README.vms and related updates (from Peter Prymmer <pvhp@best.com>)
[p5sagit/p5-mst-13.2.git] / pod / perlfaq5.pod
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68dc0745 1=head1 NAME
2
d92eb7b0 3perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date: 1999/05/23 16:08:30 $)
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?
68dc0745 11
12The C standard I/O library (stdio) normally buffers characters sent to
13devices. This is done for efficiency reasons, so that there isn't a
14system call for each byte. Any time you use print() or write() in
15Perl, you go though this buffering. syswrite() circumvents stdio and
16buffering.
17
5a964f20 18In most stdio implementations, the type of output buffering and the size of
68dc0745 19the buffer varies according to the type of device. Disk files are block
20buffered, often with a buffer size of more than 2k. Pipes and sockets
21are often buffered with a buffer size between 1/2 and 2k. Serial devices
22(e.g. modems, terminals) are normally line-buffered, and stdio sends
23the entire line when it gets the newline.
24
25Perl does not support truly unbuffered output (except insofar as you can
26C<syswrite(OUT, $char, 1)>). What it does instead support is "command
27buffering", in which a physical write is performed after every output
28command. This isn't as hard on your system as unbuffering, but does
29get the output where you want it when you want it.
30
31If you expect characters to get to your device when you print them there,
5a964f20 32you'll want to autoflush its handle.
33Use select() and the C<$|> variable to control autoflushing
34(see L<perlvar/$|> and L<perlfunc/select>):
35
36 $old_fh = select(OUTPUT_HANDLE);
37 $| = 1;
38 select($old_fh);
39
40Or using the traditional idiom:
41
42 select((select(OUTPUT_HANDLE), $| = 1)[0]);
43
44Or if don't mind slowly loading several thousand lines of module code
45just because you're afraid of the C<$|> variable:
68dc0745 46
47 use FileHandle;
5a964f20 48 open(DEV, "+</dev/tty"); # ceci n'est pas une pipe
68dc0745 49 DEV->autoflush(1);
50
51or the newer IO::* modules:
52
53 use IO::Handle;
54 open(DEV, ">/dev/printer"); # but is this?
55 DEV->autoflush(1);
56
57or even this:
58
59 use IO::Socket; # this one is kinda a pipe?
60 $sock = IO::Socket::INET->new(PeerAddr => 'www.perl.com',
61 PeerPort => 'http(80)',
62 Proto => 'tcp');
63 die "$!" unless $sock;
64
65 $sock->autoflush();
5a964f20 66 print $sock "GET / HTTP/1.0" . "\015\012" x 2;
67 $document = join('', <$sock>);
68dc0745 68 print "DOC IS: $document\n";
69
5a964f20 70Note the bizarrely hardcoded carriage return and newline in their octal
71equivalents. This is the ONLY way (currently) to assure a proper flush
d92eb7b0 72on all platforms, including Macintosh. That's the way things work in
5a964f20 73network programming: you really should specify the exact bit pattern
74on the network line terminator. In practice, C<"\n\n"> often works,
75but this is not portable.
68dc0745 76
5a964f20 77See L<perlfaq9> for other examples of fetching URLs over the web.
68dc0745 78
79=head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
80
65acb1b1 81Those are operations of a text editor. Perl is not a text editor.
82Perl is a programming language. You have to decompose the problem into
83low-level calls to read, write, open, close, and seek.
84
68dc0745 85Although humans have an easy time thinking of a text file as being a
65acb1b1 86sequence of lines that operates much like a stack of playing cards -- or
87punch cards -- computers usually see the text file as a sequence of bytes.
88In general, there's no direct way for Perl to seek to a particular line
89of a file, insert text into a file, or remove text from a file.
68dc0745 90
5a964f20 91(There are exceptions in special circumstances. You can add or remove at
92the very end of the file. Another is replacing a sequence of bytes with
93another sequence of the same length. Another is using the C<$DB_RECNO>
94array bindings as documented in L<DB_File>. Yet another is manipulating
95files with all lines the same length.)
68dc0745 96
97The general solution is to create a temporary copy of the text file with
5a964f20 98the changes you want, then copy that over the original. This assumes
99no locking.
68dc0745 100
101 $old = $file;
102 $new = "$file.tmp.$$";
65acb1b1 103 $bak = "$file.orig";
68dc0745 104
105 open(OLD, "< $old") or die "can't open $old: $!";
106 open(NEW, "> $new") or die "can't open $new: $!";
107
108 # Correct typos, preserving case
109 while (<OLD>) {
110 s/\b(p)earl\b/${1}erl/i;
111 (print NEW $_) or die "can't write to $new: $!";
112 }
113
114 close(OLD) or die "can't close $old: $!";
115 close(NEW) or die "can't close $new: $!";
116
117 rename($old, $bak) or die "can't rename $old to $bak: $!";
118 rename($new, $old) or die "can't rename $new to $old: $!";
119
120Perl can do this sort of thing for you automatically with the C<-i>
46fc3d4c 121command-line switch or the closely-related C<$^I> variable (see
68dc0745 122L<perlrun> for more details). Note that
123C<-i> may require a suffix on some non-Unix systems; see the
124platform-specific documentation that came with your port.
125
126 # Renumber a series of tests from the command line
127 perl -pi -e 's/(^\s+test\s+)\d+/ $1 . ++$count /e' t/op/taint.t
128
129 # form a script
65acb1b1 130 local($^I, @ARGV) = ('.orig', glob("*.c"));
68dc0745 131 while (<>) {
132 if ($. == 1) {
133 print "This line should appear at the top of each file\n";
134 }
135 s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
136 print;
137 close ARGV if eof; # Reset $.
