3 perlfaq5 - Files and Formats ($Revision: 1.30 $, $Date: 2003/11/23 08:07:46 $)
7 This section deals with I/O and the "f" issues: filehandles, flushing,
10 =head2 How do I flush/unbuffer an output filehandle? Why must I do this?
12 Perl does not support truly unbuffered output (except
13 insofar as you can C<syswrite(OUT, $char, 1)>), although it
14 does support is "command buffering", in which a physical
15 write is performed after every output command.
17 The C standard I/O library (stdio) normally buffers
18 characters sent to devices so that there isn't a system call
19 for each byte. In most stdio implementations, the type of
20 output buffering and the size of the buffer varies according
21 to the type of device. Perl's print() and write() functions
22 normally buffer output, while syswrite() bypasses buffering
25 If you want your output to be sent immediately when you
26 execute print() or write() (for instance, for some network
27 protocols), you must set the handle's autoflush flag. This
28 flag is the Perl variable $| and when it is set to a true
29 value, Perl will flush the handle's buffer after each
30 print() or write(). Setting $| affects buffering only for
31 the currently selected default file handle. You choose this
32 handle with the one argument select() call (see
33 L<perlvar/$E<verbar>> and L<perlfunc/select>).
35 Use select() to choose the desired handle, then set its
36 per-filehandle variables.
38 $old_fh = select(OUTPUT_HANDLE);
42 Some idioms can handle this in a single statement:
44 select((select(OUTPUT_HANDLE), $| = 1)[0]);
46 $| = 1, select $_ for select OUTPUT_HANDLE;
48 Some modules offer object-oriented access to handles and their
49 variables, although they may be overkill if this is the only
50 thing you do with them. You can use IO::Handle:
53 open(DEV, ">/dev/printer"); # but is this?
58 use IO::Socket; # this one is kinda a pipe?
59 my $sock = IO::Socket::INET->new( 'www.example.com:80' ) ;
63 =head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
65 Use the Tie::File module, which is included in the standard
66 distribution since Perl 5.8.0.
68 =head2 How do I count the number of lines in a file?
70 One fairly efficient way is to count newlines in the file. The
71 following program uses a feature of tr///, as documented in L<perlop>.
72 If your text file doesn't end with a newline, then it's not really a
73 proper text file, so this may report one fewer line than you expect.
76 open(FILE, $filename) or die "Can't open `$filename': $!";
77 while (sysread FILE, $buffer, 4096) {
78 $lines += ($buffer =~ tr/\n//);
82 This assumes no funny games with newline translations.
84 =head2 How can I use Perl's C<-i> option from within a program?
86 C<-i> sets the value of Perl's C<$^I> variable, which in turn affects
87 the behavior of C<< <> >>; see L<perlrun> for more details. By
88 modifying the appropriate variables directly, you can get the same
89 behavior within a larger program. For example:
93 local($^I, @ARGV) = ('.orig', glob("*.c"));
96 print "This line should appear at the top of each file\n";
98 s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
100 close ARGV if eof; # Reset $.
103 # $^I and @ARGV return to their old values here
105 This block modifies all the C<.c> files in the current directory,
106 leaving a backup of the original data from each file in a new
109 =head2 How do I make a temporary file name?
111 Use the File::Temp module, see L<File::Temp> for more information.
113 use File::Temp qw/ tempfile tempdir /;
115 $dir = tempdir( CLEANUP => 1 );
116 ($fh, $filename) = tempfile( DIR => $dir );
118 # or if you don't need to know the filename
120 $fh = tempfile( DIR => $dir );
122 The File::Temp has been a standard module since Perl 5.6.1. If you
123 don't have a modern enough Perl installed, use the C<new_tmpfile>
124 class method from the IO::File module to get a filehandle opened for
125 reading and writing. Use it if you don't need to know the file's name:
128 $fh = IO::File->new_tmpfile()
129 or die "Unable to make new temporary file: $!";
131 If you're committed to creating a temporary file by hand, use the
132 process ID and/or the current time-value. If you need to have many
133 temporary files in one process, use a counter:
137 my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
138 my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
142 until (defined(fileno(FH)) || $count++ > 100) {
143 $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
144 sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
146 if (defined(fileno(FH))
147 return (*FH, $base_name);
154 =head2 How can I manipulate fixed-record-length files?
