Some may find it convenient to have roscore automatically launched as a system service on startup. To do this:

rosrun robot_upstart install --job roscore robosub/launch/roscore.launch

Once installed, you can use the normal service tools to manage the service:

sudo service roscore status
sudo service roscore start
sudo service roscore restart
sudo service roscore stop

To uninstall the service:

rosrun robot_upstart uninstall roscore

References: http://docs.ros.org/jade/api/robot_upstart/html/