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

The following instructions assume you already have some version of Nix installed locally, so that you can use it to set up the development environment. If you don't have it installed, follow the installation instructions.

Nix with flakes

This section assumes you are using Nix with flakes enabled. See the next section for equivalent instructions which don't require flakes.

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

$ nix develop

This shell also adds ./outputs/bin/nix to your $PATH so you can run nix immediately after building it.

To get a shell with one of the other supported compilation environments:

$ nix develop .#native-clang11StdenvPackages

Note

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 $NIX_BUILD_CORES
[nix-shell]$ nix --version
nix (Nix) 2.12

To build a release version of Nix:

$ nix build

You can also build Nix for one of the supported target platforms.

Classic Nix

This section is for Nix without flakes.

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

To get a shell with one of the other supported compilation environments:

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

Note

You can use native-ccacheStdenvPackages 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) 2.12

To build Nix for the current operating system and CPU architecture use

$ nix-build

You can also build Nix for one of the supported target platforms.

Platforms

As specified in flake.nix, Nix can be built for various platforms:

aarch64-linux
i686-linux
x86_64-darwin
x86_64-linux

In order to build Nix for a different platform than the one you're currently on, you need to have some way for your system Nix to build code for that platform. Common solutions include remote builders and binfmt emulation (only supported on NixOS).

These solutions let Nix perform builds as if you're on the native platform, so executing the build is as simple as

$ nix build .#packages.aarch64-linux.default

for flake-enabled Nix, or

$ nix-build -A packages.aarch64-linux.default

for classic Nix.

You can use any of the other supported platforms in place of aarch64-linux.

Cross-compiled builds are available for ARMv6 and ARMv7, and Nix on unsupported platforms can be bootstrapped by adding more crossSystems in flake.nix.

Compilation environments

Nix can be compiled using multiple environments:

stdenv: default;
gccStdenv: force the use of gcc compiler;
clangStdenv: force the use of clang compiler;
ccacheStdenv: enable [ccache], a compiler cache to speed up compilation.

To build with one of those environments, you can use

$ nix build .#nix-ccacheStdenv

for flake-enabled Nix, or

$ nix-build -A nix-ccacheStdenv

for classic Nix.

You can use any of the other supported environments in place of nix-ccacheStdenv.

Editor integration

The clangd LSP server is installed by default on the clang-based devShells. See supported compilation environments and instructions how to set up a shell with flakes or in classic Nix.

To use the LSP with your editor, you first need to set up clangd by running:

make clean && bear -- make -j$NIX_BUILD_CORES install

Configure your editor to use the clangd from the shell, either by running it inside the development shell, or by using nix-direnv and the appropriate editor plugin.

Note

For some editors (e.g. Visual Studio Code), you may need to install a special extension for the editor to interact with clangd. Some other editors (e.g. Emacs, Vim) need a plugin to support LSP servers in general (e.g. lsp-mode for Emacs and vim-lsp for vim). Editor-specific setup is typically opinionated, so we will not cover it here in more detail.

Running tests

Unit-tests

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

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. Each test is a bash script.

The whole test suite can be run with:

$ make install && make installcheck
ran test tests/foo.sh... [PASS]
ran test tests/bar.sh... [PASS]
...

Individual tests can be run with make:

$ make tests/${testName}.sh.test
ran test tests/${testName}.sh... [PASS]

or without make:

$ ./mk/run-test.sh tests/${testName}.sh
ran test tests/${testName}.sh... [PASS]

To see the complete output, one can also run:

$ ./mk/debug-test.sh tests/${testName}.sh
+ foo
output from foo
+ bar
output from bar
...

The test script will then be traced with set -x and the output displayed as it happens, regardless of whether the test succeeds or fails.

Debugging failing functional tests

When a functional test fails, it usually does so somewhere in the middle of the script.

To figure out what's wrong, it is convenient to run the test regularly up to the failing nix command, and then run that command with a debugger like GDB.

For example, if the script looks like:

foo
nix blah blub
bar

edit it like so:

 foo
-nix blah blub
+gdb --args nix blah blub
 bar

Then, running the test with ./mk/debug-test.sh will drop you into GDB once the script reaches that point:

$ ./mk/debug-test.sh tests/${testName}.sh
...
+ gdb blash blub
GNU gdb (GDB) 12.1
...
(gdb)

One can debug the Nix invocation in all the usual ways. For example, enter run to start the Nix invocation.

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

Have a GitHub account with a fork of the Nix repository.
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.
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:

Click into the detailed view of the CI run.
Click into any installer_test run (the URL you're here to extract will be the same in all of them).
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...)
Copy the value of install_url

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

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

Checking links in the manual

The build checks for broken internal links. This happens late in the process, so nix build is not suitable for iterating. To build the manual incrementally, run:

make html -j $NIX_BUILD_CORES

In order to reflect changes to the Makefile, clear all generated files before re-building:

rm $(git ls-files doc/manual/ -o | grep -F '.md') && rmdir doc/manual/src/command-ref/new-cli && make html -j $NIX_BUILD_CORES

mdbook-linkcheck does not implement checking URI fragments yet.

`@docroot@` variable

@docroot@ provides a base path for links that occur in reusable snippets or other documentation that doesn't have a base path of its own.

If a broken link occurs in a snippet that was inserted into multiple generated files in different directories, use @docroot@ to reference the doc/manual/src directory.

If the @docroot@ literal appears in an error message from the mdbook-linkcheck tool, the @docroot@ replacement needs to be applied to the generated source file that mentions it. See existing @docroot@ logic in the Makefile. Regular markdown files used for the manual have a base path of their own and they can use relative paths instead of @docroot@.

Nix Reference Manual