138 }
139
140If you need to seek to an arbitrary line of a file that changes
141infrequently, you could build up an index of byte positions of where
142the line ends are in the file. If the file is large, an index of
143every tenth or hundredth line end would allow you to seek and read
144fairly efficiently. If the file is sorted, try the look.pl library
145(part of the standard perl distribution).
146
147In the unique case of deleting lines at the end of a file, you
148can use tell() and truncate(). The following code snippet deletes
149the last line of a file without making a copy or reading the
150whole file into memory:
151
152 open (FH, "+< $file");
54310121 153 while ( <FH> ) { $addr = tell(FH) unless eof(FH) }
68dc0745 154 truncate(FH, $addr);
155
156Error checking is left as an exercise for the reader.
157
158=head2 How do I count the number of lines in a file?
159
160One fairly efficient way is to count newlines in the file. The
161following program uses a feature of tr///, as documented in L<perlop>.
162If your text file doesn't end with a newline, then it's not really a
163proper text file, so this may report one fewer line than you expect.
164
165 $lines = 0;
166 open(FILE, $filename) or die "Can't open `$filename': $!";
167 while (sysread FILE, $buffer, 4096) {
168 $lines += ($buffer =~ tr/\n//);
169 }
170 close FILE;
171
5a964f20 172This assumes no funny games with newline translations.
173
68dc0745 174=head2 How do I make a temporary file name?
175
5a964f20 176Use the C<new_tmpfile> class method from the IO::File module to get a
177filehandle opened for reading and writing. Use this if you don't
178need to know the file's name.
68dc0745 179
65acb1b1 180 use IO::File;
5a964f20 181 $fh = IO::File->new_tmpfile()
65acb1b1 182 or die "Unable to make new temporary file: $!";
5a964f20 183
184Or you can use the C<tmpnam> function from the POSIX module to get a
185filename that you then open yourself. Use this if you do need to know
186the file's name.
187
188 use Fcntl;
189 use POSIX qw(tmpnam);
190
191 # try new temporary filenames until we get one that didn't already
192 # exist; the check should be unnecessary, but you can't be too careful
193 do { $name = tmpnam() }
194 until sysopen(FH, $name, O_RDWR|O_CREAT|O_EXCL);
195
196 # install atexit-style handler so that when we exit or die,
197 # we automatically delete this temporary file
198 END { unlink($name) or die "Couldn't unlink $name : $!" }
199
200 # now go on to use the file ...
201
202If you're committed to doing this by hand, use the process ID and/or
203the current time-value. If you need to have many temporary files in
204one process, use a counter:
205
206 BEGIN {
68dc0745 207 use Fcntl;
208 my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} || $ENV{TEMP};
209 my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
210 sub temp_file {
5a964f20 211 local *FH;
68dc0745 212 my $count = 0;
5a964f20 213 until (defined(fileno(FH)) || $count++ > 100) {
68dc0745 214 $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
5a964f20 215 sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
68dc0745 216 }
5a964f20 217 if (defined(fileno(FH))
218 return (*FH, $base_name);
68dc0745 219 } else {
220 return ();
221 }
222 }
223 }
224
68dc0745 225=head2 How can I manipulate fixed-record-length files?
226
5a964f20 227The most efficient way is using pack() and unpack(). This is faster than
65acb1b1 228using substr() when taking many, many strings. It is slower for just a few.
5a964f20 229
230Here is a sample chunk of code to break up and put back together again
231some fixed-format input lines, in this case from the output of a normal,
232Berkeley-style ps:
68dc0745 233
234 # sample input line:
235 # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
236 $PS_T = 'A6 A4 A7 A5 A*';
237 open(PS, "ps|");
5a964f20 238 print scalar <PS>;
68dc0745 239 while (<PS>) {
240 ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
241 for $var (qw!pid tt stat time command!) {
242 print "$var: <$$var>\n";
243 }
244 print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
245 "\n";
246 }
247
5a964f20 248We've used C<$$var> in a way that forbidden by C<use strict 'refs'>.
249That is, we've promoted a string to a scalar variable reference using
250symbolic references. This is ok in small programs, but doesn't scale
251well. It also only works on global variables, not lexicals.
252
68dc0745 253=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?
254
5a964f20 255The fastest, simplest, and most direct way is to localize the typeglob
256of the filehandle in question:
68dc0745 257
5a964f20 258 local *TmpHandle;
68dc0745 259
5a964f20 260Typeglobs are fast (especially compared with the alternatives) and
261reasonably easy to use, but they also have one subtle drawback. If you
262had, for example, a function named TmpHandle(), or a variable named
263%TmpHandle, you just hid it from yourself.
68dc0745 264
68dc0745 265 sub findme {
5a964f20 266 local *HostFile;
267 open(HostFile, "</etc/hosts") or die "no /etc/hosts: $!";
268 local $_; # <- VERY IMPORTANT
269 while (<HostFile>) {
68dc0745 270 print if /\b127\.(0\.0\.)?1\b/;
271 }
5a964f20 272 # *HostFile automatically closes/disappears here
273 }
274
275Here's how to use this in a loop to open and store a bunch of
276filehandles. We'll use as values of the hash an ordered
277pair to make it easy to sort the hash in insertion order.
