3 perlfaq5 - Files and Formats ($Revision: 1.31 $, $Date: 2004/02/07 04:29:50 $)
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 $| affects buffering only for
31 the currently selected default file handle. You choose this
32 handle with the one argument select() call (see
33 L<perlvar/$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 # O_EXCL is required for security reasons.
145 sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
147 if (defined(fileno(FH))
148 return (*FH, $base_name);
155 =head2 How can I manipulate fixed-record-length files?
157 The most efficient way is using L<pack()|perlfunc/"pack"> and
158 L<unpack()|perlfunc/"unpack">. This is faster than using
159 L<substr()|perlfunc/"substr"> when taking many, many strings. It is
160 slower for just a few.
162 Here is a sample chunk of code to break up and put back together again
163 some fixed-format input lines, in this case from the output of a normal,
167 # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
168 my $PS_T = 'A6 A4 A7 A5 A*';
169 open my $ps, '-|', 'ps';
171 my @fields = qw( pid tt stat time command );
174 @process{@fields} = unpack($PS_T, $_);
175 for my $field ( @fields ) {
176 print "$field: <$process{$field}>\n";
178 print 'line=', pack($PS_T, @process{@fields} ), "\n";
181 We've used a hash slice in order to easily handle the fields of each row.
182 Storing the keys in an array means it's easy to operate on them as a
183 group or loop over them with for. It also avoids polluting the program
184 with global variables and using symbolic references.
186 =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?
188 As of perl5.6, open() autovivifies file and directory handles
189 as references if you pass it an uninitialized scalar variable.
190 You can then pass these references just like any other scalar,
191 and use them in the place of named handles.
193 open my $fh, $file_name;
195 open local $fh, $file_name;
197 print $fh "Hello World!\n";
201 Before perl5.6, you had to deal with various typeglob idioms
202 which you may see in older code.
204 open FILE, "> $filename";
205 process_typeglob( *FILE );
206 process_reference( \*FILE );
208 sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
209 sub process_reference { local $fh = shift; print $fh "Reference!" }
211 If you want to create many anonymous handles, you should
212 check out the Symbol or IO::Handle modules.
214 =head2 How can I use a filehandle indirectly?
216 An indirect filehandle is using something other than a symbol
217 in a place that a filehandle is expected. Here are ways
218 to get indirect filehandles:
220 $fh = SOME_FH; # bareword is strict-subs hostile
221 $fh = "SOME_FH"; # strict-refs hostile; same package only
222 $fh = *SOME_FH; # typeglob
223 $fh = \*SOME_FH; # ref to typeglob (bless-able)
224 $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
226 Or, you can use the C<new> method from one of the IO::* modules to
227 create an anonymous filehandle, store that in a scalar variable,
228 and use it as though it were a normal filehandle.
230 use IO::Handle; # 5.004 or higher
231 $fh = IO::Handle->new();
233 Then use any of those as you would a normal filehandle. Anywhere that
234 Perl is expecting a filehandle, an indirect filehandle may be used
235 instead. An indirect filehandle is just a scalar variable that contains
236 a filehandle. Functions like C<print>, C<open>, C<seek>, or
237 the C<< <FH> >> diamond operator will accept either a named filehandle
238 or a scalar variable containing one:
240 ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
241 print $ofh "Type it: ";
243 print $efh "What was that: $got";
245 If you're passing a filehandle to a function, you can write
246 the function in two ways:
250 print $fh "Sending to indirect filehandle\n";
253 Or it can localize a typeglob and use the filehandle directly:
257 print FH "Sending to localized filehandle\n";
260 Both styles work with either objects or typeglobs of real filehandles.
261 (They might also work with strings under some circumstances, but this
267 In the examples above, we assigned the filehandle to a scalar variable
268 before using it. That is because only simple scalar variables, not
269 expressions or subscripts of hashes or arrays, can be used with
270 built-ins like C<print>, C<printf>, or the diamond operator. Using
271 something other than a simple scalar variable as a filehandle is
272 illegal and won't even compile:
274 @fd = (*STDIN, *STDOUT, *STDERR);
275 print $fd[1] "Type it: "; # WRONG
276 $got = <$fd[0]> # WRONG
277 print $fd[2] "What was that: $got"; # WRONG
279 With C<print> and C<printf>, you get around this by using a block and
280 an expression where you would place the filehandle:
282 print { $fd[1] } "funny stuff\n";
283 printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
284 # Pity the poor deadbeef.
