Diz "nenhum tal PID" ou há um erro, como este trabalho?
kill -TERM -- -GPID
Observe também, conforme (enfatize o meu) man 1 :
"[…] When an argument of the form '-n' is given, and it is meant to denote a process group […]"
homem 2 :
"[…] If pid is less than -1, then sig is sent to every process in the process group whose ID is -pid. […]"
homem 3 :
"[…] If pid is negative, but not -1, sig shall be sent to all processes (excluding an unspecified set of system processes) whose process group ID is equal to the absolute value of pid, […]"
Como em, não PID, mas ID do grupo de processos.
Talvez você possa se divertir tanto com / proc / [pid] / stat
ppid: awk '{gsub(/\([^)]+\)/,"_"); print $4}' /proc/3955/stat
pgrp: awk '{gsub(/\([^)]+\)/,"_"); print $5}' /proc/3955/stat
pkill -TERM -g PGRP