278
279 @names = qw(motd termcap passwd hosts);
280 my $i = 0;
281 foreach $filename (@names) {
282 local *FH;
283 open(FH, "/etc/$filename") || die "$filename: $!";
284 $file{$filename} = [ $i++, *FH ];
68dc0745 285 }
286
5a964f20 287 # Using the filehandles in the array
288 foreach $name (sort { $file{$a}[0] <=> $file{$b}[0] } keys %file) {
289 my $fh = $file{$name}[1];
290 my $line = <$fh>;
291 print "$name $. $line";
292 }
293
c8db1d39 294For passing filehandles to functions, the easiest way is to
65acb1b1 295preface them with a star, as in func(*STDIN). See L<perlfaq7/"Passing
296Filehandles"> for details.
c8db1d39 297
65acb1b1 298If you want to create many anonymous handles, you should check out the
5a964f20 299Symbol, FileHandle, or IO::Handle (etc.) modules. Here's the equivalent
300code with Symbol::gensym, which is reasonably light-weight:
301
302 foreach $filename (@names) {
303 use Symbol;
304 my $fh = gensym();
305 open($fh, "/etc/$filename") || die "open /etc/$filename: $!";
306 $file{$filename} = [ $i++, $fh ];
307 }
68dc0745 308
65acb1b1 309Or here using the semi-object-oriented FileHandle module, which certainly
310isn't light-weight:
46fc3d4c 311
312 use FileHandle;
313
46fc3d4c 314 foreach $filename (@names) {
5a964f20 315 my $fh = FileHandle->new("/etc/$filename") or die "$filename: $!";
316 $file{$filename} = [ $i++, $fh ];
46fc3d4c 317 }
318
5a964f20 319Please understand that whether the filehandle happens to be a (probably
320localized) typeglob or an anonymous handle from one of the modules,
321in no way affects the bizarre rules for managing indirect handles.
322See the next question.
323
324=head2 How can I use a filehandle indirectly?
325
326An indirect filehandle is using something other than a symbol
327in a place that a filehandle is expected. Here are ways
328to get those:
329
330 $fh = SOME_FH; # bareword is strict-subs hostile
331 $fh = "SOME_FH"; # strict-refs hostile; same package only
332 $fh = *SOME_FH; # typeglob
333 $fh = \*SOME_FH; # ref to typeglob (bless-able)
334 $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
335
336Or to use the C<new> method from the FileHandle or IO modules to
337create an anonymous filehandle, store that in a scalar variable,
338and use it as though it were a normal filehandle.
339
340 use FileHandle;
341 $fh = FileHandle->new();
342
343 use IO::Handle; # 5.004 or higher
344 $fh = IO::Handle->new();
345
346Then use any of those as you would a normal filehandle. Anywhere that
347Perl is expecting a filehandle, an indirect filehandle may be used
348instead. An indirect filehandle is just a scalar variable that contains
368c9434 349a filehandle. Functions like C<print>, C<open>, C<seek>, or
65acb1b1 350the C<E<lt>FHE<gt>> diamond operator will accept either a read filehandle
5a964f20 351or a scalar variable containing one:
352
353 ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
354 print $ofh "Type it: ";
355 $got = <$ifh>
356 print $efh "What was that: $got";
357
368c9434 358If you're passing a filehandle to a function, you can write
5a964f20 359the function in two ways:
360
361 sub accept_fh {
362 my $fh = shift;
363 print $fh "Sending to indirect filehandle\n";
46fc3d4c 364 }
365
5a964f20 366Or it can localize a typeglob and use the filehandle directly:
46fc3d4c 367
5a964f20 368 sub accept_fh {
369 local *FH = shift;
370 print FH "Sending to localized filehandle\n";
46fc3d4c 371 }
372
5a964f20 373Both styles work with either objects or typeglobs of real filehandles.
374(They might also work with strings under some circumstances, but this
375is risky.)
376
377 accept_fh(*STDOUT);
378 accept_fh($handle);
379
380In the examples above, we assigned the filehandle to a scalar variable
381before using it. That is because only simple scalar variables,
382not expressions or subscripts into hashes or arrays, can be used with
383built-ins like C<print>, C<printf>, or the diamond operator. These are
384illegal and won't even compile:
385
386 @fd = (*STDIN, *STDOUT, *STDERR);
387 print $fd[1] "Type it: "; # WRONG
388 $got = <$fd[0]> # WRONG
389 print $fd[2] "What was that: $got"; # WRONG
390
391With C<print> and C<printf>, you get around this by using a block and
392an expression where you would place the filehandle:
393
394 print { $fd[1] } "funny stuff\n";
395 printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
396 # Pity the poor deadbeef.
397
398That block is a proper block like any other, so you can put more
399complicated code there. This sends the message out to one of two places:
400
401 $ok = -x "/bin/cat";
402 print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
403 print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
404
405This approach of treating C<print> and C<printf> like object methods
406calls doesn't work for the diamond operator. That's because it's a
407real operator, not just a function with a comma-less argument. Assuming
408you've been storing typeglobs in your structure as we did above, you
409can use the built-in function named C<readline> to reads a record just
410as C<E<lt>E<gt>> does. Given the initialization shown above for @fd, this
411would work, but only because readline() require a typeglob. It doesn't
412work with objects or strings, which might be a bug we haven't fixed yet.
413
414 $got = readline($fd[0]);
415
416Let it be noted that the flakiness of indirect filehandles is not
417related to whether they're strings, typeglobs, objects, or anything else.
418It's the syntax of the fundamental operators. Playing the object
419game doesn't help you at all here.
46fc3d4c 420
68dc0745 421=head2 How can I set up a footer format to be used with write()?
422
54310121 423There's no builtin way to do this, but L<perlform> has a couple of
68dc0745 424techniques to make it possible for the intrepid hacker.
425
426=head2 How can I write() into a string?
427
65acb1b1 428See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
68dc0745 429
430=head2 How can I output my numbers with commas added?
