3 perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date: 1999/05/23 16:08:30 $)
7 This section deals with I/O and the "f" issues: filehandles, flushing,
10 =head2 How do I flush/unbuffer an output filehandle? Why must I do this?
12 The C standard I/O library (stdio) normally buffers characters sent to
13 devices. This is done for efficiency reasons, so that there isn't a
14 system call for each byte. Any time you use print() or write() in
15 Perl, you go though this buffering. syswrite() circumvents stdio and
18 In most stdio implementations, the type of output buffering and the size of
19 the buffer varies according to the type of device. Disk files are block
20 buffered, often with a buffer size of more than 2k. Pipes and sockets
21 are 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
23 the entire line when it gets the newline.
25 Perl does not support truly unbuffered output (except insofar as you can
26 C<syswrite(OUT, $char, 1)>). What it does instead support is "command
27 buffering", in which a physical write is performed after every output
28 command. This isn't as hard on your system as unbuffering, but does
29 get the output where you want it when you want it.
31 If you expect characters to get to your device when you print them there,
32 you'll want to autoflush its handle.
33 Use select() and the C<$|> variable to control autoflushing
34 (see L<perlvar/$|> and L<perlfunc/select>):
36 $old_fh = select(OUTPUT_HANDLE);
40 Or using the traditional idiom:
42 select((select(OUTPUT_HANDLE), $| = 1)[0]);
44 Or if don't mind slowly loading several thousand lines of module code
45 just because you're afraid of the C<$|> variable:
48 open(DEV, "+</dev/tty"); # ceci n'est pas une pipe
51 or the newer IO::* modules:
54 open(DEV, ">/dev/printer"); # but is this?
59 use IO::Socket; # this one is kinda a pipe?
60 $sock = IO::Socket::INET->new(PeerAddr => 'www.perl.com',
61 PeerPort => 'http(80)',
63 die "$!" unless $sock;
66 print $sock "GET / HTTP/1.0" . "\015\012" x 2;
67 $document = join('', <$sock>);
68 print "DOC IS: $document\n";
70 Note the bizarrely hardcoded carriage return and newline in their octal
71 equivalents. This is the ONLY way (currently) to assure a proper flush
72 on all platforms, including Macintosh. That's the way things work in
73 network programming: you really should specify the exact bit pattern
74 on the network line terminator. In practice, C<"\n\n"> often works,
75 but this is not portable.
77 See L<perlfaq9> for other examples of fetching URLs over the web.
79 =head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
81 Those are operations of a text editor. Perl is not a text editor.
82 Perl is a programming language. You have to decompose the problem into
83 low-level calls to read, write, open, close, and seek.
85 Although humans have an easy time thinking of a text file as being a
86 sequence of lines that operates much like a stack of playing cards -- or
87 punch cards -- computers usually see the text file as a sequence of bytes.
88 In general, there's no direct way for Perl to seek to a particular line
89 of a file, insert text into a file, or remove text from a file.
91 (There are exceptions in special circumstances. You can add or remove at
92 the very end of the file. Another is replacing a sequence of bytes with
93 another sequence of the same length. Another is using the C<$DB_RECNO>
94 array bindings as documented in L<DB_File>. Yet another is manipulating
95 files with all lines the same length.)
97 The general solution is to create a temporary copy of the text file with
98 the changes you want, then copy that over the original. This assumes
102 $new = "$file.tmp.$$";
105 open(OLD, "< $old") or die "can't open $old: $!";
106 open(NEW, "> $new") or die "can't open $new: $!";
108 # Correct typos, preserving case
110 s/\b(p)earl\b/${1}erl/i;
111 (print NEW $_) or die "can't write to $new: $!";
114 close(OLD) or die "can't close $old: $!";
115 close(NEW) or die "can't close $new: $!";
117 rename($old, $bak) or die "can't rename $old to $bak: $!";
118 rename($new, $old) or die "can't rename $new to $old: $!";
120 Perl can do this sort of thing for you automatically with the C<-i>
121 command-line switch or the closely-related C<$^I> variable (see
122 L<perlrun> for more details). Note that
123 C<-i> may require a suffix on some non-Unix systems; see the
124 platform-specific documentation that came with your port.
