perlfaq5.pod

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days as a floating point number. Some platforms may not have
all of these times.  See L<perlport> for details. To
retrieve the "raw" time in seconds since the epoch, you
would call the stat function, then use localtime(),
gmtime(), or POSIX::strftime() to convert this into
human-readable form.

Here's an example:

	$write_secs = (stat($file))[9];
	printf "file %s updated at %s\n", $file,
	scalar localtime($write_secs);

If you prefer something more legible, use the File::stat module
(part of the standard distribution in version 5.004 and later):

	# error checking left as an exercise for reader.
	use File::stat;
	use Time::localtime;
	$date_string = ctime(stat($file)->mtime);
	print "file $file updated at $date_string\n";

The POSIX::strftime() approach has the benefit of being,
in theory, independent of the current locale.  See L<perllocale>
for details.

=head2 How do I set a file's timestamp in perl?
X<timestamp> X<file, timestamp>

You use the utime() function documented in L<perlfunc/utime>.
By way of example, here's a little program that copies the
read and write times from its first argument to all the rest
of them.

	if (@ARGV < 2) {
		die "usage: cptimes timestamp_file other_files ...\n";
		}
	$timestamp = shift;
	($atime, $mtime) = (stat($timestamp))[8,9];
	utime $atime, $mtime, @ARGV;

Error checking is, as usual, left as an exercise for the reader.

The perldoc for utime also has an example that has the same
effect as touch(1) on files that I<already exist>.

Certain file systems have a limited ability to store the times
on a file at the expected level of precision.  For example, the
FAT and HPFS filesystem are unable to create dates on files with
a finer granularity than two seconds.  This is a limitation of
the filesystems, not of utime().

=head2 How do I print to more than one file at once?
X<print, to multiple files>

To connect one filehandle to several output filehandles,
you can use the IO::Tee or Tie::FileHandle::Multiplex modules.

If you only have to do this once, you can print individually
to each filehandle.

	for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }

=head2 How can I read in an entire file all at once?
X<slurp> X<file, slurping>

You can use the File::Slurp module to do it in one step.

	use File::Slurp;

	$all_of_it = read_file($filename); # entire file in scalar
	@all_lines = read_file($filename); # one line perl element

The customary Perl approach for processing all the lines in a file is to
do so one line at a time:

	open (INPUT, $file) 	|| die "can't open $file: $!";
	while (<INPUT>) {
		chomp;
		# do something with $_
		}
	close(INPUT)	    	|| die "can't close $file: $!";

This is tremendously more efficient than reading the entire file into
memory as an array of lines and then processing it one element at a time,
which is often--if not almost always--the wrong approach.  Whenever
you see someone do this:

	@lines = <INPUT>;

you should think long and hard about why you need everything loaded at
once.  It's just not a scalable solution.  You might also find it more
fun to use the standard Tie::File module, or the DB_File module's
$DB_RECNO bindings, which allow you to tie an array to a file so that
accessing an element the array actually accesses the corresponding
line in the file.

You can read the entire filehandle contents into a scalar.

	{
	local(*INPUT, $/);
	open (INPUT, $file) 	|| die "can't open $file: $!";
	$var = <INPUT>;
	}

That temporarily undefs your record separator, and will automatically
close the file at block exit.  If the file is already open, just use this:

	$var = do { local $/; <INPUT> };

For ordinary files you can also use the read function.

	read( INPUT, $var, -s INPUT );

The third argument tests the byte size of the data on the INPUT filehandle
and reads that many bytes into the buffer $var.

=head2 How can I read in a file by paragraphs?
X<file, reading by paragraphs>

Use the C<$/> variable (see L<perlvar> for details).  You can either
set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
for instance, gets treated as two paragraphs and not three), or
C<"\n\n"> to accept empty paragraphs.

Note that a blank line must have no blanks in it.  Thus
S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.

=head2 How can I read a single character from a file?  From the keyboard?
X<getc> X<file, reading one character at a time>

You can use the builtin C<getc()> function for most filehandles, but
it won't (easily) work on a terminal device.  For STDIN, either use
the Term::ReadKey module from CPAN or use the sample code in
L<perlfunc/getc>.

If your system supports the portable operating system programming
interface (POSIX), you can use the following code, which you'll note
turns off echo processing as well.

