note strict/lax version requirements in documentation
[p5sagit/p5-mst-13.2.git] / pod / perlfaq5.pod
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68dc0745 1=head1 NAME
2
109f0441 3perlfaq5 - Files and Formats
68dc0745 4
5=head1 DESCRIPTION
6
7This section deals with I/O and the "f" issues: filehandles, flushing,
8formats, and footers.
9
5a964f20 10=head2 How do I flush/unbuffer an output filehandle? Why must I do this?
d74e8afc 11X<flush> X<buffer> X<unbuffer> X<autoflush>
68dc0745 12
109f0441 13(contributed by brian d foy)
5a964f20 14
109f0441 15You might like to read Mark Jason Dominus's "Suffering From Buffering"
16at http://perl.plover.com/FAQs/Buffering.html .
68dc0745 17
109f0441 18Perl normally buffers output so it doesn't make a system call for every
19bit of output. By saving up output, it makes fewer expensive system calls.
20For instance, in this little bit of code, you want to print a dot to the
21screen for every line you process to watch the progress of your program.
22Instead of seeing a dot for every line, Perl buffers the output and you
23have a long wait before you see a row of 50 dots all at once:
24
25 # long wait, then row of dots all at once
26 while( <> ) {
27 print ".";
28 print "\n" unless ++$count % 50;
29
30 #... expensive line processing operations
31 }
32
33To get around this, you have to unbuffer the output filehandle, in this
34case, C<STDOUT>. You can set the special variable C<$|> to a true value
35(mnemonic: making your filehandles "piping hot"):
36
37 $|++;
38
39 # dot shown immediately
40 while( <> ) {
41 print ".";
42 print "\n" unless ++$count % 50;
43
44 #... expensive line processing operations
45 }
46
47The C<$|> is one of the per-filehandle special variables, so each
48filehandle has its own copy of its value. If you want to merge
49standard output and standard error for instance, you have to unbuffer
50each (although STDERR might be unbuffered by default):
51
52 {
53 my $previous_default = select(STDOUT); # save previous default
54 $|++; # autoflush STDOUT
55 select(STDERR);
56 $|++; # autoflush STDERR, to be sure
57 select($previous_default); # restore previous default
58 }
68dc0745 59
109f0441 60 # now should alternate . and +
61 while( 1 )
62 {
63 sleep 1;
64 print STDOUT ".";
65 print STDERR "+";
66 print STDOUT "\n" unless ++$count % 25;
67 }
68
69Besides the C<$|> special variable, you can use C<binmode> to give
70your filehandle a C<:unix> layer, which is unbuffered:
71
72 binmode( STDOUT, ":unix" );
68dc0745 73
109f0441 74 while( 1 ) {
75 sleep 1;
76 print ".";
77 print "\n" unless ++$count % 50;
78 }
68dc0745 79
109f0441 80For more information on output layers, see the entries for C<binmode>
81and C<open> in L<perlfunc>, and the C<PerlIO> module documentation.
68dc0745 82
109f0441 83If you are using C<IO::Handle> or one of its subclasses, you can
84call the C<autoflush> method to change the settings of the
85filehandle:
c195e131 86
87 use IO::Handle;
109f0441 88 open my( $io_fh ), ">", "output.txt";
89 $io_fh->autoflush(1);
90
91The C<IO::Handle> objects also have a C<flush> method. You can flush
92the buffer any time you want without auto-buffering
c195e131 93
109f0441 94 $io_fh->flush;
487af187 95
e573f903 96=head2 How do I change, delete, or insert a line in a file, or append to the beginning of a file?
d74e8afc 97X<file, editing>
68dc0745 98
e573f903 99(contributed by brian d foy)
100
101The basic idea of inserting, changing, or deleting a line from a text
102file involves reading and printing the file to the point you want to
103make the change, making the change, then reading and printing the rest
104of the file. Perl doesn't provide random access to lines (especially
105since the record input separator, C<$/>, is mutable), although modules
106such as C<Tie::File> can fake it.
107
108A Perl program to do these tasks takes the basic form of opening a
109file, printing its lines, then closing the file:
110
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: $!";
113
114 while( <$in> )
115 {
116 print $out $_;
117 }
118
119 close $out;
120
121Within that basic form, add the parts that you need to insert, change,
122or delete lines.
123
124To prepend lines to the beginning, print those lines before you enter
125the loop that prints the existing lines.
126
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: $!";
129
109f0441 130 print $out "# Add this line to the top\n"; # <--- HERE'S THE MAGIC
e573f903 131
132 while( <$in> )
133 {
134 print $out $_;
135 }
136
137 close $out;
138
139To change existing lines, insert the code to modify the lines inside
140the C<while> loop. In this case, the code finds all lowercased
141versions of "perl" and uppercases them. The happens for every line, so
142be sure that you're supposed to do that on every line!
143
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: $!";
146
109f0441 147 print $out "# Add this line to the top\n";
e573f903 148
149 while( <$in> )
150 {
151 s/\b(perl)\b/Perl/g;
152 print $out $_;
153 }
154
155 close $out;
156
157To change only a particular line, the input line number, C<$.>, is
ee891a00 158useful. First read and print the lines up to the one you want to
159change. Next, read the single line you want to change, change it, and
160print it. After that, read the rest of the lines and print those:
e573f903 161
ee891a00 162 while( <$in> ) # print the lines before the change
e573f903 163 {
e573f903 164 print $out $_;
ee891a00 165 last if $. == 4; # line number before change
e573f903 166 }
167
ee891a00 168 my $line = <$in>;
169 $line =~ s/\b(perl)\b/Perl/g;
170 print $out $line;
171
172 while( <$in> ) # print the rest of the lines
173 {
174 print $out $_;
175 }
109f0441 176
e573f903 177To skip lines, use the looping controls. The C<next> in this example
178skips comment lines, and the C<last> stops all processing once it
179encounters either C<__END__> or C<__DATA__>.
180
181 while( <$in> )
182 {
183 next if /^\s+#/; # skip comment lines
184 last if /^__(END|DATA)__$/; # stop at end of code marker
185 print $out $_;
186 }
187
188Do the same sort of thing to delete a particular line by using C<next>
189to skip the lines you don't want to show up in the output. This
190example skips every fifth line:
191
192 while( <$in> )
193 {
194 next unless $. % 5;
195 print $out $_;
196 }
197
198If, for some odd reason, you really want to see the whole file at once
f12f5f55 199rather than processing line-by-line, you can slurp it in (as long as
e573f903 200you can fit the whole thing in memory!):
201
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: $!";
204
205 my @lines = do { local $/; <$in> }; # slurp!
206
207 # do your magic here
208
209 print $out @lines;
210
211Modules such as C<File::Slurp> and C<Tie::File> can help with that
212too. If you can, however, avoid reading the entire file at once. Perl
213won't give that memory back to the operating system until the process
214finishes.
215
216You can also use Perl one-liners to modify a file in-place. The
217following changes all 'Fred' to 'Barney' in F<inFile.txt>, overwriting
218the file with the new contents. With the C<-p> switch, Perl wraps a
219C<while> loop around the code you specify with C<-e>, and C<-i> turns
220on in-place editing. The current line is in C<$_>. With C<-p>, Perl
221automatically prints the value of C<$_> at the end of the loop. See
222L<perlrun> for more details.
