Ubuntu – Track and close children terminals. Spare the parent

I don't thing zombie process is much of a headache. A zombie process does not take up any resources. It is just that it has it's entry in the process table.

A Zombie process is not an orphan process, it does have a parent.

kill, skill pkill will not work since the process is already killed, just that it's entry has not been removed.

Zombie process can be killed by sending SIGCHLD signal to parent. I think the signal number of SIGCHLD is 17 or 18

If this also fails, then you might want to kill the parent itself.

From Wikipedia on SIGCHLD signal:

When a child process terminates before the parent has called wait, the kernel retains some information about the process to enable its parent to call wait later. Because the child is still consuming system resources but not executing it is known as a zombie process.

EDIT 1: The system resources consumed is mostly the process table entry. If anyone knows if it consumes more than that - memory or CPU cycle, then please add an explanation. AFAIK it hardly takes up any significant system resources.

EDIT 2: Quoting from Wikipedia

On Unix and Unix-like computer operating systems, a zombie process or defunct process is a process that has completed execution but still has an entry in the process table. This entry is still needed to allow the process that started the (now zombie) process to read its exit status.

So the entry is kept so that the parent process can know the exit status because the moment the child exits, the parent is probably not in a state or not ready to read it's exit status.

EDIT 3

Till date I never experienced a zombie process taking 100% of the CPU. Seeing this for the first time.

Try doing a killall utorrent.exe

I can see that there are two instances of utorrent.exe and one of them is zombie. Probably the second one (child). killall should kill the parent since the child(zombie) cannot be killed.

EDIT 4

Looks like the killall did not work since it was giving TERM signal instead of KILL.

Try out killall --signal=KILL utorrent.exe

If this does not work then try killing the process selectivly.

Get the list of utorrent.exe process PID

ps -e | grep -i utorrent

You should get two process like

xxxx ?        aa:bb:cc utorrent.exe defunct
yyyy ?        aa:bb:cc utorrent.exe

So the second one is the parent. Kill it using

kill -9 yyyy

EDIT 5

Please try finding the process's Parent Id by this bash command

cat /proc/{defunctpid}/status | grep -i ppid

in your case is

cat /proc/7298/status | grep -i ppid

If the output comes like

PPid: 1

Then sadly I think you are out of luck. Process Id 1 belongs to init without which your system cannot run

You are killing the program SeondApp, but you are not killing the terminal it is running in. The two are separate things. For example, this is the process tree of running gedit in a terminal:

$ gedit &
[1] 13064
$ pstree -s 13064
systemd───systemd───gnome-terminal-───bash───gedit───4*[{gedit}]

Ignore the systemd, that's the init process, everything running on your machine is a child of systemd. Then, what you see there is that gnome-terminal has launched bash which then runs gedit. If you now kill gedit, that won't affect its parents. However, if you kill one of the parents, that will also kill the child.

Normally, what you would do is to use $!, a special variable that holds the PID of the last process launched to the background. Unfortunately, that doesn't with gnome-terminal which seems to have some sort of complicated launching procedure:

$ gnome-terminal &
[1] 23861
$ ps aux | grep 23861
terdon   24341  0.0  0.0   8684  2348 pts/11   S+   10:59   0:00 grep --color 23861
$ pgrep gnome-terminal
23866

As you can see above, gnome-terminal seem to re-launch itself after launching and uses a different PID. No idea why, but another good reason to use a different terminal.

So, since the standard approach won't work, we need a workaround. What you can do is use kill -$PID which will kil all processes in the process group (from man kill):

-n  where n is larger than 1.  All processes in process  group  n  are
    signaled.   When  an argument of the form '-n' is given, and it is
    meant to denote a process group, either a signal must be specified
    first,  or  the argument must be preceded by a '--' option, other‐
    wise it will be taken as the signal to send.

Putting all this together, here's a working version of your script:

#!/bin/bash
gnome-terminal --geometry=50x30 --working-directory=/$HOME/TEST --title terminal1 \
               -e  ' sh -c "./FirstAPP; exec bash"'
while true; do
    if ! pgrep SecondAPP; then
        gnome-terminal --geometry=50x30 --working-directory=/$HOME/TEST \
            --title terminal2 -e  'SecondAPP' &
        for ((i=0; i<3600; i+=5)); do
          sleep 5
          if ! pgrep SecondAPP; then
            break
          fi
        done
        ## Now, get the PID of SecondAPP
        pid=$(pgrep SecondAPP)
        ## Using -$pid will kill all process in the process group of $pid, so
        ## it will kill SecondAPP and its parent process, the terminal.
        kill -- -$pid
    fi
    sleep 5
done

Note that I also remove the [ ] around ! pgrep since that was wrong syntax.

I don't see why you are launching terminals at all though. Here's the same idea, without terminals:

#!/bin/bash

$HOME/TEST/FirstAPP
while true; do
    if ! pgrep SecondAPP; then
      #$HOME/TEST/SecondAPP &
      SecondAPP &
      pid=$!
        for ((i=0; i<3600; i+=5)); do
          sleep 5
          if ! pgrep SecondAPP; then
            break
          fi
        done
        kill $pid
    fi
    sleep 5
done

Finally, this feels like a strange way of doing things. You might want to ask a new question, explain what you are trying to do and why and we can see if we can find a simpler approach for whatever it is you need.