3 perlfaq5 - Files and Formats ($Revision: 1.17 $, $Date: 2002/05/23 19:33:50 $)
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 $| affects buffering only for
31 the currently selected default file handle. You choose this
32 handle with the one argument select() call (see
33 L<perlvar/$|> 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 do I make a temporary file name?
86 Use the File::Temp module, see L<File::Temp> for more information.
88 use File::Temp qw/ tempfile tempdir /;
90 $dir = tempdir( CLEANUP => 1 );
91 ($fh, $filename) = tempfile( DIR => $dir );
93 # or if you don't need to know the filename
95 $fh = tempfile( DIR => $dir );
97 The File::Temp has been a standard module since Perl 5.6.1. If you
98 don't have a modern enough Perl installed, use the C<new_tmpfile>
99 class method from the IO::File module to get a filehandle opened for
100 reading and writing. Use it if you don't need to know the file's name:
103 $fh = IO::File->new_tmpfile()
104 or die "Unable to make new temporary file: $!";
106 If you're committed to creating a temporary file by hand, use the
107 process ID and/or the current time-value. If you need to have many
108 temporary files in one process, use a counter:
112 my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
113 my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
117 until (defined(fileno(FH)) || $count++ > 100) {
118 $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
119 sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
121 if (defined(fileno(FH))
122 return (*FH, $base_name);
129 =head2 How can I manipulate fixed-record-length files?
131 The most efficient way is using pack() and unpack(). This is faster than
132 using substr() when taking many, many strings. It is slower for just a few.
134 Here is a sample chunk of code to break up and put back together again
135 some fixed-format input lines, in this case from the output of a normal,
139 # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
140 $PS_T = 'A6 A4 A7 A5 A*';
144 ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
145 for $var (qw!pid tt stat time command!) {
146 print "$var: <$$var>\n";
148 print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
152 We've used C<$$var> in a way that forbidden by C<use strict 'refs'>.
153 That is, we've promoted a string to a scalar variable reference using
154 symbolic references. This is okay in small programs, but doesn't scale
155 well. It also only works on global variables, not lexicals.
157 =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?
159 As of perl5.6, open() autovivifies file and directory handles
160 as references if you pass it an uninitialized scalar variable.
161 You can then pass these references just like any other scalar,
162 and use them in the place of named handles.
164 open my $fh, $file_name;
166 open local $fh, $file_name;
168 print $fh "Hello World!\n";
172 Before perl5.6, you had to deal with various typeglob idioms
173 which you may see in older code.
175 open FILE, "> $filename";
176 process_typeglob( *FILE );
177 process_reference( \*FILE );
179 sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
180 sub process_reference { local $fh = shift; print $fh "Reference!" }
182 If you want to create many anonymous handles, you should
183 check out the Symbol or IO::Handle modules.
185 =head2 How can I use a filehandle indirectly?
187 An indirect filehandle is using something other than a symbol
188 in a place that a filehandle is expected. Here are ways
189 to get indirect filehandles:
191 $fh = SOME_FH; # bareword is strict-subs hostile
192 $fh = "SOME_FH"; # strict-refs hostile; same package only
193 $fh = *SOME_FH; # typeglob
194 $fh = \*SOME_FH; # ref to typeglob (bless-able)
195 $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
197 Or, you can use the C<new> method from one of the IO::* modules to
198 create an anonymous filehandle, store that in a scalar variable,
199 and use it as though it were a normal filehandle.
