3 perlfaq5 - Files and Formats ($Revision: 6019 $)
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?
11 X<flush> X<buffer> X<unbuffer> X<autoflush>
13 Perl does not support truly unbuffered output (except
14 insofar as you can C<syswrite(OUT, $char, 1)>), although it
15 does support is "command buffering", in which a physical
16 write is performed after every output command.
18 The C standard I/O library (stdio) normally buffers
19 characters sent to devices so that there isn't a system call
20 for each byte. In most stdio implementations, the type of
21 output buffering and the size of the buffer varies according
22 to the type of device. Perl's print() and write() functions
23 normally buffer output, while syswrite() bypasses buffering
26 If you want your output to be sent immediately when you
27 execute print() or write() (for instance, for some network
28 protocols), you must set the handle's autoflush flag. This
29 flag is the Perl variable $| and when it is set to a true
30 value, Perl will flush the handle's buffer after each
31 print() or write(). Setting $| affects buffering only for
32 the currently selected default file handle. You choose this
33 handle with the one argument select() call (see
34 L<perlvar/$E<verbar>> and L<perlfunc/select>).
36 Use select() to choose the desired handle, then set its
37 per-filehandle variables.
39 $old_fh = select(OUTPUT_HANDLE);
43 Some modules offer object-oriented access to handles and their
44 variables, although they may be overkill if this is the only
45 thing you do with them. You can use IO::Handle:
48 open(DEV, ">/dev/printer"); # but is this?
53 use IO::Socket; # this one is kinda a pipe?
54 my $sock = IO::Socket::INET->new( 'www.example.com:80' );
58 =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?
61 Use the Tie::File module, which is included in the standard
62 distribution since Perl 5.8.0.
64 =head2 How do I count the number of lines in a file?
65 X<file, counting lines> X<lines> X<line>
67 One fairly efficient way is to count newlines in the file. The
68 following program uses a feature of tr///, as documented in L<perlop>.
69 If your text file doesn't end with a newline, then it's not really a
70 proper text file, so this may report one fewer line than you expect.
73 open(FILE, $filename) or die "Can't open `$filename': $!";
74 while (sysread FILE, $buffer, 4096) {
75 $lines += ($buffer =~ tr/\n//);
79 This assumes no funny games with newline translations.
81 =head2 How can I use Perl's C<-i> option from within a program?
84 C<-i> sets the value of Perl's C<$^I> variable, which in turn affects
85 the behavior of C<< <> >>; see L<perlrun> for more details. By
86 modifying the appropriate variables directly, you can get the same
87 behavior within a larger program. For example:
91 local($^I, @ARGV) = ('.orig', glob("*.c"));
94 print "This line should appear at the top of each file\n";
96 s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
98 close ARGV if eof; # Reset $.
101 # $^I and @ARGV return to their old values here
103 This block modifies all the C<.c> files in the current directory,
104 leaving a backup of the original data from each file in a new
107 =head2 How can I copy a file?
108 X<copy> X<file, copy>
110 (contributed by brian d foy)
112 Use the File::Copy module. It comes with Perl and can do a
113 true copy across file systems, and it does its magic in
118 copy( $original, $new_copy ) or die "Copy failed: $!";
120 If you can't use File::Copy, you'll have to do the work yourself:
121 open the original file, open the destination file, then print
122 to the destination file as you read the original.
124 =head2 How do I make a temporary file name?
127 If you don't need to know the name of the file, you can use C<open()>
128 with C<undef> in place of the file name. The C<open()> function
129 creates an anonymous temporary file.
131 open my $tmp, '+>', undef or die $!;
133 Otherwise, you can use the File::Temp module.
135 use File::Temp qw/ tempfile tempdir /;
137 $dir = tempdir( CLEANUP => 1 );
138 ($fh, $filename) = tempfile( DIR => $dir );
140 # or if you don't need to know the filename
142 $fh = tempfile( DIR => $dir );
144 The File::Temp has been a standard module since Perl 5.6.1. If you
145 don't have a modern enough Perl installed, use the C<new_tmpfile>
146 class method from the IO::File module to get a filehandle opened for
147 reading and writing. Use it if you don't need to know the file's name:
150 $fh = IO::File->new_tmpfile()
151 or die "Unable to make new temporary file: $!";
153 If you're committed to creating a temporary file by hand, use the
154 process ID and/or the current time-value. If you need to have many
155 temporary files in one process, use a counter:
159 my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
160 my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
165 until (defined(fileno(FH)) || $count++ > 100) {
166 $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
167 # O_EXCL is required for security reasons.
