ps aux | awk '{ print $8 "" $2 }' | grep -w Z
From: http://www.cyberciti.biz/tips/killing-zombie-process.html
From the comments an improved one:
for p in $(ps jauxww | grep Z | grep -v PID | awk '{print $3}'); do for every in $(ps auxw | grep $p | grep cron | awk '{print $2}'); do kill -9 $every; done;done;Careful though: this one also kills the proces.