201 use IO::Handle; # 5.004 or higher
202 $fh = IO::Handle->new();
204 Then use any of those as you would a normal filehandle. Anywhere that
205 Perl is expecting a filehandle, an indirect filehandle may be used
206 instead. An indirect filehandle is just a scalar variable that contains
207 a filehandle. Functions like C<print>, C<open>, C<seek>, or
208 the C<< <FH> >> diamond operator will accept either a named filehandle
209 or a scalar variable containing one:
211 ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
212 print $ofh "Type it: ";
214 print $efh "What was that: $got";
216 If you're passing a filehandle to a function, you can write
217 the function in two ways:
221 print $fh "Sending to indirect filehandle\n";
224 Or it can localize a typeglob and use the filehandle directly:
228 print FH "Sending to localized filehandle\n";
231 Both styles work with either objects or typeglobs of real filehandles.
232 (They might also work with strings under some circumstances, but this
238 In the examples above, we assigned the filehandle to a scalar variable
239 before using it. That is because only simple scalar variables, not
240 expressions or subscripts of hashes or arrays, can be used with
241 built-ins like C<print>, C<printf>, or the diamond operator. Using
242 something other than a simple scalar variable as a filehandle is
243 illegal and won't even compile:
245 @fd = (*STDIN, *STDOUT, *STDERR);
246 print $fd[1] "Type it: "; # WRONG
247 $got = <$fd[0]> # WRONG
248 print $fd[2] "What was that: $got"; # WRONG
250 With C<print> and C<printf>, you get around this by using a block and
251 an expression where you would place the filehandle:
253 print { $fd[1] } "funny stuff\n";
254 printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
255 # Pity the poor deadbeef.
257 That block is a proper block like any other, so you can put more
258 complicated code there. This sends the message out to one of two places:
261 print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
262 print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
264 This approach of treating C<print> and C<printf> like object methods
265 calls doesn't work for the diamond operator. That's because it's a
266 real operator, not just a function with a comma-less argument. Assuming
267 you've been storing typeglobs in your structure as we did above, you
268 can use the built-in function named C<readline> to read a record just
269 as C<< <> >> does. Given the initialization shown above for @fd, this
270 would work, but only because readline() requires a typeglob. It doesn't
271 work with objects or strings, which might be a bug we haven't fixed yet.
273 $got = readline($fd[0]);
275 Let it be noted that the flakiness of indirect filehandles is not
276 related to whether they're strings, typeglobs, objects, or anything else.
277 It's the syntax of the fundamental operators. Playing the object
278 game doesn't help you at all here.
280 =head2 How can I set up a footer format to be used with write()?
282 There's no builtin way to do this, but L<perlform> has a couple of
283 techniques to make it possible for the intrepid hacker.
285 =head2 How can I write() into a string?
287 See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
289 =head2 How can I output my numbers with commas added?
291 This one from Benjamin Goldberg will do it for you:
293 s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
295 or written verbosely:
298 ^[-+]? # beginning of number.
299 \d{1,3}? # first digits before first comma
300 (?= # followed by, (but not included in the match) :
301 (?>(?:\d{3})+) # some positive multiple of three digits.
302 (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever.
305 \G\d{3} # after the last group, get three digits
306 (?=\d) # but they have to have more digits after them.
309 =head2 How can I translate tildes (~) in a filename?
311 Use the <> (glob()) operator, documented in L<perlfunc>. Older
312 versions of Perl require that you have a shell installed that groks
313 tildes. Recent perl versions have this feature built in. The
314 File::KGlob module (available from CPAN) gives more portable glob
317 Within Perl, you may use this directly:
320 ^ ~ # find a leading tilde
322 [^/] # a non-slash character
323 * # repeated 0 or more times (0 means me)
328 : ( $ENV{HOME} || $ENV{LOGDIR} )
331 =head2 How come when I open a file read-write it wipes it out?
333 Because you're using something like this, which truncates the file and
334 I<then> gives you read-write access:
336 open(FH, "+> /path/name"); # WRONG (almost always)
338 Whoops. You should instead use this, which will fail if the file
341 open(FH, "+< /path/name"); # open for update
343 Using ">" always clobbers or creates. Using "<" never does
344 either. The "+" doesn't change this.