168 sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
171 if (defined(fileno(FH))
172 return (*FH, $base_name);
180 =head2 How can I manipulate fixed-record-length files?
181 X<fixed-length> X<file, fixed-length records>
183 The most efficient way is using L<pack()|perlfunc/"pack"> and
184 L<unpack()|perlfunc/"unpack">. This is faster than using
185 L<substr()|perlfunc/"substr"> when taking many, many strings. It is
186 slower for just a few.
188 Here is a sample chunk of code to break up and put back together again
189 some fixed-format input lines, in this case from the output of a normal,
193 # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
194 my $PS_T = 'A6 A4 A7 A5 A*';
195 open my $ps, '-|', 'ps';
197 my @fields = qw( pid tt stat time command );
200 @process{@fields} = unpack($PS_T, $_);
201 for my $field ( @fields ) {
202 print "$field: <$process{$field}>\n";
204 print 'line=', pack($PS_T, @process{@fields} ), "\n";
207 We've used a hash slice in order to easily handle the fields of each row.
208 Storing the keys in an array means it's easy to operate on them as a
209 group or loop over them with for. It also avoids polluting the program
210 with global variables and using symbolic references.
212 =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?
213 X<filehandle, local> X<filehandle, passing> X<filehandle, reference>
215 As of perl5.6, open() autovivifies file and directory handles
216 as references if you pass it an uninitialized scalar variable.
217 You can then pass these references just like any other scalar,
218 and use them in the place of named handles.
220 open my $fh, $file_name;
222 open local $fh, $file_name;
224 print $fh "Hello World!\n";
228 If you like, you can store these filehandles in an array or a hash.
229 If you access them directly, they aren't simple scalars and you
230 need to give C<print> a little help by placing the filehandle
231 reference in braces. Perl can only figure it out on its own when
232 the filehandle reference is a simple scalar.
234 my @fhs = ( $fh1, $fh2, $fh3 );
236 for( $i = 0; $i <= $#fhs; $i++ ) {
237 print {$fhs[$i]} "just another Perl answer, \n";
241 Before perl5.6, you had to deal with various typeglob idioms
242 which you may see in older code.
244 open FILE, "> $filename";
245 process_typeglob( *FILE );
246 process_reference( \*FILE );
248 sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
249 sub process_reference { local $fh = shift; print $fh "Reference!" }
251 If you want to create many anonymous handles, you should
252 check out the Symbol or IO::Handle modules.
254 =head2 How can I use a filehandle indirectly?
255 X<filehandle, indirect>
257 An indirect filehandle is using something other than a symbol
258 in a place that a filehandle is expected. Here are ways
259 to get indirect filehandles:
261 $fh = SOME_FH; # bareword is strict-subs hostile
262 $fh = "SOME_FH"; # strict-refs hostile; same package only
263 $fh = *SOME_FH; # typeglob
264 $fh = \*SOME_FH; # ref to typeglob (bless-able)
265 $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
267 Or, you can use the C<new> method from one of the IO::* modules to
268 create an anonymous filehandle, store that in a scalar variable,
269 and use it as though it were a normal filehandle.
