1perlfaq5(3) User Contributed Perl Documentation perlfaq5(3)
2
6 perlfaq5 - Files and Formats
7
9 version 5.20201107
10
12 This section deals with I/O and the "f" issues: filehandles, flushing,
13 formats, and footers.
14 How do I flush/unbuffer an output filehandle? Why must I do this?
16 (contributed by brian d foy)
17 You might like to read Mark Jason Dominus's "Suffering From Buffering"
19 at <http://perl.plover.com/FAQs/Buffering.html> .
20 Perl normally buffers output so it doesn't make a system call for every
22 bit of output. By saving up output, it makes fewer expensive system
23 calls. For instance, in this little bit of code, you want to print a
24 dot to the screen for every line you process to watch the progress of
25 your program. Instead of seeing a dot for every line, Perl buffers the
26 output and you have a long wait before you see a row of 50 dots all at
27 once:
28 # long wait, then row of dots all at once
30 while( <> ) {
31 print ".";
32 print "\n" unless ++$count % 50;
33 #... expensive line processing operations
35 }
36 To get around this, you have to unbuffer the output filehandle, in this
38 case, "STDOUT". You can set the special variable $| to a true value
39 (mnemonic: making your filehandles "piping hot"):
40 $|++;
42 # dot shown immediately
44 while( <> ) {
45 print ".";
46 print "\n" unless ++$count % 50;
47 #... expensive line processing operations
49 }
50 The $| is one of the per-filehandle special variables, so each
52 filehandle has its own copy of its value. If you want to merge standard
53 output and standard error for instance, you have to unbuffer each
54 (although STDERR might be unbuffered by default):
55 {
57 my $previous_default = select(STDOUT); # save previous default
58 $|++; # autoflush STDOUT
59 select(STDERR);
60 $|++; # autoflush STDERR, to be sure
61 select($previous_default); # restore previous default
62 }
63 # now should alternate . and +
65 while( 1 ) {
66 sleep 1;
67 print STDOUT ".";
68 print STDERR "+";
69 print STDOUT "\n" unless ++$count % 25;
70 }
71 Besides the $| special variable, you can use "binmode" to give your
73 filehandle a ":unix" layer, which is unbuffered:
74 binmode( STDOUT, ":unix" );
76 while( 1 ) {
78 sleep 1;
79 print ".";
80 print "\n" unless ++$count % 50;
81 }
82 For more information on output layers, see the entries for "binmode"
84 and open in perlfunc, and the PerlIO module documentation.
85 If you are using IO::Handle or one of its subclasses, you can call the
87 "autoflush" method to change the settings of the filehandle:
88 use IO::Handle;
90 open my( $io_fh ), ">", "output.txt";
91 $io_fh->autoflush(1);
92 The IO::Handle objects also have a "flush" method. You can flush the
94 buffer any time you want without auto-buffering
95 $io_fh->flush;
97 How do I change, delete, or insert a line in a file, or append to the
99 beginning of a file?
100 (contributed by brian d foy)
101 The basic idea of inserting, changing, or deleting a line from a text
103 file involves reading and printing the file to the point you want to
104 make the change, making the change, then reading and printing the rest
105 of the file. Perl doesn't provide random access to lines (especially
106 since the record input separator, $/, is mutable), although modules
107 such as Tie::File can fake it.
108 A Perl program to do these tasks takes the basic form of opening a
110 file, printing its lines, then closing the file:
111 open my $in, '<', $file or die "Can't read old file: $!";
113 open my $out, '>', "$file.new" or die "Can't write new file: $!";
114 while( <$in> ) {
116 print $out $_;
117 }
118 close $out;
120 Within that basic form, add the parts that you need to insert, change,
122 or delete lines.
123 To prepend lines to the beginning, print those lines before you enter
125 the loop that prints the existing lines.
126 open my $in, '<', $file or die "Can't read old file: $!";
128 open my $out, '>', "$file.new" or die "Can't write new file: $!";
129 print $out "# Add this line to the top\n"; # <--- HERE'S THE MAGIC
131 while( <$in> ) {
133 print $out $_;
134 }
135 close $out;
137 To change existing lines, insert the code to modify the lines inside
139 the "while" loop. In this case, the code finds all lowercased versions
140 of "perl" and uppercases them. The happens for every line, so be sure
141 that you're supposed to do that on every line!
142 open my $in, '<', $file or die "Can't read old file: $!";
144 open my $out, '>', "$file.new" or die "Can't write new file: $!";
145 print $out "# Add this line to the top\n";
147 while( <$in> ) {
149 s/\b(perl)\b/Perl/g;
150 print $out $_;
151 }
152 close $out;
154 To change only a particular line, the input line number, $., is useful.
156 First read and print the lines up to the one you want to change. Next,
157 read the single line you want to change, change it, and print it. After
158 that, read the rest of the lines and print those:
159 while( <$in> ) { # print the lines before the change
161 print $out $_;
162 last if $. == 4; # line number before change
163 }
164 my $line = <$in>;
166 $line =~ s/\b(perl)\b/Perl/g;
167 print $out $line;
168 while( <$in> ) { # print the rest of the lines
170 print $out $_;
171 }
172 To skip lines, use the looping controls. The "next" in this example
174 skips comment lines, and the "last" stops all processing once it
175 encounters either "__END__" or "__DATA__".
176 while( <$in> ) {
178 next if /^\s+#/; # skip comment lines
179 last if /^__(END|DATA)__$/; # stop at end of code marker
180 print $out $_;
181 }
182 Do the same sort of thing to delete a particular line by using "next"
184 to skip the lines you don't want to show up in the output. This example
185 skips every fifth line:
186 while( <$in> ) {
188 next unless $. % 5;
189 print $out $_;
190 }
191 If, for some odd reason, you really want to see the whole file at once
193 rather than processing line-by-line, you can slurp it in (as long as
194 you can fit the whole thing in memory!):
195 open my $in, '<', $file or die "Can't read old file: $!"
197 open my $out, '>', "$file.new" or die "Can't write new file: $!";
198 my $content = do { local $/; <$in> }; # slurp!
200 # do your magic here
202 print $out $content;
204 Modules such as Path::Tiny and Tie::File can help with that too. If you
206 can, however, avoid reading the entire file at once. Perl won't give
207 that memory back to the operating system until the process finishes.