346 Here are examples of many kinds of file opens. Those using sysopen()
351 To open file for reading:
353 open(FH, "< $path") || die $!;
354 sysopen(FH, $path, O_RDONLY) || die $!;
356 To open file for writing, create new file if needed or else truncate old file:
358 open(FH, "> $path") || die $!;
359 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
360 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
362 To open file for writing, create new file, file must not exist:
364 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
365 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
367 To open file for appending, create if necessary:
369 open(FH, ">> $path") || die $!;
370 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
371 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
373 To open file for appending, file must exist:
375 sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
377 To open file for update, file must exist:
379 open(FH, "+< $path") || die $!;
380 sysopen(FH, $path, O_RDWR) || die $!;
382 To open file for update, create file if necessary:
384 sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
385 sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
387 To open file for update, file must not exist:
389 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
390 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
392 To open a file without blocking, creating if necessary:
394 sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT)
395 or die "can't open /tmp/somefile: $!":
397 Be warned that neither creation nor deletion of files is guaranteed to
398 be an atomic operation over NFS. That is, two processes might both
399 successfully create or unlink the same file! Therefore O_EXCL
400 isn't as exclusive as you might wish.
402 See also the new L<perlopentut> if you have it (new for 5.6).
404 =head2 Why do I sometimes get an "Argument list too long" when I use <*>?
406 The C<< <> >> operator performs a globbing operation (see above).
407 In Perl versions earlier than v5.6.0, the internal glob() operator forks
408 csh(1) to do the actual glob expansion, but
409 csh can't handle more than 127 items and so gives the error message
410 C<Argument list too long>. People who installed tcsh as csh won't
411 have this problem, but their users may be surprised by it.
413 To get around this, either upgrade to Perl v5.6.0 or later, do the glob
414 yourself with readdir() and patterns, or use a module like File::KGlob,
415 one that doesn't use the shell to do globbing.
417 =head2 Is there a leak/bug in glob()?
419 Due to the current implementation on some operating systems, when you
420 use the glob() function or its angle-bracket alias in a scalar
421 context, you may cause a memory leak and/or unpredictable behavior. It's
422 best therefore to use glob() only in list context.
424 =head2 How can I open a file with a leading ">" or trailing blanks?
426 Normally perl ignores trailing blanks in filenames, and interprets
427 certain leading characters (or a trailing "|") to mean something
430 The three argument form of open() lets you specify the mode
431 separately from the filename. The open() function treats
432 special mode characters and whitespace in the filename as
435 open FILE, "<", " file "; # filename is " file "
436 open FILE, ">", ">file"; # filename is ">file"
438 It may be a lot clearer to use sysopen(), though:
441 $badpath = "<<<something really wicked ";
442 sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
443 or die "can't open $badpath: $!";
445 =head2 How can I reliably rename a file?
447 If your operating system supports a proper mv(1) utility or its functional
448 equivalent, this works:
450 rename($old, $new) or system("mv", $old, $new);
452 It may be more portable to use the File::Copy module instead.
453 You just copy to the new file to the new name (checking return
454 values), then delete the old one. This isn't really the same
455 semantically as a rename(), which preserves meta-information like
456 permissions, timestamps, inode info, etc.
458 Newer versions of File::Copy export a move() function.
460 =head2 How can I lock a file?
462 Perl's builtin flock() function (see L<perlfunc> for details) will call
463 flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
464 later), and lockf(3) if neither of the two previous system calls exists.
465 On some systems, it may even use a different form of native locking.
466 Here are some gotchas with Perl's flock():
472 Produces a fatal error if none of the three system calls (or their
473 close equivalent) exists.
477 lockf(3) does not provide shared locking, and requires that the
478 filehandle be open for writing (or appending, or read/writing).
482 Some versions of flock() can't lock files over a network (e.g. on NFS file
483 systems), so you'd need to force the use of fcntl(2) when you build Perl.
484 But even this is dubious at best. See the flock entry of L<perlfunc>
485 and the F<INSTALL> file in the source distribution for information on
486 building Perl to do this.