431
432This one will do it for you:
433
434 sub commify {
435 local $_ = shift;
65acb1b1 436 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
68dc0745 437 return $_;
438 }
439
440 $n = 23659019423.2331;
441 print "GOT: ", commify($n), "\n";
442
443 GOT: 23,659,019,423.2331
444
445You can't just:
446
65acb1b1 447 s/^([-+]?\d+)(\d{3})/$1,$2/g;
68dc0745 448
449because you have to put the comma in and then recalculate your
450position.
451
46fc3d4c 452Alternatively, this commifies all numbers in a line regardless of
453whether they have decimal portions, are preceded by + or -, or
454whatever:
455
456 # from Andrew Johnson <ajohnson@gpu.srv.ualberta.ca>
457 sub commify {
458 my $input = shift;
459 $input = reverse $input;
460 $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g;
65acb1b1 461 return scalar reverse $input;
46fc3d4c 462 }
463
68dc0745 464=head2 How can I translate tildes (~) in a filename?
465
466Use the E<lt>E<gt> (glob()) operator, documented in L<perlfunc>. This
467requires that you have a shell installed that groks tildes, meaning
468csh or tcsh or (some versions of) ksh, and thus may have portability
469problems. The Glob::KGlob module (available from CPAN) gives more
470portable glob functionality.
471
472Within Perl, you may use this directly:
473
474 $filename =~ s{
475 ^ ~ # find a leading tilde
476 ( # save this in $1
477 [^/] # a non-slash character
478 * # repeated 0 or more times (0 means me)
479 )
480 }{
481 $1
482 ? (getpwnam($1))[7]
483 : ( $ENV{HOME} || $ENV{LOGDIR} )
484 }ex;
485
5a964f20 486=head2 How come when I open a file read-write it wipes it out?
68dc0745 487
488Because you're using something like this, which truncates the file and
489I<then> gives you read-write access:
490
5a964f20 491 open(FH, "+> /path/name"); # WRONG (almost always)
68dc0745 492
493Whoops. You should instead use this, which will fail if the file
d92eb7b0 494doesn't exist.
495
496 open(FH, "+< /path/name"); # open for update
497
498Using "E<gt>" always clobbers or creates. Using "E<lt>" never does
499either. The "+" doesn't change this.
68dc0745 500
5a964f20 501Here are examples of many kinds of file opens. Those using sysopen()
502all assume
68dc0745 503
5a964f20 504 use Fcntl;
68dc0745 505
5a964f20 506To open file for reading:
68dc0745 507
5a964f20 508 open(FH, "< $path") || die $!;
509 sysopen(FH, $path, O_RDONLY) || die $!;
510
511To open file for writing, create new file if needed or else truncate old file:
512
513 open(FH, "> $path") || die $!;
514 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
515 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
516
517To open file for writing, create new file, file must not exist:
518
519 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
520 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
521
522To open file for appending, create if necessary:
523
524 open(FH, ">> $path") || die $!;
525 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
526 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
527
528To open file for appending, file must exist:
529
530 sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
531
532To open file for update, file must exist:
533
534 open(FH, "+< $path") || die $!;
535 sysopen(FH, $path, O_RDWR) || die $!;
536
537To open file for update, create file if necessary:
538
539 sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
540 sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
541
542To open file for update, file must not exist:
543
544 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
545 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
546
547To open a file without blocking, creating if necessary:
548
549 sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT)
550 or die "can't open /tmp/somefile: $!":
551
552Be warned that neither creation nor deletion of files is guaranteed to
553be an atomic operation over NFS. That is, two processes might both
554successful create or unlink the same file! Therefore O_EXCL
555isn't so exclusive as you might wish.
68dc0745 556
87275199 557See also the new L<perlopentut> if you have it (new for 5.6).
65acb1b1 558
559=head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
68dc0745 560
561The C<E<lt>E<gt>> operator performs a globbing operation (see above).
562By default glob() forks csh(1) to do the actual glob expansion, but
563csh can't handle more than 127 items and so gives the error message
564C<Argument list too long>. People who installed tcsh as csh won't
565have this problem, but their users may be surprised by it.
566
65acb1b1 567To get around this, either do the glob yourself with readdir() and
68dc0745 568patterns, or use a module like Glob::KGlob, one that doesn't use the
65acb1b1 569shell to do globbing. This is expected to be fixed soon.
68dc0745 570
571=head2 Is there a leak/bug in glob()?
572
573Due to the current implementation on some operating systems, when you
574use the glob() function or its angle-bracket alias in a scalar
575context, you may cause a leak and/or unpredictable behavior. It's
576best therefore to use glob() only in list context.
577
578=head2 How can I open a file with a leading "E<gt>" or trailing blanks?
579
580Normally perl ignores trailing blanks in filenames, and interprets
581certain leading characters (or a trailing "|") to mean something
582special. To avoid this, you might want to use a routine like this.
583It makes incomplete pathnames into explicit relative ones, and tacks a
584trailing null byte on the name to make perl leave it alone:
585
586 sub safe_filename {
587 local $_ = shift;
65acb1b1 588 s#^([^./])#./$1#;
589 $_ .= "\0";
590 return $_;
68dc0745 591 }
592
65acb1b1 593 $badpath = "<<<something really wicked ";
594 $fn = safe_filename($badpath");
595 open(FH, "> $fn") or "couldn't open $badpath: $!";
596
597This assumes that you are using POSIX (portable operating systems
598interface) paths. If you are on a closed, non-portable, proprietary
599system, you may have to adjust the C<"./"> above.
600
601It would be a lot clearer to use sysopen(), though:
602
603 use Fcntl;
604 $badpath = "<<<something really wicked ";
605 open (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
606 or die "can't open $badpath: $!";
68dc0745 607
65acb1b1 608For more information, see also the new L<perlopentut> if you have it
87275199 609(new for 5.6).
68dc0745 610
611=head2 How can I reliably rename a file?