208 You can also use Perl one-liners to modify a file in-place. The
210 following changes all 'Fred' to 'Barney' in inFile.txt, overwriting the
211 file with the new contents. With the "-p" switch, Perl wraps a "while"
212 loop around the code you specify with "-e", and "-i" turns on in-place
213 editing. The current line is in $_. With "-p", Perl automatically
214 prints the value of $_ at the end of the loop. See perlrun for more
215 details.
216 perl -pi -e 's/Fred/Barney/' inFile.txt
218 To make a backup of "inFile.txt", give "-i" a file extension to add:
220 perl -pi.bak -e 's/Fred/Barney/' inFile.txt
222 To change only the fifth line, you can add a test checking $., the
224 input line number, then only perform the operation when the test
225 passes:
226 perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt
228 To add lines before a certain line, you can add a line (or lines!)
230 before Perl prints $_:
231 perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt
233 You can even add a line to the beginning of a file, since the current
235 line prints at the end of the loop:
236 perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt
238 To insert a line after one already in the file, use the "-n" switch.
240 It's just like "-p" except that it doesn't print $_ at the end of the
241 loop, so you have to do that yourself. In this case, print $_ first,
242 then print the line that you want to add.
243 perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt
245 To delete lines, only print the ones that you want.
247 perl -ni -e 'print if /d/' inFile.txt
249 How do I count the number of lines in a file?
251 (contributed by brian d foy)
252 Conceptually, the easiest way to count the lines in a file is to simply
254 read them and count them:
255 my $count = 0;
257 while( <$fh> ) { $count++; }
258 You don't really have to count them yourself, though, since Perl
260 already does that with the $. variable, which is the current line
261 number from the last filehandle read:
262 1 while( <$fh> );
264 my $count = $.;
265 If you want to use $., you can reduce it to a simple one-liner, like
267 one of these:
268 % perl -lne '} print $.; {' file
270 % perl -lne 'END { print $. }' file
272 Those can be rather inefficient though. If they aren't fast enough for
274 you, you might just read chunks of data and count the number of
275 newlines:
276 my $lines = 0;
278 open my($fh), '<:raw', $filename or die "Can't open $filename: $!";
279 while( sysread $fh, $buffer, 4096 ) {
280 $lines += ( $buffer =~ tr/\n// );
281 }
282 close $fh;
283 However, that doesn't work if the line ending isn't a newline. You
285 might change that "tr///" to a "s///" so you can count the number of
286 times the input record separator, $/, shows up:
287 my $lines = 0;
289 open my($fh), '<:raw', $filename or die "Can't open $filename: $!";
290 while( sysread $fh, $buffer, 4096 ) {
291 $lines += ( $buffer =~ s|$/||g; );
292 }
293 close $fh;
294 If you don't mind shelling out, the "wc" command is usually the
296 fastest, even with the extra interprocess overhead. Ensure that you
297 have an untainted filename though:
298 #!perl -T
300 $ENV{PATH} = undef;
302 my $lines;
304 if( $filename =~ /^([0-9a-z_.]+)\z/ ) {
305 $lines = `/usr/bin/wc -l $1`
306 chomp $lines;
307 }
308 How do I delete the last N lines from a file?
310 (contributed by brian d foy)
311 The easiest conceptual solution is to count the lines in the file then
313 start at the beginning and print the number of lines (minus the last N)
314 to a new file.
315 Most often, the real question is how you can delete the last N lines
317 without making more than one pass over the file, or how to do it
318 without a lot of copying. The easy concept is the hard reality when you
319 might have millions of lines in your file.
320 One trick is to use File::ReadBackwards, which starts at the end of the
322 file. That module provides an object that wraps the real filehandle to
323 make it easy for you to move around the file. Once you get to the spot
324 you need, you can get the actual filehandle and work with it as normal.
325 In this case, you get the file position at the end of the last line you
326 want to keep and truncate the file to that point:
327 use File::ReadBackwards;
329 my $filename = 'test.txt';
331 my $Lines_to_truncate = 2;
332 my $bw = File::ReadBackwards->new( $filename )
334 or die "Could not read backwards in [$filename]: $!";
335 my $lines_from_end = 0;
337 until( $bw->eof or $lines_from_end == $Lines_to_truncate ) {
338 print "Got: ", $bw->readline;
339 $lines_from_end++;
340 }
341 truncate( $filename, $bw->tell );
343 The File::ReadBackwards module also has the advantage of setting the
345 input record separator to a regular expression.
346 You can also use the Tie::File module which lets you access the lines
348 through a tied array. You can use normal array operations to modify
349 your file, including setting the last index and using "splice".
350 How can I use Perl's "-i" option from within a program?
352 "-i" sets the value of Perl's $^I variable, which in turn affects the
353 behavior of "<>"; see perlrun for more details. By modifying the
354 appropriate variables directly, you can get the same behavior within a
355 larger program. For example:
356 # ...
358 {
359 local($^I, @ARGV) = ('.orig', glob("*.c"));
360 while (<>) {
361 if ($. == 1) {
362 print "This line should appear at the top of each file\n";
363 }
364 s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
365 print;
366 close ARGV if eof; # Reset $.
367 }
368 }
369 # $^I and @ARGV return to their old values here
370 This block modifies all the ".c" files in the current directory,
372 leaving a backup of the original data from each file in a new ".c.orig"
373 file.
374 How can I copy a file?
376 (contributed by brian d foy)
377 Use the File::Copy module. It comes with Perl and can do a true copy
379 across file systems, and it does its magic in a portable fashion.
380 use File::Copy;
382 copy( $original, $new_copy ) or die "Copy failed: $!";
384 If you can't use File::Copy, you'll have to do the work yourself: open
386 the original file, open the destination file, then print to the
387 destination file as you read the original. You also have to remember to
388 copy the permissions, owner, and group to the new file.