271 use IO::Handle; # 5.004 or higher
272 $fh = IO::Handle->new();
274 Then use any of those as you would a normal filehandle. Anywhere that
275 Perl is expecting a filehandle, an indirect filehandle may be used
276 instead. An indirect filehandle is just a scalar variable that contains
277 a filehandle. Functions like C<print>, C<open>, C<seek>, or
278 the C<< <FH> >> diamond operator will accept either a named filehandle
279 or a scalar variable containing one:
281 ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
282 print $ofh "Type it: ";
284 print $efh "What was that: $got";
286 If you're passing a filehandle to a function, you can write
287 the function in two ways:
291 print $fh "Sending to indirect filehandle\n";
294 Or it can localize a typeglob and use the filehandle directly:
298 print FH "Sending to localized filehandle\n";
301 Both styles work with either objects or typeglobs of real filehandles.
302 (They might also work with strings under some circumstances, but this
308 In the examples above, we assigned the filehandle to a scalar variable
309 before using it. That is because only simple scalar variables, not
310 expressions or subscripts of hashes or arrays, can be used with
311 built-ins like C<print>, C<printf>, or the diamond operator. Using
312 something other than a simple scalar variable as a filehandle is
313 illegal and won't even compile:
315 @fd = (*STDIN, *STDOUT, *STDERR);
316 print $fd[1] "Type it: "; # WRONG
317 $got = <$fd[0]> # WRONG
318 print $fd[2] "What was that: $got"; # WRONG
320 With C<print> and C<printf>, you get around this by using a block and
321 an expression where you would place the filehandle:
323 print { $fd[1] } "funny stuff\n";
324 printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
325 # Pity the poor deadbeef.
327 That block is a proper block like any other, so you can put more
328 complicated code there. This sends the message out to one of two places:
331 print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
332 print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
334 This approach of treating C<print> and C<printf> like object methods
335 calls doesn't work for the diamond operator. That's because it's a
336 real operator, not just a function with a comma-less argument. Assuming
337 you've been storing typeglobs in your structure as we did above, you
338 can use the built-in function named C<readline> to read a record just
339 as C<< <> >> does. Given the initialization shown above for @fd, this
340 would work, but only because readline() requires a typeglob. It doesn't
341 work with objects or strings, which might be a bug we haven't fixed yet.
343 $got = readline($fd[0]);
345 Let it be noted that the flakiness of indirect filehandles is not
346 related to whether they're strings, typeglobs, objects, or anything else.
347 It's the syntax of the fundamental operators. Playing the object
348 game doesn't help you at all here.
350 =head2 How can I set up a footer format to be used with write()?
353 There's no builtin way to do this, but L<perlform> has a couple of
354 techniques to make it possible for the intrepid hacker.
356 =head2 How can I write() into a string?
357 X<write, into a string>
359 See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
361 =head2 How can I output my numbers with commas added?
364 (contributed by brian d foy and Benjamin Goldberg)
366 You can use L<Number::Format> to separate places in a number.
367 It handles locale information for those of you who want to insert
368 full stops instead (or anything else that they want to use,
371 This subroutine will add commas to your number:
375 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
379 This regex from Benjamin Goldberg will add commas to numbers:
381 s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
383 It is easier to see with comments:
386 ^[-+]? # beginning of number.
387 \d+? # first digits before first comma
388 (?= # followed by, (but not included in the match) :
389 (?>(?:\d{3})+) # some positive multiple of three digits.
390 (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever.
393 \G\d{3} # after the last group, get three digits
394 (?=\d) # but they have to have more digits after them.
397 =head2 How can I translate tildes (~) in a filename?
398 X<tilde> X<tilde expansion>
400 Use the <> (glob()) operator, documented in L<perlfunc>. Older
401 versions of Perl require that you have a shell installed that groks
402 tildes. Recent perl versions have this feature built in. The
403 File::KGlob module (available from CPAN) gives more portable glob
406 Within Perl, you may use this directly:
409 ^ ~ # find a leading tilde
411 [^/] # a non-slash character
412 * # repeated 0 or more times (0 means me)
417 : ( $ENV{HOME} || $ENV{LOGDIR} )
420 =head2 How come when I open a file read-write it wipes it out?
421 X<clobber> X<read-write> X<clobbering> X<truncate> X<truncating>
423 Because you're using something like this, which truncates the file and
424 I<then> gives you read-write access:
426 open(FH, "+> /path/name"); # WRONG (almost always)
428 Whoops. You should instead use this, which will fail if the file
431 open(FH, "+< /path/name"); # open for update
433 Using ">" always clobbers or creates. Using "<" never does
434 either. The "+" doesn't change this.
