Would Nix work well with GitHub Actions? Or is it more of a replacement? How do you automate running tests and deploying to dev on every push, for example?

> Would Nix work well with GitHub Actions?

You can use Nix with GitHub actions since there is a Nix GitHub action: https://github.com/marketplace/actions/install-nix. Every time the action is triggered, Nix rebuilds everything, but thanks to its caching (need to be configured), it only rebuilds targets that has changed.

> How do you automate running tests and deploying to dev on every push

Nix is a build tool and it's main purpose is not to deploy artifacts. There are however a lot of tools to deploy artifacts built by Nix: https://github.com/nix-community/awesome-nix?tab=readme-ov-f...

Note there are also several Nix CI that can do a better job than a raw GitHub actions, because they are designed for Nix (Hydra, Garnix, Hercules, ...).

One neat Nix feature is development shells, which let you define isolated shell environments that can be activated by invoking `nix develop` (or via direnv upon entering a directory):

    devShells.default = pkgs.mkShell {
      packages = with pkgs; [ opentofu terragrunt ];
    };

I can then use these tools inside the devShell from my jobs like so:

    jobs:
      terragrunt-plan:
        runs-on: [self-hosted, Linux, X64]
        defaults:
          run:
            shell: nix develop --command bash -e {0}
        steps:
          - name: Checkout
            uses: actions/checkout@v4
          - name: Plan
            run: terragrunt --terragrunt-non-interactive run-all plan

Since I'm doing this within a Nix flake all of the dependencies for this environment are recorded in a lock file. Provided my clone of the repo is up to date I should have the same versions.

You can combine this with direnv and auto-activate the nix environment when you `cd` into directories as well. We do this, and just activate the shell in ci environments with a cache. Works great.

Yes. GitHub actions can be just a thin wrapper to call any Nix commands that you can run locally.

> How do you automate running tests

You just build the Nix derivation that runs your tests, e.g. `nix build #tests` or `nix flake check` in your workflow file.

> deploying to dev on every push

You can set up a Nix `devShell` as a staging area for any operations you'd need to perform for a deployment. You can use the same devShell both locally and in CI. You'd have to inject any required secrets into the Action environment in your repository settings, still. It doesn't matter what your staging environment is comprised of, Nix can handle it.