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1=head1 NAME
2
d92eb7b0 3perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date: 1999/05/23 16:08:30 $)
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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?
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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
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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, $char, 1)>). What it does instead support is "command
27buffering", in which a physical write is performed after every output
28command. This isn't as hard on your system as unbuffering, but does
29get the output where you want it when you want it.
30
31If you expect characters to get to your device when you print them there,
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32you'll want to autoflush its handle.
33Use select() and the C<$|> 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:
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46
47 use FileHandle;
5a964f20 48 open(DEV, "+</dev/tty"); # ceci n'est pas une pipe
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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();
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66 print $sock "GET / HTTP/1.0" . "\015\012" x 2;
67 $document = join('', <$sock>);
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68 print "DOC IS: $document\n";
69
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70Note the bizarrely hardcoded carriage return and newline in their octal
71equivalents. This is the ONLY way (currently) to assure a proper flush
d92eb7b0 72on all platforms, including Macintosh. That's the way things work in
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73network programming: you really should specify the exact bit pattern
74on the network line terminator. In practice, C<"\n\n"> often works,
75but this is not portable.
68dc0745 76
5a964f20 77See L<perlfaq9> for other examples of fetching URLs over the web.
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78
79=head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
80
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81Those are operations of a text editor. Perl is not a text editor.
82Perl is a programming language. You have to decompose the problem into
83low-level calls to read, write, open, close, and seek.
84
68dc0745 85Although humans have an easy time thinking of a text file as being a
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86sequence of lines that operates much like a stack of playing cards -- or
87punch cards -- computers usually see the text file as a sequence of bytes.
88In general, there's no direct way for Perl to seek to a particular line
89of a file, insert text into a file, or remove text from a file.
68dc0745 90
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91(There are exceptions in special circumstances. You can add or remove at
92the very end of the file. Another is replacing a sequence of bytes with
93another sequence of the same length. Another is using the C<$DB_RECNO>
94array bindings as documented in L<DB_File>. Yet another is manipulating
95files with all lines the same length.)
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96
97The general solution is to create a temporary copy of the text file with
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98the changes you want, then copy that over the original. This assumes
99no locking.
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100
101 $old = $file;
102 $new = "$file.tmp.$$";
65acb1b1 103 $bak = "$file.orig";
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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
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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"));
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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) }
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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
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172This assumes no funny games with newline translations.
173
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174=head2 How do I make a temporary file name?
175
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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: $!";
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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 {
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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 }
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217 if (defined(fileno(FH))
218 return (*FH, $base_name);
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219 } else {
220 return ();
221 }
222 }
223 }
224
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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.
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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:
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233
234 # sample input line:
235 # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
236 $PS_T = 'A6 A4 A7 A5 A*';
237 open(PS, "ps|");
5a964f20 238 print scalar <PS>;
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239 while (<PS>) {
240 ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
241 for $var (qw!pid tt stat time command!) {
242 print "$var: <$$var>\n";
243 }
244 print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
245 "\n";
246 }
247
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248We've used C<$$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
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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
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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
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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 {
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266 local *HostFile;
267 open(HostFile, "</etc/hosts") or die "no /etc/hosts: $!";
268 local $_; # <- VERY IMPORTANT
269 while (<HostFile>) {
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270 print if /\b127\.(0\.0\.)?1\b/;
271 }
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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 ];
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285 }
286
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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
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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
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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
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309Or here using the semi-object-oriented FileHandle module, which certainly
310isn't light-weight:
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311
312 use FileHandle;
313
46fc3d4c 314 foreach $filename (@names) {
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315 my $fh = FileHandle->new("/etc/$filename") or die "$filename: $!";
316 $file{$filename} = [ $i++, $fh ];
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317 }
318
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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 $fh = IO::Handle->new();
345
346Then use any of those as you would a normal filehandle. Anywhere that
347Perl is expecting a filehandle, an indirect filehandle may be used
348instead. An indirect filehandle is just a scalar variable that contains
368c9434 349a filehandle. Functions like C<print>, C<open>, C<seek>, or
c47ff5f1 350the C<< <FH> >> diamond operator will accept either a read filehandle
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351or a scalar variable containing one:
352
353 ($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
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359the function in two ways:
360
361 sub accept_fh {
362 my $fh = shift;
363 print $fh "Sending to indirect filehandle\n";
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364 }
365
5a964f20 366Or it can localize a typeglob and use the filehandle directly:
46fc3d4c 367
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368 sub accept_fh {
369 local *FH = shift;
370 print FH "Sending to localized filehandle\n";
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371 }
372
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373Both styles work with either objects or typeglobs of real filehandles.
