Environment variables are another way that you can pass state into an application beyond reading data in from a file or passing it explicitly over the command line. This recipe will explore some very basic getting and setting of environment variables and then work with the highly useful third-party library envconfig (https://github.com/kelseyhightower/envconfig).
We'll build an application that can read a config file via JSON or through environment variables. The next recipe will explore alternative formats, including TOML and YAML.