use-nix-shell-action

This action applies the environment of a nix shell to your GitHub Actions job.

This action works like workflow/nix-shell-action with one crucial difference: rather than only "applying" the provided shell to a script passed to the action's step, the shell is "applied" to the rest of the job.

This allows you to use nix to provide binaries that other GitHub Actions require and also allows you to run your action's steps in a nix shell without needing to wrap every step in a nix-shell action or bundle all your commands under one step.

Example

Note If nix isn't found, this action will handle installing nix for you using cachix/install-nix-action.

However it's recommended that you run this action yourself.

name: "Test"
on: [push]
jobs:
  example:
    runs-on: ubuntu-latest
    steps:
      - uses: cachix/install-nix-action@v18
        with:
          nix_path: nixpkgs=channel:nixos-unstable
      - name: Print env, before
        run: env
      # To make the environment in your flake's devShell available to future
      # steps:
      - uses: rrbutani/use-nix-shell-action@v1
        with:
          devShell: .#default # this is the default
      - name: Print env, after
        run: env
      - name: Run hello
        run: hello
      - name: Print env var
        run: echo $SOME_ENV_VAR

      # Alternatively you can also run a script directly in the shell; choosing
      # whether to preserve the environment outside the shell or not:
      - uses: rrbutani/use-nix-shell-action@v1
        with:
          devShell: .#
          exportEnv: false
          interpreter: python3
          clearEnvForScript: true # `SOME_ENV_VAR` will not be visible; nor will
                                  # `hello` be on `$PATH`
          script: |
            import os
            print(os.sys.version)
            print(os.environ['PATH'])

Options (with: ...)

Source

These options describe the shell that use-nix-shell-action should use.

Note You can only specify one of these options.

• packages: Comma-separated list of packages to install in your shell.
  • i.e. packages: bash,python3,python3Packages.numpy
    • spaces will be stripped so bash, python3, python3Packages.numpy works too
  • these packages are sourced from <nixpkgs>; see cachix/install-nix-action for easy ways to influence this channel or consider using flakes
• flakes: Comma-separated list of flake references to install in your shell.
  • i.e. nixpkgs#hello, github:edolstra/nix-serve, .#myPackage
• devShell: Flake reference to a devShell output. (default)
  • i.e. .# or github:NixOS/nix or some/path#someShell or .#devShells.x86_64-linux.someSpecialShell
• file: Path to a nix file producing a shell.
  • i.e. shell.nix

Export Options

• exportEnv: Boolean specifying whether use-nix-shell-action should export the shell given to your environment.
  • defaults to true
• preserveDefaultPath: Boolean controlling whether the shell's environment overrides $PATH in the job (false) or appends to it (true).

  Warning Be careful with this option; if your shell doesn't include bash, node, docker, etc. those tools will not be on the PATH after this step; this can break other actions.

  • note: things added to $GITHUB_PATH (i.e. by other actions) will be preserved regardless
  • defaults to true

Script Options

use-nix-shell-action can also, optionally, run a script of your choosing under your nix shell. This is the functionality provided by workflow/nix-shell-action but with some small mechanical differences; this action provides ways to run scripts under flake dev shells, for example.

• script: A script to run under the nix shell specified.
  • note: this runs after the environment is exported
    • if you wish to have your script affect the environment you'll need to update $GITHUB_ENV yourself
• interpreter: The interpreter under which to run script.
  • this should be present in your shell's $PATH
  • defaults to bash
• clearEnvForScript: Boolean specifying whether to preserve existing env vars when running the provided script.
  • note: this does not influence the environment that's exported and cannot be used to provide a "pure" shell for future steps in your action
  • defaults to true

Other options

• extraNixOptions: Escape hatch that you can use to specify extra flags to be passed to the command producing the shell.
  • for example --impure or extra --options
  • see the options for nix print-dev-env and nix shell

FAQ

Does this work on self-hosted runners?

Yes!

Are bash functions set in the shell preserved?

Nope, sorry. Just regular (non-array) env vars. Anything that env prints out.

Note that this also does not preserve the readonly property of env vars.

How does this work?

Essentially just constructs a nix shell, one way or another, and then dumps the contents of env into $GITHUB_ENV.

We reuse this logic from nix-direnv.

When should I use script instead of just exporting the env and running the script in a separate step?

i.e. this:

- uses: rrbutani/use-nix-shell-action@v1
  with:
    script: ./foo.sh

verus this:

- uses: rrbutani/use-nix-shell-action@v1
- run: ./foo.sh

The key difference here is "purity" (i.e. of the environment that foo.sh is run in). The former is run in the GitHub Actions environment (with the nix shell's environment layered on) while the latter is run with nix develop --ignore-environment (unless clearEnvForScript is set to false).