3 perlfaq5 - Files and Formats
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 (contributed by brian d foy)
15 You might like to read Mark Jason Dominus's "Suffering From Buffering"
16 at http://perl.plover.com/FAQs/Buffering.html .
18 Perl normally buffers output so it doesn't make a system call for every
19 bit of output. By saving up output, it makes fewer expensive system calls.
20 For instance, in this little bit of code, you want to print a dot to the
21 screen for every line you process to watch the progress of your program.
22 Instead of seeing a dot for every line, Perl buffers the output and you
23 have a long wait before you see a row of 50 dots all at once:
25 # long wait, then row of dots all at once
28 print "\n" unless ++$count % 50;
30 #... expensive line processing operations
33 To get around this, you have to unbuffer the output filehandle, in this
34 case, C<STDOUT>. You can set the special variable C<$|> to a true value
35 (mnemonic: making your filehandles "piping hot"):
39 # dot shown immediately
42 print "\n" unless ++$count % 50;
44 #... expensive line processing operations
47 The C<$|> is one of the per-filehandle special variables, so each
48 filehandle has its own copy of its value. If you want to merge
49 standard output and standard error for instance, you have to unbuffer
50 each (although STDERR might be unbuffered by default):
53 my $previous_default = select(STDOUT); # save previous default
54 $|++; # autoflush STDOUT
56 $|++; # autoflush STDERR, to be sure
57 select($previous_default); # restore previous default
60 # now should alternate . and +
66 print STDOUT "\n" unless ++$count % 25;
69 Besides the C<$|> special variable, you can use C<binmode> to give
70 your filehandle a C<:unix> layer, which is unbuffered:
72 binmode( STDOUT, ":unix" );
77 print "\n" unless ++$count % 50;
80 For more information on output layers, see the entries for C<binmode>
81 and C<open> in L<perlfunc>, and the C<PerlIO> module documentation.
83 If you are using C<IO::Handle> or one of its subclasses, you can
84 call the C<autoflush> method to change the settings of the
88 open my( $io_fh ), ">", "output.txt";
91 The C<IO::Handle> objects also have a C<flush> method. You can flush
92 the buffer any time you want without auto-buffering
96 =head2 How do I change, delete, or insert a line in a file, or append to the beginning of a file?
99 (contributed by brian d foy)
101 The basic idea of inserting, changing, or deleting a line from a text
102 file involves reading and printing the file to the point you want to
103 make the change, making the change, then reading and printing the rest
104 of the file. Perl doesn't provide random access to lines (especially
105 since the record input separator, C<$/>, is mutable), although modules
106 such as C<Tie::File> can fake it.
108 A Perl program to do these tasks takes the basic form of opening a
109 file, printing its lines, then closing the file:
111 open my $in, '<', $file or die "Can't read old file: $!";
112 open my $out, '>', "$file.new" or die "Can't write new file: $!";
121 Within that basic form, add the parts that you need to insert, change,
124 To prepend lines to the beginning, print those lines before you enter
125 the loop that prints the existing lines.
127 open my $in, '<', $file or die "Can't read old file: $!";
128 open my $out, '>', "$file.new" or die "Can't write new file: $!";
130 print $out "# Add this line to the top\n"; # <--- HERE'S THE MAGIC
139 To change existing lines, insert the code to modify the lines inside
140 the C<while> loop. In this case, the code finds all lowercased
141 versions of "perl" and uppercases them. The happens for every line, so
142 be sure that you're supposed to do that on every line!
144 open my $in, '<', $file or die "Can't read old file: $!";
145 open my $out, '>', "$file.new" or die "Can't write new file: $!";
147 print $out "# Add this line to the top\n";
157 To change only a particular line, the input line number, C<$.>, is
158 useful. First read and print the lines up to the one you want to
159 change. Next, read the single line you want to change, change it, and
160 print it. After that, read the rest of the lines and print those:
162 while( <$in> ) # print the lines before the change
165 last if $. == 4; # line number before change
169 $line =~ s/\b(perl)\b/Perl/g;
172 while( <$in> ) # print the rest of the lines
177 To skip lines, use the looping controls. The C<next> in this example
178 skips comment lines, and the C<last> stops all processing once it
179 encounters either C<__END__> or C<__DATA__>.
183 next if /^\s+#/; # skip comment lines
184 last if /^__(END|DATA)__$/; # stop at end of code marker
188 Do the same sort of thing to delete a particular line by using C<next>
189 to skip the lines you don't want to show up in the output. This
190 example skips every fifth line:
198 If, for some odd reason, you really want to see the whole file at once
199 rather than processing line by line, you can slurp it in (as long as
200 you can fit the whole thing in memory!):
202 open my $in, '<', $file or die "Can't read old file: $!"
203 open my $out, '>', "$file.new" or die "Can't write new file: $!";
205 my @lines = do { local $/; <$in> }; # slurp!