223
224 perl -pi -e 's/Fred/Barney/' inFile.txt
225
226To make a backup of C<inFile.txt>, give C<-i> a file extension to add:
227
228 perl -pi.bak -e 's/Fred/Barney/' inFile.txt
229
230To change only the fifth line, you can add a test checking C<$.>, the
231input line number, then only perform the operation when the test
232passes:
233
234 perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt
235
236To add lines before a certain line, you can add a line (or lines!)
237before Perl prints C<$_>:
238
239 perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt
240
241You can even add a line to the beginning of a file, since the current
242line prints at the end of the loop:
243
244 perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt
245
246To insert a line after one already in the file, use the C<-n> switch.
247It's just like C<-p> except that it doesn't print C<$_> at the end of
248the loop, so you have to do that yourself. In this case, print C<$_>
249first, then print the line that you want to add.
250
251 perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt
252
253To delete lines, only print the ones that you want.
254
255 perl -ni -e 'print unless /d/' inFile.txt
256
257 ... or ...
258
259 perl -pi -e 'next unless /d/' inFile.txt
68dc0745 260
261=head2 How do I count the number of lines in a file?
d74e8afc 262X<file, counting lines> X<lines> X<line>
68dc0745 263
8d2e243f 264(contributed by brian d foy)
265
266Conceptually, the easiest way to count the lines in a file is to
267simply read them and count them:
268
269 my $count = 0;
270 while( <$fh> ) { $count++; }
271
272You don't really have to count them yourself, though, since Perl
273already does that with the C<$.> variable, which is the current line
274number from the last filehandle read:
275
276 1 while( <$fh> );
277 my $count = $.;
278
279If you want to use C<$.>, you can reduce it to a simple one-liner,
280like one of these:
281
282 % perl -lne '} print $.; {' file
283
284 % perl -lne 'END { print $. }' file
285
286Those can be rather inefficient though. If they aren't fast enough for
287you, you might just read chunks of data and count the number of
288newlines:
289
290 my $lines = 0;
291 open my($fh), '<:raw', $filename or die "Can't open $filename: $!";
292 while( sysread $fh, $buffer, 4096 ) {
293 $lines += ( $buffer =~ tr/\n// );
500071f4 294 }
295 close FILE;
68dc0745 296
8d2e243f 297However, that doesn't work if the line ending isn't a newline. You
298might change that C<tr///> to a C<s///> so you can count the number of
299times the input record separator, C<$/>, shows up:
300
301 my $lines = 0;
302 open my($fh), '<:raw', $filename or die "Can't open $filename: $!";
303 while( sysread $fh, $buffer, 4096 ) {
304 $lines += ( $buffer =~ s|$/||g; );
305 }
306 close FILE;
307
308If you don't mind shelling out, the C<wc> command is usually the
309fastest, even with the extra interprocess overhead. Ensure that you
310have an untainted filename though:
311
312 #!perl -T
313
314 $ENV{PATH} = undef;
315
316 my $lines;
317 if( $filename =~ /^([0-9a-z_.]+)\z/ ) {
318 $lines = `/usr/bin/wc -l $1`
319 chomp $lines;
320 }
5a964f20 321
589a5df2 322=head2 How do I delete the last N lines from a file?
323X<lines> X<file>
324
325(contributed by brian d foy)
326
327The easiest conceptual solution is to count the lines in the
328file then start at the beginning and print the number of lines
329(minus the last N) to a new file.
330
331Most often, the real question is how you can delete the last N
332lines without making more than one pass over the file, or how to
333do it with a lot of copying. The easy concept is the hard reality when
334you might have millions of lines in your file.
335
336One trick is to use C<File::ReadBackwards>, which starts at the end of
337the file. That module provides an object that wraps the real filehandle
338to make it easy for you to move around the file. Once you get to the
339spot you need, you can get the actual filehandle and work with it as
340normal. In this case, you get the file position at the end of the last
341line you want to keep and truncate the file to that point:
342
343 use File::ReadBackwards;
344
345 my $filename = 'test.txt';
346 my $Lines_to_truncate = 2;
347
348 my $bw = File::ReadBackwards->new( $filename )
349 or die "Could not read backwards in [$filename]: $!";
350
351 my $lines_from_end = 0;
352 until( $bw->eof or $lines_from_end == $Lines_to_truncate )
353 {
354 print "Got: ", $bw->readline;
355 $lines_from_end++;
356 }
357
358 truncate( $filename, $bw->tell );
359
360The C<File::ReadBackwards> module also has the advantage of setting
361the input record separator to a regular expression.
362
363You can also use the C<Tie::File> module which lets you access
364the lines through a tied array. You can use normal array operations
365to modify your file, including setting the last index and using
366C<splice>.
367
4750257b 368=head2 How can I use Perl's C<-i> option from within a program?
d74e8afc 369X<-i> X<in-place>
4750257b 370
371C<-i> sets the value of Perl's C<$^I> variable, which in turn affects
372the behavior of C<< <> >>; see L<perlrun> for more details. By
373modifying the appropriate variables directly, you can get the same
374behavior within a larger program. For example:
375
500071f4 376 # ...
377 {
378 local($^I, @ARGV) = ('.orig', glob("*.c"));
379 while (<>) {
380 if ($. == 1) {
381 print "This line should appear at the top of each file\n";
382 }
383 s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
384 print;
385 close ARGV if eof; # Reset $.
386 }
387 }
388 # $^I and @ARGV return to their old values here
4750257b 389
390This block modifies all the C<.c> files in the current directory,
391leaving a backup of the original data from each file in a new
392C<.c.orig> file.
393
7678cced 394=head2 How can I copy a file?
109f0441 395X<copy> X<file, copy> X<File::Copy>
7678cced 396
397(contributed by brian d foy)
398
109f0441 399Use the C<File::Copy> module. It comes with Perl and can do a
7678cced 400true copy across file systems, and it does its magic in
401a portable fashion.
402
403 use File::Copy;
404
405 copy( $original, $new_copy ) or die "Copy failed: $!";
406
109f0441 407If you can't use C<File::Copy>, you'll have to do the work yourself:
7678cced 408open the original file, open the destination file, then print
109f0441 409to the destination file as you read the original. You also have to
410remember to copy the permissions, owner, and group to the new file.
7678cced 411
68dc0745 412=head2 How do I make a temporary file name?
d74e8afc 413X<file, temporary>
68dc0745 414
7678cced 415If you don't need to know the name of the file, you can use C<open()>
109f0441 416with C<undef> in place of the file name. In Perl 5.8 or later, the
417C<open()> function creates an anonymous temporary file:
7678cced 418
419 open my $tmp, '+>', undef or die $!;
6670e5e7 420
7678cced 421Otherwise, you can use the File::Temp module.
68dc0745 422
500071f4 423 use File::Temp qw/ tempfile tempdir /;
a6dd486b 424
500071f4 425 $dir = tempdir( CLEANUP => 1 );
426 ($fh, $filename) = tempfile( DIR => $dir );
5a964f20 427
500071f4 428 # or if you don't need to know the filename
5a964f20 429
500071f4 430 $fh = tempfile( DIR => $dir );
5a964f20 431
16394a69 432The File::Temp has been a standard module since Perl 5.6.1. If you
433don't have a modern enough Perl installed, use the C<new_tmpfile>
434class method from the IO::File module to get a filehandle opened for
435reading and writing. Use it if you don't need to know the file's name:
5a964f20 436
500071f4 437 use IO::File;
438 $fh = IO::File->new_tmpfile()
16394a69 439 or die "Unable to make new temporary file: $!";
5a964f20 440
a6dd486b 441If you're committed to creating a temporary file by hand, use the
442process ID and/or the current time-value. If you need to have many
443temporary files in one process, use a counter:
5a964f20 444
500071f4 445 BEGIN {
68dc0745 446 use Fcntl;
16394a69 447 my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
c195e131 448 my $base_name = sprintf "%s/%d-%d-0000", $temp_dir, $$, time;
500071f4 449
68dc0745 450 sub temp_file {
500071f4 451 local *FH;
452 my $count = 0;
c195e131 453 until( defined(fileno(FH)) || $count++ > 100 ) {
454 $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
455 # O_EXCL is required for security reasons.