612
d92eb7b0 613Well, usually you just use Perl's rename() function. But that may not
614work everywhere, in particular, renaming files across file systems.
615Some sub-Unix systems have broken ports that corrupt the semantics of
616rename() -- for example, WinNT does this right, but Win95 and Win98
617are broken. (The last two parts are not surprising, but the first is. :-)
618
619If your operating system supports a proper mv(1) program or its moral
620equivalent, this works:
68dc0745 621
622 rename($old, $new) or system("mv", $old, $new);
623
624It may be more compelling to use the File::Copy module instead. You
625just copy to the new file to the new name (checking return values),
626then delete the old one. This isn't really the same semantics as a
627real rename(), though, which preserves metainformation like
628permissions, timestamps, inode info, etc.
629
65acb1b1 630The newer version of File::Copy exports a move() function.
5a964f20 631
68dc0745 632=head2 How can I lock a file?
633
54310121 634Perl's builtin flock() function (see L<perlfunc> for details) will call
68dc0745 635flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
636later), and lockf(3) if neither of the two previous system calls exists.
637On some systems, it may even use a different form of native locking.
638Here are some gotchas with Perl's flock():
639
640=over 4
641
642=item 1
643
644Produces a fatal error if none of the three system calls (or their
645close equivalent) exists.
646
647=item 2
648
649lockf(3) does not provide shared locking, and requires that the
650filehandle be open for writing (or appending, or read/writing).
651
652=item 3
653
d92eb7b0 654Some versions of flock() can't lock files over a network (e.g. on NFS file
655systems), so you'd need to force the use of fcntl(2) when you build Perl.
656But even this is dubious at best. See the flock entry of L<perlfunc>,
657and the F<INSTALL> file in the source distribution for information on
658building Perl to do this.
659
660Two potentially non-obvious but traditional flock semantics are that
661it waits indefinitely until the lock is granted, and that its locks
662I<merely advisory>. Such discretionary locks are more flexible, but
663offer fewer guarantees. This means that files locked with flock() may
664be modified by programs that do not also use flock(). Cars that stop
665for red lights get on well with each other, but not with cars that don't
666stop for red lights. See the perlport manpage, your port's specific
667documentation, or your system-specific local manpages for details. It's
668best to assume traditional behavior if you're writing portable programs.
669(But if you're not, you should as always feel perfectly free to write
670for your own system's idiosyncrasies (sometimes called "features").
671Slavish adherence to portability concerns shouldn't get in the way of
672your getting your job done.)
68dc0745 673
65acb1b1 674For more information on file locking, see also L<perlopentut/"File
87275199 675Locking"> if you have it (new for 5.6).
65acb1b1 676
68dc0745 677=back
678
65acb1b1 679=head2 Why can't I just open(FH, ">file.lock")?
68dc0745 680
681A common bit of code B<NOT TO USE> is this:
682
683 sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
684 open(LCK, "> file.lock"); # THIS BROKEN CODE
685
686This is a classic race condition: you take two steps to do something
687which must be done in one. That's why computer hardware provides an
688atomic test-and-set instruction. In theory, this "ought" to work:
689
5a964f20 690 sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
68dc0745 691 or die "can't open file.lock: $!":
692
693except that lamentably, file creation (and deletion) is not atomic
694over NFS, so this won't work (at least, not every time) over the net.
65acb1b1 695Various schemes involving link() have been suggested, but
46fc3d4c 696these tend to involve busy-wait, which is also subdesirable.
68dc0745 697
fc36a67e 698=head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
68dc0745 699
46fc3d4c 700Didn't anyone ever tell you web-page hit counters were useless?
5a964f20 701They don't count number of hits, they're a waste of time, and they serve
702only to stroke the writer's vanity. Better to pick a random number.
703It's more realistic.
68dc0745 704
5a964f20 705Anyway, this is what you can do if you can't help yourself.
68dc0745 706
65acb1b1 707 use Fcntl ':flock';
5a964f20 708 sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
65acb1b1 709 flock(FH, LOCK_EX) or die "can't flock numfile: $!";
68dc0745 710 $num = <FH> || 0;
711 seek(FH, 0, 0) or die "can't rewind numfile: $!";
712 truncate(FH, 0) or die "can't truncate numfile: $!";
713 (print FH $num+1, "\n") or die "can't write numfile: $!";
65acb1b1 714 # Perl as of 5.004 automatically flushes before unlocking
715 flock(FH, LOCK_UN) or die "can't flock numfile: $!";
68dc0745 716 close FH or die "can't close numfile: $!";
717
46fc3d4c 718Here's a much better web-page hit counter:
68dc0745 719
720 $hits = int( (time() - 850_000_000) / rand(1_000) );
721
722If the count doesn't impress your friends, then the code might. :-)
723
724=head2 How do I randomly update a binary file?
725
726If you're just trying to patch a binary, in many cases something as
727simple as this works:
728
729 perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
730
731However, if you have fixed sized records, then you might do something more
732like this:
733
734 $RECSIZE = 220; # size of record, in bytes
735 $recno = 37; # which record to update
736 open(FH, "+<somewhere") || die "can't update somewhere: $!";
737 seek(FH, $recno * $RECSIZE, 0);
738 read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
739 # munge the record
65acb1b1 740 seek(FH, -$RECSIZE, 1);
68dc0745 741 print FH $record;
742 close FH;
743
744Locking and error checking are left as an exercise for the reader.
745Don't forget them, or you'll be quite sorry.
746
68dc0745 747=head2 How do I get a file's timestamp in perl?
