. On insecure systems, these
354 restrictions may be relaxed, but this is not a portable assumption.
358 Returns the character represented by that NUMBER in the character set.
359 For example, C<chr(65)> is "A" in ASCII.
361 =item chroot FILENAME
363 This function works as the system call by the same name: it makes the
364 named directory the new root directory for all further pathnames that
365 begin with a "/" by your process and all of its children. (It doesn't
366 change your current working directory is unaffected.) For security
367 reasons, this call is restricted to the superuser. If FILENAME is
368 omitted, does chroot to $_.
370 =item close FILEHANDLE
372 Closes the file or pipe associated with the file handle, returning TRUE
373 only if stdio successfully flushes buffers and closes the system file
374 descriptor. You don't have to close FILEHANDLE if you are immediately
375 going to do another open() on it, since open() will close it for you. (See
376 open().) However, an explicit close on an input file resets the line
377 counter ($.), while the implicit close done by open() does not. Also,
378 closing a pipe will wait for the process executing on the pipe to
379 complete, in case you want to look at the output of the pipe
380 afterwards. Closing a pipe explicitly also puts the status value of
381 the command into C<$?>. Example:
383 open(OUTPUT, '|sort >foo'); # pipe to sort
384 ... # print stuff to output
385 close OUTPUT; # wait for sort to finish
386 open(INPUT, 'foo'); # get sort's results
388 FILEHANDLE may be an expression whose value gives the real filehandle name.
390 =item closedir DIRHANDLE
392 Closes a directory opened by opendir().
394 =item connect SOCKET,NAME
396 Attempts to connect to a remote socket, just as the connect system call
397 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
398 packed address of the appropriate type for the socket. See the examples in
399 L<perlipc/"Sockets: Client/Server Communication">.
403 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
406 =item crypt PLAINTEXT,SALT
408 Encrypts a string exactly like the crypt(3) function in the C library
409 (assuming that you actually have a version there that has not been
410 extirpated as a potential munition). This can prove useful for checking
411 the password file for lousy passwords, amongst other things. Only the
412 guys wearing white hats should do this.
414 Here's an example that makes sure that whoever runs this program knows
417 $pwd = (getpwuid($<))[1];
418 $salt = substr($pwd, 0, 2);
422 chop($word = <STDIN>);
426 if (crypt($word, $salt) ne $pwd) {
432 Of course, typing in your own password to whoever asks you
435 =item dbmclose ASSOC_ARRAY
437 [This function has been superseded by the untie() function.]
439 Breaks the binding between a DBM file and an associative array.
441 =item dbmopen ASSOC,DBNAME,MODE
443 [This function has been superseded by the tie() function.]
445 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an associative array. ASSOC is the
446 name of the associative array. (Unlike normal open, the first argument
447 is I<NOT> a filehandle, even though it looks like one). DBNAME is the
448 name of the database (without the F<.dir> or F<.pag> extension if any). If the
449 database does not exist, it is created with protection specified by
450 MODE (as modified by the umask()). If your system only supports the
451 older DBM functions, you may perform only one dbmopen() in your program.
452 In order versions of Perl,
453 if your system had neither DBM nor ndbm, calling dbmopen() produced a
454 fatal error; it now falls back to sdbm(3).
456 If you don't have write access to the DBM file, you can only read
457 associative array variables, not set them. If you want to test whether
458 you can write, either use file tests or try setting a dummy array entry
459 inside an eval(), which will trap the error.
461 Note that functions such as keys() and values() may return huge array
462 values when used on large DBM files. You may prefer to use the each()
463 function to iterate over large DBM files. Example:
465 # print out history file offsets
466 dbmopen(%HIST,'/usr/lib/news/history',0666);
467 while (($key,$val) = each %HIST) {
468 print $key, ' = ', unpack('L',$val), "\n";
472 See also L<DB_File> for many other interesting possibilities.
476 Returns a boolean value saying whether the lvalue EXPR has a real value
477 or not. Many operations return the undefined value under exceptional
478 conditions, such as end of file, uninitialized variable, system error
479 and such. This function allows you to distinguish between an undefined
480 null scalar and a defined null scalar with operations that might return
481 a real null string, such as referencing elements of an array. You may
482 also check to see if arrays or subroutines exist. Use of defined on
483 predefined variables is not guaranteed to produce intuitive results.
485 When used on a hash array element, it tells you whether the value
486 is defined, not whether the key exists in the hash. Use exists() for that.
490 print if defined $switch{'D'};
491 print "$val\n" while defined($val = pop(@ary));
492 die "Can't readlink $sym: $!"
493 unless defined($value = readlink $sym);
494 eval '@foo = ()' if defined(@foo);
495 die "No XYZ package defined" unless defined %_XYZ;
496 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
502 Deletes the specified value from its hash array. Returns the deleted
503 value, or the undefined value if nothing was deleted. Deleting from
504 C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM
505 file deletes the entry from the DBM file. (But deleting from a tie()d
506 hash doesn't necessarily return anything.)
508 The following deletes all the values of an associative array:
510 foreach $key (keys %ARRAY) {
514 (But it would be faster to use the undef() command.) Note that the
515 EXPR can be arbitrarily complicated as long as the final operation is
518 delete $ref->[$x][$y]{$key};
522 Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
523 the current value of $! (errno). If $! is 0, exits with the value of
524 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> is 0,
525 exits with 255. Inside an eval(), the error message is stuffed into C<$@>,
526 and the eval() is terminated with the undefined value; this makes die()
527 the way to raise an exception.
531 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
532 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
534 If the value of EXPR does not end in a newline, the current script line
535 number and input line number (if any) are also printed, and a newline
536 is supplied. Hint: sometimes appending ", stopped" to your message
537 will cause it to make better sense when the string "at foo line 123" is
538 appended. Suppose you are running script "canasta".
540 die "/etc/games is no good";
541 die "/etc/games is no good, stopped";
543 produce, respectively
545 /etc/games is no good at canasta line 123.
546 /etc/games is no good, stopped at canasta line 123.
548 See also exit() and warn().
552 Not really a function. Returns the value of the last command in the
553 sequence of commands indicated by BLOCK. When modified by a loop
554 modifier, executes the BLOCK once before testing the loop condition.
555 (On other statements the loop modifiers test the conditional first.)
557 =item do SUBROUTINE(LIST)
559 A deprecated form of subroutine call. See L<perlsub>.
563 Uses the value of EXPR as a filename and executes the contents of the
564 file as a Perl script. Its primary use is to include subroutines
565 from a Perl subroutine library.
573 except that it's more efficient, more concise, keeps track of the
574 current filename for error messages, and searches all the B<-I>
575 libraries if the file isn't in the current directory (see also the @INC
576 array in L<perlvar/Predefined Names>). It's the same, however, in that it does
577 reparse the file every time you call it, so you probably don't want to
578 do this inside a loop.
580 Note that inclusion of library modules is better done with the
581 use() and require() operators, which also do error checking
582 and raise an exception if there's a problem.
586 This causes an immediate core dump. Primarily this is so that you can
587 use the B<undump> program to turn your core dump into an executable binary
588 after having initialized all your variables at the beginning of the
589 program. When the new binary is executed it will begin by executing a
590 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
591 it as a goto with an intervening core dump and reincarnation. If LABEL
592 is omitted, restarts the program from the top. WARNING: any files
593 opened at the time of the dump will NOT be open any more when the
594 program is reincarnated, with possible resulting confusion on the part
595 of Perl. See also B<-u> option in L<perlrun>.
612 dump QUICKSTART if $ARGV[0] eq '-d';
617 =item each ASSOC_ARRAY
619 Returns a 2-element array consisting of the key and value for the next
620 value of an associative array, so that you can iterate over it.
621 Entries are returned in an apparently random order. When the array is
622 entirely read, a null array is returned (which when assigned produces a
623 FALSE (0) value). The next call to each() after that will start
624 iterating again. The iterator can be reset only by reading all the
625 elements from the array. You should not add elements to an array while
626 you're iterating over it. There is a single iterator for each
627 associative array, shared by all each(), keys() and values() function
628 calls in the program. The following prints out your environment like
629 the printenv(1) program, only in a different order:
631 while (($key,$value) = each %ENV) {
632 print "$key=$value\n";
635 See also keys() and values().
643 Returns 1 if the next read on FILEHANDLE will return end of file, or if
644 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
645 gives the real filehandle name. (Note that this function actually
646 reads a character and then ungetc()s it, so it is not very useful in an
647 interactive context.) Do not read from a terminal file (or call
648 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
649 as terminals may lose the end-of-file condition if you do.
651 An C<eof> without an argument uses the last file read as argument.
652 Empty parentheses () may be used to indicate
653 the pseudofile formed of the files listed on the command line, i.e.
654 C<eof()> is reasonable to use inside a while (<>) loop to detect the end
655 of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
656 test I<EACH> file in a while (<>) loop. Examples:
658 # reset line numbering on each input file
661 close(ARGV) if (eof); # Not eof().
664 # insert dashes just before last line of last file
667 print "--------------\n";
668 close(ARGV); # close or break; is needed if we
669 # are reading from the terminal
674 Practical hint: you almost never need to use C<eof> in Perl, because the
675 input operators return undef when they run out of data. Testing C<eof>
681 EXPR is parsed and executed as if it were a little Perl program. It
682 is executed in the context of the current Perl program, so that any
683 variable settings, subroutine or format definitions remain afterwards.
684 The value returned is the value of the last expression evaluated, or a
685 return statement may be used, just as with subroutines.
687 If there is a syntax error or runtime error, or a die() statement is
688 executed, an undefined value is returned by eval(), and C<$@> is set to the
689 error message. If there was no error, C<$@> is guaranteed to be a null
690 string. If EXPR is omitted, evaluates $_. The final semicolon, if
691 any, may be omitted from the expression.