389 How do I make a temporary file name?
391 If you don't need to know the name of the file, you can use "open()"
392 with "undef" in place of the file name. In Perl 5.8 or later, the
393 "open()" function creates an anonymous temporary file:
394 open my $tmp, '+>', undef or die $!;
396 Otherwise, you can use the File::Temp module.
398 use File::Temp qw/ tempfile tempdir /;
400 my $dir = tempdir( CLEANUP => 1 );
402 ($fh, $filename) = tempfile( DIR => $dir );
403 # or if you don't need to know the filename
405 my $fh = tempfile( DIR => $dir );
407 The File::Temp has been a standard module since Perl 5.6.1. If you
409 don't have a modern enough Perl installed, use the "new_tmpfile" class
410 method from the IO::File module to get a filehandle opened for reading
411 and writing. Use it if you don't need to know the file's name:
412 use IO::File;
414 my $fh = IO::File->new_tmpfile()
415 or die "Unable to make new temporary file: $!";
416 If you're committed to creating a temporary file by hand, use the
418 process ID and/or the current time-value. If you need to have many
419 temporary files in one process, use a counter:
420 BEGIN {
422 use Fcntl;
423 use File::Spec;
424 my $temp_dir = File::Spec->tmpdir();
425 my $file_base = sprintf "%d-%d-0000", $$, time;
426 my $base_name = File::Spec->catfile($temp_dir, $file_base);
427 sub temp_file {
429 my $fh;
430 my $count = 0;
431 until( defined(fileno($fh)) || $count++ > 100 ) {
432 $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
433 # O_EXCL is required for security reasons.
434 sysopen $fh, $base_name, O_WRONLY|O_EXCL|O_CREAT;
435 }
436 if( defined fileno($fh) ) {
438 return ($fh, $base_name);
439 }
440 else {
441 return ();
442 }
443 }
444 }
445 How can I manipulate fixed-record-length files?
447 The most efficient way is using pack() and unpack(). This is faster
448 than using substr() when taking many, many strings. It is slower for
449 just a few.
450 Here is a sample chunk of code to break up and put back together again
452 some fixed-format input lines, in this case from the output of a
453 normal, Berkeley-style ps:
454 # sample input line:
456 # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
457 my $PS_T = 'A6 A4 A7 A5 A*';
458 open my $ps, '-|', 'ps';
459 print scalar <$ps>;
460 my @fields = qw( pid tt stat time command );
461 while (<$ps>) {
462 my %process;
463 @process{@fields} = unpack($PS_T, $_);
464 for my $field ( @fields ) {
465 print "$field: <$process{$field}>\n";
466 }
467 print 'line=', pack($PS_T, @process{@fields} ), "\n";
468 }
469 We've used a hash slice in order to easily handle the fields of each
471 row. Storing the keys in an array makes it easy to operate on them as
472 a group or loop over them with "for". It also avoids polluting the
473 program with global variables and using symbolic references.
474 How can I make a filehandle local to a subroutine? How do I pass
476 filehandles between subroutines? How do I make an array of filehandles?
477 As of perl5.6, open() autovivifies file and directory handles as
478 references if you pass it an uninitialized scalar variable. You can
479 then pass these references just like any other scalar, and use them in
480 the place of named handles.
481 open my $fh, $file_name;
483 open local $fh, $file_name;
485 print $fh "Hello World!\n";
487 process_file( $fh );
489 If you like, you can store these filehandles in an array or a hash. If
491 you access them directly, they aren't simple scalars and you need to
492 give "print" a little help by placing the filehandle reference in
493 braces. Perl can only figure it out on its own when the filehandle
494 reference is a simple scalar.
495 my @fhs = ( $fh1, $fh2, $fh3 );
497 for( $i = 0; $i <= $#fhs; $i++ ) {
499 print {$fhs[$i]} "just another Perl answer, \n";
500 }
501 Before perl5.6, you had to deal with various typeglob idioms which you
503 may see in older code.
504 open FILE, "> $filename";
506 process_typeglob( *FILE );
507 process_reference( \*FILE );
508 sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
510 sub process_reference { local $fh = shift; print $fh "Reference!" }
511 If you want to create many anonymous handles, you should check out the
513 Symbol or IO::Handle modules.
514 How can I use a filehandle indirectly?
516 An indirect filehandle is the use of something other than a symbol in a
517 place that a filehandle is expected. Here are ways to get indirect
518 filehandles:
519 $fh = SOME_FH; # bareword is strict-subs hostile
521 $fh = "SOME_FH"; # strict-refs hostile; same package only
522 $fh = *SOME_FH; # typeglob
523 $fh = \*SOME_FH; # ref to typeglob (bless-able)
524 $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
525 Or, you can use the "new" method from one of the IO::* modules to
527 create an anonymous filehandle and store that in a scalar variable.
528 use IO::Handle; # 5.004 or higher
530 my $fh = IO::Handle->new();
531 Then use any of those as you would a normal filehandle. Anywhere that
533 Perl is expecting a filehandle, an indirect filehandle may be used
534 instead. An indirect filehandle is just a scalar variable that contains
535 a filehandle. Functions like "print", "open", "seek", or the "<FH>"
536 diamond operator will accept either a named filehandle or a scalar
537 variable containing one:
538 ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
540 print $ofh "Type it: ";
541 my $got = <$ifh>
542 print $efh "What was that: $got";
543 If you're passing a filehandle to a function, you can write the
545 function in two ways:
546 sub accept_fh {
548 my $fh = shift;
549 print $fh "Sending to indirect filehandle\n";
550 }
551 Or it can localize a typeglob and use the filehandle directly:
553 sub accept_fh {
555 local *FH = shift;
556 print FH "Sending to localized filehandle\n";
557 }
558 Both styles work with either objects or typeglobs of real filehandles.
560 (They might also work with strings under some circumstances, but this
561 is risky.)
562 accept_fh(*STDOUT);
564 accept_fh($handle);
565 In the examples above, we assigned the filehandle to a scalar variable
567 before using it. That is because only simple scalar variables, not
568 expressions or subscripts of hashes or arrays, can be used with built-
569 ins like "print", "printf", or the diamond operator. Using something
570 other than a simple scalar variable as a filehandle is illegal and
571 won't even compile:
572 my @fd = (*STDIN, *STDOUT, *STDERR);
574 print $fd[1] "Type it: "; # WRONG
575 my $got = <$fd[0]> # WRONG
576 print $fd[2] "What was that: $got"; # WRONG
577 With "print" and "printf", you get around this by using a block and an
579 expression where you would place the filehandle:
580 print { $fd[1] } "funny stuff\n";
582 printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
583 # Pity the poor deadbeef.
584 That block is a proper block like any other, so you can put more
586 complicated code there. This sends the message out to one of two
587 places:
588 my $ok = -x "/bin/cat";
590 print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
591 print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
592 This approach of treating "print" and "printf" like object methods
594 calls doesn't work for the diamond operator. That's because it's a real
595 operator, not just a function with a comma-less argument. Assuming
596 you've been storing typeglobs in your structure as we did above, you
597 can use the built-in function named "readline" to read a record just as
598 "<>" does. Given the initialization shown above for @fd, this would
599 work, but only because readline() requires a typeglob. It doesn't work
600 with objects or strings, which might be a bug we haven't fixed yet.
