The Null Flake is a simple Nix flake designed to provide an empty attribute set as an output. It's especially useful for overriding specific inputs, such as secrets, in nixosConfiguration tests without introducing unnecessary complexity.
When testing NixOS configurations, you might encounter situations where certain inputs need to be overridden with a flake that has no meaningful content (e.g., secrets or sensitive data). The Null Flake serves this purpose by providing a placeholder flake that you can easily integrate into your build process.
Below is an example of how to use the Null Flake in a Dockerfile to override an input when checking the build of a NixOS configuration:
RUN nix --extra-experimental-features flakes --extra-experimental-features nix-command flake lock --override-input sec "github:devtechnica/nullflake?shallow=1"
CMD nix --extra-experimental-features flakes --extra-experimental-features nix-command build '.#nixosConfigurations.hostname.config.system.build.toplevel' --no-link