3 perlfaq5 - Files and Formats ($Revision: 1.28 $, $Date: 2003/01/26 17:45:46 $)
7 This section deals with I/O and the "f" issues: filehandles, flushing,
10 =head2 How do I flush/unbuffer an output filehandle? Why must I do this?
12 Perl does not support truly unbuffered output (except
13 insofar as you can C<syswrite(OUT, $char, 1)>), although it
14 does support is "command buffering", in which a physical
15 write is performed after every output command.
17 The C standard I/O library (stdio) normally buffers
18 characters sent to devices so that there isn't a system call
19 for each byte. In most stdio implementations, the type of
20 output buffering and the size of the buffer varies according
21 to the type of device. Perl's print() and write() functions
22 normally buffer output, while syswrite() bypasses buffering
25 If you want your output to be sent immediately when you
26 execute print() or write() (for instance, for some network
27 protocols), you must set the handle's autoflush flag. This
28 flag is the Perl variable $| and when it is set to a true
29 value, Perl will flush the handle's buffer after each
30 print() or write(). Setting $| affects buffering only for
31 the currently selected default file handle. You choose this
32 handle with the one argument select() call (see
33 L<perlvar/$E<verbar>> and L<perlfunc/select>).
35 Use select() to choose the desired handle, then set its
36 per-filehandle variables.
38 $old_fh = select(OUTPUT_HANDLE);
42 Some idioms can handle this in a single statement:
44 select((select(OUTPUT_HANDLE), $| = 1)[0]);
46 $| = 1, select $_ for select OUTPUT_HANDLE;
48 Some modules offer object-oriented access to handles and their
49 variables, although they may be overkill if this is the only
50 thing you do with them. You can use IO::Handle:
53 open(DEV, ">/dev/printer"); # but is this?
58 use IO::Socket; # this one is kinda a pipe?
59 my $sock = IO::Socket::INET->new( 'www.example.com:80' ) ;
63 =head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
65 Use the Tie::File module, which is included in the standard
66 distribution since Perl 5.8.0.
68 =head2 How do I count the number of lines in a file?
70 One fairly efficient way is to count newlines in the file. The
71 following program uses a feature of tr///, as documented in L<perlop>.
72 If your text file doesn't end with a newline, then it's not really a
73 proper text file, so this may report one fewer line than you expect.
76 open(FILE, $filename) or die "Can't open `$filename': $!";
77 while (sysread FILE, $buffer, 4096) {
78 $lines += ($buffer =~ tr/\n//);
82 This assumes no funny games with newline translations.
84 =head2 How can I use Perl's C<-i> option from within a program?
86 C<-i> sets the value of Perl's C<$^I> variable, which in turn affects
87 the behavior of C<< <> >>; see L<perlrun> for more details. By
88 modifying the appropriate variables directly, you can get the same
89 behavior within a larger program. For example:
93 local($^I, @ARGV) = ('.orig', glob("*.c"));
96 print "This line should appear at the top of each file\n";
98 s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
100 close ARGV if eof; # Reset $.
103 # $^I and @ARGV return to their old values here
105 This block modifies all the C<.c> files in the current directory,
106 leaving a backup of the original data from each file in a new
109 =head2 How do I make a temporary file name?
111 Use the File::Temp module, see L<File::Temp> for more information.
113 use File::Temp qw/ tempfile tempdir /;
115 $dir = tempdir( CLEANUP => 1 );
116 ($fh, $filename) = tempfile( DIR => $dir );
118 # or if you don't need to know the filename
120 $fh = tempfile( DIR => $dir );
122 The File::Temp has been a standard module since Perl 5.6.1. If you
123 don't have a modern enough Perl installed, use the C<new_tmpfile>
124 class method from the IO::File module to get a filehandle opened for
125 reading and writing. Use it if you don't need to know the file's name:
128 $fh = IO::File->new_tmpfile()
129 or die "Unable to make new temporary file: $!";
131 If you're committed to creating a temporary file by hand, use the
132 process ID and/or the current time-value. If you need to have many
133 temporary files in one process, use a counter:
137 my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
138 my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
142 until (defined(fileno(FH)) || $count++ > 100) {
143 $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
144 sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
146 if (defined(fileno(FH))
147 return (*FH, $base_name);
154 =head2 How can I manipulate fixed-record-length files?
