Congratulations! We are done with the ROS installation, but what next? 

The ROS installation mainly consists of scripts and executables, which are mostly installed to /opt/ros/<ros_version>.

To get access to these commands and scripts, we should add ROS environment variables to the Ubuntu Terminal. It's easy to do this. To access ROS commands from inside the Terminal, we have to source the /opt/ros/<ros_version>/setup.bash file.

Here's the command to do so: 

$ source /opt/ros/melodic/setup.bash 

But in order to get the ROS environment in multiple Terminals, we should add the command to the .bashrc script, which is in the home folder. The .bashrc script will be sourced whenever a new Terminal opens: 

$ echo "source /opt/ros/melodic/setup.bash" >> ~/.bashrc 
$ source ~/.bashrc 

We can install multiple ROS distributions on Ubuntu. If there are multiple distributions, we can switch to each ROS distribution by changing the distribution name in the preceding command.