There are exactly three reasons why kill -9 will not kill a process.
(Old man pages labeled this as "Kill with extreme prejudice". A process can not avoid -9, but only delay its death.)
You can check on the process with ps -O stat -p 1234 for example. Examine the status (S) column:
The three reasons why a process does not immediately die from -9 are:
- It's already dead -- it's a zombie. (status=Z) It will go away when its parent waits for it (and collects the zombie) or the parent process itself dies.
- It's stuck in a device wait. (status=D) The only way to kill these is to get the device to release it or reboot. This may require waiting, or physically manipulating the device, or some other out of band way to "kick" the stuck device. (And in rare cases, a reboot will fail and hang, and you'll have to power cycle.)
- It's suspended (status=T), in which case kill -9 does kill it, but it doesn't actually die until the process is resumed (kill -CONT)
The kill command does actually return something, although if it succeeds, doesn't print anything. All commands have a return value. You can check it with echo $? immediately after running the command. If the return value is 0, the kill succeeded and nothing prints. Any other value is an error, and it should print the error; possible errors might be Permission denied (or operation not permitted), and No such process. pkill has a few errors that are slightly more interesting.