211 Modules such as C<File::Slurp> and C<Tie::File> can help with that
212 too. If you can, however, avoid reading the entire file at once. Perl
213 won't give that memory back to the operating system until the process
216 You can also use Perl one-liners to modify a file in-place. The
217 following changes all 'Fred' to 'Barney' in F<inFile.txt>, overwriting
218 the file with the new contents. With the C<-p> switch, Perl wraps a
219 C<while> loop around the code you specify with C<-e>, and C<-i> turns
220 on in-place editing. The current line is in C<$_>. With C<-p>, Perl
221 automatically prints the value of C<$_> at the end of the loop. See
222 L<perlrun> for more details.
224 perl -pi -e 's/Fred/Barney/' inFile.txt
226 To make a backup of C<inFile.txt>, give C<-i> a file extension to add:
228 perl -pi.bak -e 's/Fred/Barney/' inFile.txt
230 To change only the fifth line, you can add a test checking C<$.>, the
231 input line number, then only perform the operation when the test
234 perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt
236 To add lines before a certain line, you can add a line (or lines!)
237 before Perl prints C<$_>:
239 perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt
241 You can even add a line to the beginning of a file, since the current
242 line prints at the end of the loop:
244 perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt
246 To insert a line after one already in the file, use the C<-n> switch.
247 It's just like C<-p> except that it doesn't print C<$_> at the end of
248 the loop, so you have to do that yourself. In this case, print C<$_>
249 first, then print the line that you want to add.
251 perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt
253 To delete lines, only print the ones that you want.
255 perl -ni -e 'print unless /d/' inFile.txt
259 perl -pi -e 'next unless /d/' inFile.txt
261 =head2 How do I count the number of lines in a file?
262 X<file, counting lines> X<lines> X<line>
264 One fairly efficient way is to count newlines in the file. The
265 following program uses a feature of tr///, as documented in L<perlop>.
266 If your text file doesn't end with a newline, then it's not really a
267 proper text file, so this may report one fewer line than you expect.
270 open(FILE, $filename) or die "Can't open `$filename': $!";
271 while (sysread FILE, $buffer, 4096) {
272 $lines += ($buffer =~ tr/\n//);
276 This assumes no funny games with newline translations.
278 =head2 How can I use Perl's C<-i> option from within a program?
281 C<-i> sets the value of Perl's C<$^I> variable, which in turn affects
282 the behavior of C<< <> >>; see L<perlrun> for more details. By
283 modifying the appropriate variables directly, you can get the same
284 behavior within a larger program. For example:
288 local($^I, @ARGV) = ('.orig', glob("*.c"));
291 print "This line should appear at the top of each file\n";
293 s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
295 close ARGV if eof; # Reset $.
298 # $^I and @ARGV return to their old values here
300 This block modifies all the C<.c> files in the current directory,
301 leaving a backup of the original data from each file in a new
304 =head2 How can I copy a file?
305 X<copy> X<file, copy> X<File::Copy>
307 (contributed by brian d foy)
309 Use the C<File::Copy> module. It comes with Perl and can do a
310 true copy across file systems, and it does its magic in
315 copy( $original, $new_copy ) or die "Copy failed: $!";
317 If you can't use C<File::Copy>, you'll have to do the work yourself:
318 open the original file, open the destination file, then print
319 to the destination file as you read the original. You also have to
320 remember to copy the permissions, owner, and group to the new file.
322 =head2 How do I make a temporary file name?
325 If you don't need to know the name of the file, you can use C<open()>
326 with C<undef> in place of the file name. In Perl 5.8 or later, the
327 C<open()> function creates an anonymous temporary file:
329 open my $tmp, '+>', undef or die $!;
331 Otherwise, you can use the File::Temp module.
333 use File::Temp qw/ tempfile tempdir /;
335 $dir = tempdir( CLEANUP => 1 );
336 ($fh, $filename) = tempfile( DIR => $dir );
338 # or if you don't need to know the filename
340 $fh = tempfile( DIR => $dir );
342 The File::Temp has been a standard module since Perl 5.6.1. If you
343 don't have a modern enough Perl installed, use the C<new_tmpfile>
344 class method from the IO::File module to get a filehandle opened for
345 reading and writing. Use it if you don't need to know the file's name:
348 $fh = IO::File->new_tmpfile()
349 or die "Unable to make new temporary file: $!";
351 If you're committed to creating a temporary file by hand, use the
352 process ID and/or the current time-value. If you need to have many
353 temporary files in one process, use a counter:
357 my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
358 my $base_name = sprintf "%s/%d-%d-0000", $temp_dir, $$, time;
363 until( defined(fileno(FH)) || $count++ > 100 ) {
364 $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
365 # O_EXCL is required for security reasons.
366 sysopen FH, $base_name, O_WRONLY|O_EXCL|O_CREAT;
369 if( defined fileno(FH) ) {
370 return (*FH, $base_name);
379 =head2 How can I manipulate fixed-record-length files?
380 X<fixed-length> X<file, fixed-length records>
382 The most efficient way is using L<pack()|perlfunc/"pack"> and
383 L<unpack()|perlfunc/"unpack">. This is faster than using
384 L<substr()|perlfunc/"substr"> when taking many, many strings. It is
385 slower for just a few.
387 Here is a sample chunk of code to break up and put back together again
388 some fixed-format input lines, in this case from the output of a normal,
392 # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
393 my $PS_T = 'A6 A4 A7 A5 A*';
394 open my $ps, '-|', 'ps';
396 my @fields = qw( pid tt stat time command );
399 @process{@fields} = unpack($PS_T, $_);
400 for my $field ( @fields ) {
401 print "$field: <$process{$field}>\n";
403 print 'line=', pack($PS_T, @process{@fields} ), "\n";
406 We've used a hash slice in order to easily handle the fields of each row.
407 Storing the keys in an array means it's easy to operate on them as a
408 group or loop over them with for. It also avoids polluting the program
409 with global variables and using symbolic references.