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
130 local($^I, @ARGV) = ('.orig', glob("*.c"));
133 print "This line should appear at the top of each file\n";
135 s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
137 close ARGV if eof; # Reset $.
140 If you need to seek to an arbitrary line of a file that changes
141 infrequently, you could build up an index of byte positions of where
142 the line ends are in the file. If the file is large, an index of
143 every tenth or hundredth line end would allow you to seek and read
144 fairly efficiently. If the file is sorted, try the look.pl library
145 (part of the standard perl distribution).
147 In the unique case of deleting lines at the end of a file, you
148 can use tell() and truncate(). The following code snippet deletes
149 the last line of a file without making a copy or reading the
150 whole file into memory:
152 open (FH, "+< $file");
153 while ( <FH> ) { $addr = tell(FH) unless eof(FH) }
156 Error checking is left as an exercise for the reader.
158 =head2 How do I count the number of lines in a file?
160 One fairly efficient way is to count newlines in the file. The
161 following program uses a feature of tr///, as documented in L<perlop>.
162 If your text file doesn't end with a newline, then it's not really a
163 proper text file, so this may report one fewer line than you expect.
166 open(FILE, $filename) or die "Can't open `$filename': $!";
167 while (sysread FILE, $buffer, 4096) {
168 $lines += ($buffer =~ tr/\n//);
172 This assumes no funny games with newline translations.
174 =head2 How do I make a temporary file name?
176 Use the C<new_tmpfile> class method from the IO::File module to get a
177 filehandle opened for reading and writing. Use this if you don't
178 need to know the file's name.
181 $fh = IO::File->new_tmpfile()
182 or die "Unable to make new temporary file: $!";
184 Or you can use the C<tmpnam> function from the POSIX module to get a
185 filename that you then open yourself. Use this if you do need to know
189 use POSIX qw(tmpnam);
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);
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 : $!" }
200 # now go on to use the file ...
202 If you're committed to doing this by hand, use the process ID and/or
203 the current time-value. If you need to have many temporary files in
204 one process, use a counter:
208 my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} || $ENV{TEMP};
209 my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
213 until (defined(fileno(FH)) || $count++ > 100) {
214 $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
215 sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
217 if (defined(fileno(FH))
218 return (*FH, $base_name);
225 =head2 How can I manipulate fixed-record-length files?
227 The most efficient way is using pack() and unpack(). This is faster than
228 using substr() when taking many, many strings. It is slower for just a few.
230 Here is a sample chunk of code to break up and put back together again
231 some fixed-format input lines, in this case from the output of a normal,
235 # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
236 $PS_T = 'A6 A4 A7 A5 A*';
240 ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
241 for $var (qw!pid tt stat time command!) {
242 print "$var: <$$var>\n";
244 print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
248 We've used C<$$var> in a way that forbidden by C<use strict 'refs'>.
249 That is, we've promoted a string to a scalar variable reference using
250 symbolic references. This is ok in small programs, but doesn't scale
251 well. It also only works on global variables, not lexicals.
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?
255 The fastest, simplest, and most direct way is to localize the typeglob
256 of the filehandle in question:
260 Typeglobs are fast (especially compared with the alternatives) and
261 reasonably easy to use, but they also have one subtle drawback. If you
262 had, for example, a function named TmpHandle(), or a variable named
263 %TmpHandle, you just hid it from yourself.
267 open(HostFile, "</etc/hosts") or die "no /etc/hosts: $!";
268 local $_; # <- VERY IMPORTANT
270 print if /\b127\.(0\.0\.)?1\b/;
272 # *HostFile automatically closes/disappears here
275 Here's how to use this in a loop to open and store a bunch of
276 filehandles. We'll use as values of the hash an ordered
277 pair to make it easy to sort the hash in insertion order.