286 That block is a proper block like any other, so you can put more
287 complicated code there. This sends the message out to one of two places:
290 print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
291 print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
293 This approach of treating C<print> and C<printf> like object methods
294 calls doesn't work for the diamond operator. That's because it's a
295 real operator, not just a function with a comma-less argument. Assuming
296 you've been storing typeglobs in your structure as we did above, you
297 can use the built-in function named C<readline> to read a record just
298 as C<< <> >> does. Given the initialization shown above for @fd, this
299 would work, but only because readline() requires a typeglob. It doesn't
300 work with objects or strings, which might be a bug we haven't fixed yet.
302 $got = readline($fd[0]);
304 Let it be noted that the flakiness of indirect filehandles is not
305 related to whether they're strings, typeglobs, objects, or anything else.
306 It's the syntax of the fundamental operators. Playing the object
307 game doesn't help you at all here.
309 =head2 How can I set up a footer format to be used with write()?
311 There's no builtin way to do this, but L<perlform> has a couple of
312 techniques to make it possible for the intrepid hacker.
314 =head2 How can I write() into a string?
316 See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
318 =head2 How can I output my numbers with commas added?
320 This subroutine will add commas to your number:
324 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
328 This regex from Benjamin Goldberg will add commas to numbers:
330 s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
332 It is easier to see with comments:
335 ^[-+]? # beginning of number.
336 \d{1,3}? # first digits before first comma
337 (?= # followed by, (but not included in the match) :
338 (?>(?:\d{3})+) # some positive multiple of three digits.
339 (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever.
342 \G\d{3} # after the last group, get three digits
343 (?=\d) # but they have to have more digits after them.
346 =head2 How can I translate tildes (~) in a filename?
348 Use the <> (glob()) operator, documented in L<perlfunc>. Older
349 versions of Perl require that you have a shell installed that groks
350 tildes. Recent perl versions have this feature built in. The
351 File::KGlob module (available from CPAN) gives more portable glob
354 Within Perl, you may use this directly:
357 ^ ~ # find a leading tilde
359 [^/] # a non-slash character
360 * # repeated 0 or more times (0 means me)
365 : ( $ENV{HOME} || $ENV{LOGDIR} )
368 =head2 How come when I open a file read-write it wipes it out?
370 Because you're using something like this, which truncates the file and
371 I<then> gives you read-write access:
373 open(FH, "+> /path/name"); # WRONG (almost always)
375 Whoops. You should instead use this, which will fail if the file
378 open(FH, "+< /path/name"); # open for update
380 Using ">" always clobbers or creates. Using "<" never does
381 either. The "+" doesn't change this.
383 Here are examples of many kinds of file opens. Those using sysopen()
388 To open file for reading:
390 open(FH, "< $path") || die $!;
391 sysopen(FH, $path, O_RDONLY) || die $!;
393 To open file for writing, create new file if needed or else truncate old file:
395 open(FH, "> $path") || die $!;
396 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
397 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
399 To open file for writing, create new file, file must not exist:
401 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
402 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
404 To open file for appending, create if necessary:
406 open(FH, ">> $path") || die $!;
407 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
408 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
410 To open file for appending, file must exist:
412 sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
414 To open file for update, file must exist:
416 open(FH, "+< $path") || die $!;
417 sysopen(FH, $path, O_RDWR) || die $!;
419 To open file for update, create file if necessary:
421 sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
422 sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
424 To open file for update, file must not exist:
426 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
427 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
429 To open a file without blocking, creating if necessary:
431 sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
432 or die "can't open /foo/somefile: $!":
434 Be warned that neither creation nor deletion of files is guaranteed to
435 be an atomic operation over NFS. That is, two processes might both
436 successfully create or unlink the same file! Therefore O_EXCL
437 isn't as exclusive as you might wish.
439 See also the new L<perlopentut> if you have it (new for 5.6).
441 =head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
443 The C<< <> >> operator performs a globbing operation (see above).
444 In Perl versions earlier than v5.6.0, the internal glob() operator forks
445 csh(1) to do the actual glob expansion, but
446 csh can't handle more than 127 items and so gives the error message
447 C<Argument list too long>. People who installed tcsh as csh won't
448 have this problem, but their users may be surprised by it.
450 To get around this, either upgrade to Perl v5.6.0 or later, do the glob
451 yourself with readdir() and patterns, or use a module like File::KGlob,
452 one that doesn't use the shell to do globbing.
