Nix Flakes¶

A Nix Flake is a feature in the Nix package manager that provides a standardized and declarative way to define and manage projects, packages, dependencies, and development environments. It simplifies the way Nix projects are packaged, versioned, and shared by introducing a flake as a well-structured, self-contained unit of code or configuration.

Key Features of Nix Flakes:¶

Reproducibility: Flakes ensure that the same environment or package can be recreated anywhere, providing consistency across different systems and users.
Dependency Pinning: Flakes allow you to lock dependencies to specific versions, ensuring that builds remain reproducible even if upstream packages change.
Composability: Flakes can reference other flakes, allowing for easier reuse of configuration or packages across different projects. This makes it simpler to manage dependencies and compose complex projects from smaller, reusable parts.
Standard Structure: A Nix Flake has a standardized structure, usually defined in a flake.nix file, which clearly outlines how the package or project is built, what dependencies are required, and what outputs are expected (e.g., applications, libraries, development shells, etc.).
Multiple Outputs: Flakes support defining multiple outputs, such as:
- Packages: Pre-built binaries or libraries.
- Development Shells: Development environments that can be instantiated with nix develop.
- Apps: Executable programs.
- Tests: Automated tests for the flake.
Easy Sharing: Flakes simplify sharing Nix configurations by using the flake.nix file as a single source of truth, which can be distributed or referenced easily via Git repositories or other remote locations.

Structure of a Nix Flake:¶

A Nix Flake is defined in a flake.nix file, which typically includes information like:
- Inputs: The dependencies of the flake (other flakes or packages).
- Outputs: What the flake provides, such as packages, applications, or development environments.

Here’s an example structure of a flake.nix file:

{
  description = "A simple example Nix Flake";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs";
  };

  outputs = { self, nixpkgs }: {
    packages.default = nixpkgs.lib.mkShell {
      buildInputs = [ nixpkgs.hello ];
    };

    devShell = nixpkgs.mkShell {
      buildInputs = [ nixpkgs.gcc nixpkgs.cmake ];
    };
  };
}

Inputs: The nixpkgs input references the standard Nix package collection from GitHub.
Outputs: It defines a default package and a development shell, each using dependencies from nixpkgs.

How Nix Flakes are Used¶

Package Management: Flakes make it easier to manage project dependencies by pinning specific versions and providing a consistent environment.
Development Environments: Developers can use flakes to define and share reproducible development environments, ensuring that everyone on a team uses the same tooling and dependencies.
Continuous Integration/Deployment: Flakes simplify CI/CD pipelines by providing a consistent, declarative way to define how applications are built and tested.

Using Flakes¶

To use a Nix Flake, you’ll need Nix 2.4 or newer, as flakes are a relatively new feature that requires enabling specific functionality in Nix. Below is a step-by-step guide on how to use a flake.

1. Enable Flakes in Nix¶

Before using flakes, you need to enable them. To do this, add the following to your Nix configuration:

For single commands, you can enable flakes by passing the --experimental-features flag:

nix --experimental-features 'nix-command flakes' <command>

To enable flakes globally (recommended):
Edit or create the ~/.config/nix/nix.conf file and add the following line:

mkdir -p ~/.config/nix/
echo 'experimental-features = nix-command flakes' \
    >> ~/.config/nix/nix.conf

2. Basic Commands to Use a Flake¶

Once flakes are enabled, here’s how you can use them in different contexts:

2.1. Clone and Explore a Flake¶

To fetch and inspect a flake from a remote repository, you can use the following command:

nix flake info github:<user>/<repo>

For example:

nix flake info github:NixOS/nixpkgs

This will show details about the flake, such as its inputs, outputs, and versions.

2.2. Run a Flake Application¶

If a flake provides applications, you can run them directly using the nix run command:

nix run github:<user>/<repo>#<app>

For example, to run the hello application from a flake:

nix run github:NixOS/nixpkgs#hello

This will download and execute the hello package from the specified flake.

2.3. Enter a Development Environment (DevShell)¶

If a flake defines a development shell, you can enter it using the nix develop command:

nix develop github:<user>/<repo>

This command drops you into a shell where all the dependencies required for development (as specified by the flake) are available. For example:

nix develop github:NixOS/nixpkgs

This is particularly useful for ensuring that all developers working on a project have the same development environment.

2.4. Build a Flake¶

If the flake provides build outputs (such as applications, packages, or containers), you can build them using the nix build command:

nix build github:<user>/<repo>#<package>

For example:

nix build github:NixOS/nixpkgs#hello

This command fetches the source code and dependencies, builds the application, and stores the resulting output in the ./result symlink, which you can run or inspect.

3. Create Your Own Flake¶

You can also create your own flake. Here’s how to get started:

3.1. Create a `flake.nix` File¶

In your project’s directory, create a flake.nix file. Below is a simple example that provides a development shell and an application:

{
  description = "A simple example Nix Flake";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs";
  };

  outputs = { self, nixpkgs }: {
    packages = {
      default = nixpkgs.hello;
    };

    devShell = nixpkgs.mkShell {
      buildInputs = [ nixpkgs.gcc nixpkgs.cmake ];
    };
  };
}

Inputs: This defines external dependencies (in this case, nixpkgs from GitHub).
Outputs: This defines the flake’s outputs:
packages.default: Points to a package (in this case, the hello application).
devShell: Defines a development shell with gcc and cmake installed.

3.2. Build Your Flake¶

You can build your flake with:

nix build

This will build the default package specified in the flake.nix file.

3.3. Enter the DevShell¶

To enter the development shell defined in the flake:

nix develop

This will provide you with a shell where gcc and cmake are pre-installed.

4. Pin Dependencies with `flake.lock`¶

When you use or create a flake, Nix will automatically generate a flake.lock file. This file locks the versions of all inputs (such as nixpkgs), ensuring that builds remain reproducible even if the upstream repositories change.

For example, if you’re using the latest version of nixpkgs, flake.lock ensures that subsequent builds use the same version unless you manually update the lock file.

To update your dependencies, run:

nix flake update

5. Use Local Flakes¶

If you’re working on a local flake, you can reference it using a path:

nix develop ./my-flake

This lets you test local flakes and ensure they are working as expected before sharing them.

Summary of Common Commands:¶

Command	Description
`nix flake info <flake>`	Show information about a flake (e.g., inputs, outputs).
`nix run <flake>#<app>`	Run an application provided by a flake.
`nix develop <flake>`	Enter a development shell provided by a flake.
`nix build <flake>#<package>`	Build a specific package from the flake.
`nix flake update`	Update the lock file to use the latest versions of inputs.

Conclusion¶

Nix Flakes introduce a more structured and user-friendly approach to managing Nix projects by providing reproducibility, dependency pinning, and ease of sharing. They help simplify complex workflows, making it easier to manage environments, packages, and deployments across different systems while ensuring consistency and reliability. Using a Nix Flake allows you to manage dependencies, build environments, and share configurations in a declarative and reproducible way. Whether you’re running pre-built applications or setting up a consistent development environment, flakes help streamline and standardize your workflow, ensuring consistency across different systems.

Page last modified: 2024-09-18 11:49:35