693 Note that, since eval() traps otherwise-fatal errors, it is useful for
694 determining whether a particular feature (such as socket() or symlink())
695 is implemented. It is also Perl's exception trapping mechanism, where
696 the die operator is used to raise exceptions.
698 If the code to be executed doesn't vary, you may use the eval-BLOCK
699 form to trap run-time errors without incurring the penalty of
700 recompiling each time. The error, if any, is still returned in C<$@>.
703 # make divide-by-zero non-fatal
704 eval { $answer = $a / $b; }; warn $@ if $@;
706 # same thing, but less efficient
707 eval '$answer = $a / $b'; warn $@ if $@;
709 # a compile-time error
713 eval '$answer ='; # sets $@
715 With an eval(), you should be especially careful to remember what's
716 being looked at when:
722 eval { $x }; # CASE 4
724 eval "\$$x++" # CASE 5
727 Cases 1 and 2 above behave identically: they run the code contained in the
728 variable $x. (Although case 2 has misleading double quotes making the
729 reader wonder what else might be happening (nothing is).) Cases 3 and 4
730 likewise behave in the same way: they run the code <$x>, which does
731 nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
732 is a place where normally you I<WOULD> like to use double quotes, except
733 in that particular situation, you can just use symbolic references
734 instead, as in case 6.
738 The exec() function executes a system command I<AND NEVER RETURNS>. Use
739 the system() function if you want it to return.
741 If there is more than one argument in LIST, or if LIST is an array with
742 more than one value, calls execvp(3) with the arguments in LIST. If
743 there is only one scalar argument, the argument is checked for shell
744 metacharacters. If there are any, the entire argument is passed to
745 C</bin/sh -c> for parsing. If there are none, the argument is split
746 into words and passed directly to execvp(), which is more efficient.
747 Note: exec() (and system(0) do not flush your output buffer, so you may
748 need to set C<$|> to avoid lost output. Examples:
750 exec '/bin/echo', 'Your arguments are: ', @ARGV;
751 exec "sort $outfile | uniq";
753 If you don't really want to execute the first argument, but want to lie
754 to the program you are executing about its own name, you can specify
755 the program you actually want to run as an "indirect object" (without a
756 comma) in front of the LIST. (This always forces interpretation of the
757 LIST as a multi-valued list, even if there is only a single scalar in
761 exec $shell '-sh'; # pretend it's a login shell
765 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
769 Returns TRUE if the specified hash key exists in its hash array, even
770 if the corresponding value is undefined.
772 print "Exists\n" if exists $array{$key};
773 print "Defined\n" if defined $array{$key};
774 print "True\n" if $array{$key};
776 A hash element can only be TRUE if it's defined, and defined if
777 it exists, but the reverse doesn't necessarily hold true.
779 Note that the EXPR can be arbitrarily complicated as long as the final
780 operation is a hash key lookup:
782 if (exists $ref->[$x][$y]{$key}) { ... }
786 Evaluates EXPR and exits immediately with that value. (Actually, it
787 calls any defined C<END> routines first, but the C<END> routines may not
788 abort the exit. Likewise any object destructors that need to be called
789 are called before exit.) Example:
792 exit 0 if $ans =~ /^[Xx]/;
794 See also die(). If EXPR is omitted, exits with 0 status.
798 Returns I<e> (the natural logarithm base) to the power of EXPR.
799 If EXPR is omitted, gives C<exp($_)>.
801 =item fcntl FILEHANDLE,FUNCTION,SCALAR
803 Implements the fcntl(2) function. You'll probably have to say
807 first to get the correct function definitions. Argument processing and
808 value return works just like ioctl() below. Note that fcntl() will produce
809 a fatal error if used on a machine that doesn't implement fcntl(2).
813 fcntl($filehandle, F_GETLK, $packed_return_buffer);
815 =item fileno FILEHANDLE
817 Returns the file descriptor for a filehandle. This is useful for
818 constructing bitmaps for select(). If FILEHANDLE is an expression, the
819 value is taken as the name of the filehandle.
821 =item flock FILEHANDLE,OPERATION
823 Calls flock(2) on FILEHANDLE. See L<flock(2)> for definition of
824 OPERATION. Returns TRUE for success, FALSE on failure. Will produce a
825 fatal error if used on a machine that doesn't implement either flock(2) or
826 fcntl(2). (fcntl(2) will be automatically used if flock(2) is missing.)
828 Here's a mailbox appender for BSD systems.
836 flock(MBOX,$LOCK_EX);
837 # and, in case someone appended
838 # while we were waiting...
843 flock(MBOX,$LOCK_UN);
846 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
847 or die "Can't open mailbox: $!";
850 print MBOX $msg,"\n\n";
853 Note that many versions of flock() cannot lock things over the network.
854 You need to do locking with fcntl() for that.
858 Does a fork(2) system call. Returns the child pid to the parent process
859 and 0 to the child process, or C<undef> if the fork is unsuccessful.
860 Note: unflushed buffers remain unflushed in both processes, which means
861 you may need to set C<$|> ($AUTOFLUSH in English) or call the
862 autoflush() FileHandle method to avoid duplicate output.
864 If you fork() without ever waiting on your children, you will accumulate
867 $SIG{CHLD} = sub { wait };
869 There's also the double-fork trick (error checking on
870 fork() returns omitted);
872 unless ($pid = fork) {
874 exec "what you really wanna do";
877 ## (some_perl_code_here)
885 =item formline PICTURE, LIST
887 This is an internal function used by C<format>s, though you may call it
888 too. It formats (see L<perlform>) a list of values according to the
889 contents of PICTURE, placing the output into the format output
890 accumulator, C<$^A> (or $ACCUMULATOR in English).
891 Eventually, when a write() is done, the contents of
892 C<$^A> are written to some filehandle, but you could also read C<$^A>
893 yourself and then set C<$^A> back to "". Note that a format typically
894 does one formline() per line of form, but the formline() function itself
895 doesn't care how many newlines are embedded in the PICTURE. This means
896 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
897 You may therefore need to use multiple formlines to implement a single
898 record format, just like the format compiler.
900 Be careful if you put double quotes around the picture, since an "C<@>"
901 character may be taken to mean the beginning of an array name.
902 formline() always returns TRUE. See L<perlform> for other examples.
904 =item getc FILEHANDLE
908 Returns the next character from the input file attached to FILEHANDLE,
909 or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
910 This is not particularly efficient. It cannot be used to get unbuffered
914 system "stty cbreak </dev/tty >/dev/tty 2>&1";
917 system "stty", '-icanon',
918 system "stty", 'eol', "\001";
924 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
927 system "stty", 'icanon';
928 system "stty", 'eol', '^@'; # ascii null
932 Determination of whether to whether $BSD_STYLE should be set
933 is left as an exercise to the reader.
937 Returns the current login from F</etc/utmp>, if any. If null, use
940 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
942 Do not consider getlogin() for authorentication: it is not as
943 secure as getpwuid().
945 =item getpeername SOCKET
947 Returns the packed sockaddr address of other end of the SOCKET connection.
950 $hersockaddr = getpeername(SOCK);
951 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
952 $herhostname = gethostbyaddr($iaddr, AF_INET);
953 $herstraddr = inet_ntoa($iaddr);
957 Returns the current process group for the specified PID, 0 for the
958 current process. Will raise an exception if used on a machine that
959 doesn't implement getpgrp(2). If PID is omitted, returns process
960 group of current process.
964 Returns the process id of the parent process.
966 =item getpriority WHICH,WHO
968 Returns the current priority for a process, a process group, or a user.
969 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
970 machine that doesn't implement getpriority(2).
976 =item gethostbyname NAME
978 =item getnetbyname NAME
980 =item getprotobyname NAME
986 =item getservbyname NAME,PROTO
988 =item gethostbyaddr ADDR,ADDRTYPE
990 =item getnetbyaddr ADDR,ADDRTYPE
992 =item getprotobynumber NUMBER
994 =item getservbyport PORT,PROTO
1012 =item sethostent STAYOPEN
1014 =item setnetent STAYOPEN
1016 =item setprotoent STAYOPEN
1018 =item setservent STAYOPEN
1032 These routines perform the same functions as their counterparts in the
1033 system library. Within a list context, the return values from the
1034 various get routines are as follows:
1036 ($name,$passwd,$uid,$gid,
1037 $quota,$comment,$gcos,$dir,$shell) = getpw*
1038 ($name,$passwd,$gid,$members) = getgr*
1039 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1040 ($name,$aliases,$addrtype,$net) = getnet*
1041 ($name,$aliases,$proto) = getproto*
1042 ($name,$aliases,$port,$proto) = getserv*
1044 (If the entry doesn't exist you get a null list.)
1046 Within a scalar context, you get the name, unless the function was a
1047 lookup by name, in which case you get the other thing, whatever it is.
1048 (If the entry doesn't exist you get the undefined value.) For example:
1058 The $members value returned by I<getgr*()> is a space separated list of
1059 the login names of the members of the group.
1061 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1062 C, it will be returned to you via C<$?> if the function call fails. The
1063 @addrs value returned by a successful call is a list of the raw
1064 addresses returned by the corresponding system library call. In the
1065 Internet domain, each address is four bytes long and you can unpack it
1066 by saying something like:
1068 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1070 =item getsockname SOCKET
1072 Returns the packed sockaddr address of this end of the SOCKET connection.
1075 $mysockaddr = getsockname(SOCK);
1076 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1078 =item getsockopt SOCKET,LEVEL,OPTNAME
1080 Returns the socket option requested, or undefined if there is an error.
1084 Returns the value of EXPR with filename expansions such as a shell
1085 would do. This is the internal function implementing the <*.*>
1086 operator, except it's easier to use.
1090 Converts a time as returned by the time function to a 9-element array
1091 with the time localized for the standard Greenwich timezone.
1092 Typically used as follows:
1095 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1098 All array elements are numeric, and come straight out of a struct tm.