454 =head2 Is there a leak/bug in glob()?
456 Due to the current implementation on some operating systems, when you
457 use the glob() function or its angle-bracket alias in a scalar
458 context, you may cause a memory leak and/or unpredictable behavior. It's
459 best therefore to use glob() only in list context.
461 =head2 How can I open a file with a leading ">" or trailing blanks?
463 Normally perl ignores trailing blanks in filenames, and interprets
464 certain leading characters (or a trailing "|") to mean something
467 The three argument form of open() lets you specify the mode
468 separately from the filename. The open() function treats
469 special mode characters and whitespace in the filename as
472 open FILE, "<", " file "; # filename is " file "
473 open FILE, ">", ">file"; # filename is ">file"
475 It may be a lot clearer to use sysopen(), though:
478 $badpath = "<<<something really wicked ";
479 sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
480 or die "can't open $badpath: $!";
482 =head2 How can I reliably rename a file?
484 If your operating system supports a proper mv(1) utility or its
485 functional equivalent, this works:
487 rename($old, $new) or system("mv", $old, $new);
489 It may be more portable to use the File::Copy module instead.
490 You just copy to the new file to the new name (checking return
491 values), then delete the old one. This isn't really the same
492 semantically as a rename(), which preserves meta-information like
493 permissions, timestamps, inode info, etc.
495 Newer versions of File::Copy export a move() function.
497 =head2 How can I lock a file?
499 Perl's builtin flock() function (see L<perlfunc> for details) will call
500 flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
501 later), and lockf(3) if neither of the two previous system calls exists.
502 On some systems, it may even use a different form of native locking.
503 Here are some gotchas with Perl's flock():
509 Produces a fatal error if none of the three system calls (or their
510 close equivalent) exists.
514 lockf(3) does not provide shared locking, and requires that the
515 filehandle be open for writing (or appending, or read/writing).
519 Some versions of flock() can't lock files over a network (e.g. on NFS file
520 systems), so you'd need to force the use of fcntl(2) when you build Perl.
521 But even this is dubious at best. See the flock entry of L<perlfunc>
522 and the F<INSTALL> file in the source distribution for information on
523 building Perl to do this.
525 Two potentially non-obvious but traditional flock semantics are that
526 it waits indefinitely until the lock is granted, and that its locks are
527 I<merely advisory>. Such discretionary locks are more flexible, but
528 offer fewer guarantees. This means that files locked with flock() may
529 be modified by programs that do not also use flock(). Cars that stop
530 for red lights get on well with each other, but not with cars that don't
531 stop for red lights. See the perlport manpage, your port's specific
532 documentation, or your system-specific local manpages for details. It's
533 best to assume traditional behavior if you're writing portable programs.
534 (If you're not, you should as always feel perfectly free to write
535 for your own system's idiosyncrasies (sometimes called "features").
536 Slavish adherence to portability concerns shouldn't get in the way of
537 your getting your job done.)
539 For more information on file locking, see also
540 L<perlopentut/"File Locking"> if you have it (new for 5.6).
544 =head2 Why can't I just open(FH, "E<gt>file.lock")?
546 A common bit of code B<NOT TO USE> is this:
548 sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
549 open(LCK, "> file.lock"); # THIS BROKEN CODE
551 This is a classic race condition: you take two steps to do something
552 which must be done in one. That's why computer hardware provides an
553 atomic test-and-set instruction. In theory, this "ought" to work:
555 sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
556 or die "can't open file.lock: $!";
558 except that lamentably, file creation (and deletion) is not atomic
559 over NFS, so this won't work (at least, not every time) over the net.
560 Various schemes involving link() have been suggested, but
561 these tend to involve busy-wait, which is also subdesirable.
563 =head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
565 Didn't anyone ever tell you web-page hit counters were useless?
566 They don't count number of hits, they're a waste of time, and they serve
567 only to stroke the writer's vanity. It's better to pick a random number;
568 they're more realistic.
570 Anyway, this is what you can do if you can't help yourself.
572 use Fcntl qw(:DEFAULT :flock);
573 sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
574 flock(FH, LOCK_EX) or die "can't flock numfile: $!";
576 seek(FH, 0, 0) or die "can't rewind numfile: $!";
577 truncate(FH, 0) or die "can't truncate numfile: $!";
578 (print FH $num+1, "\n") or die "can't write numfile: $!";
579 close FH or die "can't close numfile: $!";
581 Here's a much better web-page hit counter:
583 $hits = int( (time() - 850_000_000) / rand(1_000) );
585 If the count doesn't impress your friends, then the code might. :-)
587 =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?
