.env files were introduced by Heroku in 2012.

If you have a .env, you'll see instructions how to enable integration:

  dotenv.enable = true;

  # Optionally, you can choose which filename to load.
  # dotenv.filename = ".env.production";

When the developer environment is loaded, environment variables from .env will be loaded and set into config.env.

Variables from .env are set using lib.mkDefault, meaning that any existing env variables set in devenv.nix will have priority over them.