1099 In particular this means that $mon has the range 0..11 and $wday has
1100 the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1108 The goto-LABEL form finds the statement labeled with LABEL and resumes
1109 execution there. It may not be used to go into any construct that
1110 requires initialization, such as a subroutine or a foreach loop. It
1111 also can't be used to go into a construct that is optimized away. It
1112 can be used to go almost anywhere else within the dynamic scope,
1113 including out of subroutines, but it's usually better to use some other
1114 construct such as last or die. The author of Perl has never felt the
1115 need to use this form of goto (in Perl, that is--C is another matter).
1117 The goto-EXPR form expects a label name, whose scope will be resolved
1118 dynamically. This allows for computed gotos per FORTRAN, but isn't
1119 necessarily recommended if you're optimizing for maintainability:
1121 goto ("FOO", "BAR", "GLARCH")[$i];
1123 The goto-&NAME form is highly magical, and substitutes a call to the
1124 named subroutine for the currently running subroutine. This is used by
1125 AUTOLOAD subroutines that wish to load another subroutine and then
1126 pretend that the other subroutine had been called in the first place
1127 (except that any modifications to @_ in the current subroutine are
1128 propagated to the other subroutine.) After the goto, not even caller()
1129 will be able to tell that this routine was called first.
1131 =item grep BLOCK LIST
1133 =item grep EXPR,LIST
1135 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1136 $_ to each element) and returns the list value consisting of those
1137 elements for which the expression evaluated to TRUE. In a scalar
1138 context, returns the number of times the expression was TRUE.
1140 @foo = grep(!/^#/, @bar); # weed out comments
1144 @foo = grep {!/^#/} @bar; # weed out comments
1146 Note that, since $_ is a reference into the list value, it can be used
1147 to modify the elements of the array. While this is useful and
1148 supported, it can cause bizarre results if the LIST is not a named
1153 Interprets EXPR as a hex string and returns the corresponding decimal
1154 value. (To convert strings that might start with 0 or 0x see
1155 oct().) If EXPR is omitted, uses $_.
1159 There is no built-in import() function. It is merely an ordinary
1160 method (subroutine) defined (or inherited) by modules that wish to export
1161 names to another module. The use() function calls the import() method
1162 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
1164 =item index STR,SUBSTR,POSITION
1166 =item index STR,SUBSTR
1168 Returns the position of the first occurrence of SUBSTR in STR at or after
1169 POSITION. If POSITION is omitted, starts searching from the beginning of
1170 the string. The return value is based at 0 (or whatever you've set the $[
1171 variable to--but don't do that). If the substring is not found, returns
1172 one less than the base, ordinarily -1.
1176 Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1178 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1180 Implements the ioctl(2) function. You'll probably have to say
1182 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1184 first to get the correct function definitions. If F<ioctl.ph> doesn't
1185 exist or doesn't have the correct definitions you'll have to roll your
1186 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1187 (There is a Perl script called B<h2ph> that comes with the Perl kit which
1188 may help you in this, but it's non-trivial.) SCALAR will be read and/or
1189 written depending on the FUNCTION--a pointer to the string value of SCALAR
1190 will be passed as the third argument of the actual ioctl call. (If SCALAR
1191 has no string value but does have a numeric value, that value will be
1192 passed rather than a pointer to the string value. To guarantee this to be
1193 TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1194 functions are useful for manipulating the values of structures used by
1195 ioctl(). The following example sets the erase character to DEL.
1199 die "NO TIOCGETP" if $@ || !$getp;
1200 $sgttyb_t = "ccccs"; # 4 chars and a short
1201 if (ioctl(STDIN,$getp,$sgttyb)) {
1202 @ary = unpack($sgttyb_t,$sgttyb);
1204 $sgttyb = pack($sgttyb_t,@ary);
1205 ioctl(STDIN,&TIOCSETP,$sgttyb)
1206 || die "Can't ioctl: $!";
1209 The return value of ioctl (and fcntl) is as follows:
1211 if OS returns: then Perl returns:
1213 0 string "0 but true"
1214 anything else that number
1216 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1217 still easily determine the actual value returned by the operating
1220 ($retval = ioctl(...)) || ($retval = -1);
1221 printf "System returned %d\n", $retval;
1223 =item join EXPR,LIST
1225 Joins the separate strings of LIST or ARRAY into a single string with
1226 fields separated by the value of EXPR, and returns the string.
1229 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1231 See L<perlfunc/split>.
1233 =item keys ASSOC_ARRAY
1235 Returns a normal array consisting of all the keys of the named
1236 associative array. (In a scalar context, returns the number of keys.)
1237 The keys are returned in an apparently random order, but it is the same
1238 order as either the values() or each() function produces (given that
1239 the associative array has not been modified). Here is yet another way
1240 to print your environment:
1243 @values = values %ENV;
1244 while ($#keys >= 0) {
1245 print pop(@keys), '=', pop(@values), "\n";
1248 or how about sorted by key:
1250 foreach $key (sort(keys %ENV)) {
1251 print $key, '=', $ENV{$key}, "\n";
1254 To sort an array by value, you'll need to use a C<sort{}>
1255 function. Here's a descending numeric sort by value:
1257 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1258 printf "%4d %s\n", $hash{$key}, $key;
1263 Sends a signal to a list of processes. The first element of
1264 the list must be the signal to send. Returns the number of
1265 processes successfully signaled.
1267 $cnt = kill 1, $child1, $child2;
1270 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1271 process groups instead of processes. (On System V, a negative I<PROCESS>
1272 number will also kill process groups, but that's not portable.) That
1273 means you usually want to use positive not negative signals. You may also
1274 use a signal name in quotes. See the L<perlipc/"Signals"> man page for details.
1280 The C<last> command is like the C<break> statement in C (as used in
1281 loops); it immediately exits the loop in question. If the LABEL is
1282 omitted, the command refers to the innermost enclosing loop. The
1283 C<continue> block, if any, is not executed:
1285 LINE: while (<STDIN>) {
1286 last LINE if /^$/; # exit when done with header
1292 Returns an lowercased version of EXPR. This is the internal function
1293 implementing the \L escape in double-quoted strings.
1294 Should respect any POSIX setlocale() settings.
1298 Returns the value of EXPR with the first character lowercased. This is
1299 the internal function implementing the \l escape in double-quoted strings.
1300 Should respect any POSIX setlocale() settings.
1304 Returns the length in characters of the value of EXPR. If EXPR is
1305 omitted, returns length of $_.
1307 =item link OLDFILE,NEWFILE
1309 Creates a new filename linked to the old filename. Returns 1 for
1310 success, 0 otherwise.
1312 =item listen SOCKET,QUEUESIZE
1314 Does the same thing that the listen system call does. Returns TRUE if
1315 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1319 In general, you should be using "my" instead of "local", because it's
1320 faster and safer. Format variables often use "local" though, as
1321 do other variables whose current value must be visible to called
1322 subroutines. This is known as dynamic scoping. Lexical scoping is
1323 done with "my", which works more like C's auto declarations.
1325 A local modifies the listed variables to be local to the enclosing block,
1326 subroutine, eval or "do". If more than one value is listed, the list
1327 must be placed in parens. All the listed elements must be legal
1328 lvalues. This operator works by saving the current values of those
1329 variables in LIST on a hidden stack and restoring them upon exiting the
1330 block, subroutine or eval. This means that called subroutines can also
1331 reference the local variable, but not the global one. The LIST may be
1332 assigned to if desired, which allows you to initialize your local
1333 variables. (If no initializer is given for a particular variable, it
1334 is created with an undefined value.) Commonly this is used to name the
1335 parameters to a subroutine. Examples:
1338 local($min, $max, $thunk) = @_;
1342 # Presumably $thunk makes reference to $i
1344 for ($i = $min; $i < $max; $i++) {
1345 $result .= eval $thunk;
1353 # init local array with global array
1354 local @ARGV = @ARGV;
1355 unshift(@ARGV,'echo');
1361 # temporarily add to digits associative array
1363 # (NOTE: not claiming this is efficient!)
1364 local(%digits) = (%digits,'t',10,'e',11);
1368 Note that local() is a run-time command, and so gets executed every
1369 time through a loop. In Perl 4 it used more stack storage each
1370 time until the loop was exited. Perl 5 reclaims the space each time
1371 through, but it's still more efficient to declare your variables
1374 A local is simply a modifier on an lvalue expression.
1375 When you assign to a localized EXPR, the local doesn't change whether
1376 EXPR is viewed as a scalar or an array. So
1378 local($foo) = <STDIN>;
1379 local @FOO = <STDIN>;
1381 both supply a list context to the righthand side, while
1383 local $foo = <STDIN>;
1385 supplies a scalar context.
1387 =item localtime EXPR
1389 Converts a time as returned by the time function to a 9-element array
1390 with the time analyzed for the local timezone. Typically used as
1393 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1396 All array elements are numeric, and come straight out of a struct tm.
1397 In particular this means that $mon has the range 0..11 and $wday has
1398 the range 0..6. If EXPR is omitted, does localtime(time).
1400 In a scalar context, prints out the ctime(3) value:
1402 $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
1404 See also L<perlmod/timelocal> and the strftime(3) function available
1405 via the POSIX modulie.
1409 Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1412 =item lstat FILEHANDLE
1416 Does the same thing as the stat() function, but stats a symbolic link
1417 instead of the file the symbolic link points to. If symbolic links are
1418 unimplemented on your system, a normal stat() is done.
1422 The match operator. See L<perlop>.
1424 =item map BLOCK LIST
1428 Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1429 element) and returns the list value composed of the results of each such
1430 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1431 may produce zero, one, or more elements in the returned value.
1433 @chars = map(chr, @nums);
1435 translates a list of numbers to the corresponding characters. And
1437 %hash = map { getkey($_) => $_ } @array;
1439 is just a funny way to write
1442 foreach $_ (@array) {
1443 $hash{getkey($_)} = $_;
1446 =item mkdir FILENAME,MODE
1448 Creates the directory specified by FILENAME, with permissions specified
1449 by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1450 it returns 0 and sets $! (errno).