411 =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?
412 X<filehandle, local> X<filehandle, passing> X<filehandle, reference>
414 As of perl5.6, open() autovivifies file and directory handles
415 as references if you pass it an uninitialized scalar variable.
416 You can then pass these references just like any other scalar,
417 and use them in the place of named handles.
419 open my $fh, $file_name;
421 open local $fh, $file_name;
423 print $fh "Hello World!\n";
427 If you like, you can store these filehandles in an array or a hash.
428 If you access them directly, they aren't simple scalars and you
429 need to give C<print> a little help by placing the filehandle
430 reference in braces. Perl can only figure it out on its own when
431 the filehandle reference is a simple scalar.
433 my @fhs = ( $fh1, $fh2, $fh3 );
435 for( $i = 0; $i <= $#fhs; $i++ ) {
436 print {$fhs[$i]} "just another Perl answer, \n";
439 Before perl5.6, you had to deal with various typeglob idioms
440 which you may see in older code.
442 open FILE, "> $filename";
443 process_typeglob( *FILE );
444 process_reference( \*FILE );
446 sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
447 sub process_reference { local $fh = shift; print $fh "Reference!" }
449 If you want to create many anonymous handles, you should
450 check out the Symbol or IO::Handle modules.
452 =head2 How can I use a filehandle indirectly?
453 X<filehandle, indirect>
455 An indirect filehandle is using something other than a symbol
456 in a place that a filehandle is expected. Here are ways
457 to get indirect filehandles:
459 $fh = SOME_FH; # bareword is strict-subs hostile
460 $fh = "SOME_FH"; # strict-refs hostile; same package only
461 $fh = *SOME_FH; # typeglob
462 $fh = \*SOME_FH; # ref to typeglob (bless-able)
463 $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
465 Or, you can use the C<new> method from one of the IO::* modules to
466 create an anonymous filehandle, store that in a scalar variable,
467 and use it as though it were a normal filehandle.
469 use IO::Handle; # 5.004 or higher
470 $fh = IO::Handle->new();
472 Then use any of those as you would a normal filehandle. Anywhere that
473 Perl is expecting a filehandle, an indirect filehandle may be used
474 instead. An indirect filehandle is just a scalar variable that contains
475 a filehandle. Functions like C<print>, C<open>, C<seek>, or
476 the C<< <FH> >> diamond operator will accept either a named filehandle
477 or a scalar variable containing one:
479 ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
480 print $ofh "Type it: ";
482 print $efh "What was that: $got";
484 If you're passing a filehandle to a function, you can write
485 the function in two ways:
489 print $fh "Sending to indirect filehandle\n";
492 Or it can localize a typeglob and use the filehandle directly:
496 print FH "Sending to localized filehandle\n";
499 Both styles work with either objects or typeglobs of real filehandles.
500 (They might also work with strings under some circumstances, but this
506 In the examples above, we assigned the filehandle to a scalar variable
507 before using it. That is because only simple scalar variables, not
508 expressions or subscripts of hashes or arrays, can be used with
509 built-ins like C<print>, C<printf>, or the diamond operator. Using
510 something other than a simple scalar variable as a filehandle is
511 illegal and won't even compile:
513 @fd = (*STDIN, *STDOUT, *STDERR);
514 print $fd[1] "Type it: "; # WRONG
515 $got = <$fd[0]> # WRONG
516 print $fd[2] "What was that: $got"; # WRONG
518 With C<print> and C<printf>, you get around this by using a block and
519 an expression where you would place the filehandle:
521 print { $fd[1] } "funny stuff\n";
522 printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
523 # Pity the poor deadbeef.
525 That block is a proper block like any other, so you can put more
526 complicated code there. This sends the message out to one of two places:
529 print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
530 print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
532 This approach of treating C<print> and C<printf> like object methods
533 calls doesn't work for the diamond operator. That's because it's a
534 real operator, not just a function with a comma-less argument. Assuming
535 you've been storing typeglobs in your structure as we did above, you
536 can use the built-in function named C<readline> to read a record just
537 as C<< <> >> does. Given the initialization shown above for @fd, this
538 would work, but only because readline() requires a typeglob. It doesn't
539 work with objects or strings, which might be a bug we haven't fixed yet.
541 $got = readline($fd[0]);
543 Let it be noted that the flakiness of indirect filehandles is not
544 related to whether they're strings, typeglobs, objects, or anything else.
545 It's the syntax of the fundamental operators. Playing the object
546 game doesn't help you at all here.
548 =head2 How can I set up a footer format to be used with write()?
551 There's no builtin way to do this, but L<perlform> has a couple of
552 techniques to make it possible for the intrepid hacker.
554 =head2 How can I write() into a string?
555 X<write, into a string>
557 See L<perlform/"Accessing Formatting Internals"> for an C<swrite()> function.
559 =head2 How can I open a filehandle to a string?
560 X<string> X<open> X<IO::String> X<filehandle>
562 (contributed by Peter J. Holzer, hjp-usenet2@hjp.at)
564 Since Perl 5.8.0 a file handle referring to a string can be created by
565 calling open with a reference to that string instead of the filename.