436 Here are examples of many kinds of file opens. Those using sysopen()
441 To open file for reading:
443 open(FH, "< $path") || die $!;
444 sysopen(FH, $path, O_RDONLY) || die $!;
446 To open file for writing, create new file if needed or else truncate old file:
448 open(FH, "> $path") || die $!;
449 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
450 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
452 To open file for writing, create new file, file must not exist:
454 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
455 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
457 To open file for appending, create if necessary:
459 open(FH, ">> $path") || die $!;
460 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
461 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
463 To open file for appending, file must exist:
465 sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
467 To open file for update, file must exist:
469 open(FH, "+< $path") || die $!;
470 sysopen(FH, $path, O_RDWR) || die $!;
472 To open file for update, create file if necessary:
474 sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
475 sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
477 To open file for update, file must not exist:
479 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
480 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
482 To open a file without blocking, creating if necessary:
484 sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
485 or die "can't open /foo/somefile: $!":
487 Be warned that neither creation nor deletion of files is guaranteed to
488 be an atomic operation over NFS. That is, two processes might both
489 successfully create or unlink the same file! Therefore O_EXCL
490 isn't as exclusive as you might wish.
492 See also the new L<perlopentut> if you have it (new for 5.6).
494 =head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
495 X<argument list too long>
497 The C<< <> >> operator performs a globbing operation (see above).
498 In Perl versions earlier than v5.6.0, the internal glob() operator forks
499 csh(1) to do the actual glob expansion, but
500 csh can't handle more than 127 items and so gives the error message
501 C<Argument list too long>. People who installed tcsh as csh won't
502 have this problem, but their users may be surprised by it.
504 To get around this, either upgrade to Perl v5.6.0 or later, do the glob
505 yourself with readdir() and patterns, or use a module like File::KGlob,
506 one that doesn't use the shell to do globbing.
508 =head2 Is there a leak/bug in glob()?
511 Due to the current implementation on some operating systems, when you
512 use the glob() function or its angle-bracket alias in a scalar
513 context, you may cause a memory leak and/or unpredictable behavior. It's
514 best therefore to use glob() only in list context.
516 =head2 How can I open a file with a leading ">" or trailing blanks?
517 X<filename, special characters>
519 (contributed by Brian McCauley)
521 The special two argument form of Perl's open() function ignores
522 trailing blanks in filenames and infers the mode from certain leading
523 characters (or a trailing "|"). In older versions of Perl this was the
524 only version of open() and so it is prevalent in old code and books.
526 Unless you have a particular reason to use the two argument form you
527 should use the three argument form of open() which does not treat any
528 charcters in the filename as special.
530 open FILE, "<", " file "; # filename is " file "
531 open FILE, ">", ">file"; # filename is ">file"
533 =head2 How can I reliably rename a file?
534 X<rename> X<mv> X<move> X<file, rename> X<ren>
536 If your operating system supports a proper mv(1) utility or its
537 functional equivalent, this works:
539 rename($old, $new) or system("mv", $old, $new);
541 It may be more portable to use the File::Copy module instead.
542 You just copy to the new file to the new name (checking return
543 values), then delete the old one. This isn't really the same
544 semantically as a rename(), which preserves meta-information like
545 permissions, timestamps, inode info, etc.
547 Newer versions of File::Copy export a move() function.
549 =head2 How can I lock a file?
550 X<lock> X<file, lock> X<flock>
552 Perl's builtin flock() function (see L<perlfunc> for details) will call
553 flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
554 later), and lockf(3) if neither of the two previous system calls exists.
555 On some systems, it may even use a different form of native locking.
556 Here are some gotchas with Perl's flock():
562 Produces a fatal error if none of the three system calls (or their
563 close equivalent) exists.
567 lockf(3) does not provide shared locking, and requires that the
568 filehandle be open for writing (or appending, or read/writing).
572 Some versions of flock() can't lock files over a network (e.g. on NFS file
573 systems), so you'd need to force the use of fcntl(2) when you build Perl.
574 But even this is dubious at best. See the flock entry of L<perlfunc>
575 and the F<INSTALL> file in the source distribution for information on
576 building Perl to do this.