488 Two potentially non-obvious but traditional flock semantics are that
489 it waits indefinitely until the lock is granted, and that its locks are
490 I<merely advisory>. Such discretionary locks are more flexible, but
491 offer fewer guarantees. This means that files locked with flock() may
492 be modified by programs that do not also use flock(). Cars that stop
493 for red lights get on well with each other, but not with cars that don't
494 stop for red lights. See the perlport manpage, your port's specific
495 documentation, or your system-specific local manpages for details. It's
496 best to assume traditional behavior if you're writing portable programs.
497 (If you're not, you should as always feel perfectly free to write
498 for your own system's idiosyncrasies (sometimes called "features").
499 Slavish adherence to portability concerns shouldn't get in the way of
500 your getting your job done.)
502 For more information on file locking, see also
503 L<perlopentut/"File Locking"> if you have it (new for 5.6).
507 =head2 Why can't I just open(FH, ">file.lock")?
509 A common bit of code B<NOT TO USE> is this:
511 sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
512 open(LCK, "> file.lock"); # THIS BROKEN CODE
514 This is a classic race condition: you take two steps to do something
515 which must be done in one. That's why computer hardware provides an
516 atomic test-and-set instruction. In theory, this "ought" to work:
518 sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
519 or die "can't open file.lock: $!":
521 except that lamentably, file creation (and deletion) is not atomic
522 over NFS, so this won't work (at least, not every time) over the net.
523 Various schemes involving link() have been suggested, but
524 these tend to involve busy-wait, which is also subdesirable.
526 =head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
528 Didn't anyone ever tell you web-page hit counters were useless?
529 They don't count number of hits, they're a waste of time, and they serve
530 only to stroke the writer's vanity. It's better to pick a random number;
531 they're more realistic.
533 Anyway, this is what you can do if you can't help yourself.
535 use Fcntl qw(:DEFAULT :flock);
536 sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
537 flock(FH, LOCK_EX) or die "can't flock numfile: $!";
539 seek(FH, 0, 0) or die "can't rewind numfile: $!";
540 truncate(FH, 0) or die "can't truncate numfile: $!";
541 (print FH $num+1, "\n") or die "can't write numfile: $!";
542 close FH or die "can't close numfile: $!";
544 Here's a much better web-page hit counter:
546 $hits = int( (time() - 850_000_000) / rand(1_000) );
548 If the count doesn't impress your friends, then the code might. :-)
550 =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?
552 If you are on a system that correctly implements flock() and you use the
553 example appending code from "perldoc -f flock" everything will be OK
554 even if the OS you are on doesn't implement append mode correctly (if
555 such a system exists.) So if you are happy to restrict yourself to OSs
556 that implement flock() (and that's not really much of a restriction)
557 then that is what you should do.
559 If you know you are only going to use a system that does correctly
560 implement appending (i.e. not Win32) then you can omit the seek() from
563 If you know you are only writing code to run on an OS and filesystem that
564 does implement append mode correctly (a local filesystem on a modern
565 Unix for example), and you keep the file in block-buffered mode and you
566 write less than one buffer-full of output between each manual flushing
567 of the buffer then each bufferload is almost guaranteed to be written to
568 the end of the file in one chunk without getting intermingled with
569 anyone else's output. You can also use the syswrite() function which is
570 simply a wrapper around your systems write(2) system call.
572 There is still a small theoretical chance that a signal will interrupt
573 the system level write() operation before completion. There is also a
574 possibility that some STDIO implementations may call multiple system
575 level write()s even if the buffer was empty to start. There may be some
576 systems where this probability is reduced to zero.
578 =head2 How do I randomly update a binary file?
580 If you're just trying to patch a binary, in many cases something as
581 simple as this works:
583 perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
585 However, if you have fixed sized records, then you might do something more
588 $RECSIZE = 220; # size of record, in bytes
589 $recno = 37; # which record to update
590 open(FH, "+<somewhere") || die "can't update somewhere: $!";
591 seek(FH, $recno * $RECSIZE, 0);
592 read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
594 seek(FH, -$RECSIZE, 1);
598 Locking and error checking are left as an exercise for the reader.
599 Don't forget them or you'll be quite sorry.
601 =head2 How do I get a file's timestamp in perl?