374(They might also work with strings under some circumstances, but this
375is risky.)
376
377 accept_fh(*STDOUT);
378 accept_fh($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
c47ff5f1 410as C<< <> >> does. Given the initialization shown above for @fd, this
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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
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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
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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.
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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/;
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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;
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448
449because you have to put the comma in and then recalculate your
450position.
451
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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;
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462 }
463
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464=head2 How can I translate tildes (~) in a filename?
465
c47ff5f1 466Use the <> (glob()) operator, documented in L<perlfunc>. This
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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?
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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)
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492
493Whoops. You should instead use this, which will fail if the file
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494doesn't exist.
495
496 open(FH, "+< /path/name"); # open for update
497
c47ff5f1 498Using ">" always clobbers or creates. Using "<" never does
d92eb7b0 499either. The "+" doesn't change this.
68dc0745 500
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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
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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
c47ff5f1 559=head2 Why do I sometimes get an "Argument list too long" when I use <*>?
68dc0745 560
c47ff5f1 561The C<< <> >> operator performs a globbing operation (see above).
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562In Perl versions earlier than v5.6.0, the internal glob() operator forks
563csh(1) to do the actual glob expansion, but
68dc0745
PP
564csh can't handle more than 127 items and so gives the error message
565C<Argument list too long>. People who installed tcsh as csh won't
566have this problem, but their users may be surprised by it.
567
3a4b19e4
GS
568To get around this, either upgrade to Perl v5.6.0 or later, do the glob
569yourself with readdir() and patterns, or use a module like Glob::KGlob,
570one that doesn't use the shell to do globbing.
68dc0745
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571
572=head2 Is there a leak/bug in glob()?
573
574Due to the current implementation on some operating systems, when you
575use the glob() function or its angle-bracket alias in a scalar
576context, you may cause a leak and/or unpredictable behavior. It's
577best therefore to use glob() only in list context.
578
c47ff5f1 579=head2 How can I open a file with a leading ">" or trailing blanks?
68dc0745
PP
580
581Normally perl ignores trailing blanks in filenames, and interprets
582certain leading characters (or a trailing "|") to mean something
583special. To avoid this, you might want to use a routine like this.
584It makes incomplete pathnames into explicit relative ones, and tacks a
585trailing null byte on the name to make perl leave it alone:
586
587 sub safe_filename {
588 local $_ = shift;
65acb1b1
TC
589 s#^([^./])#./$1#;
590 $_ .= "\0";
591 return $_;
68dc0745
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592 }
593
65acb1b1
TC
594 $badpath = "<<<something really wicked ";
595 $fn = safe_filename($badpath");
596 open(FH, "> $fn") or "couldn't open $badpath: $!";
597
598This assumes that you are using POSIX (portable operating systems
599interface) paths. If you are on a closed, non-portable, proprietary
600system, you may have to adjust the C<"./"> above.
601
602It would be a lot clearer to use sysopen(), though:
603
604 use Fcntl;
605 $badpath = "<<<something really wicked ";
606 open (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
607 or die "can't open $badpath: $!";
68dc0745 608
65acb1b1 609For more information, see also the new L<perlopentut> if you have it
87275199 610(new for 5.6).
