Adapted from the Command Line Essentials Companion Guide . You install something, open a fresh terminal, type the command, and get bash: python3: command not found — or on Windows, 'python3' is not recognized as an internal or external command . The installer said it finished successfully. You can probably even find the program in your applications folder. And yet the terminal insists it doesn't exist. The instinct at this point is usually to reinstall, or install a second copy fro...