1452 =item msgctl ID,CMD,ARG
1454 Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
1455 must be a variable which will hold the returned msqid_ds structure.
1456 Returns like ioctl: the undefined value for error, "0 but true" for
1457 zero, or the actual return value otherwise.
1459 =item msgget KEY,FLAGS
1461 Calls the System V IPC function msgget(2). Returns the message queue id,
1462 or the undefined value if there is an error.
1464 =item msgsnd ID,MSG,FLAGS
1466 Calls the System V IPC function msgsnd to send the message MSG to the
1467 message queue ID. MSG must begin with the long integer message type,
1468 which may be created with C<pack("L", $type)>. Returns TRUE if
1469 successful, or FALSE if there is an error.
1471 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1473 Calls the System V IPC function msgrcv to receive a message from
1474 message queue ID into variable VAR with a maximum message size of
1475 SIZE. Note that if a message is received, the message type will be the
1476 first thing in VAR, and the maximum length of VAR is SIZE plus the size
1477 of the message type. Returns TRUE if successful, or FALSE if there is
1482 A "my" declares the listed variables to be local (lexically) to the
1483 enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If more than one value is
1484 listed, the list must be placed in parens. All the listed elements
1485 must be legal lvalues. Only alphanumeric identifiers may be lexically
1486 scoped--magical builtins like $/ must be localized with "local"
1487 instead. You also cannot use my() on a package variable.
1488 In particular, you're not allowed to say
1491 my $pack::$var; # Illegal!
1493 Unlike the "local" declaration, variables declared with "my"
1494 are totally hidden from the outside world, including any called
1495 subroutines (even if it's the same subroutine--every call gets its own
1498 (An eval(), however, can see the lexical variables of the scope it is
1499 being evaluated in so long as the names aren't hidden by declarations within
1500 the eval() itself. See L<perlref>.)
1502 The EXPR may be assigned to if desired, which allows you to initialize
1503 your variables. (If no initializer is given for a particular
1504 variable, it is created with an undefined value.) Commonly this is
1505 used to name the parameters to a subroutine. Examples:
1508 my($min, $max, $thunk) = @_;
1512 # Presumably $thunk makes reference to $i
1514 for ($i = $min; $i < $max; $i++) {
1515 $result .= eval $thunk;
1523 # init my array with global array
1525 unshift(@ARGV,'echo');
1528 # Outer @ARGV again visible
1530 The "my" is simply a modifier on something you might assign to.
1531 So when you do assign to the EXPR, the "my" doesn't change whether
1532 EXPR is viewed as a scalar or an array. So
1534 my ($foo) = <STDIN>;
1537 both supply a list context to the righthand side, while
1541 supplies a scalar context. But the following only declares one variable:
1545 That has the same effect as
1550 The declared variable is not introduced (is not visible) until after
1551 the current statement. Thus,
1555 can be used to initialize the new $x with the value of the old $x, and
1558 my $x = 123 and $x == 123
1560 is false unless the old $x happened to have the value 123.
1562 Some users may wish to encourage the use of lexically scoped variables.
1563 As an aid to catching implicit references to package variables,
1568 then any variable reference from there to the end of the enclosing
1569 block must either refer to a lexical variable, or must be fully
1570 qualified with the package name. A compilation error results
1571 otherwise. An inner block may countermand this with S<"no strict 'vars'">.
1573 Variables declared with "my" are not part of any package and
1574 are therefore never fully qualified with the package name.
1575 However, you may declare a "my" variable at the outer most
1576 scope of a file to totally hide any such identifiers from the
1577 outside world. This is similar to a C's static variables
1578 at the file level. To do this with a subroutine requires the
1579 use of a closure (anonymous function):
1581 my $secret_version = '1.001-beta';
1582 my $secret_sub = { print $secret_version };
1585 This does not work with object methods, however;
1586 all object methods have to be in the symbol table of some
1587 package to be found.
1589 Just because the "my" variable is lexically scoped doesn't mean that
1590 within a function it works like a C static. Here's a mechanism for giving
1591 a function private variables with both lexical scoping and a static
1600 print "$var (call # $count)";
1611 The C<next> command is like the C<continue> statement in C; it starts
1612 the next iteration of the loop:
1614 LINE: while (<STDIN>) {
1615 next LINE if /^#/; # discard comments
1619 Note that if there were a C<continue> block on the above, it would get
1620 executed even on discarded lines. If the LABEL is omitted, the command
1621 refers to the innermost enclosing loop.
1623 =item no Module LIST
1625 See the "use" function, which "no" is the opposite of.
1629 Interprets EXPR as an octal string and returns the corresponding
1630 decimal value. (If EXPR happens to start off with 0x, interprets it as
1631 a hex string instead.) The following will handle decimal, octal, and
1632 hex in the standard Perl or C notation:
1634 $val = oct($val) if $val =~ /^0/;
1636 If EXPR is omitted, uses $_.
1638 =item open FILEHANDLE,EXPR
1640 =item open FILEHANDLE
1642 Opens the file whose filename is given by EXPR, and associates it with
1643 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name
1644 of the real filehandle wanted. If EXPR is omitted, the scalar variable of
1645 the same name as the FILEHANDLE contains the filename. If the filename
1646 begins with "<" or nothing, the file is opened for input. If the filename
1647 begins with ">", the file is opened for output. If the filename begins
1648 with ">>", the file is opened for appending. (You can put a '+' in front
1649 of the '>' or '<' to indicate that you want both read and write access to
1650 the file.) If the filename begins with "|", the filename is interpreted
1651 as a command to which output is to be piped, and if the filename ends with
1652 a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
1653 for more examples of this. as command which pipes input to us. (You may
1654 not have a command that pipes both in and out, but see See L<open2>,
1655 L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)
1656 Opening '-' opens STDIN and opening '>-' opens STDOUT. Open returns
1657 non-zero upon success, the undefined value otherwise. If the open
1658 involved a pipe, the return value happens to be the pid of the
1659 subprocess. Examples:
1662 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1663 while (<ARTICLE>) {...
1665 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1667 open(ARTICLE, "caesar <$article |"); # decrypt article
1669 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
1671 # process argument list of files along with any includes
1673 foreach $file (@ARGV) {
1674 process($file, 'fh00');
1678 local($filename, $input) = @_;
1679 $input++; # this is a string increment
1680 unless (open($input, $filename)) {
1681 print STDERR "Can't open $filename: $!\n";
1685 while (<$input>) { # note use of indirection
1686 if (/^#include "(.*)"/) {
1687 process($1, $input);
1694 You may also, in the Bourne shell tradition, specify an EXPR beginning
1695 with ">&", in which case the rest of the string is interpreted as the
1696 name of a filehandle (or file descriptor, if numeric) which is to be
1697 duped and opened. You may use & after >, >>, <, +>, +>> and +<. The
1698 mode you specify should match the mode of the original filehandle.
1699 Here is a script that saves, redirects, and restores STDOUT and
1703 open(SAVEOUT, ">&STDOUT");
1704 open(SAVEERR, ">&STDERR");
1706 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1707 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1709 select(STDERR); $| = 1; # make unbuffered
1710 select(STDOUT); $| = 1; # make unbuffered
1712 print STDOUT "stdout 1\n"; # this works for
1713 print STDERR "stderr 1\n"; # subprocesses too
1718 open(STDOUT, ">&SAVEOUT");
1719 open(STDERR, ">&SAVEERR");
1721 print STDOUT "stdout 2\n";
1722 print STDERR "stderr 2\n";
1725 If you specify "<&=N", where N is a number, then Perl will do an
1726 equivalent of C's fdopen() of that file descriptor; this is more
1727 parsimonious of file descriptors. For example:
1729 open(FILEHANDLE, "<&=$fd")
1731 If you open a pipe on the command "-", i.e. either "|-" or "-|", then
1732 there is an implicit fork done, and the return value of open is the pid
1733 of the child within the parent process, and 0 within the child
1734 process. (Use defined($pid) to determine whether the open was successful.)
1735 The filehandle behaves normally for the parent, but i/o to that
1736 filehandle is piped from/to the STDOUT/STDIN of the child process.
1737 In the child process the filehandle isn't opened--i/o happens from/to
1738 the new STDOUT or STDIN. Typically this is used like the normal
1739 piped open when you want to exercise more control over just how the
1740 pipe command gets executed, such as when you are running setuid, and
1741 don't want to have to scan shell commands for metacharacters.
1742 The following pairs are more or less equivalent:
1744 open(FOO, "|tr '[a-z]' '[A-Z]'");
1745 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1747 open(FOO, "cat -n '$file'|");
1748 open(FOO, "-|") || exec 'cat', '-n', $file;
1750 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1752 Explicitly closing any piped filehandle causes the parent process to
1753 wait for the child to finish, and returns the status value in $?.
1754 Note: on any operation which may do a fork, unflushed buffers remain
1755 unflushed in both processes, which means you may need to set $| to
1756 avoid duplicate output.
1758 The filename that is passed to open will have leading and trailing
1759 whitespace deleted. In order to open a file with arbitrary weird
1760 characters in it, it's necessary to protect any leading and trailing
1763 $file =~ s#^(\s)#./$1#;
1764 open(FOO, "< $file\0");
1766 =item opendir DIRHANDLE,EXPR
1768 Opens a directory named EXPR for processing by readdir(), telldir(),
1769 seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
1770 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1774 Returns the numeric ascii value of the first character of EXPR. If
1775 EXPR is omitted, uses $_.
1777 =item pack TEMPLATE,LIST
1779 Takes an array or list of values and packs it into a binary structure,
1780 returning the string containing the structure. The TEMPLATE is a
1781 sequence of characters that give the order and type of values, as
1784 A An ascii string, will be space padded.
1785 a An ascii string, will be null padded.
1786 b A bit string (ascending bit order, like vec()).
