How to setup ROS environment variable on Ubuntu 20.04?

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I tried following this guide to install ROS, but even after adding ROS source.list and its key

sudo apt install ros-melodic-desktop-full

gave error.

E: Unable to locate package ros-melodic-desktop-full

Then I ran this command

sudo apt search ros

to see if any such package exists. I couldn't find ros-melodic-desktop-full but I found another similar package ros-desktop-full. So I installed it instead. The installation went smooth without giving any errors.

Next step in the guide is to set-up ROS environment variable, but I have no such directory

/opt/ros

So how do I setup the environment variable?

P.S.

I also installed some tools and dependencies with this command

sudo apt install python3-rosdep python3-rosinstall python3-rosinstall-generator python3-wstool build-essential

and initialized rosdep

sudo rosdep init
rosdep update