279 @names = qw(motd termcap passwd hosts);
281 foreach $filename (@names) {
283 open(FH, "/etc/$filename") || die "$filename: $!";
284 $file{$filename} = [ $i++, *FH ];
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];
291 print "$name $. $line";
294 For passing filehandles to functions, the easiest way is to
295 preface them with a star, as in func(*STDIN). See L<perlfaq7/"Passing
296 Filehandles"> for details.
298 If you want to create many anonymous handles, you should check out the
299 Symbol, FileHandle, or IO::Handle (etc.) modules. Here's the equivalent
300 code with Symbol::gensym, which is reasonably light-weight:
302 foreach $filename (@names) {
305 open($fh, "/etc/$filename") || die "open /etc/$filename: $!";
306 $file{$filename} = [ $i++, $fh ];
309 Or here using the semi-object-oriented FileHandle module, which certainly
314 foreach $filename (@names) {
315 my $fh = FileHandle->new("/etc/$filename") or die "$filename: $!";
316 $file{$filename} = [ $i++, $fh ];
319 Please understand that whether the filehandle happens to be a (probably
320 localized) typeglob or an anonymous handle from one of the modules,
321 in no way affects the bizarre rules for managing indirect handles.
322 See the next question.
324 =head2 How can I use a filehandle indirectly?
326 An indirect filehandle is using something other than a symbol
327 in a place that a filehandle is expected. Here are ways
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
336 Or to use the C<new> method from the FileHandle or IO modules to
337 create an anonymous filehandle, store that in a scalar variable,
338 and use it as though it were a normal filehandle.
341 $fh = FileHandle->new();
343 use IO::Handle; # 5.004 or higher
344 $fh = IO::Handle->new();
346 Then use any of those as you would a normal filehandle. Anywhere that
347 Perl is expecting a filehandle, an indirect filehandle may be used
348 instead. An indirect filehandle is just a scalar variable that contains
349 a filehandle. Functions like C<print>, C<open>, C<seek>, or
350 the C<< <FH> >> diamond operator will accept either a read filehandle
351 or a scalar variable containing one:
353 ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
354 print $ofh "Type it: ";
356 print $efh "What was that: $got";
358 If you're passing a filehandle to a function, you can write
359 the function in two ways:
363 print $fh "Sending to indirect filehandle\n";
366 Or it can localize a typeglob and use the filehandle directly:
370 print FH "Sending to localized filehandle\n";
373 Both styles work with either objects or typeglobs of real filehandles.
374 (They might also work with strings under some circumstances, but this
380 In the examples above, we assigned the filehandle to a scalar variable
381 before using it. That is because only simple scalar variables,
382 not expressions or subscripts into hashes or arrays, can be used with
383 built-ins like C<print>, C<printf>, or the diamond operator. These are
384 illegal and won't even compile:
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
391 With C<print> and C<printf>, you get around this by using a block and
392 an expression where you would place the filehandle:
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.
398 That block is a proper block like any other, so you can put more
399 complicated code there. This sends the message out to one of two places:
402 print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
403 print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
405 This approach of treating C<print> and C<printf> like object methods
406 calls doesn't work for the diamond operator. That's because it's a
407 real operator, not just a function with a comma-less argument. Assuming
408 you've been storing typeglobs in your structure as we did above, you
409 can use the built-in function named C<readline> to reads a record just
410 as C<< <> >> does. Given the initialization shown above for @fd, this
411 would work, but only because readline() require a typeglob. It doesn't
412 work with objects or strings, which might be a bug we haven't fixed yet.
414 $got = readline($fd[0]);
416 Let it be noted that the flakiness of indirect filehandles is not
417 related to whether they're strings, typeglobs, objects, or anything else.
418 It's the syntax of the fundamental operators. Playing the object
419 game doesn't help you at all here.
421 =head2 How can I set up a footer format to be used with write()?
423 There's no builtin way to do this, but L<perlform> has a couple of
424 techniques to make it possible for the intrepid hacker.
426 =head2 How can I write() into a string?
428 See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
430 =head2 How can I output my numbers with commas added?
432 This one will do it for you:
436 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
440 $n = 23659019423.2331;
441 print "GOT: ", commify($n), "\n";
443 GOT: 23,659,019,423.2331
447 s/^([-+]?\d+)(\d{3})/$1,$2/g;
449 because you have to put the comma in and then recalculate your
452 Alternatively, this commifies all numbers in a line regardless of
453 whether they have decimal portions, are preceded by + or -, or
456 # from Andrew Johnson <ajohnson@gpu.srv.ualberta.ca>
459 $input = reverse $input;
460 $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g;
461 return scalar reverse $input;
464 =head2 How can I translate tildes (~) in a filename?