156 The most efficient way is using pack() and unpack(). This is faster than
157 using substr() when taking many, many strings. It is slower for just a few.
159 Here is a sample chunk of code to break up and put back together again
160 some fixed-format input lines, in this case from the output of a normal,
164 # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
165 $PS_T = 'A6 A4 A7 A5 A*';
169 ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
170 for $var (qw!pid tt stat time command!) {
171 print "$var: <$$var>\n";
173 print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
177 We've used C<$$var> in a way that forbidden by C<use strict 'refs'>.
178 That is, we've promoted a string to a scalar variable reference using
179 symbolic references. This is okay in small programs, but doesn't scale
180 well. It also only works on global variables, not lexicals.
182 =head2 How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles?
184 As of perl5.6, open() autovivifies file and directory handles
185 as references if you pass it an uninitialized scalar variable.
186 You can then pass these references just like any other scalar,
187 and use them in the place of named handles.
189 open my $fh, $file_name;
191 open local $fh, $file_name;
193 print $fh "Hello World!\n";
197 Before perl5.6, you had to deal with various typeglob idioms
198 which you may see in older code.
200 open FILE, "> $filename";
201 process_typeglob( *FILE );
202 process_reference( \*FILE );
204 sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
205 sub process_reference { local $fh = shift; print $fh "Reference!" }
207 If you want to create many anonymous handles, you should
208 check out the Symbol or IO::Handle modules.
210 =head2 How can I use a filehandle indirectly?
212 An indirect filehandle is using something other than a symbol
213 in a place that a filehandle is expected. Here are ways
214 to get indirect filehandles:
216 $fh = SOME_FH; # bareword is strict-subs hostile
217 $fh = "SOME_FH"; # strict-refs hostile; same package only
218 $fh = *SOME_FH; # typeglob
219 $fh = \*SOME_FH; # ref to typeglob (bless-able)
220 $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
222 Or, you can use the C<new> method from one of the IO::* modules to
223 create an anonymous filehandle, store that in a scalar variable,
224 and use it as though it were a normal filehandle.
226 use IO::Handle; # 5.004 or higher
227 $fh = IO::Handle->new();
229 Then use any of those as you would a normal filehandle. Anywhere that
230 Perl is expecting a filehandle, an indirect filehandle may be used
231 instead. An indirect filehandle is just a scalar variable that contains
232 a filehandle. Functions like C<print>, C<open>, C<seek>, or
233 the C<< <FH> >> diamond operator will accept either a named filehandle
234 or a scalar variable containing one:
236 ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
237 print $ofh "Type it: ";
239 print $efh "What was that: $got";
241 If you're passing a filehandle to a function, you can write
242 the function in two ways:
246 print $fh "Sending to indirect filehandle\n";
249 Or it can localize a typeglob and use the filehandle directly:
253 print FH "Sending to localized filehandle\n";
256 Both styles work with either objects or typeglobs of real filehandles.
257 (They might also work with strings under some circumstances, but this
263 In the examples above, we assigned the filehandle to a scalar variable
264 before using it. That is because only simple scalar variables, not
265 expressions or subscripts of hashes or arrays, can be used with
266 built-ins like C<print>, C<printf>, or the diamond operator. Using
267 something other than a simple scalar variable as a filehandle is
268 illegal and won't even compile:
270 @fd = (*STDIN, *STDOUT, *STDERR);
271 print $fd[1] "Type it: "; # WRONG
272 $got = <$fd[0]> # WRONG
273 print $fd[2] "What was that: $got"; # WRONG
275 With C<print> and C<printf>, you get around this by using a block and
276 an expression where you would place the filehandle:
278 print { $fd[1] } "funny stuff\n";
279 printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
280 # Pity the poor deadbeef.
282 That block is a proper block like any other, so you can put more
283 complicated code there. This sends the message out to one of two places:
286 print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
287 print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
289 This approach of treating C<print> and C<printf> like object methods
290 calls doesn't work for the diamond operator. That's because it's a
291 real operator, not just a function with a comma-less argument. Assuming
292 you've been storing typeglobs in your structure as we did above, you
293 can use the built-in function named C<readline> to read a record just
294 as C<< <> >> does. Given the initialization shown above for @fd, this
295 would work, but only because readline() requires a typeglob. It doesn't
296 work with objects or strings, which might be a bug we haven't fixed yet.