603 If you want to retrieve the time at which the file was last
604 read, written, or had its meta-data (owner, etc) changed,
605 you use the B<-M>, B<-A>, or B<-C> file test operations as
606 documented in L<perlfunc>. These retrieve the age of the
607 file (measured against the start-time of your program) in
608 days as a floating point number. Some platforms may not have
609 all of these times. See L<perlport> for details. To
610 retrieve the "raw" time in seconds since the epoch, you
611 would call the stat function, then use localtime(),
612 gmtime(), or POSIX::strftime() to convert this into
617 $write_secs = (stat($file))[9];
618 printf "file %s updated at %s\n", $file,
619 scalar localtime($write_secs);
621 If you prefer something more legible, use the File::stat module
622 (part of the standard distribution in version 5.004 and later):
624 # error checking left as an exercise for reader.
627 $date_string = ctime(stat($file)->mtime);
628 print "file $file updated at $date_string\n";
630 The POSIX::strftime() approach has the benefit of being,
631 in theory, independent of the current locale. See L<perllocale>
634 =head2 How do I set a file's timestamp in perl?
636 You use the utime() function documented in L<perlfunc/utime>.
637 By way of example, here's a little program that copies the
638 read and write times from its first argument to all the rest
642 die "usage: cptimes timestamp_file other_files ...\n";
645 ($atime, $mtime) = (stat($timestamp))[8,9];
646 utime $atime, $mtime, @ARGV;
648 Error checking is, as usual, left as an exercise for the reader.
650 Note that utime() currently doesn't work correctly with Win95/NT
651 ports. A bug has been reported. Check it carefully before using
652 utime() on those platforms.
654 =head2 How do I print to more than one file at once?
656 If you only have to do this once, you can do this:
658 for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
660 To connect up to one filehandle to several output filehandles, it's
661 easiest to use the tee(1) program if you have it, and let it take care
664 open (FH, "| tee file1 file2 file3");
668 # make STDOUT go to three files, plus original STDOUT
669 open (STDOUT, "| tee file1 file2 file3") or die "Teeing off: $!\n";
670 print "whatever\n" or die "Writing: $!\n";
671 close(STDOUT) or die "Closing: $!\n";
673 Otherwise you'll have to write your own multiplexing print
674 function--or your own tee program--or use Tom Christiansen's,
675 at http://www.cpan.org/authors/id/TOMC/scripts/tct.gz , which is
676 written in Perl and offers much greater functionality
677 than the stock version.
679 =head2 How can I read in an entire file all at once?
681 The customary Perl approach for processing all the lines in a file is to
682 do so one line at a time:
684 open (INPUT, $file) || die "can't open $file: $!";
687 # do something with $_
689 close(INPUT) || die "can't close $file: $!";
691 This is tremendously more efficient than reading the entire file into
692 memory as an array of lines and then processing it one element at a time,
693 which is often--if not almost always--the wrong approach. Whenever
694 you see someone do this:
698 you should think long and hard about why you need everything loaded at
699 once. It's just not a scalable solution. You might also find it more
700 fun to use the standard Tie::File module, or the DB_File module's
701 $DB_RECNO bindings, which allow you to tie an array to a file so that
702 accessing an element the array actually accesses the corresponding
705 You can read the entire filehandle contents into a scalar.
709 open (INPUT, $file) || die "can't open $file: $!";
713 That temporarily undefs your record separator, and will automatically
714 close the file at block exit. If the file is already open, just use this:
716 $var = do { local $/; <INPUT> };
718 For ordinary files you can also use the read function.
720 read( INPUT, $var, -s INPUT );
722 The third argument tests the byte size of the data on the INPUT filehandle
723 and reads that many bytes into the buffer $var.