1787 B A bit string (descending bit order).
1788 h A hex string (low nybble first).
1789 H A hex string (high nybble first).
1791 c A signed char value.
1792 C An unsigned char value.
1793 s A signed short value.
1794 S An unsigned short value.
1795 i A signed integer value.
1796 I An unsigned integer value.
1797 l A signed long value.
1798 L An unsigned long value.
1800 n A short in "network" order.
1801 N A long in "network" order.
1802 v A short in "VAX" (little-endian) order.
1803 V A long in "VAX" (little-endian) order.
1805 f A single-precision float in the native format.
1806 d A double-precision float in the native format.
1808 p A pointer to a null-terminated string.
1809 P A pointer to a structure (fixed-length string).
1811 u A uuencoded string.
1815 @ Null fill to absolute position.
1817 Each letter may optionally be followed by a number which gives a repeat
1818 count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
1819 pack function will gobble up that many values from the LIST. A * for the
1820 repeat count means to use however many items are left. The "a" and "A"
1821 types gobble just one value, but pack it as a string of length count,
1822 padding with nulls or spaces as necessary. (When unpacking, "A" strips
1823 trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
1824 fields pack a string that many bits long. The "h" and "H" fields pack a
1825 string that many nybbles long. The "P" packs a pointer to a structure of
1826 the size indicated by the length. Real numbers (floats and doubles) are
1827 in the native machine format only; due to the multiplicity of floating
1828 formats around, and the lack of a standard "network" representation, no
1829 facility for interchange has been made. This means that packed floating
1830 point data written on one machine may not be readable on another - even if
1831 both use IEEE floating point arithmetic (as the endian-ness of the memory
1832 representation is not part of the IEEE spec). Note that Perl uses doubles
1833 internally for all numeric calculation, and converting from double into
1834 float and thence back to double again will lose precision (i.e.
1835 C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
1839 $foo = pack("cccc",65,66,67,68);
1841 $foo = pack("c4",65,66,67,68);
1844 $foo = pack("ccxxcc",65,66,67,68);
1847 $foo = pack("s2",1,2);
1848 # "\1\0\2\0" on little-endian
1849 # "\0\1\0\2" on big-endian
1851 $foo = pack("a4","abcd","x","y","z");
1854 $foo = pack("aaaa","abcd","x","y","z");
1857 $foo = pack("a14","abcdefg");
1858 # "abcdefg\0\0\0\0\0\0\0"
1860 $foo = pack("i9pl", gmtime);
1861 # a real struct tm (on my system anyway)
1864 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
1867 The same template may generally also be used in the unpack function.
1869 =item pipe READHANDLE,WRITEHANDLE
1871 Opens a pair of connected pipes like the corresponding system call.
1872 Note that if you set up a loop of piped processes, deadlock can occur
1873 unless you are very careful. In addition, note that Perl's pipes use
1874 stdio buffering, so you may need to set $| to flush your WRITEHANDLE
1875 after each command, depending on the application.
1877 See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
1878 for examples of such things.
1882 Pops and returns the last value of the array, shortening the array by
1883 1. Has a similar effect to
1885 $tmp = $ARRAY[$#ARRAY--];
1887 If there are no elements in the array, returns the undefined value.
1891 Returns the offset of where the last C<m//g> search left off for the variable
1892 in question. May be modified to change that offset.
1894 =item print FILEHANDLE LIST
1900 Prints a string or a comma-separated list of strings. Returns non-zero
1901 if successful. FILEHANDLE may be a scalar variable name, in which case
1902 the variable contains the name of the filehandle, thus introducing one
1903 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
1904 token is a term, it may be misinterpreted as an operator unless you
1905 interpose a + or put parens around the arguments.) If FILEHANDLE is
1906 omitted, prints by default to standard output (or to the last selected
1907 output channel--see select()). If LIST is also omitted, prints $_ to
1908 STDOUT. To set the default output channel to something other than
1909 STDOUT use the select operation. Note that, because print takes a
1910 LIST, anything in the LIST is evaluated in a list context, and any
1911 subroutine that you call will have one or more of its expressions
1912 evaluated in a list context. Also be careful not to follow the print
1913 keyword with a left parenthesis unless you want the corresponding right
1914 parenthesis to terminate the arguments to the print--interpose a + or
1915 put parens around all the arguments.
1917 Note that if you're storing FILEHANDLES in an array or other expression,
1918 you will have to use a block returning its value instead
1920 print { $files[$i] } "stuff\n";
1921 print { $OK ? STDOUT : STDERR } "stuff\n";
1923 =item printf FILEHANDLE LIST
1927 Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
1928 of the list will be interpreted as the printf format.
1930 =item push ARRAY,LIST
1932 Treats ARRAY as a stack, and pushes the values of LIST
1933 onto the end of ARRAY. The length of ARRAY increases by the length of
1934 LIST. Has the same effect as
1937 $ARRAY[++$#ARRAY] = $value;
1940 but is more efficient. Returns the new number of elements in the array.
1950 Generalized quotes. See L<perlop>.
1952 =item quotemeta EXPR
1954 Returns the value of EXPR with with all regular expression
1955 metacharacters backslashed. This is the internal function implementing
1956 the \Q escape in double-quoted strings.
1962 Returns a random fractional number between 0 and the value of EXPR.
1963 (EXPR should be positive.) If EXPR is omitted, returns a value between
1964 0 and 1. This function produces repeatable sequences unless srand()
1965 is invoked. See also srand().
1967 (Note: if your rand function consistently returns numbers that are too
1968 large or too small, then your version of Perl was probably compiled
1969 with the wrong number of RANDBITS. As a workaround, you can usually
1970 multiply EXPR by the correct power of 2 to get the range you want.
1971 This will make your script unportable, however. It's better to recompile
1974 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
1976 =item read FILEHANDLE,SCALAR,LENGTH
1978 Attempts to read LENGTH bytes of data into variable SCALAR from the
1979 specified FILEHANDLE. Returns the number of bytes actually read, or
1980 undef if there was an error. SCALAR will be grown or shrunk to the
1981 length actually read. An OFFSET may be specified to place the read
1982 data at some other place than the beginning of the string. This call
1983 is actually implemented in terms of stdio's fread call. To get a true
1984 read system call, see sysread().
1986 =item readdir DIRHANDLE
1988 Returns the next directory entry for a directory opened by opendir().
1989 If used in a list context, returns all the rest of the entries in the
1990 directory. If there are no more entries, returns an undefined value in
1991 a scalar context or a null list in a list context.
1995 Returns the value of a symbolic link, if symbolic links are
1996 implemented. If not, gives a fatal error. If there is some system
1997 error, returns the undefined value and sets $! (errno). If EXPR is
2000 =item recv SOCKET,SCALAR,LEN,FLAGS
2002 Receives a message on a socket. Attempts to receive LENGTH bytes of
2003 data into variable SCALAR from the specified SOCKET filehandle.
2004 Actually does a C recvfrom(), so that it can returns the address of the
2005 sender. Returns the undefined value if there's an error. SCALAR will
2006 be grown or shrunk to the length actually read. Takes the same flags
2007 as the system call of the same name.
2008 See L<perlipc/"UDP: Message Passing"> for examples.
2014 The C<redo> command restarts the loop block without evaluating the
2015 conditional again. The C<continue> block, if any, is not executed. If
2016 the LABEL is omitted, the command refers to the innermost enclosing
2017 loop. This command is normally used by programs that want to lie to
2018 themselves about what was just input:
2020 # a simpleminded Pascal comment stripper
2021 # (warning: assumes no { or } in strings)
2022 LINE: while (<STDIN>) {
2023 while (s|({.*}.*){.*}|$1 |) {}
2028 if (/}/) { # end of comment?
2039 Returns a TRUE value if EXPR is a reference, FALSE otherwise. The value
2040 returned depends on the type of thing the reference is a reference to.
2041 Builtin types include:
2050 If the referenced object has been blessed into a package, then that package
2051 name is returned instead. You can think of ref() as a typeof() operator.
2053 if (ref($r) eq "HASH") {
2054 print "r is a reference to an associative array.\n";
2057 print "r is not a reference at all.\n";
2060 See also L<perlref>.
2062 =item rename OLDNAME,NEWNAME
2064 Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2065 not work across filesystem boundaries.
2071 Demands some semantics specified by EXPR, or by $_ if EXPR is not
2072 supplied. If EXPR is numeric, demands that the current version of Perl
2073 ($] or $PERL_VERSION) be equal or greater than EXPR.
2075 Otherwise, demands that a library file be included if it hasn't already
2076 been included. The file is included via the do-FILE mechanism, which is
2077 essentially just a variety of eval(). Has semantics similar to the following
2081 local($filename) = @_;
2082 return 1 if $INC{$filename};
2083 local($realfilename,$result);
2085 foreach $prefix (@INC) {
2086 $realfilename = "$prefix/$filename";
2087 if (-f $realfilename) {
2088 $result = do $realfilename;
2092 die "Can't find $filename in \@INC";
2095 die "$filename did not return true value" unless $result;
2096 $INC{$filename} = $realfilename;
2100 Note that the file will not be included twice under the same specified
2101 name. The file must return TRUE as the last statement to indicate
2102 successful execution of any initialization code, so it's customary to
2103 end such a file with "1;" unless you're sure it'll return TRUE
2104 otherwise. But it's better just to put the "C<1;>", in case you add more
2107 If EXPR is a bare word, the require assumes a "F<.pm>" extension for you,
2108 to make it easy to load standard modules. This form of loading of
2109 modules does not risk altering your namespace.