156 The most efficient way is using L<pack()|perlfunc/"pack"> and
157 L<unpack()|perlfunc/"unpack">. This is faster than using
158 L<substr()|perlfunc/"substr"> when taking many, many strings. It is
159 slower for just a few.
161 Here is a sample chunk of code to break up and put back together again
162 some fixed-format input lines, in this case from the output of a normal,
166 # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
167 my $PS_T = 'A6 A4 A7 A5 A*';
168 open my $ps, '-|', 'ps';
170 my @fields = qw( pid tt stat time command );
173 @process{@fields} = unpack($PS_T, $_);
174 for my $field ( @fields ) {
175 print "$field: <$process{$field}>\n";
177 print 'line=', pack($PS_T, @process{@fields} ), "\n";
180 We've used a hash slice in order to easily handle the fields of each row.
181 Storing the keys in an array means it's easy to operate on them as a
182 group or loop over them with for. It also avoids polluting the program
183 with global variables and using symbolic references.
185 =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?
187 As of perl5.6, open() autovivifies file and directory handles
188 as references if you pass it an uninitialized scalar variable.
189 You can then pass these references just like any other scalar,
190 and use them in the place of named handles.
192 open my $fh, $file_name;
194 open local $fh, $file_name;
196 print $fh "Hello World!\n";
200 Before perl5.6, you had to deal with various typeglob idioms
201 which you may see in older code.
203 open FILE, "> $filename";
204 process_typeglob( *FILE );
205 process_reference( \*FILE );
207 sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
208 sub process_reference { local $fh = shift; print $fh "Reference!" }
210 If you want to create many anonymous handles, you should
211 check out the Symbol or IO::Handle modules.
213 =head2 How can I use a filehandle indirectly?
215 An indirect filehandle is using something other than a symbol
216 in a place that a filehandle is expected. Here are ways
217 to get indirect filehandles:
219 $fh = SOME_FH; # bareword is strict-subs hostile
220 $fh = "SOME_FH"; # strict-refs hostile; same package only
221 $fh = *SOME_FH; # typeglob
222 $fh = \*SOME_FH; # ref to typeglob (bless-able)
223 $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
225 Or, you can use the C<new> method from one of the IO::* modules to
226 create an anonymous filehandle, store that in a scalar variable,
227 and use it as though it were a normal filehandle.
229 use IO::Handle; # 5.004 or higher
230 $fh = IO::Handle->new();
232 Then use any of those as you would a normal filehandle. Anywhere that
233 Perl is expecting a filehandle, an indirect filehandle may be used
234 instead. An indirect filehandle is just a scalar variable that contains
235 a filehandle. Functions like C<print>, C<open>, C<seek>, or
236 the C<< <FH> >> diamond operator will accept either a named filehandle
237 or a scalar variable containing one:
239 ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
240 print $ofh "Type it: ";
242 print $efh "What was that: $got";
244 If you're passing a filehandle to a function, you can write
245 the function in two ways:
249 print $fh "Sending to indirect filehandle\n";
252 Or it can localize a typeglob and use the filehandle directly:
256 print FH "Sending to localized filehandle\n";
259 Both styles work with either objects or typeglobs of real filehandles.
260 (They might also work with strings under some circumstances, but this
266 In the examples above, we assigned the filehandle to a scalar variable
267 before using it. That is because only simple scalar variables, not
268 expressions or subscripts of hashes or arrays, can be used with
269 built-ins like C<print>, C<printf>, or the diamond operator. Using
270 something other than a simple scalar variable as a filehandle is
271 illegal and won't even compile:
273 @fd = (*STDIN, *STDOUT, *STDERR);
274 print $fd[1] "Type it: "; # WRONG
275 $got = <$fd[0]> # WRONG
276 print $fd[2] "What was that: $got"; # WRONG
278 With C<print> and C<printf>, you get around this by using a block and
279 an expression where you would place the filehandle:
281 print { $fd[1] } "funny stuff\n";
282 printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
283 # Pity the poor deadbeef.
285 That block is a proper block like any other, so you can put more
286 complicated code there. This sends the message out to one of two places:
289 print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
290 print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
292 This approach of treating C<print> and C<printf> like object methods
293 calls doesn't work for the diamond operator. That's because it's a
294 real operator, not just a function with a comma-less argument. Assuming
295 you've been storing typeglobs in your structure as we did above, you
296 can use the built-in function named C<readline> to read a record just
297 as C<< <> >> does. Given the initialization shown above for @fd, this
298 would work, but only because readline() requires a typeglob. It doesn't
299 work with objects or strings, which might be a bug we haven't fixed yet.