601 $got = readline($fd[0]);
603 Let it be noted that the flakiness of indirect filehandles is not
605 related to whether they're strings, typeglobs, objects, or anything
606 else. It's the syntax of the fundamental operators. Playing the object
607 game doesn't help you at all here.
608 How can I open a filehandle to a string?
610 (contributed by Peter J. Holzer, hjp-usenet2@hjp.at)
611 Since Perl 5.8.0 a file handle referring to a string can be created by
613 calling open with a reference to that string instead of the filename.
614 This file handle can then be used to read from or write to the string:
615 open(my $fh, '>', \$string) or die "Could not open string for writing";
617 print $fh "foo\n";
618 print $fh "bar\n"; # $string now contains "foo\nbar\n"
619 open(my $fh, '<', \$string) or die "Could not open string for reading";
621 my $x = <$fh>; # $x now contains "foo\n"
622 With older versions of Perl, the IO::String module provides similar
624 functionality.
625 How can I set up a footer format to be used with write()?
627 There's no builtin way to do this, but perlform has a couple of
628 techniques to make it possible for the intrepid hacker.
629 How can I write() into a string?
631 (contributed by brian d foy)
632 If you want to "write" into a string, you just have to <open> a
634 filehandle to a string, which Perl has been able to do since Perl 5.6:
635 open FH, '>', \my $string;
637 write( FH );
638 Since you want to be a good programmer, you probably want to use a
640 lexical filehandle, even though formats are designed to work with
641 bareword filehandles since the default format names take the filehandle
642 name. However, you can control this with some Perl special per-
643 filehandle variables: $^, which names the top-of-page format, and $~
644 which shows the line format. You have to change the default filehandle
645 to set these variables:
646 open my($fh), '>', \my $string;
648 { # set per-filehandle variables
650 my $old_fh = select( $fh );
651 $~ = 'ANIMAL';
652 $^ = 'ANIMAL_TOP';
653 select( $old_fh );
654 }
655 format ANIMAL_TOP =
657 ID Type Name
658 .
659 format ANIMAL =
661 @## @<<< @<<<<<<<<<<<<<<
662 $id, $type, $name
663 .
664 Although write can work with lexical or package variables, whatever
666 variables you use have to scope in the format. That most likely means
667 you'll want to localize some package variables:
668 {
670 local( $id, $type, $name ) = qw( 12 cat Buster );
671 write( $fh );
672 }
673 print $string;
675 There are also some tricks that you can play with "formline" and the
677 accumulator variable $^A, but you lose a lot of the value of formats
678 since "formline" won't handle paging and so on. You end up
679 reimplementing formats when you use them.
680 How can I output my numbers with commas added?
682 (contributed by brian d foy and Benjamin Goldberg)
683 You can use Number::Format to separate places in a number. It handles
685 locale information for those of you who want to insert full stops
686 instead (or anything else that they want to use, really).
687 This subroutine will add commas to your number:
689 sub commify {
691 local $_ = shift;
692 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
693 return $_;
694 }
695 This regex from Benjamin Goldberg will add commas to numbers:
697 s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
699 It is easier to see with comments:
701 s/(
703 ^[-+]? # beginning of number.
704 \d+? # first digits before first comma
705 (?= # followed by, (but not included in the match) :
706 (?>(?:\d{3})+) # some positive multiple of three digits.
707 (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever.
708 )
709 | # or:
710 \G\d{3} # after the last group, get three digits
711 (?=\d) # but they have to have more digits after them.
712 )/$1,/xg;
713 How can I translate tildes (~) in a filename?
715 Use the <> ("glob()") operator, documented in perlfunc. Versions of
716 Perl older than 5.6 require that you have a shell installed that groks
717 tildes. Later versions of Perl have this feature built in. The
718 File::KGlob module (available from CPAN) gives more portable glob
719 functionality.
720 Within Perl, you may use this directly:
722 $filename =~ s{
724 ^ ~ # find a leading tilde
725 ( # save this in $1
726 [^/] # a non-slash character
727 * # repeated 0 or more times (0 means me)
728 )
729 }{
730 $1
731 ? (getpwnam($1))[7]
732 : ( $ENV{HOME} || $ENV{LOGDIR} )
733 }ex;
734 How come when I open a file read-write it wipes it out?
736 Because you're using something like this, which truncates the file then
737 gives you read-write access:
738 open my $fh, '+>', '/path/name'; # WRONG (almost always)
740 Whoops. You should instead use this, which will fail if the file
742 doesn't exist:
743 open my $fh, '+<', '/path/name'; # open for update
745 Using ">" always clobbers or creates. Using "<" never does either. The
747 "+" doesn't change this.
748 Here are examples of many kinds of file opens. Those using "sysopen"
750 all assume that you've pulled in the constants from Fcntl:
751 use Fcntl;
753 To open file for reading:
755 open my $fh, '<', $path or die $!;
757 sysopen my $fh, $path, O_RDONLY or die $!;
758 To open file for writing, create new file if needed or else truncate
760 old file:
761 open my $fh, '>', $path or die $!;
763 sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT or die $!;
764 sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666 or die $!;
765 To open file for writing, create new file, file must not exist:
767 sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT or die $!;
769 sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT, 0666 or die $!;
770 To open file for appending, create if necessary:
772 open my $fh, '>>', $path or die $!;
774 sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT or die $!;
775 sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT, 0666 or die $!;
776 To open file for appending, file must exist:
778 sysopen my $fh, $path, O_WRONLY|O_APPEND or die $!;
780 To open file for update, file must exist:
782 open my $fh, '+<', $path or die $!;
784 sysopen my $fh, $path, O_RDWR or die $!;
785 To open file for update, create file if necessary:
787 sysopen my $fh, $path, O_RDWR|O_CREAT or die $!;
789 sysopen my $fh, $path, O_RDWR|O_CREAT, 0666 or die $!;
790 To open file for update, file must not exist:
792 sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT or die $!;
794 sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT, 0666 or die $!;
795 To open a file without blocking, creating if necessary:
797 sysopen my $fh, '/foo/somefile', O_WRONLY|O_NDELAY|O_CREAT
799 or die "can't open /foo/somefile: $!":
800 Be warned that neither creation nor deletion of files is guaranteed to
802 be an atomic operation over NFS. That is, two processes might both
803 successfully create or unlink the same file! Therefore O_EXCL isn't as
804 exclusive as you might wish.