466 Use the <> (glob()) operator, documented in L<perlfunc>. This
467 requires that you have a shell installed that groks tildes, meaning
468 csh or tcsh or (some versions of) ksh, and thus may have portability
469 problems. The Glob::KGlob module (available from CPAN) gives more
470 portable glob functionality.
472 Within Perl, you may use this directly:
475 ^ ~ # find a leading tilde
477 [^/] # a non-slash character
478 * # repeated 0 or more times (0 means me)
483 : ( $ENV{HOME} || $ENV{LOGDIR} )
486 =head2 How come when I open a file read-write it wipes it out?
488 Because you're using something like this, which truncates the file and
489 I<then> gives you read-write access:
491 open(FH, "+> /path/name"); # WRONG (almost always)
493 Whoops. You should instead use this, which will fail if the file
496 open(FH, "+< /path/name"); # open for update
498 Using ">" always clobbers or creates. Using "<" never does
499 either. The "+" doesn't change this.
501 Here are examples of many kinds of file opens. Those using sysopen()
506 To open file for reading:
508 open(FH, "< $path") || die $!;
509 sysopen(FH, $path, O_RDONLY) || die $!;
511 To open file for writing, create new file if needed or else truncate old file:
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 $!;
517 To open file for writing, create new file, file must not exist:
519 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
520 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
522 To open file for appending, create if necessary:
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 $!;
528 To open file for appending, file must exist:
530 sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
532 To open file for update, file must exist:
534 open(FH, "+< $path") || die $!;
535 sysopen(FH, $path, O_RDWR) || die $!;
537 To open file for update, create file if necessary:
539 sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
540 sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
542 To open file for update, file must not exist:
544 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
545 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
547 To open a file without blocking, creating if necessary:
549 sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT)
550 or die "can't open /tmp/somefile: $!":
552 Be warned that neither creation nor deletion of files is guaranteed to
553 be an atomic operation over NFS. That is, two processes might both
554 successful create or unlink the same file! Therefore O_EXCL
555 isn't so exclusive as you might wish.
557 See also the new L<perlopentut> if you have it (new for 5.6).
559 =head2 Why do I sometimes get an "Argument list too long" when I use <*>?
561 The C<< <> >> operator performs a globbing operation (see above).
562 In Perl versions earlier than v5.6.0, the internal glob() operator forks
563 csh(1) to do the actual glob expansion, but
564 csh can't handle more than 127 items and so gives the error message
565 C<Argument list too long>. People who installed tcsh as csh won't
566 have this problem, but their users may be surprised by it.
568 To get around this, either upgrade to Perl v5.6.0 or later, do the glob
569 yourself with readdir() and patterns, or use a module like Glob::KGlob,
570 one that doesn't use the shell to do globbing.
572 =head2 Is there a leak/bug in glob()?
574 Due to the current implementation on some operating systems, when you
575 use the glob() function or its angle-bracket alias in a scalar
576 context, you may cause a leak and/or unpredictable behavior. It's
577 best therefore to use glob() only in list context.
579 =head2 How can I open a file with a leading ">" or trailing blanks?
581 Normally perl ignores trailing blanks in filenames, and interprets
582 certain leading characters (or a trailing "|") to mean something
583 special. To avoid this, you might want to use a routine like this.
584 It makes incomplete pathnames into explicit relative ones, and tacks a
585 trailing null byte on the name to make perl leave it alone:
594 $badpath = "<<<something really wicked ";
595 $fn = safe_filename($badpath");
596 open(FH, "> $fn") or "couldn't open $badpath: $!";
598 This assumes that you are using POSIX (portable operating systems
599 interface) paths. If you are on a closed, non-portable, proprietary
600 system, you may have to adjust the C<"./"> above.
602 It would be a lot clearer to use sysopen(), though:
605 $badpath = "<<<something really wicked ";
606 open (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
607 or die "can't open $badpath: $!";
609 For more information, see also the new L<perlopentut> if you have it
612 =head2 How can I reliably rename a file?
614 Well, usually you just use Perl's rename() function. But that may not
615 work everywhere, in particular, renaming files across file systems.