725 =head2 How can I read in a file by paragraphs?
727 Use the C<$/> variable (see L<perlvar> for details). You can either
728 set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
729 for instance, gets treated as two paragraphs and not three), or
730 C<"\n\n"> to accept empty paragraphs.
732 Note that a blank line must have no blanks in it. Thus
733 S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
735 =head2 How can I read a single character from a file? From the keyboard?
737 You can use the builtin C<getc()> function for most filehandles, but
738 it won't (easily) work on a terminal device. For STDIN, either use
739 the Term::ReadKey module from CPAN or use the sample code in
742 If your system supports the portable operating system programming
743 interface (POSIX), you can use the following code, which you'll note
744 turns off echo processing as well.
758 use POSIX qw(:termios_h);
760 my ($term, $oterm, $echo, $noecho, $fd_stdin);
762 $fd_stdin = fileno(STDIN);
764 $term = POSIX::Termios->new();
765 $term->getattr($fd_stdin);
766 $oterm = $term->getlflag();
768 $echo = ECHO | ECHOK | ICANON;
769 $noecho = $oterm & ~$echo;
772 $term->setlflag($noecho);
773 $term->setcc(VTIME, 1);
774 $term->setattr($fd_stdin, TCSANOW);
778 $term->setlflag($oterm);
779 $term->setcc(VTIME, 0);
780 $term->setattr($fd_stdin, TCSANOW);
786 sysread(STDIN, $key, 1);
795 The Term::ReadKey module from CPAN may be easier to use. Recent versions
796 include also support for non-portable systems as well.
799 open(TTY, "</dev/tty");
800 print "Gimme a char: ";
802 $key = ReadKey 0, *TTY;
804 printf "\nYou said %s, char number %03d\n",
807 =head2 How can I tell whether there's a character waiting on a filehandle?
809 The very first thing you should do is look into getting the Term::ReadKey
810 extension from CPAN. As we mentioned earlier, it now even has limited
811 support for non-portable (read: not open systems, closed, proprietary,
812 not POSIX, not Unix, etc) systems.
814 You should also check out the Frequently Asked Questions list in
815 comp.unix.* for things like this: the answer is essentially the same.
816 It's very system dependent. Here's one solution that works on BSD
821 vec($rin, fileno(STDIN), 1) = 1;
822 return $nfd = select($rin,undef,undef,0);
825 If you want to find out how many characters are waiting, there's
826 also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
827 comes with Perl tries to convert C include files to Perl code, which
828 can be C<require>d. FIONREAD ends up defined as a function in the
829 I<sys/ioctl.ph> file:
831 require 'sys/ioctl.ph';
833 $size = pack("L", 0);
834 ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
835 $size = unpack("L", $size);
837 If I<h2ph> wasn't installed or doesn't work for you, you can
838 I<grep> the include files by hand:
840 % grep FIONREAD /usr/include/*/*
841 /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
843 Or write a small C program using the editor of champions:
846 #include <sys/ioctl.h>
848 printf("%#08x\n", FIONREAD);
851 % cc -o fionread fionread.c
855 And then hard code it, leaving porting as an exercise to your successor.
857 $FIONREAD = 0x4004667f; # XXX: opsys dependent
859 $size = pack("L", 0);
860 ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
861 $size = unpack("L", $size);
863 FIONREAD requires a filehandle connected to a stream, meaning that sockets,
864 pipes, and tty devices work, but I<not> files.
866 =head2 How do I do a C<tail -f> in perl?
872 The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
873 but it does clear the end-of-file condition on the handle, so that the
874 next <GWFILE> makes Perl try again to read something.
876 If that doesn't work (it relies on features of your stdio implementation),
877 then you need something more like this:
880 for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
881 # search for some stuff and put it into files
884 seek(GWFILE, $curpos, 0); # seek to where we had been
887 If this still doesn't work, look into the POSIX module. POSIX defines
888 the clearerr() method, which can remove the end of file condition on a
889 filehandle. The method: read until end of file, clearerr(), read some
890 more. Lather, rinse, repeat.
