I have a zombie process that refuses to be removed, have tried killall -9 1913 and pkill with no effect. Any help would be very much appreciated.Running Ubuntu 12.04.2 LTS. Thank you.

aruna@aruna-desktop:~$ ps -aux | grep Z
Warning: bad ps syntax, perhaps a bogus '-'? See http://procps.sf.net/faq.html
aruna     1913  0.0  0.0      0     0 ?        Z    12:07   0:00 [lightdm-session] <defunct>
  • This is a special case for a known bug, still not fixed. More references here, here and here Oct 10 '14 at 20:18

It looks like the root cause of this zombie process is a bug that is described here.

Getting Rid of Zombie Processes:

You can’t kill zombie processes as you can kill normal processes with the SIGKILL signal — zombie processes are already dead. Bear in mind that you don’t need to get rid of zombie processes unless you have a large amount on your system – a few zombies are harmless. However, there are a few ways you can get rid of zombie processes.

One way is by sending the SIGCHLD signal to the parent process. This signal tells the parent process to execute the wait() system call and clean up its zombie children. Send the signal with the kill command, replacing pid in the command below with the parent process’s PID:

kill -s SIGCHLD pid

However, if the parent process isn’t programmed properly and is ignoring SIGCHLD signals, this won’t help. You’ll have to kill or close the zombies’ parent process. When the process that created the zombies ends, init inherits the zombie processes and becomes their new parent. (init is the first process started on Linux at boot and is assigned PID 1.) init periodically executes the wait() system call to clean up its zombie children, so init will make short work of the zombies. You can restart the parent process after closing it.

If a parent process continues to create zombies, it should be fixed so that it properly calls wait() to reap its zombie children. File a bug report if a program on your system keeps creating zombies.

You can find parent process id when you use l option with ps that is ps -l, it will show in PID and child process will show in PPID

You can also see process tree by using pstree command.

  • Thank you for the answer, it's way over my head right now but I understand some of it. This may sound simple to one who is experienced but HOW do I find the parent process ID of the zombie ? Jul 6 '13 at 18:13
  • I have updated the answer
    – Raza
    Jul 6 '13 at 18:24
  • Thank's again. Tried everything it still remains, am not woorired anymore since a zombie does not consume system resources and I just learnt a whole lot of stuff so am happy :-) Jul 6 '13 at 18:38

Well, I would have said "you can't do that" before I googled up this stackoverflow question. It appears that you can use gdb to call the waitpid() system call in the parent process of the zombie process. Good trick.

My advice is to not do this: whatever "lightdm" is, it probably cleans up after exited child processes every so often, or maybe it assumes it does. If you do the waitpid() with gdb, you may hose up lightdm.

  • Many thank's very useful and I will look through the gdb trick and study it first before I try it.. Jul 6 '13 at 18:36

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