616 Some sub-Unix systems have broken ports that corrupt the semantics of
617 rename() -- for example, WinNT does this right, but Win95 and Win98
618 are broken. (The last two parts are not surprising, but the first is. :-)
620 If your operating system supports a proper mv(1) program or its moral
621 equivalent, this works:
623 rename($old, $new) or system("mv", $old, $new);
625 It may be more compelling to use the File::Copy module instead. You
626 just copy to the new file to the new name (checking return values),
627 then delete the old one. This isn't really the same semantics as a
628 real rename(), though, which preserves metainformation like
629 permissions, timestamps, inode info, etc.
631 The newer version of File::Copy exports a move() function.
633 =head2 How can I lock a file?
635 Perl's builtin flock() function (see L<perlfunc> for details) will call
636 flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
637 later), and lockf(3) if neither of the two previous system calls exists.
638 On some systems, it may even use a different form of native locking.
639 Here are some gotchas with Perl's flock():
645 Produces a fatal error if none of the three system calls (or their
646 close equivalent) exists.
650 lockf(3) does not provide shared locking, and requires that the
651 filehandle be open for writing (or appending, or read/writing).
655 Some versions of flock() can't lock files over a network (e.g. on NFS file
656 systems), so you'd need to force the use of fcntl(2) when you build Perl.
657 But even this is dubious at best. See the flock entry of L<perlfunc>,
658 and the F<INSTALL> file in the source distribution for information on
659 building Perl to do this.
661 Two potentially non-obvious but traditional flock semantics are that
662 it waits indefinitely until the lock is granted, and that its locks
663 I<merely advisory>. Such discretionary locks are more flexible, but
664 offer fewer guarantees. This means that files locked with flock() may
665 be modified by programs that do not also use flock(). Cars that stop
666 for red lights get on well with each other, but not with cars that don't
667 stop for red lights. See the perlport manpage, your port's specific
668 documentation, or your system-specific local manpages for details. It's
669 best 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
671 for your own system's idiosyncrasies (sometimes called "features").
672 Slavish adherence to portability concerns shouldn't get in the way of
673 your getting your job done.)
675 For more information on file locking, see also L<perlopentut/"File
676 Locking"> if you have it (new for 5.6).
680 =head2 Why can't I just open(FH, ">file.lock")?
682 A common bit of code B<NOT TO USE> is this:
684 sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
685 open(LCK, "> file.lock"); # THIS BROKEN CODE
687 This is a classic race condition: you take two steps to do something
688 which must be done in one. That's why computer hardware provides an
689 atomic test-and-set instruction. In theory, this "ought" to work:
691 sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
692 or die "can't open file.lock: $!":
694 except that lamentably, file creation (and deletion) is not atomic
695 over NFS, so this won't work (at least, not every time) over the net.
696 Various schemes involving link() have been suggested, but
697 these tend to involve busy-wait, which is also subdesirable.
699 =head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
701 Didn't anyone ever tell you web-page hit counters were useless?
702 They don't count number of hits, they're a waste of time, and they serve
703 only to stroke the writer's vanity. Better to pick a random number.
706 Anyway, this is what you can do if you can't help yourself.
709 sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
710 flock(FH, LOCK_EX) or die "can't flock numfile: $!";
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: $!";
715 # Perl as of 5.004 automatically flushes before unlocking
716 flock(FH, LOCK_UN) or die "can't flock numfile: $!";
717 close FH or die "can't close numfile: $!";
719 Here's a much better web-page hit counter:
721 $hits = int( (time() - 850_000_000) / rand(1_000) );
723 If the count doesn't impress your friends, then the code might. :-)
725 =head2 How do I randomly update a binary file?
727 If you're just trying to patch a binary, in many cases something as
728 simple as this works:
730 perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
732 However, if you have fixed sized records, then you might do something more
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: $!";
741 seek(FH, -$RECSIZE, 1);
745 Locking and error checking are left as an exercise for the reader.
746 Don't forget them, or you'll be quite sorry.
748 =head2 How do I get a file's timestamp in perl?
750 If you want to retrieve the time at which the file was last read,
751 written, or had its meta-data (owner, etc) changed, you use the B<-M>,
752 B<-A>, or B<-C> filetest operations as documented in L<perlfunc>. These
753 retrieve the age of the file (measured against the start-time of your
754 program) in days as a floating point number. To retrieve the "raw"
755 time in seconds since the epoch, you would call the stat function,
756 then use localtime(), gmtime(), or POSIX::strftime() to convert this
757 into human-readable form.