301 $got = readline($fd[0]);
303 Let it be noted that the flakiness of indirect filehandles is not
304 related to whether they're strings, typeglobs, objects, or anything else.
305 It's the syntax of the fundamental operators. Playing the object
306 game doesn't help you at all here.
308 =head2 How can I set up a footer format to be used with write()?
310 There's no builtin way to do this, but L<perlform> has a couple of
311 techniques to make it possible for the intrepid hacker.
313 =head2 How can I write() into a string?
315 See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
317 =head2 How can I output my numbers with commas added?
319 This subroutine will add commas to your number:
323 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
327 This regex from Benjamin Goldberg will add commas to numbers:
329 s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
331 It is easier to see with comments:
334 ^[-+]? # beginning of number.
335 \d{1,3}? # first digits before first comma
336 (?= # followed by, (but not included in the match) :
337 (?>(?:\d{3})+) # some positive multiple of three digits.
338 (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever.
341 \G\d{3} # after the last group, get three digits
342 (?=\d) # but they have to have more digits after them.
345 =head2 How can I translate tildes (~) in a filename?
347 Use the <> (glob()) operator, documented in L<perlfunc>. Older
348 versions of Perl require that you have a shell installed that groks
349 tildes. Recent perl versions have this feature built in. The
350 File::KGlob module (available from CPAN) gives more portable glob
353 Within Perl, you may use this directly:
356 ^ ~ # find a leading tilde
358 [^/] # a non-slash character
359 * # repeated 0 or more times (0 means me)
364 : ( $ENV{HOME} || $ENV{LOGDIR} )
367 =head2 How come when I open a file read-write it wipes it out?
369 Because you're using something like this, which truncates the file and
370 I<then> gives you read-write access:
372 open(FH, "+> /path/name"); # WRONG (almost always)
374 Whoops. You should instead use this, which will fail if the file
377 open(FH, "+< /path/name"); # open for update
379 Using ">" always clobbers or creates. Using "<" never does
380 either. The "+" doesn't change this.
382 Here are examples of many kinds of file opens. Those using sysopen()
387 To open file for reading:
389 open(FH, "< $path") || die $!;
390 sysopen(FH, $path, O_RDONLY) || die $!;
392 To open file for writing, create new file if needed or else truncate old file:
394 open(FH, "> $path") || die $!;
395 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
396 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
398 To open file for writing, create new file, file must not exist:
400 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
401 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
403 To open file for appending, create if necessary:
405 open(FH, ">> $path") || die $!;
406 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
407 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
409 To open file for appending, file must exist:
411 sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
413 To open file for update, file must exist:
415 open(FH, "+< $path") || die $!;
416 sysopen(FH, $path, O_RDWR) || die $!;
418 To open file for update, create file if necessary:
420 sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
421 sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
423 To open file for update, file must not exist:
425 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
426 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
428 To open a file without blocking, creating if necessary:
430 sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT)
431 or die "can't open /tmp/somefile: $!":
433 Be warned that neither creation nor deletion of files is guaranteed to
434 be an atomic operation over NFS. That is, two processes might both
435 successfully create or unlink the same file! Therefore O_EXCL
436 isn't as exclusive as you might wish.
438 See also the new L<perlopentut> if you have it (new for 5.6).
440 =head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
442 The C<< <> >> operator performs a globbing operation (see above).
443 In Perl versions earlier than v5.6.0, the internal glob() operator forks
444 csh(1) to do the actual glob expansion, but
445 csh can't handle more than 127 items and so gives the error message
446 C<Argument list too long>. People who installed tcsh as csh won't
447 have this problem, but their users may be surprised by it.
449 To get around this, either upgrade to Perl v5.6.0 or later, do the glob
450 yourself with readdir() and patterns, or use a module like File::KGlob,
451 one that doesn't use the shell to do globbing.
453 =head2 Is there a leak/bug in glob()?
455 Due to the current implementation on some operating systems, when you
456 use the glob() function or its angle-bracket alias in a scalar
457 context, you may cause a memory leak and/or unpredictable behavior. It's
458 best therefore to use glob() only in list context.
