The rosdep tool in ROS helps us easily install the dependencies of the packages that we are going to compile. This tool is also necessary for some core components of ROS.

This command launches rosdep: 

$ sudo rosdep init 
$ rosdep update

Here, while the first command was called, a file called 20-default.list was created in /etc/ros/rosdep/sources.list.d/, with a list of links that connect to the respective ros-distros.