589 If you are on a system that correctly implements flock() and you use the
590 example appending code from "perldoc -f flock" everything will be OK
591 even if the OS you are on doesn't implement append mode correctly (if
592 such a system exists.) So if you are happy to restrict yourself to OSs
593 that implement flock() (and that's not really much of a restriction)
594 then that is what you should do.
596 If you know you are only going to use a system that does correctly
597 implement appending (i.e. not Win32) then you can omit the seek() from
600 If you know you are only writing code to run on an OS and filesystem that
601 does implement append mode correctly (a local filesystem on a modern
602 Unix for example), and you keep the file in block-buffered mode and you
603 write less than one buffer-full of output between each manual flushing
604 of the buffer then each bufferload is almost guaranteed to be written to
605 the end of the file in one chunk without getting intermingled with
606 anyone else's output. You can also use the syswrite() function which is
607 simply a wrapper around your systems write(2) system call.
609 There is still a small theoretical chance that a signal will interrupt
610 the system level write() operation before completion. There is also a
611 possibility that some STDIO implementations may call multiple system
612 level write()s even if the buffer was empty to start. There may be some
613 systems where this probability is reduced to zero.
615 =head2 How do I randomly update a binary file?
617 If you're just trying to patch a binary, in many cases something as
618 simple as this works:
620 perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
622 However, if you have fixed sized records, then you might do something more
625 $RECSIZE = 220; # size of record, in bytes
626 $recno = 37; # which record to update
627 open(FH, "+<somewhere") || die "can't update somewhere: $!";
628 seek(FH, $recno * $RECSIZE, 0);
629 read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
631 seek(FH, -$RECSIZE, 1);
635 Locking and error checking are left as an exercise for the reader.
636 Don't forget them or you'll be quite sorry.
638 =head2 How do I get a file's timestamp in perl?
640 If you want to retrieve the time at which the file was last
641 read, written, or had its meta-data (owner, etc) changed,
642 you use the B<-M>, B<-A>, or B<-C> file test operations as
643 documented in L<perlfunc>. These retrieve the age of the
644 file (measured against the start-time of your program) in
645 days as a floating point number. Some platforms may not have
646 all of these times. See L<perlport> for details. To
647 retrieve the "raw" time in seconds since the epoch, you
648 would call the stat function, then use localtime(),
649 gmtime(), or POSIX::strftime() to convert this into
654 $write_secs = (stat($file))[9];
655 printf "file %s updated at %s\n", $file,
656 scalar localtime($write_secs);
658 If you prefer something more legible, use the File::stat module
659 (part of the standard distribution in version 5.004 and later):
661 # error checking left as an exercise for reader.
664 $date_string = ctime(stat($file)->mtime);
665 print "file $file updated at $date_string\n";
667 The POSIX::strftime() approach has the benefit of being,
668 in theory, independent of the current locale. See L<perllocale>
671 =head2 How do I set a file's timestamp in perl?
673 You use the utime() function documented in L<perlfunc/utime>.
674 By way of example, here's a little program that copies the
675 read and write times from its first argument to all the rest
679 die "usage: cptimes timestamp_file other_files ...\n";
682 ($atime, $mtime) = (stat($timestamp))[8,9];
683 utime $atime, $mtime, @ARGV;
685 Error checking is, as usual, left as an exercise for the reader.
687 Note that utime() currently doesn't work correctly with Win95/NT
688 ports. A bug has been reported. Check it carefully before using
689 utime() on those platforms.
691 =head2 How do I print to more than one file at once?
693 To connect one filehandle to several output filehandles,
694 you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
696 If you only have to do this once, you can print individually
699 for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
701 =head2 How can I read in an entire file all at once?