456 sysopen FH, $base_name, O_WRONLY|O_EXCL|O_CREAT;
457 }
458
459 if( defined fileno(FH) ) {
460 return (*FH, $base_name);
461 }
462 else {
463 return ();
464 }
500071f4 465 }
109f0441 466
500071f4 467 }
68dc0745 468
68dc0745 469=head2 How can I manipulate fixed-record-length files?
d74e8afc 470X<fixed-length> X<file, fixed-length records>
68dc0745 471
793f5136 472The most efficient way is using L<pack()|perlfunc/"pack"> and
473L<unpack()|perlfunc/"unpack">. This is faster than using
474L<substr()|perlfunc/"substr"> when taking many, many strings. It is
475slower for just a few.
5a964f20 476
477Here is a sample chunk of code to break up and put back together again
478some fixed-format input lines, in this case from the output of a normal,
479Berkeley-style ps:
68dc0745 480
500071f4 481 # sample input line:
482 # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
483 my $PS_T = 'A6 A4 A7 A5 A*';
484 open my $ps, '-|', 'ps';
485 print scalar <$ps>;
486 my @fields = qw( pid tt stat time command );
487 while (<$ps>) {
488 my %process;
489 @process{@fields} = unpack($PS_T, $_);
793f5136 490 for my $field ( @fields ) {
500071f4 491 print "$field: <$process{$field}>\n";
68dc0745 492 }
793f5136 493 print 'line=', pack($PS_T, @process{@fields} ), "\n";
500071f4 494 }
68dc0745 495
793f5136 496We've used a hash slice in order to easily handle the fields of each row.
497Storing the keys in an array means it's easy to operate on them as a
498group or loop over them with for. It also avoids polluting the program
499with global variables and using symbolic references.
5a964f20 500
ac9dac7f 501=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?
d74e8afc 502X<filehandle, local> X<filehandle, passing> X<filehandle, reference>
68dc0745 503
c90536be 504As of perl5.6, open() autovivifies file and directory handles
505as references if you pass it an uninitialized scalar variable.
506You can then pass these references just like any other scalar,
507and use them in the place of named handles.
68dc0745 508
c90536be 509 open my $fh, $file_name;
818c4caa 510
c90536be 511 open local $fh, $file_name;
818c4caa 512
c90536be 513 print $fh "Hello World!\n";
818c4caa 514
c90536be 515 process_file( $fh );
68dc0745 516
500071f4 517If you like, you can store these filehandles in an array or a hash.
518If you access them directly, they aren't simple scalars and you
ac9dac7f 519need to give C<print> a little help by placing the filehandle
500071f4 520reference in braces. Perl can only figure it out on its own when
521the filehandle reference is a simple scalar.
522
523 my @fhs = ( $fh1, $fh2, $fh3 );
ac9dac7f 524
500071f4 525 for( $i = 0; $i <= $#fhs; $i++ ) {
526 print {$fhs[$i]} "just another Perl answer, \n";
527 }
528
c90536be 529Before perl5.6, you had to deal with various typeglob idioms
530which you may see in older code.
68dc0745 531
c90536be 532 open FILE, "> $filename";
533 process_typeglob( *FILE );
534 process_reference( \*FILE );
818c4caa 535
c90536be 536 sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
537 sub process_reference { local $fh = shift; print $fh "Reference!" }
5a964f20 538
c90536be 539If you want to create many anonymous handles, you should
540check out the Symbol or IO::Handle modules.
5a964f20 541
542=head2 How can I use a filehandle indirectly?
d74e8afc 543X<filehandle, indirect>
5a964f20 544
545An indirect filehandle is using something other than a symbol
546in a place that a filehandle is expected. Here are ways
a6dd486b 547to get indirect filehandles:
5a964f20 548
500071f4 549 $fh = SOME_FH; # bareword is strict-subs hostile
550 $fh = "SOME_FH"; # strict-refs hostile; same package only
551 $fh = *SOME_FH; # typeglob
552 $fh = \*SOME_FH; # ref to typeglob (bless-able)
553 $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
5a964f20 554
c90536be 555Or, you can use the C<new> method from one of the IO::* modules to
5a964f20 556create an anonymous filehandle, store that in a scalar variable,
557and use it as though it were a normal filehandle.
558
500071f4 559 use IO::Handle; # 5.004 or higher
560 $fh = IO::Handle->new();
5a964f20 561
562Then use any of those as you would a normal filehandle. Anywhere that
563Perl is expecting a filehandle, an indirect filehandle may be used
564instead. An indirect filehandle is just a scalar variable that contains
368c9434 565a filehandle. Functions like C<print>, C<open>, C<seek>, or
c90536be 566the C<< <FH> >> diamond operator will accept either a named filehandle
5a964f20 567or a scalar variable containing one:
568
500071f4 569 ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
570 print $ofh "Type it: ";
571 $got = <$ifh>
572 print $efh "What was that: $got";
5a964f20 573
368c9434 574If you're passing a filehandle to a function, you can write
5a964f20 575the function in two ways:
576
500071f4 577 sub accept_fh {
578 my $fh = shift;
579 print $fh "Sending to indirect filehandle\n";
580 }
46fc3d4c 581
5a964f20 582Or it can localize a typeglob and use the filehandle directly:
46fc3d4c 583
500071f4 584 sub accept_fh {
585 local *FH = shift;
586 print FH "Sending to localized filehandle\n";
587 }
46fc3d4c 588
5a964f20 589Both styles work with either objects or typeglobs of real filehandles.
590(They might also work with strings under some circumstances, but this
591is risky.)
592
500071f4 593 accept_fh(*STDOUT);
594 accept_fh($handle);
5a964f20 595
596In the examples above, we assigned the filehandle to a scalar variable
a6dd486b 597before using it. That is because only simple scalar variables, not
598expressions or subscripts of hashes or arrays, can be used with
599built-ins like C<print>, C<printf>, or the diamond operator. Using
8305e449 600something other than a simple scalar variable as a filehandle is
5a964f20 601illegal and won't even compile:
602
500071f4 603 @fd = (*STDIN, *STDOUT, *STDERR);
604 print $fd[1] "Type it: "; # WRONG
605 $got = <$fd[0]> # WRONG
606 print $fd[2] "What was that: $got"; # WRONG
5a964f20 607
608With C<print> and C<printf>, you get around this by using a block and
609an expression where you would place the filehandle:
610
500071f4 611 print { $fd[1] } "funny stuff\n";
612 printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
613 # Pity the poor deadbeef.
