Some process rows are not removed even by kill -9, and they carry a STAT letter you have not met in earlier lessons. Look at such a row and try to remove it with the KILL signal.
A Z row and kill -9
In ~/zombie-lab, run three commands: the rows of the spooler program, kill -9 on the process from zombie.pid, and its row again. $(cat zombie.pid) puts the file's contents, a process number, into the command.
First, predict
What will the last command show after kill -9?
Now start the exercise, run the command and compare your prediction with the output.
Z means zombie. The process has already exited: its program no longer runs, and its memory is freed. What is left in the process table is a row with the exit status, which the parent has to collect. You did the same in the shell: wait got the exit status of a background job.
Until the parent collects the status, the row hangs on. A signal will not help: there is nobody to receive it, so kill -9 reported success while the row stayed. If the parent exits, its zombies pass to systemd, PID 1, which collects their statuses right away.
One zombie is harmless: it takes neither CPU nor memory. The trouble is a parent that does not collect statuses, so zombies pile up. That is why you deal with the parent: find the program by PPID and see whether it can collect the statuses on request. Ending the parent is a last resort, since its work stops along with it.
A Z row hangs in ps, and its parent is a working program. What will remove the row?
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The Z row is an entry in the process table, not tied to the program file. The parent removes it by collecting the code.
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The row hangs while the parent lives and has not collected the code. The parent collects it, or systemd once the parent is gone.
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A zombie has already exited, and a signal does nothing to it: in the exercise the Z row stayed in place after
kill -9.
A zombie has already exited and waits for its parent to collect the exit status. So you deal with the parent: find it by PPID and see whether it can collect the code without exiting.