How to kill a zombie in one row. Execute the below command as root.

# kill -HUP `ps -A -ostat,ppid | grep -e '^[Zz]' | awk '{print $2}'`

Other good-to-know commands:

Who is a zombie?
# ps aux | awk '{ print $8 " " $2 }' | grep -w Z

Kill the zombie.
# kill -9 PID

Kill the zombie's parent (process).
# ps ef

Find the zombie.

# ps aux | grep 'Z'

or

# ps aux | grep 'defunct'

which gives

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND

backuppc 12397  0.0  0.0      0     0 ?        Z    Apr16   1:33 [BackupPC_dump] <defunct>
backuppc 12749  0.0  0.0      0     0 ?        Z    Apr13   6:44 [BackupPC_dump] <defunct>
backuppc 16986  0.0  0.0      0     0 ?        Z    Apr16   2:28 [BackupPC_dump] <defunct>
root     19409  0.0  0.0 103308   860 pts/1    S+   07:59   0:00 grep Z
root@mach217 ~/ [0]#
 
# pstree -H 12397 -p
which gives eg
 
init(1)---cnid_metad(1311)---cnid_dbd(5145)
 
Kill immediately with
# kill -9 5145