Environment setup
You must source this script in every bash terminal you use ROS in.
source /opt/ros/noetic/setup.bash
It can be convenient to automatically source this script every time a new shell is launched. These commands will do that for you.
Bash
If you have more than one ROS distribution installed, ~/.bashrc must only source the setup.bash for the version you are currently using.
echo "source /opt/ros/noetic/setup.bash" >> ~/.bashrc source ~/.bashrc
zsh
echo "source /opt/ros/noetic/setup.zsh" >> ~/.zshrc source ~/.zshrc