566 This file handle can then be used to read from or write to the string:
568 open(my $fh, '>', \$string) or die "Could not open string for writing";
570 print $fh "bar\n"; # $string now contains "foo\nbar\n"
572 open(my $fh, '<', \$string) or die "Could not open string for reading";
573 my $x = <$fh>; # $x now contains "foo\n"
575 With older versions of Perl, the C<IO::String> module provides similar
578 =head2 How can I output my numbers with commas added?
581 (contributed by brian d foy and Benjamin Goldberg)
583 You can use L<Number::Format> to separate places in a number.
584 It handles locale information for those of you who want to insert
585 full stops instead (or anything else that they want to use,
588 This subroutine will add commas to your number:
592 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
596 This regex from Benjamin Goldberg will add commas to numbers:
598 s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
600 It is easier to see with comments:
603 ^[-+]? # beginning of number.
604 \d+? # first digits before first comma
605 (?= # followed by, (but not included in the match) :
606 (?>(?:\d{3})+) # some positive multiple of three digits.
607 (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever.
610 \G\d{3} # after the last group, get three digits
611 (?=\d) # but they have to have more digits after them.
614 =head2 How can I translate tildes (~) in a filename?
615 X<tilde> X<tilde expansion>
617 Use the E<lt>E<gt> (C<glob()>) operator, documented in L<perlfunc>.
618 Versions of Perl older than 5.6 require that you have a shell
619 installed that groks tildes. Later versions of Perl have this feature
620 built in. The C<File::KGlob> module (available from CPAN) gives more
621 portable glob functionality.
623 Within Perl, you may use this directly:
626 ^ ~ # find a leading tilde
628 [^/] # a non-slash character
629 * # repeated 0 or more times (0 means me)
634 : ( $ENV{HOME} || $ENV{LOGDIR} )
637 =head2 How come when I open a file read-write it wipes it out?
638 X<clobber> X<read-write> X<clobbering> X<truncate> X<truncating>
640 Because you're using something like this, which truncates the file and
641 I<then> gives you read-write access:
643 open(FH, "+> /path/name"); # WRONG (almost always)
645 Whoops. You should instead use this, which will fail if the file
648 open(FH, "+< /path/name"); # open for update
650 Using ">" always clobbers or creates. Using "<" never does
651 either. The "+" doesn't change this.
653 Here are examples of many kinds of file opens. Those using sysopen()
658 To open file for reading:
660 open(FH, "< $path") || die $!;
661 sysopen(FH, $path, O_RDONLY) || die $!;
663 To open file for writing, create new file if needed or else truncate old file:
665 open(FH, "> $path") || die $!;
666 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
667 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
669 To open file for writing, create new file, file must not exist:
671 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
672 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
674 To open file for appending, create if necessary:
676 open(FH, ">> $path") || die $!;
677 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
678 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
680 To open file for appending, file must exist:
682 sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
684 To open file for update, file must exist:
686 open(FH, "+< $path") || die $!;
687 sysopen(FH, $path, O_RDWR) || die $!;
689 To open file for update, create file if necessary:
691 sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
692 sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
694 To open file for update, file must not exist:
696 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
697 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
699 To open a file without blocking, creating if necessary:
701 sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
702 or die "can't open /foo/somefile: $!":
704 Be warned that neither creation nor deletion of files is guaranteed to
705 be an atomic operation over NFS. That is, two processes might both
706 successfully create or unlink the same file! Therefore O_EXCL
707 isn't as exclusive as you might wish.
709 See also the new L<perlopentut> if you have it (new for 5.6).
711 =head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
712 X<argument list too long>
714 The C<< <> >> operator performs a globbing operation (see above).
715 In Perl versions earlier than v5.6.0, the internal glob() operator forks
716 csh(1) to do the actual glob expansion, but
717 csh can't handle more than 127 items and so gives the error message
718 C<Argument list too long>. People who installed tcsh as csh won't
719 have this problem, but their users may be surprised by it.
721 To get around this, either upgrade to Perl v5.6.0 or later, do the glob
722 yourself with readdir() and patterns, or use a module like File::KGlob,
723 one that doesn't use the shell to do globbing.
725 =head2 Is there a leak/bug in glob()?
728 Due to the current implementation on some operating systems, when you
729 use the glob() function or its angle-bracket alias in a scalar
730 context, you may cause a memory leak and/or unpredictable behavior. It's
731 best therefore to use glob() only in list context.
733 =head2 How can I open a file with a leading ">" or trailing blanks?
734 X<filename, special characters>
736 (contributed by Brian McCauley)
738 The special two argument form of Perl's open() function ignores
739 trailing blanks in filenames and infers the mode from certain leading
740 characters (or a trailing "|"). In older versions of Perl this was the
741 only version of open() and so it is prevalent in old code and books.
743 Unless you have a particular reason to use the two argument form you
744 should use the three argument form of open() which does not treat any
745 characters in the filename as special.
747 open FILE, "<", " file "; # filename is " file "
748 open FILE, ">", ">file"; # filename is ">file"
750 =head2 How can I reliably rename a file?
751 X<rename> X<mv> X<move> X<file, rename> X<ren>
753 If your operating system supports a proper mv(1) utility or its
754 functional equivalent, this works:
756 rename($old, $new) or system("mv", $old, $new);
758 It may be more portable to use the File::Copy module instead.
759 You just copy to the new file to the new name (checking return
760 values), then delete the old one. This isn't really the same
761 semantically as a rename(), which preserves meta-information like
762 permissions, timestamps, inode info, etc.