2111 For a yet-more-powerful import facility, see the L</use()> and
2118 Generally used in a C<continue> block at the end of a loop to clear
2119 variables and reset ?? searches so that they work again. The
2120 expression is interpreted as a list of single characters (hyphens
2121 allowed for ranges). All variables and arrays beginning with one of
2122 those letters are reset to their pristine state. If the expression is
2123 omitted, one-match searches (?pattern?) are reset to match again. Only
2124 resets variables or searches in the current package. Always returns
2127 reset 'X'; # reset all X variables
2128 reset 'a-z'; # reset lower case variables
2129 reset; # just reset ?? searches
2131 Resetting "A-Z" is not recommended since you'll wipe out your
2132 ARGV and ENV arrays. Only resets package variables--lexical variables
2133 are unaffected, but they clean themselves up on scope exit anyway,
2134 so anymore you probably want to use them instead. See L</my>.
2138 Returns from a subroutine or eval with the value specified. (Note that
2139 in the absence of a return a subroutine or eval() will automatically
2140 return the value of the last expression evaluated.)
2144 In a list context, returns a list value consisting of the elements
2145 of LIST in the opposite order. In a scalar context, returns a string
2146 value consisting of the bytes of the first element of LIST in the
2149 print reverse <>; # line tac
2152 print scalar reverse scalar <>; # byte tac
2154 =item rewinddir DIRHANDLE
2156 Sets the current position to the beginning of the directory for the
2157 readdir() routine on DIRHANDLE.
2159 =item rindex STR,SUBSTR,POSITION
2161 =item rindex STR,SUBSTR
2163 Works just like index except that it returns the position of the LAST
2164 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2165 last occurrence at or before that position.
2167 =item rmdir FILENAME
2169 Deletes the directory specified by FILENAME if it is empty. If it
2170 succeeds it returns 1, otherwise it returns 0 and sets $! (errno). If
2171 FILENAME is omitted, uses $_.
2175 The substitution operator. See L<perlop>.
2179 Forces EXPR to be interpreted in a scalar context and returns the value
2182 =item seek FILEHANDLE,POSITION,WHENCE
2184 Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2185 call of stdio. FILEHANDLE may be an expression whose value gives the name
2186 of the filehandle. The values for WHENCE are 0 to set the file pointer to
2187 POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2188 plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
2189 this from POSIX module. Returns 1 upon success, 0 otherwise.
2191 =item seekdir DIRHANDLE,POS
2193 Sets the current position for the readdir() routine on DIRHANDLE. POS
2194 must be a value returned by telldir(). Has the same caveats about
2195 possible directory compaction as the corresponding system library
2198 =item select FILEHANDLE
2202 Returns the currently selected filehandle. Sets the current default
2203 filehandle for output, if FILEHANDLE is supplied. This has two
2204 effects: first, a C<write> or a C<print> without a filehandle will
2205 default to this FILEHANDLE. Second, references to variables related to
2206 output will refer to this output channel. For example, if you have to
2207 set the top of form format for more than one output channel, you might
2215 FILEHANDLE may be an expression whose value gives the name of the
2216 actual filehandle. Thus:
2218 $oldfh = select(STDERR); $| = 1; select($oldfh);
2220 Some programmers may prefer to think of filehandles as objects with
2221 methods, preferring to write the last example as:
2224 STDERR->autoflush(1);
2226 =item select RBITS,WBITS,EBITS,TIMEOUT
2228 This calls the select(2) system call with the bitmasks specified, which
2229 can be constructed using fileno() and vec(), along these lines:
2231 $rin = $win = $ein = '';
2232 vec($rin,fileno(STDIN),1) = 1;
2233 vec($win,fileno(STDOUT),1) = 1;
2236 If you want to select on many filehandles you might wish to write a
2240 local(@fhlist) = split(' ',$_[0]);
2243 vec($bits,fileno($_),1) = 1;
2247 $rin = fhbits('STDIN TTY SOCK');
2251 ($nfound,$timeleft) =
2252 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2254 or to block until something becomes ready:
2256 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2258 Any of the bitmasks can also be undef. The timeout, if specified, is
2259 in seconds, which may be fractional. Note: not all implementations are
2260 capable of returning the $timeleft. If not, they always return
2261 $timeleft equal to the supplied $timeout.
2263 You can effect a 250-microsecond sleep this way:
2265 select(undef, undef, undef, 0.25);
2268 =item semctl ID,SEMNUM,CMD,ARG
2270 Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2271 &GETALL, then ARG must be a variable which will hold the returned
2272 semid_ds structure or semaphore value array. Returns like ioctl: the
2273 undefined value for error, "0 but true" for zero, or the actual return
2276 =item semget KEY,NSEMS,FLAGS
2278 Calls the System V IPC function semget. Returns the semaphore id, or
2279 the undefined value if there is an error.
2281 =item semop KEY,OPSTRING
2283 Calls the System V IPC function semop to perform semaphore operations
2284 such as signaling and waiting. OPSTRING must be a packed array of
2285 semop structures. Each semop structure can be generated with
2286 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2287 operations is implied by the length of OPSTRING. Returns TRUE if
2288 successful, or FALSE if there is an error. As an example, the
2289 following code waits on semaphore $semnum of semaphore id $semid:
2291 $semop = pack("sss", $semnum, -1, 0);
2292 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2294 To signal the semaphore, replace "-1" with "1".
2296 =item send SOCKET,MSG,FLAGS,TO
2298 =item send SOCKET,MSG,FLAGS
2300 Sends a message on a socket. Takes the same flags as the system call
2301 of the same name. On unconnected sockets you must specify a
2302 destination to send TO, in which case it does a C sendto(). Returns
2303 the number of characters sent, or the undefined value if there is an
2305 See L<perlipc/"UDP: Message Passing"> for examples.
2307 =item setpgrp PID,PGRP
2309 Sets the current process group for the specified PID, 0 for the current
2310 process. Will produce a fatal error if used on a machine that doesn't
2311 implement setpgrp(2).
2313 =item setpriority WHICH,WHO,PRIORITY
2315 Sets the current priority for a process, a process group, or a user.
2316 (See setpriority(2).) Will produce a fatal error if used on a machine
2317 that doesn't implement setpriority(2).
2319 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2321 Sets the socket option requested. Returns undefined if there is an
2322 error. OPTVAL may be specified as undef if you don't want to pass an
2329 Shifts the first value of the array off and returns it, shortening the
2330 array by 1 and moving everything down. If there are no elements in the
2331 array, returns the undefined value. If ARRAY is omitted, shifts the
2332 @ARGV array in the main program, and the @_ array in subroutines.
2333 (This is determined lexically.) See also unshift(), push(), and pop().
2334 Shift() and unshift() do the same thing to the left end of an array
2335 that push() and pop() do to the right end.
2337 =item shmctl ID,CMD,ARG
2339 Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2340 must be a variable which will hold the returned shmid_ds structure.
2341 Returns like ioctl: the undefined value for error, "0 but true" for
2342 zero, or the actual return value otherwise.
2344 =item shmget KEY,SIZE,FLAGS
2346 Calls the System V IPC function shmget. Returns the shared memory
2347 segment id, or the undefined value if there is an error.
2349 =item shmread ID,VAR,POS,SIZE
2351 =item shmwrite ID,STRING,POS,SIZE
2353 Reads or writes the System V shared memory segment ID starting at
2354 position POS for size SIZE by attaching to it, copying in/out, and
2355 detaching from it. When reading, VAR must be a variable which will
2356 hold the data read. When writing, if STRING is too long, only SIZE
2357 bytes are used; if STRING is too short, nulls are written to fill out
2358 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2360 =item shutdown SOCKET,HOW
2362 Shuts down a socket connection in the manner indicated by HOW, which
2363 has the same interpretation as in the system call of the same name.
2367 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2374 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2375 May be interrupted by sending the process a SIGALRM. Returns the
2376 number of seconds actually slept. You probably cannot mix alarm() and
2377 sleep() calls, since sleep() is often implemented using alarm().
2379 On some older systems, it may sleep up to a full second less than what
2380 you requested, depending on how it counts seconds. Most modern systems
2381 always sleep the full amount.
2383 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2385 Opens a socket of the specified kind and attaches it to filehandle
2386 SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
2387 system call of the same name. You should "use Socket;" first to get
2388 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2390 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2392 Creates an unnamed pair of sockets in the specified domain, of the
2393 specified type. DOMAIN, TYPE and PROTOCOL are specified the same as
2394 for the system call of the same name. If unimplemented, yields a fatal
2395 error. Returns TRUE if successful.
2397 =item sort SUBNAME LIST
2399 =item sort BLOCK LIST
2403 Sorts the LIST and returns the sorted list value. Nonexistent values
2404 of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2405 in standard string comparison order. If SUBNAME is specified, it
2406 gives the name of a subroutine that returns an integer less than, equal
2407 to, or greater than 0, depending on how the elements of the array are
2408 to be ordered. (The <=> and cmp operators are extremely useful in such
2409 routines.) SUBNAME may be a scalar variable name, in which case the
2410 value provides the name of the subroutine to use. In place of a
2411 SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2414 In the interests of efficiency the normal calling code for subroutines
2415 is bypassed, with the following effects: the subroutine may not be a
2416 recursive subroutine, and the two elements to be compared are passed
2417 into the subroutine not via @_ but as $a and $b (see example below).