68dc0745
PP
611
612=head2 How can I reliably rename a file?
613
d92eb7b0
GS
614Well, usually you just use Perl's rename() function. But that may not
615work everywhere, in particular, renaming files across file systems.
616Some sub-Unix systems have broken ports that corrupt the semantics of
617rename() -- for example, WinNT does this right, but Win95 and Win98
618are broken. (The last two parts are not surprising, but the first is. :-)
619
620If your operating system supports a proper mv(1) program or its moral
621equivalent, this works:
68dc0745
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622
623 rename($old, $new) or system("mv", $old, $new);
624
625It may be more compelling to use the File::Copy module instead. You
626just copy to the new file to the new name (checking return values),
627then delete the old one. This isn't really the same semantics as a
628real rename(), though, which preserves metainformation like
629permissions, timestamps, inode info, etc.
630
65acb1b1 631The newer version of File::Copy exports a move() function.
5a964f20 632
68dc0745
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633=head2 How can I lock a file?
634
54310121 635Perl's builtin flock() function (see L<perlfunc> for details) will call
68dc0745
PP
636flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
637later), and lockf(3) if neither of the two previous system calls exists.
638On some systems, it may even use a different form of native locking.
639Here are some gotchas with Perl's flock():
640
641=over 4
642
643=item 1
644
645Produces a fatal error if none of the three system calls (or their
646close equivalent) exists.
647
648=item 2
649
650lockf(3) does not provide shared locking, and requires that the
651filehandle be open for writing (or appending, or read/writing).
652
653=item 3
654
d92eb7b0
GS
655Some versions of flock() can't lock files over a network (e.g. on NFS file
656systems), so you'd need to force the use of fcntl(2) when you build Perl.
657But even this is dubious at best. See the flock entry of L<perlfunc>,
658and the F<INSTALL> file in the source distribution for information on
659building Perl to do this.
660
661Two potentially non-obvious but traditional flock semantics are that
662it waits indefinitely until the lock is granted, and that its locks
663I<merely advisory>. Such discretionary locks are more flexible, but
664offer fewer guarantees. This means that files locked with flock() may
665be modified by programs that do not also use flock(). Cars that stop
666for red lights get on well with each other, but not with cars that don't
667stop for red lights. See the perlport manpage, your port's specific
668documentation, or your system-specific local manpages for details. It's
669best to assume traditional behavior if you're writing portable programs.
670(But if you're not, you should as always feel perfectly free to write
671for your own system's idiosyncrasies (sometimes called "features").
672Slavish adherence to portability concerns shouldn't get in the way of
673your getting your job done.)
68dc0745 674
65acb1b1 675For more information on file locking, see also L<perlopentut/"File
87275199 676Locking"> if you have it (new for 5.6).
65acb1b1 677
68dc0745
PP
678=back
679
65acb1b1 680=head2 Why can't I just open(FH, ">file.lock")?
68dc0745
PP
681
682A common bit of code B<NOT TO USE> is this:
683
684 sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
685 open(LCK, "> file.lock"); # THIS BROKEN CODE
686
687This is a classic race condition: you take two steps to do something
688which must be done in one. That's why computer hardware provides an
689atomic test-and-set instruction. In theory, this "ought" to work:
690
5a964f20 691 sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
68dc0745
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692 or die "can't open file.lock: $!":
693
694except that lamentably, file creation (and deletion) is not atomic
695over NFS, so this won't work (at least, not every time) over the net.
65acb1b1 696Various schemes involving link() have been suggested, but
46fc3d4c 697these tend to involve busy-wait, which is also subdesirable.
68dc0745 698
fc36a67e 699=head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
68dc0745 700
46fc3d4c 701Didn't anyone ever tell you web-page hit counters were useless?
5a964f20
TC
702They don't count number of hits, they're a waste of time, and they serve
703only to stroke the writer's vanity. Better to pick a random number.