460 =head2 How can I open a file with a leading ">" or trailing blanks?
462 Normally perl ignores trailing blanks in filenames, and interprets
463 certain leading characters (or a trailing "|") to mean something
466 The three argument form of open() lets you specify the mode
467 separately from the filename. The open() function treats
468 special mode characters and whitespace in the filename as
471 open FILE, "<", " file "; # filename is " file "
472 open FILE, ">", ">file"; # filename is ">file"
474 It may be a lot clearer to use sysopen(), though:
477 $badpath = "<<<something really wicked ";
478 sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
479 or die "can't open $badpath: $!";
481 =head2 How can I reliably rename a file?
483 If your operating system supports a proper mv(1) utility or its
484 functional equivalent, this works:
486 rename($old, $new) or system("mv", $old, $new);
488 It may be more portable to use the File::Copy module instead.
489 You just copy to the new file to the new name (checking return
490 values), then delete the old one. This isn't really the same
491 semantically as a rename(), which preserves meta-information like
492 permissions, timestamps, inode info, etc.
494 Newer versions of File::Copy export a move() function.
496 =head2 How can I lock a file?
498 Perl's builtin flock() function (see L<perlfunc> for details) will call
499 flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
500 later), and lockf(3) if neither of the two previous system calls exists.
501 On some systems, it may even use a different form of native locking.
502 Here are some gotchas with Perl's flock():
508 Produces a fatal error if none of the three system calls (or their
509 close equivalent) exists.
513 lockf(3) does not provide shared locking, and requires that the
514 filehandle be open for writing (or appending, or read/writing).
518 Some versions of flock() can't lock files over a network (e.g. on NFS file
519 systems), so you'd need to force the use of fcntl(2) when you build Perl.
520 But even this is dubious at best. See the flock entry of L<perlfunc>
521 and the F<INSTALL> file in the source distribution for information on
522 building Perl to do this.
524 Two potentially non-obvious but traditional flock semantics are that
525 it waits indefinitely until the lock is granted, and that its locks are
526 I<merely advisory>. Such discretionary locks are more flexible, but
527 offer fewer guarantees. This means that files locked with flock() may
528 be modified by programs that do not also use flock(). Cars that stop
529 for red lights get on well with each other, but not with cars that don't
530 stop for red lights. See the perlport manpage, your port's specific
531 documentation, or your system-specific local manpages for details. It's
532 best to assume traditional behavior if you're writing portable programs.
533 (If you're not, you should as always feel perfectly free to write
534 for your own system's idiosyncrasies (sometimes called "features").
535 Slavish adherence to portability concerns shouldn't get in the way of
536 your getting your job done.)
538 For more information on file locking, see also
539 L<perlopentut/"File Locking"> if you have it (new for 5.6).
543 =head2 Why can't I just open(FH, "E<gt>file.lock")?
545 A common bit of code B<NOT TO USE> is this:
547 sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
548 open(LCK, "> file.lock"); # THIS BROKEN CODE
550 This is a classic race condition: you take two steps to do something
551 which must be done in one. That's why computer hardware provides an
552 atomic test-and-set instruction. In theory, this "ought" to work:
554 sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
555 or die "can't open file.lock: $!";
557 except that lamentably, file creation (and deletion) is not atomic
558 over NFS, so this won't work (at least, not every time) over the net.
559 Various schemes involving link() have been suggested, but
560 these tend to involve busy-wait, which is also subdesirable.
562 =head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
564 Didn't anyone ever tell you web-page hit counters were useless?
565 They don't count number of hits, they're a waste of time, and they serve
566 only to stroke the writer's vanity. It's better to pick a random number;
567 they're more realistic.
569 Anyway, this is what you can do if you can't help yourself.
571 use Fcntl qw(:DEFAULT :flock);
572 sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
573 flock(FH, LOCK_EX) or die "can't flock numfile: $!";
575 seek(FH, 0, 0) or die "can't rewind numfile: $!";
576 truncate(FH, 0) or die "can't truncate numfile: $!";
577 (print FH $num+1, "\n") or die "can't write numfile: $!";
578 close FH or die "can't close numfile: $!";
580 Here's a much better web-page hit counter:
582 $hits = int( (time() - 850_000_000) / rand(1_000) );
584 If the count doesn't impress your friends, then the code might. :-)
586 =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?
