The launch files in ROS are a very useful feature for launching more than one node. In the preceding examples, we have seen a maximum of two ROS nodes, but imagine a scenario in which we have to launch 10 or 20 nodes for a robot. It will be difficult if we run each node in a terminal one by one. Instead, we can write all nodes inside an XML-based file called launch files and, using a command called roslaunch, we can parse this file and launch the nodes.
The roslaunch command will automatically start the ROS Master and the parameter server. So, in essence, there is no need to start the roscore command and individual node; if we launch the file, all operations will be done in a single command.
Let's start creating launch files. Switch to the package folder and create a new launch file called demo_topic.launch to launch two ROS nodes that are publishing and subscribing an integer value. We keep the launch files in a launch folder, which is inside the package:
$ roscd mastering_ros_demo_pkg...