805 See also perlopentut.
807 Why do I sometimes get an "Argument list too long" when I use <*>?
809 The "<>" operator performs a globbing operation (see above). In Perl
810 versions earlier than v5.6.0, the internal glob() operator forks csh(1)
811 to do the actual glob expansion, but csh can't handle more than 127
812 items and so gives the error message "Argument list too long". People
813 who installed tcsh as csh won't have this problem, but their users may
814 be surprised by it.
815 To get around this, either upgrade to Perl v5.6.0 or later, do the glob
817 yourself with readdir() and patterns, or use a module like File::Glob,
818 one that doesn't use the shell to do globbing.
819 How can I open a file named with a leading ">" or trailing blanks?
821 (contributed by Brian McCauley)
822 The special two-argument form of Perl's open() function ignores
824 trailing blanks in filenames and infers the mode from certain leading
825 characters (or a trailing "|"). In older versions of Perl this was the
826 only version of open() and so it is prevalent in old code and books.
827 Unless you have a particular reason to use the two-argument form you
829 should use the three-argument form of open() which does not treat any
830 characters in the filename as special.
831 open my $fh, "<", " file "; # filename is " file "
833 open my $fh, ">", ">file"; # filename is ">file"
834 How can I reliably rename a file?
836 If your operating system supports a proper mv(1) utility or its
837 functional equivalent, this works:
838 rename($old, $new) or system("mv", $old, $new);
840 It may be more portable to use the File::Copy module instead. You just
842 copy to the new file to the new name (checking return values), then
843 delete the old one. This isn't really the same semantically as a
844 "rename()", which preserves meta-information like permissions,
845 timestamps, inode info, etc.
846 How can I lock a file?
848 Perl's builtin flock() function (see perlfunc for details) will call
849 flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004
850 and later), and lockf(3) if neither of the two previous system calls
851 exists. On some systems, it may even use a different form of native
852 locking. Here are some gotchas with Perl's flock():
853 1. Produces a fatal error if none of the three system calls (or their
855 close equivalent) exists.
856 2. lockf(3) does not provide shared locking, and requires that the
858 filehandle be open for writing (or appending, or read/writing).
859 3. Some versions of flock() can't lock files over a network (e.g. on
861 NFS file systems), so you'd need to force the use of fcntl(2) when
862 you build Perl. But even this is dubious at best. See the flock
863 entry of perlfunc and the INSTALL file in the source distribution
864 for information on building Perl to do this.
865 Two potentially non-obvious but traditional flock semantics are
867 that it waits indefinitely until the lock is granted, and that its
868 locks are merely advisory. Such discretionary locks are more
869 flexible, but offer fewer guarantees. This means that files locked
870 with flock() may be modified by programs that do not also use
871 flock(). Cars that stop for red lights get on well with each other,
872 but not with cars that don't stop for red lights. See the perlport
873 manpage, your port's specific documentation, or your system-
874 specific local manpages for details. It's best to assume
875 traditional behavior if you're writing portable programs. (If
876 you're not, you should as always feel perfectly free to write for
877 your own system's idiosyncrasies (sometimes called "features").
878 Slavish adherence to portability concerns shouldn't get in the way
879 of your getting your job done.)
880 For more information on file locking, see also "File Locking" in
882 perlopentut if you have it (new for 5.6).
883 Why can't I just open(FH, ">file.lock")?
885 A common bit of code NOT TO USE is this:
886 sleep(3) while -e 'file.lock'; # PLEASE DO NOT USE
888 open my $lock, '>', 'file.lock'; # THIS BROKEN CODE
889 This is a classic race condition: you take two steps to do something
891 which must be done in one. That's why computer hardware provides an
892 atomic test-and-set instruction. In theory, this "ought" to work:
893 sysopen my $fh, "file.lock", O_WRONLY|O_EXCL|O_CREAT
895 or die "can't open file.lock: $!";
896 except that lamentably, file creation (and deletion) is not atomic over
898 NFS, so this won't work (at least, not every time) over the net.
899 Various schemes involving link() have been suggested, but these tend to
900 involve busy-wait, which is also less than desirable.
901 I still don't get locking. I just want to increment the number in the file.
903 How can I do this?
904 Didn't anyone ever tell you web-page hit counters were useless? They
905 don't count number of hits, they're a waste of time, and they serve
906 only to stroke the writer's vanity. It's better to pick a random
907 number; they're more realistic.
908 Anyway, this is what you can do if you can't help yourself.
910 use Fcntl qw(:DEFAULT :flock);
912 sysopen my $fh, "numfile", O_RDWR|O_CREAT or die "can't open numfile: $!";
913 flock $fh, LOCK_EX or die "can't flock numfile: $!";
914 my $num = <$fh> || 0;
915 seek $fh, 0, 0 or die "can't rewind numfile: $!";
916 truncate $fh, 0 or die "can't truncate numfile: $!";
917 (print $fh $num+1, "\n") or die "can't write numfile: $!";
918 close $fh or die "can't close numfile: $!";
919 Here's a much better web-page hit counter:
921 $hits = int( (time() - 850_000_000) / rand(1_000) );
923 If the count doesn't impress your friends, then the code might. :-)
925 All I want to do is append a small amount of text to the end of a file. Do
927 I still have to use locking?
928 If you are on a system that correctly implements "flock" and you use
929 the example appending code from "perldoc -f flock" everything will be
930 OK even if the OS you are on doesn't implement append mode correctly
931 (if such a system exists). So if you are happy to restrict yourself to
932 OSs that implement "flock" (and that's not really much of a
933 restriction) then that is what you should do.
934 If you know you are only going to use a system that does correctly
936 implement appending (i.e. not Win32) then you can omit the "seek" from
937 the code in the previous answer.
938 If you know you are only writing code to run on an OS and filesystem
940 that does implement append mode correctly (a local filesystem on a
941 modern Unix for example), and you keep the file in block-buffered mode
942 and you write less than one buffer-full of output between each manual
943 flushing of the buffer then each bufferload is almost guaranteed to be
944 written to the end of the file in one chunk without getting
945 intermingled with anyone else's output. You can also use the "syswrite"
946 function which is simply a wrapper around your system's write(2) system
947 call.
