This is a sample program I wrote to learn ROS. I have used linux joystick api to directly capture values of the analog stick of my joystick (F710 Wireless Gamepad). These data are then directly send to the turtlesim/cmd_velocity as a geometry_msgs/Twist type. NB: There are ready to use ROS packages available to capture joystick key events (which are easy to implement). This is to just start coding with ROS.
A personal blog having my hobby projects.