os/doc/tests.md

# Testing

Testing is always an important aspect of any software development project, and
NixOS offers some incredibly powerful tools to write tests for your
configuration, and, optionally, run them in
[CI](./integrations/hercules.md).

## Unit Tests
Unit tests can be created from regular derivations, and they can do
almost anything you can imagine. By convention, it is best to test your
packages during their [check phase][check]. All packages and their tests will
be built during CI.

## Integration Tests
All your profiles defined in suites will be tested in a NixOS VM.

You can write integration tests for one or more NixOS VMs that can,
optionally, be networked together, and yes, it's as awesome as it sounds!

Be sure to use the `mkTest` function from digga, `digga.lib.pkgs-lib.mkTest`
which wraps the official [testing-python][testing-python] function to ensure
that the system is setup exactly as it is for a bare DevOS system. There are
already great resources for learning how to use these tests effectively,
including the official [docs][test-doc], a fantastic [blog post][test-blog],
and the examples in [nixpkgs][nixos-tests].

[test-doc]: https://nixos.org/manual/nixos/stable/index.html#sec-nixos-tests
[test-blog]: https://www.haskellforall.com/2020/11/how-to-use-nixos-for-lightweight.html
[default]: https://github.com/divnix/devos/tree/master/tests/default.nix
[run-test]: https://github.com/NixOS/nixpkgs/blob/6571462647d7316aff8b8597ecdf5922547bf365/lib/debug.nix#L154-L166
[nixos-tests]: https://github.com/NixOS/nixpkgs/tree/master/nixos/tests
[testing-python]: https://github.com/NixOS/nixpkgs/tree/master/nixos/lib/testing-python.nix
[check]: https://nixos.org/manual/nixpkgs/stable/#ssec-check-phase