Question

I need to modify an existing Perl program. I want to pipe a string (which can contain multiple lines) through an external program and read the output from this program. This external program is used to modify the string. Let’s simply use cat as a filter program. I tried it like this, but it doesn’t work. (Output of cat goes to STDOUT instead of being read by perl.)

#!/usr/bin/perl

open(MESSAGE, "| cat |") or die("cat failed\n");
print MESSAGE "Line 1\nLine 2\n";
my $message = "";
while (<MESSAGE>)
{
    $message .= $_;
}
close(MESSAGE);
print "This is the message: $message\n";

I’ve read that this isn’t supported by Perl because it may end up in a deadlock, and I can understand it. But how do I do it then?

Answer 1

You can use IPC::Open3 to achieve bi-directional communication with child.

use strict;
use IPC::Open3;

my $pid = open3(\*CHLD_IN, \*CHLD_OUT, \*CHLD_ERR, 'cat')
    or die "open3() failed $!";

my $r;

for(my $i=1;$i<10;$i++) {
    print CHLD_IN "$i\n";
    $r = <CHLD_OUT>;
    print "Got $r from child\n";
}