5a964f20 614
615That block is a proper block like any other, so you can put more
616complicated code there. This sends the message out to one of two places:
617
500071f4 618 $ok = -x "/bin/cat";
619 print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
620 print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
5a964f20 621
622This approach of treating C<print> and C<printf> like object methods
623calls doesn't work for the diamond operator. That's because it's a
624real operator, not just a function with a comma-less argument. Assuming
625you've been storing typeglobs in your structure as we did above, you
c90536be 626can use the built-in function named C<readline> to read a record just
c47ff5f1 627as C<< <> >> does. Given the initialization shown above for @fd, this
c90536be 628would work, but only because readline() requires a typeglob. It doesn't
5a964f20 629work with objects or strings, which might be a bug we haven't fixed yet.
630
500071f4 631 $got = readline($fd[0]);
5a964f20 632
633Let it be noted that the flakiness of indirect filehandles is not
634related to whether they're strings, typeglobs, objects, or anything else.
635It's the syntax of the fundamental operators. Playing the object
636game doesn't help you at all here.
46fc3d4c 637
68dc0745 638=head2 How can I set up a footer format to be used with write()?
d74e8afc 639X<footer>
68dc0745 640
54310121 641There's no builtin way to do this, but L<perlform> has a couple of
68dc0745 642techniques to make it possible for the intrepid hacker.
643
644=head2 How can I write() into a string?
d74e8afc 645X<write, into a string>
68dc0745 646
c195e131 647See L<perlform/"Accessing Formatting Internals"> for an C<swrite()> function.
68dc0745 648
c195e131 649=head2 How can I open a filehandle to a string?
109f0441 650X<string> X<open> X<IO::String> X<filehandle>
c195e131 651
652(contributed by Peter J. Holzer, hjp-usenet2@hjp.at)
653
109f0441 654Since Perl 5.8.0 a file handle referring to a string can be created by
655calling open with a reference to that string instead of the filename.
656This file handle can then be used to read from or write to the string:
c195e131 657
658 open(my $fh, '>', \$string) or die "Could not open string for writing";
659 print $fh "foo\n";
660 print $fh "bar\n"; # $string now contains "foo\nbar\n"
661
662 open(my $fh, '<', \$string) or die "Could not open string for reading";
663 my $x = <$fh>; # $x now contains "foo\n"
664
665With older versions of Perl, the C<IO::String> module provides similar
666functionality.
487af187 667
68dc0745 668=head2 How can I output my numbers with commas added?
d74e8afc 669X<number, commify>
68dc0745 670
b68463f7 671(contributed by brian d foy and Benjamin Goldberg)
672
673You can use L<Number::Format> to separate places in a number.
674It handles locale information for those of you who want to insert
675full stops instead (or anything else that they want to use,
676really).
677
49d635f9 678This subroutine will add commas to your number:
679
680 sub commify {
500071f4 681 local $_ = shift;
682 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
683 return $_;
684 }
49d635f9 685
686This regex from Benjamin Goldberg will add commas to numbers:
68dc0745 687
500071f4 688 s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
68dc0745 689
49d635f9 690It is easier to see with comments:
68dc0745 691
500071f4 692 s/(
693 ^[-+]? # beginning of number.
694 \d+? # first digits before first comma
695 (?= # followed by, (but not included in the match) :
696 (?>(?:\d{3})+) # some positive multiple of three digits.
697 (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever.
698 )
699 | # or:
700 \G\d{3} # after the last group, get three digits
701 (?=\d) # but they have to have more digits after them.
702 )/$1,/xg;
46fc3d4c 703
68dc0745 704=head2 How can I translate tildes (~) in a filename?
d74e8afc 705X<tilde> X<tilde expansion>
68dc0745 706
109f0441 707Use the E<lt>E<gt> (C<glob()>) operator, documented in L<perlfunc>.
708Versions of Perl older than 5.6 require that you have a shell
709installed that groks tildes. Later versions of Perl have this feature
710built in. The C<File::KGlob> module (available from CPAN) gives more
711portable glob functionality.
68dc0745 712
713Within Perl, you may use this directly:
714
715 $filename =~ s{
716 ^ ~ # find a leading tilde
717 ( # save this in $1
718 [^/] # a non-slash character
719 * # repeated 0 or more times (0 means me)
720 )
721 }{
722 $1
723 ? (getpwnam($1))[7]
724 : ( $ENV{HOME} || $ENV{LOGDIR} )
725 }ex;
726
5a964f20 727=head2 How come when I open a file read-write it wipes it out?
d74e8afc 728X<clobber> X<read-write> X<clobbering> X<truncate> X<truncating>
68dc0745 729
730Because you're using something like this, which truncates the file and
731I<then> gives you read-write access:
732
500071f4 733 open(FH, "+> /path/name"); # WRONG (almost always)
68dc0745 734
735Whoops. You should instead use this, which will fail if the file
197aec24 736doesn't exist.
d92eb7b0 737
500071f4 738 open(FH, "+< /path/name"); # open for update
d92eb7b0 739
c47ff5f1 740Using ">" always clobbers or creates. Using "<" never does
d92eb7b0 741either. The "+" doesn't change this.
68dc0745 742
5a964f20 743Here are examples of many kinds of file opens. Those using sysopen()
744all assume
68dc0745 745
500071f4 746 use Fcntl;
68dc0745 747
5a964f20 748To open file for reading:
68dc0745 749
500071f4 750 open(FH, "< $path") || die $!;
751 sysopen(FH, $path, O_RDONLY) || die $!;
5a964f20 752
753To open file for writing, create new file if needed or else truncate old file:
754
500071f4 755 open(FH, "> $path") || die $!;
756 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
757 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
5a964f20 758
759To open file for writing, create new file, file must not exist:
760
500071f4 761 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
762 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
5a964f20 763
764To open file for appending, create if necessary:
765
500071f4 766 open(FH, ">> $path") || die $!;
767 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
768 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
5a964f20 769
770To open file for appending, file must exist:
771
500071f4 772 sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
5a964f20 773
774To open file for update, file must exist:
775
500071f4 776 open(FH, "+< $path") || die $!;
777 sysopen(FH, $path, O_RDWR) || die $!;
5a964f20 778
779To open file for update, create file if necessary:
780
500071f4 781 sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
782 sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
5a964f20 783
784To open file for update, file must not exist:
785
500071f4 786 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
787 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
5a964f20 788
789To open a file without blocking, creating if necessary:
790
500071f4 791 sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
2359510d 792 or die "can't open /foo/somefile: $!":
5a964f20 793
794Be warned that neither creation nor deletion of files is guaranteed to
795be an atomic operation over NFS. That is, two processes might both
a6dd486b 796successfully create or unlink the same file! Therefore O_EXCL
797isn't as exclusive as you might wish.
68dc0745 798
87275199 799See also the new L<perlopentut> if you have it (new for 5.6).
65acb1b1 800
04d666b1 801=head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
d74e8afc 802X<argument list too long>
68dc0745 803
c47ff5f1 804The C<< <> >> operator performs a globbing operation (see above).
3a4b19e4 805In Perl versions earlier than v5.6.0, the internal glob() operator forks
806csh(1) to do the actual glob expansion, but
68dc0745 807csh can't handle more than 127 items and so gives the error message
808C<Argument list too long>. People who installed tcsh as csh won't
809have this problem, but their users may be surprised by it.
810
3a4b19e4 811To get around this, either upgrade to Perl v5.6.0 or later, do the glob
d6260402 812yourself with readdir() and patterns, or use a module like File::KGlob,
3a4b19e4 813one that doesn't use the shell to do globbing.
