Each process is controlled by sending signals to it which tell it what to do. This is how the kill command has its effect; by sending a process a signal which tells it to stop, hang up or terminate etc. You can list the different types of signal that can be sent to a process by using the kill command together with its -l option. ... Signal type 1 (one) will send a process a signal to hang up ( SIGHUP ): signal type 9 (nine) is a signal to kill ( SIGKILL ) the process. ...