704It's more realistic.
68dc0745 705
5a964f20 706Anyway, this is what you can do if you can't help yourself.
68dc0745 707
e2c57c3e 708 use Fcntl qw(:DEFAULT :flock);
5a964f20 709 sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
65acb1b1 710 flock(FH, LOCK_EX) or die "can't flock numfile: $!";
68dc0745
PP
711 $num = <FH> || 0;
712 seek(FH, 0, 0) or die "can't rewind numfile: $!";
713 truncate(FH, 0) or die "can't truncate numfile: $!";
714 (print FH $num+1, "\n") or die "can't write numfile: $!";
65acb1b1
TC
715 # Perl as of 5.004 automatically flushes before unlocking
716 flock(FH, LOCK_UN) or die "can't flock numfile: $!";
68dc0745
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717 close FH or die "can't close numfile: $!";
718
46fc3d4c 719Here's a much better web-page hit counter:
68dc0745
PP
720
721 $hits = int( (time() - 850_000_000) / rand(1_000) );
722
723If the count doesn't impress your friends, then the code might. :-)
724
725=head2 How do I randomly update a binary file?
726
727If you're just trying to patch a binary, in many cases something as
728simple as this works:
729
730 perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
731
732However, if you have fixed sized records, then you might do something more
733like this:
734
735 $RECSIZE = 220; # size of record, in bytes
736 $recno = 37; # which record to update
737 open(FH, "+<somewhere") || die "can't update somewhere: $!";
738 seek(FH, $recno * $RECSIZE, 0);
739 read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
740 # munge the record
65acb1b1 741 seek(FH, -$RECSIZE, 1);
68dc0745
PP
742 print FH $record;
743 close FH;
744
745Locking and error checking are left as an exercise for the reader.
746Don't forget them, or you'll be quite sorry.
747
68dc0745
PP
748=head2 How do I get a file's timestamp in perl?
749
750If you want to retrieve the time at which the file was last read,
46fc3d4c 751written, or had its meta-data (owner, etc) changed, you use the B<-M>,
68dc0745
PP
752B<-A>, or B<-C> filetest operations as documented in L<perlfunc>. These
753retrieve the age of the file (measured against the start-time of your
754program) in days as a floating point number. To retrieve the "raw"
755time in seconds since the epoch, you would call the stat function,
756then use localtime(), gmtime(), or POSIX::strftime() to convert this
757into human-readable form.
758
759Here's an example:
760
761 $write_secs = (stat($file))[9];
c8db1d39
TC
762 printf "file %s updated at %s\n", $file,
763 scalar localtime($write_secs);
68dc0745
PP
764
765If you prefer something more legible, use the File::stat module
766(part of the standard distribution in version 5.004 and later):
767
65acb1b1 768 # error checking left as an exercise for reader.
68dc0745
PP
769 use File::stat;
770 use Time::localtime;
771 $date_string = ctime(stat($file)->mtime);
772 print "file $file updated at $date_string\n";
773
65acb1b1
TC
774The POSIX::strftime() approach has the benefit of being,
775in theory, independent of the current locale. See L<perllocale>
776for details.
68dc0745
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777
778=head2 How do I set a file's timestamp in perl?
779
780You use the utime() function documented in L<perlfunc/utime>.
781By way of example, here's a little program that copies the
782read and write times from its first argument to all the rest
783of them.
784
785 if (@ARGV < 2) {
786 die "usage: cptimes timestamp_file other_files ...\n";
787 }
788 $timestamp = shift;
789 ($atime, $mtime) = (stat($timestamp))[8,9];
790 utime $atime, $mtime, @ARGV;
791
65acb1b1 792Error checking is, as usual, left as an exercise for the reader.
68dc0745
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793
794Note that utime() currently doesn't work correctly with Win95/NT
795ports. A bug has been reported. Check it carefully before using
796it on those platforms.
