awk '/^config\ hostname:.*/ {f=1;print $3}' barnyard2.conf
prints my-units-hostname
yet when I run this:
HOSTNAME=2-bot.kernelops.org; awk -v hostname=$HOSTNAME '/config\ hostname:.*/ {f=1 && $2=="hostname:"} f {$3="\t"hostname} {print}' barnyard2.conf > newbarnyard
every single line in the file has the $HOSTNAME variable printed in column 3...