578 Two potentially non-obvious but traditional flock semantics are that
579 it waits indefinitely until the lock is granted, and that its locks are
580 I<merely advisory>. Such discretionary locks are more flexible, but
581 offer fewer guarantees. This means that files locked with flock() may
582 be modified by programs that do not also use flock(). Cars that stop
583 for red lights get on well with each other, but not with cars that don't
584 stop for red lights. See the perlport manpage, your port's specific
585 documentation, or your system-specific local manpages for details. It's
586 best to assume traditional behavior if you're writing portable programs.
587 (If you're not, you should as always feel perfectly free to write
588 for your own system's idiosyncrasies (sometimes called "features").
589 Slavish adherence to portability concerns shouldn't get in the way of
590 your getting your job done.)
592 For more information on file locking, see also
593 L<perlopentut/"File Locking"> if you have it (new for 5.6).
597 =head2 Why can't I just open(FH, "E<gt>file.lock")?
598 X<lock, lockfile race condition>
600 A common bit of code B<NOT TO USE> is this:
602 sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
603 open(LCK, "> file.lock"); # THIS BROKEN CODE
605 This is a classic race condition: you take two steps to do something
606 which must be done in one. That's why computer hardware provides an
607 atomic test-and-set instruction. In theory, this "ought" to work:
609 sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
610 or die "can't open file.lock: $!";
612 except that lamentably, file creation (and deletion) is not atomic
613 over NFS, so this won't work (at least, not every time) over the net.
614 Various schemes involving link() have been suggested, but
615 these tend to involve busy-wait, which is also subdesirable.
617 =head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
618 X<counter> X<file, counter>
620 Didn't anyone ever tell you web-page hit counters were useless?
621 They don't count number of hits, they're a waste of time, and they serve
622 only to stroke the writer's vanity. It's better to pick a random number;
623 they're more realistic.
625 Anyway, this is what you can do if you can't help yourself.
627 use Fcntl qw(:DEFAULT :flock);
628 sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
629 flock(FH, LOCK_EX) or die "can't flock numfile: $!";
631 seek(FH, 0, 0) or die "can't rewind numfile: $!";
632 truncate(FH, 0) or die "can't truncate numfile: $!";
633 (print FH $num+1, "\n") or die "can't write numfile: $!";
634 close FH or die "can't close numfile: $!";
636 Here's a much better web-page hit counter:
638 $hits = int( (time() - 850_000_000) / rand(1_000) );
640 If the count doesn't impress your friends, then the code might. :-)
642 =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?
643 X<append> X<file, append>
645 If you are on a system that correctly implements flock() and you use the
646 example appending code from "perldoc -f flock" everything will be OK
647 even if the OS you are on doesn't implement append mode correctly (if
648 such a system exists.) So if you are happy to restrict yourself to OSs
649 that implement flock() (and that's not really much of a restriction)
650 then that is what you should do.
652 If you know you are only going to use a system that does correctly
653 implement appending (i.e. not Win32) then you can omit the seek() from
656 If you know you are only writing code to run on an OS and filesystem that
657 does implement append mode correctly (a local filesystem on a modern
658 Unix for example), and you keep the file in block-buffered mode and you
659 write less than one buffer-full of output between each manual flushing
660 of the buffer then each bufferload is almost guaranteed to be written to
661 the end of the file in one chunk without getting intermingled with
662 anyone else's output. You can also use the syswrite() function which is
663 simply a wrapper around your systems write(2) system call.
665 There is still a small theoretical chance that a signal will interrupt
666 the system level write() operation before completion. There is also a
667 possibility that some STDIO implementations may call multiple system
668 level write()s even if the buffer was empty to start. There may be some
669 systems where this probability is reduced to zero.
671 =head2 How do I randomly update a binary file?
672 X<file, binary patch>
674 If you're just trying to patch a binary, in many cases something as
675 simple as this works:
677 perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
679 However, if you have fixed sized records, then you might do something more
682 $RECSIZE = 220; # size of record, in bytes
683 $recno = 37; # which record to update
684 open(FH, "+<somewhere") || die "can't update somewhere: $!";
685 seek(FH, $recno * $RECSIZE, 0);
686 read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
688 seek(FH, -$RECSIZE, 1);
692 Locking and error checking are left as an exercise for the reader.
693 Don't forget them or you'll be quite sorry.
695 =head2 How do I get a file's timestamp in perl?