761 $write_secs = (stat($file))[9];
762 printf "file %s updated at %s\n", $file,
763 scalar localtime($write_secs);
765 If you prefer something more legible, use the File::stat module
766 (part of the standard distribution in version 5.004 and later):
768 # error checking left as an exercise for reader.
771 $date_string = ctime(stat($file)->mtime);
772 print "file $file updated at $date_string\n";
774 The POSIX::strftime() approach has the benefit of being,
775 in theory, independent of the current locale. See L<perllocale>
778 =head2 How do I set a file's timestamp in perl?
780 You use the utime() function documented in L<perlfunc/utime>.
781 By way of example, here's a little program that copies the
782 read and write times from its first argument to all the rest
786 die "usage: cptimes timestamp_file other_files ...\n";
789 ($atime, $mtime) = (stat($timestamp))[8,9];
790 utime $atime, $mtime, @ARGV;
792 Error checking is, as usual, left as an exercise for the reader.
794 Note that utime() currently doesn't work correctly with Win95/NT
795 ports. A bug has been reported. Check it carefully before using
796 it on those platforms.
798 =head2 How do I print to more than one file at once?
800 If you only have to do this once, you can do this:
802 for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
804 To connect up to one filehandle to several output filehandles, it's
805 easiest to use the tee(1) program if you have it, and let it take care
808 open (FH, "| tee file1 file2 file3");
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";
817 Otherwise you'll have to write your own multiplexing print
818 function -- or your own tee program -- or use Tom Christiansen's,
819 at http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz, which is
820 written in Perl and offers much greater functionality
821 than the stock version.
823 =head2 How can I read in an entire file all at once?
825 The customary Perl approach for processing all the lines in a file is to
826 do so one line at a time:
828 open (INPUT, $file) || die "can't open $file: $!";
831 # do something with $_
833 close(INPUT) || die "can't close $file: $!";
835 This is tremendously more efficient than reading the entire file into
836 memory as an array of lines and then processing it one element at a time,
837 which is often -- if not almost always -- the wrong approach. Whenever
838 you see someone do this:
842 You should think long and hard about why you need everything loaded
843 at once. It's just not a scalable solution. You might also find it
844 more fun to use the standard DB_File module's $DB_RECNO bindings,
845 which allow you to tie an array to a file so that accessing an element
846 the array actually accesses the corresponding line in the file.
848 On very rare occasion, you may have an algorithm that demands that
849 the entire file be in memory at once as one scalar. The simplest solution
854 Being in scalar context, you get the whole thing. In list context,
855 you'd get a list of all the lines:
857 @lines = `cat $file`;
859 This tiny but expedient solution is neat, clean, and portable to
860 all systems on which decent tools have been installed. For those
861 who prefer not to use the toolbox, you can of course read the file
862 manually, although this makes for more complicated code.
866 open (INPUT, $file) || die "can't open $file: $!";
870 That temporarily undefs your record separator, and will automatically
871 close the file at block exit. If the file is already open, just use this:
873 $var = do { local $/; <INPUT> };
875 =head2 How can I read in a file by paragraphs?
877 Use the C<$/> variable (see L<perlvar> for details). You can either
878 set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
879 for instance, gets treated as two paragraphs and not three), or
880 C<"\n\n"> to accept empty paragraphs.
882 Note 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.
885 =head2 How can I read a single character from a file? From the keyboard?
887 You can use the builtin C<getc()> function for most filehandles, but
888 it won't (easily) work on a terminal device. For STDIN, either use
889 the Term::ReadKey module from CPAN, or use the sample code in
892 If your system supports the portable operating system programming
893 interface (POSIX), you can use the following code, which you'll note
894 turns off echo processing as well.
908 use POSIX qw(:termios_h);
910 my ($term, $oterm, $echo, $noecho, $fd_stdin);
912 $fd_stdin = fileno(STDIN);
914 $term = POSIX::Termios->new();
915 $term->getattr($fd_stdin);
916 $oterm = $term->getlflag();
918 $echo = ECHO | ECHOK | ICANON;
919 $noecho = $oterm & ~$echo;
922 $term->setlflag($noecho);
923 $term->setcc(VTIME, 1);
924 $term->setattr($fd_stdin, TCSANOW);
928 $term->setlflag($oterm);
929 $term->setcc(VTIME, 0);
930 $term->setattr($fd_stdin, TCSANOW);
936 sysread(STDIN, $key, 1);
945 The Term::ReadKey module from CPAN may be easier to use. Recent version
946 include also support for non-portable systems as well.