298 $got = readline($fd[0]);
300 Let it be noted that the flakiness of indirect filehandles is not
301 related to whether they're strings, typeglobs, objects, or anything else.
302 It's the syntax of the fundamental operators. Playing the object
303 game doesn't help you at all here.
305 =head2 How can I set up a footer format to be used with write()?
307 There's no builtin way to do this, but L<perlform> has a couple of
308 techniques to make it possible for the intrepid hacker.
310 =head2 How can I write() into a string?
312 See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
314 =head2 How can I output my numbers with commas added?
316 This subroutine will add commas to your number:
320 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
324 This regex from Benjamin Goldberg will add commas to numbers:
326 s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
328 It is easier to see with comments:
331 ^[-+]? # beginning of number.
332 \d{1,3}? # first digits before first comma
333 (?= # followed by, (but not included in the match) :
334 (?>(?:\d{3})+) # some positive multiple of three digits.
335 (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever.
338 \G\d{3} # after the last group, get three digits
339 (?=\d) # but they have to have more digits after them.
342 =head2 How can I translate tildes (~) in a filename?
344 Use the <> (glob()) operator, documented in L<perlfunc>. Older
345 versions of Perl require that you have a shell installed that groks
346 tildes. Recent perl versions have this feature built in. The
347 File::KGlob module (available from CPAN) gives more portable glob
350 Within Perl, you may use this directly:
353 ^ ~ # find a leading tilde
355 [^/] # a non-slash character
356 * # repeated 0 or more times (0 means me)
361 : ( $ENV{HOME} || $ENV{LOGDIR} )
364 =head2 How come when I open a file read-write it wipes it out?
366 Because you're using something like this, which truncates the file and
367 I<then> gives you read-write access:
369 open(FH, "+> /path/name"); # WRONG (almost always)
371 Whoops. You should instead use this, which will fail if the file
374 open(FH, "+< /path/name"); # open for update
376 Using ">" always clobbers or creates. Using "<" never does
377 either. The "+" doesn't change this.
379 Here are examples of many kinds of file opens. Those using sysopen()
384 To open file for reading:
386 open(FH, "< $path") || die $!;
387 sysopen(FH, $path, O_RDONLY) || die $!;
389 To open file for writing, create new file if needed or else truncate old file:
391 open(FH, "> $path") || die $!;
392 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
393 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
395 To open file for writing, create new file, file must not exist:
397 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
398 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
400 To open file for appending, create if necessary:
402 open(FH, ">> $path") || die $!;
403 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
404 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
406 To open file for appending, file must exist:
408 sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
410 To open file for update, file must exist:
412 open(FH, "+< $path") || die $!;
413 sysopen(FH, $path, O_RDWR) || die $!;
415 To open file for update, create file if necessary:
417 sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
418 sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
420 To open file for update, file must not exist:
422 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
423 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
425 To open a file without blocking, creating if necessary:
427 sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT)
428 or die "can't open /tmp/somefile: $!":
430 Be warned that neither creation nor deletion of files is guaranteed to
431 be an atomic operation over NFS. That is, two processes might both
432 successfully create or unlink the same file! Therefore O_EXCL
433 isn't as exclusive as you might wish.
435 See also the new L<perlopentut> if you have it (new for 5.6).
437 =head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
439 The C<< <> >> operator performs a globbing operation (see above).
440 In Perl versions earlier than v5.6.0, the internal glob() operator forks
441 csh(1) to do the actual glob expansion, but
442 csh can't handle more than 127 items and so gives the error message
443 C<Argument list too long>. People who installed tcsh as csh won't
444 have this problem, but their users may be surprised by it.
446 To get around this, either upgrade to Perl v5.6.0 or later, do the glob
447 yourself with readdir() and patterns, or use a module like File::KGlob,
448 one that doesn't use the shell to do globbing.
450 =head2 Is there a leak/bug in glob()?
452 Due to the current implementation on some operating systems, when you
453 use the glob() function or its angle-bracket alias in a scalar
454 context, you may cause a memory leak and/or unpredictable behavior. It's
455 best therefore to use glob() only in list context.
457 =head2 How can I open a file with a leading ">" or trailing blanks?