764 Newer versions of File::Copy export a move() function.
766 =head2 How can I lock a file?
767 X<lock> X<file, lock> X<flock>
769 Perl's builtin flock() function (see L<perlfunc> for details) will call
770 flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
771 later), and lockf(3) if neither of the two previous system calls exists.
772 On some systems, it may even use a different form of native locking.
773 Here are some gotchas with Perl's flock():
779 Produces a fatal error if none of the three system calls (or their
780 close equivalent) exists.
784 lockf(3) does not provide shared locking, and requires that the
785 filehandle be open for writing (or appending, or read/writing).
789 Some versions of flock() can't lock files over a network (e.g. on NFS file
790 systems), so you'd need to force the use of fcntl(2) when you build Perl.
791 But even this is dubious at best. See the flock entry of L<perlfunc>
792 and the F<INSTALL> file in the source distribution for information on
793 building Perl to do this.
795 Two potentially non-obvious but traditional flock semantics are that
796 it waits indefinitely until the lock is granted, and that its locks are
797 I<merely advisory>. Such discretionary locks are more flexible, but
798 offer fewer guarantees. This means that files locked with flock() may
799 be modified by programs that do not also use flock(). Cars that stop
800 for red lights get on well with each other, but not with cars that don't
801 stop for red lights. See the perlport manpage, your port's specific
802 documentation, or your system-specific local manpages for details. It's
803 best to assume traditional behavior if you're writing portable programs.
804 (If you're not, you should as always feel perfectly free to write
805 for your own system's idiosyncrasies (sometimes called "features").
806 Slavish adherence to portability concerns shouldn't get in the way of
807 your getting your job done.)
809 For more information on file locking, see also
810 L<perlopentut/"File Locking"> if you have it (new for 5.6).
814 =head2 Why can't I just open(FH, "E<gt>file.lock")?
815 X<lock, lockfile race condition>
817 A common bit of code B<NOT TO USE> is this:
819 sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
820 open(LCK, "> file.lock"); # THIS BROKEN CODE
822 This is a classic race condition: you take two steps to do something
823 which must be done in one. That's why computer hardware provides an
824 atomic test-and-set instruction. In theory, this "ought" to work:
826 sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
827 or die "can't open file.lock: $!";
829 except that lamentably, file creation (and deletion) is not atomic
830 over NFS, so this won't work (at least, not every time) over the net.
831 Various schemes involving link() have been suggested, but
832 these tend to involve busy-wait, which is also less than desirable.
834 =head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
835 X<counter> X<file, counter>
837 Didn't anyone ever tell you web-page hit counters were useless?
838 They don't count number of hits, they're a waste of time, and they serve
839 only to stroke the writer's vanity. It's better to pick a random number;
840 they're more realistic.
842 Anyway, this is what you can do if you can't help yourself.
844 use Fcntl qw(:DEFAULT :flock);
845 sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
846 flock(FH, LOCK_EX) or die "can't flock numfile: $!";
848 seek(FH, 0, 0) or die "can't rewind numfile: $!";
849 truncate(FH, 0) or die "can't truncate numfile: $!";
850 (print FH $num+1, "\n") or die "can't write numfile: $!";
851 close FH or die "can't close numfile: $!";
853 Here's a much better web-page hit counter:
855 $hits = int( (time() - 850_000_000) / rand(1_000) );
857 If the count doesn't impress your friends, then the code might. :-)
859 =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?
860 X<append> X<file, append>
862 If you are on a system that correctly implements C<flock> and you use
863 the example appending code from "perldoc -f flock" everything will be
864 OK even if the OS you are on doesn't implement append mode correctly
865 (if such a system exists.) So if you are happy to restrict yourself to
866 OSs that implement C<flock> (and that's not really much of a
867 restriction) then that is what you should do.
869 If you know you are only going to use a system that does correctly
870 implement appending (i.e. not Win32) then you can omit the C<seek>
871 from the code in the previous answer.
873 If you know you are only writing code to run on an OS and filesystem
874 that does implement append mode correctly (a local filesystem on a
875 modern Unix for example), and you keep the file in block-buffered mode
876 and you write less than one buffer-full of output between each manual
877 flushing of the buffer then each bufferload is almost guaranteed to be
878 written to the end of the file in one chunk without getting
879 intermingled with anyone else's output. You can also use the
880 C<syswrite> function which is simply a wrapper around your system's
881 C<write(2)> system call.
883 There is still a small theoretical chance that a signal will interrupt
884 the system level C<write()> operation before completion. There is also
885 a possibility that some STDIO implementations may call multiple system
886 level C<write()>s even if the buffer was empty to start. There may be
887 some systems where this probability is reduced to zero, and this is
888 not a concern when using C<:perlio> instead of your system's STDIO.
890 =head2 How do I randomly update a binary file?
891 X<file, binary patch>
893 If you're just trying to patch a binary, in many cases something as
894 simple as this works:
896 perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
898 However, if you have fixed sized records, then you might do something more
901 $RECSIZE = 220; # size of record, in bytes
902 $recno = 37; # which record to update
903 open(FH, "+<somewhere") || die "can't update somewhere: $!";
904 seek(FH, $recno * $RECSIZE, 0);
905 read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
907 seek(FH, -$RECSIZE, 1);
911 Locking and error checking are left as an exercise for the reader.
912 Don't forget them or you'll be quite sorry.
914 =head2 How do I get a file's timestamp in perl?
