How to use the command `nix why-depends` (with examples)

The nix why-depends command is used to show why a package depends on another package. It is a useful tool for understanding the dependencies of packages in the Nix ecosystem.

Use case 1: Show why the currently running NixOS system requires a certain store path

This use case allows you to see why the currently running NixOS system depends on a specific store path.

Code:

nix why-depends /run/current-system /nix/store/...

Motivation: This use case can come in handy when you want to understand why a particular store path is necessary for the functioning of your NixOS system. It can help you identify dependencies and troubleshoot any issues related to the dependencies of your system.

Explanation:

• /run/current-system: This argument specifies the currently running NixOS system.
• /nix/store/...: This argument represents the store path that you want to investigate. You need to replace ... with the specific store path you want to analyze.

Example output:

The following packages depend on '/nix/store/...':
• nixos.system
• nixos.systemPackages

In this example output, you can see that the currently running NixOS system depends on the specified store path. Additionally, it shows the specific packages (nixos.system and nixos.systemPackages) that have this dependency.

Use case 2: Show why a package from nixpkgs requires another package as a build-time dependency

This use case allows you to determine why a package from nixpkgs requires another package as a build-time dependency.

Code:

nix why-depends --derivation nixpkgs#dependent nixpkgs#dependency

Motivation: Understanding the build-time dependencies of packages is crucial for efficiently managing software development projects. This use case helps you identify the reasons for a package’s dependency on another package during the build process.

Explanation:

• --derivation nixpkgs#dependent: This argument specifies the derivation of the package from nixpkgs that you want to analyze. Replace dependent with the actual package name.
• nixpkgs#dependency: This argument represents the dependency package in nixpkgs that you want to investigate. Replace dependency with the actual package name.

Example output:

The package 'nixpkgs#dependent' has the following build-time dependencies on 'nixpkgs#dependency':
• nixpkgs#dependency1
• nixpkgs#dependency2

In this example output, you can see that the package nixpkgs#dependent requires nixpkgs#dependency1 and nixpkgs#dependency2 as build-time dependencies. This information can be useful for understanding the dependencies of a package in order to optimize the build process or manage dependencies more effectively.

Conclusion:

The nix why-depends command is a powerful tool for understanding the dependencies of packages in the Nix ecosystem. By using this command, you can investigate why a package depends on another package and gain valuable insights into the dependencies of your NixOS system or nixpkgs packages. Understanding these dependencies can help you troubleshoot issues, optimize builds, and manage dependencies more effectively.