748
749If you want to retrieve the time at which the file was last read,
46fc3d4c 750written, or had its meta-data (owner, etc) changed, you use the B<-M>,
68dc0745 751B<-A>, or B<-C> filetest operations as documented in L<perlfunc>. These
752retrieve the age of the file (measured against the start-time of your
753program) in days as a floating point number. To retrieve the "raw"
754time in seconds since the epoch, you would call the stat function,
755then use localtime(), gmtime(), or POSIX::strftime() to convert this
756into human-readable form.
757
758Here's an example:
759
760 $write_secs = (stat($file))[9];
c8db1d39 761 printf "file %s updated at %s\n", $file,
762 scalar localtime($write_secs);
68dc0745 763
764If you prefer something more legible, use the File::stat module
765(part of the standard distribution in version 5.004 and later):
766
65acb1b1 767 # error checking left as an exercise for reader.
68dc0745 768 use File::stat;
769 use Time::localtime;
770 $date_string = ctime(stat($file)->mtime);
771 print "file $file updated at $date_string\n";
772
65acb1b1 773The POSIX::strftime() approach has the benefit of being,
774in theory, independent of the current locale. See L<perllocale>
775for details.
68dc0745 776
777=head2 How do I set a file's timestamp in perl?
778
779You use the utime() function documented in L<perlfunc/utime>.
780By way of example, here's a little program that copies the
781read and write times from its first argument to all the rest
782of them.
783
784 if (@ARGV < 2) {
785 die "usage: cptimes timestamp_file other_files ...\n";
786 }
787 $timestamp = shift;
788 ($atime, $mtime) = (stat($timestamp))[8,9];
789 utime $atime, $mtime, @ARGV;
790
65acb1b1 791Error checking is, as usual, left as an exercise for the reader.
68dc0745 792
793Note that utime() currently doesn't work correctly with Win95/NT
794ports. A bug has been reported. Check it carefully before using
795it on those platforms.
796
797=head2 How do I print to more than one file at once?
798
799If you only have to do this once, you can do this:
800
801 for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
802
803To connect up to one filehandle to several output filehandles, it's
804easiest to use the tee(1) program if you have it, and let it take care
805of the multiplexing:
806
807 open (FH, "| tee file1 file2 file3");
808
5a964f20 809Or even:
810
811 # make STDOUT go to three files, plus original STDOUT
812 open (STDOUT, "| tee file1 file2 file3") or die "Teeing off: $!\n";
813 print "whatever\n" or die "Writing: $!\n";
814 close(STDOUT) or die "Closing: $!\n";
68dc0745 815
5a964f20 816Otherwise you'll have to write your own multiplexing print
817function -- or your own tee program -- or use Tom Christiansen's,
818at http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz, which is
819written in Perl and offers much greater functionality
820than the stock version.
68dc0745 821
d92eb7b0 822=head2 How can I read in an entire file all at once?
823
824The customary Perl approach for processing all the lines in a file is to
825do so one line at a time:
826
827 open (INPUT, $file) || die "can't open $file: $!";
828 while (<INPUT>) {
829 chomp;
830 # do something with $_
831 }
832 close(INPUT) || die "can't close $file: $!";
833
834This is tremendously more efficient than reading the entire file into
835memory as an array of lines and then processing it one element at a time,
836which is often -- if not almost always -- the wrong approach. Whenever
837you see someone do this:
838
839 @lines = <INPUT>;
840
841You should think long and hard about why you need everything loaded
842at once. It's just not a scalable solution. You might also find it
843more fun to use the the standard DB_File module's $DB_RECNO bindings,
844which allow you to tie an array to a file so that accessing an element
845the array actually accesses the corresponding line in the file.
846
847On very rare occasion, you may have an algorithm that demands that
848the entire file be in memory at once as one scalar. The simplest solution
849to that is:
850
851 $var = `cat $file`;
852
853Being in scalar context, you get the whole thing. In list context,
854you'd get a list of all the lines:
855
856 @lines = `cat $file`;
857
87275199 858This tiny but expedient solution is neat, clean, and portable to
859all systems on which decent tools have been installed. For those
860who prefer not to use the toolbox, you can of course read the file
861manually, although this makes for more complicated code.
d92eb7b0 862
863 {
864 local(*INPUT, $/);
865 open (INPUT, $file) || die "can't open $file: $!";
866 $var = <INPUT>;
867 }
868
869That temporarily undefs your record separator, and will automatically
870close the file at block exit. If the file is already open, just use this:
871
872 $var = do { local $/; <INPUT> };
873
68dc0745 874=head2 How can I read in a file by paragraphs?
875
65acb1b1 876Use the C<$/> variable (see L<perlvar> for details). You can either
68dc0745 877set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
878for instance, gets treated as two paragraphs and not three), or
879C<"\n\n"> to accept empty paragraphs.
880
65acb1b1 881Note that a blank line must have no blanks in it. Thus C<"fred\n
882\nstuff\n\n"> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
883
68dc0745 884=head2 How can I read a single character from a file? From the keyboard?
885
886You can use the builtin C<getc()> function for most filehandles, but
887it won't (easily) work on a terminal device. For STDIN, either use
888the Term::ReadKey module from CPAN, or use the sample code in
889L<perlfunc/getc>.
890
65acb1b1 891If your system supports the portable operating system programming
892interface (POSIX), you can use the following code, which you'll note
893turns off echo processing as well.