68dc0745 814
815=head2 Is there a leak/bug in glob()?
d74e8afc 816X<glob>
68dc0745 817
589a5df2 818(contributed by brian d foy)
f12f5f55 819
820Starting with Perl 5.6.0, C<glob> is implemented internally rather
821than relying on an external resource. As such, memory issues with
822C<glob> aren't a problem in modern perls.
68dc0745 823
c47ff5f1 824=head2 How can I open a file with a leading ">" or trailing blanks?
d74e8afc 825X<filename, special characters>
68dc0745 826
b68463f7 827(contributed by Brian McCauley)
68dc0745 828
b68463f7 829The special two argument form of Perl's open() function ignores
830trailing blanks in filenames and infers the mode from certain leading
831characters (or a trailing "|"). In older versions of Perl this was the
832only version of open() and so it is prevalent in old code and books.
65acb1b1 833
b68463f7 834Unless you have a particular reason to use the two argument form you
835should use the three argument form of open() which does not treat any
c195e131 836characters in the filename as special.
58103a2e 837
881bdbd4 838 open FILE, "<", " file "; # filename is " file "
839 open FILE, ">", ">file"; # filename is ">file"
65acb1b1 840
68dc0745 841=head2 How can I reliably rename a file?
f12f5f55 842X<rename> X<mv> X<move> X<file, rename>
68dc0745 843
49d635f9 844If your operating system supports a proper mv(1) utility or its
845functional equivalent, this works:
68dc0745 846
500071f4 847 rename($old, $new) or system("mv", $old, $new);
68dc0745 848
f12f5f55 849It may be more portable to use the C<File::Copy> module instead.
d2321c93 850You just copy to the new file to the new name (checking return
851values), then delete the old one. This isn't really the same
f12f5f55 852semantically as a C<rename()>, which preserves meta-information like
68dc0745 853permissions, timestamps, inode info, etc.
854
855=head2 How can I lock a file?
d74e8afc 856X<lock> X<file, lock> X<flock>
68dc0745 857
54310121 858Perl's builtin flock() function (see L<perlfunc> for details) will call
68dc0745 859flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
860later), and lockf(3) if neither of the two previous system calls exists.
861On some systems, it may even use a different form of native locking.
862Here are some gotchas with Perl's flock():
863
864=over 4
865
866=item 1
867
868Produces a fatal error if none of the three system calls (or their
869close equivalent) exists.
870
871=item 2
872
873lockf(3) does not provide shared locking, and requires that the
874filehandle be open for writing (or appending, or read/writing).
875
876=item 3
877
d92eb7b0 878Some versions of flock() can't lock files over a network (e.g. on NFS file
879systems), so you'd need to force the use of fcntl(2) when you build Perl.
a6dd486b 880But even this is dubious at best. See the flock entry of L<perlfunc>
d92eb7b0 881and the F<INSTALL> file in the source distribution for information on
882building Perl to do this.
883
884Two potentially non-obvious but traditional flock semantics are that
a6dd486b 885it waits indefinitely until the lock is granted, and that its locks are
d92eb7b0 886I<merely advisory>. Such discretionary locks are more flexible, but
887offer fewer guarantees. This means that files locked with flock() may
888be modified by programs that do not also use flock(). Cars that stop
889for red lights get on well with each other, but not with cars that don't
890stop for red lights. See the perlport manpage, your port's specific
891documentation, or your system-specific local manpages for details. It's
892best to assume traditional behavior if you're writing portable programs.
a6dd486b 893(If you're not, you should as always feel perfectly free to write
d92eb7b0 894for your own system's idiosyncrasies (sometimes called "features").
895Slavish adherence to portability concerns shouldn't get in the way of
896your getting your job done.)
68dc0745 897
197aec24 898For more information on file locking, see also
13a2d996 899L<perlopentut/"File Locking"> if you have it (new for 5.6).
65acb1b1 900
68dc0745 901=back
902
04d666b1 903=head2 Why can't I just open(FH, "E<gt>file.lock")?
d74e8afc 904X<lock, lockfile race condition>
68dc0745 905
906A common bit of code B<NOT TO USE> is this:
907
500071f4 908 sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
909 open(LCK, "> file.lock"); # THIS BROKEN CODE
68dc0745 910
911This is a classic race condition: you take two steps to do something
912which must be done in one. That's why computer hardware provides an
913atomic test-and-set instruction. In theory, this "ought" to work:
914
500071f4 915 sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
9b55d3ab 916 or die "can't open file.lock: $!";
68dc0745 917
918except that lamentably, file creation (and deletion) is not atomic
919over NFS, so this won't work (at least, not every time) over the net.
65acb1b1 920Various schemes involving link() have been suggested, but
c195e131 921these tend to involve busy-wait, which is also less than desirable.
68dc0745 922
fc36a67e 923=head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
d74e8afc 924X<counter> X<file, counter>
68dc0745 925
46fc3d4c 926Didn't anyone ever tell you web-page hit counters were useless?
5a964f20 927They don't count number of hits, they're a waste of time, and they serve
a6dd486b 928only to stroke the writer's vanity. It's better to pick a random number;
929they're more realistic.
68dc0745 930
5a964f20 931Anyway, this is what you can do if you can't help yourself.
68dc0745 932
500071f4 933 use Fcntl qw(:DEFAULT :flock);
934 sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
935 flock(FH, LOCK_EX) or die "can't flock numfile: $!";
936 $num = <FH> || 0;
937 seek(FH, 0, 0) or die "can't rewind numfile: $!";
938 truncate(FH, 0) or die "can't truncate numfile: $!";
939 (print FH $num+1, "\n") or die "can't write numfile: $!";
940 close FH or die "can't close numfile: $!";
68dc0745 941
46fc3d4c 942Here's a much better web-page hit counter:
68dc0745 943
500071f4 944 $hits = int( (time() - 850_000_000) / rand(1_000) );
68dc0745 945
946If the count doesn't impress your friends, then the code might. :-)
947
f52f3be2 948=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?
d74e8afc 949X<append> X<file, append>
05caf3a7 950
109f0441 951If you are on a system that correctly implements C<flock> and you use
952the example appending code from "perldoc -f flock" everything will be
953OK even if the OS you are on doesn't implement append mode correctly
954(if such a system exists.) So if you are happy to restrict yourself to
955OSs that implement C<flock> (and that's not really much of a
956restriction) then that is what you should do.
05caf3a7 957
958If you know you are only going to use a system that does correctly
109f0441 959implement appending (i.e. not Win32) then you can omit the C<seek>
960from the code in the previous answer.
961
962If you know you are only writing code to run on an OS and filesystem
963that does implement append mode correctly (a local filesystem on a
964modern Unix for example), and you keep the file in block-buffered mode
965and you write less than one buffer-full of output between each manual
966flushing of the buffer then each bufferload is almost guaranteed to be
967written to the end of the file in one chunk without getting
968intermingled with anyone else's output. You can also use the
969C<syswrite> function which is simply a wrapper around your system's
970C<write(2)> system call.
05caf3a7 971
972There is still a small theoretical chance that a signal will interrupt
109f0441 973the system level C<write()> operation before completion. There is also
974a possibility that some STDIO implementations may call multiple system
975level C<write()>s even if the buffer was empty to start. There may be
976some systems where this probability is reduced to zero, and this is
977not a concern when using C<:perlio> instead of your system's STDIO.
