# Building Nix

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

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

> **Note**
>
> 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](../installation/index.md).


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

```console
$ nix-shell
```

To get a shell with one of the other [supported compilation environments](#compilation-environments):

```console
$ nix-shell --attr devShells.x86_64-linux.native-clangStdenvPackages
```

> **Note**
>
> You can use `native-ccacheStdenv` to drastically improve rebuild time.
> By default, [ccache](https://ccache.dev) keeps artifacts in `~/.cache/ccache/`.

To build Nix itself in this shell:

```console
[nix-shell]$ mesonFlags+=" --prefix=$(pwd)/outputs/out"
[nix-shell]$ dontAddPrefix=1 configurePhase
[nix-shell]$ buildPhase
```

To test it:

```console
[nix-shell]$ checkPhase
```

To install it in `$(pwd)/outputs`:

```console
[nix-shell]$ installPhase
[nix-shell]$ ./outputs/out/bin/nix --version
nix (Nix) 2.12
```

To build a release version of Nix for the current operating system and CPU architecture:

```console
$ nix-build
```

You can also build Nix for one of the [supported platforms](#platforms).

## Building Nix with flakes

This section assumes you are using Nix with the [`flakes`] and [`nix-command`] experimental features enabled.


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

```console
$ 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](#compilation-environments):

```console
$ nix develop .#native-clangStdenv
```

> **Note**
>
> Use `ccacheStdenv` to drastically improve rebuild time.
> By default, [ccache](https://ccache.dev) keeps artifacts in `~/.cache/ccache/`.

To build Nix itself in this shell:

```console
[nix-shell]$ configurePhase
[nix-shell]$ buildPhase
```

To test it:

```console
[nix-shell]$ checkPhase
```

To install it in `$(pwd)/outputs`:

```console
[nix-shell]$ installPhase
[nix-shell]$ nix --version
nix (Nix) 2.12
```

For more information on running and filtering tests, see
[`testing.md`](./testing.md).

To build a release version of Nix for the current operating system and CPU architecture:

```console
$ nix build
```

You can also build Nix for one of the [supported platforms](#platforms).

## Platforms

Nix can be built for various platforms, as specified in [`flake.nix`]:


- `x86_64-linux`
- `x86_64-darwin`
- `i686-linux`
- `aarch64-linux`
- `aarch64-darwin`
- `armv6l-linux`
- `armv7l-linux`
- `riscv64-linux`

In order to build Nix for a different platform than the one you're currently
on, you need a way for your current Nix installation to build code for that
platform. Common solutions include [remote build machines] and [binary format emulation]
(only supported on NixOS).


Given such a setup, executing the build only requires selecting the respective attribute.
For example, to compile for `aarch64-linux`:

```console
$ nix-build --attr packages.aarch64-linux.default
```

or for Nix with the [`flakes`] and [`nix-command`] experimental features enabled:

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

Cross-compiled builds are available for:
- `armv6l-linux`
- `armv7l-linux`
- `riscv64-linux`
Add more [system types](#system-type) to `crossSystems` in `flake.nix` to bootstrap Nix on unsupported platforms.

### Building for multiple platforms at once

It is useful to perform multiple cross and native builds on the same source tree,