2418 They are passed by reference, so don't modify $a and $b.
2423 @articles = sort @files;
2425 # same thing, but with explicit sort routine
2426 @articles = sort {$a cmp $b} @files;
2428 # same thing in reversed order
2429 @articles = sort {$b cmp $a} @files;
2431 # sort numerically ascending
2432 @articles = sort {$a <=> $b} @files;
2434 # sort numerically descending
2435 @articles = sort {$b <=> $a} @files;
2437 # sort using explicit subroutine name
2439 $age{$a} <=> $age{$b}; # presuming integers
2441 @sortedclass = sort byage @class;
2443 sub backwards { $b cmp $a; }
2444 @harry = ('dog','cat','x','Cain','Abel');
2445 @george = ('gone','chased','yz','Punished','Axed');
2447 # prints AbelCaincatdogx
2448 print sort backwards @harry;
2449 # prints xdogcatCainAbel
2450 print sort @george, 'to', @harry;
2451 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2453 =item splice ARRAY,OFFSET,LENGTH,LIST
2455 =item splice ARRAY,OFFSET,LENGTH
2457 =item splice ARRAY,OFFSET
2459 Removes the elements designated by OFFSET and LENGTH from an array, and
2460 replaces them with the elements of LIST, if any. Returns the elements
2461 removed from the array. The array grows or shrinks as necessary. If
2462 LENGTH is omitted, removes everything from OFFSET onward. The
2463 following equivalencies hold (assuming $[ == 0):
2465 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2466 pop(@a) splice(@a,-1)
2467 shift(@a) splice(@a,0,1)
2468 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2469 $a[$x] = $y splice(@a,$x,1,$y);
2471 Example, assuming array lengths are passed before arrays:
2473 sub aeq { # compare two list values
2474 local(@a) = splice(@_,0,shift);
2475 local(@b) = splice(@_,0,shift);
2476 return 0 unless @a == @b; # same len?
2478 return 0 if pop(@a) ne pop(@b);
2482 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2484 =item split /PATTERN/,EXPR,LIMIT
2486 =item split /PATTERN/,EXPR
2488 =item split /PATTERN/
2492 Splits a string into an array of strings, and returns it.
2494 If not in a list context, returns the number of fields found and splits into
2495 the @_ array. (In a list context, you can force the split into @_ by
2496 using C<??> as the pattern delimiters, but it still returns the array
2497 value.) The use of implicit split to @_ is deprecated, however.
2499 If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
2500 splits on whitespace (after skipping any leading whitespace). Anything
2501 matching PATTERN is taken to be a delimiter separating the fields. (Note
2502 that the delimiter may be longer than one character.) If LIMIT is
2503 specified and is not negative, splits into no more than that many fields
2504 (though it may split into fewer). If LIMIT is unspecified, trailing null
2505 fields are stripped (which potential users of pop() would do well to
2506 remember). If LIMIT is negative, it is treated as if an arbitrarily large
2507 LIMIT had been specified.
2509 A pattern matching the null string (not to be confused with
2510 a null pattern C<//>, which is just one member of the set of patterns
2511 matching a null string) will split the value of EXPR into separate
2512 characters at each point it matches that way. For example:
2514 print join(':', split(/ */, 'hi there'));
2516 produces the output 'h:i:t:h:e:r:e'.
2518 The LIMIT parameter can be used to partially split a line
2520 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2522 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2523 one larger than the number of variables in the list, to avoid
2524 unnecessary work. For the list above LIMIT would have been 4 by
2525 default. In time critical applications it behooves you not to split
2526 into more fields than you really need.
2528 If the PATTERN contains parentheses, additional array elements are
2529 created from each matching substring in the delimiter.
2531 split(/([,-])/, "1-10,20");
2533 produces the list value
2535 (1, '-', 10, ',', 20)
2537 If you had the entire header of a normal Unix email message in $header,
2538 you could split it up into fields and their values this way:
2540 $header =~ s/\n\s+/ /g; # fix continuation lines
2541 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2543 The pattern C</PATTERN/> may be replaced with an expression to specify
2544 patterns that vary at runtime. (To do runtime compilation only once,
2545 use C</$variable/o>.)
2547 As a special case, specifying a PATTERN of space (C<' '>) will split on
2548 white space just as split with no arguments does. Thus, split(' ') can
2549 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2550 will give you as many null initial fields as there are leading spaces.
2551 A split on /\s+/ is like a split(' ') except that any leading
2552 whitespace produces a null first field. A split with no arguments
2553 really does a C<split(' ', $_)> internally.
2557 open(passwd, '/etc/passwd');
2559 ($login, $passwd, $uid, $gid, $gcos,
2560 $home, $shell) = split(/:/);
2564 (Note that $shell above will still have a newline on it. See L</chop>,
2565 L</chomp>, and L</join>.)
2567 =item sprintf FORMAT,LIST
2569 Returns a string formatted by the usual printf conventions of the C
2570 language. (The * character for an indirectly specified length is not
2571 supported, but you can get the same effect by interpolating a variable
2572 into the pattern.) Some C libraries' implementations of sprintf() can dump core
2573 when fed ludiocrous arguments.
2577 Return the square root of EXPR. If EXPR is omitted, returns square
2582 Sets the random number seed for the C<rand> operator. If EXPR is
2583 omitted, does C<srand(time)>. Of course, you'd need something much more
2584 random than that for cryptographic purposes, since it's easy to guess
2585 the current time. Checksumming the compressed output of rapidly
2586 changing operating system status programs is the usual method.
2587 Examples are posted regularly to comp.security.unix.
2589 =item stat FILEHANDLE
2593 Returns a 13-element array giving the status info for a file, either the
2594 file opened via FILEHANDLE, or named by EXPR. Returns a null list if
2595 the stat fails. Typically used as follows:
2597 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2598 $atime,$mtime,$ctime,$blksize,$blocks)
2601 If stat is passed the special filehandle consisting of an underline, no
2602 stat is done, but the current contents of the stat structure from the
2603 last stat or filetest are returned. Example:
2605 if (-x $file && (($d) = stat(_)) && $d < 0) {
2606 print "$file is executable NFS file\n";
2609 (This only works on machines for which the device number is negative under NFS.)
2615 Takes extra time to study SCALAR ($_ if unspecified) in anticipation of
2616 doing many pattern matches on the string before it is next modified.
2617 This may or may not save time, depending on the nature and number of
2618 patterns you are searching on, and on the distribution of character
2619 frequencies in the string to be searched--you probably want to compare
2620 runtimes with and without it to see which runs faster. Those loops
2621 which scan for many short constant strings (including the constant
2622 parts of more complex patterns) will benefit most. You may have only
2623 one study active at a time--if you study a different scalar the first
2624 is "unstudied". (The way study works is this: a linked list of every
2625 character in the string to be searched is made, so we know, for
2626 example, where all the 'k' characters are. From each search string,
2627 the rarest character is selected, based on some static frequency tables
2628 constructed from some C programs and English text. Only those places
2629 that contain this "rarest" character are examined.)
2631 For example, here is a loop which inserts index producing entries
2632 before any line containing a certain pattern:
2636 print ".IX foo\n" if /\bfoo\b/;
2637 print ".IX bar\n" if /\bbar\b/;
2638 print ".IX blurfl\n" if /\bblurfl\b/;
2643 In searching for /\bfoo\b/, only those locations in $_ that contain "f"
2644 will be looked at, because "f" is rarer than "o". In general, this is
2645 a big win except in pathological cases. The only question is whether
2646 it saves you more time than it took to build the linked list in the
2649 Note that if you have to look for strings that you don't know till
2650 runtime, you can build an entire loop as a string and eval that to
2651 avoid recompiling all your patterns all the time. Together with
2652 undefining $/ to input entire files as one record, this can be very
2653 fast, often faster than specialized programs like fgrep(1). The following
2654 scans a list of files (@files) for a list of words (@words), and prints
2655 out the names of those files that contain a match:
2657 $search = 'while (<>) { study;';
2658 foreach $word (@words) {
2659 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
2664 eval $search; # this screams
2665 $/ = "\n"; # put back to normal input delim
2666 foreach $file (sort keys(%seen)) {
2670 =item substr EXPR,OFFSET,LEN
2672 =item substr EXPR,OFFSET
2674 Extracts a substring out of EXPR and returns it. First character is at
2675 offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
2676 that far from the end of the string. If LEN is omitted, returns
2677 everything to the end of the string. If LEN is negative, leaves that
2678 many characters off the end of the string.
2680 You can use the substr() function
2681 as an lvalue, in which case EXPR must be an lvalue. If you assign
2682 something shorter than LEN, the string will shrink, and if you assign
2683 something longer than LEN, the string will grow to accommodate it. To
2684 keep the string the same length you may need to pad or chop your value
2687 =item symlink OLDFILE,NEWFILE
2689 Creates a new filename symbolically linked to the old filename.
2690 Returns 1 for success, 0 otherwise. On systems that don't support
2691 symbolic links, produces a fatal error at run time. To check for that,
2694 $symlink_exists = (eval 'symlink("","");', $@ eq '');
2698 Calls the system call specified as the first element of the list,
2699 passing the remaining elements as arguments to the system call. If
2700 unimplemented, produces a fatal error. The arguments are interpreted
2701 as follows: if a given argument is numeric, the argument is passed as
2702 an int. If not, the pointer to the string value is passed. You are
2703 responsible to make sure a string is pre-extended long enough to
2704 receive any result that might be written into a string. If your
2705 integer arguments are not literals and have never been interpreted in a
2706 numeric context, you may need to add 0 to them to force them to look
2709 require 'syscall.ph'; # may need to run h2ph
2710 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
2712 Note that Perl only supports passing of up to 14 arguments to your system call,
2713 which in practice should usually suffice.
2715 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
2717 =item sysread FILEHANDLE,SCALAR,LENGTH
2719 Attempts to read LENGTH bytes of data into variable SCALAR from the
2720 specified FILEHANDLE, using the system call read(2). It bypasses
2721 stdio, so mixing this with other kinds of reads may cause confusion.
2722 Returns the number of bytes actually read, or undef if there was an
2723 error. SCALAR will be grown or shrunk to the length actually read. An
2724 OFFSET may be specified to place the read data at some other place than
2725 the beginning of the string.
2729 Does exactly the same thing as "exec LIST" except that a fork is done
2730 first, and the parent process waits for the child process to complete.
2731 Note that argument processing varies depending on the number of
2732 arguments. The return value is the exit status of the program as
2733 returned by the wait() call. To get the actual exit value divide by
2734 256. See also L</exec>.