05caf3a7 978
68dc0745 979=head2 How do I randomly update a binary file?
d74e8afc 980X<file, binary patch>
68dc0745 981
982If you're just trying to patch a binary, in many cases something as
983simple as this works:
984
500071f4 985 perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
68dc0745 986
987However, if you have fixed sized records, then you might do something more
988like this:
989
500071f4 990 $RECSIZE = 220; # size of record, in bytes
991 $recno = 37; # which record to update
992 open(FH, "+<somewhere") || die "can't update somewhere: $!";
993 seek(FH, $recno * $RECSIZE, 0);
994 read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
995 # munge the record
996 seek(FH, -$RECSIZE, 1);
997 print FH $record;
998 close FH;
68dc0745 999
1000Locking and error checking are left as an exercise for the reader.
a6dd486b 1001Don't forget them or you'll be quite sorry.
68dc0745 1002
68dc0745 1003=head2 How do I get a file's timestamp in perl?
d74e8afc 1004X<timestamp> X<file, timestamp>
68dc0745 1005
589a5df2 1006If you want to retrieve the time at which the file was last read,
1007written, or had its meta-data (owner, etc) changed, you use the B<-A>,
1008B<-M>, or B<-C> file test operations as documented in L<perlfunc>.
1009These retrieve the age of the file (measured against the start-time of
1010your program) in days as a floating point number. Some platforms may
1011not have all of these times. See L<perlport> for details. To retrieve
1012the "raw" time in seconds since the epoch, you would call the stat
1013function, then use C<localtime()>, C<gmtime()>, or
1014C<POSIX::strftime()> to convert this into human-readable form.
68dc0745 1015
1016Here's an example:
1017
500071f4 1018 $write_secs = (stat($file))[9];
1019 printf "file %s updated at %s\n", $file,
c8db1d39 1020 scalar localtime($write_secs);
68dc0745 1021
1022If you prefer something more legible, use the File::stat module
1023(part of the standard distribution in version 5.004 and later):
1024
500071f4 1025 # error checking left as an exercise for reader.
1026 use File::stat;
1027 use Time::localtime;
1028 $date_string = ctime(stat($file)->mtime);
1029 print "file $file updated at $date_string\n";
68dc0745 1030
65acb1b1 1031The POSIX::strftime() approach has the benefit of being,
1032in theory, independent of the current locale. See L<perllocale>
1033for details.
68dc0745 1034
1035=head2 How do I set a file's timestamp in perl?
d74e8afc 1036X<timestamp> X<file, timestamp>
68dc0745 1037
1038You use the utime() function documented in L<perlfunc/utime>.
1039By way of example, here's a little program that copies the
1040read and write times from its first argument to all the rest
1041of them.
1042
500071f4 1043 if (@ARGV < 2) {
1044 die "usage: cptimes timestamp_file other_files ...\n";
1045 }
1046 $timestamp = shift;
1047 ($atime, $mtime) = (stat($timestamp))[8,9];
1048 utime $atime, $mtime, @ARGV;
68dc0745 1049
65acb1b1 1050Error checking is, as usual, left as an exercise for the reader.
68dc0745 1051
19a1cd16 1052The perldoc for utime also has an example that has the same
1053effect as touch(1) on files that I<already exist>.
1054
1055Certain file systems have a limited ability to store the times
1056on a file at the expected level of precision. For example, the
1057FAT and HPFS filesystem are unable to create dates on files with
1058a finer granularity than two seconds. This is a limitation of
1059the filesystems, not of utime().
68dc0745 1060
1061=head2 How do I print to more than one file at once?
d74e8afc 1062X<print, to multiple files>
68dc0745 1063
49d635f9 1064To connect one filehandle to several output filehandles,
1065you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
68dc0745 1066
49d635f9 1067If you only have to do this once, you can print individually
1068to each filehandle.
68dc0745 1069
500071f4 1070 for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
5a964f20 1071
49d635f9 1072=head2 How can I read in an entire file all at once?
d74e8afc 1073X<slurp> X<file, slurping>
68dc0745 1074
49d635f9 1075You can use the File::Slurp module to do it in one step.
68dc0745 1076
49d635f9 1077 use File::Slurp;
197aec24 1078
49d635f9 1079 $all_of_it = read_file($filename); # entire file in scalar
109f0441 1080 @all_lines = read_file($filename); # one line per element
d92eb7b0 1081
1082The customary Perl approach for processing all the lines in a file is to
1083do so one line at a time:
1084
500071f4 1085 open (INPUT, $file) || die "can't open $file: $!";
1086 while (<INPUT>) {
1087 chomp;
1088 # do something with $_
1089 }
1090 close(INPUT) || die "can't close $file: $!";
d92eb7b0 1091
1092This is tremendously more efficient than reading the entire file into
1093memory as an array of lines and then processing it one element at a time,
a6dd486b 1094which is often--if not almost always--the wrong approach. Whenever
d92eb7b0 1095you see someone do this:
1096
500071f4 1097 @lines = <INPUT>;
d92eb7b0 1098
30852c57 1099you should think long and hard about why you need everything loaded at
1100once. It's just not a scalable solution. You might also find it more
1101fun to use the standard Tie::File module, or the DB_File module's
1102$DB_RECNO bindings, which allow you to tie an array to a file so that
1103accessing an element the array actually accesses the corresponding
1104line in the file.
d92eb7b0 1105
f05bbc40 1106You can read the entire filehandle contents into a scalar.
d92eb7b0 1107
500071f4 1108 {
d92eb7b0 1109 local(*INPUT, $/);
1110 open (INPUT, $file) || die "can't open $file: $!";
1111 $var = <INPUT>;
500071f4 1112 }
d92eb7b0 1113
197aec24 1114That temporarily undefs your record separator, and will automatically
d92eb7b0 1115close the file at block exit. If the file is already open, just use this:
1116
500071f4 1117 $var = do { local $/; <INPUT> };
d92eb7b0 1118
f05bbc40 1119For ordinary files you can also use the read function.
1120
1121 read( INPUT, $var, -s INPUT );
1122
1123The third argument tests the byte size of the data on the INPUT filehandle
1124and reads that many bytes into the buffer $var.
1125
68dc0745 1126=head2 How can I read in a file by paragraphs?
d74e8afc 1127X<file, reading by paragraphs>
68dc0745 1128
65acb1b1 1129Use the C<$/> variable (see L<perlvar> for details). You can either
68dc0745 1130set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
1131for instance, gets treated as two paragraphs and not three), or
1132C<"\n\n"> to accept empty paragraphs.
1133
197aec24 1134Note that a blank line must have no blanks in it. Thus
c4db748a 1135S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
65acb1b1 1136
68dc0745 1137=head2 How can I read a single character from a file? From the keyboard?
d74e8afc 1138X<getc> X<file, reading one character at a time>
68dc0745 1139
1140You can use the builtin C<getc()> function for most filehandles, but
1141it won't (easily) work on a terminal device. For STDIN, either use
a6dd486b 1142the Term::ReadKey module from CPAN or use the sample code in
68dc0745 1143L<perlfunc/getc>.
1144
65acb1b1 1145If your system supports the portable operating system programming
1146interface (POSIX), you can use the following code, which you'll note
1147turns off echo processing as well.
