How to get a specific PID #

[Ralf Mardorf]

On Wed, 30 Dec 2015 12:09:39 +0100, I wrote:
>>xkill  
>
>If the OP wants to know what PID belongs to what window, then wmctrl is
>the way to go. There are exceptions, when wmctrl does not provide the
>PID, but usually it does provide it. However, I doubt that xkill would
>work, if a program doesn't provide a real window anymore, but wmctrl
>usually allows to at least determine the PID of the instances that
>still work as expected, so the OP just needs to kill the remaining PID
>by htop, kill or another command line tool.

Assumed somebody wants to know an exceptional case when wmctrl doesn't
show the PID:

https://lists.archlinux.org/pipermail/arch-general/2015-December/040347.html

I can't reboot to my Ubuntu install now, so I only could test it with
the Linux I'm currently running.