588 If you are on a system that correctly implements flock() and you use the
589 example appending code from "perldoc -f flock" everything will be OK
590 even if the OS you are on doesn't implement append mode correctly (if
591 such a system exists.) So if you are happy to restrict yourself to OSs
592 that implement flock() (and that's not really much of a restriction)
593 then that is what you should do.
595 If you know you are only going to use a system that does correctly
596 implement appending (i.e. not Win32) then you can omit the seek() from
599 If you know you are only writing code to run on an OS and filesystem that
600 does implement append mode correctly (a local filesystem on a modern
601 Unix for example), and you keep the file in block-buffered mode and you
602 write less than one buffer-full of output between each manual flushing
603 of the buffer then each bufferload is almost guaranteed to be written to
604 the end of the file in one chunk without getting intermingled with
605 anyone else's output. You can also use the syswrite() function which is
606 simply a wrapper around your systems write(2) system call.
608 There is still a small theoretical chance that a signal will interrupt
609 the system level write() operation before completion. There is also a
610 possibility that some STDIO implementations may call multiple system
611 level write()s even if the buffer was empty to start. There may be some
612 systems where this probability is reduced to zero.
614 =head2 How do I randomly update a binary file?
616 If you're just trying to patch a binary, in many cases something as
617 simple as this works:
619 perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
621 However, if you have fixed sized records, then you might do something more
624 $RECSIZE = 220; # size of record, in bytes
625 $recno = 37; # which record to update
626 open(FH, "+<somewhere") || die "can't update somewhere: $!";
627 seek(FH, $recno * $RECSIZE, 0);
628 read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
630 seek(FH, -$RECSIZE, 1);
634 Locking and error checking are left as an exercise for the reader.
635 Don't forget them or you'll be quite sorry.
637 =head2 How do I get a file's timestamp in perl?
639 If you want to retrieve the time at which the file was last
640 read, written, or had its meta-data (owner, etc) changed,
641 you use the B<-M>, B<-A>, or B<-C> file test operations as
642 documented in L<perlfunc>. These retrieve the age of the
643 file (measured against the start-time of your program) in
644 days as a floating point number. Some platforms may not have
645 all of these times. See L<perlport> for details. To
646 retrieve the "raw" time in seconds since the epoch, you
647 would call the stat function, then use localtime(),
648 gmtime(), or POSIX::strftime() to convert this into
653 $write_secs = (stat($file))[9];
654 printf "file %s updated at %s\n", $file,
655 scalar localtime($write_secs);
657 If you prefer something more legible, use the File::stat module
658 (part of the standard distribution in version 5.004 and later):
660 # error checking left as an exercise for reader.
663 $date_string = ctime(stat($file)->mtime);
664 print "file $file updated at $date_string\n";
666 The POSIX::strftime() approach has the benefit of being,
667 in theory, independent of the current locale. See L<perllocale>
670 =head2 How do I set a file's timestamp in perl?
672 You use the utime() function documented in L<perlfunc/utime>.
673 By way of example, here's a little program that copies the
674 read and write times from its first argument to all the rest
678 die "usage: cptimes timestamp_file other_files ...\n";
681 ($atime, $mtime) = (stat($timestamp))[8,9];
682 utime $atime, $mtime, @ARGV;
684 Error checking is, as usual, left as an exercise for the reader.
686 Note that utime() currently doesn't work correctly with Win95/NT
687 ports. A bug has been reported. Check it carefully before using
688 utime() on those platforms.
690 =head2 How do I print to more than one file at once?
692 To connect one filehandle to several output filehandles,
693 you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
695 If you only have to do this once, you can print individually
698 for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
700 =head2 How can I read in an entire file all at once?
702 You can use the File::Slurp module to do it in one step.
706 $all_of_it = read_file($filename); # entire file in scalar
707 @all_lines = read_file($filename); # one line perl element
709 The customary Perl approach for processing all the lines in a file is to
710 do so one line at a time:
712 open (INPUT, $file) || die "can't open $file: $!";
715 # do something with $_
717 close(INPUT) || die "can't close $file: $!";
719 This is tremendously more efficient than reading the entire file into
720 memory as an array of lines and then processing it one element at a time,
721 which is often--if not almost always--the wrong approach. Whenever
722 you see someone do this:
726 you should think long and hard about why you need everything loaded at
727 once. It's just not a scalable solution. You might also find it more
728 fun to use the standard Tie::File module, or the DB_File module's
729 $DB_RECNO bindings, which allow you to tie an array to a file so that
730 accessing an element the array actually accesses the corresponding
733 You can read the entire filehandle contents into a scalar.
737 open (INPUT, $file) || die "can't open $file: $!";
741 That temporarily undefs your record separator, and will automatically
742 close the file at block exit. If the file is already open, just use this:
744 $var = do { local $/; <INPUT> };
746 For ordinary files you can also use the read function.
748 read( INPUT, $var, -s INPUT );
750 The third argument tests the byte size of the data on the INPUT filehandle
751 and reads that many bytes into the buffer $var.