892 There's also a File::Tail module from CPAN.
894 =head2 How do I dup() a filehandle in Perl?
896 If you check L<perlfunc/open>, you'll see that several of the ways
897 to call open() should do the trick. For example:
899 open(LOG, ">>/tmp/logfile");
900 open(STDERR, ">&LOG");
902 Or even with a literal numeric descriptor:
904 $fd = $ENV{MHCONTEXTFD};
905 open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
907 Note that "<&STDIN" makes a copy, but "<&=STDIN" make
908 an alias. That means if you close an aliased handle, all
909 aliases become inaccessible. This is not true with
912 Error checking, as always, has been left as an exercise for the reader.
914 =head2 How do I close a file descriptor by number?
916 This should rarely be necessary, as the Perl close() function is to be
917 used for things that Perl opened itself, even if it was a dup of a
918 numeric descriptor as with MHCONTEXT above. But if you really have
919 to, you may be able to do this:
921 require 'sys/syscall.ph';
922 $rc = syscall(&SYS_close, $fd + 0); # must force numeric
923 die "can't sysclose $fd: $!" unless $rc == -1;
925 Or, just use the fdopen(3S) feature of open():
929 open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
933 =head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
935 Whoops! You just put a tab and a formfeed into that filename!
936 Remember that within double quoted strings ("like\this"), the
937 backslash is an escape character. The full list of these is in
938 L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
939 have a file called "c:(tab)emp(formfeed)oo" or
940 "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
942 Either single-quote your strings, or (preferably) use forward slashes.
943 Since all DOS and Windows versions since something like MS-DOS 2.0 or so
944 have treated C</> and C<\> the same in a path, you might as well use the
945 one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++,
946 awk, Tcl, Java, or Python, just to mention a few. POSIX paths
947 are more portable, too.
949 =head2 Why doesn't glob("*.*") get all the files?
951 Because even on non-Unix ports, Perl's glob function follows standard
952 Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
953 files. This makes glob() portable even to legacy systems. Your
954 port may include proprietary globbing functions as well. Check its
955 documentation for details.
957 =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
959 This is elaborately and painstakingly described in the
960 F<file-dir-perms> article in the "Far More Than You Ever Wanted To
961 Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz .
963 The executive summary: learn how your filesystem works. The
964 permissions on a file say what can happen to the data in that file.
965 The permissions on a directory say what can happen to the list of
966 files in that directory. If you delete a file, you're removing its
967 name from the directory (so the operation depends on the permissions
968 of the directory, not of the file). If you try to write to the file,
969 the permissions of the file govern whether you're allowed to.
971 =head2 How do I select a random line from a file?
973 Here's an algorithm from the Camel Book:
976 rand($.) < 1 && ($line = $_) while <>;
978 This has a significant advantage in space over reading the whole
979 file in. A simple proof by induction is available upon
980 request if you doubt the algorithm's correctness.
982 =head2 Why do I get weird spaces when I print an array of lines?
988 joins together the elements of C<@lines> with a space between them.
989 If C<@lines> were C<("little", "fluffy", "clouds")> then the above
990 statement would print
994 but if each element of C<@lines> was a line of text, ending a newline
995 character C<("little\n", "fluffy\n", "clouds\n")> then it would print:
1001 If your array contains lines, just print them:
1005 =head1 AUTHOR AND COPYRIGHT
1007 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
1008 All rights reserved.
1010 This documentation is free; you can redistribute it and/or modify it
1011 under the same terms as Perl itself.
1013 Irrespective of its distribution, all code examples here are in the public
1014 domain. You are permitted and encouraged to use this code and any
1015 derivatives thereof in your own programs for fun or for profit as you
1016 see fit. A simple comment in the code giving credit to the FAQ would
1017 be courteous but is not required.