68dc0745 894
895 #!/usr/bin/perl -w
896 use strict;
897 $| = 1;
898 for (1..4) {
899 my $got;
900 print "gimme: ";
901 $got = getone();
902 print "--> $got\n";
903 }
904 exit;
905
906 BEGIN {
907 use POSIX qw(:termios_h);
908
909 my ($term, $oterm, $echo, $noecho, $fd_stdin);
910
911 $fd_stdin = fileno(STDIN);
912
913 $term = POSIX::Termios->new();
914 $term->getattr($fd_stdin);
915 $oterm = $term->getlflag();
916
917 $echo = ECHO | ECHOK | ICANON;
918 $noecho = $oterm & ~$echo;
919
920 sub cbreak {
921 $term->setlflag($noecho);
922 $term->setcc(VTIME, 1);
923 $term->setattr($fd_stdin, TCSANOW);
924 }
925
926 sub cooked {
927 $term->setlflag($oterm);
928 $term->setcc(VTIME, 0);
929 $term->setattr($fd_stdin, TCSANOW);
930 }
931
932 sub getone {
933 my $key = '';
934 cbreak();
935 sysread(STDIN, $key, 1);
936 cooked();
937 return $key;
938 }
939
940 }
941
942 END { cooked() }
943
65acb1b1 944The Term::ReadKey module from CPAN may be easier to use. Recent version
945include also support for non-portable systems as well.
68dc0745 946
947 use Term::ReadKey;
948 open(TTY, "</dev/tty");
949 print "Gimme a char: ";
950 ReadMode "raw";
951 $key = ReadKey 0, *TTY;
952 ReadMode "normal";
953 printf "\nYou said %s, char number %03d\n",
954 $key, ord $key;
955
65acb1b1 956For legacy DOS systems, Dan Carson <dbc@tc.fluke.COM> reports the following:
68dc0745 957
958To put the PC in "raw" mode, use ioctl with some magic numbers gleaned
959from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes
960across the net every so often):
961
962 $old_ioctl = ioctl(STDIN,0,0); # Gets device info
963 $old_ioctl &= 0xff;
964 ioctl(STDIN,1,$old_ioctl | 32); # Writes it back, setting bit 5
965
966Then to read a single character:
967
968 sysread(STDIN,$c,1); # Read a single character
969
970And to put the PC back to "cooked" mode:
971
972 ioctl(STDIN,1,$old_ioctl); # Sets it back to cooked mode.
973
974So now you have $c. If C<ord($c) == 0>, you have a two byte code, which
975means you hit a special key. Read another byte with C<sysread(STDIN,$c,1)>,
976and that value tells you what combination it was according to this
977table:
978
979 # PC 2-byte keycodes = ^@ + the following:
980
981 # HEX KEYS
982 # --- ----
983 # 0F SHF TAB
984 # 10-19 ALT QWERTYUIOP
985 # 1E-26 ALT ASDFGHJKL
986 # 2C-32 ALT ZXCVBNM
987 # 3B-44 F1-F10
988 # 47-49 HOME,UP,PgUp
989 # 4B LEFT
990 # 4D RIGHT
991 # 4F-53 END,DOWN,PgDn,Ins,Del
992 # 54-5D SHF F1-F10
993 # 5E-67 CTR F1-F10
994 # 68-71 ALT F1-F10
995 # 73-77 CTR LEFT,RIGHT,END,PgDn,HOME
996 # 78-83 ALT 1234567890-=
997 # 84 CTR PgUp
998
999This is all trial and error I did a long time ago, I hope I'm reading the
1000file that worked.
1001
65acb1b1 1002=head2 How can I tell whether there's a character waiting on a filehandle?
68dc0745 1003
5a964f20 1004The very first thing you should do is look into getting the Term::ReadKey
65acb1b1 1005extension from CPAN. As we mentioned earlier, it now even has limited
1006support for non-portable (read: not open systems, closed, proprietary,
1007not POSIX, not Unix, etc) systems.
5a964f20 1008
1009You should also check out the Frequently Asked Questions list in
68dc0745 1010comp.unix.* for things like this: the answer is essentially the same.
1011It's very system dependent. Here's one solution that works on BSD
1012systems:
1013
1014 sub key_ready {
1015 my($rin, $nfd);
1016 vec($rin, fileno(STDIN), 1) = 1;
1017 return $nfd = select($rin,undef,undef,0);
1018 }
1019
65acb1b1 1020If you want to find out how many characters are waiting, there's
1021also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
1022comes with Perl tries to convert C include files to Perl code, which
1023can be C<require>d. FIONREAD ends up defined as a function in the
1024I<sys/ioctl.ph> file:
68dc0745 1025
5a964f20 1026 require 'sys/ioctl.ph';
68dc0745 1027
5a964f20 1028 $size = pack("L", 0);
1029 ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
1030 $size = unpack("L", $size);
68dc0745 1031
5a964f20 1032If I<h2ph> wasn't installed or doesn't work for you, you can
1033I<grep> the include files by hand:
68dc0745 1034
5a964f20 1035 % grep FIONREAD /usr/include/*/*
1036 /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
68dc0745 1037
5a964f20 1038Or write a small C program using the editor of champions:
68dc0745 1039
5a964f20 1040 % cat > fionread.c
1041 #include <sys/ioctl.h>
1042 main() {
1043 printf("%#08x\n", FIONREAD);
1044 }
1045 ^D
65acb1b1 1046 % cc -o fionread fionread.c
5a964f20 1047 % ./fionread
1048 0x4004667f
1049
1050And then hard-code it, leaving porting as an exercise to your successor.
1051
1052 $FIONREAD = 0x4004667f; # XXX: opsys dependent
1053
1054 $size = pack("L", 0);
1055 ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
1056 $size = unpack("L", $size);
1057
1058FIONREAD requires a filehandle connected to a stream, meaning sockets,
1059pipes, and tty devices work, but I<not> files.
68dc0745 1060
1061=head2 How do I do a C<tail -f> in perl?
1062
1063First try
1064
1065 seek(GWFILE, 0, 1);
1066
1067The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
1068but it does clear the end-of-file condition on the handle, so that the
1069next <GWFILE> makes Perl try again to read something.