948 There is still a small theoretical chance that a signal will interrupt
950 the system-level "write()" operation before completion. There is also a
951 possibility that some STDIO implementations may call multiple system
952 level "write()"s even if the buffer was empty to start. There may be
953 some systems where this probability is reduced to zero, and this is not
954 a concern when using ":perlio" instead of your system's STDIO.
955 How do I randomly update a binary file?
957 If you're just trying to patch a binary, in many cases something as
958 simple as this works:
959 perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
961 However, if you have fixed sized records, then you might do something
963 more like this:
964 my $RECSIZE = 220; # size of record, in bytes
966 my $recno = 37; # which record to update
967 open my $fh, '+<', 'somewhere' or die "can't update somewhere: $!";
968 seek $fh, $recno * $RECSIZE, 0;
969 read $fh, $record, $RECSIZE == $RECSIZE or die "can't read record $recno: $!";
970 # munge the record
971 seek $fh, -$RECSIZE, 1;
972 print $fh $record;
973 close $fh;
974 Locking and error checking are left as an exercise for the reader.
976 Don't forget them or you'll be quite sorry.
977 How do I get a file's timestamp in perl?
979 If you want to retrieve the time at which the file was last read,
980 written, or had its meta-data (owner, etc) changed, you use the -A, -M,
981 or -C file test operations as documented in perlfunc. These retrieve
982 the age of the file (measured against the start-time of your program)
983 in days as a floating point number. Some platforms may not have all of
984 these times. See perlport for details. To retrieve the "raw" time in
985 seconds since the epoch, you would call the stat function, then use
986 "localtime()", "gmtime()", or "POSIX::strftime()" to convert this into
987 human-readable form.
988 Here's an example:
990 my $write_secs = (stat($file))[9];
992 printf "file %s updated at %s\n", $file,
993 scalar localtime($write_secs);
994 If you prefer something more legible, use the File::stat module (part
996 of the standard distribution in version 5.004 and later):
997 # error checking left as an exercise for reader.
999 use File::stat;
1000 use Time::localtime;
1001 my $date_string = ctime(stat($file)->mtime);
1002 print "file $file updated at $date_string\n";
1003 The POSIX::strftime() approach has the benefit of being, in theory,
1005 independent of the current locale. See perllocale for details.
1006 How do I set a file's timestamp in perl?
1008 You use the utime() function documented in "utime" in perlfunc. By way
1009 of example, here's a little program that copies the read and write
1010 times from its first argument to all the rest of them.
1011 if (@ARGV < 2) {
1013 die "usage: cptimes timestamp_file other_files ...\n";
1014 }
1015 my $timestamp = shift;
1016 my($atime, $mtime) = (stat($timestamp))[8,9];
1017 utime $atime, $mtime, @ARGV;
1018 Error checking is, as usual, left as an exercise for the reader.
1020 The perldoc for utime also has an example that has the same effect as
1022 touch(1) on files that already exist.
1023 Certain file systems have a limited ability to store the times on a
1025 file at the expected level of precision. For example, the FAT and HPFS
1026 filesystem are unable to create dates on files with a finer granularity
1027 than two seconds. This is a limitation of the filesystems, not of
1028 utime().
1029 How do I print to more than one file at once?
1031 To connect one filehandle to several output filehandles, you can use
1032 the IO::Tee or Tie::FileHandle::Multiplex modules.
1033 If you only have to do this once, you can print individually to each
1035 filehandle.
1036 for my $fh ($fh1, $fh2, $fh3) { print $fh "whatever\n" }
1038 How can I read in an entire file all at once?
1040 The customary Perl approach for processing all the lines in a file is
1041 to do so one line at a time:
1042 open my $input, '<', $file or die "can't open $file: $!";
1044 while (<$input>) {
1045 chomp;
1046 # do something with $_
1047 }
1048 close $input or die "can't close $file: $!";
1049 This is tremendously more efficient than reading the entire file into
1051 memory as an array of lines and then processing it one element at a
1052 time, which is often--if not almost always--the wrong approach.
1053 Whenever you see someone do this:
1054 my @lines = <INPUT>;
1056 You should think long and hard about why you need everything loaded at
1058 once. It's just not a scalable solution.
1059 If you "mmap" the file with the File::Map module from CPAN, you can
1061 virtually load the entire file into a string without actually storing
1062 it in memory:
1063 use File::Map qw(map_file);
1065 map_file my $string, $filename;
1067 Once mapped, you can treat $string as you would any other string.
1069 Since you don't necessarily have to load the data, mmap-ing can be very
1070 fast and may not increase your memory footprint.
1071 You might also find it more fun to use the standard Tie::File module,
1073 or the DB_File module's $DB_RECNO bindings, which allow you to tie an
1074 array to a file so that accessing an element of the array actually
1075 accesses the corresponding line in the file.
1076 If you want to load the entire file, you can use the Path::Tiny module
1078 to do it in one simple and efficient step:
1079 use Path::Tiny;
1081 my $all_of_it = path($filename)->slurp; # entire file in scalar
1083 my @all_lines = path($filename)->lines; # one line per element
1084 Or you can read the entire file contents into a scalar like this:
1086 my $var;
1088 {
1089 local $/;
1090 open my $fh, '<', $file or die "can't open $file: $!";
1091 $var = <$fh>;
1092 }
1093 That temporarily undefs your record separator, and will automatically
1095 close the file at block exit. If the file is already open, just use
1096 this:
1097 my $var = do { local $/; <$fh> };
1099 You can also use a localized @ARGV to eliminate the "open":
1101 my $var = do { local( @ARGV, $/ ) = $file; <> };
1103 For ordinary files you can also use the "read" function.
1105 read( $fh, $var, -s $fh );
1107 That third argument tests the byte size of the data on the $fh
1109 filehandle and reads that many bytes into the buffer $var.
1110 How can I read in a file by paragraphs?
1112 Use the $/ variable (see perlvar for details). You can either set it to
1113 "" to eliminate empty paragraphs ("abc\n\n\n\ndef", for instance, gets
1114 treated as two paragraphs and not three), or "\n\n" to accept empty
1115 paragraphs.
1116 Note that a blank line must have no blanks in it. Thus
1118 "fred\n \nstuff\n\n" is one paragraph, but "fred\n\nstuff\n\n" is two.
1119 How can I read a single character from a file? From the keyboard?
1121 You can use the builtin "getc()" function for most filehandles, but it
1122 won't (easily) work on a terminal device. For STDIN, either use the
1123 Term::ReadKey module from CPAN or use the sample code in "getc" in
1124 perlfunc.
