Killing a defunct zombie process means identifying and killing the parent process. In this article, I will show how you can force a zombie process so that you can practice removing it.
A simple way of forcing a zombie process is to start a process that has a short sleep in the background, immediately invoking a child process that last longer. The command below will create a zombie child process for a minute.
# use subshell, otherwise kill will end the session $(sleep 1 & exec /bin/sleep 60)
During this 60 seconds, you can use ps to show the defunct zombie child process and its parent process id. top will also report the zombie process.
# shows defunct zombie child process
$ ps ux | awk '{if($8=="Z+") print}'
fabian 1209381 0.0 0.0 0 0 pts/1 Z+ 08:01 0:00 [sleep] <defunct>
# shows child and then parent process id
$ ps -A -ostat,pid,ppid | grep -e '[zZ]'
Z+ 1209381 1209380
# kill parent process
$ kill -9 1209380
REFERENCES
sobyte.net, how to find and kill zombie processes
linux.stackexchange, creating zombie in bash
stackoverflow, zombie process in c
NOTES
another way to find the parent process of a child PID
ps -o ppid= -p <child_id>