915 X<timestamp> X<file, timestamp>
917 If you want to retrieve the time at which the file was last
918 read, written, or had its meta-data (owner, etc) changed,
919 you use the B<-A>, B<-M>, or B<-C> file test operations as
920 documented in L<perlfunc>. These retrieve the age of the
921 file (measured against the start-time of your program) in
922 days as a floating point number. Some platforms may not have
923 all of these times. See L<perlport> for details. To
924 retrieve the "raw" time in seconds since the epoch, you
925 would call the stat function, then use localtime(),
926 gmtime(), or POSIX::strftime() to convert this into
931 $write_secs = (stat($file))[9];
932 printf "file %s updated at %s\n", $file,
933 scalar localtime($write_secs);
935 If you prefer something more legible, use the File::stat module
936 (part of the standard distribution in version 5.004 and later):
938 # error checking left as an exercise for reader.
941 $date_string = ctime(stat($file)->mtime);
942 print "file $file updated at $date_string\n";
944 The POSIX::strftime() approach has the benefit of being,
945 in theory, independent of the current locale. See L<perllocale>
948 =head2 How do I set a file's timestamp in perl?
949 X<timestamp> X<file, timestamp>
951 You use the utime() function documented in L<perlfunc/utime>.
952 By way of example, here's a little program that copies the
953 read and write times from its first argument to all the rest
957 die "usage: cptimes timestamp_file other_files ...\n";
960 ($atime, $mtime) = (stat($timestamp))[8,9];
961 utime $atime, $mtime, @ARGV;
963 Error checking is, as usual, left as an exercise for the reader.
965 The perldoc for utime also has an example that has the same
966 effect as touch(1) on files that I<already exist>.
968 Certain file systems have a limited ability to store the times
969 on a file at the expected level of precision. For example, the
970 FAT and HPFS filesystem are unable to create dates on files with
971 a finer granularity than two seconds. This is a limitation of
972 the filesystems, not of utime().
974 =head2 How do I print to more than one file at once?
975 X<print, to multiple files>
977 To connect one filehandle to several output filehandles,
978 you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
980 If you only have to do this once, you can print individually
983 for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
985 =head2 How can I read in an entire file all at once?
986 X<slurp> X<file, slurping>
988 You can use the File::Slurp module to do it in one step.
992 $all_of_it = read_file($filename); # entire file in scalar
993 @all_lines = read_file($filename); # one line per element
995 The customary Perl approach for processing all the lines in a file is to
996 do so one line at a time:
998 open (INPUT, $file) || die "can't open $file: $!";
1001 # do something with $_
1003 close(INPUT) || die "can't close $file: $!";
1005 This is tremendously more efficient than reading the entire file into
1006 memory as an array of lines and then processing it one element at a time,
1007 which is often--if not almost always--the wrong approach. Whenever
1008 you see someone do this:
1012 you should think long and hard about why you need everything loaded at
1013 once. It's just not a scalable solution. You might also find it more
1014 fun to use the standard Tie::File module, or the DB_File module's
1015 $DB_RECNO bindings, which allow you to tie an array to a file so that
1016 accessing an element the array actually accesses the corresponding
1019 You can read the entire filehandle contents into a scalar.
1023 open (INPUT, $file) || die "can't open $file: $!";
1027 That temporarily undefs your record separator, and will automatically
1028 close the file at block exit. If the file is already open, just use this:
1030 $var = do { local $/; <INPUT> };
1032 For ordinary files you can also use the read function.
1034 read( INPUT, $var, -s INPUT );
1036 The third argument tests the byte size of the data on the INPUT filehandle
1037 and reads that many bytes into the buffer $var.
1039 =head2 How can I read in a file by paragraphs?
1040 X<file, reading by paragraphs>
1042 Use the C<$/> variable (see L<perlvar> for details). You can either
1043 set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
1044 for instance, gets treated as two paragraphs and not three), or
1045 C<"\n\n"> to accept empty paragraphs.
1047 Note that a blank line must have no blanks in it. Thus
1048 S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
1050 =head2 How can I read a single character from a file? From the keyboard?
1051 X<getc> X<file, reading one character at a time>
1053 You can use the builtin C<getc()> function for most filehandles, but
1054 it won't (easily) work on a terminal device. For STDIN, either use
1055 the Term::ReadKey module from CPAN or use the sample code in
1058 If your system supports the portable operating system programming
1059 interface (POSIX), you can use the following code, which you'll note
1060 turns off echo processing as well.
1074 use POSIX qw(:termios_h);
1076 my ($term, $oterm, $echo, $noecho, $fd_stdin);
1078 $fd_stdin = fileno(STDIN);
1080 $term = POSIX::Termios->new();
1081 $term->getattr($fd_stdin);
1082 $oterm = $term->getlflag();
1084 $echo = ECHO | ECHOK | ICANON;
1085 $noecho = $oterm & ~$echo;
1088 $term->setlflag($noecho);
1089 $term->setcc(VTIME, 1);
1090 $term->setattr($fd_stdin, TCSANOW);
1094 $term->setlflag($oterm);
1095 $term->setcc(VTIME, 0);
1096 $term->setattr($fd_stdin, TCSANOW);
1102 sysread(STDIN, $key, 1);
1111 The Term::ReadKey module from CPAN may be easier to use. Recent versions
1112 include also support for non-portable systems as well.