459 Normally perl ignores trailing blanks in filenames, and interprets
460 certain leading characters (or a trailing "|") to mean something
463 The three argument form of open() lets you specify the mode
464 separately from the filename. The open() function treats
465 special mode characters and whitespace in the filename as
468 open FILE, "<", " file "; # filename is " file "
469 open FILE, ">", ">file"; # filename is ">file"
471 It may be a lot clearer to use sysopen(), though:
474 $badpath = "<<<something really wicked ";
475 sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
476 or die "can't open $badpath: $!";
478 =head2 How can I reliably rename a file?
480 If your operating system supports a proper mv(1) utility or its
481 functional equivalent, this works:
483 rename($old, $new) or system("mv", $old, $new);
485 It may be more portable to use the File::Copy module instead.
486 You just copy to the new file to the new name (checking return
487 values), then delete the old one. This isn't really the same
488 semantically as a rename(), which preserves meta-information like
489 permissions, timestamps, inode info, etc.
491 Newer versions of File::Copy export a move() function.
493 =head2 How can I lock a file?
495 Perl's builtin flock() function (see L<perlfunc> for details) will call
496 flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
497 later), and lockf(3) if neither of the two previous system calls exists.
498 On some systems, it may even use a different form of native locking.
499 Here are some gotchas with Perl's flock():
505 Produces a fatal error if none of the three system calls (or their
506 close equivalent) exists.
510 lockf(3) does not provide shared locking, and requires that the
511 filehandle be open for writing (or appending, or read/writing).
515 Some versions of flock() can't lock files over a network (e.g. on NFS file
516 systems), so you'd need to force the use of fcntl(2) when you build Perl.
517 But even this is dubious at best. See the flock entry of L<perlfunc>
518 and the F<INSTALL> file in the source distribution for information on
519 building Perl to do this.
521 Two potentially non-obvious but traditional flock semantics are that
522 it waits indefinitely until the lock is granted, and that its locks are
523 I<merely advisory>. Such discretionary locks are more flexible, but
524 offer fewer guarantees. This means that files locked with flock() may
525 be modified by programs that do not also use flock(). Cars that stop
526 for red lights get on well with each other, but not with cars that don't
527 stop for red lights. See the perlport manpage, your port's specific
528 documentation, or your system-specific local manpages for details. It's
529 best to assume traditional behavior if you're writing portable programs.
530 (If you're not, you should as always feel perfectly free to write
531 for your own system's idiosyncrasies (sometimes called "features").
532 Slavish adherence to portability concerns shouldn't get in the way of
533 your getting your job done.)
535 For more information on file locking, see also
536 L<perlopentut/"File Locking"> if you have it (new for 5.6).
540 =head2 Why can't I just open(FH, "E<gt>file.lock")?
542 A common bit of code B<NOT TO USE> is this:
544 sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
545 open(LCK, "> file.lock"); # THIS BROKEN CODE
547 This is a classic race condition: you take two steps to do something
548 which must be done in one. That's why computer hardware provides an
549 atomic test-and-set instruction. In theory, this "ought" to work:
551 sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
552 or die "can't open file.lock: $!":
554 except that lamentably, file creation (and deletion) is not atomic
555 over NFS, so this won't work (at least, not every time) over the net.
556 Various schemes involving link() have been suggested, but
557 these tend to involve busy-wait, which is also subdesirable.
559 =head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
561 Didn't anyone ever tell you web-page hit counters were useless?
562 They don't count number of hits, they're a waste of time, and they serve
563 only to stroke the writer's vanity. It's better to pick a random number;
564 they're more realistic.
566 Anyway, this is what you can do if you can't help yourself.
568 use Fcntl qw(:DEFAULT :flock);
569 sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
570 flock(FH, LOCK_EX) or die "can't flock numfile: $!";
572 seek(FH, 0, 0) or die "can't rewind numfile: $!";
573 truncate(FH, 0) or die "can't truncate numfile: $!";
574 (print FH $num+1, "\n") or die "can't write numfile: $!";
575 close FH or die "can't close numfile: $!";
577 Here's a much better web-page hit counter:
579 $hits = int( (time() - 850_000_000) / rand(1_000) );
581 If the count doesn't impress your friends, then the code might. :-)
583 =head2 All I want to do is append a small amount of text to the end of a file. Do I still have to use locking?