68dc0745 1148
500071f4 1149 #!/usr/bin/perl -w
1150 use strict;
1151 $| = 1;
1152 for (1..4) {
1153 my $got;
1154 print "gimme: ";
1155 $got = getone();
1156 print "--> $got\n";
1157 }
68dc0745 1158 exit;
1159
500071f4 1160 BEGIN {
68dc0745 1161 use POSIX qw(:termios_h);
1162
1163 my ($term, $oterm, $echo, $noecho, $fd_stdin);
1164
1165 $fd_stdin = fileno(STDIN);
1166
1167 $term = POSIX::Termios->new();
1168 $term->getattr($fd_stdin);
1169 $oterm = $term->getlflag();
1170
1171 $echo = ECHO | ECHOK | ICANON;
1172 $noecho = $oterm & ~$echo;
1173
1174 sub cbreak {
500071f4 1175 $term->setlflag($noecho);
1176 $term->setcc(VTIME, 1);
1177 $term->setattr($fd_stdin, TCSANOW);
1178 }
ac9dac7f 1179
68dc0745 1180 sub cooked {
500071f4 1181 $term->setlflag($oterm);
1182 $term->setcc(VTIME, 0);
1183 $term->setattr($fd_stdin, TCSANOW);
1184 }
68dc0745 1185
1186 sub getone {
500071f4 1187 my $key = '';
1188 cbreak();
1189 sysread(STDIN, $key, 1);
1190 cooked();
1191 return $key;
1192 }
68dc0745 1193
500071f4 1194 }
68dc0745 1195
500071f4 1196 END { cooked() }
68dc0745 1197
a6dd486b 1198The Term::ReadKey module from CPAN may be easier to use. Recent versions
65acb1b1 1199include also support for non-portable systems as well.
68dc0745 1200
500071f4 1201 use Term::ReadKey;
1202 open(TTY, "</dev/tty");
1203 print "Gimme a char: ";
1204 ReadMode "raw";
1205 $key = ReadKey 0, *TTY;
1206 ReadMode "normal";
1207 printf "\nYou said %s, char number %03d\n",
1208 $key, ord $key;
68dc0745 1209
65acb1b1 1210=head2 How can I tell whether there's a character waiting on a filehandle?
68dc0745 1211
5a964f20 1212The very first thing you should do is look into getting the Term::ReadKey
65acb1b1 1213extension from CPAN. As we mentioned earlier, it now even has limited
1214support for non-portable (read: not open systems, closed, proprietary,
589a5df2 1215not POSIX, not Unix, etc.) systems.
5a964f20 1216
1217You should also check out the Frequently Asked Questions list in
68dc0745 1218comp.unix.* for things like this: the answer is essentially the same.
1219It's very system dependent. Here's one solution that works on BSD
1220systems:
1221
500071f4 1222 sub key_ready {
1223 my($rin, $nfd);
1224 vec($rin, fileno(STDIN), 1) = 1;
1225 return $nfd = select($rin,undef,undef,0);
1226 }
68dc0745 1227
65acb1b1 1228If you want to find out how many characters are waiting, there's
1229also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
1230comes with Perl tries to convert C include files to Perl code, which
1231can be C<require>d. FIONREAD ends up defined as a function in the
1232I<sys/ioctl.ph> file:
68dc0745 1233
500071f4 1234 require 'sys/ioctl.ph';
68dc0745 1235
500071f4 1236 $size = pack("L", 0);
1237 ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
1238 $size = unpack("L", $size);
68dc0745 1239
5a964f20 1240If I<h2ph> wasn't installed or doesn't work for you, you can
1241I<grep> the include files by hand:
68dc0745 1242
500071f4 1243 % grep FIONREAD /usr/include/*/*
1244 /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
68dc0745 1245
5a964f20 1246Or write a small C program using the editor of champions:
68dc0745 1247
500071f4 1248 % cat > fionread.c
1249 #include <sys/ioctl.h>
1250 main() {
1251 printf("%#08x\n", FIONREAD);
1252 }
1253 ^D
1254 % cc -o fionread fionread.c
1255 % ./fionread
1256 0x4004667f
5a964f20 1257
8305e449 1258And then hard code it, leaving porting as an exercise to your successor.
5a964f20 1259
500071f4 1260 $FIONREAD = 0x4004667f; # XXX: opsys dependent
5a964f20 1261
500071f4 1262 $size = pack("L", 0);
1263 ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
1264 $size = unpack("L", $size);
5a964f20 1265
a6dd486b 1266FIONREAD requires a filehandle connected to a stream, meaning that sockets,
5a964f20 1267pipes, and tty devices work, but I<not> files.
68dc0745 1268
1269=head2 How do I do a C<tail -f> in perl?
ac9dac7f 1270X<tail> X<IO::Handle> X<File::Tail> X<clearerr>
68dc0745 1271
1272First try
1273
500071f4 1274 seek(GWFILE, 0, 1);
68dc0745 1275
1276The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
1277but it does clear the end-of-file condition on the handle, so that the
ac9dac7f 1278next C<< <GWFILE> >> makes Perl try again to read something.
68dc0745 1279
1280If that doesn't work (it relies on features of your stdio implementation),
1281then you need something more like this:
1282
1283 for (;;) {
1284 for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
1285 # search for some stuff and put it into files
1286 }
1287 # sleep for a while
1288 seek(GWFILE, $curpos, 0); # seek to where we had been
1289 }
1290
ac9dac7f 1291If this still doesn't work, look into the C<clearerr> method
1292from C<IO::Handle>, which resets the error and end-of-file states
1293on the handle.
68dc0745 1294
ac9dac7f 1295There's also a C<File::Tail> module from CPAN.
65acb1b1 1296
68dc0745 1297=head2 How do I dup() a filehandle in Perl?
d74e8afc 1298X<dup>
68dc0745 1299
1300If you check L<perlfunc/open>, you'll see that several of the ways
1301to call open() should do the trick. For example:
1302
500071f4 1303 open(LOG, ">>/foo/logfile");
1304 open(STDERR, ">&LOG");
68dc0745 1305
1306Or even with a literal numeric descriptor:
1307
1308 $fd = $ENV{MHCONTEXTFD};
1309 open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
1310
c47ff5f1 1311Note that "<&STDIN" makes a copy, but "<&=STDIN" make
5a964f20 1312an alias. That means if you close an aliased handle, all
197aec24 1313aliases become inaccessible. This is not true with
5a964f20 1314a copied one.
1315
1316Error checking, as always, has been left as an exercise for the reader.
68dc0745 1317
1318=head2 How do I close a file descriptor by number?
ee891a00 1319X<file, closing file descriptors> X<POSIX> X<close>
1320
1321If, for some reason, you have a file descriptor instead of a
1322filehandle (perhaps you used C<POSIX::open>), you can use the
1323C<close()> function from the C<POSIX> module:
68dc0745 1324
ee891a00 1325 use POSIX ();
109f0441 1326
ee891a00 1327 POSIX::close( $fd );
109f0441 1328
ac003c96 1329This should rarely be necessary, as the Perl C<close()> function is to be
68dc0745 1330used for things that Perl opened itself, even if it was a dup of a
ac003c96 1331numeric descriptor as with C<MHCONTEXT> above. But if you really have
68dc0745 1332to, you may be able to do this:
1333
500071f4 1334 require 'sys/syscall.ph';
1335 $rc = syscall(&SYS_close, $fd + 0); # must force numeric
1336 die "can't sysclose $fd: $!" unless $rc == -1;
68dc0745 1337
ee891a00 1338Or, just use the fdopen(3S) feature of C<open()>:
d92eb7b0 1339
500071f4 1340 {
ee891a00 1341 open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!";
1342 close $fh;
500071f4 1343 }
d92eb7b0 1344
883f1635 1345=head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
d74e8afc 1346X<filename, DOS issues>
68dc0745 1347
1348Whoops! You just put a tab and a formfeed into that filename!
