Testing with Environment Variables

It’s a common (and very good) practice to configure applications with environment variables. To simplify writing tests for code that depends on environment variables, Go provides a helper method on testing.T. Call t.Setenv to register a value for an environment variable for your test. Behind the scenes, it calls Cleanup to revert the environment variable to its previous state when the test exits:

// assume ProcessEnvVars is a function that processes environment variables
// and returns a struct with an OutputFormat field
func TestProcessEnvVar(t *testing.T) {
    t.Setenv("OUTPUT_FORMAT", "JSON")
    cfg := ProcessEnvVars()
    if cfg.OutputFormat != "JSON" {
        t.Error("OutputFormat not set correctly")
    }
    // value of OUTPUT_FORMAT is reset when the test function exits
}

Note

While it is good to use environment variables to configure your application, it is also good to make sure that most of your code is entirely unaware of them. Be sure to copy the values of environment variables into configuration structs before your program starts its work, in your main function or soon afterward. Doing so makes it easier to reuse and test code, since how the code is configured has been abstracted away from what the code is doing.

Rather than writing this code yourself, you should strongly consider using a third-party configuration library, like Viper or envconfig. Also, look at GoDotEnv as a way to store environment variables in.env files for development or continuous integration machines.