797
798=head2 How do I print to more than one file at once?
799
800If you only have to do this once, you can do this:
801
802 for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
803
804To connect up to one filehandle to several output filehandles, it's
805easiest to use the tee(1) program if you have it, and let it take care
806of the multiplexing:
807
808 open (FH, "| tee file1 file2 file3");
809
5a964f20
TC
810Or even:
811
812 # make STDOUT go to three files, plus original STDOUT
813 open (STDOUT, "| tee file1 file2 file3") or die "Teeing off: $!\n";
814 print "whatever\n" or die "Writing: $!\n";
815 close(STDOUT) or die "Closing: $!\n";
68dc0745 816
5a964f20
TC
817Otherwise you'll have to write your own multiplexing print
818function -- or your own tee program -- or use Tom Christiansen's,
819at http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz, which is
820written in Perl and offers much greater functionality
821than the stock version.
68dc0745 822
d92eb7b0
GS
823=head2 How can I read in an entire file all at once?
824
825The customary Perl approach for processing all the lines in a file is to
826do so one line at a time:
827
828 open (INPUT, $file) || die "can't open $file: $!";
829 while (<INPUT>) {
830 chomp;
831 # do something with $_
832 }
833 close(INPUT) || die "can't close $file: $!";
834
835This is tremendously more efficient than reading the entire file into
836memory as an array of lines and then processing it one element at a time,
837which is often -- if not almost always -- the wrong approach. Whenever
838you see someone do this:
839
840 @lines = <INPUT>;
841
842You should think long and hard about why you need everything loaded
843at once. It's just not a scalable solution. You might also find it
106325ad 844more fun to use the standard DB_File module's $DB_RECNO bindings,
d92eb7b0
GS
845which allow you to tie an array to a file so that accessing an element
846the array actually accesses the corresponding line in the file.
847
848On very rare occasion, you may have an algorithm that demands that
849the entire file be in memory at once as one scalar. The simplest solution
850to that is:
851
852 $var = `cat $file`;
853
854Being in scalar context, you get the whole thing. In list context,
855you'd get a list of all the lines:
856
857 @lines = `cat $file`;
858
87275199
GS
859This tiny but expedient solution is neat, clean, and portable to
860all systems on which decent tools have been installed. For those
861who prefer not to use the toolbox, you can of course read the file
862manually, although this makes for more complicated code.
d92eb7b0
GS
863
864 {
865 local(*INPUT, $/);
866 open (INPUT, $file) || die "can't open $file: $!";
867 $var = <INPUT>;
868 }
869
870That temporarily undefs your record separator, and will automatically
871close the file at block exit. If the file is already open, just use this:
872
873 $var = do { local $/; <INPUT> };
874
68dc0745
PP
875=head2 How can I read in a file by paragraphs?
876
65acb1b1 877Use the C<$/> variable (see L<perlvar> for details). You can either
68dc0745
PP
878set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
879for instance, gets treated as two paragraphs and not three), or
880C<"\n\n"> to accept empty paragraphs.
881
65acb1b1
TC
882Note that a blank line must have no blanks in it. Thus C<"fred\n
883\nstuff\n\n"> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
884
68dc0745
PP
885=head2 How can I read a single character from a file? From the keyboard?
886
887You can use the builtin C<getc()> function for most filehandles, but
888it won't (easily) work on a terminal device. For STDIN, either use
889the Term::ReadKey module from CPAN, or use the sample code in
890L<perlfunc/getc>.
891
65acb1b1
TC
892If your system supports the portable operating system programming
893interface (POSIX), you can use the following code, which you'll note
894turns off echo processing as well.