1349Remember that within double quoted strings ("like\this"), the
1350backslash is an escape character. The full list of these is in
1351L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
1352have a file called "c:(tab)emp(formfeed)oo" or
65acb1b1 1353"c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
68dc0745 1354
1355Either single-quote your strings, or (preferably) use forward slashes.
46fc3d4c 1356Since all DOS and Windows versions since something like MS-DOS 2.0 or so
68dc0745 1357have treated C</> and C<\> the same in a path, you might as well use the
a6dd486b 1358one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++,
65acb1b1 1359awk, Tcl, Java, or Python, just to mention a few. POSIX paths
1360are more portable, too.
68dc0745 1361
1362=head2 Why doesn't glob("*.*") get all the files?
d74e8afc 1363X<glob>
68dc0745 1364
1365Because even on non-Unix ports, Perl's glob function follows standard
46fc3d4c 1366Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
65acb1b1 1367files. This makes glob() portable even to legacy systems. Your
1368port may include proprietary globbing functions as well. Check its
1369documentation for details.
68dc0745 1370
1371=head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
1372
06a5f41f 1373This is elaborately and painstakingly described in the
1374F<file-dir-perms> article in the "Far More Than You Ever Wanted To
49d635f9 1375Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
68dc0745 1376
1377The executive summary: learn how your filesystem works. The
1378permissions on a file say what can happen to the data in that file.
1379The permissions on a directory say what can happen to the list of
1380files in that directory. If you delete a file, you're removing its
1381name from the directory (so the operation depends on the permissions
1382of the directory, not of the file). If you try to write to the file,
1383the permissions of the file govern whether you're allowed to.
1384
1385=head2 How do I select a random line from a file?
d74e8afc 1386X<file, selecting a random line>
68dc0745 1387
109f0441 1388Short of loading the file into a database or pre-indexing the lines in
1389the file, there are a couple of things that you can do.
1390
1391Here's a reservoir-sampling algorithm from the Camel Book:
68dc0745 1392
500071f4 1393 srand;
1394 rand($.) < 1 && ($line = $_) while <>;
68dc0745 1395
49d635f9 1396This has a significant advantage in space over reading the whole file
1397in. You can find a proof of this method in I<The Art of Computer
1398Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth.
1399
109f0441 1400You can use the C<File::Random> module which provides a function
49d635f9 1401for that algorithm:
1402
1403 use File::Random qw/random_line/;
1404 my $line = random_line($filename);
1405
109f0441 1406Another way is to use the C<Tie::File> module, which treats the entire
49d635f9 1407file as an array. Simply access a random array element.
68dc0745 1408
65acb1b1 1409=head2 Why do I get weird spaces when I print an array of lines?
1410
109f0441 1411(contributed by brian d foy)
1412
1413If you are seeing spaces between the elements of your array when
1414you print the array, you are probably interpolating the array in
1415double quotes:
1416
1417 my @animals = qw(camel llama alpaca vicuna);
1418 print "animals are: @animals\n";
65acb1b1 1419
109f0441 1420It's the double quotes, not the C<print>, doing this. Whenever you
1421interpolate an array in a double quote context, Perl joins the
1422elements with spaces (or whatever is in C<$">, which is a space by
1423default):
65acb1b1 1424
109f0441 1425 animals are: camel llama alpaca vicuna
65acb1b1 1426
109f0441 1427This is different than printing the array without the interpolation:
65acb1b1 1428
109f0441 1429 my @animals = qw(camel llama alpaca vicuna);
1430 print "animals are: ", @animals, "\n";
65acb1b1 1431
109f0441 1432Now the output doesn't have the spaces between the elements because
1433the elements of C<@animals> simply become part of the list to
1434C<print>:
65acb1b1 1435
109f0441 1436 animals are: camelllamaalpacavicuna
1437
1438You might notice this when each of the elements of C<@array> end with
1439a newline. You expect to print one element per line, but notice that
1440every line after the first is indented:
1441
1442 this is a line
1443 this is another line
1444 this is the third line
1445
1446That extra space comes from the interpolation of the array. If you
1447don't want to put anything between your array elements, don't use the
1448array in double quotes. You can send it to print without them:
65acb1b1 1449
500071f4 1450 print @lines;
1451
109f0441 1452=head2 How do I traverse a directory tree?
1453
1454(contributed by brian d foy)
1455
1456The C<File::Find> module, which comes with Perl, does all of the hard
1457work to traverse a directory structure. It comes with Perl. You simply
1458call the C<find> subroutine with a callback subroutine and the
1459directories you want to traverse:
1460
1461 use File::Find;
1462
1463 find( \&wanted, @directories );
1464
1465 sub wanted {
1466 # full path in $File::Find::name
1467 # just filename in $_
1468 ... do whatever you want to do ...
1469 }
1470
1471The C<File::Find::Closures>, which you can download from CPAN, provides
1472many ready-to-use subroutines that you can use with C<File::Find>.
1473
1474The C<File::Finder>, which you can download from CPAN, can help you
1475create the callback subroutine using something closer to the syntax of
1476the C<find> command-line utility:
1477
1478 use File::Find;
1479 use File::Finder;
1480
1481 my $deep_dirs = File::Finder->depth->type('d')->ls->exec('rmdir','{}');
1482
1483 find( $deep_dirs->as_options, @places );
1484
1485The C<File::Find::Rule> module, which you can download from CPAN, has
1486a similar interface, but does the traversal for you too:
1487
1488 use File::Find::Rule;
1489
1490 my @files = File::Find::Rule->file()
1491 ->name( '*.pm' )
1492 ->in( @INC );
1493
1494=head2 How do I delete a directory tree?
1495
1496(contributed by brian d foy)
1497
8d2e243f 1498If you have an empty directory, you can use Perl's built-in C<rmdir>.
1499If the directory is not empty (so, no files or subdirectories), you
1500either have to empty it yourself (a lot of work) or use a module to
1501help you.
109f0441 1502
8d2e243f 1503The C<File::Path> module, which comes with Perl, has a C<remove_tree>
1504which can take care of all of the hard work for you:
109f0441 1505
8d2e243f 1506 use File::Path qw(remove_tree);
109f0441 1507
8d2e243f 1508 remove_tree( @directories );
109f0441 1509
8d2e243f 1510The C<File::Path> module also has a legacy interface to the older
1511C<rmtree> subroutine.
109f0441 1512
1513=head2 How do I copy an entire directory?
1514
1515(contributed by Shlomi Fish)
1516
1517To do the equivalent of C<cp -R> (i.e. copy an entire directory tree
1518recursively) in portable Perl, you'll either need to write something yourself
1519or find a good CPAN module such as L<File::Copy::Recursive>.
65acb1b1 1520
68dc0745 1521=head1 AUTHOR AND COPYRIGHT
1522
8d2e243f 1523Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and
7678cced 1524other authors as noted. All rights reserved.
5a964f20 1525
5a7beb56 1526This documentation is free; you can redistribute it and/or modify it
1527under the same terms as Perl itself.
c8db1d39 1528
87275199 1529Irrespective of its distribution, all code examples here are in the public
c8db1d39 1530domain. You are permitted and encouraged to use this code and any
1531derivatives thereof in your own programs for fun or for profit as you
1532see fit. A simple comment in the code giving credit to the FAQ would
1533be courteous but is not required.