	#!/usr/bin/perl -w
	use strict;
	$| = 1;
	for (1..4) {
		my $got;
		print "gimme: ";
		$got = getone();
		print "--> $got\n";
		}
    exit;

	BEGIN {
	use POSIX qw(:termios_h);

	my ($term, $oterm, $echo, $noecho, $fd_stdin);

	$fd_stdin = fileno(STDIN);

	$term     = POSIX::Termios->new();
	$term->getattr($fd_stdin);
	$oterm     = $term->getlflag();

	$echo     = ECHO | ECHOK | ICANON;
	$noecho   = $oterm & ~$echo;

	sub cbreak {
		$term->setlflag($noecho);
		$term->setcc(VTIME, 1);
		$term->setattr($fd_stdin, TCSANOW);
		}

	sub cooked {
		$term->setlflag($oterm);
		$term->setcc(VTIME, 0);
		$term->setattr($fd_stdin, TCSANOW);
		}

	sub getone {
		my $key = '';
		cbreak();
		sysread(STDIN, $key, 1);
		cooked();
		return $key;
		}

	}

	END { cooked() }

The Term::ReadKey module from CPAN may be easier to use.  Recent versions
include also support for non-portable systems as well.

	use Term::ReadKey;
	open(TTY, "</dev/tty");
	print "Gimme a char: ";
	ReadMode "raw";
	$key = ReadKey 0, *TTY;
	ReadMode "normal";
	printf "\nYou said %s, char number %03d\n",
		$key, ord $key;

=head2 How can I tell whether there's a character waiting on a filehandle?

The very first thing you should do is look into getting the Term::ReadKey
extension from CPAN.  As we mentioned earlier, it now even has limited
support for non-portable (read: not open systems, closed, proprietary,
not POSIX, not Unix, etc) systems.

You should also check out the Frequently Asked Questions list in
comp.unix.* for things like this: the answer is essentially the same.
It's very system dependent.  Here's one solution that works on BSD
systems:

	sub key_ready {
		my($rin, $nfd);
		vec($rin, fileno(STDIN), 1) = 1;
		return $nfd = select($rin,undef,undef,0);
		}

If you want to find out how many characters are waiting, there's
also the FIONREAD ioctl call to be looked at.  The I<h2ph> tool that
comes with Perl tries to convert C include files to Perl code, which
can be C<require>d.  FIONREAD ends up defined as a function in the
I<sys/ioctl.ph> file:

	require 'sys/ioctl.ph';

	$size = pack("L", 0);
	ioctl(FH, FIONREAD(), $size)    or die "Couldn't call ioctl: $!\n";
	$size = unpack("L", $size);

If I<h2ph> wasn't installed or doesn't work for you, you can
I<grep> the include files by hand:

	% grep FIONREAD /usr/include/*/*
	/usr/include/asm/ioctls.h:#define FIONREAD      0x541B

Or write a small C program using the editor of champions:

	% cat > fionread.c
	#include <sys/ioctl.h>
	main() {
	    printf("%#08x\n", FIONREAD);
	}
	^D
	% cc -o fionread fionread.c
	% ./fionread
	0x4004667f

And then hard code it, leaving porting as an exercise to your successor.

	$FIONREAD = 0x4004667f;         # XXX: opsys dependent

	$size = pack("L", 0);
	ioctl(FH, $FIONREAD, $size)     or die "Couldn't call ioctl: $!\n";
	$size = unpack("L", $size);

FIONREAD requires a filehandle connected to a stream, meaning that sockets,
pipes, and tty devices work, but I<not> files.

=head2 How do I do a C<tail -f> in perl?
X<tail> X<IO::Handle> X<File::Tail> X<clearerr>

First try

	seek(GWFILE, 0, 1);

The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
but it does clear the end-of-file condition on the handle, so that the
next C<< <GWFILE> >> makes Perl try again to read something.

If that doesn't work (it relies on features of your stdio implementation),
then you need something more like this:

	for (;;) {
	  for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
	    # search for some stuff and put it into files
	  }
	  # sleep for a while
	  seek(GWFILE, $curpos, 0);  # seek to where we had been
	}

If this still doesn't work, look into the C<clearerr> method
from C<IO::Handle>, which resets the error and end-of-file states
on the handle.

There's also a C<File::Tail> module from CPAN.