68dc0745
PP
895
896 #!/usr/bin/perl -w
897 use strict;
898 $| = 1;
899 for (1..4) {
900 my $got;
901 print "gimme: ";
902 $got = getone();
903 print "--> $got\n";
904 }
905 exit;
906
907 BEGIN {
908 use POSIX qw(:termios_h);
909
910 my ($term, $oterm, $echo, $noecho, $fd_stdin);
911
912 $fd_stdin = fileno(STDIN);
913
914 $term = POSIX::Termios->new();
915 $term->getattr($fd_stdin);
916 $oterm = $term->getlflag();
917
918 $echo = ECHO | ECHOK | ICANON;
919 $noecho = $oterm & ~$echo;
920
921 sub cbreak {
922 $term->setlflag($noecho);
923 $term->setcc(VTIME, 1);
924 $term->setattr($fd_stdin, TCSANOW);
925 }
926
927 sub cooked {
928 $term->setlflag($oterm);
929 $term->setcc(VTIME, 0);
930 $term->setattr($fd_stdin, TCSANOW);
931 }
932
933 sub getone {
934 my $key = '';
935 cbreak();
936 sysread(STDIN, $key, 1);
937 cooked();
938 return $key;
939 }
940
941 }
942
943 END { cooked() }
944
65acb1b1
TC
945The Term::ReadKey module from CPAN may be easier to use. Recent version
946include also support for non-portable systems as well.
68dc0745
PP
947
948 use Term::ReadKey;
949 open(TTY, "</dev/tty");
950 print "Gimme a char: ";
951 ReadMode "raw";
952 $key = ReadKey 0, *TTY;
953 ReadMode "normal";
954 printf "\nYou said %s, char number %03d\n",
955 $key, ord $key;
956
65acb1b1 957For legacy DOS systems, Dan Carson <dbc@tc.fluke.COM> reports the following:
68dc0745
PP
958
959To put the PC in "raw" mode, use ioctl with some magic numbers gleaned
960from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes
961across the net every so often):
962
963 $old_ioctl = ioctl(STDIN,0,0); # Gets device info
964 $old_ioctl &= 0xff;
965 ioctl(STDIN,1,$old_ioctl | 32); # Writes it back, setting bit 5
966
967Then to read a single character:
968
969 sysread(STDIN,$c,1); # Read a single character
970
971And to put the PC back to "cooked" mode:
972
973 ioctl(STDIN,1,$old_ioctl); # Sets it back to cooked mode.
974
975So now you have $c. If C<ord($c) == 0>, you have a two byte code, which
976means you hit a special key. Read another byte with C<sysread(STDIN,$c,1)>,
977and that value tells you what combination it was according to this
978table:
979
980 # PC 2-byte keycodes = ^@ + the following:
981
982 # HEX KEYS
983 # --- ----
984 # 0F SHF TAB
985 # 10-19 ALT QWERTYUIOP
986 # 1E-26 ALT ASDFGHJKL
987 # 2C-32 ALT ZXCVBNM
988 # 3B-44 F1-F10
989 # 47-49 HOME,UP,PgUp
990 # 4B LEFT
991 # 4D RIGHT
992 # 4F-53 END,DOWN,PgDn,Ins,Del
993 # 54-5D SHF F1-F10
994 # 5E-67 CTR F1-F10
995 # 68-71 ALT F1-F10
996 # 73-77 CTR LEFT,RIGHT,END,PgDn,HOME
997 # 78-83 ALT 1234567890-=
998 # 84 CTR PgUp
999
1000This is all trial and error I did a long time ago, I hope I'm reading the
1001file that worked.
1002
65acb1b1 1003=head2 How can I tell whether there's a character waiting on a filehandle?
68dc0745 1004
5a964f20 1005The very first thing you should do is look into getting the Term::ReadKey
65acb1b1
TC
1006extension from CPAN. As we mentioned earlier, it now even has limited
1007support for non-portable (read: not open systems, closed, proprietary,
1008not POSIX, not Unix, etc) systems.