585 If you are on a system that correctly implements flock() and you use the
586 example appending code from "perldoc -f flock" everything will be OK
587 even if the OS you are on doesn't implement append mode correctly (if
588 such a system exists.) So if you are happy to restrict yourself to OSs
589 that implement flock() (and that's not really much of a restriction)
590 then that is what you should do.
592 If you know you are only going to use a system that does correctly
593 implement appending (i.e. not Win32) then you can omit the seek() from
596 If you know you are only writing code to run on an OS and filesystem that
597 does implement append mode correctly (a local filesystem on a modern
598 Unix for example), and you keep the file in block-buffered mode and you
599 write less than one buffer-full of output between each manual flushing
600 of the buffer then each bufferload is almost guaranteed to be written to
601 the end of the file in one chunk without getting intermingled with
602 anyone else's output. You can also use the syswrite() function which is
603 simply a wrapper around your systems write(2) system call.
605 There is still a small theoretical chance that a signal will interrupt
606 the system level write() operation before completion. There is also a
607 possibility that some STDIO implementations may call multiple system
608 level write()s even if the buffer was empty to start. There may be some
609 systems where this probability is reduced to zero.
611 =head2 How do I randomly update a binary file?
613 If you're just trying to patch a binary, in many cases something as
614 simple as this works:
616 perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
618 However, if you have fixed sized records, then you might do something more
621 $RECSIZE = 220; # size of record, in bytes
622 $recno = 37; # which record to update
623 open(FH, "+<somewhere") || die "can't update somewhere: $!";
624 seek(FH, $recno * $RECSIZE, 0);
625 read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
627 seek(FH, -$RECSIZE, 1);
631 Locking and error checking are left as an exercise for the reader.
632 Don't forget them or you'll be quite sorry.
634 =head2 How do I get a file's timestamp in perl?
636 If you want to retrieve the time at which the file was last
637 read, written, or had its meta-data (owner, etc) changed,
638 you use the B<-M>, B<-A>, or B<-C> file test operations as
639 documented in L<perlfunc>. These retrieve the age of the
640 file (measured against the start-time of your program) in
641 days as a floating point number. Some platforms may not have
642 all of these times. See L<perlport> for details. To
643 retrieve the "raw" time in seconds since the epoch, you
644 would call the stat function, then use localtime(),
645 gmtime(), or POSIX::strftime() to convert this into
650 $write_secs = (stat($file))[9];
651 printf "file %s updated at %s\n", $file,
652 scalar localtime($write_secs);
654 If you prefer something more legible, use the File::stat module
655 (part of the standard distribution in version 5.004 and later):
657 # error checking left as an exercise for reader.
660 $date_string = ctime(stat($file)->mtime);
661 print "file $file updated at $date_string\n";
663 The POSIX::strftime() approach has the benefit of being,
664 in theory, independent of the current locale. See L<perllocale>
667 =head2 How do I set a file's timestamp in perl?
669 You use the utime() function documented in L<perlfunc/utime>.
670 By way of example, here's a little program that copies the
671 read and write times from its first argument to all the rest
675 die "usage: cptimes timestamp_file other_files ...\n";
678 ($atime, $mtime) = (stat($timestamp))[8,9];
679 utime $atime, $mtime, @ARGV;
681 Error checking is, as usual, left as an exercise for the reader.
683 Note that utime() currently doesn't work correctly with Win95/NT
684 ports. A bug has been reported. Check it carefully before using
685 utime() on those platforms.
687 =head2 How do I print to more than one file at once?
689 To connect one filehandle to several output filehandles,
690 you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
692 If you only have to do this once, you can print individually
695 for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
697 =head2 How can I read in an entire file all at once?
699 You can use the File::Slurp module to do it in one step.
703 $all_of_it = read_file($filename); # entire file in scalar
704 @all_lines = read_file($filename); # one line perl element
706 The customary Perl approach for processing all the lines in a file is to
707 do so one line at a time:
709 open (INPUT, $file) || die "can't open $file: $!";
712 # do something with $_
714 close(INPUT) || die "can't close $file: $!";
716 This is tremendously more efficient than reading the entire file into
717 memory as an array of lines and then processing it one element at a time,
718 which is often--if not almost always--the wrong approach. Whenever
719 you see someone do this:
723 you should think long and hard about why you need everything loaded at
724 once. It's just not a scalable solution. You might also find it more
725 fun to use the standard Tie::File module, or the DB_File module's
726 $DB_RECNO bindings, which allow you to tie an array to a file so that
727 accessing an element the array actually accesses the corresponding
730 You can read the entire filehandle contents into a scalar.
734 open (INPUT, $file) || die "can't open $file: $!";
738 That temporarily undefs your record separator, and will automatically
739 close the file at block exit. If the file is already open, just use this:
741 $var = do { local $/; <INPUT> };
743 For ordinary files you can also use the read function.
745 read( INPUT, $var, -s INPUT );
747 The third argument tests the byte size of the data on the INPUT filehandle
748 and reads that many bytes into the buffer $var.