=head2 How do I dup() a filehandle in Perl?
X<dup>

If you check L<perlfunc/open>, you'll see that several of the ways
to call open() should do the trick.  For example:

	open(LOG, ">>/foo/logfile");
	open(STDERR, ">&LOG");

Or even with a literal numeric descriptor:

   $fd = $ENV{MHCONTEXTFD};
   open(MHCONTEXT, "<&=$fd");	# like fdopen(3S)

Note that "<&STDIN" makes a copy, but "<&=STDIN" make
an alias.  That means if you close an aliased handle, all
aliases become inaccessible.  This is not true with
a copied one.

Error checking, as always, has been left as an exercise for the reader.

=head2 How do I close a file descriptor by number?
X<file, closing file descriptors> X<POSIX> X<close>

If, for some reason, you have a file descriptor instead of a
filehandle (perhaps you used C<POSIX::open>), you can use the
C<close()> function from the C<POSIX> module:

	use POSIX ();
	
	POSIX::close( $fd );
	
This should rarely be necessary, as the Perl C<close()> function is to be
used for things that Perl opened itself, even if it was a dup of a
numeric descriptor as with C<MHCONTEXT> above.  But if you really have
to, you may be able to do this:

	require 'sys/syscall.ph';
	$rc = syscall(&SYS_close, $fd + 0);  # must force numeric
	die "can't sysclose $fd: $!" unless $rc == -1;

Or, just use the fdopen(3S) feature of C<open()>:

	{
	open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!";
	close $fh;
	}

=head2 Why can't I use "C:\temp\foo" in DOS paths?  Why doesn't `C:\temp\foo.exe` work?
X<filename, DOS issues>

Whoops!  You just put a tab and a formfeed into that filename!
Remember that within double quoted strings ("like\this"), the
backslash is an escape character.  The full list of these is in
L<perlop/Quote and Quote-like Operators>.  Unsurprisingly, you don't
have a file called "c:(tab)emp(formfeed)oo" or
"c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.

Either single-quote your strings, or (preferably) use forward slashes.
Since all DOS and Windows versions since something like MS-DOS 2.0 or so
have treated C</> and C<\> the same in a path, you might as well use the
one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++,
awk, Tcl, Java, or Python, just to mention a few.  POSIX paths
are more portable, too.

=head2 Why doesn't glob("*.*") get all the files?
X<glob>

Because even on non-Unix ports, Perl's glob function follows standard
Unix globbing semantics.  You'll need C<glob("*")> to get all (non-hidden)
files.  This makes glob() portable even to legacy systems.  Your
port may include proprietary globbing functions as well.  Check its
documentation for details.

=head2 Why does Perl let me delete read-only files?  Why does C<-i> clobber protected files?  Isn't this a bug in Perl?

This is elaborately and painstakingly described in the
F<file-dir-perms> article in the "Far More Than You Ever Wanted To
Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .

The executive summary: learn how your filesystem works.  The
permissions on a file say what can happen to the data in that file.
The permissions on a directory say what can happen to the list of
files in that directory.  If you delete a file, you're removing its
name from the directory (so the operation depends on the permissions
of the directory, not of the file).  If you try to write to the file,
the permissions of the file govern whether you're allowed to.

=head2 How do I select a random line from a file?
X<file, selecting a random line>

Here's an algorithm from the Camel Book:

	srand;
	rand($.) < 1 && ($line = $_) while <>;

This has a significant advantage in space over reading the whole file
in.  You can find a proof of this method in I<The Art of Computer
Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth.

You can use the File::Random module which provides a function
for that algorithm:

	use File::Random qw/random_line/;
	my $line = random_line($filename);

Another way is to use the Tie::File module, which treats the entire
file as an array.  Simply access a random array element.

=head2 Why do I get weird spaces when I print an array of lines?

Saying

	print "@lines\n";

joins together the elements of C<@lines> with a space between them.
If C<@lines> were C<("little", "fluffy", "clouds")> then the above
statement would print

	little fluffy clouds

but if each element of C<@lines> was a line of text, ending a newline
character C<("little\n", "fluffy\n", "clouds\n")> then it would print:

	little
	 fluffy
	 clouds

If your array contains lines, just print them:

	print @lines;

=head1 REVISION

Revision: $Revision: 10126 $

Date: $Date: 2007-10-27 21:29:20 +0200 (Sat, 27 Oct 2007) $

See L<perlfaq> for source control details and availability.

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.

This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.

Irrespective of its distribution, all code examples here are in the public
domain.  You are permitted and encouraged to use this code and any
derivatives thereof in your own programs for fun or for profit as you
see fit.  A simple comment in the code giving credit to the FAQ would
be courteous but is not required.