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 can be tested against an individual host.
Simply use digga's pre-baked `digga.lib.allProfilesTest` like so:

```nix
{
  hosts = {
    Morty.tests = [ allProfilesTest ];
  };
}
```

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.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/core/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