Let's create and build a catkin workspace:
$ mkdir -p ~/catkin_ws/src $ cd ~/catkin_ws/ $ catkin_make
Python 3 users in ROS Melodic and earlier: note, if you are building ROS from source to achieve Python 3 compatibility, and have setup your system appropriately (ie: have the Python 3 versions of all the required ROS Python packages installed, such as catkin) the first catkin_make command in a clean catkin workspace must be:
$ catkin_make -DPYTHON_EXECUTABLE=/usr/bin/python3
This will configure catkin_make with Python 3. You may then proceed to use just catkin_make for subsequent builds.
Additionally, if you look in your current directory you should now have a 'build' and 'devel' folder. Inside the 'devel' folder you can see that there are now several setup.*sh files. Sourcing any of these files will overlay this workspace on top of your environment. To understand more about this see the general catkin documentation: catkin. Before continuing source your new setup.*sh file:
$ source devel/setup.bash
To make sure your workspace is properly overlayed by the setup script, make sure ROS_PACKAGE_PATH environment variable includes the directory you're in.
$ echo $ROS_PACKAGE_PATH /home/youruser/catkin_ws/src:/opt/ros/kinetic/share