Can a child be killed by a not-parent process? - unix

Question

I'm making a project in which the parent forks a terminator process that kills one random child of the parent. It seems that it creates some problems.
Is it allowed?

Answer 1

Not only is that allowed: any process running as the same user or as the root user can kill any other process.
Running as my user ID in a terminal I can kill anything with my user ID. Even the terminal that I am running in. Or my GUI process.
The only process in Unix OS types that is immune to kill is PID 1 aka init. Because killing init would result in an immediate kernel panic. If PID 1 exits for any reason, such as an internal bug and segmentation fault, there's an immediate kernel panic.

Answer 2

It is allowed. Write the following code in parent.sh
terminator() {
sleep 2;
echo "(terminator) Going to kill Pid $1"
kill -9 "$1" && echo "(terminator) Pid $1 killed"
}
sleep 7 &
sleep 7 &
sleep 7 &
pid=$!
echo "Random pid=${pid} will be killed"
sleep 7 &
sleep 7 &
terminator ${pid} &
echo "All started"
ps -ef | sed -n '1p; /sleep 7/p'
sleep 3
echo "After kill"
ps -ef | sed -n '1p; /sleep 7/p'
Background processes are childs. The terminator child will kill a random other child after 2 seconds.
Random pid=6781 will be killed
All started
UID PID PPID C STIME TTY TIME CMD
notroot 6779 6777 0 16:59 pts/0 00:00:00 sleep 7
notroot 6780 6777 0 16:59 pts/0 00:00:00 sleep 7
notroot 6781 6777 0 16:59 pts/0 00:00:00 sleep 7
notroot 6782 6777 0 16:59 pts/0 00:00:00 sleep 7
notroot 6783 6777 0 16:59 pts/0 00:00:00 sleep 7
(terminator) Going to kill Pid 6781
(terminator) Pid 6781 killed
parent.sh: line ...: 6781 Killed sleep 7
After kill
UID PID PPID C STIME TTY TIME CMD
notroot 6779 6777 0 16:59 pts/0 00:00:00 sleep 7
notroot 6780 6777 0 16:59 pts/0 00:00:00 sleep 7
notroot 6782 6777 0 16:59 pts/0 00:00:00 sleep 7
notroot 6783 6777 0 16:59 pts/0 00:00:00 sleep 7