use IPC::Open3;
use IO::Select;

$cmd="find /fst/home/ -name '*ooo*'";

$pid = open3(*CMD_IN, *CMD_OUT, *CMD_ERR, $cmd);
close(CMD_IN);

$SIG{CHLD} = sub {
    print "REAPER: status $? on $pid\n" if waitpid($pid, 0) > 0
};

$selector = IO::Select->new();
$selector->add(*CMD_ERR, *CMD_OUT);

while (@ready = $selector->can_read) {
    foreach $fh (@ready) {
    $line = scalar <$fh>;
    if (fileno($fh) == fileno(CMD_ERR)) {print "STDERR: ", $line}
    else {print "STDOUT: ", $line}
    ($line) || $selector->remove($fh);
    }
}

close(CMD_OUT);
close(CMD_ERR);
[download]

• Annoyance: upon completion there are STDOUT: STDERR: in front of the prompt (easy to clean out in the next step)
• Observation of strange behavior:
The messages only come out in the "right" order the first time the program is run in a shell. Subsequent calls mess up the order. Thus I have to exit and reenter the shell each time I debug. Weird.

So when I use open3, I create a program (sniped straight from the recipe only with my find command):

use IPC::Open3;
use IO::Select;

$cmd="find /fst/home/ -name '*ooo*'";

system("$cmd");

print "@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
+@@@@@@@@@@@@@@@@@@@@@@@@@@@\n";

$pid = open3(*CMD_IN, *CMD_OUT, *CMD_ERR, $cmd);

$SIG{CHLD} = sub {
    print "REAPER: status $? on $pid\n" if waitpid($pid, 0) > 0
};

#print CMD_IN "This line has a vt33 lurking in it\n";
close(CMD_IN);

$selector = IO::Select->new();
$selector->add(*CMD_ERR, *CMD_OUT);

while (@ready = $selector->can_read) {
    foreach $fh (@ready) {
        if (fileno($fh) == fileno(CMD_ERR)) {print "STDERR: ", scalar 
+<CMD_ERR>}
        else                                {print "STDOUT: ", scalar 
+<CMD_OUT>}
        $selector->remove($fh) if eof($fh);
    }
}
close(CMD_OUT);
close(CMD_ERR);
[download]

In this command, there are several dozen lines of output, and two of them go to STDOUT; the rest go to STDERR. The output looks something like

find: xxxxxxxx
..
/path/to/match1
find: xxxxxxxx
..
/path/to/match2
find: xxxxxxxx
[download]

however, running the program above, it simply hangs after the second line of output to STDOUT (the rest of STDERR is "left hanging?")

Edited by planetscape - added code tags

• Please the <code> tags around your code. (It's custom to PerlMonks.) It makes it (more) readable, preserves whitespace, handles HTML escaping, wraps lines if needed, and provides a mechanism to view the code unwrapped.

• One problem I see is that you use <FH> instead of sysread. See the warning at the bottom of the documentation for select.

• Another problem I see if that it only prints "STDOUT: " or "STDERR: " for every chunk, not necessarily every line. You need to split on newlines what you get.

• I never figured out the use of some methods in IO::Select. What's the use of can_read if you can't check for errors? The proper usage seems to be:

  use IO::Select;
  
  $selector = IO::Select->new();
  $selector->add(*CMD_ERR, *CMD_OUT);
  
  while ($selector->count()) {
     my ($readable, undef, $error) = 
        IO::Select::select($selector, undef, $selector);
  
     $selector->remove($_)
        foreach @$error;
  
     foreach my $fh (@$readable) {
        ...
     }
  )
  [download]

• I don't know what you mean by "left hanging".

OK, sorry about the code tags thing. I just tried it for the output of the strace (I can't edit the posts I made as Anynymous). So the code using

print "STDXXX: " , scalar <FH>
[download]

is straight from the recipe in the Perl Cookbook! Anyway, how do I use sysread instead of scalar <FH> ? As far as left hanging, I mean the program simply stops executing and the rest of the STDERR output is sort of left hanging in the ether, I guess, never to be outputted.

Ok so I ran an strace on the program and the last lines are

<p>
read(5, "/fst/home/m1jas05/.openoffice/sh"..., 4096) = 45
write(1, "STDOUT: /fst/home/m1jas05/.openo"..., 53STDOUT: /fst/home/m1
+jas05/.openoffice/share/dict/ooo
) = 53
read(5, 
<p>
[download]

so it looks like some kind of deadlock, but the child is not reading anything, only the parent is.

Man I am confused.