1070
1071If that doesn't work (it relies on features of your stdio implementation),
1072then you need something more like this:
1073
1074 for (;;) {
1075 for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
1076 # search for some stuff and put it into files
1077 }
1078 # sleep for a while
1079 seek(GWFILE, $curpos, 0); # seek to where we had been
1080 }
1081
1082If this still doesn't work, look into the POSIX module. POSIX defines
1083the clearerr() method, which can remove the end of file condition on a
1084filehandle. The method: read until end of file, clearerr(), read some
1085more. Lather, rinse, repeat.
1086
65acb1b1 1087There's also a File::Tail module from CPAN.
1088
68dc0745 1089=head2 How do I dup() a filehandle in Perl?
1090
1091If you check L<perlfunc/open>, you'll see that several of the ways
1092to call open() should do the trick. For example:
1093
1094 open(LOG, ">>/tmp/logfile");
1095 open(STDERR, ">&LOG");
1096
1097Or even with a literal numeric descriptor:
1098
1099 $fd = $ENV{MHCONTEXTFD};
1100 open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
1101
5a964f20 1102Note that "E<lt>&STDIN" makes a copy, but "E<lt>&=STDIN" make
1103an alias. That means if you close an aliased handle, all
1104aliases become inaccessible. This is not true with
1105a copied one.
1106
1107Error checking, as always, has been left as an exercise for the reader.
68dc0745 1108
1109=head2 How do I close a file descriptor by number?
1110
1111This should rarely be necessary, as the Perl close() function is to be
1112used for things that Perl opened itself, even if it was a dup of a
1113numeric descriptor, as with MHCONTEXT above. But if you really have
1114to, you may be able to do this:
1115
1116 require 'sys/syscall.ph';
1117 $rc = syscall(&SYS_close, $fd + 0); # must force numeric
1118 die "can't sysclose $fd: $!" unless $rc == -1;
1119
d92eb7b0 1120Or just use the fdopen(3S) feature of open():
1121
1122 {
1123 local *F;
1124 open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
1125 close F;
1126 }
1127
46fc3d4c 1128=head2 Why can't I use "C:\temp\foo" in DOS paths? What doesn't `C:\temp\foo.exe` work?
68dc0745 1129
1130Whoops! You just put a tab and a formfeed into that filename!
1131Remember that within double quoted strings ("like\this"), the
1132backslash is an escape character. The full list of these is in
1133L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
1134have a file called "c:(tab)emp(formfeed)oo" or
65acb1b1 1135"c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
68dc0745 1136
1137Either single-quote your strings, or (preferably) use forward slashes.
46fc3d4c 1138Since all DOS and Windows versions since something like MS-DOS 2.0 or so
68dc0745 1139have treated C</> and C<\> the same in a path, you might as well use the
1140one that doesn't clash with Perl -- or the POSIX shell, ANSI C and C++,
65acb1b1 1141awk, Tcl, Java, or Python, just to mention a few. POSIX paths
1142are more portable, too.
68dc0745 1143
1144=head2 Why doesn't glob("*.*") get all the files?
1145
1146Because even on non-Unix ports, Perl's glob function follows standard
46fc3d4c 1147Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
65acb1b1 1148files. This makes glob() portable even to legacy systems. Your
1149port may include proprietary globbing functions as well. Check its
1150documentation for details.
68dc0745 1151
1152=head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
1153
1154This is elaborately and painstakingly described in the "Far More Than
7b8d334a 1155You Ever Wanted To Know" in
68dc0745 1156http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .
1157
1158The executive summary: learn how your filesystem works. The
1159permissions on a file say what can happen to the data in that file.
1160The permissions on a directory say what can happen to the list of
1161files in that directory. If you delete a file, you're removing its
1162name from the directory (so the operation depends on the permissions
1163of the directory, not of the file). If you try to write to the file,
1164the permissions of the file govern whether you're allowed to.
1165
1166=head2 How do I select a random line from a file?
1167
1168Here's an algorithm from the Camel Book:
1169
1170 srand;
1171 rand($.) < 1 && ($line = $_) while <>;
1172
1173This has a significant advantage in space over reading the whole
5a964f20 1174file in. A simple proof by induction is available upon
1175request if you doubt its correctness.
68dc0745 1176
65acb1b1 1177=head2 Why do I get weird spaces when I print an array of lines?
1178
1179Saying
1180
1181 print "@lines\n";
1182
1183joins together the elements of C<@lines> with a space between them.
1184If C<@lines> were C<("little", "fluffy", "clouds")> then the above
1185statement would print:
1186
1187 little fluffy clouds
1188
1189but if each element of C<@lines> was a line of text, ending a newline
1190character C<("little\n", "fluffy\n", "clouds\n")> then it would print:
1191
1192 little
1193 fluffy
1194 clouds
1195
1196If your array contains lines, just print them:
1197
1198 print @lines;
1199
68dc0745 1200=head1 AUTHOR AND COPYRIGHT
1201
65acb1b1 1202Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
5a964f20 1203All rights reserved.
1204
c8db1d39 1205When included as an integrated part of the Standard Distribution
d92eb7b0 1206of Perl or of its documentation (printed or otherwise), this works is
1207covered under Perl's Artistic License. For separate distributions of
c8db1d39 1208all or part of this FAQ outside of that, see L<perlfaq>.
1209
87275199 1210Irrespective of its distribution, all code examples here are in the public
c8db1d39 1211domain. You are permitted and encouraged to use this code and any
1212derivatives thereof in your own programs for fun or for profit as you
1213see fit. A simple comment in the code giving credit to the FAQ would
1214be courteous but is not required.