696 X<timestamp> X<file, timestamp>
698 If you want to retrieve the time at which the file was last
699 read, written, or had its meta-data (owner, etc) changed,
700 you use the B<-A>, B<-M>, or B<-C> file test operations as
701 documented in L<perlfunc>. These retrieve the age of the
702 file (measured against the start-time of your program) in
703 days as a floating point number. Some platforms may not have
704 all of these times. See L<perlport> for details. To
705 retrieve the "raw" time in seconds since the epoch, you
706 would call the stat function, then use localtime(),
707 gmtime(), or POSIX::strftime() to convert this into
712 $write_secs = (stat($file))[9];
713 printf "file %s updated at %s\n", $file,
714 scalar localtime($write_secs);
716 If you prefer something more legible, use the File::stat module
717 (part of the standard distribution in version 5.004 and later):
719 # error checking left as an exercise for reader.
722 $date_string = ctime(stat($file)->mtime);
723 print "file $file updated at $date_string\n";
725 The POSIX::strftime() approach has the benefit of being,
726 in theory, independent of the current locale. See L<perllocale>
729 =head2 How do I set a file's timestamp in perl?
730 X<timestamp> X<file, timestamp>
732 You use the utime() function documented in L<perlfunc/utime>.
733 By way of example, here's a little program that copies the
734 read and write times from its first argument to all the rest
738 die "usage: cptimes timestamp_file other_files ...\n";
741 ($atime, $mtime) = (stat($timestamp))[8,9];
742 utime $atime, $mtime, @ARGV;
744 Error checking is, as usual, left as an exercise for the reader.
746 The perldoc for utime also has an example that has the same
747 effect as touch(1) on files that I<already exist>.
749 Certain file systems have a limited ability to store the times
750 on a file at the expected level of precision. For example, the
751 FAT and HPFS filesystem are unable to create dates on files with
752 a finer granularity than two seconds. This is a limitation of
753 the filesystems, not of utime().
755 =head2 How do I print to more than one file at once?
756 X<print, to multiple files>
758 To connect one filehandle to several output filehandles,
759 you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
761 If you only have to do this once, you can print individually
764 for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
766 =head2 How can I read in an entire file all at once?
767 X<slurp> X<file, slurping>
769 You can use the File::Slurp module to do it in one step.
773 $all_of_it = read_file($filename); # entire file in scalar
774 @all_lines = read_file($filename); # one line perl element
776 The customary Perl approach for processing all the lines in a file is to
777 do so one line at a time:
779 open (INPUT, $file) || die "can't open $file: $!";
782 # do something with $_
784 close(INPUT) || die "can't close $file: $!";
786 This is tremendously more efficient than reading the entire file into
787 memory as an array of lines and then processing it one element at a time,
788 which is often--if not almost always--the wrong approach. Whenever
789 you see someone do this:
793 you should think long and hard about why you need everything loaded at
794 once. It's just not a scalable solution. You might also find it more
795 fun to use the standard Tie::File module, or the DB_File module's
796 $DB_RECNO bindings, which allow you to tie an array to a file so that
797 accessing an element the array actually accesses the corresponding
800 You can read the entire filehandle contents into a scalar.
804 open (INPUT, $file) || die "can't open $file: $!";
808 That temporarily undefs your record separator, and will automatically
809 close the file at block exit. If the file is already open, just use this:
811 $var = do { local $/; <INPUT> };
813 For ordinary files you can also use the read function.
815 read( INPUT, $var, -s INPUT );
817 The third argument tests the byte size of the data on the INPUT filehandle
818 and reads that many bytes into the buffer $var.
820 =head2 How can I read in a file by paragraphs?
821 X<file, reading by paragraphs>
823 Use the C<$/> variable (see L<perlvar> for details). You can either
824 set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
825 for instance, gets treated as two paragraphs and not three), or
826 C<"\n\n"> to accept empty paragraphs.
828 Note that a blank line must have no blanks in it. Thus
829 S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
831 =head2 How can I read a single character from a file? From the keyboard?