753 =head2 How can I read in a file by paragraphs?
755 Use the C<$/> variable (see L<perlvar> for details). You can either
756 set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
757 for instance, gets treated as two paragraphs and not three), or
758 C<"\n\n"> to accept empty paragraphs.
760 Note that a blank line must have no blanks in it. Thus
761 S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
763 =head2 How can I read a single character from a file? From the keyboard?
765 You can use the builtin C<getc()> function for most filehandles, but
766 it won't (easily) work on a terminal device. For STDIN, either use
767 the Term::ReadKey module from CPAN or use the sample code in
770 If your system supports the portable operating system programming
771 interface (POSIX), you can use the following code, which you'll note
772 turns off echo processing as well.
786 use POSIX qw(:termios_h);
788 my ($term, $oterm, $echo, $noecho, $fd_stdin);
790 $fd_stdin = fileno(STDIN);
792 $term = POSIX::Termios->new();
793 $term->getattr($fd_stdin);
794 $oterm = $term->getlflag();
796 $echo = ECHO | ECHOK | ICANON;
797 $noecho = $oterm & ~$echo;
800 $term->setlflag($noecho);
801 $term->setcc(VTIME, 1);
802 $term->setattr($fd_stdin, TCSANOW);
806 $term->setlflag($oterm);
807 $term->setcc(VTIME, 0);
808 $term->setattr($fd_stdin, TCSANOW);
814 sysread(STDIN, $key, 1);
823 The Term::ReadKey module from CPAN may be easier to use. Recent versions
824 include also support for non-portable systems as well.
827 open(TTY, "</dev/tty");
828 print "Gimme a char: ";
830 $key = ReadKey 0, *TTY;
832 printf "\nYou said %s, char number %03d\n",
835 =head2 How can I tell whether there's a character waiting on a filehandle?
837 The very first thing you should do is look into getting the Term::ReadKey
838 extension from CPAN. As we mentioned earlier, it now even has limited
839 support for non-portable (read: not open systems, closed, proprietary,
840 not POSIX, not Unix, etc) systems.
842 You should also check out the Frequently Asked Questions list in
843 comp.unix.* for things like this: the answer is essentially the same.
844 It's very system dependent. Here's one solution that works on BSD
849 vec($rin, fileno(STDIN), 1) = 1;
850 return $nfd = select($rin,undef,undef,0);
853 If you want to find out how many characters are waiting, there's
854 also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
855 comes with Perl tries to convert C include files to Perl code, which
856 can be C<require>d. FIONREAD ends up defined as a function in the
857 I<sys/ioctl.ph> file:
859 require 'sys/ioctl.ph';
861 $size = pack("L", 0);
862 ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
863 $size = unpack("L", $size);
865 If I<h2ph> wasn't installed or doesn't work for you, you can
866 I<grep> the include files by hand:
868 % grep FIONREAD /usr/include/*/*
869 /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
871 Or write a small C program using the editor of champions:
874 #include <sys/ioctl.h>
876 printf("%#08x\n", FIONREAD);
879 % cc -o fionread fionread.c
883 And then hard code it, leaving porting as an exercise to your successor.
885 $FIONREAD = 0x4004667f; # XXX: opsys dependent
887 $size = pack("L", 0);
888 ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
889 $size = unpack("L", $size);
891 FIONREAD requires a filehandle connected to a stream, meaning that sockets,
892 pipes, and tty devices work, but I<not> files.
894 =head2 How do I do a C<tail -f> in perl?
900 The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
901 but it does clear the end-of-file condition on the handle, so that the
902 next <GWFILE> makes Perl try again to read something.