703 You can use the File::Slurp module to do it in one step.
707 $all_of_it = read_file($filename); # entire file in scalar
708 @all_lines = read_file($filename); # one line perl element
710 The customary Perl approach for processing all the lines in a file is to
711 do so one line at a time:
713 open (INPUT, $file) || die "can't open $file: $!";
716 # do something with $_
718 close(INPUT) || die "can't close $file: $!";
720 This is tremendously more efficient than reading the entire file into
721 memory as an array of lines and then processing it one element at a time,
722 which is often--if not almost always--the wrong approach. Whenever
723 you see someone do this:
727 you should think long and hard about why you need everything loaded at
728 once. It's just not a scalable solution. You might also find it more
729 fun to use the standard Tie::File module, or the DB_File module's
730 $DB_RECNO bindings, which allow you to tie an array to a file so that
731 accessing an element the array actually accesses the corresponding
734 You can read the entire filehandle contents into a scalar.
738 open (INPUT, $file) || die "can't open $file: $!";
742 That temporarily undefs your record separator, and will automatically
743 close the file at block exit. If the file is already open, just use this:
745 $var = do { local $/; <INPUT> };
747 For ordinary files you can also use the read function.
749 read( INPUT, $var, -s INPUT );
751 The third argument tests the byte size of the data on the INPUT filehandle
752 and reads that many bytes into the buffer $var.
754 =head2 How can I read in a file by paragraphs?
756 Use the C<$/> variable (see L<perlvar> for details). You can either
757 set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
758 for instance, gets treated as two paragraphs and not three), or
759 C<"\n\n"> to accept empty paragraphs.
761 Note that a blank line must have no blanks in it. Thus
762 S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
764 =head2 How can I read a single character from a file? From the keyboard?
766 You can use the builtin C<getc()> function for most filehandles, but
767 it won't (easily) work on a terminal device. For STDIN, either use
768 the Term::ReadKey module from CPAN or use the sample code in
771 If your system supports the portable operating system programming
772 interface (POSIX), you can use the following code, which you'll note
773 turns off echo processing as well.
787 use POSIX qw(:termios_h);
789 my ($term, $oterm, $echo, $noecho, $fd_stdin);
791 $fd_stdin = fileno(STDIN);
793 $term = POSIX::Termios->new();
794 $term->getattr($fd_stdin);
795 $oterm = $term->getlflag();
797 $echo = ECHO | ECHOK | ICANON;
798 $noecho = $oterm & ~$echo;
801 $term->setlflag($noecho);
802 $term->setcc(VTIME, 1);
803 $term->setattr($fd_stdin, TCSANOW);
807 $term->setlflag($oterm);
808 $term->setcc(VTIME, 0);
809 $term->setattr($fd_stdin, TCSANOW);
815 sysread(STDIN, $key, 1);
824 The Term::ReadKey module from CPAN may be easier to use. Recent versions
825 include also support for non-portable systems as well.
828 open(TTY, "</dev/tty");
829 print "Gimme a char: ";
831 $key = ReadKey 0, *TTY;
833 printf "\nYou said %s, char number %03d\n",
836 =head2 How can I tell whether there's a character waiting on a filehandle?
838 The very first thing you should do is look into getting the Term::ReadKey
839 extension from CPAN. As we mentioned earlier, it now even has limited
840 support for non-portable (read: not open systems, closed, proprietary,
841 not POSIX, not Unix, etc) systems.
843 You should also check out the Frequently Asked Questions list in
844 comp.unix.* for things like this: the answer is essentially the same.
845 It's very system dependent. Here's one solution that works on BSD
850 vec($rin, fileno(STDIN), 1) = 1;
851 return $nfd = select($rin,undef,undef,0);
854 If you want to find out how many characters are waiting, there's
855 also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
856 comes with Perl tries to convert C include files to Perl code, which
857 can be C<require>d. FIONREAD ends up defined as a function in the
858 I<sys/ioctl.ph> file:
860 require 'sys/ioctl.ph';
862 $size = pack("L", 0);
863 ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
864 $size = unpack("L", $size);
866 If I<h2ph> wasn't installed or doesn't work for you, you can
867 I<grep> the include files by hand:
869 % grep FIONREAD /usr/include/*/*
870 /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
872 Or write a small C program using the editor of champions:
875 #include <sys/ioctl.h>
877 printf("%#08x\n", FIONREAD);
880 % cc -o fionread fionread.c
884 And then hard code it, leaving porting as an exercise to your successor.
886 $FIONREAD = 0x4004667f; # XXX: opsys dependent
888 $size = pack("L", 0);
889 ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
890 $size = unpack("L", $size);
892 FIONREAD requires a filehandle connected to a stream, meaning that sockets,
893 pipes, and tty devices work, but I<not> files.
895 =head2 How do I do a C<tail -f> in perl?
901 The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
902 but it does clear the end-of-file condition on the handle, so that the
903 next <GWFILE> makes Perl try again to read something.