2736 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
2738 =item syswrite FILEHANDLE,SCALAR,LENGTH
2740 Attempts to write LENGTH bytes of data from variable SCALAR to the
2741 specified FILEHANDLE, using the system call write(2). It bypasses
2742 stdio, so mixing this with prints may cause confusion. Returns the
2743 number of bytes actually written, or undef if there was an error. An
2744 OFFSET may be specified to place the read data at some other place than
2745 the beginning of the string.
2747 =item tell FILEHANDLE
2751 Returns the current file position for FILEHANDLE. FILEHANDLE may be an
2752 expression whose value gives the name of the actual filehandle. If
2753 FILEHANDLE is omitted, assumes the file last read.
2755 =item telldir DIRHANDLE
2757 Returns the current position of the readdir() routines on DIRHANDLE.
2758 Value may be given to seekdir() to access a particular location in a
2759 directory. Has the same caveats about possible directory compaction as
2760 the corresponding system library routine.
2762 =item tie VARIABLE,CLASSNAME,LIST
2764 This function binds a variable to a package class that will provide the
2765 implementation for the variable. VARIABLE is the name of the variable
2766 to be enchanted. CLASSNAME is the name of a class implementing objects
2767 of correct type. Any additional arguments are passed to the "new"
2768 method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
2769 Typically these are arguments such as might be passed to the dbm_open()
2770 function of C. The object returned by the "new" method +is also
2771 returned by the tie() function, which would be useful if you +want to
2772 access other methods in CLASSNAME.
2774 Note that functions such as keys() and values() may return huge array
2775 values when used on large objects, like DBM files. You may prefer to
2776 use the each() function to iterate over such. Example:
2778 # print out history file offsets
2780 tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0);
2781 while (($key,$val) = each %HIST) {
2782 print $key, ' = ', unpack('L',$val), "\n";
2786 A class implementing an associative array should have the following
2789 TIEHASH classname, LIST
2792 STORE this, key, value
2796 NEXTKEY this, lastkey
2798 A class implementing an ordinary array should have the following methods:
2800 TIEARRAY classname, LIST
2803 STORE this, key, value
2806 A class implementing a scalar should have the following methods:
2808 TIESCALAR classname, LIST
2813 Unlike dbmopen(), the tie() function will not use or require a module
2814 for you--you need to do that explicitly yourself. See L<DB_File>
2815 or the F<Config> module for interesting tie() implementations.
2819 Returns the number of non-leap seconds since 00:00:00 UTC, January 1,
2820 1970. Suitable for feeding to gmtime() and localtime().
2824 Returns a four-element array giving the user and system times, in
2825 seconds, for this process and the children of this process.
2827 ($user,$system,$cuser,$csystem) = times;
2831 The translation operator. See L<perlop>.
2833 =item truncate FILEHANDLE,LENGTH
2835 =item truncate EXPR,LENGTH
2837 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
2838 specified length. Produces a fatal error if truncate isn't implemented
2843 Returns an uppercased version of EXPR. This is the internal function
2844 implementing the \U escape in double-quoted strings.
2845 Should respect any POSIX setlocale() settings.
2849 Returns the value of EXPR with the first character uppercased. This is
2850 the internal function implementing the \u escape in double-quoted strings.
2851 Should respect any POSIX setlocale() settings.
2857 Sets the umask for the process and returns the old one. If EXPR is
2858 omitted, merely returns current umask.
2864 Undefines the value of EXPR, which must be an lvalue. Use only on a
2865 scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
2866 will probably not do what you expect on most predefined variables or
2867 DBM list values, so don't do that.) Always returns the undefined value. You can omit
2868 the EXPR, in which case nothing is undefined, but you still get an
2869 undefined value that you could, for instance, return from a
2870 subroutine. Examples:
2873 undef $bar{'blurfl'};
2877 return (wantarray ? () : undef) if $they_blew_it;
2881 Deletes a list of files. Returns the number of files successfully
2884 $cnt = unlink 'a', 'b', 'c';
2888 Note: unlink will not delete directories unless you are superuser and
2889 the B<-U> flag is supplied to Perl. Even if these conditions are
2890 met, be warned that unlinking a directory can inflict damage on your
2891 filesystem. Use rmdir instead.
2893 =item unpack TEMPLATE,EXPR
2895 Unpack does the reverse of pack: it takes a string representing a
2896 structure and expands it out into a list value, returning the array
2897 value. (In a scalar context, it merely returns the first value
2898 produced.) The TEMPLATE has the same format as in the pack function.
2899 Here's a subroutine that does substring:
2902 local($what,$where,$howmuch) = @_;
2903 unpack("x$where a$howmuch", $what);
2908 sub ordinal { unpack("c",$_[0]); } # same as ord()
2910 In addition, you may prefix a field with a %<number> to indicate that
2911 you want a <number>-bit checksum of the items instead of the items
2912 themselves. Default is a 16-bit checksum. For example, the following
2913 computes the same number as the System V sum program:
2916 $checksum += unpack("%16C*", $_);
2920 The following efficiently counts the number of set bits in a bit vector:
2922 $setbits = unpack("%32b*", $selectmask);
2924 =item untie VARIABLE
2926 Breaks the binding between a variable and a package. (See tie().)
2928 =item unshift ARRAY,LIST
2930 Does the opposite of a C<shift>. Or the opposite of a C<push>,
2931 depending on how you look at it. Prepends list to the front of the
2932 array, and returns the new number of elements in the array.
2934 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
2936 Note the LIST is prepended whole, not one element at a time, so the
2937 prepended elements stay in the same order. Use reverse to do the
2940 =item use Module LIST
2944 Imports some semantics into the current package from the named module,
2945 generally by aliasing certain subroutine or variable names into your
2946 package. It is exactly equivalent to
2948 BEGIN { require Module; import Module LIST; }
2950 If you don't want your namespace altered, use require instead.
2952 The BEGIN forces the require and import to happen at compile time. The
2953 require makes sure the module is loaded into memory if it hasn't been
2954 yet. The import is not a builtin--it's just an ordinary static method
2955 call into the "Module" package to tell the module to import the list of
2956 features back into the current package. The module can implement its
2957 import method any way it likes, though most modules just choose to
2958 derive their import method via inheritance from the Exporter class that
2959 is defined in the Exporter module.
2961 Because this is a wide-open interface, pragmas (compiler directives)
2962 are also implemented this way. Currently implemented pragmas are:
2966 use sigtrap qw(SEGV BUS);
2967 use strict qw(subs vars refs);
2968 use subs qw(afunc blurfl);
2970 These pseudomodules import semantics into the current block scope, unlike
2971 ordinary modules, which import symbols into the current package (which are
2972 effective through the end of the file).
2974 There's a corresponding "no" command that unimports meanings imported
2980 See L<perlmod> for a list of standard modules and pragmas.
2984 Changes the access and modification times on each file of a list of
2985 files. The first two elements of the list must be the NUMERICAL access
2986 and modification times, in that order. Returns the number of files
2987 successfully changed. The inode modification time of each file is set
2988 to the current time. Example of a "touch" command:
2992 utime $now, $now, @ARGV;
2994 =item values ASSOC_ARRAY
2996 Returns a normal array consisting of all the values of the named
2997 associative array. (In a scalar context, returns the number of
2998 values.) The values are returned in an apparently random order, but it
2999 is the same order as either the keys() or each() function would produce
3000 on the same array. See also keys() and each().
3002 =item vec EXPR,OFFSET,BITS
3004 Treats a string as a vector of unsigned integers, and returns the value
3005 of the bitfield specified. May also be assigned to. BITS must be a
3006 power of two from 1 to 32.
3008 Vectors created with vec() can also be manipulated with the logical
3009 operators |, & and ^, which will assume a bit vector operation is
3010 desired when both operands are strings.
3012 To transform a bit vector into a string or array of 0's and 1's, use these:
3014 $bits = unpack("b*", $vector);
3015 @bits = split(//, unpack("b*", $vector));
3017 If you know the exact length in bits, it can be used in place of the *.
3021 Waits for a child process to terminate and returns the pid of the
3022 deceased process, or -1 if there are no child processes. The status is
3025 =item waitpid PID,FLAGS
3027 Waits for a particular child process to terminate and returns the pid
3028 of the deceased process, or -1 if there is no such child process. The
3029 status is returned in $?. If you say
3033 waitpid(-1,&WNOHANG);
3035 then you can do a non-blocking wait for any process. Non-blocking wait
3036 is only available on machines supporting either the waitpid(2) or
3037 wait4(2) system calls. However, waiting for a particular pid with
3038 FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3039 by remembering the status values of processes that have exited but have
3040 not been harvested by the Perl script yet.)
3044 Returns TRUE if the context of the currently executing subroutine is
3045 looking for a list value. Returns FALSE if the context is looking
3048 return wantarray ? () : undef;
3052 Produces a message on STDERR just like die(), but doesn't exit or
3055 =item write FILEHANDLE
3061 Writes a formatted record (possibly multi-line) to the specified file,
3062 using the format associated with that file. By default the format for
3063 a file is the one having the same name is the filehandle, but the
3064 format for the current output channel (see the select() function) may be set
3065 explicitly by assigning the name of the format to the $~ variable.
3067 Top of form processing is handled automatically: if there is
3068 insufficient room on the current page for the formatted record, the
3069 page is advanced by writing a form feed, a special top-of-page format
3070 is used to format the new page header, and then the record is written.
3071 By default the top-of-page format is the name of the filehandle with
3072 "_TOP" appended, but it may be dynamically set to the format of your
3073 choice by assigning the name to the $^ variable while the filehandle is
3074 selected. The number of lines remaining on the current page is in
3075 variable $-, which can be set to 0 to force a new page.
3077 If FILEHANDLE is unspecified, output goes to the current default output
3078 channel, which starts out as STDOUT but may be changed by the
3079 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3080 is evaluated and the resulting string is used to look up the name of
3081 the FILEHANDLE at run time. For more on formats, see L<perlform>.
3083 Note that write is I<NOT> the opposite of read. Unfortunately.
3087 The translation operator. See L<perlop/tr///>.