904 If that doesn't work (it relies on features of your stdio implementation),
905 then you need something more like this:
908 for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
909 # search for some stuff and put it into files
912 seek(GWFILE, $curpos, 0); # seek to where we had been
915 If this still doesn't work, look into the POSIX module. POSIX defines
916 the clearerr() method, which can remove the end of file condition on a
917 filehandle. The method: read until end of file, clearerr(), read some
918 more. Lather, rinse, repeat.
920 There's also a File::Tail module from CPAN.
922 =head2 How do I dup() a filehandle in Perl?
924 If you check L<perlfunc/open>, you'll see that several of the ways
925 to call open() should do the trick. For example:
927 open(LOG, ">>/tmp/logfile");
928 open(STDERR, ">&LOG");
930 Or even with a literal numeric descriptor:
932 $fd = $ENV{MHCONTEXTFD};
933 open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
935 Note that "<&STDIN" makes a copy, but "<&=STDIN" make
936 an alias. That means if you close an aliased handle, all
937 aliases become inaccessible. This is not true with
940 Error checking, as always, has been left as an exercise for the reader.
942 =head2 How do I close a file descriptor by number?
944 This should rarely be necessary, as the Perl close() function is to be
945 used for things that Perl opened itself, even if it was a dup of a
946 numeric descriptor as with MHCONTEXT above. But if you really have
947 to, you may be able to do this:
949 require 'sys/syscall.ph';
950 $rc = syscall(&SYS_close, $fd + 0); # must force numeric
951 die "can't sysclose $fd: $!" unless $rc == -1;
953 Or, just use the fdopen(3S) feature of open():
957 open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
961 =head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
963 Whoops! You just put a tab and a formfeed into that filename!
964 Remember that within double quoted strings ("like\this"), the
965 backslash is an escape character. The full list of these is in
966 L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
967 have a file called "c:(tab)emp(formfeed)oo" or
968 "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
970 Either single-quote your strings, or (preferably) use forward slashes.
971 Since all DOS and Windows versions since something like MS-DOS 2.0 or so
972 have treated C</> and C<\> the same in a path, you might as well use the
973 one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++,
974 awk, Tcl, Java, or Python, just to mention a few. POSIX paths
975 are more portable, too.
977 =head2 Why doesn't glob("*.*") get all the files?
979 Because even on non-Unix ports, Perl's glob function follows standard
980 Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
981 files. This makes glob() portable even to legacy systems. Your
982 port may include proprietary globbing functions as well. Check its
983 documentation for details.
985 =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
987 This is elaborately and painstakingly described in the
988 F<file-dir-perms> article in the "Far More Than You Ever Wanted To
989 Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
991 The executive summary: learn how your filesystem works. The
992 permissions on a file say what can happen to the data in that file.
993 The permissions on a directory say what can happen to the list of
994 files in that directory. If you delete a file, you're removing its
995 name from the directory (so the operation depends on the permissions
996 of the directory, not of the file). If you try to write to the file,
997 the permissions of the file govern whether you're allowed to.
999 =head2 How do I select a random line from a file?
1001 Here's an algorithm from the Camel Book:
1004 rand($.) < 1 && ($line = $_) while <>;
1006 This has a significant advantage in space over reading the whole file
1007 in. You can find a proof of this method in I<The Art of Computer
1008 Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth.
1010 You can use the File::Random module which provides a function
1013 use File::Random qw/random_line/;
1014 my $line = random_line($filename);
1016 Another way is to use the Tie::File module, which treats the entire
1017 file as an array. Simply access a random array element.
1019 =head2 Why do I get weird spaces when I print an array of lines?
1025 joins together the elements of C<@lines> with a space between them.
1026 If C<@lines> were C<("little", "fluffy", "clouds")> then the above
1027 statement would print
1029 little fluffy clouds
1031 but if each element of C<@lines> was a line of text, ending a newline
1032 character C<("little\n", "fluffy\n", "clouds\n")> then it would print:
1038 If your array contains lines, just print them:
1042 =head1 AUTHOR AND COPYRIGHT
1044 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
1045 All rights reserved.
1047 This documentation is free; you can redistribute it and/or modify it
1048 under the same terms as Perl itself.
1050 Irrespective of its distribution, all code examples here are in the public
1051 domain. You are permitted and encouraged to use this code and any
1052 derivatives thereof in your own programs for fun or for profit as you
1053 see fit. A simple comment in the code giving credit to the FAQ would
1054 be courteous but is not required.