1115 open(TTY, "</dev/tty");
1116 print "Gimme a char: ";
1118 $key = ReadKey 0, *TTY;
1120 printf "\nYou said %s, char number %03d\n",
1123 =head2 How can I tell whether there's a character waiting on a filehandle?
1125 The very first thing you should do is look into getting the Term::ReadKey
1126 extension from CPAN. As we mentioned earlier, it now even has limited
1127 support for non-portable (read: not open systems, closed, proprietary,
1128 not POSIX, not Unix, etc) systems.
1130 You should also check out the Frequently Asked Questions list in
1131 comp.unix.* for things like this: the answer is essentially the same.
1132 It's very system dependent. Here's one solution that works on BSD
1137 vec($rin, fileno(STDIN), 1) = 1;
1138 return $nfd = select($rin,undef,undef,0);
1141 If you want to find out how many characters are waiting, there's
1142 also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
1143 comes with Perl tries to convert C include files to Perl code, which
1144 can be C<require>d. FIONREAD ends up defined as a function in the
1145 I<sys/ioctl.ph> file:
1147 require 'sys/ioctl.ph';
1149 $size = pack("L", 0);
1150 ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
1151 $size = unpack("L", $size);
1153 If I<h2ph> wasn't installed or doesn't work for you, you can
1154 I<grep> the include files by hand:
1156 % grep FIONREAD /usr/include/*/*
1157 /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
1159 Or write a small C program using the editor of champions:
1162 #include <sys/ioctl.h>
1164 printf("%#08x\n", FIONREAD);
1167 % cc -o fionread fionread.c
1171 And then hard code it, leaving porting as an exercise to your successor.
1173 $FIONREAD = 0x4004667f; # XXX: opsys dependent
1175 $size = pack("L", 0);
1176 ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
1177 $size = unpack("L", $size);
1179 FIONREAD requires a filehandle connected to a stream, meaning that sockets,
1180 pipes, and tty devices work, but I<not> files.
1182 =head2 How do I do a C<tail -f> in perl?
1183 X<tail> X<IO::Handle> X<File::Tail> X<clearerr>
1189 The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
1190 but it does clear the end-of-file condition on the handle, so that the
1191 next C<< <GWFILE> >> makes Perl try again to read something.
1193 If that doesn't work (it relies on features of your stdio implementation),
1194 then you need something more like this:
1197 for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
1198 # search for some stuff and put it into files
1201 seek(GWFILE, $curpos, 0); # seek to where we had been
1204 If this still doesn't work, look into the C<clearerr> method
1205 from C<IO::Handle>, which resets the error and end-of-file states
1208 There's also a C<File::Tail> module from CPAN.
1210 =head2 How do I dup() a filehandle in Perl?
1213 If you check L<perlfunc/open>, you'll see that several of the ways
1214 to call open() should do the trick. For example:
1216 open(LOG, ">>/foo/logfile");
1217 open(STDERR, ">&LOG");
1219 Or even with a literal numeric descriptor:
1221 $fd = $ENV{MHCONTEXTFD};
1222 open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
1224 Note that "<&STDIN" makes a copy, but "<&=STDIN" make
1225 an alias. That means if you close an aliased handle, all
1226 aliases become inaccessible. This is not true with
1229 Error checking, as always, has been left as an exercise for the reader.
1231 =head2 How do I close a file descriptor by number?
1232 X<file, closing file descriptors> X<POSIX> X<close>
1234 If, for some reason, you have a file descriptor instead of a
1235 filehandle (perhaps you used C<POSIX::open>), you can use the
1236 C<close()> function from the C<POSIX> module:
1240 POSIX::close( $fd );
1242 This should rarely be necessary, as the Perl C<close()> function is to be
1243 used for things that Perl opened itself, even if it was a dup of a
1244 numeric descriptor as with C<MHCONTEXT> above. But if you really have
1245 to, you may be able to do this:
1247 require 'sys/syscall.ph';
1248 $rc = syscall(&SYS_close, $fd + 0); # must force numeric
1249 die "can't sysclose $fd: $!" unless $rc == -1;
1251 Or, just use the fdopen(3S) feature of C<open()>:
1254 open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!";
1258 =head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
1259 X<filename, DOS issues>
1261 Whoops! You just put a tab and a formfeed into that filename!
1262 Remember that within double quoted strings ("like\this"), the
1263 backslash is an escape character. The full list of these is in
1264 L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
1265 have a file called "c:(tab)emp(formfeed)oo" or
1266 "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
1268 Either single-quote your strings, or (preferably) use forward slashes.
1269 Since all DOS and Windows versions since something like MS-DOS 2.0 or so
1270 have treated C</> and C<\> the same in a path, you might as well use the
1271 one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++,
1272 awk, Tcl, Java, or Python, just to mention a few. POSIX paths
1273 are more portable, too.
1275 =head2 Why doesn't glob("*.*") get all the files?
1278 Because even on non-Unix ports, Perl's glob function follows standard
1279 Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
1280 files. This makes glob() portable even to legacy systems. Your
1281 port may include proprietary globbing functions as well. Check its
1282 documentation for details.
