extract, alter, publish

hey everyone, im forever trying to subscribe to a topic (gazebo/link_states) alter the info and publish it again so i can use it for other nodes. however. i cant get it to work with a seperate subscriber and publisher, as the subscriber node doesnt, obviously, publish any data. and neither do i succeed with a publisher node alone. because; how do i extract info from the link_states topic in a publisher node without having to use a rospy.subscriber? ive been trying for over 4 hours and cant seem to figure it out, is the whole publisher/subscriber approach wrong? any answers are welcome! greetings girpon