nix-develop (for GitHub Actions)

This is the most explicit and compatible way I know of to load the environment of a nix flake devShell into a GitHub Actions job. It works just like other setup-* actions: use it in a GitHub Actions job, and in all the following steps of that job, PATH will include all the dependencies of your project's default devShell, and the environment will include all variables set in that devShell.

Usage

You can use this repository as a GitHub Action...

  - uses: nicknovitski/nix-develop@v1

...or run it as a nix app...

  - run: nix run github:nicknovitski/nix-develop/v1
  # This works even on action runners with nothing besides nix installed!

...or for the nixiest possible approach, add it to your flake's inputs, expose its packages.default output as one of your own packages, and nix run it that way.

# flake.nix
{
  inputs = {
    nix-develop-gha.url = "github:nicknovitski/nix-develop";
    nix-develop-gha.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = { nixpkgs, nix-develop-gha, ... }:
    in {
      packages.x86_64-linux.nix-develop-gha = nix-develop-gha.packages.x86_64-linux.default;
      devShells.default = let
        pkgs = nixpkgs.legacyPackages.x86_64-linux;
      in pkgs.mkShell {
          packages = [
            # your development dependencies
          ];
        };
      };
    });
}

  - run: nix run .#nix-develop-gha

Arguments

You can pass arbitrary command-line arguments to the underlying nix develop command.

  - uses: nicknovitski/nix-develop@v1
    with:
      arguments: "github:DeterminateSystems/zero-to-nix#multi"
  - run: nix run github:nicknovitski/nix-develop -- github:DeterminateSystems/zero-to-nix#multi

Why?

Why would you load a nix shell environment into a GitHub Actions job?

If you haven't heard of nix, then you don't need this action. Nix lets you write succint and reliably reproducible cross-platform shell environments, among many other things. This can help you manage build dependencies very well.

If you have heard about nix, and you already have all your builds and tests expressed as derivations, then you don't need this action. Your GitHub CI workflows are just checking out the code and running nix-build or nix flake check! Only keep reading if you're curious.

I made this action for the people who know the value of specifying shell environments in nix and using nix develop, and need to run any commands in GitHub actions besides nix-build and nix flake check.

Those people can now, instead of doing things like this...

      - run: |
          nix develop --command \
            cargo fmt --check
      - run: |
          nix develop --command \
            cargo-deny check
      - run: |
          nix develop --command \
            eclint \
              -exclude "Cargo.lock"
      - run: |
          nix develop --command \
            codespell \
              --skip target,.git \
              --ignore-words-list crate

...or even this...

      - run: cargo fmt --check
        shell: nix develop --command bash -e {0}
      - run: cargo-deny check
        shell: nix develop --command bash -e {0}
      - run: eclint \
               -exclude "Cargo.lock"
        shell: nix develop --command bash -e {0}
      - run: codespell \
              --skip target,.git \
              --ignore-words-list crate
        shell: nix develop --command bash -e {0}

...can instead do this:

      - uses: nicknovitski/nix-develop@v1
      - run: cargo fmt --check
      - run: cargo-deny check
      - run: eclint \
               -exclude "Cargo.lock"
      - run: codespell \
              --skip target,.git \
              --ignore-words-list crate

Besides just being less repetitive, only the nix-develop action makes the devShell environment available in all subsequent steps in the job, including ones that use: third-party actions. So the other approaches aren't fully compatible with the GitHub Actions ecosystem.

(That's originally why I wrote this action: I was working on a project which used yarn and GitHub Actions, and I wanted to install node and yarn with nix, but then use the setup-node action to handle caching of node modules. For that to work, setup-node needed to have yarn in PATH.)

How?

How does the action work?

First it runs nix develop. Without additional arguments, this evaluates and if necessary builds the devShells.default of the flake in the current working directory, or as a fall-back, the packages.default.

For each variable in the build environment of the nix develop target besides PATH, if the same variable is either unset or set to a different value in the step's current environment, the target's variable value is echo'd to GITHUB_ENV, setting that variable for all subsequent steps in the job.

For each entry in the build environment's PATH variable, starting from the last and ending with the first, if the entry is not present in the PATH of the step's current environment, it is echo'd to GITHUB_PATH, prepending it to the PATH variable in all subsequent steps of the job.

Contributing

Feel free! The script can be run locally with any arguments you want to test, and unsurprisingly, running nix develop will give you the same dependencies used to test changes in CI.

If you use direnv, you can also bring those dependencies into your own shell with nix-direnv-reload.