750 =head2 How can I read in a file by paragraphs?
752 Use the C<$/> variable (see L<perlvar> for details). You can either
753 set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
754 for instance, gets treated as two paragraphs and not three), or
755 C<"\n\n"> to accept empty paragraphs.
757 Note that a blank line must have no blanks in it. Thus
758 S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
760 =head2 How can I read a single character from a file? From the keyboard?
762 You can use the builtin C<getc()> function for most filehandles, but
763 it won't (easily) work on a terminal device. For STDIN, either use
764 the Term::ReadKey module from CPAN or use the sample code in
767 If your system supports the portable operating system programming
768 interface (POSIX), you can use the following code, which you'll note
769 turns off echo processing as well.
783 use POSIX qw(:termios_h);
785 my ($term, $oterm, $echo, $noecho, $fd_stdin);
787 $fd_stdin = fileno(STDIN);
789 $term = POSIX::Termios->new();
790 $term->getattr($fd_stdin);
791 $oterm = $term->getlflag();
793 $echo = ECHO | ECHOK | ICANON;
794 $noecho = $oterm & ~$echo;
797 $term->setlflag($noecho);
798 $term->setcc(VTIME, 1);
799 $term->setattr($fd_stdin, TCSANOW);
803 $term->setlflag($oterm);
804 $term->setcc(VTIME, 0);
805 $term->setattr($fd_stdin, TCSANOW);
811 sysread(STDIN, $key, 1);
820 The Term::ReadKey module from CPAN may be easier to use. Recent versions
821 include also support for non-portable systems as well.
824 open(TTY, "</dev/tty");
825 print "Gimme a char: ";
827 $key = ReadKey 0, *TTY;
829 printf "\nYou said %s, char number %03d\n",
832 =head2 How can I tell whether there's a character waiting on a filehandle?
834 The very first thing you should do is look into getting the Term::ReadKey
835 extension from CPAN. As we mentioned earlier, it now even has limited
836 support for non-portable (read: not open systems, closed, proprietary,
837 not POSIX, not Unix, etc) systems.
839 You should also check out the Frequently Asked Questions list in
840 comp.unix.* for things like this: the answer is essentially the same.
841 It's very system dependent. Here's one solution that works on BSD
846 vec($rin, fileno(STDIN), 1) = 1;
847 return $nfd = select($rin,undef,undef,0);
850 If you want to find out how many characters are waiting, there's
851 also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
852 comes with Perl tries to convert C include files to Perl code, which
853 can be C<require>d. FIONREAD ends up defined as a function in the
854 I<sys/ioctl.ph> file:
856 require 'sys/ioctl.ph';
858 $size = pack("L", 0);
859 ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
860 $size = unpack("L", $size);
862 If I<h2ph> wasn't installed or doesn't work for you, you can
863 I<grep> the include files by hand:
865 % grep FIONREAD /usr/include/*/*
866 /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
868 Or write a small C program using the editor of champions:
871 #include <sys/ioctl.h>
873 printf("%#08x\n", FIONREAD);
876 % cc -o fionread fionread.c
880 And then hard code it, leaving porting as an exercise to your successor.
882 $FIONREAD = 0x4004667f; # XXX: opsys dependent
884 $size = pack("L", 0);
885 ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
886 $size = unpack("L", $size);
888 FIONREAD requires a filehandle connected to a stream, meaning that sockets,
889 pipes, and tty devices work, but I<not> files.
891 =head2 How do I do a C<tail -f> in perl?
897 The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
898 but it does clear the end-of-file condition on the handle, so that the
899 next <GWFILE> makes Perl try again to read something.
