ROS2: What is '. install/setup.bash' doing?

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Source the environment with '. install/setup.bash'

When colcon has completed building successfully, the output will be in the install directory. Before you can use any of the installed executables or libraries, you will need to add them to your path and library paths. colcon will have generated bash/bat files in the install directory to help setup the environment. These files will add all of the required elements to your path and library paths as well as provide any bash or shell commands exported by packages.