832 X<getc> X<file, reading one character at a time>
834 You can use the builtin C<getc()> function for most filehandles, but
835 it won't (easily) work on a terminal device. For STDIN, either use
836 the Term::ReadKey module from CPAN or use the sample code in
839 If your system supports the portable operating system programming
840 interface (POSIX), you can use the following code, which you'll note
841 turns off echo processing as well.
855 use POSIX qw(:termios_h);
857 my ($term, $oterm, $echo, $noecho, $fd_stdin);
859 $fd_stdin = fileno(STDIN);
861 $term = POSIX::Termios->new();
862 $term->getattr($fd_stdin);
863 $oterm = $term->getlflag();
865 $echo = ECHO | ECHOK | ICANON;
866 $noecho = $oterm & ~$echo;
869 $term->setlflag($noecho);
870 $term->setcc(VTIME, 1);
871 $term->setattr($fd_stdin, TCSANOW);
875 $term->setlflag($oterm);
876 $term->setcc(VTIME, 0);
877 $term->setattr($fd_stdin, TCSANOW);
883 sysread(STDIN, $key, 1);
892 The Term::ReadKey module from CPAN may be easier to use. Recent versions
893 include also support for non-portable systems as well.
896 open(TTY, "</dev/tty");
897 print "Gimme a char: ";
899 $key = ReadKey 0, *TTY;
901 printf "\nYou said %s, char number %03d\n",
904 =head2 How can I tell whether there's a character waiting on a filehandle?
906 The very first thing you should do is look into getting the Term::ReadKey
907 extension from CPAN. As we mentioned earlier, it now even has limited
908 support for non-portable (read: not open systems, closed, proprietary,
909 not POSIX, not Unix, etc) systems.
911 You should also check out the Frequently Asked Questions list in
912 comp.unix.* for things like this: the answer is essentially the same.
913 It's very system dependent. Here's one solution that works on BSD
918 vec($rin, fileno(STDIN), 1) = 1;
919 return $nfd = select($rin,undef,undef,0);
922 If you want to find out how many characters are waiting, there's
923 also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
924 comes with Perl tries to convert C include files to Perl code, which
925 can be C<require>d. FIONREAD ends up defined as a function in the
926 I<sys/ioctl.ph> file:
928 require 'sys/ioctl.ph';
930 $size = pack("L", 0);
931 ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
932 $size = unpack("L", $size);
934 If I<h2ph> wasn't installed or doesn't work for you, you can
935 I<grep> the include files by hand:
937 % grep FIONREAD /usr/include/*/*
938 /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
940 Or write a small C program using the editor of champions:
943 #include <sys/ioctl.h>
945 printf("%#08x\n", FIONREAD);
948 % cc -o fionread fionread.c
952 And then hard code it, leaving porting as an exercise to your successor.
954 $FIONREAD = 0x4004667f; # XXX: opsys dependent
956 $size = pack("L", 0);
957 ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
958 $size = unpack("L", $size);
960 FIONREAD requires a filehandle connected to a stream, meaning that sockets,
961 pipes, and tty devices work, but I<not> files.
963 =head2 How do I do a C<tail -f> in perl?
964 X<tail> X<IO::Handle> X<File::Tail> X<clearerr>
970 The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
971 but it does clear the end-of-file condition on the handle, so that the
972 next C<< <GWFILE> >> makes Perl try again to read something.
974 If that doesn't work (it relies on features of your stdio implementation),
975 then you need something more like this:
978 for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
979 # search for some stuff and put it into files
982 seek(GWFILE, $curpos, 0); # seek to where we had been
985 If this still doesn't work, look into the C<clearerr> method
986 from C<IO::Handle>, which resets the error and end-of-file states
989 There's also a C<File::Tail> module from CPAN.
991 =head2 How do I dup() a filehandle in Perl?