901 If that doesn't work (it relies on features of your stdio implementation),
902 then you need something more like this:
905 for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
906 # search for some stuff and put it into files
909 seek(GWFILE, $curpos, 0); # seek to where we had been
912 If this still doesn't work, look into the POSIX module. POSIX defines
913 the clearerr() method, which can remove the end of file condition on a
914 filehandle. The method: read until end of file, clearerr(), read some
915 more. Lather, rinse, repeat.
917 There's also a File::Tail module from CPAN.
919 =head2 How do I dup() a filehandle in Perl?
921 If you check L<perlfunc/open>, you'll see that several of the ways
922 to call open() should do the trick. For example:
924 open(LOG, ">>/tmp/logfile");
925 open(STDERR, ">&LOG");
927 Or even with a literal numeric descriptor:
929 $fd = $ENV{MHCONTEXTFD};
930 open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
932 Note that "<&STDIN" makes a copy, but "<&=STDIN" make
933 an alias. That means if you close an aliased handle, all
934 aliases become inaccessible. This is not true with
937 Error checking, as always, has been left as an exercise for the reader.
939 =head2 How do I close a file descriptor by number?
941 This should rarely be necessary, as the Perl close() function is to be
942 used for things that Perl opened itself, even if it was a dup of a
943 numeric descriptor as with MHCONTEXT above. But if you really have
944 to, you may be able to do this:
946 require 'sys/syscall.ph';
947 $rc = syscall(&SYS_close, $fd + 0); # must force numeric
948 die "can't sysclose $fd: $!" unless $rc == -1;
950 Or, just use the fdopen(3S) feature of open():
954 open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
958 =head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
960 Whoops! You just put a tab and a formfeed into that filename!
961 Remember that within double quoted strings ("like\this"), the
962 backslash is an escape character. The full list of these is in
963 L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
964 have a file called "c:(tab)emp(formfeed)oo" or
965 "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
967 Either single-quote your strings, or (preferably) use forward slashes.
968 Since all DOS and Windows versions since something like MS-DOS 2.0 or so
969 have treated C</> and C<\> the same in a path, you might as well use the
970 one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++,
971 awk, Tcl, Java, or Python, just to mention a few. POSIX paths
972 are more portable, too.
974 =head2 Why doesn't glob("*.*") get all the files?
976 Because even on non-Unix ports, Perl's glob function follows standard
977 Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
978 files. This makes glob() portable even to legacy systems. Your
979 port may include proprietary globbing functions as well. Check its
980 documentation for details.
982 =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
984 This is elaborately and painstakingly described in the
985 F<file-dir-perms> article in the "Far More Than You Ever Wanted To
986 Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
988 The executive summary: learn how your filesystem works. The
989 permissions on a file say what can happen to the data in that file.
990 The permissions on a directory say what can happen to the list of
991 files in that directory. If you delete a file, you're removing its
992 name from the directory (so the operation depends on the permissions
993 of the directory, not of the file). If you try to write to the file,
994 the permissions of the file govern whether you're allowed to.
996 =head2 How do I select a random line from a file?
998 Here's an algorithm from the Camel Book:
1001 rand($.) < 1 && ($line = $_) while <>;
1003 This has a significant advantage in space over reading the whole file
1004 in. You can find a proof of this method in I<The Art of Computer
1005 Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth.
1007 You can use the File::Random module which provides a function
1010 use File::Random qw/random_line/;
1011 my $line = random_line($filename);
1013 Another way is to use the Tie::File module, which treats the entire
1014 file as an array. Simply access a random array element.
1016 =head2 Why do I get weird spaces when I print an array of lines?
1022 joins together the elements of C<@lines> with a space between them.
1023 If C<@lines> were C<("little", "fluffy", "clouds")> then the above
1024 statement would print
1026 little fluffy clouds
1028 but if each element of C<@lines> was a line of text, ending a newline
1029 character C<("little\n", "fluffy\n", "clouds\n")> then it would print:
1035 If your array contains lines, just print them:
1039 =head1 AUTHOR AND COPYRIGHT
1041 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
1042 All rights reserved.
1044 This documentation is free; you can redistribute it and/or modify it
1045 under the same terms as Perl itself.
1047 Irrespective of its distribution, all code examples here are in the public
1048 domain. You are permitted and encouraged to use this code and any
1049 derivatives thereof in your own programs for fun or for profit as you
1050 see fit. A simple comment in the code giving credit to the FAQ would
1051 be courteous but is not required.