949 open(TTY, "</dev/tty");
950 print "Gimme a char: ";
952 $key = ReadKey 0, *TTY;
954 printf "\nYou said %s, char number %03d\n",
957 For legacy DOS systems, Dan Carson <dbc@tc.fluke.COM> reports the following:
959 To put the PC in "raw" mode, use ioctl with some magic numbers gleaned
960 from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes
961 across the net every so often):
963 $old_ioctl = ioctl(STDIN,0,0); # Gets device info
965 ioctl(STDIN,1,$old_ioctl | 32); # Writes it back, setting bit 5
967 Then to read a single character:
969 sysread(STDIN,$c,1); # Read a single character
971 And to put the PC back to "cooked" mode:
973 ioctl(STDIN,1,$old_ioctl); # Sets it back to cooked mode.
975 So now you have $c. If C<ord($c) == 0>, you have a two byte code, which
976 means you hit a special key. Read another byte with C<sysread(STDIN,$c,1)>,
977 and that value tells you what combination it was according to this
980 # PC 2-byte keycodes = ^@ + the following:
985 # 10-19 ALT QWERTYUIOP
986 # 1E-26 ALT ASDFGHJKL
992 # 4F-53 END,DOWN,PgDn,Ins,Del
996 # 73-77 CTR LEFT,RIGHT,END,PgDn,HOME
997 # 78-83 ALT 1234567890-=
1000 This is all trial and error I did a long time ago, I hope I'm reading the
1003 =head2 How can I tell whether there's a character waiting on a filehandle?
1005 The very first thing you should do is look into getting the Term::ReadKey
1006 extension from CPAN. As we mentioned earlier, it now even has limited
1007 support for non-portable (read: not open systems, closed, proprietary,
1008 not POSIX, not Unix, etc) systems.
1010 You should also check out the Frequently Asked Questions list in
1011 comp.unix.* for things like this: the answer is essentially the same.
1012 It's very system dependent. Here's one solution that works on BSD
1017 vec($rin, fileno(STDIN), 1) = 1;
1018 return $nfd = select($rin,undef,undef,0);
1021 If you want to find out how many characters are waiting, there's
1022 also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
1023 comes with Perl tries to convert C include files to Perl code, which
1024 can be C<require>d. FIONREAD ends up defined as a function in the
1025 I<sys/ioctl.ph> file:
1027 require 'sys/ioctl.ph';
1029 $size = pack("L", 0);
1030 ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
1031 $size = unpack("L", $size);
1033 If I<h2ph> wasn't installed or doesn't work for you, you can
1034 I<grep> the include files by hand:
1036 % grep FIONREAD /usr/include/*/*
1037 /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
1039 Or write a small C program using the editor of champions:
1042 #include <sys/ioctl.h>
1044 printf("%#08x\n", FIONREAD);
1047 % cc -o fionread fionread.c
1051 And then hard-code it, leaving porting as an exercise to your successor.
1053 $FIONREAD = 0x4004667f; # XXX: opsys dependent
1055 $size = pack("L", 0);
1056 ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
1057 $size = unpack("L", $size);
1059 FIONREAD requires a filehandle connected to a stream, meaning sockets,
1060 pipes, and tty devices work, but I<not> files.
1062 =head2 How do I do a C<tail -f> in perl?
1068 The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
1069 but it does clear the end-of-file condition on the handle, so that the
1070 next <GWFILE> makes Perl try again to read something.
1072 If that doesn't work (it relies on features of your stdio implementation),
1073 then you need something more like this:
1076 for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
1077 # search for some stuff and put it into files
1080 seek(GWFILE, $curpos, 0); # seek to where we had been
1083 If this still doesn't work, look into the POSIX module. POSIX defines
1084 the clearerr() method, which can remove the end of file condition on a
1085 filehandle. The method: read until end of file, clearerr(), read some
1086 more. Lather, rinse, repeat.