994 If you check L<perlfunc/open>, you'll see that several of the ways
995 to call open() should do the trick. For example:
997 open(LOG, ">>/foo/logfile");
998 open(STDERR, ">&LOG");
1000 Or even with a literal numeric descriptor:
1002 $fd = $ENV{MHCONTEXTFD};
1003 open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
1005 Note that "<&STDIN" makes a copy, but "<&=STDIN" make
1006 an alias. That means if you close an aliased handle, all
1007 aliases become inaccessible. This is not true with
1010 Error checking, as always, has been left as an exercise for the reader.
1012 =head2 How do I close a file descriptor by number?
1013 X<file, closing file descriptors>
1015 This should rarely be necessary, as the Perl close() function is to be
1016 used for things that Perl opened itself, even if it was a dup of a
1017 numeric descriptor as with MHCONTEXT above. But if you really have
1018 to, you may be able to do this:
1020 require 'sys/syscall.ph';
1021 $rc = syscall(&SYS_close, $fd + 0); # must force numeric
1022 die "can't sysclose $fd: $!" unless $rc == -1;
1024 Or, just use the fdopen(3S) feature of open():
1028 open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
1032 =head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
1033 X<filename, DOS issues>
1035 Whoops! You just put a tab and a formfeed into that filename!
1036 Remember that within double quoted strings ("like\this"), the
1037 backslash is an escape character. The full list of these is in
1038 L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
1039 have a file called "c:(tab)emp(formfeed)oo" or
1040 "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
1042 Either single-quote your strings, or (preferably) use forward slashes.
1043 Since all DOS and Windows versions since something like MS-DOS 2.0 or so
1044 have treated C</> and C<\> the same in a path, you might as well use the
1045 one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++,
1046 awk, Tcl, Java, or Python, just to mention a few. POSIX paths
1047 are more portable, too.
1049 =head2 Why doesn't glob("*.*") get all the files?
1052 Because even on non-Unix ports, Perl's glob function follows standard
1053 Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
1054 files. This makes glob() portable even to legacy systems. Your
1055 port may include proprietary globbing functions as well. Check its
1056 documentation for details.
1058 =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
1060 This is elaborately and painstakingly described in the
1061 F<file-dir-perms> article in the "Far More Than You Ever Wanted To
1062 Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
1064 The executive summary: learn how your filesystem works. The
1065 permissions on a file say what can happen to the data in that file.
1066 The permissions on a directory say what can happen to the list of
1067 files in that directory. If you delete a file, you're removing its
1068 name from the directory (so the operation depends on the permissions
1069 of the directory, not of the file). If you try to write to the file,
1070 the permissions of the file govern whether you're allowed to.
1072 =head2 How do I select a random line from a file?
1073 X<file, selecting a random line>
1075 Here's an algorithm from the Camel Book:
1078 rand($.) < 1 && ($line = $_) while <>;
1080 This has a significant advantage in space over reading the whole file
1081 in. You can find a proof of this method in I<The Art of Computer
1082 Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth.
1084 You can use the File::Random module which provides a function
1087 use File::Random qw/random_line/;
1088 my $line = random_line($filename);
1090 Another way is to use the Tie::File module, which treats the entire
1091 file as an array. Simply access a random array element.
1093 =head2 Why do I get weird spaces when I print an array of lines?
1099 joins together the elements of C<@lines> with a space between them.
1100 If C<@lines> were C<("little", "fluffy", "clouds")> then the above
1101 statement would print
1103 little fluffy clouds
1105 but if each element of C<@lines> was a line of text, ending a newline
1106 character C<("little\n", "fluffy\n", "clouds\n")> then it would print:
1112 If your array contains lines, just print them:
1118 Revision: $Revision: 6019 $
1120 Date: $Date: 2006-05-04 19:04:31 +0200 (jeu, 04 mai 2006) $
1122 See L<perlfaq> for source control details and availability.
1124 =head1 AUTHOR AND COPYRIGHT
1126 Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
1127 other authors as noted. All rights reserved.
1129 This documentation is free; you can redistribute it and/or modify it
1130 under the same terms as Perl itself.
1132 Irrespective of its distribution, all code examples here are in the public
1133 domain. You are permitted and encouraged to use this code and any
1134 derivatives thereof in your own programs for fun or for profit as you
1135 see fit. A simple comment in the code giving credit to the FAQ would
1136 be courteous but is not required.