Hacking

This section provides some notes on how to hack on Nix. To get the latest version of Nix from GitHub:

$ git clone https://github.com/NixOS/nix.git
$ cd nix

To build Nix for the current operating system/architecture use

$ nix-build

or if you have a flake-enabled nix:

$ nix build

This will build defaultPackage attribute defined in the flake.nix file. To build for other platforms add one of the following suffixes to it: aarch64-linux, i686-linux, x86_64-darwin, x86_64-linux. i.e.

$ nix-build -A defaultPackage.x86_64-linux

To build all dependencies and start a shell in which all environment variables are set up so that those dependencies can be found:

$ nix-shell

or if you have a flake-enabled nix:

$ nix develop

To get a shell with a different compilation environment (e.g. stdenv, gccStdenv, clangStdenv, clang11Stdenv, ccacheStdenv):

$ nix-shell -A devShells.x86_64-linux.clang11StdenvPackages

or if you have a flake-enabled nix:

$ nix develop .#clang11StdenvPackages

Note: you can use ccacheStdenv to drastically improve rebuild time. By default, ccache keeps artifacts in ~/.cache/ccache/.

To build Nix itself in this shell:

[nix-shell]$ ./bootstrap.sh
[nix-shell]$ ./configure $configureFlags --prefix=$(pwd)/outputs/out
[nix-shell]$ make -j $NIX_BUILD_CORES

To install it in $(pwd)/outputs and test it:

[nix-shell]$ make install
[nix-shell]$ make installcheck -j $NIX_BUILD_CORES
[nix-shell]$ ./outputs/out/bin/nix --version
nix (Nix) 3.0

If you have a flakes-enabled Nix you can replace:

$ nix-shell

by:

$ nix develop

Running tests

Unit-tests

The unit-tests for each Nix library (libexpr, libstore, etc..) are defined under src/{library_name}/tests using the googletest framework.

You can run the whole testsuite with make check, or the tests for a specific component with make libfoo-tests_RUN. Finer-grained filtering is also possible using the --gtest_filter command-line option.

Functional tests

The functional tests reside under the tests directory and are listed in tests/local.mk. The whole testsuite can be run with make install && make installcheck. Individual tests can be run with make tests/{testName}.sh.test.

Integration tests

The integration tests are defined in the Nix flake under the hydraJobs.tests attribute. These tests include everything that needs to interact with external services or run Nix in a non-trivial distributed setup. Because these tests are expensive and require more than what the standard github-actions setup provides, they only run on the master branch (on https://hydra.nixos.org/jobset/nix/master).

You can run them manually with nix build .#hydraJobs.tests.{testName} or nix-build -A hydraJobs.tests.{testName}

Installer tests

After a one-time setup, the Nix repository's GitHub Actions continuous integration (CI) workflow can test the installer each time you push to a branch.

Creating a Cachix cache for your installer tests and adding its authorization token to GitHub enables two installer-specific jobs in the CI workflow:

  • The installer job generates installers for the platforms below and uploads them to your Cachix cache:

    • x86_64-linux
    • armv6l-linux
    • armv7l-linux
    • x86_64-darwin
  • The installer_test job (which runs on ubuntu-latest and macos-latest) will try to install Nix with the cached installer and run a trivial Nix command.

One-time setup

  1. Have a GitHub account with a fork of the Nix repository.
  2. At cachix.org:
    • Create or log in to an account.
    • Create a Cachix cache using the format <github-username>-nix-install-tests.
    • Navigate to the new cache > Settings > Auth Tokens.
    • Generate a new Cachix auth token and copy the generated value.
  3. At github.com:
    • Navigate to your Nix fork > Settings > Secrets > Actions > New repository secret.
    • Name the secret CACHIX_AUTH_TOKEN.
    • Paste the copied value of the Cachix cache auth token.

Using the CI-generated installer for manual testing

After the CI run completes, you can check the output to extract the installer URL:

  1. Click into the detailed view of the CI run.

  2. Click into any installer_test run (the URL you're here to extract will be the same in all of them).

  3. Click into the Run cachix/install-nix-action@v... step and click the detail triangle next to the first log line (it will also be Run cachix/install-nix-action@v...)

  4. Copy the value of install_url

  5. To generate an install command, plug this install_url and your GitHub username into this template:

    sh <(curl -L <install_url>) --tarball-url-prefix https://<github-username>-nix-install-tests.cachix.org/serve