perlfaq5.pod

MSYS在windows下模拟了一个类unix的终端

    # make STDOUT go to three files, plus original STDOUT
    open (STDOUT, "| tee file1 file2 file3") or die "Teeing off: $!\n";
    print "whatever\n"                       or die "Writing: $!\n";
    close(STDOUT)                            or die "Closing: $!\n";

Otherwise you'll have to write your own multiplexing print
function--or your own tee program--or use Tom Christiansen's,
at http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz , which is
written in Perl and offers much greater functionality
than the stock version.

=head2 How can I read in an entire file all at once?

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 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.

On very rare occasion, you may have an algorithm that demands that
the entire file be in memory at once as one scalar.  The simplest solution
to that is

    $var = `cat $file`;

Being in scalar context, you get the whole thing.  In list context,
you'd get a list of all the lines:

    @lines = `cat $file`;

This tiny but expedient solution is neat, clean, and portable to
all systems on which decent tools have been installed.  For those
who prefer not to use the toolbox, you can of course read the file
manually, although this makes for more complicated code.

    {
	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> };

=head2 How can I read in a file 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 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?

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;

For legacy DOS systems, Dan Carson <[e-mail]> reports the following:

To put the PC in "raw" mode, use ioctl with some magic numbers gleaned
from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes
across the net every so often):

    $old_ioctl = ioctl(STDIN,0,0);     # Gets device info
    $old_ioctl &= 0xff;
    ioctl(STDIN,1,$old_ioctl | 32);    # Writes it back, setting bit 5

Then to read a single character:

    sysread(STDIN,$c,1);               # Read a single character

And to put the PC back to "cooked" mode:

    ioctl(STDIN,1,$old_ioctl);         # Sets it back to cooked mode.

So now you have $c.  If C<ord($c) == 0>, you have a two byte code, which
means you hit a special key.  Read another byte with C<sysread(STDIN,$c,1)>,
and that value tells you what combination it was according to this
table:

    # PC 2-byte keycodes = ^@ + the following:

    # HEX     KEYS
    # ---     ----
    # 0F      SHF TAB
    # 10-19   ALT QWERTYUIOP
    # 1E-26   ALT ASDFGHJKL
    # 2C-32   ALT ZXCVBNM
    # 3B-44   F1-F10
    # 47-49   HOME,UP,PgUp
    # 4B      LEFT
    # 4D      RIGHT
    # 4F-53   END,DOWN,PgDn,Ins,Del
    # 54-5D   SHF F1-F10
    # 5E-67   CTR F1-F10
    # 68-71   ALT F1-F10
    # 73-77   CTR LEFT,RIGHT,END,PgDn,HOME
    # 78-83   ALT 1234567890-=
    # 84      CTR PgUp

This is all trial and error I did a long time ago; I hope I'm reading the
file that worked...

=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?

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 <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 POSIX module.  POSIX defines
the clearerr() method, which can remove the end of file condition on a
filehandle.  The method: read until end of file, clearerr(), read some
more.  Lather, rinse, repeat.

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

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

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, ">>/tmp/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?

This should rarely be necessary, as the Perl close() function is to be
used for things that Perl opened itself, even if it was a dup of a
numeric descriptor as with 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 open():

    { 
	local *F; 
	open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
	close F;
    }

=head2 Why can't I use "C:\temp\foo" in DOS paths?  What doesn't `C:\temp\foo.exe` work?

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?

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 "Far More Than
You Ever Wanted To Know" in
http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .

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?

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.  A simple proof by induction is available upon 
request if you doubt the algorithm's correctness.

=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 AUTHOR AND COPYRIGHT

Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
All rights reserved.

When included as an integrated part of the Standard Distribution
of Perl or of its documentation (printed or otherwise), this works is
covered under Perl's Artistic License.  For separate distributions of
all or part of this FAQ outside of that, see L<perlfaq>.

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.