1088 There's also a File::Tail module from CPAN.
1090 =head2 How do I dup() a filehandle in Perl?
1092 If you check L<perlfunc/open>, you'll see that several of the ways
1093 to call open() should do the trick. For example:
1095 open(LOG, ">>/tmp/logfile");
1096 open(STDERR, ">&LOG");
1098 Or even with a literal numeric descriptor:
1100 $fd = $ENV{MHCONTEXTFD};
1101 open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
1103 Note that "<&STDIN" makes a copy, but "<&=STDIN" make
1104 an alias. That means if you close an aliased handle, all
1105 aliases become inaccessible. This is not true with
1108 Error checking, as always, has been left as an exercise for the reader.
1110 =head2 How do I close a file descriptor by number?
1112 This should rarely be necessary, as the Perl close() function is to be
1113 used for things that Perl opened itself, even if it was a dup of a
1114 numeric descriptor, as with MHCONTEXT above. But if you really have
1115 to, you may be able to do this:
1117 require 'sys/syscall.ph';
1118 $rc = syscall(&SYS_close, $fd + 0); # must force numeric
1119 die "can't sysclose $fd: $!" unless $rc == -1;
1121 Or just use the fdopen(3S) feature of open():
1125 open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
1129 =head2 Why can't I use "C:\temp\foo" in DOS paths? What doesn't `C:\temp\foo.exe` work?
1131 Whoops! You just put a tab and a formfeed into that filename!
1132 Remember that within double quoted strings ("like\this"), the
1133 backslash is an escape character. The full list of these is in
1134 L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
1135 have a file called "c:(tab)emp(formfeed)oo" or
1136 "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
1138 Either single-quote your strings, or (preferably) use forward slashes.
1139 Since all DOS and Windows versions since something like MS-DOS 2.0 or so
1140 have treated C</> and C<\> the same in a path, you might as well use the
1141 one that doesn't clash with Perl -- or the POSIX shell, ANSI C and C++,
1142 awk, Tcl, Java, or Python, just to mention a few. POSIX paths
1143 are more portable, too.
1145 =head2 Why doesn't glob("*.*") get all the files?
1147 Because even on non-Unix ports, Perl's glob function follows standard
1148 Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
1149 files. This makes glob() portable even to legacy systems. Your
1150 port may include proprietary globbing functions as well. Check its
1151 documentation for details.
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?
1155 This is elaborately and painstakingly described in the "Far More Than
1156 You Ever Wanted To Know" in
1157 http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .
1159 The executive summary: learn how your filesystem works. The
1160 permissions on a file say what can happen to the data in that file.
1161 The permissions on a directory say what can happen to the list of
1162 files in that directory. If you delete a file, you're removing its
1163 name from the directory (so the operation depends on the permissions
1164 of the directory, not of the file). If you try to write to the file,
1165 the permissions of the file govern whether you're allowed to.
1167 =head2 How do I select a random line from a file?
1169 Here's an algorithm from the Camel Book:
1172 rand($.) < 1 && ($line = $_) while <>;
1174 This has a significant advantage in space over reading the whole
1175 file in. A simple proof by induction is available upon
1176 request if you doubt its correctness.
1178 =head2 Why do I get weird spaces when I print an array of lines?
1184 joins together the elements of C<@lines> with a space between them.
1185 If C<@lines> were C<("little", "fluffy", "clouds")> then the above
1186 statement would print:
1188 little fluffy clouds
1190 but if each element of C<@lines> was a line of text, ending a newline
1191 character C<("little\n", "fluffy\n", "clouds\n")> then it would print:
1197 If your array contains lines, just print them:
1201 =head1 AUTHOR AND COPYRIGHT
1203 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1204 All rights reserved.
1206 When included as an integrated part of the Standard Distribution
1207 of Perl or of its documentation (printed or otherwise), this works is
1208 covered under Perl's Artistic License. For separate distributions of
1209 all or part of this FAQ outside of that, see L<perlfaq>.
1211 Irrespective of its distribution, all code examples here are in the public
1212 domain. You are permitted and encouraged to use this code and any
1213 derivatives thereof in your own programs for fun or for profit as you
1214 see fit. A simple comment in the code giving credit to the FAQ would
1215 be courteous but is not required.