5a964f20
TC
1009
1010You should also check out the Frequently Asked Questions list in
68dc0745
PP
1011comp.unix.* for things like this: the answer is essentially the same.
1012It's very system dependent. Here's one solution that works on BSD
1013systems:
1014
1015 sub key_ready {
1016 my($rin, $nfd);
1017 vec($rin, fileno(STDIN), 1) = 1;
1018 return $nfd = select($rin,undef,undef,0);
1019 }
1020
65acb1b1
TC
1021If you want to find out how many characters are waiting, there's
1022also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
1023comes with Perl tries to convert C include files to Perl code, which
1024can be C<require>d. FIONREAD ends up defined as a function in the
1025I<sys/ioctl.ph> file:
68dc0745 1026
5a964f20 1027 require 'sys/ioctl.ph';
68dc0745 1028
5a964f20
TC
1029 $size = pack("L", 0);
1030 ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
1031 $size = unpack("L", $size);
68dc0745 1032
5a964f20
TC
1033If I<h2ph> wasn't installed or doesn't work for you, you can
1034I<grep> the include files by hand:
68dc0745 1035
5a964f20
TC
1036 % grep FIONREAD /usr/include/*/*
1037 /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
68dc0745 1038
5a964f20 1039Or write a small C program using the editor of champions:
68dc0745 1040
5a964f20
TC
1041 % cat > fionread.c
1042 #include <sys/ioctl.h>
1043 main() {
1044 printf("%#08x\n", FIONREAD);
1045 }
1046 ^D
65acb1b1 1047 % cc -o fionread fionread.c
5a964f20
TC
1048 % ./fionread
1049 0x4004667f
1050
1051And then hard-code it, leaving porting as an exercise to your successor.
1052
1053 $FIONREAD = 0x4004667f; # XXX: opsys dependent
1054
1055 $size = pack("L", 0);
1056 ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
1057 $size = unpack("L", $size);
1058
1059FIONREAD requires a filehandle connected to a stream, meaning sockets,
1060pipes, and tty devices work, but I<not> files.
68dc0745
PP
1061
1062=head2 How do I do a C<tail -f> in perl?
1063
1064First try
1065
1066 seek(GWFILE, 0, 1);
1067
1068The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
1069but it does clear the end-of-file condition on the handle, so that the
1070next <GWFILE> makes Perl try again to read something.
1071
1072If that doesn't work (it relies on features of your stdio implementation),
1073then you need something more like this:
1074
1075 for (;;) {
1076 for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
1077 # search for some stuff and put it into files
1078 }
1079 # sleep for a while
1080 seek(GWFILE, $curpos, 0); # seek to where we had been
1081 }
1082
1083If this still doesn't work, look into the POSIX module. POSIX defines
1084the clearerr() method, which can remove the end of file condition on a
1085filehandle. The method: read until end of file, clearerr(), read some
1086more. Lather, rinse, repeat.
1087
65acb1b1
TC
1088There's also a File::Tail module from CPAN.
1089
68dc0745
PP
1090=head2 How do I dup() a filehandle in Perl?
1091
1092If you check L<perlfunc/open>, you'll see that several of the ways
1093to call open() should do the trick. For example:
1094
1095 open(LOG, ">>/tmp/logfile");
1096 open(STDERR, ">&LOG");
1097
1098Or even with a literal numeric descriptor:
1099
1100 $fd = $ENV{MHCONTEXTFD};
1101 open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
1102
c47ff5f1 1103Note that "<&STDIN" makes a copy, but "<&=STDIN" make
5a964f20
TC
1104an alias. That means if you close an aliased handle, all
1105aliases become inaccessible. This is not true with
1106a copied one.
1107
1108Error checking, as always, has been left as an exercise for the reader.
68dc0745
PP
1109
1110=head2 How do I close a file descriptor by number?
