In previous sections, we created nodes and have been executing them in different shells. Imagine working with 20 nodes and the nightmare of executing each one in a shell! However, with the launch file, we can do it in the same shell by launching a configuration file with the extension .launch. The launch file is a useful feature in ROS for launching more than one node.

To learn about the ROS launch file, we will create a new folder in the chapter2_tutorials package, as follows:

$ roscd chapter2_tutorials/ 
$ mkdir launch 
$ cd launch 
$ vim chapter2.launch 

Add the following code inside the chapter2.launch file:

<?xml version="1.0"?> 
<launch> 
   <node name ="example1a" pkg="chapter2_tutorials" type="example1a" output="screen"/> 
   <node name ="example1b" pkg="chapter2_tutorials" type="example1b" output="screen"/> 
</launch> 

This file is simple, although we would have to write a very complex file to, for example, control a complete robot, such as PR2. These could be real robots or simulated in ROS.

The ROS launch is an XML file with the .launch extension and has a launch tag. Inside this tag, we will find the node tag, which is used to launch a node from a package, for example, the example1a node from the chapter2_tutorials package.

The chapter2.launch file will execute two nodes—the first two examples of this chapter :example1a and example1b.

To launch the file, use the following command:

 $ roslaunch chapter2_tutorials chapter2.launch

We will see something similar to the following screenshot on our screens:

The running nodes are listed in the screenshot. When you launch a launch file, it is not necessary to execute it before the roscore command; roslaunch does it for us.