Defunct (Zombie) process, preap and orphan process

A defunct (or zombie) process is one whose exit status has yet to be reaped by its parent. So when a process shows as "defunct" in ps, you need to reap it. Here's how:

preap(1) - force a defunct process to be reaped by its parent

Syntax: /usr/bin/preap PID

So, to get rid of all zombies on our system, all we have to do is script this to reap all process marked as defunct:

/usr/bin/preap $(ps -ef | grep defunct | awk '{ print $2 }' | xargs)

So, what's an orphan process then? If the parent of a defunct process happens to exit, it becomes an orphan, and is asigned PID 1 as a parent - see init(1M).