How to kill a background process created in a script

Question

Suppose I input the following in a shell

(while true; do echo hahaha; sleep 1; done)&

Then I know I can kill it by

fg;          


        

Answer 1

(while true; do echo hahaha; sleep 1; done)&
RUNNING_PID=$!
kill ${RUNNING_PID}

$! will pick up the PID of the process that is running so you can do with it as you wish