905 If that doesn't work (it relies on features of your stdio implementation),
906 then you need something more like this:
909 for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
910 # search for some stuff and put it into files
913 seek(GWFILE, $curpos, 0); # seek to where we had been
916 If this still doesn't work, look into the POSIX module. POSIX defines
917 the clearerr() method, which can remove the end of file condition on a
918 filehandle. The method: read until end of file, clearerr(), read some
919 more. Lather, rinse, repeat.
921 There's also a File::Tail module from CPAN.
923 =head2 How do I dup() a filehandle in Perl?
925 If you check L<perlfunc/open>, you'll see that several of the ways
926 to call open() should do the trick. For example:
928 open(LOG, ">>/foo/logfile");
929 open(STDERR, ">&LOG");
931 Or even with a literal numeric descriptor:
933 $fd = $ENV{MHCONTEXTFD};
934 open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
936 Note that "<&STDIN" makes a copy, but "<&=STDIN" make
937 an alias. That means if you close an aliased handle, all
938 aliases become inaccessible. This is not true with
941 Error checking, as always, has been left as an exercise for the reader.
943 =head2 How do I close a file descriptor by number?
945 This should rarely be necessary, as the Perl close() function is to be
946 used for things that Perl opened itself, even if it was a dup of a
947 numeric descriptor as with MHCONTEXT above. But if you really have
948 to, you may be able to do this:
950 require 'sys/syscall.ph';
951 $rc = syscall(&SYS_close, $fd + 0); # must force numeric
952 die "can't sysclose $fd: $!" unless $rc == -1;
954 Or, just use the fdopen(3S) feature of open():
958 open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
962 =head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
964 Whoops! You just put a tab and a formfeed into that filename!
965 Remember that within double quoted strings ("like\this"), the
966 backslash is an escape character. The full list of these is in
967 L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
968 have a file called "c:(tab)emp(formfeed)oo" or
969 "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
971 Either single-quote your strings, or (preferably) use forward slashes.
972 Since all DOS and Windows versions since something like MS-DOS 2.0 or so
973 have treated C</> and C<\> the same in a path, you might as well use the
974 one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++,
975 awk, Tcl, Java, or Python, just to mention a few. POSIX paths
976 are more portable, too.
978 =head2 Why doesn't glob("*.*") get all the files?
980 Because even on non-Unix ports, Perl's glob function follows standard
981 Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
982 files. This makes glob() portable even to legacy systems. Your
983 port may include proprietary globbing functions as well. Check its
984 documentation for details.
986 =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
988 This is elaborately and painstakingly described in the
989 F<file-dir-perms> article in the "Far More Than You Ever Wanted To
990 Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
992 The executive summary: learn how your filesystem works. The
993 permissions on a file say what can happen to the data in that file.
994 The permissions on a directory say what can happen to the list of
995 files in that directory. If you delete a file, you're removing its
996 name from the directory (so the operation depends on the permissions
997 of the directory, not of the file). If you try to write to the file,
998 the permissions of the file govern whether you're allowed to.
1000 =head2 How do I select a random line from a file?
1002 Here's an algorithm from the Camel Book:
1005 rand($.) < 1 && ($line = $_) while <>;
1007 This has a significant advantage in space over reading the whole file
1008 in. You can find a proof of this method in I<The Art of Computer
1009 Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth.
1011 You can use the File::Random module which provides a function
1014 use File::Random qw/random_line/;
1015 my $line = random_line($filename);
1017 Another way is to use the Tie::File module, which treats the entire
1018 file as an array. Simply access a random array element.
1020 =head2 Why do I get weird spaces when I print an array of lines?
1026 joins together the elements of C<@lines> with a space between them.
1027 If C<@lines> were C<("little", "fluffy", "clouds")> then the above
1028 statement would print
1030 little fluffy clouds
1032 but if each element of C<@lines> was a line of text, ending a newline
1033 character C<("little\n", "fluffy\n", "clouds\n")> then it would print:
1039 If your array contains lines, just print them:
1043 =head1 AUTHOR AND COPYRIGHT
1045 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
1046 All rights reserved.
1048 This documentation is free; you can redistribute it and/or modify it
1049 under the same terms as Perl itself.
1051 Irrespective of its distribution, all code examples here are in the public
1052 domain. You are permitted and encouraged to use this code and any
1053 derivatives thereof in your own programs for fun or for profit as you
1054 see fit. A simple comment in the code giving credit to the FAQ would
1055 be courteous but is not required.