perlのfork&execで `echo "show databases" | mysql -uroot` を再現してみた

gistfile1.pl

use strict;
use warnings;
use Data::Dumper;
use IO::Handle;

my ($parent_in, $parent_out);
my ($child_in, $child_out);
pipe $parent_out, $child_in;
pipe $child_out, $parent_in;
$parent_in->autoflush(1);
$child_in->autoflush(1);

if (my $pid = fork) {
    close $parent_out;
    close $parent_in;

    my ($parent_in2, $child_out2);
    pipe $child_out2, $parent_in2;
    $parent_in2->autoflush(1);

    if (my $pid2 = fork) {
        close $child_out;
        close $parent_in2;

        my @lines = <$child_out2>;
        print "@lines";
        waitpid($pid2,0);
    }
    else {
        close $child_out2;

        open STDIN, '<&', $child_out;
        open STDOUT, '>&', $parent_in2;

        exec ('mysql', '-uroot');
    }
    waitpid($pid,0);
}
else {
    close $child_out;
    close $child_in;

    open STDIN, '<&', $parent_out;
    open STDOUT, '>&', $parent_in;

    exec ('echo', 'show databases');
}