1111
1112This should rarely be necessary, as the Perl close() function is to be
1113used for things that Perl opened itself, even if it was a dup of a
1114numeric descriptor, as with MHCONTEXT above. But if you really have
1115to, you may be able to do this:
1116
1117 require 'sys/syscall.ph';
1118 $rc = syscall(&SYS_close, $fd + 0); # must force numeric
1119 die "can't sysclose $fd: $!" unless $rc == -1;
1120
d92eb7b0
GS
1121Or just use the fdopen(3S) feature of open():
1122
1123 {
1124 local *F;
1125 open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
1126 close F;
1127 }
1128
46fc3d4c 1129=head2 Why can't I use "C:\temp\foo" in DOS paths? What doesn't `C:\temp\foo.exe` work?
68dc0745
PP
1130
1131Whoops! You just put a tab and a formfeed into that filename!
1132Remember that within double quoted strings ("like\this"), the
1133backslash is an escape character. The full list of these is in
1134L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
1135have a file called "c:(tab)emp(formfeed)oo" or
65acb1b1 1136"c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
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1137
1138Either single-quote your strings, or (preferably) use forward slashes.
46fc3d4c 1139Since all DOS and Windows versions since something like MS-DOS 2.0 or so
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1140have treated C</> and C<\> the same in a path, you might as well use the
1141one that doesn't clash with Perl -- or the POSIX shell, ANSI C and C++,
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1142awk, Tcl, Java, or Python, just to mention a few. POSIX paths
1143are more portable, too.
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1144
1145=head2 Why doesn't glob("*.*") get all the files?
1146
1147Because even on non-Unix ports, Perl's glob function follows standard
46fc3d4c 1148Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
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1149files. This makes glob() portable even to legacy systems. Your
1150port may include proprietary globbing functions as well. Check its
1151documentation for details.
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1152
1153=head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
1154
1155This is elaborately and painstakingly described in the "Far More Than
7b8d334a 1156You Ever Wanted To Know" in
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1157http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .
1158
1159The executive summary: learn how your filesystem works. The
1160permissions on a file say what can happen to the data in that file.
1161The permissions on a directory say what can happen to the list of
1162files in that directory. If you delete a file, you're removing its
1163name from the directory (so the operation depends on the permissions
1164of the directory, not of the file). If you try to write to the file,
1165the permissions of the file govern whether you're allowed to.
1166
1167=head2 How do I select a random line from a file?
1168
1169Here's an algorithm from the Camel Book:
1170
1171 srand;
1172 rand($.) < 1 && ($line = $_) while <>;
1173
1174This has a significant advantage in space over reading the whole
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1175file in. A simple proof by induction is available upon
1176request if you doubt its correctness.
68dc0745 1177
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1178=head2 Why do I get weird spaces when I print an array of lines?
1179
1180Saying
1181
1182 print "@lines\n";
1183
1184joins together the elements of C<@lines> with a space between them.
1185If C<@lines> were C<("little", "fluffy", "clouds")> then the above
1186statement would print:
1187
1188 little fluffy clouds
1189
1190but if each element of C<@lines> was a line of text, ending a newline
1191character C<("little\n", "fluffy\n", "clouds\n")> then it would print:
1192
1193 little
1194 fluffy
1195 clouds
1196
1197If your array contains lines, just print them:
1198
1199 print @lines;
1200
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1201=head1 AUTHOR AND COPYRIGHT
1202
65acb1b1 1203Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
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1204All rights reserved.
1205
c8db1d39 1206When included as an integrated part of the Standard Distribution
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1207of Perl or of its documentation (printed or otherwise), this works is
1208covered under Perl's Artistic License. For separate distributions of
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1209all or part of this FAQ outside of that, see L<perlfaq>.
1210
87275199 1211Irrespective of its distribution, all code examples here are in the public
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1212domain. You are permitted and encouraged to use this code and any
1213derivatives thereof in your own programs for fun or for profit as you
1214see fit. A simple comment in the code giving credit to the FAQ would
1215be courteous but is not required.