1284 =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
1286 This is elaborately and painstakingly described in the
1287 F<file-dir-perms> article in the "Far More Than You Ever Wanted To
1288 Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
1290 The executive summary: learn how your filesystem works. The
1291 permissions on a file say what can happen to the data in that file.
1292 The permissions on a directory say what can happen to the list of
1293 files in that directory. If you delete a file, you're removing its
1294 name from the directory (so the operation depends on the permissions
1295 of the directory, not of the file). If you try to write to the file,
1296 the permissions of the file govern whether you're allowed to.
1298 =head2 How do I select a random line from a file?
1299 X<file, selecting a random line>
1301 Short of loading the file into a database or pre-indexing the lines in
1302 the file, there are a couple of things that you can do.
1304 Here's a reservoir-sampling algorithm from the Camel Book:
1307 rand($.) < 1 && ($line = $_) while <>;
1309 This has a significant advantage in space over reading the whole file
1310 in. You can find a proof of this method in I<The Art of Computer
1311 Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth.
1313 You can use the C<File::Random> module which provides a function
1316 use File::Random qw/random_line/;
1317 my $line = random_line($filename);
1319 Another way is to use the C<Tie::File> module, which treats the entire
1320 file as an array. Simply access a random array element.
1322 =head2 Why do I get weird spaces when I print an array of lines?
1324 (contributed by brian d foy)
1326 If you are seeing spaces between the elements of your array when
1327 you print the array, you are probably interpolating the array in
1330 my @animals = qw(camel llama alpaca vicuna);
1331 print "animals are: @animals\n";
1333 It's the double quotes, not the C<print>, doing this. Whenever you
1334 interpolate an array in a double quote context, Perl joins the
1335 elements with spaces (or whatever is in C<$">, which is a space by
1338 animals are: camel llama alpaca vicuna
1340 This is different than printing the array without the interpolation:
1342 my @animals = qw(camel llama alpaca vicuna);
1343 print "animals are: ", @animals, "\n";
1345 Now the output doesn't have the spaces between the elements because
1346 the elements of C<@animals> simply become part of the list to
1349 animals are: camelllamaalpacavicuna
1351 You might notice this when each of the elements of C<@array> end with
1352 a newline. You expect to print one element per line, but notice that
1353 every line after the first is indented:
1356 this is another line
1357 this is the third line
1359 That extra space comes from the interpolation of the array. If you
1360 don't want to put anything between your array elements, don't use the
1361 array in double quotes. You can send it to print without them:
1365 =head2 How do I traverse a directory tree?
1367 (contributed by brian d foy)
1369 The C<File::Find> module, which comes with Perl, does all of the hard
1370 work to traverse a directory structure. It comes with Perl. You simply
1371 call the C<find> subroutine with a callback subroutine and the
1372 directories you want to traverse:
1376 find( \&wanted, @directories );
1379 # full path in $File::Find::name
1380 # just filename in $_
1381 ... do whatever you want to do ...
1384 The C<File::Find::Closures>, which you can download from CPAN, provides
1385 many ready-to-use subroutines that you can use with C<File::Find>.
1387 The C<File::Finder>, which you can download from CPAN, can help you
1388 create the callback subroutine using something closer to the syntax of
1389 the C<find> command-line utility:
1394 my $deep_dirs = File::Finder->depth->type('d')->ls->exec('rmdir','{}');
1396 find( $deep_dirs->as_options, @places );
1398 The C<File::Find::Rule> module, which you can download from CPAN, has
1399 a similar interface, but does the traversal for you too:
1401 use File::Find::Rule;
1403 my @files = File::Find::Rule->file()
1407 =head2 How do I delete a directory tree?
1409 (contributed by brian d foy)
1411 If you have an empty directory, you can use Perl's built-in C<rmdir>. If
1412 the directory is not empty (so, no files or subdirectories), you either
1413 have to empty it yourself (a lot of work) or use a module to help you.
1415 The C<File::Path> module, which comes with Perl, has a C<rmtree> which
1416 can take care of all of the hard work for you:
1418 use File::Path qw(rmtree);
1420 rmtree( \@directories, 0, 0 );
1422 The first argument to C<rmtree> is either a string representing a directory path
1423 or an array reference. The second argument controls progress messages, and the
1424 third argument controls the handling of files you don't have permissions to
1425 delete. See the C<File::Path> module for the details.
1427 =head2 How do I copy an entire directory?
1429 (contributed by Shlomi Fish)
1431 To do the equivalent of C<cp -R> (i.e. copy an entire directory tree
1432 recursively) in portable Perl, you'll either need to write something yourself
1433 or find a good CPAN module such as L<File::Copy::Recursive>.
1436 Revision: $Revision$
1440 See L<perlfaq> for source control details and availability.
1442 =head1 AUTHOR AND COPYRIGHT
1444 Copyright (c) 1997-2009 Tom Christiansen, Nathan Torkington, and
1445 other authors as noted. All rights reserved.
1447 This documentation is free; you can redistribute it and/or modify it
1448 under the same terms as Perl itself.
1450 Irrespective of its distribution, all code examples here are in the public
1451 domain. You are permitted and encouraged to use this code and any
1452 derivatives thereof in your own programs for fun or for profit as you
1453 see fit. A simple comment in the code giving credit to the FAQ would
1454 be courteous but is not required.