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 most shells (including Bourne and C), the PID of the last subprocess you launched in the background will be stored in the special variable $!.

#!/bin/bash
./app1 &
PID=$!
# ...
kill $PID

There is some information here under the Special Variables section.