1125 If your system supports the portable operating system programming
1127 interface (POSIX), you can use the following code, which you'll note
1128 turns off echo processing as well.
1129 #!/usr/bin/perl -w
1131 use strict;
1132 $| = 1;
1133 for (1..4) {
1134 print "gimme: ";
1135 my $got = getone();
1136 print "--> $got\n";
1137 }
1138 exit;
1139 BEGIN {
1141 use POSIX qw(:termios_h);
1142 my ($term, $oterm, $echo, $noecho, $fd_stdin);
1144 my $fd_stdin = fileno(STDIN);
1146 $term = POSIX::Termios->new();
1148 $term->getattr($fd_stdin);
1149 $oterm = $term->getlflag();
1150 $echo = ECHO | ECHOK | ICANON;
1152 $noecho = $oterm & ~$echo;
1153 sub cbreak {
1155 $term->setlflag($noecho);
1156 $term->setcc(VTIME, 1);
1157 $term->setattr($fd_stdin, TCSANOW);
1158 }
1159 sub cooked {
1161 $term->setlflag($oterm);
1162 $term->setcc(VTIME, 0);
1163 $term->setattr($fd_stdin, TCSANOW);
1164 }
1165 sub getone {
1167 my $key = '';
1168 cbreak();
1169 sysread(STDIN, $key, 1);
1170 cooked();
1171 return $key;
1172 }
1173 }
1174 END { cooked() }
1176 The Term::ReadKey module from CPAN may be easier to use. Recent
1178 versions include also support for non-portable systems as well.
1179 use Term::ReadKey;
1181 open my $tty, '<', '/dev/tty';
1182 print "Gimme a char: ";
1183 ReadMode "raw";
1184 my $key = ReadKey 0, $tty;
1185 ReadMode "normal";
1186 printf "\nYou said %s, char number %03d\n",
1187 $key, ord $key;
1188 How can I tell whether there's a character waiting on a filehandle?
1190 The very first thing you should do is look into getting the
1191 Term::ReadKey extension from CPAN. As we mentioned earlier, it now even
1192 has limited support for non-portable (read: not open systems, closed,
1193 proprietary, not POSIX, not Unix, etc.) systems.
1194 You should also check out the Frequently Asked Questions list in
1196 comp.unix.* for things like this: the answer is essentially the same.
1197 It's very system-dependent. Here's one solution that works on BSD
1198 systems:
1199 sub key_ready {
1201 my($rin, $nfd);
1202 vec($rin, fileno(STDIN), 1) = 1;
1203 return $nfd = select($rin,undef,undef,0);
1204 }
1205 If you want to find out how many characters are waiting, there's also
1207 the FIONREAD ioctl call to be looked at. The h2ph tool that comes with
1208 Perl tries to convert C include files to Perl code, which can be
1209 "require"d. FIONREAD ends up defined as a function in the sys/ioctl.ph
1210 file:
1211 require './sys/ioctl.ph';
1213 $size = pack("L", 0);
1215 ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
1216 $size = unpack("L", $size);
1217 If h2ph wasn't installed or doesn't work for you, you can grep the
1219 include files by hand:
1220 % grep FIONREAD /usr/include/*/*
1222 /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
1223 Or write a small C program using the editor of champions:
1225 % cat > fionread.c
1227 #include <sys/ioctl.h>
1228 main() {
1229 printf("%#08x\n", FIONREAD);
1230 }
1231 ^D
1232 % cc -o fionread fionread.c
1233 % ./fionread
1234 0x4004667f
1235 And then hard-code it, leaving porting as an exercise to your
1237 successor.
1238 $FIONREAD = 0x4004667f; # XXX: opsys dependent
1240 $size = pack("L", 0);
1242 ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
1243 $size = unpack("L", $size);
1244 FIONREAD requires a filehandle connected to a stream, meaning that
1246 sockets, pipes, and tty devices work, but not files.
1247 How do I do a "tail -f" in perl?
1249 First try
1250 seek($gw_fh, 0, 1);
1252 The statement "seek($gw_fh, 0, 1)" doesn't change the current position,
1254 but it does clear the end-of-file condition on the handle, so that the
1255 next "<$gw_fh>" makes Perl try again to read something.
1256 If that doesn't work (it relies on features of your stdio
1258 implementation), then you need something more like this:
1259 for (;;) {
1261 for ($curpos = tell($gw_fh); <$gw_fh>; $curpos =tell($gw_fh)) {
1262 # search for some stuff and put it into files
1263 }
1264 # sleep for a while
1265 seek($gw_fh, $curpos, 0); # seek to where we had been
1266 }
1267 If this still doesn't work, look into the "clearerr" method from
1269 IO::Handle, which resets the error and end-of-file states on the
1270 handle.
1271 There's also a File::Tail module from CPAN.
1273 How do I dup() a filehandle in Perl?
1275 If you check "open" in perlfunc, you'll see that several of the ways to
1276 call open() should do the trick. For example:
1277 open my $log, '>>', '/foo/logfile';
1279 open STDERR, '>&', $log;
1280 Or even with a literal numeric descriptor:
1282 my $fd = $ENV{MHCONTEXTFD};
1284 open $mhcontext, "<&=$fd"; # like fdopen(3S)
1285 Note that "<&STDIN" makes a copy, but "<&=STDIN" makes an alias. That
1287 means if you close an aliased handle, all aliases become inaccessible.
1288 This is not true with a copied one.
1289 Error checking, as always, has been left as an exercise for the reader.
1291 How do I close a file descriptor by number?
1293 If, for some reason, you have a file descriptor instead of a filehandle
1294 (perhaps you used "POSIX::open"), you can use the "close()" function
1295 from the POSIX module:
1296 use POSIX ();
1298 POSIX::close( $fd );
1300 This should rarely be necessary, as the Perl "close()" function is to
1302 be used for things that Perl opened itself, even if it was a dup of a
1303 numeric descriptor as with "MHCONTEXT" above. But if you really have
1304 to, you may be able to do this:
1305 require './sys/syscall.ph';
1307 my $rc = syscall(SYS_close(), $fd + 0); # must force numeric
1308 die "can't sysclose $fd: $!" unless $rc == -1;
1309 Or, just use the fdopen(3S) feature of "open()":
1311 {
1313 open my $fh, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
1314 close $fh;
1315 }
1316 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe`
1318 work?
