Killing process in Shell Script

Question

I have a very simple problem: When I run a shell script I start a program which runs in an infinite loop. After a while I wanna stop then this program before I can it again with

Answer 1

In bash $! expands to the PID of the last process started in the background. So you can do:

./app1 param1 &
APP1PID=$!
# ...
kill $APP1PID