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Introduction to Flakes

The flakes experimental feature is a major development for Nix, it introduces a policy for managing dependencies between Nix expressions, it improves reproducibility, composability and usability in the Nix ecosystem. Although it's still an experimental feature, flakes have been widely used by the Nix community.[^1]

Flakes is one of the most significant changes the nix project has ever seen.[^2]

A Word of Caution about Flakes

The benefits of Flakes are evident, and the entire NixOS community has embraced it wholeheartedly. Currently, more than half of the users utilize Flakes[^3], providing assurance that Flakes will not be deprecated.

⚠️ However, it's important to note that Flakes is still an experimental feature. Some issues persist, and there is a possibility of introducing breaking changes during the stabilization process. The extent of these breaking changes remains uncertain.

Overall, I strongly recommend everyone to use Flakes, especially since this book revolves around NixOS and Flakes. However, it's crucial to be prepared for potential problems that may arise due to forthcoming breaking changes.

Nix Flakes and Classic Nix

Since the nix-command and flakes features are still experimental, the official documentation lacks detailed coverage, and the community's documentation on them is also scattered. From the perspective of reproducibility, ease of management, and maintenance, the classic Nix package structure and CLI are no longer recommended. Therefore, I will not delve into the usage of classic Nix. Beginners are advised to start with nix-command and flakes while disregarding any content related to classic Nix.

The following are classic Nix commands and associated concepts that are no longer necessary after enabling nix-command and flakes. When searching for information, you can safely ignore them:

  1. nix-channel: nix-channel manages software package versions through stable/unstable/test channels, similar to other package management tools such as apt/yum/pacman.
    1. In Flakes, the functionality of nix-channel is entirely replaced by the inputs section in flake.nix.
  2. nix-env: nix-env is a core command-line tool for classic Nix used to manage software packages in the user environment.
    1. It installs packages from the data sources added by nix-channel, causing the installed package's version to be influenced by the channel. Packages installed with nix-env are not automatically recorded in Nix's declarative configuration and are completely independent of its control, making them challenging to reproduce on other machines. Therefore, it is not recommended to use this tool.
    2. The corresponding command in Flakes is nix profile, which is also not recommended for use.
  3. nix-shell: nix-shell creates a temporary shell environment, which is useful for development and testing.
    1. In Flakes, this tool is divided into three sub-commands: nix develop, nix shell, and nix run. We will discuss these three commands in detail in the "Development" chapter.
  4. nix-build: nix-build builds Nix packages and places the build results in /nix/store, but it does not record them in Nix's declarative configuration.
    1. In Flakes, nix-build is replaced by nix build.
  5. ...

NOTE: nix-env -qa may still be useful in some cases, as it returns all packages installed in the system.

When Will Flakes Be Stabilized?

I delved into some details regarding Flakes:

After reviewing these resources, it seems likely that Flakes will be stabilized within one or two years, possibly accompanied by some breaking changes[^1][^2][^3].