1319 Whoops! You just put a tab and a formfeed into that filename!
1320 Remember that within double quoted strings ("like\this"), the backslash
1321 is an escape character. The full list of these is in "Quote and Quote-
1322 like Operators" in perlop. Unsurprisingly, you don't have a file called
1323 "c:(tab)emp(formfeed)oo" or "c:(tab)emp(formfeed)oo.exe" on your legacy
1324 DOS filesystem.
1325 Either single-quote your strings, or (preferably) use forward slashes.
1327 Since all DOS and Windows versions since something like MS-DOS 2.0 or
1328 so have treated "/" and "\" the same in a path, you might as well use
1329 the one that doesn't clash with Perl--or the POSIX shell, ANSI C and
1330 C++, awk, Tcl, Java, or Python, just to mention a few. POSIX paths are
1331 more portable, too.
1332 Why doesn't glob("*.*") get all the files?
1334 Because even on non-Unix ports, Perl's glob function follows standard
1335 Unix globbing semantics. You'll need "glob("*")" to get all (non-
1336 hidden) files. This makes glob() portable even to legacy systems. Your
1337 port may include proprietary globbing functions as well. Check its
1338 documentation for details.
1339 Why does Perl let me delete read-only files? Why does "-i" clobber
1341 protected files? Isn't this a bug in Perl?
1342 This is elaborately and painstakingly described in the file-dir-perms
1343 article in the "Far More Than You Ever Wanted To Know" collection in
1344 <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz> .
1345 The executive summary: learn how your filesystem works. The permissions
1347 on a file say what can happen to the data in that file. The
1348 permissions on a directory say what can happen to the list of files in
1349 that directory. If you delete a file, you're removing its name from the
1350 directory (so the operation depends on the permissions of the
1351 directory, not of the file). If you try to write to the file, the
1352 permissions of the file govern whether you're allowed to.
1353 How do I select a random line from a file?
1355 Short of loading the file into a database or pre-indexing the lines in
1356 the file, there are a couple of things that you can do.
1357 Here's a reservoir-sampling algorithm from the Camel Book:
1359 srand;
1361 rand($.) < 1 && ($line = $_) while <>;
1362 This has a significant advantage in space over reading the whole file
1364 in. You can find a proof of this method in The Art of Computer
1365 Programming, Volume 2, Section 3.4.2, by Donald E. Knuth.
1366 You can use the File::Random module which provides a function for that
1368 algorithm:
1369 use File::Random qw/random_line/;
1371 my $line = random_line($filename);
1372 Another way is to use the Tie::File module, which treats the entire
1374 file as an array. Simply access a random array element.
1375 Why do I get weird spaces when I print an array of lines?
1377 (contributed by brian d foy)
1378 If you are seeing spaces between the elements of your array when you
1380 print the array, you are probably interpolating the array in double
1381 quotes:
1382 my @animals = qw(camel llama alpaca vicuna);
1384 print "animals are: @animals\n";
1385 It's the double quotes, not the "print", doing this. Whenever you
1387 interpolate an array in a double quote context, Perl joins the elements
1388 with spaces (or whatever is in $", which is a space by default):
1389 animals are: camel llama alpaca vicuna
1391 This is different than printing the array without the interpolation:
1393 my @animals = qw(camel llama alpaca vicuna);
1395 print "animals are: ", @animals, "\n";
1396 Now the output doesn't have the spaces between the elements because the
1398 elements of @animals simply become part of the list to "print":
1399 animals are: camelllamaalpacavicuna
1401 You might notice this when each of the elements of @array end with a
1403 newline. You expect to print one element per line, but notice that
1404 every line after the first is indented:
1405 this is a line
1407 this is another line
1408 this is the third line
1409 That extra space comes from the interpolation of the array. If you
1411 don't want to put anything between your array elements, don't use the
1412 array in double quotes. You can send it to print without them:
1413 print @lines;
1415 How do I traverse a directory tree?
1417 (contributed by brian d foy)
1418 The File::Find module, which comes with Perl, does all of the hard work
1420 to traverse a directory structure. It comes with Perl. You simply call
1421 the "find" subroutine with a callback subroutine and the directories
1422 you want to traverse:
1423 use File::Find;
1425 find( \&wanted, @directories );
1427 sub wanted {
1429 # full path in $File::Find::name
1430 # just filename in $_
1431 ... do whatever you want to do ...
1432 }
1433 The File::Find::Closures, which you can download from CPAN, provides
1435 many ready-to-use subroutines that you can use with File::Find.
1436 The File::Finder, which you can download from CPAN, can help you create
1438 the callback subroutine using something closer to the syntax of the
1439 "find" command-line utility:
1440 use File::Find;
1442 use File::Finder;
1443 my $deep_dirs = File::Finder->depth->type('d')->ls->exec('rmdir','{}');
1445 find( $deep_dirs->as_options, @places );
1447 The File::Find::Rule module, which you can download from CPAN, has a
1449 similar interface, but does the traversal for you too:
1450 use File::Find::Rule;
1452 my @files = File::Find::Rule->file()
1454 ->name( '*.pm' )
1455 ->in( @INC );
1456 How do I delete a directory tree?
1458 (contributed by brian d foy)
1459 If you have an empty directory, you can use Perl's built-in "rmdir".
1461 If the directory is not empty (so, with files or subdirectories), you
1462 either have to empty it yourself (a lot of work) or use a module to
1463 help you.
1464 The File::Path module, which comes with Perl, has a "remove_tree" which
1466 can take care of all of the hard work for you:
1467 use File::Path qw(remove_tree);
1469 remove_tree( @directories );
1471 The File::Path module also has a legacy interface to the older "rmtree"
1473 subroutine.
1474 How do I copy an entire directory?
1476 (contributed by Shlomi Fish)
1477 To do the equivalent of "cp -R" (i.e. copy an entire directory tree
1479 recursively) in portable Perl, you'll either need to write something
1480 yourself or find a good CPAN module such as File::Copy::Recursive.
1481
1483 Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and other
1484 authors as noted. All rights reserved.
1485 This documentation is free; you can redistribute it and/or modify it
1487 under the same terms as Perl itself.
1488 Irrespective of its distribution, all code examples here are in the
1490 public domain. You are permitted and encouraged to use this code and
1491 any derivatives thereof in your own programs for fun or for profit as
1492 you see fit. A simple comment in the code giving credit to the FAQ
1493 would be courteous but is not required.
1494perl v5.32.0 2020-11-09 perlfaq5(3)